OMC 3 litre GM Breaks down under load, Backfires through carb

[Kelly Ferris]

We have an '85 Citation with an OMC 3 litre (GM). It runs fine when ideling, but breaks down under load. It is also backfiring thru the carb. Any suggestions????

 

[duckman]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

Kelly, our 7.4L merc was doing the same thing..<br /><br />Plugs, wires, distributor Cap... and a carb clean did the trick.. I guess first thing make sure all plug wires are secure... if that doesn't do it.. look at plugs, wires, etc...

 

[Kelly Ferris]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

Thanks! I'll let my husband know. We just bought the boat this summer and it has been doing this for 5 weeks. The mechanic can't seem to find the problem. We just want to get it in the water and enjoy it.

 

[duckman]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

I'd inspect myself... there are many honest mechanics out there... but at least as many will rob you... just a caveat.. I've had mechanics say they checked something when in fact they didn't...<br /><br />about 4 weeks ago when this started while underway I thought I hit something... (the backfire)... I ran the boat at idle home (after inspecting the lower unit in the water for damage).. grabbed the scuba gear and went for a closer look... I couldn't find anything wrong.. so I called a "mercruiser specialist" on our lake.. (they deal in Fountains, etc).. after waiting for Two hours for a mechanic to even look at the boat he came up with this..(keep in mind he just looked in the engine boot, and ran it out on the lake for 10 minutes)...<br />Poor Compression (just from the sound?)<br />bad distributor<br />bad coil<br />bad plugs and wires<br />needs gimbal bearing<br />probably needs U-joints<br />possible ring job (doesn't smoke)<br /><br />I asked for a quote, he went to his service writer and made sure I couldn't hear him...<br /><br />The service writer came up with...<br />$425+tax for the gimble bearing<br />$55 for distributor cap<br />$50 for wires<br />$125 for rotor (this is where I smelled something)<br /><br />I called to some other local shops and got $150-225 for the gimble<br /><br />35 for cap<br />50 for wires<br />10 for rotor<br /><br />needless to say we did our own tune up...(we found two wires off of plugs) runs like a champ... took to another mechanic... clicking noise we hear... exhaust flappers...

 

[galaxy225]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

if it is backfiring throw carb i would check to see if timing is off........

 

[sea wolf]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

ever change points? make sure their adjusted properly. solved my backfiring problem. i would also get a complete tune up including new dist. cap. u can get parts through napa. very reasonable.

 

[consumidor]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

I had the same problem as well, with OMC 2.5 L- before it destroyed itself, The carburator was bad.

 

[fishingsalt]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

Here are 4 good tests<br />1) get a timming light on it if the pointer moves around you have a timming chain problem!<br />2)have someone give engine gas not to much/long <br />make shore you have muffs on same with water <br />youre timming should advance? if not fix it.<br />3)put a vacuum gauge to manafoild vacuum should <br />should stay steddy if not may have bad/sticky valves!<br />4)all check out head for cap coil rotor!

 

[steve 2]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

I have a 3.0 o.m.c cobra with the same sort of problem,when you check the points,first remove the points back plate and make sure it is clean,the plate gets a tarnish and a bad connection to ground is made,while the plate is off also check the centrifugal weights are o.k.Check compression,a burnt valve may be the problem.hope this helps,Going to try a new carb on mine,tried everthing else.

 

[spinakersheet]

Re: OMC 3 litre GM Breaks down under load, Backfires through carb

I had the same problem. There were 2 teeth on the new timing belt that were off. Therefor my timing was off. Backfire also could be weak spark, check points, and coil. I was getting backfire and than it just stopped and wouldnt start. Put in a new coil boom fires right up and no more backfire or flat spots.<br />Good luck.