poor idle driving me nuttier

L

larry harding

Senior Member
we rebuilt the engine on my 31 pickup this winter its been installed for a while and just starting to drive it some. new oversized pistons,b cam high comp head, lightened flywheel and 12 v electrics. the engine idles rough and wont go down to the A counting em off speed at all. i'm using a tillotson x after trying a
zenith (didn't like the stalling at stop lights) both carbs have been cleaned and new repair kits. i have checked my ign timing several times,new plugs, and ign coil no more than a year or so old. i made a plate to fit the intake so i could pressure check for leaks. it runs best with the idle air screw all the way in, but screwing it out makes very little difference. i've got to have the idle speed faster than i like and when i try slowing it down there is a point it just shuts down, never reaching the tick over counting off that sounds so good on an A. there is something i'm missing, please help.
 
If your engine now has a great deal more capacity...you might need a Weber downdraft Carburetor and a electric fuel pump in order to put enough fuel and air into the larger engine...The major suppliers have them along with the intake and exhaust manifolds required...Ernie in Arizona
 
A properly restored and tuned Zenith or Tillotson are plenty adequate unless you are building a full race engine. How did it idle before the rebuild. Consider a possible intake leak.
 
Larry, you mentioned that you rebuilt the carb but did you have the jets flow tested. Get Rex Reheis's book on rebuilding the Zenith carb. For years I had similar problems and poor running was finally eliminated with a thorough cleaning and proper flowing jets. Ed
 
Sure sounds like leaky manifold. Take a LP torch, turn it on without lighting and with the engine running direct the gas around the manifold gasket area. If engine RPM changes, you have a leak.
 
eagle said:
Sure sounds like leaky manifold. Take a LP torch, turn it on without lighting and with the engine running direct the gas around the manifold gasket area. If engine RPM changes, you have a leak.
Click to expand...



I could be wrong, but wouldn't LP gas be dangerous around a running engine, especially if the exhaust is hot or got into the distributor?


How about using a water mister/spray bottle or something else?


I would be concerned about fire/explosion.
 
30 Closed Cab PU said:
I could be wrong, but wouldn't LP gas be dangerous around a running engine, especially if the exhaust is hot or got into the distributor?


How about using a water mister/spray bottle or something else?


I would be concerned about fire/explosion.
Click to expand...

That's a good method to find an intake system air leak...it won't ignite !!
Propane is not that volatile, it would take a large concentration of propane vs air to light up.
I remember a Chrysler service manual described the same way to check for valve seal/guide leaks. Blowing inside a valve cover on OHV engines.
 
I would suggest a vacuum gauge: http://www.fordgarage.com/pages/vacuumtune.htm It is only $15.99 on Amazon. It may not tell you the exact problem, but will allow you to eliminate a number of possible problems.
I had a similar issue twice with engines that I did not know. The first time, my newly rebuilt stock engine was just too tight. It finally started to idle better after about 500 miles.
The second time, the vacuum gauge helped with the diagnosis. it was valves on a my 30 pickup not properly seating. There was a lot of carbon build up because of running too rich for years. Once I ran it at speed for a couple of hours with the correct fuel mixture, it would idle properly.
Good Luck
 
thanks for the input guys, but i've have tried to cover most of what you stated. i am only .080" over to clean up, not hot rod boring. also i put the plate on the intake carb. flange and put about 10 psi. and other than a few bubbles around the wiper motor there was no leaking sound.(i even pinched off the wiper hose) i sandpapered the carb. mounting flange as well as the lower half of the carb on a glass surface. i have had an old engine in the truck before that would do the count em off, but not so far with this one. i put new repair kits in both carbs so that should take care of the jets. this engine starts easily and runs good, just not happy withe idle and it sometimes wants to die for no reason, but not as bad as the zenith at stops. i have another zenith that belongs to a friend i may try tomorrow to see how it runs.
 
whidbey island? i just went to their website, its nice but either i'm an idiot, or their"s no info on carb service or trouble shooting. also i had a vacuum gage many years ago, gone now. i'm willing to buy one but don't know how it would help.... what am i looking for with a vacuum gage?
 
A zenith that stops at stoplights needs the float messed with. usually lower the fuel in the bowl a bit. Have you tried the propane test?
 
It sounds to me like your idle circuit is very lean. You're idling fast because you're using the main jets to idle. If the air corrector jet on the Idle jet is all the way in and it makes little or no difference to your idle circuit then your to lean. I don,t know what is causing your problem but I had the same symptoms with my zenith carburetor. I opened the idle jet from a number 75 drill to a number 74 and it made all the difference in the world. I can now idle at a much slower RPM and I can stop without stalling. Try a slightly larger idle jet and see if it helps. Hope this Helps.
 
Larry -

Hopefully with lots of good input from the forum you'll get your idle smooth out! But one thing that I don't think has been mentioned is that with the higher compression head you will get a more modern sounding engine and that cut-em-off idle sound that we all love will be gone!

Don.
 
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