OK … where to start? So my uncle bought a 31 phaeton a few years ago to go with his collection of 31’s. Problem is we have never been able to get it to run correctly since we got it. The problem has been popping out of the carb. At first it was at all conditions, starting the car rolling and running down the road. Over time we have changed the carb to one form one of the other 31’s. Then found out we had problems with the head when we found water in the oil and started working on the head last fall. We found out that the head was cracked which might have been one of our problems to start with.

Fast forward to current time, we still have a popping coming out of the carb at steady speed over 30. Driving 30 or less you don’t have the problem. Otherwise it runs great at idle. We’ve gone through the head completely, new guides, new valves, new springs, new seats. Checked for vacuum leaks, new spark plug wires, new plugs, new points, and new condenser.

So the one thing which I have noticed today is the problem gets worst after driving for a few minutes. Come home and let is sit it will run great and start getting worst again. This is telling me something is getting hot and opening up causing the problem.

The only things which I have left to try is taking the electrolock out of the loop and changing the coil.

Can anyone think of anything else to look at or try?

Thank you,
Bruce

I would definately try changing the spark coil. I have had bad coils do things like that when they got hot.

Hey Brucie! Ya, try changing the coil. I know you talked about doing that several months ago.

OK, tried the coil tonight, not difference. Tried changing the distributor with no difference. Took the electrolock out of the loop, no difference.

Now what? Please come up with the solution.

Possible solutions:
1) Don't drive over 30 mph.
2) Turn up the radio.
3) or... analyze the problem.

After it gets hot, there is either a) too much gas, or b) too little gas, or c) early spark, or d) late spark (or no spark).

Something new that you can try. Hook up the dwell meter and tape it to the windshield with electrical tape. Drive at 30 mph and watch the dwell. See if it is STEADY.

Then, switch the dial to Tach and do the same thing again, but observe the RPM to see if it remains constant.

You're looking to see if the electrical conditions change as the car heats up. If you don't see anything with either of these, then shift your focus to fuel.

Now what is the definition, or sympton of "popping". Help me out guys. Is it excessive gas, or lack of gas? I'd guess it could be either. But the RPM gauge should tell you whether it's lack of gas because the R's should decrease or flutter if it is being starved..

Who's next? Continue with a detailed analysis for poor 'ol Brucie. :-)

Don't drive over 30?

I suspect a leak or crack in the intake system or restriction that is causing the mixture in cylinder(s) to lean at higher flow rates. Lean mixture is by far the most common cause of popping through the carb. Ignition timing (which is coordination of initiation of burn by spark of the plug to the position of the piston and valves) is another.

A second suspicion is the valve timing. Have you checked that? Is it possible that the cam/crank are not properly timed? Is the cam lift up to spec? If the cylinders can't properly take in gas/air mixture the burn mixture will be lean. Cam lift and duration and timing all have a effect on the amount of fresh gas/air mixture to each cylinder.

If you can't find the problem take it to an emissions testing facility and have them test at various engine speeds. The ratio of unburned hydrocarbons, NOx, CO2 and CO can indicate the mixture ratio which should be 14-14.7:1. Lean mixtures cause sharp rises in the amount of nitrogen oxides (NOx). If the mixture becomes too lean, the engine may fail to ignite the mixture, causing misfire and a large increase in unburned hydrocarbon (HC) emissions. Lean mixtures burn hotter and may cause poor performance, rough idle, hard starting and stalling.

There are a few other things that come to mind but are not as likely. Pre-ignition due to rough combustion chamber (which causes a hot spot) is possible cause. Restriction in exhaust is another but it typically shows as loss of power at higher speeds. Loose baffle in muffler can also result in irregular performance.

OK, Thanks Bill and Chip.

Chip your going down the same road that I was going. The manifold is the only thing left which I have not changed. So the reason why I'm thinking the manifold is that I still can't get more then 15 inches of vacuum. With the compress that I'm getting and all the new parts in the head I can't see why I'm getting a vacuum so low. Also the manifold is not a Chevrolet manifold.

As to the valve timing I don't know and really don't want to take the front of the car off to check. But I'm just about there.

Bruce,
I added a few other comments to the post likely when you were writing your response. So you might re-read it.

You can check the cam lift and timing with a dial indicator on a rocker arm. Determine TDC and then rotate the engine slowly noting when the particular valve (No. 1 cylinder is by far the best) begins to open and close. Note the max. lift distance. All you need to do for timing is one valve. Actually you don't even need the dial indicator if you really pay close attention to the push rod (rocker) position. If you do that then I will need to look up the cam timing and lift. Don't remember where I put it for a '31 engine. Maybe someone else will have it handy.

