Mercruiser 'Bucks' at 2800-3200 RPM...

My 5.7L Mercruiser in my rinker 28' with the alpha one drive is bucking when in the rpm ranges 2800-3200. Im pretty sure its engine only related. It inconsistantly at random bucks if your in this range, it also only lasts for half a second. It sounds like a cough, for real. Engine will achieve WOT without any problems (GM Scream). In any other range its fine. Does the carb need some attention? Is it starving for fuel for that one spot? is it running lean? is the distributor or wires in need of a change?

Sound of it:


MMMMMMMMMMMMMMMMMMMMMMMM-CHFF-MMMMMMMMMMMMMMMMMMMMMMMMM-CHFF-MMMMMMMMMMMM.....

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

What year?

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

1988 (boat)

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

the short answer is... you need to make sure your engine is in tune before you go ahead and start diagnosing.
By by in tune I mean all Filters at least once a year (depending on useage, with average usage once a year is usually ok) Cap, rotor, plugs, wires all in accordance with manufactorers specs. (usually every 2 to 3 years) Compression check every year or couple hundred hours.
Start by making sure your engine is in tune, then take it from there.

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

And don't go back and edit out your original question. Makes all the replies really worthless for anyone reading the thread.

http://forums.iboats.com/showthread.php?t=232732

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

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Re: Mercruiser 'Bucks' at 2800-3200 RPM...

it honestly sounds like a backfire. We've listened and its like a 'bang'...Im going to do oil / fuel filter / clean carbs / air filter / plugs / cap / rotor and see what happens. did not think of checking the tach, will keep that in mind. Won't do it any other RPM range. really weird...

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

Misfire... Question is why. Lean would be my guess in terms of fuel... Tune up and see what happens.

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

......

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

plugs, wires do not seem to help. im going to try the fuel filter next...what is the magic to ensuring you dont get any gas runs when you take it off? im paranoid beyond belief about an explosion.... had an atv burst into flames on me cause the carb was dumping fuel through the intake.... not fun... What is this about the tach i should be checking? I cleaned the frame arrestor and shot some throttle body cleaner into the carb...it seems sluggish when you open it up under load... it does this when its under heavy load always at 3,000rpm, not a hint of trouble otherwise. Revs excellent to 4,600rpm once the carb picks up.... im convinced it's a fuel problem...But im also not sure...it will idle and run at any other rpm for hours and hours without a hint of trouble, miss, or anything. its just when you dump it or your at 3,000 - the optimal speed i like to run it at... and it will only bang every minute or so...

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

Im also wondering if perhaps there could be an air leak somewhere causing it to lean out? whats the protocol for checking this?

Re: Mercruiser 'Bucks' at 2800-3200 RPM...

im toying with the idea of just abandoning this carb and putting a new one on, i was told rebuilding them yourself can be hit and miss.

Im freaked that one of these times the backfire is going to make it too far so i havent been using it. I understand its been sitting all winter...but this problem has been here since we bought it. ive been reading the forums about backfires and it makes my head spin.

Also, is the Thunderbolt II ignition electronic or points?