VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Mixture and idle speed

Few questions for the experts on my 1930 Caoch. I just got my car out of the garage (for the first time), I made a few adjustments on the idle and feel that it is idleing OK, but how can I determine if the idle is set correctly (i.e not to fast or to slow). How can I also determine if my mixture is set correctly as well? I was not blowing any black smoke therefore, I am confindent it is not set to rich, but how to I know if is set to lean?

Typically if it is too lean it will knock under load and power will peter out quite quickly when you accelerate. As for idle speed, i do it 'by feel'. i set it low enough to still run smooth, and not too low as to stumble when i accelerate from a stop.

The idle speed can be set to the lowest engine speed where when rapidly increasing the engine speed and then rapidly letting it go back to idle it does not stumble. Another way is so it will idle smoothly and have enough power so you can release the clutch very easily on flat surface and the engine will not die.

The idle mixture screw is usually adjusted to the point where the engine just reaches its highest idle speed. Turning the screw in either direction will reduce the idle speed.

Thanks for the advice. I plan on registering the car tomorrow and hopefully can take it down the road to see how it actually does. I have driven it around the yard, and it seen to respond well. A road test will say alot.

Good evening, speaking of idle problems. I can not get my 31 chevy truck to idle. It throttles up but will not idle unless I choke the fire out of it. Does anyone know what vacume # should be on the manifold port? I have been thru the carb. I think I have a leak some where. Thanks Steve

I suspect you have a blockage in the idle circuit. Take the idle adjustment screw out and squirt a large dose of aerosol carburetor cleaner in the hole. Try to get as much to go down the verticle passage as possible. I recommend Berryman's B12 brand or Wal-Mart. Most of the others I have tried are worthless. It is best to take the carb off so you can block off the slit on the inside with a finger when you squirt but sometimes you can get enough with the carb still in place. Let it sit for a minute or two and then squirt again. Blowing compressed air into the idle screw hole may also help dislodge the blockage.

If the above does not work then I suggest you send the carb to a knowledgeable rebuilder. In most cases trying to take out the low speed jet (commonly called the "idle tube") just gets the screw leaving the tube in place. It then requires a special puller to get the tube out. There are two good rebuilders who frequent these forums. Either will do a good job for a reasonable cost.

Thanks for the advice, I put a vac guage on it last night and it had 40/45 lbs. Steve

40/45 pounds? The vacuum on the manifold is measured in inches of mercury, and the vacuum reading should be around 17 to 20 inches of mercury at idle.

Now you have me confused. The max. vacuum you should get is 15 lbs. That is atmospheric pressure. Anything greater is not possible here on earth. Are you maybe reading 30" of Hg? or 30 ft. of water? They are in the same ballpark as 15 psig.

Or are you reading pressure in each cylinder?

I will double check the guage and get back to you. Thanks Steve

I need glasses and a good mech. You were right. The 40# was the guage for checking the fuel pressure. I have 17/18 on the vac. guage. Thanks Steve

Yep.......that's more like it.

That is much closer. Guess the calibration is is bit off but that is a minor problem. Looks like you have max vacuum so that should not be the problem.

Thanks, Can you recammend someone to rebuild this carb. It looks like a Kane carb. There are no #s on it, just a Pat.#. Steve

I see your not a VCCA member yet and there are two re-builders of carburetors listed in the G&D which is short for the monthly magazine Generator & Distributor put out by the VCCA Club Anyway try contacting Chipper or the Junkyard Dog maybe they could help. And consider joining the VCCA best investment you could make in your car.

I had a similar situation concerning the idle. The motor idled and ran fine when I removed manifolds and accessories to repaint. I also disassembled the entire car to paint so it was a year before I reinstalled the carb. It refused to idle. The low speed idle tube was stuck in the carb body before this problem so I doubted that was the problem in it's self. I soaked, sprayed, doused, and cussed that carb with negative results. Finally, I removed the float bowl (for the 3rd or 4th time), removed the low speed idle tube threaded plug and ran a wire into the tube to rod it out. Forced carb cleaner into the tube via the nozzle straw and reintalled the carb. BINGO It now idled great. Obviously there had been an obstruction in the tube that only the wire could clear.

Thanks for the reply. I just went thru the carb for the 4th time today. I to ran a wire thru the tube ,it is now idling the way it should. It does not want to throttle up like it should now. I must have other issues. I have just started this chevy project and I can see it a must to join VCCA. Thanks Steve

I have made adjustments to my carb as recommended in the tread and I can say that I think I have the car running pretty good. I still need to figure out the correct settings on the choke, throttle and spark positions when starting, but that will come with time as I start to run the car more frequently. When the engine is "hot", I just touch the starter switch and it starts up within seconds. My only question on the carb now is that when I start to accelerate, if I do it to quickly it "stumbles" a little and then rapidly catches up.

I tend to think this has to do with the accelerator pump. I am missing the boot on the top of the pump, does it perform any function other than keeping debris out(i.e keeping vacuum)? Any suggestions?

You might want to replace the accelerator pump leather and spring and then your car will probably accelerate as it should.

The rubber boot on top of the accelerator pump is to keep the debris out and to make the acclerator pump cylinder more of a sealed system.

I have already replaced the leather when I perfromed a rebuild on the carb. Is there anything else I can try?

inside the float bowl is a screw in brass fitting that contains a check ball. that ball needs to be free in the fitting. also, the interior of the pump housing should be as smooth as possible. with the updraft carb it's difficult to observe the pump stream.

Make sure that the small orifice on the pump jet is not partially obstructed also.

The carb. that I have is a Marvel Sch.Are there parts for these? Thanks Steve

oooooo....... never mind what I said. I haven't a clue

Same here. Sounds like you need to find the correct carburetor for your 1931. It should be a Carter 150-S.