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#1 |
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Join Date: Apr 2015
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I'm having trouble getting my 81A to idle properly.
My 81A has a crab distributor and a later model Holly 94 carb. We've had it a couple of years but the more I work on it the more I realise that it has never been set up properly and was more or less pieced together to run but not well, so I'm working through each system in turn to make it more reliable. A couple of weeks ago I noticed a type of surging at idle. I have long suspected the carburettor needed some TLC so I have overhauled this myself with a kit from Daytona parts. I also got the carb casings vacuum blasted cause I like that sort of thing. While that was away I also pulled out the ignition loom holders and painted them and replaced the loom with parts from the hotrod company. I particularly wanted the right angled plugs for the crab distributor because previously it had straight ones on it and that end of the ignition wiring was way too close to the water pump pulleys. The distributor and coil are from Skip and have been working beautifully. Putting it all back together resulted in a first turn firing, and it appears to run well with a bit of choke and a bit of throttle but when it warms up and I open the choke and start to lower the revs it starts to die. It often recovers and then starts to die again and this cycle repeats itself. It seems to be a bit similar to the surging before I rebuilt the carb. I have attached a vacuum gauge to the mount for the carb (shown in photo). There is also a vacuum takeoff from the front of the intake manifold that is connected to the retard/advance mechanism at the front of the block for the leather plunger in the distributor (could someone let me know if it retards or advances on vacuum cause I'm confused about this also). The engine runs well at about 1100 RPM. I am measuring this with an optical RPM meter with a piece of reflective tape on the end of the crankshaft. I have wondered if this could be reading double but I doubt it but just to throw all the facts in the mix. I have set the idle mixture screws at this point to just off stumbling and then I start the process of backing down the idle adjustment with the expectation that I will need to readjust the idle mixtures as the idle drops. I can get the idle down to about 820rpm but its quite sensitive and doesn't seem to be stable at this speed. Intake manifold vacuum is also only about 14" at this point and I start to experience the dying. Winding the mixture screws in any further worsens the stability. Winding them further out seems to have minimum effect. At this point they are about 1 full turn from closed. When the engine starts to die the vacuum drops away to almost zero then most of the time the engine will pick back up and the vacuum rises to 14" again and then the cycle repeats. Some times it dies completely. Any further reduction on the idle screw results in it dying. My first thoughts are that I have an inlet manifold leak and certainly one of the inlet manifold bolts is missing (cause I had to steal it for another inlet manifold socket which holds the rear of the RH ignition holder which I found had been partially stripped. I had thought that the gasket would likely hold vacuum with one bolt missing but of course the problem might be staring me in the face. In any case I thought that before I go down any further rabbit holes I'd better get some advice. Sorry for the long winded post but I'm fairly new to this although I have reasonable mechanical skills and I'm very keen to learn. Regards David |
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#2 |
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I would suspect that you have a rubber tipped inlet valve needle and it is partially sticking causing the float level to change and inducing the surging.
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#3 |
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Its a brand new one from Daytona Parts. https://daytonaparts.com/daytona-car...oat-valve.html. It does appear to be rubber but I have understood from others that this is the one to use. Does it need to be broken in in some way? I was pretty careful setting the float height btw.
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#4 |
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If you adjust the idle mixture screws for the highest stable vacuum reading is that reading over 14? If not check spark plug gap make sure they are about 0.028" Has the distributor been set up on a machine and rebuilt? Get the missing bolt replaced and make sure all the intake bolts are tight.
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#5 |
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14" is too low. You're wanting something around 20". There's vacuum gauge reading diagnosis all over the web so look those up and pick the one that best matches what you're experiencing. Your engine looks great...is this a fresh engine or is it getting tired? With the engine idling at 850 rpm and the MP at 14" my guess is you've got a vacuum leak somewhere. I believe Skip sets the vacuum advance on the distributors he does so I wouldn't mess with it...that's not your problem.
