engine intermittently revs on its own! Im baffled!
 

I just installed my 455 which is basically a stock rebuild. I got it fired up, and after getting it to idle I noticed it would idle at 700 rpms and then out of nowhere rev up to 1200 then back down again. Almost like how you would do a quick rev on your motor.

It has 10 degrees initial timing.

Also too, im now noticing an occasional knock sound from the lower end of the engine. it has like one knock noise every 5 seconds.

I need some help. I have been playing with hot rods for 10 years now and have never had an engine rev on its own like this or have such a weird occasional knock.

Anybody ever seen this? Thanks.

Sounds like you're getting a sudden spurt or drip of extra gas. I had a bit of a gas drip after a rebuild on a carb once and the idle would go up and down every now and then till I got the throttle plates closed.

Check the timing as well, to verify the advance curve isn't coming in and out, which is a pretty common problem often mis-diagnosed as a carb issue.....Cliff

Sounds like a problem I had in the past. I found a vacuum leak. Mine was at the carb to intake gasket. I think I found it by spraying carb cleaner near the potential vacuum leaks, and my RPM changed.
Good luck.

Could be all of the above.

1) If the rpm stayed up until you blipped the throttle, that typically meant an advance weight was hanging (keeping the timing advanced).

2) If a booster was dripping, the engine would typically run rough all the time at idle and would smell fuel rich.

3) A vacuum leak is a funny deal in that it can be intermittent. Extra air from anyplace will raise the rpm; (PCV VALVE, vacuum leak at a gasket, hose off, intake manifold bolts loose).

Tom Vaught

The rpm didnt stay up until i blipped the throttle, The engine was doing that on its own.

I posted #1 in case someone else, at some point, might have an issue with the rpm staying up. I was not referring to your problem specifically.

Tom Vaught

Does the knocking noise and RPM change happen near the same time? A bad vacuum leak at a gasket can make an noise like that. Almost like a muffled popping sound. Don't let me steer you wrong, maybe a different problem completly.

yes, actually, it sounds like a muffled pop! I actually thought it was coming from the exhaust at first, but its coming from the engine. Thats why im stumped. Tomorrow im going to spray some areas with carb cleaner.

Lean fuel mixture can cause surging.

When the engine surges, the centrifugal advance activates and makes the surge worse.

FIRST you need to get the carb sorted out. That might take care of everything--but if not, slightly stiffer springs for the centrifugal advance would reduce the surging.

I just found one of the distributor cap terminals loose. I will replace it. Im wondering if this can be the problem.

Block off the PCV valve, see if anything changes

what would that do for me? Also too, i might have the wrong intake gaskets. I dont have the exhust crosover blocked off.

Smell your dipstick, if it smells like gas you may have a leak in the fuel pump which is putting gas in the crankcase. The crankcase vapors get rich enough to change the mixture through the PCV or the breather to the aircleaner. The added rpm's pull the vapors from the crankcase leaning the mixture back down and the rpm's drop back.

If the diaphragm in the pump leaks, it will pump gas into the crankcase. The scenario I postulated is possible, but not probable. Everything would have to be just right. It would explain why the rpm's drop after they climb. It's a little difficult to come up with a reason for that happening.

As mentioned, block the PCV off. Remove the vent breather to the carb as well. I would check, also, to see if the knock is coming from the fuel pump. You can use a lo-o-o-ng screwdriver as a sounding rod.

Good luck, and don't forget to let us know what the problem turns out to be.

I would spray carb cleaner around your carb base and intake gaskets. If the rpm drops to idle then you know there is a leak in that area. If that doesnt work, you may want to consider looking at your carb more carefully and consider taking it apart for inspection. Good luck!

Also check for collapsing vacuum lines

Did you fix it??