Most troublesome carb!

Dave in MN

Dave in MN

Well-known member
Greetings all,
I have a Zenith -2 Carb giving me trouble. The darn carb causes any engine I put it on to backfire upon quick let-off of the accelerator. Over the years, I have rebuilt many carbs and always flow test the jets, clean out the passages, follow Steve Parteger's booklet guidelines and refer back to Moler's book and the Model-A.org info on line when necessary.
The jet settings on the carb giving me trouble are: 168ml/mn on the main jet flow, 142ml/mn on the compensator jet, 44ml/mn on the idle, 170ml/mn on the cap jet and the float is set to provide the correct fuel level height when measured with an outside sight level tube.
The carb idles great, the mid range power is smooth and strong. The top end power is good. The GAV adjustment (closed to 2-3 turns open) causes no change in the backfiring problem. The GAV causes the typical reactions to adjustment when the engine is cold at start-up but limited change once the engine is warm. This would typically indicate a lean condition. My next step is to oversize the main jet to see if it will correct the problem but.... With the jets I have in it...it should not be lean unless something else is entering into the picture. Slightly choking the engine is real difficult to do when driving and letting off of the gas quickly...I can't tell if it helps much.
I have two Model A's with very good running carbs. When I switch their carbs to this particular owner's car the car will not backfire. Likewise when I switch this offensive carb to one of my vehicles, it will cause backfiring on my A's. The problem must be in the carb...but what am I missing? Thanks in advance for any help offered!
Is there such a thing as a "devil" carb?
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Good Day!
Dave in MN
 
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Just a thought, but could it be a crack in the housing? especially the upper housing. It must be drawing air somewhere its not suppose to. This would cause a lean condition that couldn't be tuned out. Good luck.
 
my second guess was a crack in the housing also....First guess... if it came off Bob A*****'s then it IS possessed.
 
I did check the carb over pretty good...and I see nothing suspect. I even completely stripped it and soaked it a second time in carb cleaner, glass beadblasted and re-painted it. In other words, I looked at the bare casting also.
I'll look it over again for a crack that might open up as the carb gets hot. Thanks!

Anyone else?

Dave in MN

Preacher...not Bob's carb...his was actually pretty good.
It's Joe C***A's from Stillwater...
 
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Lack of response from the GAV when hot might indicate that this circuit is restricted when hot. I cannot think of anything that would cause this. I don't have a Zenith on a car right now, so I can't check to see how sensitive the GAV control should be when hot. Do the "good" carburetors that you tested have GAV response when hot?
 
OK, a little logic here. The problem is ONLY with the throttle fully closed and high vac/ engine speed. The closed throttle eliminates the comp, cap, and main jets and circuit. That leaves the idle jet, the mix chamber and it's air feed, and the gasket seal between carb halves for that circuit as suspect. Hmm.. only backfires with a higher than idle vacuum. Things I'd check for and try: Flatness of carb halves with steel straightedge; defective gasket; crack in upper half parallel to threaded carb assembly bolt hole; messed up idle drill holes in throttle bore; crud anywhere in idle circuit; gremlins.
 
How about a warped flange sucking air where it bolts to manifold. Try a flat file across it and see what you have.
 
I try to get the main jet around 159, comp 151, cap 173, idle 44-48. Rex Reheis says to set the float 1-5/16" form the top surface to the solder seam on the float, this is what I have done for years of carburetor rebuilding.
 
Have you checked the position of the throttle shaft in relation to the idle ports in the throttle bore? Maybe the factory drilled one or the other in the wrong place. Compare your other carbs for how far the throttle plate is open at idle. If the problem child is significantly different, investigate further.
 
Put an o ring on the throttle shaft (outside away from the engine) so that you for sure seal up the throttle shaft from sucking air.

Also sounds like an idle circuit TOO LEAN condition to me.

Try two gaskets where it connects to the manifold.

Also might try an o ring under the spring of the idle jet adjusting screw to make sure there is no air leaking there as well.

Let us know what it eventually was...as you will find it.

Lastly, try switching top and bottom halves of your known to be good carbs with this one and see if the problem follows bottom or top half of the offending carb.

Dennis L Oberer
Green Bay WI
 
If it were mine I'd be looking for a vacuum leak, Manafold to block or maybe the vacuum line to the wiper, if you have one.
just my W.A.G.
JB
 
OR, just let it bring out the kid in you. A high point of driving a Model A back in the day was to go down a hill with the GAV shut and spark lever all the way up and listen to it pop and bang! I remember a classmate impressed with a buddies' A because "it popped 40 times going down a hill!".
 
Thanks Guys...
I'll go back and try again...paying particular attention to the idle circuit and the also check the carb/carb gasket @ the 1/2 line. I'll let you know what I find.
Dave in MN
 
Dave - there are good suggestions listed above, but there is one I don't see. Have you checked the length of the jets? If they are not of the correct height, then float adjustment won't be right.

Just a thought.
 
Ron/IA said:
Dave - there are good suggestions listed above, but there is one I don't see. Have you checked the length of the jets? If they are not of the correct height, then float adjustment won't be right.

Just a thought.
Click to expand...
I did adjust them a bit with varying washer thickness and multiples...along with adjusting the float a bit up and down to provide a variation in the fuel height in the bowl to jet relationship.
I am going to try fresh gaskets at the carb 1/2s and refresh the gasket at the manifold to carb tonight.
Thanks Ron,
Dave in MN
 
When you have it off look at the throttle valve disc, sometimes the edges near the throttle shaft wear , more flow there, less flow over the idle port.

Hold it up to light and check how well it fits the bore.

Having a torn gasket hasn't made any difference to how my car runs --it is broke in several places some stuck to the top-some to the bottom --i just wiggle it so the breaks match up then tighten the bolt ---i have a new gasket ---somewhere
 
Most troublesome carb! Some improvement....

Greetings all,
Thanks for all the suggestions so far. Last night I re-checked all of the following: installed jet height in the lower body: main @ 3/8" & cap 7/16" below parting line, throttle shaft fit was tight, almost 100% sealed at throttle vane, idle port opening looked good but it is one of the larger (opening size) versions. I did get considerable improvement when I placed a 56ml/mn idle jet in the carb. The idle air ajustment settled in at 1.5 turns out from the seat. This flow is about 16% richer than the maximum spec in Pargater's booklet. My next step is to move up to about 60-64ml/mn flow on the idle jet and re-test. I should have tried a larger idle jet earlier...I was stuck on not going out of the suggested range of flow with the jets. At this point, I am about 10ml/mn rich on the combined flow of the compensator and main jets. I will bring those back into spec and keep the idle jet over spec by 16-33%. I will post again and let you know the results.
Dave in MN
 
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