C2: L36 miss at low RPM

[Dan Pepper]

I'm experiencing a slight miss at idle up to about 1500 RPM. Above that mark, everything is smooth and pulls very strong. Have checked and re-checked everything in the ignition system, looked for vacuum leaks, adjusted the carb, checked the fuel filter, etc. etc. The carb is newly rebuilt (1500 miles ago).

The car has 52k orgininal miles and a K19 system, but I've removed the vanes from the pump and pluged the tree inlets to the exhaust manifold.

Any other ideas? PS- Just installed an NOS set of N11. What a sweet, sweet sound! Crisp but throaty...

[Duke W.]

Re: C2: L36 miss at low RPM

Your symptoms keep saying "vacuum leak" to me. The leak is leaning out the mixture at idle and low load, which is causing a misfire.

Possible sources of vacuum leaks could be the manifold gaskets, vacuum advance can, vacuum hoses and connections, carb to manifold gasket, or something internal in the carb.

You say the carb was rebuilt 1500 miles ago. Did the problem start when you installed the rebuilt carb.

Spraying carbuetor cleaner at suspect leaks can be used to troubleshoot. The liquid will be drawn in through the leak and richen the mixture, which will increase engine speed by momentarily eliminating the overlean condition.

Duke

[Dan Pepper]

Re: C2: L36 miss at low RPM

Duke,

I've replaced the carb gasket and re-checked the vac lines - all appears okay. Here's a bit more info; the miss seems to diminish as the car warms up. What say? Thanks for the wisdom.

[Duke W.]

Re: C2: L36 miss at low RPM

I don't know how you've tested, but I would recommend that you use a vacuum gage to check that the vacuum advance, choke vacuum break, and brake booster hold vacuum. Then with the engine running spray paint thinner or carb cleaner on the inlet manifold/head interface and carb to manifold interface. If engine revs increase momentarily, you've probably found a vacuum leak.

You didn't say if this problem started with the rebuild carb, but there's also a possibility that there could be a leak in the carb due to improper assembly such as leaving out a gasket, etc.

Duke

[Jerry M.]

Re: C2: L36 miss at low RPM

I had the same problem. Mine still missed even when the choke was on and was running rich. I swichted to electrical. To make a long story short it is not characteristic for coil to arc at low r's but mine was. I think the higher r's masked it. When I replaced it, it smoothed right out and is still running well.

[Dan Pepper]

Re: C2: L36 miss at low RPM

Sounds strangly familiar... A while back I had a miss (through all whole rev range) and found the coil wire was arcing through the wire boot onto the shielding cover. Only found the problem by noticing a small burn mark on the underside of the lid. I replaced the wire and things went back to normal. Perhaps the same problem is happening again... I'll check it out. May be able to solve the problem by lowering the coil in the bracket to reduce interference on the wire boot.

[Jerry M.]

Re: C2: L36 miss at low RPM

Dan, Mine was arcing internally.