Acceleration problem

[Joe Flanagan]

Hello everyone,

 

I've been away from the forum for a long time, mostly because my Plymouth is done and I didn't drive it at all over the winter because our weather was so bad and there was salt on the roads for months and months. But I've been back on the road for the last few weeks and I've developed a problem:

The engine idles perfectly but when I accelerate, it surges. For example, if you open the throttle half way, the engine will accelerate and fade, accelerate and fade repeatedly. It does this in a perfect rhythm, like it's surging. It sounds to me like a carburetor problem, like maybe something wrong with the jet/plunger that allows you to accelerate. I will be pulling the air cleaner and examining the carb this weekend but I just wanted to check in quickly and see if anyone's had this problem and can point me to something specific. I'm going to watch for the stream of fuel down the throat of the carb when I accelerate as a first test. Like I said, it idles perfectly, but accelerating produces a surge that goes in a very regular cycle. 

[MarkAubuchon]

That's what I would check. Start with the accelerator pump, one thing I keep an eye on is when you pull the top of the carb, if you see any dirt or debris I would rebuild it and make sure everything is clear.

[Don Coatney]

Accelerator pump is the first thing I would look at. Remove it, soak the leather in oil, reinstall it and see if the problem has disappeared.

 

[Jeff Balazs]

I am not sure if this could actually be caused by the accelerator pump itself. Seems to me that the cycling effect would more likely be caused by the float level being too low? Or perhaps dirt in the accelerator circuit ? Or maybe a fault with the fuel pump......not keeping up with demand under load.

 

Jeff

[Joe Flanagan]

Will likely do this on Saturday. I'll report back with whatever I find. Thanks for the responses.

[Niel Hoback]

Make sure you have a good vacuum signal getting to the power piston.

[ledfootslim]

Thats exactly what mine has been doing. I hope you have luck with yours- mine benefitted from having the float reset a couple of days ago, so I hope hat will help.

[Labrauer]

I have had the same problem with my 48 Plymouth a while back and took the carburetor off to do a rebuild. Put the carb back on and like magic the problem went away. I would go with Don's answer and look at the accelerator pump.

[Joe Flanagan]

This carb was rebuilt some years ago and it hasn't been run very much. I've got 2,000 trouble free miles out of it so far. 

[Bad Apache]

If you're holding the throttle steady and the engine is surging, it is not the accelerator pump circuit, but rather the cruise circuit. Probably a dirt issue. Looking forward to your find!

 

Lon

[Joe Flanagan]

This morning i removed the carburetor and examined it. The accelerator pump works fine and squirts a healthy amount of gas when it's depressed. Everything else looks good, too. I removed the step up jet and checked it out. It looks fine. There is no debris on the needle/seat. I took a can of compressed air and blew out all the passages thoroughly, reassembled the carb and put it back on. No change. I have verified that the accelerator pump shoots gas down the throat of the carb and with the throttle open, gas flows out of the two jets in the tube that goes across the top. Everything else looks good. I've confirmed that the line to the vacuum advance is tight. I pulled off the distributor cap and took a look inside but didn't see anything out of order. When I move the breaker plates, though, there is a clicking sound and I'm not sure that's normal. 

[Jeff Balazs]

Did you check the float level? and verify if the check balls were in place and seating properly? If all is truly well in the carb then I would look towards the fuel pump as the next step. Could be a fuel delivery issue?

 

Jeff

[Joe Flanagan]

Yes, float level is good and the check balls seem to be where they're supposed to be. I'll check the fuel pump next.

[Young Ed]

Joe my car does this too and has for most or all of the time I've owned it. My current theory is extra air entering the carb via worn throttle shaft.

[Don Coatney]

Joe my car does this too and has for most or all of the time I've owned it. My current theory is extra air entering the carb via worn throttle shaft.

Spray some heavy oil around the throttle shaft and see if the problem disappears for a short time until the oil is sucked in.

[Young Ed]

Spray some heavy oil around the throttle shaft and see if the problem disappears for a short time until the oil is sucked in.

