Can't get the idle right
 

I wonder if I can get some advice. I just rebuilt two Zenith carbs for the 31 Tudor. I can't get that sewing machine idle from either carb. Power is good, no backfiring or stuttering. Good throttle response. It idles but not as smooth as I think it should. I installed flow tested jets from Renners and the float is set properly. No leaks. The GAV is at 1/4 open and changing it doesn't smooth it out. The engine has about 400 miles since it was rebuilt but it sat for 15 years until 4 month ago when I started working on it. Timing is set according to Les Andrews videos and it has a new intake manifold, rotor cap, rotor, plugs, points and condenser. I set the points to .018. Can't find any vacuum leaks but I did not bush the throttle and there is a very small amount of play but I don't think it is enough to create a rough idle. The idle mixture screw doesn't seem to make much difference except it will stall like it should when fully closed. Is this the way Model A's idle or am I missing something?

Any ideas are appreciated.

Joe

Re: Can't get the idle right
 

Fully retarded while idling? If so - check your timing AGAIN.

You don't say if you have an upgraded cam - or a high compression head. Both of these can affect idle adversely.

Joe K

Re: Can't get the idle right
 

The carb idle mixture screw when turned in makes the mixture richer.
The screw controls the air admitted to the mixture, unlike most other carbs, where the idle mixture screw controls the fuel.

Re: Can't get the idle right
 

No upgraded cam or high compression head. I also have two fuel filters. One in the sediment bowl and an inline after the bowl. Gas tank was flushed out before I started it after the long storage so I know it's not contaminated fuel. Would an idle jet problem cause this? They are new flow tested jets. Maybe too small? Running out of ideas other than this is the way these cars run.

Re: Can't get the idle right
 

With the aid of an unlit propane torch apply a touch of propane around the carb and intake manifold. Any air leaks will become evident with faster and smooth idle. A vacuum wiper?/can cause problems too. An no matter how old the rebuild with only 400mi. Can't be expected to idle perfectly any way, assuming that the rebuild was done correctly.
My engine didn't behave properly until it had approx. 2100 miles under it's belt.

Chet

Re: Can't get the idle right
 

You can check the throttle shaft for leaks by using a unlit propane tourch. If there is a change in idle when you pass the propne over the area the leak is a issue. You can also test the carb to intake gasket the same way. When you rebuilt the carb did you make sure the passage way in the carb throught was open and clean? Many forget to check it. There are many passage ways each and every one must be spotless clean for a carb to work 100%. And almost clean carb will, run an engine but for example may not idel proper. On another point, are you sure your spark is good and hot and perfectly timed? This includes no slop in the distributor shaft or rotor and cap issues. A lot of what looks like carb problems are really on the spark side.

Re: Can't get the idle right
 

Reclean the carb. Are you removing the access plugs and clearing each circuit? Try a known good running carb. Take out the filters and see if that helps. You could pull the throttle shaft and bush it.

Re: Can't get the idle right
 

Even though the jets are flow tested to the correct rate, are the ends of the jets the correct shape. The ends of the tube wall need to be rounded from inside to out, similar to cutting a doughnut in half horizontally. If they are square, a small amount of fuel will slowly build up on the end of the jet, and when it gets big enough, will fly off into the airflow and cause a random stammer in the idle. This is not the only cause, but just one possible cause.

Re: Can't get the idle right
 

One thing you might want to check is the gap between the tip of the rotor and each of the contact points in the distributor upper half. Many of the repos are notorious for not being even. Each gap should be the same. I have seen a number of gap values stated, however, I think it is more important that they all be the same. Find the one that is the furthest away and file the contact point down on the others to match. Somewhere around .025 is nominal.

Tom Endy

Re: Can't get the idle right
 

Just went out to try the propane trick and there is fuel all over the floor. Great and I put a new Viton float valve in. It didn't leak when I tested the float level. I set it for 5/8 below the top housing. I let it sit for over an hour to test for leaks. I would think there is contamination in the fuel but the tank was flushed and I have two filters on the line. The carb was sitting in Chem Dip for two days and rinsed with water then immediately sprayed with carb cleaner. I put a wire through all the passageways and re assembled it. It's got to be clean.

