28SS

check to make sure all plugs wires in good shape . make sure wires sepperated. check coil for correect ohm. cracked cap. finnaly start looking @ valve train. If only @ idle also check voltage to coil @ idle / 1500 rpm

Panther

What they all just said. Also, check to make sure you have the proper firing order. If you cross #5 and #7 you will have a rough idle and it will pop in the exhaust.

BTW, It is funny as hell when you do the switcharoo when your buddy just built a new motor and ran it on the ground. I would switch the wires before it was dropped in the boat and the other mechanics would sit there scratching their heads after they installed the motor. It was pretty funny!

Darkhorse468

Panther,
Thats just plain WRONG. I would probably have a mild stroke and break down at the same time...then the wrenchs would start to fly. Yeah, now that I think of it, that would be pretty funny! All the advice above is right on...if the motors are fresh, a valve can easily hang up, also the order can be off, but I've seen ALOT of this; the cap is cracked, but you can't see it o the outside, check inside for carbon "tracking" soot that goes in little lines around the inside...those are the cracks. Any moisture while compound the results. EVEN NEW CAPS!!!

barron

Replaced,plugs.wires,dist caps,roters, and checked firing order,thats my limit to what I can do any thing else and I'll cause more damage ! Thanks for all the imput.

28SS

like I said check voltage to coils @ idle the altenater starts putting out more voltage @ approx 1200 rpm.
so compare results from idle and 1500 rpm

If you have twins swap coils

this is easy free stuff to check.

barron

28SS I have replaced the coil. Thanks!

barron

TTT

rmbuilder

Barron,
Are your symptoms consistent(or intermittent) throughout the rpm range or just occur @ idle? Does the engine clean out above a certain range 1500/2000/2500 rpm? Is the cam in the engine the crate cam or was it replaced?
Bob

Helmwurst

I chased this problem for 2 month's on a friends boat. What plugs are you running. I cannot make this any easier, MAKE sure you are running extended tip plugs.
Ex: R44TS
Same exact symptoms you describe. I asked him at least 10 times, did you change the plugs? Yes I did! I finally pulled one and said, What the F%$k are these????
Put in the R44TS, hit the key reset the carbs and son-of-biscuit has not missed a lick since.
Do not drive it if it keeps backfiring, or you will be spending the big bucks.
If you have already done this, then start with the ignition, then fuel, then mechanical stuff.
Good luck.
PM me for tele# if you want to talk

Pure Energy

Mine did the same thing. Would start, and idle but when I went to excellerate they would pop though the carbs. My mechanic said it was the carbs. I don't know excactly what he did but it has been fine.