'89 454 Mercruiser has raw gas smell in exhaust

[MtkaBoater]

:huh: I have '89 340 EC with twin Merc 454. I had replaced plugs and wires last year on starboard eng--cap and rotor this year--started out fine this year but have now devloped the smell of raw gas in the exhaust--(I changed the cap and rotor after discovering this issue). Max RPM has been reduced to around 3100 plus a slight shake when under power (assume it is misfiring)---it has been suggested to rebuild the Rochester Quadrajet carb--Any Suggestions or other trouble shooting tips? Has any one else run into this?

 

[wileecoyote]

:huh: I have '89 340 EC with twin Merc 454. I had replaced plugs and wires last year on starboard eng--cap and rotor this year--started out fine this year but have now devloped the smell of raw gas in the exhaust--(I changed the cap and rotor after discovering this issue). Max RPM has been reduced to around 3100 plus a slight shake when under power (assume it is misfiring)---it has been suggested to rebuild the Rochester Quadrajet carb--Any Suggestions or other trouble shooting tips? Has any one else run into this?

Carb rebuild is always a good idea. Misfiring is usually something different. Usually means ignition. Does it sound like it bogs down, or does it misfire? Also If you are like me you have the Hurth Trans, both engines are standard rotation. If you have the B/W I believe one is counter rotation. You may have it wired incorrectly. It may still run, but like crap. (I'm not sure it would even run, but good to check. Raw fuel smell could be ignition not burning the fuel, or carb dumping too much. Do you by chance have FloScans?

 

[MtkaBoater]

Thaks for the reply---I do not have Flowscans and I don't recall the transmision-Everything was working fine until lately-Think I will start with checking plugs-wires first then go to the rebuild if needed. Nerver worked on Rochesters before.

 

[wileecoyote]

The Quads are fairly straight forward. There is actually a step by step on the internet somewhere. The best thing is the metering rods set the mixture (except ide mixture) so you don't have to fool around with it too much. I would recommend you do that anyhow, even if your problem is elsewhere. Just make sure you clean all the ports good

 

[fwebster]

This sounds more like an ignition or timing problem. Unless the ignition is right, the engine is going to run rich at high rpms, thus the fuel smell in the exhaust. The richer it runs the more likely the plugs are fouled.

Mercruiser carburetors are not saddled with all the smog features on auto carbs, and they are really very dependable, unless they've been sitting with old gas in them for a while.......6 months+. Personally, I'd do everything in the book to avoid tearing down a carburetor on a guess.

Also, just because the wires were new last year doesn't make them immune from breaking down, particularly if they are aftermarket plug wires. Try cranking the engines with the hatches open at night and no other light on the engines........Mercruiser's ignition can light up an engine compartment like blinking Christmas tree lights when broken down wires arc to ground.

 

[Chris Dauth]

Check the fuel pump vent line to the carb. You may have a split diaphram in the fuel pump and dumping fuel into the carb. This will make them run rich and will flood the engine depending how large the split is.

 

[capecodcruiser]

First thing I would look at is the firing order. I will bet something got swapped when you were replacing the dist. cap. Find what your firing order is and start from scratch with placement on the cap. I have worked on engines for years and still mess this up sometimes!!

 

[MtkaBoater]

Thanks everyone----I found the issue to be a bad coil---it became evident when lifting plug wires-nice arch appeared on the coil---so replaced Coil, plugs and it now runs great.

 

[wileecoyote]

Great news!