LT1 Miss

[Dan A.]

My 72 LT1 has developed a miss when first starting off from a dead stop. If the RPM's drop below around 800 it acts like it is dropping a few cylinders until the engine hits around 1200 rpm at which point it runs well again. If you keep the RPM's over 1000 and slide the clutch it runs OK. It did not do this until recently, and it does it with either of my carbs. I have checked everything in the ignition including timing, cap, rotor, coil etc. New plugs didn't help, and the wires are are also relatively new. Other than this it runs great. Anyone have any other ideas?
Thanks.

[Edward J.]

Re: LT1 Miss

My 72 LT1 has developed a miss when first starting off from a dead stop. If the RPM's drop below around 800 it acts like it is dropping a few cylinders until the engine hits around 1200 rpm at which point it runs well again. If you keep the RPM's over 1000 and slide the clutch it runs OK. It did not do this until recently, and it does it with either of my carbs. I have checked everything in the ignition including timing, cap, rotor, coil etc. New plugs didn't help, and the wires are are also relatively new. Other than this it runs great. Anyone have any other ideas?
Thanks.

Dan, Maybe the dist. pick-up coil is bad. what happens if you apply 15"vacuum to the vacuum advance while idling? By moving the advance plate it will sometimes tug on the wires on the pick-up coil, the wires do dry out over time and teeter enough to break the cooper strands this will cause drivability problems. just food for thought.

[Dan A.]

Re: LT1 Miss

Dan, Maybe the dist. pick-up coil is bad. what happens if you apply 15"vacuum to the vacuum advance while idling? By moving the advance plate it will sometimes tug on the wires on the pick-up coil, the wires do dry out over time and teeter enough to break the cooper strands this will cause drivability problems. just food for thought.

Thanks for the reply, any ideas appreciated. I neglected to include the fact that the car has a Lectric Limited electronic ignition conversion that was added when the distributor was rebuilt a few years ago by a reputable Corvette shop in New Mexico.

[Duke W.]

Re: LT1 Miss

Don't confuse miss with startoff stumble. That's common on high overlap cam engines if you startoff with minumum clutch slippage. Go through the non-emission idle speed-mixture procedure with a targe idle speed of 900, then test. If the problem is start off stumble and it continues, try richening the mixture in quarter turn increments and see if it goes away.

I don't like electronic points conversions, so I also recommend you install a conventional set of points to see if the problem goes away. Electronic points conversions are a solution to a non-problem.

One of the nice things about vintage Corvettes is than they don't have any black boxes to fail or cause weird problems. Think about that.

Duke

[Edward J.]

Re: LT1 Miss

Dan, take a peek under the Dist. cap and look to see if any bare wires with the installation worn through because of the breaker plate movement. and grounding out. try driving the car with the vacuum advance disconnected and plugged to see if the stumble goes away.another thing to do is check the ignition wires , this can be done in the darkness of night, arcing may show up easier if you have a bad wire or plug on that cylinder.

[Timothy B.]

Re: LT1 Miss

Dan,

My first thought is something in the ignition, you may want to check the voltage to the coil + terminal just to make sure there is no wire resistance problem. My experience with the breakerless SE is good so I would be surprised if there is a problem but you may want to give the M&H people a call to discuss.

If the problem happens with two carburetors that pretty much rules that out. Could the fuel be old in the tank or something like the rubber hose that plugs to the vacuum advance be old and egg shaped causing a leak. If the engine does not do this when the choke is partially closed there could very well be a vacuum leak. Good idea to drive the car with the vacuum advance plugged to eliminate a leak there.

[Dan A.]

Re: LT1 Miss

Thanks to all who replied. The breakerless conversion was done prior to my ownership, and I have had mixed experiences with them in the past. I checked the voltage at the + terminal and it was 12 volts +, so I think we are OK there. The fuel is fresh, so that should be OK also. However, the comment about the condition's existence with the choke partially closed is interesting, as it doesn't exhibit the problematical behavior until the engine is fully warmed up. I will go back and look for a vaccum leak again.

[Terry M.]

Re: LT1 Miss

Disconnect the body vacuum system at the intake manifold and cap the intake port. Vacuum leaks in the body vacuum system can be insidious and ultimately get to the point of deteriorating engine performance. If you do the road test in the daylight and with no rain you will not need the body vacuum system during the test.

Do you have power brakes? that could be another source of vacuum leak.

[Dan A.]

Re: LT1 Miss

I had to chance to work on the car last night and have pretty much eliminated the vaccum theories. I ran the car with all of the vacuum accessories capped off and there was no change. I have discovered that the car runs great for about the first 5-10 minutes (much longer than it needs for the choke to fully open) and then the driveability issues start and become progressively worse. I guess I will look at the coil and distributor more closely.
Thanks again.

[Timothy B.]

Re: LT1 Miss

Dan,

If you can find a spare coil just to swap and eliminate that's a good idea.

One other thing to think about is the metering block gasket on the Holley. When the casting warms, vacuum is finding it's way making a air leak. If you have a nice new metering block gasket give some thought to changing it. Many times from sitting these gaskets dry and don't seal to the main body, not saying that's the problem but it's typical Holley issues. Check the screws that hold the bowl on the primary and secondary to make sure they are snug but don't strip them, remember these are cheap castings.