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#6 |
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David...you've got perfect compression and a Skips rebuilt distributor and a fresh carb. Fuel, Ignition, Timing and Compression...doesn't sound like a cam issue and with 14" at 850 idle...sounds to me like a vacuum leak somewhere. Is the vacuum gauge steady at 14" at idle or is it fluctuating all over the place? A vacuum gauge is like an OBD2 for old engines!
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#8 |
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Is the throttleshaft tight in the housing ?
If its worn you get symptoms like this to. |
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#9 |
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Thank you everyone - very much appreciated. I've run some more tests this afternoon. I've removed all 16 of the inlet manifold bolts and re-torqued them to 25 ftpounds after removing the spring washers that were under bolt heads. They measured a bit short to me - I checked each for bottoming out first. so I think the manifold is bolted down well. I then disconnected the front manifold vacuum port which drives the advance (?) on the distributor and plugged it to isolate the distributor as a source of the leak. I checked all the mounting bolts and carb section bolts and tightened most of them and was a bit surprised that they seemed to be looser than I had set them. Maybe a few heat cycles can loosen them? Then started the engine. At 850 still about 14" and stable although as the engine starts to die so does the vacuum. At 1100 stable from memory about 16" vacuum. Interestingly at 850 it would slowly die to almost no vacuum and then pick back up again from very low revs - never seen an engine do that. Flatheadmurre, I looked carefully at the throttle shaft when I reassembled it and there was some very small but still noticeable lateral play in the RHS of the shaft to housing fitting. I had assumed it ok but maybe not. Thanks again.
PS if I disconnected the (electric) fuel pump and cranked the engine for a while should I expect to get 19" of vacuum if I didn't have any leaks and had the main butterfly valves closed? |
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#10 |
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Youre not going to get full vacuum from just cranking.
Did it work great in the past idling correctly ? |
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#11 |
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Thank you. Never perfectly but certainly not as bad as is it now. It had however started surging at idle and it was pronounced enough for me to feel that I'd better have a crack at the carb. It would never idle much below 800 but I have previously run a vacuum test on it and it was stable and didn't show any problems but i didn't note down the readings.
There were a few things I noticed while working on the carb. The return spring on the choke shaft was not attached so the choke could flop around quite a bit. The accelerator pump linkage was extremely loose with a lot of play in it. (it certainly used to stumble on acceleration from idle). Idle was not being set by the idle screw as there was a significant gap between the end of this and the throttle mechanism. The gaskets typically crumbled as I removed the carb. I'm not sure if any of this is relevant but I appreciate the support and guidance. |
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#12 |
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Can confirm your vaccum is way low. Mine is showing a steady 19 to 20 inch vaccum at idle rpms around 600 approx.
Could be worth a try to swap out the carb with a spare if you have one as your distributor side has been done and should be good. A suggestion to contact the Early Ford V8 Club in Wellington as there will be guys who can assist and perhaps provide good advice and assistance. Phil NZ |
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#13 |
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Hi Phil, yes good advice. Yes the vacuum is way low - around 14". I went through my old notes and I had run a vacuum test before which I noted "steady just below the green" and the green on my gauge starts at 20. I also have a spare distributor from skip which I can change to. I pretty much want a spare of everything
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#14 |
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The 81A intake manifold has two vacuum ports one on the RH front for the distributor vacuum brake that slows mechanical advance. The second is RH rear for the windshield wiper. Is the port under the Carburetor something you added?? Do you have the wiper vacuum port?
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Three different sources (Haynes, Chiltons and Equus) state the following for low but steady vacuum.
1) a lower reading indicates possible incorrect timing, incorrect valve timing or adjustment, incorrect setting of idle mixture, worn piston rings, or leak in intake manifold. 2) gauge reading low (15-20 in Hg) but steady indicates late ignition or valve timing, low compression, stuck throttle valve, leaking carburetor or manifold gasket. 3) low steady reading usually indicates leaking gasket between intake manifold and carburetor or throttle body, leaky vacuum hose, or incorrect camshaft timing.
