engine surging

[billfish]

My 51 stock 8ba has developed a surging problem over the past few weeks. I'm calling it a surging sensation because I don't no how else to explain it. When cruising at a steady speed or when accelerating slowly the engine seems to pick up and than drop rpm on it's own. Starts and runs fine otherwise. Any ideas?

[scicala]

If you're idle has gotten worse or rougher also, it could be a ruptured power valve in the carburetor.

Sal

[billfish]

Any recommendations on a source for new or re-built carburetor, stock 2 barrel.

[big job]

Originally Posted by billfish

Any recommendations on a source for new or re-built carburetor, stock 2 barrel.

We bought a new one from Dennis Carpenter; end of problem runs like a
kitten right out of the box. I say the price isn't a not bad ouch, try a new
4 bbl Holley now thats an ouch.....sam

[scooder]

Mixture to weak (lean) and to much ignition advance will also creates surge at cruise.
Try a cruise, when it starts to surge, pull the choke knob out, does it get better or worse when you try this?
For better than new 94, Charlie NY definitely or Sal posted above I think can do a 94.
Martin.

[scicala]

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Yes, I would be glad to help out if you find it is a carb problem.
Try Scooder's advise too.

Sal

[billfish]

With timing set on pulley mark engine idles and runs rough. With timing advanced 1/2 inch on pulley engine idles and runs good. The surging is less noticeable with the spark advanced but still there at a constant speed in 2nd or 3rd gear. Pulling out choke seems to help the surging. Also I notice a black wet oily deposit on garage floor from exhaust.

[Init1]

Sounds like a blown power valve.
Easy to fix.

[billfish]

Great advice from all, I appreciate the help. Whats involved in replacing the power valve, I assume It's available on line. Thanks again

Bill

[scicala]

If pulling out the choke makes it better, I kind of doubt it's a bad power valve. With a bad power valve, pulling out the choke should make it worse.

Yes. power valves are available online or locally. I believe a #75 would be correct for a stock engine (opens at 7.5" of manifold vacuum).

They are easy to replace. remove carb from engine and separate the bowl (center section of carb) from the cast iron base. The power valve is screwed into the bottom of the bowl. If you see wet fuel in the recess of the cast iron base below the power valve, then the valve is bad.

Sal

[flatheadmurre]

If you dont have any starting issues at all i dont think its a broken powervalve.
The symptoms can as well be a to weak spark not able to ignite the lean mixture.
Advancing the spark gives it more time to ignite the fuel.

[billfish]

Great information--any thoughts on the black oily, wet discharge from the exhaust?

[flatheadmurre]

Unburnt fuel/oil coming out of the engine probably.
An engine taking a little oil usually burns it now it just spits it out.

[billfish]

I checked the timing this morning and found the timing not advancing. If I connect the distributor to the wiper motor vacuum port on the carburetor the advance works correctly. Vacuum at this port is about 18 in's and at the distributor vacuum port is about 5 in's and goes up to 10 in's when revved up. Does this indicate a sticky vacuum advance in distributor or something else?

[billfish]

I know there is a spring adjustment in the distributor, could this be the problem?

[flatheadmurre]

Its not as simple as a leak in the diaphragm and at the restricted flow it doesnt work and with a higher direct vacuum it moves ?

[deuce_roadster]

I would tend to suspect distributor problem or maybe vacuum leak that is intermittent instead of fuel problem.

[flatheadmurre]

Test the diaphragm for leak first.
Dont start messing with the springs in the distributor that will not turn out good...

[billfish]

The vacuum advance does work when I apply 18 in's of vacuum from wiper vacuum port on carburetor. But does not work with the 4 in's supplied by the distributer port. How do I determine if vacuum advance is the problem.

[Tony, NY]

Originally Posted by billfish

Hate to be a pain in the a, but how do I test diaphragm and is this difficult to replace?

Use a hand held vacuum pump connected to the diaphragm and see if it holds vacuum on the gauge. http://mityvac.com/hq_images/MV8000.jpg