I guess I am back to the drawing board. I will disassemble it again and spray it with carb cleaner. I am not too thrilled with the mechanics of this Zenith. I'll put the other carb on and try and get the idle right otherwise I have a Tilitson I could rebuild and put on. Maybe they made Tilitsons because of the problems with these Zenith carb. They don't seem very reliable.

By the way when I look at the design of this carb I would like to know, when coming to a stop what stops the float from shutting off the fuel? I have heard of stalling problems when coming to a stop. Makes sense because of the design. Why wouldn't it stall?

Tom Thanks for the tip. When I gapped the rotor and cap I do remember that one of the contact points was gapped to far but then I had the other three perfect. I will get set the contact with the larger gap first and then file the other contacts. That might be the problem.

Everyone thanks for the help. I really appreciate all of you taking your time to help me.

Joe

Re: Can't get the idle right
 

Be sure to shake the float to be sure it didnt gas in it and sink on you.
More than likely you are right its trash.
There are just as many cussing Tillotsons ( I'm one) as there are Zeniths.:)
Dont give up on it, you''ll get it figured out. Good Luck!

Re: Can't get the idle right
 

"By the way when I look at the design of this carb I would like to know, when coming to a stop what stops the float from shutting off the fuel? I have heard of stalling problems when coming to a stop. Makes sense because of the design. Why wouldn't it stall?"


I have heard that story about the mini tidal wave inside the Zenith causing the car to stall when coming to a stop. I don't believe it though because a properly functioning Zenith will not stall when coming to a stop.

However, stalling when coming to a stop is a common problem that I have encountered many times when road testing a Zenith I had just rebuilt.

I don't have an answer to the problem. I am sure it has to do with the secondary circuit. The secondary well fills with fuel when coming to a stop and the idle jet picks up its fuel from there. Usually what I do when I encounter the problem is I take the carburetor apart and go through all the passageways again.

The car will stall coming to a stop if you have an air leak somewhere. However when testing a rebuilt carburetor I run it on my Vic that runs fine with its own carburetor, therefore the rebuilt should run as well.

Tom Endy

Re: Can't get the idle right
 

Does the arm of the distributor top plate make contact with both sides of gap (opening) in the distributor body when moving the advance rod from fully retarded to fully advanced? If not, your timing and idle will not be correct . The fix is to rotate the steering column to get the correct throw. Search this forum for the "Les Andrews timing video". It will give you step by step instruction for this fix.

Re: Can't get the idle right
 

I had the same problem with idling and found the gas was leaking out of the carb after I overhauled one. A member on line had suggested I remove the top of the carb and fill the bottom portion to see if there is any leaks around the bas of the two jets. I had overtightened the cap jet and it was cracked at the base which allowed the gas to run out. Whenever I end up with a poor idling situation its usually too much gas.

Re: Can't get the idle right
 

The float is good. I had it submerged for 2 days and no leaks. Nobubbles when submerged in water.

The advance arm makes contact with both sides of the distributor. That was the first thing I adjusted before start up.

I will re inspect the cap jet for cracks but I would think it would leak right away as soon as I reinstalled it and turned the fuel on.

Is it possible that all the Zeniths are just worn out from so much use? If so has anyone tried a brand new Zenith or Tillitson from the supply houses?

Getting this thing right is like trying to make the wife happy during childbirth!!!

Re: Can't get the idle right
 

I have a friend that bought a new Tillotson, but it doesnt have provision for the GAV. He says it runs good, but I wouldnt want it. I dont know anybody that bought a new Zenith.
I think if I was determined to start over with another carburetor, I'd try to find a "B" model Zenith. I like the one I have.
But you should be able to fix the one you have. There's a lot of them out there working fine.Good Luck.:)

Re: Can't get the idle right
 

People here swear by the rebuilt one's from Bert's.
Many also say float height has a lot to do with stalling when stopping.
Your leak is most likely because of the Viton tip float valve. FWIW

Paul in CT

Re: Can't get the idle right
 

a common cause of stalling at stops is the float level is to high.. i know you set it at 5/8 how many gaskets are under the valve??

Re: Can't get the idle right
 

I agree with Growley Bear. My rebuilt engine only has 700 miles and it has smoothed out a lot in the last 100 miles or so. I am still not satisfied, but much more so than when it had 400 miles. Mine still stalls when coming to a stop.... ugh.