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#16 |
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If you think it might be a vacuum leak ... and it does sound suspicious ... spray some Brake Clean on all the likely areas with engine idling at the lowest speed it can maintain. Gaskets, shafts, etc.
Engine should react if you hit the spot. Standard trouble-shooting for vacuum leaks.
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#17 |
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Thank you all.
I've just spent 1/2 day rebuilding the carb with extreme care and checking all the passages are clear etc. Assembled it with gasket sealant as well this time but no change in engine behaviour. I tried the brake clean squirt thingy and initially thought that I was onto it as the engine would burst into life and the vacuum would rise significantly but it turned out that it was sucking the vaporised cleaner in the air intake and it liked it. Didn't matter where I sprayed I got the same result and as soon as the electric fan turned on it stopped having an effect because the vapour got blown away. It seems to run better at higher revs and (say >1500) and when up there I am seeing 18" of vacuum. As I let the revs reduce gradually I am also hearing the odd miss and this seems to become more noticeable as the revs reduce. I am going to change the distributor as I have a spare one next just to eliminate that and then I will pull the plugs and check them as well and then change the coil to try and eliminate the ignition system as a potential source of trouble. I don't think it will make any difference but I'm an engineer by trade and we have to do this stuff to be sure don't we.... Terry - I went over the manifold carefully and there is only the one port at the front. Given I have no knowledge of the background of the engine I can't even say for sure that the manifold is correct for an 81A. Phil, I put you wrong on the statement that I was getting an ok manifold vacuum previously. I double checked the vacuum gauge and the green starts at 18" so I think that the reading previously would have been around 17" so this low manifold pressure at idle may have been around for some time. On the compression I have previously tested this and allowing for the fact that there are different compression heads on each side (yeah I know...) I got spot on 80lbs dry on all four of the left and 100lb dry on the right with less than 1lb variation on each side. Regards all Last edited by david.skinner; 08-23-2016 at 09:31 PM. |
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#18 |
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More info and I feel like Alice going down the rabbit hole....
Changed the distributor, coil, and checked the plugs - bit dirty but I think ok so I'm going to rule out electrical. When I rev the engine up and then release the throttle I get just over 20" of vacuum while the engine is slowing up. Once it reaches idle the vacuum drops to between 14" and 16" depending on what idle revs I've set. Does the fact that it will hold 20" for a few seconds mean that its less likely to be a leak? 2nd gotcha and this one might make you laugh (or cry). I disconnected the vacuum feed to the distributor and plugged in my gauge there and guess what - no vacuum at all. I think the feed has been drilled in the wrong place in the manifold and has never worked. You can see the vacuum port for my gauge in the base of the generator mount immediately in front of the oil filler in Picture 1. Long shot from my limited experience but could the inability to set idle relate to the fact that the distributor has no vacuum at idle?. Picture 2 shows the two existing vacuum ports on the manifold under the rear of the carburettor. They are two different sizes. Perhaps I could use the smaller one as the feed to the distributor? I have many photos of the carb in pieces and as I reassembled it in case this was useful. I did notice a slight difference between the two idle tube (jets?) in that the top hole was in line with the screwdriver slot on one and perpendicular on the other. Couldn't see any difference otherwise (pic 3). I had thought that perhaps needing to idle the engine so high that the idle mixtures would not be active but in fact I can easily get a stumble on both sides so its clear that they are having some effect. Over to you... Thanks |
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#19 |
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I don't think you will pick up any Vac there ,Does your manifold have a heat riser hole between the carb studs that needs to be covered ,Ted
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#20 |
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Yeah, I didn't put it there Ted. Thats where it was
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#21 |
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Bring it over the hill to me in Masterton. I'll sort it, Brian
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#22 |
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David,
That "vacuum" port by the oil fill tube is NOT a vacuum port. That is the engine end of the crankcase ventilator that was fitted to some military stuff. Your manifold is a Canadian war time job. The vacuum ports should be on the back of the manifold riser just below the carb, I believe. I can check later when I get home as I have this manifold. Vacuum leaks can occur at the valve chamber side of the inlet ports, had this happen, no amount of spray stuff will find these ones. Only manifold removal. Martin. |
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#23 |
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See in your pictures, the two vacuum ports on the riser. Small ones is for dizzy and wipers, Big one for crankcase ventilator (factory PCV)
Martin. |
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#25 |
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Right - new inlet manifold gasket on the way - will change tomorrow. Is it best practice to use a gasket sealer with these? I certainly do on my motorcycle restoration.
