VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1929-1932 Carburetor and rough idle

Now that I have some great braking action on my 31, I’ve turned my attention back to trying to get it running a little better. After going through the ignition system, and setting gap and advance as recommended in these forums, it’s still difficult to start, doesn’t idle as smoothly as it should, and seems to lack power.

Now, thinking the problem could be fuel related, I couldn’t locate any vacuum leaks around the manifold, but did find that spraying some WD40 around the throttle shaft results in an increased and smoother idle. The idle mixture screw is full rich, and turning out more than about a quarter turn produces a very rough idle. Pulling the choke on a little smooths out the idle for a short time. The previous owner had the carburetor refurbished, and looks clean, bowl is clean and float adjusted correctly, but no telling if something deeper was missed or incorrect.

Wear at the throttle shaft doesn’t appear excessive, but could that create enough vacuum leak to run too lean? Any other recommendations on what to look for?

Thanks!

Test the compression

What carburetor are you running?

The Carter RJH08

Yeah, I’ll have to stop and pick up a compression tester tomorrow. Thanks

It does not take much wear on the throttle shaft or carburetor body to effect the idle mixture. As long as the carburetor will give a reasonable idle I typically don't bush the body and replace the throttle shaft when I rebuild a RJH-08 carburetor. Does not make economic sense to spent North of $ 100 just to get a little adjustment to the idle mixture screw. However I will do the work if the owner wants it done. If the idle is not acceptable then spending $ 100 plus may be desired by the owner. In your case it can be the source of your problem.

Here's a hokey idea: smear some really thick grease on the throttle shaft ends. Maybe it'll plug up the gap.

Sludge to the rescue!!

Cheers, Dean

Ha! That thought had crossed my mind, even just as a temporary solution to determine if the wear in the shaft was really making a difference.

The Carter brass bowl updraft carburetors have an idle tube (idle jet) that could be problematic when new, and certainly can give a "restorer" with little experience on these carbs fits.

Possible issues:

(1) I have listings for 138 Carter brass bowl updraft carburetors, and probably no more than 3 or 4 ever used the same idle tube. In other words, there are a bunch of different idle tubes, all of which are wrong except for one. There are some reproduction tubes available, but as I have a good supply of NOS, I have not checked the quality and applications.
(2) The tube is soldered to a plug. Heat is often used to aid in the removal of the tube, and solder can flow covering the tiny hole at the bottom of the tube where the plug is soldered.
(3) These tubes, as well as the idle tubes used on the Carter W-1, are inserted into a TAPERED channel. When the tube is tightened the last partial rotation of a turn, the soft brass is deformed at the top cutting a seal. Tubes may be removed, and reused, but if reused, the top of the tube MUST be expanded roughly 0.003 to allow the tube to reseal when installed. Failure to expand the top will result in a poor seal at the top of the tube; which results in a lean idle mixture. Carter always recommended the installation of a new tube. The trick of the expansion of the top was taught to me by the Carter School chief instructor some 50 years ago.

The fact that the OP has the idle set full rich leads me to believe #3 above MIGHT be the culprit; enough so that I would suggest contacting the rebuilder to inquire about the procedure used.

EDIT: Just reread the entire thread. Concerning the "play" (tolerance) in the fit of the throttle shaft to the throttle body. Carter "design" specifications were 0.004~0.006 inch. In practice, wear up to an additional 0.003 will not cause an issue. So, adding 0.003 to 0.006 gives a "maximum" of 0.009 inch. In the FWIW category, when I was still restoring carbs, I found that, after bushing and replacing the throttle shaft, less than 0.005 COULD occasionally bind; so we would ream the bushings to give a final fit of 0.055 inch.

Jon.

Thanks for the input Jon. The PO either purchased a rebuilt or had sent this one off, and was installing it when I looked at the car. Unfortunately I have no idea who did the work. Is there anything with the idle tube or other components that could be verified or resolved by the average guy with some tools, or would it be a curiosity killed the carburetor type of thing?

Steve - after you have done more testing (compression, etc.) and you are really CERTAIN that the carburetor is the culprit:

No offense meant at all, but the Carter brass bowl updraft carbs are best left to professionals or very skilled amateurs. Not grinding my own ax, as I no longer restore. If it comes to this, I have had dealings/conversations with both Chipper & the Junkyard Dog, both whom have participated in this thread. I would suggest contacting one of them.

EDIT: Should you opt to ignore the above, here is one of the documents you will find useful: http://www.thecarburetorshop.com/Service_Carter_Brass_Bowl_updraft.pdf
The other document would be the original Carter parts print for your carburetor. By the way, the Carter literature posted in the above link was posted to the internet WITH WRITTEN AUTHORITY FROM CARTER, and may be downloaded and printed for individual use. I don't believe in ignoring copyrights.

Jon.

