Engine Stumble. Carb?

[I66coupe]

Ok, got the car driving today. Its been almost 2 years now. I think the carb may now need some work.

I got the car driving and there is a serious stumble that is very bad during acceleration. At cruise its not as bad but still there. I have set the point gap and the timing. I have adjusted the timing and gap a bit either side of specs and no difference. This leads me to beleive, its the carb. Sitting for 2 years can't have been good for it.

Its not like a backfire, more like a misfire that slows the car down. It is not continuous, its intermittant. Like stabbing the gas and then letting off real quick and then stabbing it again, over and over again.

Tomorrow, I'll install a new fuel filter and see if that helps any. Perhaps some rust formed during its extended down time and has clogged it.

Any thoughts?

 

[Anonymous]

You're probably right but just so you have some other ideas, last time my ride had a stumbling problem the fuel pump was the culprit. It's a possibility.

 

[JoeTrojan]

The Autolite 1100 (like most carbs) uses a pump to squirt a little slug of fuel into the throat of the carb when the throttle is depressed. this provides fuel on acceleration (because the heavy fuel doesn't gain speed in the venturi fast enough to keep up with the lighter air that responds almost immediately to the throttle plates opening). This system is called the accelerator pump.

the system is pretty simple. Often the accel. pump gets fouled when fuel dries out during storage. the pump uses a rubber diaphram to seal it. this piece is replaced in a rebuild.

to determine if your accel pump is working right:
1 - remove air cleaner and run car at idle.
2 - use flashlight to look into carb throat. you may have difficulty finding it, but look for pinhole sized opening in the side of the throat that is at the 3 o'clock position if you view the choke housing at 12 o'clock.

3 - exercise the throttle while looking down the throat of the carb.

if you see slight bubbling or a very weak or short squirt of fuel from the pinhole, then your accel pump is not working. rebuild kits for the whole carb are cheap (less than $20) and rebuilding the pump alone may get you by.

if you see a solid squirt of fuel that form a nice straight line of fuel the width of a paperclip that lasts for half a second or more, then the accel pump is fine and you may have another carb related vaccuum leak, or I don't know what.

What you describe is classic accel pump problem...good luck.

 

[I66coupe]

Yep Joe, I am familiar with the accelerator pump. I know that it is bad but this problem is different. I do have the bad accelerator pump but this problem I am asking about is present at steady road speeds. Its worse during acceleration but still present at steady speeds. I think its just a dirty carb. Not getting a proper mixture into the intake.

I'm just going to go ahead and rebuild the carb (accelerator pump needs repair anyway). Good cleaning, new gaskets, float setting, etc. I was hoping for something easy to repair on a Sunday but will have to wait now 'til next weekend as no local parts house had a kit on the shelf.

Probably grab a new fuel pump at the same time and add an inline filter before the pump too. Just another "might-as-well".

 

[JoeTrojan]

didn't mean to offend...i'd consider a vacuum leak in the carb body. a way to isolate that is the squirt carb cleaner at the gasket seam while idling. If idle picks up at some point, then the cleaner sealed the vacuum leak momentarily, with flammable liquid.

you are right, no matter the cause, a rebuild eliminates most all of 'em...good luck.

 

[I66coupe]

No offence taken here Joe. Just letting you and everyone else know that I am familiar with the effects of a bad accelerator pump and that this stumble is different from that.

It just seems to be a fuel delivery problem. Took my older, hotrodder brother for a drive and he concurred that it seemed to be a lean fuel mixture.

I'm not surprised the carb needs work. The car sat unused for almost 2 years. Looks like about a $20 kit should do it. Just have to get one coming tomorrow.

Also going to have my brother bring his old Sun diagnostic center over to where the car is and hook it up for a thorough check.

 

[Anonymous]

I had the same stumble at 50 mph on the road. It felt like a miss.

Two things I did that seemed to solve it:

Replaced a bad plug wire (shocked the crap out of me as the car was running one day)

Replaced the fuel filter with a clear one. Now I can actually see some rust in there, and it is accumulating that I can see. The old gold filter on my 1100 let the crap run back down the line to the fuel pump.

The clear one holds it in the filter base for your observation.

r/
Steve-O

 

[Anonymous]

when I had a msss. it was a fouled plug.