140 Mercruiser Surges/Backfires at WOT

[drivin79]

hey guys, I've already posted on this same subject but haven't gotten enough replies so hopfully I can get some more advice. To update everyone, I have a Wellcraft america 170 with a 140hp Merc 3.0 I/O. Problem being at W.O.T. it surges and backfires through the carb. Idles great, starts great, medium throttle is great, just when you open it up it startes lurching and RPMS jump up and down and backfires when it reaches peak RPMS.

Heres what i've done so far:.....
Carb Rebuilt, Adjusted Carb, New Cap, New Rotor, New Fuel Filter, New Anti Siphon Valve, New Plugs, Oil and oil filter, bought a timing light and timed to specs 6 degrees BTDC, went with a higher octane, Checked the springs under the rotor, good condition and not sticking. Is there anything I'm missing? I'm about to give up on this boat.

 

[drivin79]

Re: 140 Mercruiser Surges/Backfires at WOT

whats the recomended oil weight to use in the merc, When I changed the oil in summerization I used castrol 10W-30? Wrong?

 

[Bondo]

Re: 140 Mercruiser Surges/Backfires at WOT

When I changed the oil in summerization I used castrol 10W-30? Wrong?

Ayuh,... That's Fine,... You have a Fuel Delivery Issue...

 

[fat fanny]

Re: 140 Mercruiser Surges/Backfires at WOT

Have done a compression test on all 4 cylinders? I had this issue a few years back and found I had a blown head gasket between #3 & #4 cylinders not a very big seapage but just enough to give that very same symptom. Hope this helps but I would also check your fuel delivery pressure to the carb as well Good luck!!.

 

[drivin79]

Re: 140 Mercruiser Surges/Backfires at WOT

Thanks for the replies everyone. To answer your question fanny, when I had the carb work done I had them replace the valve cover gasket and the head gasket as well. No I haven't done a compression test yet. I'm going to work my way away from the fuel then go from there. I've been trying to get the valve off where the anti siphon valve meets up at the fuel tank to check the tube inside the fuel tank to see how that looks, but it seems to be cross threaded and wont come off. Once I do this I figured I could run an alternative fuel tank and see how it handles. I'll also change the oil bubba and go back to factory spec of 30 w. It really makes me think that this could have something to do with this, the previous owner said that he never had any of the problems i've described. Once I recieved the boat in early april I started doing a tune up including oil.

 

[mbboat]

Re: 140 Mercruiser Surges/Backfires at WOT

You may have some blockage in the Gas tank pickup tube. There is a mesh screen in it. You maybe getting blockage the faster it runs.