The Stovebolt Site Forums The Shop Area Engine Shop Stumble off of idle

My bone stock truck is having a slight stumble when revved from idle. If revved when already above idle, it revs great. It also idles fine, and runs fine at high rpm.

Should I tear into my carb for a third time to triple check that the off-idle circuit is unclogged, or should I be looking at my float level (which was on the gnat's @ss at 1-9/32"). Some of you may have seen in another post that my power piston spring and power piston check ball spring are not the original length anymore....time has taken it's toll on them. But I want to eliminate other problems before I shell out the cash for new springs

Maybe a vacuum leak?

I know about some of the inherent problems that come up with the Rochester B's. I'm getting a good sealing line on the gasket between the airhorn and float bowl. My throttle, and choke shafts don't appear to have excessive wear.

My carb seems to make a pretty healthy sucking sound when you first start to open the throttle flap....normal?

Things I've done:

-Rebuilt Carb with a kit from NAPA
-Completely de-rusted and cleaned the fuel system, tank, lines, pump
-Installed 1 inline filter between the tank and pump, and a sediment bowl before the carb
-Completely new ignition system (except coil, and dist housing)
-Valve lash adjustment
-Timing set to 0 deg.
-Points gap set by dwell to 31-34 degrees
-Plugs are black & sooty, but not wet

I've been dealing with this for a while now. Thanks a TON in advance! or a 1/2 ton haha

If you have an original 216 or 235 the timing should be set to the ball while running. Is that what you mean by zero degrees?

I would be checking the operation of the accelerator pump. It has two springs and sometimes it will hang up a bit. You should be able to see a little stream come out when you tweak the throttle and it is not running. Also the early carburetors had a check ball right in the pump bore.

Yeah, it's a real sod when that hesitation just won't go away.

I had the same thing for ages , then one day I just tweeked the idle mixture screw a LITTLE, (gave it some more gas)and behold ..... no more stumble.

Good Luck

Off-idle hesitation sometimes is caused by high resistance in the coil secondary wire. Make sure the terminal connections are not burned off or broken. Low ignition current can also contribute to that condition. You should find about 4 amps of static current- that is key on, engine off, points closed.

Harvester

Timing was set to the ball while the truck was idling.
I have a good stream when the gas pedal is mashed

As I said before, almost all of the ignition system is new, but I will check to see if I have adequate voltage at the coil.

any other suggestions? ideas?

Well, hmmmmmm.
How about the idle mixture, set for most vacuum. For that matter, what does the gauge read when this happens.

Does this go away when the truck warms up, after 20 minutes or so. Do you have some kind of manifold heat?

Have you set the timing using a road test, as per the shop manual? Your timing may be to retarded. What with todays modern gas and all you might just find that you can move the timing up a little more.

Now I don't know about the Rochester carb you have, but on a Holley 4 bbl, if you get a hesitation off idle, it is because the squirter size is either not adequate or the cam which actuates the accelerator pump does not have an aggressive-enough ramp.

Basically, to make a long story short, it sounds to me like you are not getting enough of a fuel shot when you first stab the throttle. You might want to look at the accelerator pump circuit of your carb.

Bryan,
you shouldn't need to mash the throttle to get a good squirt from the pump system. Just moving it a little from the stop should produce a good shot. If it doesn't work like that then it may be the source of your problem.

Mr. Lang-
I set the idle screw to get a "smooth idle", as I do not have a vacuum gauge. It will definitely be the next tool that I buy. I haven't found that the problem goes away after warm-up, but I still don't trust the truck enough to drive it very far/long, so maybe I need to circle the block for a while. The flap in my exhaust manifold is seized, I'm not sure what position it is seized in, but my intake will get good and warm after running a while, so I think I am getting some heat. As I understand it, the shop manual says to time it on the steel ball...correct? incorrect? When you say to advance the timing, you mean to make it spark further BTDC...correct?

Hot Rod Dad-
I have had someone suggest to me that I need to shorten the rod between the throttle linkage and the accelerator pump. Attempting to make it squirt earlier. Anyone else tried this?

Truckernix-
I'll put your comment into the "tear into the carb AGAIN column"

Is it possible that this is being caused by my vacuum advance not working correctly? My dist moves a good amount, but if there is a small leak, maybe it's not moving enough?

I run my timing a bit advanced and it seems to cure the stumble problem I was having. Since modern gasoline has a lot more octane than the compression ratio of the 235/261 calls for, its pretty safe to do- just turn it down a bit if it does knock.

Also, it seems to happen more commonly with ethanol blend fuels. Oregon is using 10% ethanol all year round and you actually have to richen up the mixture sometimes to get rid of the stumble.

