I have a 350 chevrolet in my 68 chevelle. The engine was rebuilt some years ago and was moved to this car. It was done .30 over, performer cam and intake, 10:1 pistons, (I think - one compression ratio over stock), 3 angle valve job. It was some years between being run regular after being moved from car to car. Most accessories are of the mid 70's, so is the block and heads(intermediate performance cast iron heads) It was converted to an HEI and I did replace the resistance wire that goes to the distributer.

My problems started with an occasional stumble when hot and after a short run on the expressway. I changed the wires out with new accell 8mm wires with no change. Sems to be better cold. The problem got worse and it started to surge at about 20-40 miles per hour. It will stumble when you "goose" the throttle. It will backfire down the exaust when floored under load.

I changed the Q-jet over to a new Edelbrock 1406 with a short improvement then it seemed to go back to the same problems. Same stumbles, surges, and backfires.

I replaced the HEI distributer with a whole new one. New cap, rotor, and coil. There was no improvement. I had to put shorter plugs in to clear the headers and have an r42 equivalent in it. Advanced timing makes it a little better with a lot of advance. I currently run 87 in it, but will try 93 next tank.

I checked the compression and have 155 to 170 on all. After this check problems go worse. The motor has a transient miss all together. It comes at low acceleration and is slightly noticeable on higher. If I push the pedal slowly it will work.

I do notice anti-freeze puddles around the bolts for the intake at cylinder one. It appears to puddle there when cool, does not continue to leak when cold or hot. Almost like it sweats when the engine cools.

So I'm at a loss of where to go now. Stumbles, and misses upon acceleration, though if I acclerate slow all is well. Carb, ignition changed. Compressions good. Backfires down exaust when floored. I'm running out of options. Any ideas?

vacuum leak

I was going to say that you might have a leak around the intake...

runs better cold? intake is pooling up coolant?

Is it just loose enough to leak when it is hotter? Does that make sense?

Just a guess.

BTW, I used to live in Rochester...got family there...;):D

I have been working on this for a few weaks now. My post is in here called "Lean mid throttle". We are running very few of the same parts, but having the exact performance issue.

I read that post and will keep watching. I got a day or two before I can play with it so i'll see if anything else shows up...

btw, for me the antifreeze puddle is from an improperly sealed adjustable water neck. I can put a little pressure on the upper radiator hose and it will leak some fluid, and when I drive the car hard it will leak.

Timing, vacuum advance can, jetting, and correct carb adjustments. All related to stumble and hesitation. Coolant leak? As stated, check the t.stat housing. Customer's car just left and had a chrome one and it leaked. Don't they all? I don't think running that 87 octane is helping you at all.

I switched to 93 today, with no improvement. I tried the accelerator pump longer and shorter. No change. I tested 19 mmhg vacumme at idle, drops to zero when the trottle is pumped. Also it seems to be the engine "misses bad" when you pump the trottle. I did look closer around the top of the intake and also noted a little pooled oil. For lack of anything else I was going to pull the valve covers and see if I notice anything. If not then an intake manifold gasket.

Well $2.50 later the problem is solved. I parked her in the garage out of the sun. I pulled a valve cover to check for any problems on the top side of the head. I started her up to see how the valves operated and low in behold spark plug #1 was sparking around the base where the porcelin meets the metal. New plug - No problem. And I specifically looked at each wire and plug in the dark to see if i could see it previously. When I checked the compression it must have finished the plug off, or I got it back in a place that was eisier to see. Thank goodness and for the help from everyone here.

Man, running on 7 out of 8 will really make an engine miss, huh? Amazing. Glad you got it going, dude. :beers: