Check vacuum advance....is vacuum hose off/leaking? Is vacuum advance diaphram bad? Check diaphram by disconnecting hose to distributor. Insert another hose so you can suck on hose.....a good suck should change engine speed (because you are changing timing). If you can continuously suck air, diaphram may be ruptured. Easy change and cheap part at auto supply. Buy a good brand, however or you will be doing the change again.

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