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Old 09-29-2011, 03:17 AM   #1
Lowsquire
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Question Tuning problem with modified A engine

Hi All.

Thought I would try to get some help here to see if anyone has experiences like mine with a modified A engine..
Specs are as follows,
Standard A engine ,standard head,standard cam.
F&S electronic Ignition, 12Volt system,new Champion W18 plugs
Ansen cast aluminium single down draft intake
Single 94 holley, rebuilt, 56 mains 7.5 PV
reds headers four into two extractors, into large dual inlet muffler then out below the running board (sounds great!)

I have done a load of work on this car, and it idles and revs up beautifully.
but through the gears if revving hard,and in top at higher speeds (above 40mph)it misfires and bucks and pops, and has no power, the whole ignition sytem is new and seems to operate fine at lower speeds (it pulls like a train up to about 2-2500 rpms) then it just kind of goes fluffy and misfires, next gear and its fine again, till the revs get up and it falls apart again.
at first I thought it was leaning off, as I started with 47 mains, so swapped them to 50s, ran better, but plugs looked very lean, so now i have 56s and the plug colour is good, allowed it to run with a few more degrees advance too.
I dont have an oscilliscope and a dyno..what the hell else can I try with limited equipment? Im at a complete loss. fuel sytem is clean, carb is perfect, ignition is all brand new, compression is good, engine is quiet and feels so strong low down, but cant take the thing on a freeway as it can barely get over 40MPH. help!
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Old 09-29-2011, 04:42 AM   #2
roddyb34
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Default Re: Tuning problem with modified A engine

We have a similar set up with a single Holden Stromberg it has a 57 jet and dosent like to rev right out in 2nd gear ,it seems to starve ,going to up the jet a couple of sizes ,with the big gap from 2nd to 3rd gear it struggles to pick up in 3rd ,try the bigger jet ,it just might be still lean,,
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Old 09-29-2011, 04:55 AM   #3
Lowsquire
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Default Re: Tuning problem with modified A engine

Just went back to 51 jets and it improved, so I dont think its too lean.. I also filled it with shell ultra wammo or whatever its called, now it will get to 50mph before it starts giving up. Im starting to think its a fuel flow issue, maybe i do need a pump. is it possible that a full tank gives a little more 'head' and helps keep fuel up to the carb? Im running the float higher than stock hoping that would compensate.could it really be draining the fuel bowl faster than it can pour in? its a pretty good flow at the carb when i checked it.
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Old 09-29-2011, 04:57 AM   #4
MikeK
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Default Re: Tuning problem with modified A engine

Sounds like fuel starvation when the power demand goes up. Downdraft carbs often have problems with the gravity fuel feed in a Model A. You may need a pump.
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Old 09-29-2011, 05:03 AM   #5
roddyb34
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Default Re: Tuning problem with modified A engine

Ah yes ,,we put a pump on too ,,,,forgot about that ,,,,it sure did help ,,,as our carb is off an FJ Holden is probably smaller than a 97 we will upsize our jet ,heading off to Newstead tomorrow on the maiden voyage on a "good used "engine with a few noises ,we will see how far we get,
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Old 09-29-2011, 05:59 AM   #6
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Default Re: Tuning problem with modified A engine

Is the gas cap vented? - this could cause your fuel starvation problem! I experienced a similar problem on a car I purchased.
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Old 09-29-2011, 07:12 AM   #7
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Default Re: Tuning problem with modified A engine

I seem to remember that I had a similar problem with a Model A that was equipped with an FS Ignition. I think that you need a dedicated power source for this distributor which this particular car did not have. I rewired it-end of problem.
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Old 09-29-2011, 09:58 AM   #8
MALAK
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Default Re: Tuning problem with modified A engine

Quote:
Sounds like fuel starvation when the power demand goes up. Downdraft carbs often have problems with the gravity fuel feed in a Model A. You may need a pump.
I agree with MikeK on this one. My A has a "Farmer" downdraft conversion with a Carter W-1 carb. It exhibits the same problem which is magnified at lower fuel levels and/or going up-hill. My carb's fuel inlet is nearly level with the tank shut of valve. It's getting a pump.
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Old 09-29-2011, 10:03 AM   #9
Jim Brierley
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Default Re: Tuning problem with modified A engine

The carb jets should stay close to stock. Carb's don't know anything except how much air is going thru them and are jetted from the factory to give the proper ratio. Only minor changes should be needed when put on a different engine and/or used in multiples.
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Old 09-29-2011, 05:56 PM   #10
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Default Re: Tuning problem with modified A engine

Thanks all,
yes Im suspecting its the lack of gravity feed, the carb inlet is only about three inches below the petcock on the firewall. I really thought it would flow enough, but maybe not!
Jim, I agree on the carbs, it was a desperate measure to go to 56 mains, and ended up not being the solution.

Good luck RoddyB34..see you at Newstead!!
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Old 09-29-2011, 09:42 PM   #11
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Default Re: Tuning problem with modified A engine

i fought that for 2 yrs . yes im slow ! but the 97 was just to big , went to an 81 , electric fuel pump . all is great ! ......... steve
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Old 09-30-2011, 12:48 AM   #12
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Default Re: Tuning problem with modified A engine

One 97, size 42 mains . fuel pump on B engine , we are at sea level. No problems 15 years later. You need low preasure pump i think.
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Old 09-30-2011, 05:07 AM   #13
Tom Wesenberg
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Default Re: Tuning problem with modified A engine

I'm curious about the mods you chose? Wouldn't a high compression head have given more horsepower gain than all the other mods you used? Then you could still have the stock carb, no need for a fuel pump, and the engine would look totally stock.
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Old 09-30-2011, 12:27 PM   #14
Jim Brierley
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Default Re: Tuning problem with modified A engine

Compression will help a whole lot!
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Old 09-30-2011, 02:06 PM   #15
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Default Re: Tuning problem with modified A engine

Stock cam ??? Has it been checked for wear, Suggest you purchase a Brierley cam great value . One of my so called rebuilt engines would not do over 40 mph. It was the cam, lobes badly worn, after having it reground 65mph. no other mods. The cam is sometimes overlooked but is the heart of an engine,
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Old 09-30-2011, 02:22 PM   #16
30Murray
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Default Re: Tuning problem with modified A engine

I would first try a larger fuel line with no sediment bowl. With only 3" of head and gravity feed, you can only flow so much through the stock steel line. The fuel tank is essentially just an extension of the bowl in the carb. By the way, I've had no experience with this myself, just with water piping design which is the same principle.
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Old 09-30-2011, 10:50 PM   #17
Tom Foster
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Default Re: Tuning problem with modified A engine

With a stock cam and head, 2500rpm is about all you can expect before the engine 'runs out of breath.' A fuel pump might help the 'fluffy' feel but more bolt-ons won't help it breathe deeper.
Fearless
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Old 10-01-2011, 12:56 AM   #18
ken ct
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Default Re: Tuning problem with modified A engine

In my opinion your over carbed. ken ct.
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Old 12-06-2011, 07:39 PM   #19
Ron Engen
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Smile Re: Tuning problem with modified A engine

Try a larger power valve.
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Old 12-06-2011, 09:08 PM   #20
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Default Re: Tuning problem with modified A engine

Fuel pump! Might want a regulator also.
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