Penta 5.7 backfires at mid throttle

[terry585]

I have a 1998 5.7 GS Penta Volva. At WOT it runs fine. At idle it runs fine. However, in the mid-range at about 20mph, it ocassionally backfires and sputters. Typically if you speed up the motor, it will clear.

I tried to adjust the timing with the help of a mechanic, who changed the timing from about 8 degrees BTC to about 4 degrees BTC. He also adjusted the carb. It didn't make a huge difference...same problem. However, if anything, I would venture to see it made it worse and more consistently runs poorly in the mid range now.

Its got a Holley Carb.

Any ideas of potential problems?

Thanks very much in advance for your help.
Terry.

 

[Don S]

Re: Penta 5.7 backfires at mid throttle

Probably due to accelerator pump getting weak. When you give it throttle and the pump doesn't pump gas into the throttle bores, the engine would be lean and cause the backfire.
May also be due to weak springs on the advance weights in the distributor.

 

[terry585]

Re: Penta 5.7 backfires at mid throttle

Thank you very much for your reply.

1. Can you just get new springs for the distributor or do you have to buy a whole new distributor ($564)
2. Where is the Accelerator pump?

Now, one thing is that this doesn't require "advancing" or "moving" the throttle. Basically the motor just doesn't like to run at 20mph...it starts sputtering even with the throttle not moved. Moving the throttle actually helps to solve the problem.

 

[terry585]

Re: Penta 5.7 backfires at mid throttle

bump.