Thanks Chip,

As to the idle, it idles great and will come done to about 100 to 200 RPM with out a problem.

Now on the cam lift and timing. I tried that with a dial indicator but never could get a good reading. I never could find a good place to put the magnetic base. Any thoughts on that?

I'm will to try anything right now, could you look up the lift info? Now when you say lift your talking how far down the rocker pushes the valve?

Hi Bruce,
The valve lift is how far the valve is lifted off its seat in the cylinder head.
( so from the top rocker side, it is how far the valve is pushed Down)

Valve lift for a 1931 head with all 1931 rocker arms is .277"
(for the 1932 model it is increased to .309")

Hi Jack,

Thanks! This is great! Only one problem, its 11:30pm and I need to go to work in the morning.

Any idea how to mount the dial indicator so I can get a good reading? Maybe I need to make a bracket to mount to the rocker arm bolts.

Thanks,
Bruce

Bruce, I havent done the task, but the dial gauge normally has some sort of a clamp, maybe bend a small L shape bracket off the manifold bolts. or water outlet.
Cheers Jack

Bruce ,
My thoughts on the popping through the carby as the engine gets hotter is that maybe the valve guide clearance to stems is closing up as the engine gets hot,and maybe that is why it is linked also to the higher speeds over 30, ..more engine work and more heat. And in particular a sticking intake valve, staying open and allowing burning fuel back to the carb.

Maybe the new guides which you mentioned were installed are just a touch too tight.

I saw a 1927 chev 4 cyl at the Last anniversary rally which had sticking bronze valve guides after about 20,000 miles of use.

If you purchased your new valve guides from Egge Machine that might be the problem since guys have had problems with those in the past.

If it is valve guides wouldn't they also stick at lower engine speeds once the engine is hot?

You could pour some MMO into the intake when this occurs and see if it helps. The MMO would lubricate the guides (for a short time) and relieve the problem.
If popping back through the carb. it would be an intake that is hanging up.

hey bruce no one mentioned a rust through in the exhaust manifold a place where you can loose vacuum . if so it can be repaired just bore it out and put a sleeve in .hope this helps . i have done a couple of them and it works

Maybe it is a combination, of sticky valve guides and possible weak springs, and the heat is affecting both the guides and the springs with increase heat and RPM,s???
Maybe check all the springs on a tension machine.

Hi Guys,

This is great information, thank you all. We have been trying to fix this for about 2 years now with out finding the problem.

Jack and Skip, as to the valve guides, I changed the guides hoping to fix the problem. So just over 2 weeks ago I had original guides in the head and changed them to new ones because the old ones were moving around. So I’m very positive that the guides are not the problem. Also on the Valve springs since I changed them to new ones before I redid the head again and the problem didn’t go away. I don’t think it springs. Also tried shimming the valve springs with no change. Forgot, also measured all the new springs before installing.

Gene, as to the MMO, how would you do this with an updraft carb? :) I did put some in the gas hoping to do the same thing. Thank you for the help.

Hi pushrod, can you give me a little more details on where to look? As you might have seen Chip and I were talking last night about the manifold being the problem since I can’t get higher then 15 inches of vacuum. Also that is the last part on the car with I have not changed.

Can you pull 1 plug wire at a time and see which one if any causes the popping to stop? This would narrow it down to 1 cylinder, then I would look for a vacuum leak on or around that runner. You can also carefully spray carb cleaner around the intake gasket and see If the motor speeds up . This indicates a vacuum leak as well. Popping thru the carb is usually an indication of a lean condition.

With all of the other things you have tried then I would suggest changing out the manifolds since that is about the only thing you haven't done yet.

o k here goes on the exhaust manifold where the carb bolts on the intake passage which is the gas preheat section . exhaust goes around this section heating the gas rust out i have seen holes in this section or crack you will have to remove the manifold or remove the carb and use a mirror . i have found several to be leaking . if you cant find a replacement you can bore it out and put a sleeve in . i dont remember exactly what the hole size is some where around an inch and three eights . hope this helps

Pushrod fixed one for me by putting in a sleeve. It is still working.

Have you checked or eliminated the vacuum line for the wipers?How about connecting the vacuum gauge to the intake manifold at the wiper fitting and watching the gauge as you drive at 30 mph or whatever speed you are having the problem at.Also unless I missed it I didn't see any mention of a compression test.I had a similar problem with my 32 Sedan and found a pinhole in the wiper tubing in the windshield pillar post.