Thanks Jon, and no offense taken! Kinda figured that may be the case, so I’ll continue looking for other potential culprits before resigning myself to pulling the carb and sending off. I’ve seen many comments in the forums on Chipper and Junkyard Dog’s quality work. Thanks again!

Did a compression test this evening - 75,75,75,78,78,75

I think this is in the normal range, correct?

Next up will be a vacuum test. I assume the windshield wiper port Is the only source and acceptable?

Thanks,
Steve

The compression looks fine.

Excellent, I can mark that off then. Thanks

i had a rough idle and sprayed brake clean around the carb and found it smoothed out, as it was sucking in the brake clean and burning it off. also sprayed around the intake manifold but nothing there. seems like there is an AIR LEAK sucking in air.

BearsFan - That's where I'm at as well. Were you able to resolve it?

I'm thinking either the wear at the throttle shaft is beyond spec and leaning the mix, or, something within the carb is creating a lean mix and what WD40 is pulled in around the throttle shaft is enriching the mix enough to run smoother.

Spraying around the carb at higher RPM also results in it running smoother.

Thanks

If spraying a combustible improves the performance at both low and higher engine speeds it may not be the carburetor. Richer mixtures can be easier to ignite and burn. So looking at the spark plugs, location of the plug tip and intensity of spark at the plugs might be wise. It is more often much easier and less expensive to fix those problems than a carburetor overhaul or replacement.

If the carburetor does not produce the 15:1 (or close) air/fuel ratio then it may be the majority of the problem. Also remember that only the combustible vapor burns effectively. Particles or liquid droplets do not.

Once the engine speed increases past a fast idle the leakage at the throttle shaft becomes insignificant. Engine vacuum decreases as the throttle plate is rotated and gas begins to flow due to incoming air flow rate and not being drawn though the idle passage by the high vacuum.

Thanks Chipper. I've been through the ignition system with the tune up kit, coil, and wires from The Filling Station, and new C87 plugs gaped at .04 and timing at 18BTDC. Makes sense regarding the throttle shaft, and hadn't thought of the vacuum drop as the plate opens.

if nothing else try setting it all back to factory specs and see if ANY difference. spark gap, timing, etc... just to validate it is not that. i know my 1929 runs rough at the rec specs, i had to find a sweet spot in the middle. now that i have redone the top half of the engine, when i get it running again i am starting at STOCK specs from the manual make sure it runs then tweak it from there. Also using C87 Plugs :)

Tried playing with the gap and timing a bit. Decreasing from .040 to .032 definitely made it worse, so I tried going the other way to .045 which smoothed it out quite a bit. 18 BTDC is definitely the sweet spot for timing. Still feels like it’s strangling a bit, so i’ll continue to tinker on, but did get to put about 20 miles on it this morning.

"EDIT: Just reread the entire thread. Concerning the "play" (tolerance) in the fit of the throttle shaft to the throttle body. Carter "design" specifications were 0.004~0.006 inch. In practice, wear up to an additional 0.003 will not cause an issue. So, adding 0.003 to 0.006 gives a "maximum" of 0.009 inch. In the FWIW category, when I was still restoring carbs, I found that, after bushing and replacing the throttle shaft, less than 0.005 COULD occasionally bind; so we would ream the bushings to give a final fit of 0.055 inch.

Jon."


Jon,

Can you please explain why such a seemingly loose fit of the throttle shaft in the carb body would be required? Is there a problem with a smaller clearance? I have put bushings in some W-1 and Quadrajet carbs, enlarging the holes by holding the base in a milling machine vise then passing a drill followed by a reamer all the way through in a single operation. With the bushings pressed in and the shafts installed the shafts move freely and there is no perceptible "wiggle". Is there a risk that this setup could bind or seize under some conditions?

Ray W

I run my AC-C86 spark plugs at .045" and I have found that the vehicle runs the best at that gap. The plug gap of .040" is great too for most 1929-32 Chevys, but for my application the .045" seems to be a little better. The timing of 18 degrees is definitely spot on for today's gasoline.

Ray - as I stated, we found occasional binding when engines warmed up using the 0.004 with the Q-Jets (coefficient of linear expansion is different for different materials). Remember that these clearances were designed by the original engineers, and the carburetors were calibrated to compensate for the clearance.

I don't ever remember having to put bushings in a W-1 (cast iron throttle body). Machining a new throttle shaft always put the unit back in spec.

Jon.

In addition to the differential in metal expansion, gasoline does not make a very good lubricant. Also the throttle arm puts an off-center force on the shaft so the gap needs to be a little wider than other engine parts.

I agree with Jon. I can't recall ever needing to bush a W-1. Have bushed a few older carburetors but a few out of over 800 makes that "rare". I only replace the throttle shaft if I can't get a decent idle or the shaft is damaged. No need to raise the cost just to be able to adjust the idle mixture. No real benefit does not justify higher cost.