I have a 427-400 HP car and have had it recently profressionally rebuilt. I have a problem with it laying down between 3500-4000 RPM. I had this same problem before it was rebuilt. The motor is all to original specs. including the cam, lift and duration, as I've been told that could be a problem. I run Cam2 racing fuel, so octane is not a concern. I posted this problem earlier and got a reply from Jack Humphrey, and he told me of a similar situation with a 68 tripower car, that when the engine came up to temp the fuel overheated sitting in the carb. and spurted out and made the engine run too rich. That is exactly what is happening here. If I open it up before the engine comes up to temp. it will make the horsepower and go to redline. I recently was at a car gathering and was talking to a corvette owner who told me he knows about this problem, because he worked as a mechanic at a Chevy dealer in 67, and the same problem continued in 68 with the tripower corvette. He said that GM sent a memo to the dealership to add a metal plate or block between the carb and intake to act as a heat sink, and that corrected the problem. Has anyone heard of that fix, if so, where does one find this part? He had no idea of where to get the part, but said it was available in 68, 36 years ago. If this sounds like a probable fix I could have them machined. Has anyone heard of this, does it make sense, or a wild goose tale? Help me out. Many thanx...
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Re: C2 '67 L-68
If you're running pure race gas (vs. pump gas with an additive), there's no alcohol blended in to lower the boiling point of fuel stored in the carb bowls. I wouldn't expect the percolation issue we've seen pretty frequently on tripower cars AT ALTITUDE in Denver to be biting you....
We were able to confirm the problem by visual inspection (NOT something I recommend due to inherent danger).... We had the car on a chassis dyno and duplicated, repeatedly, the engine refusing to climb above 5000-5200 RPM. The shop mechanic donned a flame proof race suit and welding helmet. He removed the air cleaner and laid himself on the front fender with eyeballs staring STRAIGHT DOWN the carb venturis while another mechanic ran the car up to speed on the dyno and was able to CONFIRM fuel stored in the tri-power bowls randomly spewing raw gas that was sucked into the venturis causing an over-rich condition.
The dyno mechanics went a step further by soaking shop towels in water and cooling them in a refigerator. The towels were used to wrap the fuel lines and generate an artificial heat sink. The car then climbed effortless to red line on three consecutive runs before the towels warmed and the problem returned.
But all this was with a bone stock, freshly restored, '69 400 HP car running standard pump premium gas AND being at altitude. The chassis dyno was in the foothills outside Denver at approx 7000' which, by standard pressure/elevation tables, represents a 23% reduction in atmospheric pressure to the vented fuel bowls.
We too looked for an off the shelf solution that would either deflect heat at the carbs OR increase thermal dissipation. But, the end game for this car was constrained by the owner wanting it to be absolutely factory stock for judging AND a solid daily driver at altitude with today's pump gas. We found no solution given these constraints! -
Tri power carb insulater plate
Chris:
That tri-power plate you are referring to, was made to use with the '70 454 tri-power engine, that didn't come to fruition for that year. I have one of those metal plates, as does Joe Lucia.
I or Joe can give you the part #, they are fairly rare but do exist.Comment
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Re: C2 '67 L-68
I don't have much help in this situation but I would try this. I would make a stainless steel plate and cut it to fit like the gasket and intall it under the carbs. This would be a similar set-up as found on the quatrajet equiped cars that I have worked on. The other thing I would try would be to switch to Amsoil. I had a friend who raced circle track and used it. He used conventional oil and Amsoil and he also had a temperature gage for his oil. He said the Amsoil ran 20-30 degrees cooler than the conventional oil. He claimed it was a superior product. I don't know how the engine was set up but I would let it run rich on fuel. My limited understanding of things would suggest that it would lower cylinder head temperature a little. I am sure I am splitting hairs here. You would have to play with the jetting. I would make sure the fan clutch was working real well, not just looking good but actually working. I would let the engine get broken in well before getting too excited. That may take a couple thousand miles after which the operating temp may come down a bit. I am not sure how the intake gaskets are on the tri-power but some gasket set-ups came with restrictors from the exhaust port to limit the amount of heat going to the intake manifold. If they are not used in that application, you could consider installing them. Interesting problem. I would detune the engine as much as possible. Less horse power = less heat? Just my two cents worth. Consider full roller rockers or roller tip rockers if you can hide them under the valve covers (splitting hairs again). Consider a roler timing chain (splitting hair again). Some of this I am sure you may laugh at but the idea is to reduce friction I guess. Best regards, TerryComment
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Re: C2 '67 L-68
i would install a inline fuel pressure gauge to check the fuel pressure when this happens to make sure you are not losing fuel pressure as the engine heats up. you could also block the heat riser crossovers in the intake manifold to remove the heat from the carbs. make sure your heatriser valve is working correctly and not frozen.you can also try install holley part # 26-39 bowl vent screens to prevent excess fuel from coming up the bowl vents. the end carbs require you to modify the screens because the screens are made for carbs with a metering body but this can be done. we used the screens on all the 3X2 carb setups used for racing.Comment
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