Re: Can't get the idle right
 

Mitch
I have 3 gaskets under the valve. I am beginning to think the new Viton valve is bad.
I thought the Viton valves were better than the original. I was under the impression that the old valves were the reason these things leaked.

I will send one of the three Zeniths in for a rebuilt and keep trying to get one of the others up to par. I ordered a rebuild kit for the Tilitson. I'll see if it's any better.

Meanwhile anyone know a guy in Pasadena California named Klecker. He is the person that rebuilt the engine about 15 years ago. I remember him being about 85 years old and Mel Gross told me it was one of the last engines he was going to do. Anyone know if he did quality rebuilds? I don't think the engine is the problem and maybe it just needs to be driven to smooth it out. But that would mean these things ran rough when they came off the assembly line. I can't imagine that new owners would accept that.

Re: Can't get the idle right
 

thats interesting as i find using 2 red gaskets works perfect for me on my carbs. i never had trouble with the viton valves as thats all i use now////hmmmm
also when i do get any play in the throttle shaft i use an oversize

Re: Can't get the idle right
 

On the vitron tip valve, if you pushed it shut with much more force than tbe float uses you can harm it. Also a very good way to clean all the passageways is to use a glass bead blaster, then chase every passage way with a small wire tip cleaner. You would be amazed at the amount of rust, old lead from the old gas, and general dirt I have removed from a carb that was chemical boiled and cleaned before it was sent to me.

Re: Can't get the idle right
 

If the float is set too high it will cause stalling when coming to a stop. If the level of the fuel in the carburetor is too high , like higher than the top of the main and cap jets, fuel will continue to drip out the tops of the jets and out the choke end of the carburetor. Most usually think that the float is set correctly , when it is usually set too high and allowing too much gas to collect in the bowl of the carburetor. When this happens, you get stalling when coming to a stop and leaks. The only problem that I ever had with viton tipped needle valves was that that they could stick closed, this is clearly not the problem.

Re: Can't get the idle right
 

I never had much luck with those little gadgets that hook into the drain port for setting the float level, so I built a float level test stand. I mount just the top casting to it and flow fuel into it with a glass container positioned below. When the valve shuts off the float level is indicated on the side of the glass. It can be measured and accurately adjusted. It also shows if the valve is holding over a period of time and not creeping.

My experience has been that many float settings that do not hold repeatedly is because someone in the past molested the float trying to adjust the level. This causes the float to not come up straight and off-sets the valve. Unmolested new floats from Bratton's work very good and only require one fiber washer.

I also set the bottom casting into a wooden holding fixture and pour fuel into the casting just above the comp jet and let it sit awhile to see if fuel is leaking around the main jet, cap jet, and GAV assembly.

I have had very good luck with the Viton tip float valves. The first ones I bought would not shut off at all. I took one apart and found it full of brass shavings with brass shavings stuck into the rubber tip. I called and spoke to Walt Bratton about it and he complained to his supplier and the situation improved... somewhat.

However, though there was a vast improvement in later shipments it is not perfect. Walt, like everyone else is at the mercy of his supplier. I usually buy several at a time and very carefully take them apart and inspect and clean any brass debris I find in them.

Tom Endy

Re: Can't get the idle right
 

This is very good information from Tom. Many times carburetor problems are ignition related. There is an amazing difference in idle quality with even rotor gap vs uneven gap.

Chet

Re: Can't get the idle right
 

Tom
Thank you. I put the other zenith on that I rebuilt and the idle was better. Then I checked the rotor gap and yes they were not even. I set the largest gap to the correct gap and had to file the other contacts to match. It idles much better but somehow the timing has changed as now the full retard position doesn't lope the engine. I'll re time her tomorrow and see how she runs.

Thanks everyone for the great advice.

Re: Can't get the idle right
 

Check that your points haven't closed up. New points tend to close up more during the first few hundred miles. During the first hundred miles they can close up to half what you set them at. Set your points fist, then the timing. I set mine at .020. When the points close up it will change the timing and the car will run poorly. When you reset the points to what you previously had them at when you set the timing, the timing will return to the previous setting.

Tom Endy

Re: Can't get the idle right
 

I would say if it is a fresh rebuild with very little miles like my A it won't idle well,try putting your 2 fingers across the carb air intake,if the idle smoothes out it's a mixture problem,air to fuel ratio found this out playing around with my carb.I found no intake leaks at block or carb shafts.