 

My problem is the issue only happens under load like trying to climb a hill on the freeway. If you try to maintain speed it will surge if you give it more gas it won't. I went back and reread Joe's original post and it sounds like mine isn't as similar as I thought.

[Joe Flanagan]

Right, my car does this simply when revving the engine. I pulled the carburetor off again and double checked everything. Float level is where it's supposed to be. The balls are in place, passages are all clear (as far as I can tell, and I blew air through every one I could find). I also pulled out the accelerator pump and flared out the leather sleeve at the end of the piston. I'm going to have to figure out a way to spray oil on the throttle shaft while revving the engine so I can do the test Don recommends. I can't really drip it on because of the location. In the absence of oil, I sprayed carb cleaner around the throttle shaft and it didn't make a difference. Tried another solvent that looked a little thicker than carb cleaner but that didn't make a difference either. 

Given the symptoms, what's the best way to check the fuel pump? Just seeing how much gas it delivers while cranking the engine won't really give me an answer, will it?

[Don Coatney]

My vacuum gauge is a compound gauge used to check fuel pump pressure.

 

[Joe Flanagan]

Hmm. That's interesting. I don't think my gauge has that function but I'll have a look. It's been a long time since I've used it. Thanks for the picture. 

[Jeff Balazs]

Couple of ideas. You could try hooking up a temporary fuel supply that is gravity fed and see if that makes a difference. Also this would allow you to run the engine with the fuel line off and get a measure of how well the pump is delivering fuel. There are other fuel related items that could be checked as well. Condition of lines between pump and tank?

 

Wear is or should be a concern with these old carbs. One area I had some issues with was the bore and fit of the power piston. You might want to take a close look at yours to see if it is operating smoothly. I would think that if it were hanging up it could contribute to the problem you are experiencing. Sure wish someone made brand new replacments for these carbs.

Hope this is of some help to you.

 

Jeff

[Lloyd]

An accelerator pump only pumps fuel when you push down on the pedal. This gives you an additional boost in power. Thats about all it does. It does this as you push down on the gas pedal, not hold it steady.

 

If there is a constant surging while holding the throttle steady then more than likely its a plugged circuit or pulling excess air or fuel, or even a restriction in the air or fuel.

Do you get any black smoke out the exhaust when its surges? Get it to surging then shut it down, check and see if your plugs are wet.

Also if you have any sort of PCV system on there you might check the filter.

One other thing might be the main vent tube. When rebuilding my carb I seem to recall reading that if it becomes clogged it can cause an overly rich fuel mixture at higher speeds.

One last thought is an intake leak, remove all vacumn hoses and plug at the intake. Then see if it is still there. Newer engines we could squirt some starter fluid around the intake while the engine was running but since the exhaust is right there and could or probably would catch fire honestly I wouldnt know how to do that test except with a vacuum gauge.

[greg g]

Wonder if you have a stuck or broken spring on one of the centrifugal advance weights. Or your vacuum advance diaphragm is leaking.

[greg g]

How is the tension in the spring for the points? Some have mentioned some of the stuff on the market now are weak keeping them from fully closing causing a weaker coil build up at moderate rpm's.

[Joe Flanagan]

Thank you, guys, for all that. Regarding the accelerator pump, what Lloyd says is how I understand its function also. Watching it work, it is just that short burst of gasoline while the pedal is traveling downward. The pump on this carb seems to fit the bore snugly and works fine. 

Lloyd, I will do the checks you mention: wet plugs, rich exhaust, etc. I'll also do a check of the vacuum advance and see if it's leaking. I also thought it would be interesting to follow some advice that was given earlier and rig up a gravity fed fuel supply just to take the fuel pump out of the equation and see what happens. I'll also check for vacuum leaks.

[oldodge41]

I would give that distributor a good going over. Plug the vacuum advance at the carb and try it. Check  the condition of the points. Look for a little tit on one of the contact surfaces or any burning or pitting. A lot of seemingly fuel issues can be electrical and vice-versa, Maybe throw the timing light on and see if it is dancing around when it is surging.