Back fire and trans noise

So it is June 20th and I havn't had a good ride yet on my buddies boat! He has a 82 Ski Nautique 2001, 351 engine. Ran great last year! Begining of this year we changed the oil and replaced the rear 2 engine/trans mounts. Put it in the water and it was running kinda rough. Replaced the spark plugs even though visualy they were good. Took it back out today and she was idling good, rev it up in nuetral sounds great, put it in gear and give it any descent amount of gas and she back fires, bring it up to speed slowly and seems to run great, thats problem #1!. #2 worries me more clunking noise coming from trans in gear not in nuetral. Output shaft is tight and no free play. Almost sounds like a turning clunk, Like a loose torque converter would sound. (Do boats or this boat have torque converters?)(don't laugh I know cars not boats). The clunk doese seem to go away at higher speeds, but going from N to reverse or Drive shakes the whole boat. Any ideas would be helpfull. Thank you, Nick

Why do you say Carb needs to be rebuilt? It is only 2 years old. Is there any way to verify the dampner without pulling the hole trans?
Thank

Wake, there have been numerous posts on here about the dampner having springs in it that do break. You're only option is to replace it.

As far as the backfire, that sounds like a cracked dist. cap or a mech. advance that isn't advancing as it should. I've had the exact same symptom with both scenarios. Is this a point ignition or EI? Check the cap and replace if necessary and put a timing light on it and make sure the timing advances nice and smooth as you increase RPM to about 2500 RPM. My timing would just stay at 10 degrees BTDC until it hit about 2200RPM and then jump off the scale to full advance. The mechanical advance was sticky and just needed some attention.

Take the flame arrestor off and give it some throttle with the engine off. Check to see if the accelerator pump is working.

Check the timing as a matter of course.

The question about alinging is a good one.

Nick, The engine mount plus the dampner is what keyed me into asking the question about high speed shifting. It's when you shift from forward to reverse (or vice-versa) rapidly so engine RPM increases before the trans fully locks up into forward or reverse. It's like dropping the clutch on a manual trans on a car. It doesn't need to go to WOT but can be rough on the trans. Why did he need to change the rear motor mounts? Is there a previous owner?

Pete, lighten up on the high speed shifts, i got 3 mouths to feed plus the wife, and the Benz needs a tune up... your suppose to say it wont hurt anything at all
lol   eric

Im in the crappy Ohio economy, close to Cleveland, I dont own a benz either, i stay busy during the off seasons but I'll help you out if you need it