Get out your timing light, and check the timing. Rev the engine up until the timing stops advancing, and check to make sure it is around 28-34 degrees advanced, if it's higher, lower it down to where you want it. What motor are you using? On my b20b it likes 30-32 degrees maximum advance. After my setting of 30-32 degrees advanced, when idling around 950rpm it reads about 10 degrees advanced, which is the stock setting. If you don't find that your timing is advanced too much, then the only other thing that I can think of would be a vacuum leak(or three). What're you using for carburation?
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Kyle - '68 142? - Oregon Volvo Tuners?