Stubborn Zenith

TinCup

TinCup

Well-known member
I have a very nice Zenith 1 that runs great at speed but tends to struggle to idle. I have gone through carb completely as I have done many others but have never found one that I could not adjust the idle air mixture. The passage is clear with the correct length air mixture needle driver. Turning it makes no difference in the idle of the carb whatsoever. No leaks around the manifold or gaskets to allow extra air into the circuit. It used to require more GAV adjustment than most but the correct #20 jet fixed that. Float height is spot on. My last thing to try is a different secondary well. There were 3 different wells and I have a version of all 3 so that is next on the list.
 
Added a bushing there and it's very snug. I sprayed starting fluid around it and it didn't surge on me.
 
I would start with the basics,check the cylinder compression and tappet clearance,I have had the same problem,one time worn lifters with huge valve clearance and another time low compression in one cylinder.Engine ran fine but no idle,died when I would stop.
 
It may be that your engine is idling too fast for the air mixture adjustment to have any effect. If you can't reduce the idle speed, check the comp jet and GAV adjustments. Until you can reduce idle speed and have the main throttle plate closed, you won't see much effect with the idle air adjustment.
 
What camshaft are you running? Stock? Or? If a stock camshaft, what shape is it in? A badly worn camshaft can cause idle problems.
 
Had an idle issue, blamed the carb/ignition, etc. Spent weeks on the issue. Finally one day I ran a metal dryer vent hose attached/sealed to the carb intake, hung the hose from the radiator support rod to up by the radiator, Issue disappeared. Found a small exhaust leak at the exhaust manifold/muffler clamp. Was getting dirty air.
 
I'm also dealing with this right now. Yesterday ran a vacuum leak test, squirted starting fluid pretty much everywhere with no effect.

Took the carb apart, used compressed air on each jet and port. Everything looked fine. Float height normal.

Took the manifold off, looks like there's warping at either the first exhaust or the first intake, hard to tell. Also major pitting on the internal walls. Soot all over the exhaust manifold (although that might be normal). I've ordered a nice used combo manifold – gonna put it on with copper gaskets and see if that fixes things. Also planning to seal the joint at the muffler clamp. I also cleaned and re-gapped the plugs.

Such a frustrating problem to troubleshoot, could be just about anything causing it.
 
The copper gasket/gland rings are not a fix for warped leaking manifolds. All the ports need to be flat and level with each other. Check the used one your are getting before you install. And no there should not be soot around the ports.
 
Next time you have the carb off, hit the mounting flange a big file and make sure it is flat and not warped a bit. Can’t have any low spots.
 
redmodelt said:
The copper gasket/gland rings are not a fix for warped leaking manifolds. All the ports need to be flat and level with each other. Check the used one your are getting before you install. And no there should not be soot around the ports.
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I'm buying it from Bert's, so... probably it's OK... but I'll definitely check and have it surfaced if needed. I would've had the existing one surfaced, but the rust was so bad I decided to just replace.
 
I spent Sunday afternoon going through things again. Changed the idle jet, Changed the secondary well, and put a bushing on the throttle shaft. I also hooked up a vacuum gauge so I could watch the effect of adjusting the air/idle needle. I used starting fluid to spray around the intake and the carb itself to see if I have an air leak I missed and all is good on that end. I am starting to look more to the condition of the intake valves. I know when I was going through them this spring they were a little off most measured around .014 and with original solid tappets I left them alone. I think I have gotten as good as I am going to get with this carb. Another known good carb acts almost identical to this one which with the absence of a manifold leak would tend to point to valves. I really liked using the vacuum gauge when setting the idle jet. While I was taught to tune it by ear watching the impact on the vacuum gauge was really helpful.
 
Hi Tin Cup,
Be sure that the idle speed is down below 6-700 RPM or lower , and the GAV is almost closed, then set the idle air screw at 1.1/2, - just a thought.
 
Another known good carb acts almost identical to this one which with the absence of a manifold leak would tend to point to valves.
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Have you done a compression test and a leak down test? A bad valve will cause a poor idle but will be masked at speed.
 
I would flow test the idle jet to be sure it is passing 44-48 ml/min. with a 36 in. water column test jig per Steve Pargeter's recommendations in his manual "Zenith Model A Carburetor Restoration Guidelines." I like an idle jet that passes 44-45 ml/min. because it allows the idle adjusting screw to be positioned near the seat. Good luck, Ed
 
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