Carburetor stumble and idle speed

[TR7Aaron]

Lately, when I get off the gas quickly and the transmission is in neutral, the engine revs drop to about 400 - 500 and if I don't touch the throttle, the engine will sometimes die. The idle also fluctuates between 600 - 1000 rpm. It doesn't surge or flutter, but when returning to idle, sometimes it's a little higher (like around 1000) and sometimes, a little lower (like around 550 - 600).
I was driving all over today and the problem seemed to be getting a little worse. On the side of the road, I bumped the idle speed up a few hundred rpm to about 900 - 1000 and that seemed to help - it was steadier and even though it would still stumble when I got off the gas quickly, the revs only dropped to about 500 - 600 and it would recover.
I didn't have a lot of time, and I haven't studied up on these carbs, but I did take a quick look with the air cleaner off to see if anything obvious was the cause.
So, here's a real stupid, simple question - are these dual carbs designed to work in tandem at all rpms or does the front one do the low rpm work and the back one kicks in at higher revs or when demand increases? I ask because I revved the engine by hand a couple times with the air cleaner off and I could see the piston on the front carb rise and fall, but the back one hardly moved.
Thanks,
Aaron


Relax, all right? My old man is a television repairman, he's got this ultimate set of tools. I can fix it.

[Limey7]

Here are a few basics to start with. Make sure the carbs have oil in the dampers. Check the diaphrams and make sure the rubber is in good shape. Also, check the gasket where the carbs mount to the intake manifold. I removed my carbs to have them rebuilt last year and fought with an intake leak at the mounting points for quite awhile. The symptoms you describe where alot like my problem.
I'm sure you'll get more technical responses than mine but these are a few basics to check in the mean time.
Good Luck,
Matt

"The Flying Doorstop!"

[Workshop Help]

Well, Well! We seem to have our own little epidemic Z-S flu. You have done a good thing thru your diagnostic efforts by observing the rise & fall of the carburetor pistons. The next step is to remove the 4ea screws on the top of the rear carburetor and examine the rubber diaphragm. Is it cracked? Is it torn? Are there holes when you lightly stretch the rubber?

If yes to any of these conditions, you get to replace your diaphragm(s). And, since you monkeyed around with the idle setting, you get to matriculate the degree plan for your Masters in Zenith-Stromberg Tuning.

Won't that be fun?

Mildred Hargis

[Hasbeen]

Yes Aaron, they work in tandem.

To get this right, disconnect the linkage between the carbs, &
adjust the throttle stops so each carb has the same suction. Do it
by ear, with a length of hose, or with a gauge.

Once adjusted, reconnect the throttle linkages.

The following may be a local problem, due to our dry dusty climate.
I find the throttle spindles get sticky, with oil, & dust, & can
cause the idle to vary. I wash these out from the inside, with
electrical contact cleaner [pressure pack makes it easy], &
lubricate with a puff of graphite powder. Required about 5000Km.

The same occurs with the throttle cable. It will cause the
application of throttle to be "jerky" particularly from idle. I
remove the cable, wash it out, let it dry, then lubricate with
graphite powder.

I find any wet lubricant used for the above jobs will work for a
short period, with the problem returning more quickly each time, but
it will do to test if these things are your problem

Hasbeen

[TR7Aaron]

Lots of great info - thanks. I really don't know much about these types of carbs - my experience is with 'traditional' 2 & 4 barrel versions. I don't know what you mean by oil dampners and adjusting by suction. I guess I'll have to educate myself before I get in too deep over my head.
Hasbeen, I have that sticky throttle action also - you really can't make a smooth off idle transition - it's very jerky.
This also explains why this car is an absolute dog when it comes to acceleration - it's slow at all speeds but nothing really happens after 3500 rpms when you floor it - which makes sense if I'm just using one carb.
I have A LOT of work ahead of me....
Aaron



Relax, all right? My old man is a television repairman, he's got this ultimate set of tools. I can fix it.

[Hasbeen]

Don't get your hopes for rocket like acceleration too high Aaron.
The difference in the work done by each carb is high at idle,
when the linkage adjustment is out, but not as great at higher revs.
Still, it will be quicker.

Getting the carbs tuned properly makes a lot of difference of
course, as does ignition timing. However, asuming the thing is in
fair condition, I have found the most important thing for
performance with these cars, is the valve gear.

They rapidly loose performance once the valve clearance gets more
than a couple of thou out, & one car, when I bought it, had a cam,
so worn, I don't know how it dragged itself out of the shed.

Just work through it slowly, & enjoy the learning. They are fun
cars, once they are going OK.

Oh, unscrew the knob on the top of thr carb. The oil goes in there,
to dampen the piston movement. This stops the thing going too lean,
when the throttle is floored.

Hasbeen

[Hasbeen]

Sorry, I missed you didn't know how to adjust the suction.

Remove the air filter. Disconnect the junction between the carb
throttle linkages. Run the engine, & bring it up to temp.

Ger 3Ft of 3/8", or 1/2" plastic hose, [or a special tool to check
this suction].

Stick one end of the hose in your ear, & the other at the front of
each carb, in turn. [Make sure it is the same place on each carb].

Listen to the sucking noise of each carb

Open the idle stop of the carb with less noise.

Repeat. Perhaps close the idle stop on the noiser carb, to keep the
revs around 1000 RPM.

Open the throttles on both carbs, once every couple of minutes, &
run the engine at 2500 or there about, for 30 seconds, to keep the
plugs clean.

Once you have the suction noise equal, on both carbs, & the engine
idling about 800 RPM, reconnect the throttle linkages.

Do try, it's a lot easier than it sounds.

Hasbeen

[Workshop Help]

Yes, but first one must verify the condition of the diaphragms. Once that is done, then one can search thru these forums for the exhaustive tomes on carburetion tuning.

We must not get the cart before the horse. Remember, you do NOT have to remove the carburetors from their mounts to service either the top or the float bowls. Again, search these forums for this information.

Mildred Hargis

P.S.

On our U.S. model cars with the Zenith-Stromberg carburetors, DO NOT DISCONNECT THE LINKAGE! Balancing the two units is done with all connections in place.

M.H.