belated stumble and head scratching

[Anonymous]

Engine idles great, runs great, pulls hard. But when I've got it in gear (it's an auto) and holding it against the brake, like at a stop light, after a couple of minutes it will start to chug. Barely noticeable at first, but keeps getting worse and worse, eventually the idle drops down below 500 and I have to shift to neutral to keep it running. Then it slowy recovers and idles normal.

What's so weird is that it literally takes a couple minutes to start doing it--so it only happens at long red lights.

Here's the vitals:
65 200, with a 170 head
Autolite 2100, newly rebuilt by a reputable carb guy, with Clifford adapter
Pacemaker headers
DSII
FSPP 1.6 ratio rocker arms
After market power disk brakes, with manifold vacuum from the adapter teed to booster and PCV
Timing set at 13* (advancing it makes the condition worse, but retarding it further doesn't seem to help)
Mixture screws set at 1 1/2 turns out from lean stumble point (but I've tried several settings and the mixture doesn't seem to have any effect on the problem).
62 main jets.

Any ideas?

 

[JackFish]

Maybe try checking the screws on the carb, mine have come loose on occasion, creating a vacuum leak, I think, at certain pressures.

 

[Anonymous]

maybe a leak in the brake booster

 

[Stubby]

Check your accelerator linkage and accelerator pump setting, I have seen the pulsing from an engine gently pump the accelerator pump. It usualy only happens on engines with mechanical linkage combined with a lopey cam but I have seen it on mild engines if the accelerator pump linkage is pretty tight.

 

[Anonymous]

Checked the carb mount, everything is tight. Vacuum at idle (in Park) is steady at 18.5, with or without the brake booster connected. I set the mixture to max vacuum, actually leaning it out a little so now I'm down to just over a full turn out from lean stumble point. Problem persists.

Stubby--do you mean the accelerator pump linkage may need loosened? If the vibration of the engine were moving the pump, wouldn't that just make it idle faster?

I just can't figure out what would be changing in the engine as it sits in gear for a while. Temperature goes up? Runs out of fuel in the bowl? I don't have a cooling problem, just hooked up a new SunPro temp gauge and I never get above 180. What is supposed to be keeping the bowl full at idle in gear??

 

[Gene Fiore]

Years ago I encounted a very similar problem. I had a Maverick with a 250 I6 that had headers, 500 cfm direct mount and a Sig Erson cam that was probably too much cam for the street. When idling in gear at a stop light for any length of time the same thing would happen, but I just figured the plugs were loading up. I would just pop it in neutral, rev it a few times, and then it would be ok. But since you didn't mention anything about an aftermarket camshaft, not sure why yours would be doing that.

 

[Anonymous]

Gene--my cam is stock, but I do have the 1.6 ratio rocker arms so I guess the effect could be the same. If the plugs are loading up, what would be the fix--hotter, colder, more or less gap?

 

[Gene Fiore]

I don't really think the 1.6 rockers would have any effect. If I'm right I believe they just increase the amount of valve lift but would not affect how long the valves are staying open (duration). The longer duration cams could cause the plugs to load up at low idle conditions, but I don't think that is the case with your setup. I'm sure someone else on this forum might have some other suggestions.

 

[Mercury Mike]

I am having the same problems currently. It happened when my head gasket started leaking mostly. I fixed the head gasket, and my problems went away. My timing was too advanced, and kept putting out the head gasket around the #5 cylinder. I got my timing set right, idle right, and it hums. I rebuilt my carb and now I am having problems with a complete stall when I come to a stop. I'm still working on it. I think my float was adjusted wrong, and that I might have a vaccuum leak somewhere. I had 3 stripped holes in my carburetor screws, the top ones. I am putting a different autolite 1100 instead of my 1940 that my grandpa's got laying around and see if that helps any. =) Good luck.

 

[Anonymous]

Well I pulled a plug and it's black--not oily, just black with a little bit of dark brown on one side of the insulator. So I'm thinking it's running rich and loading up after it idles in gear for a while. But I've already got the mixture screws close to the lean stumble point, about a turn back.

Could it be that my jets are too big (62)--would that make it run rich regardless of the idle mixture setting?

Maybe I need hotter plugs? I'm already running Autolite 46's, I don't think they go any hotter than that do they?

Thanks for the suggestions so far, keep them coming please!

 

[Anonymous]

the size of the jets have no effect on the idle, however the float level does so that is somthing you could check, but for what its worth my 2100 is running 52 holley jets and my plugs are a nice tan

 

[Anonymous]

My 2100 had developed that same problem and it turned out the float was not seating quite all the way and at idle enough gas was overfilling and dribling into the intake progressively getting to the point of loading up the engine. The seepage at the needle wasn't enough to cause a problem except at warm idle. I had adjusted the float a time or two trying to fix the problem, then upon investigation found what looked like a hair in the float needle seat. Cleaned needle seat area and problem gone.
--The 2100 is a good carb but is very sensitive to any trash and a little prone towards problems that are trash related. I run a 108 and use the smallest jets i could find in a pile of about 10 2100 carbs- i don,t remember the size just that they are the smallest i could find--of course i do have 2 other carbs the open progressively so hi rpm demands are diffrent.

 

[Anonymous]

Thanks for the ideas. I ordered 3 sets of progressively smaller jets today and hopefully can find the sweet spot. I've got 52, 56 and 60's coming, currently running a set that's drilled out to 62.

I'll also check float level and needle/seat for debris.

My plugs aren't quite as bad as I thought. After I took it out on the road and opened it up for a while, they're all brown with some black.

The car went into the shop today for dualies with Flowmasters--can't wait to pick it up tomorrow!!!