Merc 3.0 popping through carb

[Moneypit21]

New to posting in iBoats but I love the info I usually get here. Not sure if I was suppose to start a topic with this but I've been reading for days and C ant seem to find anything with my same problem.

I had my boat out on the lake last year, it's a 17ft cheetah with a 3.0L 130hp merc. Ran great with a 17p prop but I wanted a little more top end when I wasn't pulling the tube. I out a 21p on it and took it for a spin. Noticed it had a little trouble getting up on plane but once it was there I got about 6-7 more mph out of her. Still decided to go back to the 17p. Took the boat home after running all day on the 21p. Switched it back. The next day I brought the boat to the dock and when I tried to start it .... it's popping loudly out the carb. Even shoots flames sometimes. It seems to build compression just fine because a few more times of starting it and it blew the head gasket. I can feel air coming out of it on every compression stroke. I replaced the head gasket thinking it was bad and might have been the problem. Tried starting it.... same thong until I blew the head gasket again. Please help.

 

[Rick Stephens]

Seems like a random choice to over-prop with a 21 inch. Props are chosen like gear ratio on a car. If you go to a bigger pitch, and 4 inches is a HUGE jump, then you spent the day badly lugging your motor.

The way props pitch is chosen is by running your boat with WOT (wide open throttle) and seeing what the RPM is. A 3.0L should run between 4400 and 4800 RPM at WOT. You also look at GPS speeds when at WOT. With that information you can calculate prop slippage and efficiency and whether you are in the ballpark. I guarantee that the 21 inch dropped your RPMs under 4k and the wear and tear on the motor was enormous.

That said, it sounds like either something is bad now, like a burned valve, or the timing is now advanced far enough that it is firing while the intake valves are open. Check the timing with a light and then do a compression check on the motor. That will tell us a lot.

 

[Scott Danforth]

backfireing wont blow head gaskets

overheating will

backfiring is either too lean or timing is way too advanced

what RPM were you spinning with the 17p and the 21p? my guess the 17p allowed the motor to over-rev and the 21p lugged the motor leading to detonation which eats pistons.

 

[kenny nunez]

Remove the spark plugs and perform a compression test. When you refer to “head gasket” are you talking about the valve cover? It may be either a stuck or not operating exhaust valve . From the description of the last time the boat ran, did you not use it for some time after running it with the 21” propeller?

 

[Moneypit21]

Thanks guys ! I'll do the compression test this weekend. Yes the 17p had me way over rev. Rick is correct with my WOT AND I do understand how prop pitch works. 21p had me right at 4600 perfect. To answer kennys question I'm talking about the head gasket not the valve cover gasket. I am an experienced mechanic but just wanted more opinions before I go dipping it apart again. When I originally had the problem I checked compression and had a loss between 3 and 4 that's how I knew I had a head gasket problem. I replaced it and it's still popping through the carb. I've adjusted valves just wanted another look before I go replacing everything haha. I also was not aware it could jump timing over night like that because like I said it ran fine all day... brought it home on the trailer.... went to start it the next day and it popped through the carb when it builds compression from turning over.

 

[TurtleTamer]

Did you slap a new head gasket in without having the head decked? If that's the case you could be dealing with a warped head which'll leak compression regardless of gasket condition. It MAY even be possible, if bad enough, to leak combustion gasses to the adjacent cyl on the power stroke and ignite fuel in that one prematurely, causing your backfiring. I can't say I've ever heard of it happening though, so it's a guess.

 

[harringtondav]

This sounds too simple, but I've had partially gap bridged spark plugs cause back firing. The carbon can act like a hot glow plug and cause detonation prior to intake valves closing.

 

[lister]

When the timing was wrong with my new engine, it blew fire out of the top of the carb without the arrestor on!