Low RPM and carb backfiring

[Daddy5295]

I just installed a new Jasper reman. Vortec 4.3L V6, on a 2002 Chaparral 18ft with Mercruiser alpha 1 gen 2. I set the timing (in base timing mode) 10 degrees btdc as listed in the merc manual. Ran fine on the muffs. Took it to the lake and I can only achive 3200 rpm, and the hole shot sucks !!!! After stopping and killing the engine a couple times to check things over, when starting again it backfired through the carb about 5 different times. But not everytime. Taking off from a dead stop if backfired once and died, but restarted fine. HELP.
I have replaced cap and rotor, plugs (gapped) and fuel filters and MOTOR. The carb, I just transfered straight over from the other motor. It was running fine then.

 

[Daddy5295]

Re: Low RPM and carb backfiring

I am running a 21 pitch prop, plugs look good after checking. Previous motor ran 47 mph and around 4600 best I can remember. Could only get up to 35 mph. And the crate w/new motor says 4.3 marine.

 

[cooter2506]

Re: Low RPM and carb backfiring

Backfiring thru carb is usually a lean fuel condition. Also Don has a list of WOT problems I will try to find and copy and paste for you. Unless of course he beats me to it.

 

[wallydc2]

Re: Low RPM and carb backfiring

Here is a list that Don S. sent me, I'll bet you find your problem here.


List of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

Fuel condition. Type and Octane possibly old fuel
Propeller pitch or diameter, damaged blades
Restricted fuel pickup tube or anti siphon valve Fuel System Test
Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
Marine growth on hull and outdrive
Wrong gear ratio in outdrive
Restricted carburetor air intake (clogged flame arrestor)
Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
Poor cylinder compression Compression Test
Carburetor defective, or wrong type.
Fuel pump pressure and vacuum
Boat overloaded, improperly loaded, or improperly trimmed.
Engine Overheating
Engine timing and ignition system operation
Remote control cables and linkage for proper travel to open throttle plates fully.
__________________

Don S.


Why is there never enough time to do it right the first time,
But always enough time to do it again?

 

[Daddy5295]

Re: Low RPM and carb backfiring

Here is a list that Don S. sent me, I'll bet you find your problem here.


List of possible causes of low WOT.

Engine Won't Reach Operating RPM. Check

Fuel condition. Type and Octane possibly old fuel
Propeller pitch or diameter, damaged blades
Restricted fuel pickup tube or anti siphon valve Fuel System Test
Crankcase oil volume, high oil level can cause aerated oil and lifter collapse
Marine growth on hull and outdrive
Wrong gear ratio in outdrive
Restricted carburetor air intake (clogged flame arrestor)
Restricted exhaust system (broken exhaust shutters/flappers) in engine transom shield or drive
Poor cylinder compression Compression Test
Carburetor defective, or wrong type.
Fuel pump pressure and vacuum
Boat overloaded, improperly loaded, or improperly trimmed.
Engine Overheating
Engine timing and ignition system operation
Remote control cables and linkage for proper travel to open throttle plates fully.
__________________

Don S.


Why is there never enough time to do it right the first time,
But always enough time to do it again?

Thanks for the info I was just looking through your post's to find it.
I have done all the above except fuel pump pressure and vaccum, and compression test. Because it is a new motor and the previous motor was running great when I pulled it. So I figured carb is set and ok and compression should be ok also.

 

[cooter2506]

Re: Low RPM and carb backfiring

I knew someone would beat me to it. Good luck with the list I know I needed it a few months ago and it really helped me find out some problems on mine

 

[Daddy5295]

Re: Low RPM and carb backfiring

The fuel water filter is a sierra and the numbers didnt cross with the quicksilver filter that was on it. Is it possible the filter doesnt let enough fuel to the carb? And if the cam is not oem spec is it possible that the engine may need more fuel?

 

[180shabah]

Re: Low RPM and carb backfiring

If the cam is different enough, then yes you may need to rejet.

Do you know the cam specs?

 

[Daddy5295]

Re: Low RPM and carb backfiring

No I don't know the cam specs. Gave the serial # to the dealer and it is supposed to be the same specs, but who knows. One of the other posts here has a problem with wot after changing the fuel/water filter so I'm going to find the correct # from the one that I took off and give it a try.

P.S. I got the old one out of the trash put it back on and no more backfire.

 

[Daddy5295]

Re: Low RPM and carb backfiring

Hey Don, I have a new one for you to add to your causes for low WOT. After checking most everything twice I decided to check ohms on the plug wires. HOLY smokes #2 and # 6 were crossed how on earth could I be dumb enough not to double check that? Anyways went back to the lake this evening and WOW it runs like a champ. . 49 mph 4700rpm.
Thanks everyone for all your help.

I kept saying it was running like a 4 cylinder. Duh

 

[Don S]

Re: Low RPM and carb backfiring

You can also hear crossed plug wires. There is ALWAYS a miss in the engine, even at idle. That is a dead givaway that there is a problem.

 

[Daddy5295]

Re: Low RPM and carb backfiring

If ya could have heard the motor, you wouldnt have thought so. I agree with you, but it ran smooth.