Regards David |
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#26 |
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In my experience the best...most trouble free....nozzle bars are those that sort of resemble an upside down 'V' with an integral vent tube pointing straight down towards the venturris. From the photo of your idle tubes it APPEARS as though you have the tall nozzle bars which kind of look like the letter 'N' with one vertical leg missing. Let us know which nozzle bars you actually have. The 'N' type are very fussy about which idle tube can be used and further more there are about 6 different 'N' bars and just as many variations of idle tubes to service the 'N' bars. Let us know what bars you have and we'll go from there. Charlie ny |
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#27 |
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ds,
Your vac seems good enough to me. Charlie ny |
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#28 |
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Blast the carb out with compressed air ,blocked idal jets Ted
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#29 |
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Put gasket on with grease
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#30 |
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Just normal grease Brian?
Ted - did that yesterday as part of the rebuild Charlie, they are the N ones and I did notice that the idle tubes don't sit concentrically in them. The both touch the sides of the outer tubes. I have heaps of close up photos of the disassembled carbs if that is of any use for identification. I'd be keen to know what to fit and where from. Much appreciated. Regards David |
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#31 |
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Just normal grease Brian?
Yes David. You know how when you coat the sides of a gasket with [any] coating, and then when you torque the gasket down, all that 'goop' squeezes out around the outside of the joint? - yeah, well it also squeezes through to the inside of the joint too. If you use grease on the intake manifold gasket, any excess grease that squeezes into the engine is readily assimilated and does no damage. Other products, especially silicon rubber, I shudder to think about. Grease will seal those minor imperfections, allow surfaces to 'slide' over each other as the torque is applied, and permit the gasket to easily be subsequently removed and indeed, even re-used!
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#32 |
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David, I have previously had Charlie supply me with one of his carbs, and it was a thing of beauty! You'll not do better than to get him to do you a carb, but lets have a go together on your 36 first. Be so much easier to diagnose in my shed than over the 'net.
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#33 |
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Yes - that sounds a really good idea on both counts Brian. I would like to change the manifold gasket before I head over. I would probably need the car to be running a bit better before I come over.
With the inlet manifold off is there anything to identify/check/measure? Does anyone know the inlet manifold head bolt size and pitch? I'm pretty sure I've got a partially stripped thread and I am planning to put in a thread insert and need the sizing, unless there is a reason not to? Regards |
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#34 |
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What 94 Carb do you have??? 91-99, 59 ?? How is the center body marked.
