off idle stumble

[jdl71]

If this is better suited somewhere else, please mods feel free to move it.

I have a '64 230 with a rochester B. I currently have a rebuilt carb on it that I purchased from a member here. It has a new fuel pump, clean gas tank, filter and lines. I have tried running it from a gas can with an electric pump, made no difference. I found a fairly major intake leak near the rear of the gasket. New felpro intake and exhaust gaskets, no change. I have adjusted the timing both directions, ensured the advance is working and tried the fuel/air mixture screw. I have new plugs, wires, cap, rotor condensor. I do not think I've done anything with the points. All have been done since I noticed this problem, with the exception of the plugs. I do not remember for sure if it was doing this before I changed plugs. The truck had been sitting for a long time before I got it. I do remember having a hard time finding plugs because it still had the original AC plugs in it that are a discontinued part number and the part number that is listed for my application is a longer plug. I did gap them, and as I recall they were the same heat range. Could this be the issue? It seems to me like a lean stumble as adding choke helps, but doesn't make it completely go away, and if I hammer it too hard off idle it will backfire up through the carb. Any and all help is appreciated.

http://www.youtube.com/watch?v=tuHGQkASnc4

[Jurassic-1]

Have you checked the vacuum advance. It made a big difference in my truck. Also it could be a faulty accelerator pump.

[jdl71]

I verified it is working. I have changed carbs, same issue.

[Steve66]

There's an old mechanics saying that goes something like "Most carburetor troubles are electrical in nature". But I think in this case it's probably in your carb. You say it's been rebuilt but you don't know how well. That's what I would look at first. A lot of these parts house rebuild kits you buy nowdays just aren't that good.

On another subject I'm 99% sure that I'm the guy that bought that 66 from you a few years ago. I've done an awful lot to it and it's been good to me. I'm just about done with the interior. The engine is next.

[Steve66]

Accelerator pump would be pretty high on my list too but you say it does it with both carbs. I just don't really see it being an ignition issue.

[oem4me]

Turn the engine off. Pull the air cleaner and set aside. Look down into the carb throat with a flashlight. Work the throttle fully by hand as you look into the carb. You should see a healthy stream of gas injected with each "pump" of the throttle. If you DO see a stream, I'd move onto ignition, but if you don't, that could be your problem. A bad accelerator pump in the carb, or a plugged passage.
I had nearly identical symptoms and cured it after much frustration with a fine wire right at the tiny outlet in the carb throat.

[jdl71]

Originally Posted by Steve66

There's an old mechanics saying that goes something like "Most carburetor troubles are electrical in nature". But I think in this case it's probably in your carb. You say it's been rebuilt but you don't know how well. That's what I would look at first. A lot of these parts house rebuild kits you buy nowdays just aren't that good.

On another subject I'm 99% sure that I'm the guy that bought that 66 from you a few years ago. I've done an awful lot to it and it's been good to me. I'm just about done with the interior. The engine is next.

You mean this one?



That's funny I was just thinking about that truck today and how much I miss it. It was a darn good truck and really solid for something around here. Glad it's in good hands though I'd love to see some pics of it.

As for the carb it was a professional rebuild, but like I said I had the same problem with the original carb.

[jdl71]

Originally Posted by oem4me

Turn the engine off. Pull the air cleaner and set aside. Look down into the carb throat with a flashlight. Work the throttle fully by hand as you look into the carb. You should see a healthy stream of gas injected with each "pump" of the throttle. If you DO see a stream, I'd move onto ignition, but if you don't, that could be your problem. A bad accelerator pump in the carb, or a plugged passage.
I had nearly identical symptoms and cured it after much frustration with a fine wire right at the tiny outlet in the carb throat.

That was actually the next thing I was going to try, to see if I could see the "pump shot". Where is the tiny outlet you put the wire in? Down inside the carb? Is it something I can do with the carb on the truck?

[TJ's Chevy]

Had that exact same issue. Ended up being the accelerator pump.

[Steve66]

That's it! I'm kind of busy right now but later tonight I'll give you some details on what I've done so far.

[jdl71]

Looks like the Rochester manual will be on the nightstand tonight. Thanks for the suggestions so far.

[oem4me]

Originally Posted by jdl71

That was actually the next thing I was going to try, to see if I could see the "pump shot". Where is the tiny outlet you put the wire in? Down inside the carb? Is it something I can do with the carb on the truck?

