Oversized Cam???

[tonka_shores]

I've got a Merc 888 that backfires through the carb upon quick acceleration from dead stop (like pulling a skier up). Sometimes, so violently it kills the engine. There are no problems if I accelerate slowly, and it never backfires other than that. The previous owner thinks that someone put a new cam in it. It has a Holly 4 barrel carb and high-rise manifold that's not stock.

Is there any way I can tell if there's a race cam in that engine. If so, how would I set the timing? And, would timing be the cause of my backfire issue?

 

[bruceb58]

Re: Oversized Cam???

By opening the throttle suddenly, you are creating a temporary lean condition. If you accelerator pump is not working correctly, you will get the condition(backfiring through the carb) you are experiencing. Even when the accelerator pump is working, you may still get a slight lag if you open the throttle quickly.

 

[f_inscreenname]

Re: Oversized Cam???

I agree with Bruce. If you have a carb with a accelerator pump lever with three holes in it, move the rod that attach's to it one hole closer to the carb and see if that helps.

 

[tonka_shores]

Re: Oversized Cam???

I re-built the carb thinking a new accelerator pump would solve the problem. I've adjusted the linkage so it seems to activate normally. And I would expect some hesitation with a bad pump, but backfire to the point of killing the engine?

 

[Coors]

Re: Oversized Cam???

type of carb? why do you think it has different cam? that won't cause a backfire. timing, which is probably not the problem. Sounds like a lean condition.

 

[RCSConstruction]

Re: Oversized Cam???

The littlest backfire will blow out the power valve in Holley's.
What model holley?

 

[Coors]

Re: Oversized Cam???

That's why I don't deal with Holley anymore..

 

[180shabah]

Re: Oversized Cam???

Edelbrock baby!!

 

[Boomyal]

Re: Oversized Cam???

Holley has a retrofit kit to prevent blowout of the power valve in the event of a backfire.

Still sounds to me like no accelerator pump fuel. You can check it easy enough. Actuate the trottle and watch down the primary venturis for two solid squirts of gas.

 

[jimmbo]

Re: Oversized Cam???

Do this with the engine off. Leaning over an engine prone to backfiring with eyes wide open could be a bit exciting.

 

[Don S]

Re: Oversized Cam???

Do this with the engine off. Leaning over an engine prone to backfiring with eyes wide open could be a bit exciting.

 

[jimmbo]

Re: Oversized Cam???

Roflmao!!!

 

[tonka_shores]

Re: Oversized Cam???

The venturies squirt gas with the slightest actuation of the throttle (motor not running). That's why I was thinking something else. Maybe I'll look for the retrofit. Is that an additional kit other than a standard rebuild kit?

Maybe sudden acceleration is asking too much of an old boat?

 

[SuperNova]

Re: Oversized Cam???

Hey Don, my wife wants to know where you got that picture of me. She's LHAO. You know how many times I've done just that while tuning?
--
Stan

 

[Coors]

Re: Oversized Cam???

When I was 16, I looked down a carb trying to see if accel pump was working, while I revved it- no eye brows for 4 months, they grow in slow.