Bryan,
on one of my carburetor rebuilds, I noticed that the pump was hanging up in the bore. Since it is pushed down with a spring it can hang up without the rest of the linkage being affected. I had to go through an assortment of pump pistons before I found one that worked.

Fred

Thanks for all the suggestions guys....as soon as I have time, I'm going to give them a shot. This website is awesome, I don't know what I'd do without it!

Bryan,
Just a thought...when you rebuilt the carb, were there any internal dimensions for the accelerator pump stroke/hieght? It's been a while since I looked at the specs, but seems like there is an aadjustment on this and it is often overlooked. Timing is critical in the pump shot/stroke.
Also, these old carbs are worn quite a bit, especially in all the linkage holes. Could it be that when you push the pedal, the first movement (when nothing happens) is just all the slack coming out of all the linkage connections?
Just food for thought. Let us know how it turns out when you solve it. Jim

chetman-

If there is a way to adjust the pump stroke/height, it would be to bend the arm that connects it to the throttle linkage. I've never heard of anyone here doing it, and that makes me hesitant. It is shown as part # 36 here:

http://chevy.oldcarmanualproject.com/carb/Rochester/9C305p01.htm

Your suggestion of the slack in the linkage system is a good thought, but the problem will occur even when I manipulate the throttle linkage on the carb.

OK guys,

Last night I tinkered with it a little bit. I bought a vacuum gauge, and at idle I'm getting a fairly steady 19.5" (needle only wavers 1" total at idle." Meaning it vibrates rythmically between 19 and 20). Drops to around 3-4 when revved, and shoots up to about 25 when I close the throttle flap, then settles back down to 19.5. After advancing the timing to about 6-8 degrees using the octane selector built into the distributor base, I took it for a spin. It still wasn't running right. didn't have much power, and stumbled when you try to gas it.

Then I pulled out the choke knob about 1/2 - 3/4....

IT WAS LIKE A WHOLE DIFFERENT TURCK! It ran great, pulled strong, and didn't even think about stumbling. I knew that using the choke knob helped it, but it has never run so good! I had previously had it timed on the BB, and I know that advancing it has made a huge difference.

I've plugged off the vacuum line to the wipers, just to be sure that it wasn't the source of my leak....no dice

Now to the problem at hand. Where does this leave me? Vacuum leak? Float Level? Warping Carb castings? Throttle Shaft Leaks? Weak Fuel Pump?. It seems obvious to me that I'm running lean.

It was SO encouraging to have the truck running so well. I never knew that it could run so good, it was an awesome feeling. Please help guys, I want my baby fixed!

I don't think that the vacuum should move rhythmically. Do you have a shop manual?

If your timing is too far advanced, pulling the choke might straighten things out.

What you might want to do, is put in clean plugs, set the timing as per the manual, and the idle, etc., don't drive with the choke on. Drive the truck around for 30 minutes, so its warmed up. Now, find a gentle hill, where you can accelerate at full throttle, in top gear, from about 25 mph. Now after running at WOT for a few minutes, push in the clutch, turn off the key, coast to a stop. Do not let the truck come back to idle, just shut it off. Pull out the plugs, and stick them into a piece of cardboard, labeled as to which cylinder they came from.

Now, put in your spare plugs, and drive home.

Now that you are at home, you can take your time, and look at each of the plugs. This will tell you whether you are lean, rich, etc.

Just my thoughts. Pick a hill where the above can be done safely.

vibrating is a better word for it, it would vibrate, between 19 and 20, but it was constant, hence rythmic.

yes I have a shop manual, I had been running the truck with the timing on the BB, I doubt that 6-8 degree of advance is too much. Running it advanced didn't make it run better or worse. It only ran better after I pulled out the choke knob

pulling out the choke richens the mixture. correct? which would mean that I'm currently running lean. correct?

Bryan,
I have noticed the same thing on my '41 GMC (228). I can pull on the choke a tad and climb hills much better. I have no solution yet either. I haven't had much time to figure it out. If you do, please let me know.
Jim

I went hunting for vacuum leaks last night. As expected, I found no leaks, except at the throttle and choke shafts. I don't have a micrometer or dial calipers, so I can't measure to see how much of a gap there is. When I would spray carb cleaner on the outside of the carb where the shafts exit, I would get a small bump in RPM's (about 10rpm). It didn't seem like much to me, but maybe the collective leak from all 4 penetrations is just enough to cause troubles?

I searched for the leaks with propane, MAPP gas, and carb cleaner. the leak at the shafts was only detectable with the use of the carb cleaner.

I still don't think that this is going to solve my problem, My inclination is that there are a few small things wrong, that are snowballing.

Chetman-
I feel your pain man.... I'll PM you if I ever get er' done!