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I know I"ll sound like a broken record, but check your spark first to make sure that you have the requisite FAT, BLUISH-WHITE SPARK. Check the timing (7 oBTDC @400 rpm),and make sure the advance is working properly. If all that checks out, the ignition system is working properly and we can eliminate a weak spark, icorrect timing and incorrect advance as the culprit. The look at the fuel system(enriched fuel/air ratios help weak spark performance). If any of these things are not correct, you"ll have to fix it so that you have a FAT, BLUISH-WHITE SPARK AT THE CORRECT TIME. Since your performance is helped by enrichment and we"ve hopefully eliminated the ignition system as the problem, there are two possibilities for too lean a mixture; carb power jet adjustment or an airleak around the metering system of the carb. You just rebuilt the carb and these M-S carbs are sensitive to float level, You might want to check that again and make sure you have it right. Did you install the new air seals around the throttle shaft. hat"s one source of potential; leakage around the carb metering system. The other is the connection flange joint between the carb and the intake manifold. Did you install the new gasket and did it seal? Lastly a crack in the intake manifold or a leaky manifold to cylinder head gasket can cause leaning because you have air bypassing the carb metering system. To check for these airleaks, CAREFULLY squirt unlit propane from a hand cylinder around the throttle shaft, the carb to manifold connect flange and around the intake manifold. If the engine speeds up when you squirted the propane around one of these places you"ve found the problem area. Another possibility is inadequate fuel delivery and the quick check for that is to open the tank valve,and holding a suitable container beneath the carb, open the big plug at the bottom of the carb. The fuel should CONTINUOUSLY gush out of the carb fuel bowl like a cow peein" on a flat rock. If it sdoen"t you"ll nedd to check the stariners in the fuel line, the one at the carb fuel elbow, the one above the ediment bowl and the one in the tank over the tank valve.
Do these checks systematically, one at a time and if you find a problem, fix it and check it out before making another change. Record the results for further analysis.
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