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Original Message
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| RE: Probably two or more, unrelated issues. |
| By Gerry Proctor - 06/22/2004 6:16:12 AM; IP 207.133.188.254 |
As robert pointed out, you could have too much preload and the lifters are pumping up and hanging the valve open. That could be one cause of the carb backfire. A vacuum gauge will show some rythemic bounce if that's the case. It's also possible that the valves are too tight in the guides and when the engine is reaching operating temp, the valve stems are expanding faster than the guide and are sticking. Though you'd usually hear some rocker noise if that was the case. The carb could also be running lean. A lean misfire will also cause overheating and backfiring.
The overheating and low manifold vacuum are very likely the result of retarded ignition timing. It could also be a significant vacuum leak causing a lean misfire but check the timing first. I always prefer to overadvance on initial fire and adjust as the engine is going through the cam break-in. At 2k rpm with the vacuum advance connected, you shouldn't have any problems with timing between 40 and 50 degrees. Overadvancing the timing causes far fewer issues than retarded timing. I also prefer a fat fuel curve for the same reasons...too rich will make your eyes water but the engine will live until you can dial in the carb and ignition.
So you're looking at significant mechanical issues (excessive lifter preload, valve guides, intake manifold vacuum leak) or some tuning issues (lean carb calibration, retarded timing). I'd go for the tuning issues before you start invasive surgery on the engine. Also, I'd pretty much rule out cam timing since the engine will run fine with an advanced or retarded cam as long as the fuel and spark are right. |
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This thread, so far...
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