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#35 |
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Dave,
Brian's offer is outstanding and I'm sure together you will both sort it. If by any chance any of your mates has a donor 94 with a pair of the inverted 'V' nozzle bar assemblies with holding straps grab them pronto. The 'N' nozzle bar must be fitted with just the right style idle tube....some are vented some are not. To further frustrate things the 'N' bars come in no vent single vent and multiple vent designs. Everything must match including the 94 these bars are installed in, if they do not match the issues you are facing will occur. The 'V' nozzle bars seem to be happy in most every application. If you and Brian choose to go this way ,during sorting, be sure to carefully install the 'V' bars with 2 gaskets under each leg and careful see that the bars sit nice and proper on their seats. With both bars and straps in place snug up the rear strap and attempt to rock the bars, they should be secure and solid feeling at this point. If good here back the rear strap securing screws off 1/2 turn and repeat this proceedure on the front strap. At this point the squirter should also be firm and not wiggle. I remember to always disassemble the bars and run a tag wire thru the idle tubes and be sure all openings in the bars are clear. The above is only my opinion and I certainly respect all other opinions. Charlie ny Last edited by Charlie ny; 08-25-2016 at 07:08 AM. Reason: forgot the ny |
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#36 |
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Just curious again, did you do the quick test; pull throttle out a little 'key off'
now turn engine over with your hand over carb. If its not sucking your hand, thats reason why you only have 14 inches at idle. Example we have a late Mercury in a F6 that runs like a kitten, problem is you have to tow it. When I put my hand over the carb. there is nothing then when running vac gauge is way down there. Haven't got to it yet, but my bet is the gasket failed 'inside the valley chamber' so in case any new or old carburetor needs suction to pull fuel through venturies idle passages and such. I been whatching this post and for kicks stuck a vac. gauge on my car 59A and I got steady 20 inches at idle......doctors stethoscope........ threw my timming light out 50 yrs ago . the vac gauge rules. |
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#37 |
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manifold bolts 3/8 UNC
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#38 |
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Thank you everyone - just got to get it going well enough that I can get over the hill for the 2 hr trip to Brians. Terry here are all the photo's of the carb. Of course it is possible that the components are not even from a single style of carb. The only markings on it I can see is the single '94'.
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#39 |
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and some more...
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#40 |
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Is it my bad eyes or imagination, but are there washers under the main jets? It could be the casting, but looks a different colour in the pics.
Mart. |
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#41 |
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I kind of see gaskets or washers under the jets too ,BIG no no....no gaskets or washers
under main jets. I do hope you find the 'V' style nozzle bars. Failing the above try blocking the small vent hole that you see in the nozzle bar adjacent to the idle tube.......this would be under the category of experimenting..... so do not do this on a permanent basis. This worked for me one time a long time ago. Charlie ny |
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#42 |
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Good eyes Mart - they are little black gasket seal thingys (not sure what they are called). they are soft - not washers. They came with the carb kit but it doesn't mean that I have it right. thanks.
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#43 |
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Oops - charlie thank you - none under the idle side of the spray bars either?
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#44 |
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The thin black gaskets go under the spray bars, both sides, 4 locations, like you have them.
Mart. |
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#45 |
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I think you would have some back firing if your MF gasket was gone ,I would try another carb .Ted
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#46 |
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Last edited by Terry,OH; 08-27-2016 at 06:22 AM. |
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#47 |
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Considering the couple issues found with your carb based on the pictures, I'd try another carb as well too. It's a quick and easy check and will answer a lot of questions. You may have more than one issue that you're fighting with. If a loaner carb fixes your idle issue and you're low vacuum then you have your answer. If a loaner carb fixes your idle issue but still have low vacuum then you've got things whittled down to a manageable level.
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#48 |
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Update time. The main source of the problem is a sticky and leaking throttle shaft. Complications were a small leak in the inlet manifold. No vacuum to the distributor, and numerous small faults in the carb itself. I'm just organising for a replacement base now. I found it by accidentally jamming the throttle butterflies completely closed and all of a sudden the vacuum jumped up to 20" and the idle mixture adjustments started working properly. In the meantime I also have a new inlet manifold gasket, nicely painted inlet manifold, replacement V belt of the correct length this time, and a nice new shiny vacuum feed line to the dizzy. Thanks to everyone for your guidance. Not a hope I would have got there without the support from the people on the forum. Pictures of the offending article attached.
PS: Anyone know the thread size on the small vacuum port on the manifold riser? |
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#49 |
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Get a new oversize shaft ream the base and you are good to go.
Most used ones are worn and finding a good one will take more time then getting the old one back in shape. |
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#50 |
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Temporary fixes in place and dead steady 19.5" on 600rpm - tried to upload a video of it idling I was so happy but don't think we can post video's. Thank you again to everyone. Just some spare parts to find and fit now and all fixed.
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#51 |
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Just a final update and a public thank you to CharlieNY on this forum. Charlie has been helping me with this problem and with some parts and thanks to his guidance and generosity this quite complex problem has been solved. Thank you CharlieNY.
D. |
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