Need help to set idle

[david.skinner]

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

[flatjack9]

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.

[david.skinner]

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.
Regards
David

[Terry,OH]

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.

[Capt Kirk]

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.

[david.skinner]

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Thanks Capt Kirk. That was my thinking but wanted some advice. I don't know the condition of the engine except that a compression test shows perfect compression on both sides so I think the top end is in good nick. Don't know how to test the mains. Yes Terry the distributor has been rebuilt and set up on a machine. I't didn't used to do this but the surging started before I rebuilt the carb so I think its likely something other than the carb or distributor.

[Capt Kirk]

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!

[flatheadmurre]

Is the throttleshaft tight in the housing ?
If its worn you get symptoms like this to.

[david.skinner]

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?

[flatheadmurre]

Youre not going to get full vacuum from just cranking.
Did it work great in the past idling correctly ?

[david.skinner]

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.

[Phil Gillespie]

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

[david.skinner]

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

[Terry,OH]

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?

[Capt Kirk]

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.

[Hoop]

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.

[david.skinner]

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

[david.skinner]

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

[FlatheadTed]

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

[david.skinner]

Yeah, I didn't put it there Ted. Thats where it was I'll check on the manifold.