backfire - what causes it?

Doc

Liquid Addiction
Staff member
One of the engines on my houseboat has started to backfire. The higher the rpm the more I notice the backfire. At less than 1200 rpm it does not backfire at all. It is a 350 Crusader (454cc).
Any idea what causes backfire and what might fix it? I put in new plugs this year but now am ready to do the points and all. Hoping that might fix it, but what else might cause a backfire?
 

waybomb

I'd rather be blown
Did you check the timing? Fuel pressure? If ok, pull all the plugs and do a compression test. If any seem low, do a wet test (spray some oil in the cylinder before you hook up the tester).

If one seems higher than the rest, pull that valve cover off. Make sure all the rockers are lashed correctly.
 

Doc

Liquid Addiction
Staff member
Thanks Fred. We'll check some of that this weekend and I'll let you know what we find out.
 

Bt Doctur

Super Moderator
Staff member
add: carboned/cracked cap, cross-fire from bad wires,water ingestion in the fuel supply,bad intake valve or cracked valve spring, lean running carb.
 

waybomb

I'd rather be blown
Doc
Don't skimp on engine mtce. When you have to change plugs, I'd change it all - a major tune up. Plugs, cap, rotor, points/condenser as it sounds like you have (do you have a dwell meter to adjust the points? Points adjustement affects timing), wires, fuel filters and clean the arrestor.

Hope that fixes it.
 
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