Of course I have no idea where exactly your carb is plugged, IF it's plugged, but mine was stopped up right at the end of the line. That is, right at the throat wall. I couldn't even see the pinhole outlet at first. From memory, and I could be off, it was about 1-1/2" down fron the top, toward the firewall side of the throat with the carb in place. Use a bright light and look to see a hole. Mine looked like a tiny discolored spot on the wall, so I poked at it and it broke in, revealing the clean outlet. previously, I had taken the carb off several times and "thoroughly" cleaned everything with compressed air and Gumout to no avail. This last effort with a wire was done late at night, in the driveway, with the carb installed, and was the only thing that made a difference.

[Ronw435]

Sure sounds like accelerator pump to me!! Just because you see fuel squirting out, make sure it is getting the full amount. You can adjust the stroke if needed.

[akqj9]

I had rough idle and it was caused by a crack in the vent hose from the valve cover to the manifold.

[jdl71]

I have a working theory, but a couple more questions first:

If I understand the manual correctly, this is the rod that is connected to the accelerator pump plunger, yes?



Am I also correct in thinking that it also should not be coated in nasty, varnished gasoline?

Here is a brief synopsis. Bought new (rebuilt) carb thinking it would make truck run better. (Randomly throwing parts at a vehicle always works, right?) Also figured my carb would need a good cleaning and possible rebuild and was intimidated by doing it myself so this looked like an easier path. New carb does not fix anything, but I managed to gum it up nicely by running more nasty gas through it after I had thought the tank was clean. Took tank out again, powerwashed the snot out of the inside, coated with wd40, rinsed and sloshed around with clean fresh gas, repeated 2 or 3 times. Blew out lines and replaced all rubber. Still runs like crap. Now I finally start doing what I should have done in the first place, actually troubleshooting. Find massive intake leak at the head. Replace intake/exhaust gaskets, leak fixed, truck still runs like crap because I have mucked up a freshly rebuilt carb.

Theory: had huge intake leak causing lean condition. Ran new carb with nasty gas, then fixed intake leak, but new carb is now full of trash. Same issue, different cause.

Solution: Take apart either new (rebuilt) or original carb and reclean, try again.

Thoughts?

[61_FL_Apache]

Was timing mentioned?

[jdl71]

Originally Posted by 61_FL_Apache

Was timing mentioned?

Yes, I have checked and adjusted several times. Right now it's at 4degrees BTDC, vacuum advance plugged, at idle.

[VictoriaHardware]

I recently fixed my off-idle stumble with timing. It's a '66 292 with Rochester B carb. I had set the timing to 4 deg BTDC since that was the spec. So I kept advancing it until it was gone and checked that it didn't knock when driven. It's now at 10 BTDC and happy (idle w/ vac plugged). The vacuum advance works fine and I didn't have the host of other issues you did, so that was the only thing I did.

[jdl71]

Thanks for the input. I have adjusted it both directions and seemed to make no difference. I am going to get a clean carb back on there and start over.

[TJ's Chevy]

I'd clean both of them. lol Then get a dual single barrel intake manifold.

[jdl71]

Originally Posted by TJ's Chevy

I'd clean both of them. lol Then get a dual single barrel intake manifold.

I bought a parts truck not too long ago so I actually have three!

[TJ's Chevy]

Originally Posted by jdl71

I bought a parts truck not too long ago so I actually have three!

Well you can get a 3 carb manifold! lol

[jdl71]

I pulled the carb off my running parts truck, that did not have this problem and put it on. I can hear the pump squirt when I work the throttle. No change, still falls on face when throttle off idle. In addition to that I still have an intake leak at the gasket. I have a new gasket and a new exhaust manifold as I trashed my other two breaking off studs when I was trying to fix the intake leak originally, I have the intake/exhaust to head bolts as tight as I feel comfortable with. Same for the intake to exhaust bolts. Certainly more than the recommended 30-35 ft lbs. I also have at least one exhaust leak now, either at the heat riser pivot or the intake to exhaust gasket, which is also new. As much as I wanted to keep this 6 cyl it's about to get yanked and a small block dropped in. This is getting ridiculous. I have done everything by the book and then some, and I"m going backwards not forwards.

[jtrichard]

check your vacuum advance and see if it is WORKING

[jdl71]

I have and it is.