1993 Merc 454 bogging under load

[OMARiverRat]

Hey Everybody.... more problems, and of course, right before the 4th of July! The problem child is at it again. 1993 Mercruiser 454 (not mag), bravo 1 on a 23' crownline. Earlier this year I had to replace the camshaft & lifters due to a flat lobe & worn lifter. At this same time I also had one new headgasket installed, and purchased a new Edelbrock EDL-1410 750cfm marine carburetor. This boat has been nothing but problems ever since. I took the boat out yesterday, filled it up with gas, and started heading downstream without any problems. Stop at a sandbar for a couple hours, try to leave but the boat will not get on plane. Trim tabs down, lower unit all the way down, and when you throttle-up to get on plane the RPMS go up to about 3500, and then it bogs down like its getting too much fuel. Boat has thru-transom exhaust and when you look behind it, its puffing a huge amount of black smoke. I have a terrible suspicion that I've over-carbed this motor with the 750cfm, but I've been told by many professionals that a 750cfm is the correct sized carburetor for my application.<br /><br />I have the boat out of the water now, and I'm going to try my hardest to get whatever the problem is fixed sometime before this weekend. Any suggestions would be greatly appreciated.<br /><br />Thank you all...

 

[OMARiverRat]

Re: 1993 Merc 454 bogging under load

Well, I started fiddling around with the engine last night and I found something extremely interesting - a small amount of oil & water resting between two of the intake runners. Turns out that every bolt holding the intake manifold down was extremely loose - a couple of the bolts I could move with my finger! I re-tightened all of the bolts on the intake, but by the time I was finished with this it was too dark to fire it up. Does anybody think this problem could be related to the bogging and sputtering rich condition I've been experiencing?<br /><br />Thanks guys....

 

[Mahoney]

Re: 1993 Merc 454 bogging under load

very likely I would guess, but I am not an expert. The carb is made and sole purpose is to mix fuel and air. Sounds like as you increase rpms and increase manifold vacuum, you are sucking air from other places around the intake, the carb is still dumping the fuel, but it is not being atomized by the carb and rather sucked down from a vaccum leak around the manifold. When this mixture hits the cylinders it will smoke like crazy and bog.

 

[salty87]

Re: 1993 Merc 454 bogging under load

sorry, can't help with the real problem but i also have a 454 and have switched to the edelbrock 1410....runs great. that is the correct size.

 

[OMARiverRat]

Re: 1993 Merc 454 bogging under load

Thank you both for the info. Salty, that is definitely good to hear. If tightening the intake bolts doesn't fix the problem, I'll know to look somewhere else. I think the next area to explore will be the timing advance.<br /><br />Thank you.

 

[OMARiverRat]

Re: 1993 Merc 454 bogging under load

I was able to put everything back together last night, checked the timing and it reads 10 degrees btdc at idle. At 2500rpm, it reads approx. 30 degrees. I then hooked a vacuum gage up to the intake manifold, adjusted the carburetor and I have approx. 10-11 lbs of vacuum at idle. Took it out on the water, ran excellent the first couple of runs. The last time I tried getting on plane, it started bogging again. I let back on the throttle, tried it again slowly and I was able to get on plane. I'm still a little concerned about the bogging, and I'd hate to get stuck out on the river (9mph current) and have to idle upstream to get back home. Any input, or additional things I should be checking would be greatly appreciated.