Backfire through carb

[kkline06]

I have a 69 Merc inline 4 that has a backfire through the carb. The gas tank, line, pump, filter and hose are new. The Rochester carb has been rebuilt and cleaned. Also new plugs, cap, rotor, wires, points and condensor. The coil is also new. The compression is 140 straight across. The points and timing have been set per service book. When I say new this boat has not seen water other than my driveway. Any ideas where to go from here?

 

[JustJason]

Re: Backfire through carb

either your timing is out or you have a lean lean lean condition. Check your base timing and for advance with a timing light. Then hunt on for a vacuum leak, or a fuel delivery issue. If its the original carb, its an oldie and could be time for a new one.

 

[kkline06]

Re: Backfire through carb

I have done the timing and is set to the service book. I also checked the fule pressure at the carb and it is at 4psi ant the spec is 3.5 to 6.5. I am thinking it is time for a new carb but my money tree is almost picked clean. I just want to make sure befor I buy.

 

[Maclin]

Re: Backfire through carb

Double check the dwell and timing. BOTH must be within spec. Also verify the mechanical timing advance works smoothly.

If ignition specs check out then it is probably the carb needs adjustment or cleaning or new correct parts or any combination thereof.

If you mean the backfire occurs as the throttle is opening then that is usually an accelerator pump problem (if ignition is in spec), either linkage or setting/adjustment or bad pump plunger or diaphragm depending on type of carb. For those not in the know, an accelerator pump is part of the carb, has no realtionship to the fuel pump that feeds the carb. It is activated and controlled by a linkage to the throttle. It's purpose is to squirt fuel down the nozzle as the throttle opens up and for just a bit longer then stop. The squirt must start immediately when the throttle is opened, any delay can cause that exact type of backfire.

The idle mix can also be too lean and the accelerator pump shot cannot quite cover it up immediately as the throttle opens up.

If it occurs while cruising at a constant rpm then there is a serious lean fuel condition most likely pointing to the carb or a vacuum leak. That type of backfire can also be from a valve problem but your compression numbers indiacte a healthy engine in that respect.

 

[kkline06]

Re: Backfire through carb

Maclin, thanks. I think I have over looked something in the timing so I am going back over everything. The backfire is all the time and not just trying to get to WOT. As I said the carb was rebuilt and a accelerator pump comes in all rebulid kits as id did mine. I have now went through the carb twice, just checking. I also checked for a vac leak the last time I put it back on and everything was tight. Anything else you can think of?