OMC '82 305: carb or engine problem?

[gpane]

hi,<br />I have an '82 OMC 5.0 305GM with 800 stringer drive. have owned the boat for 4 years now and have had problem with backfire thru the carb. It has a two barrel Holley and last season i rebuilt the carb and timed to 13deg BTDC, ran beautifully for about a day and a half then started to backfire again, mostly near full throttle when i eased off on throttle it steadied out, but towards end of season it backfired alot. (distributor has electronic conversion kit by Sierra). so, for this season i'm thinking of replacing the manifold with a cast iron 4-bbl and rochester quadrajet and maybe changing the ignition coil(which i never have). If anyone has any suggestions or ideas as to what could be going on I appreciate it. Also , have heard that exhaust manifolds should be replaced every 4years; haven't done mine but did take them off and clean them out a couple of years ago; did get alot of scale out but my temp. been fine. should I consider replacing them?

 

[Blue Thunder]

Re: OMC '82 305: carb or engine problem?

Sounds to me like its time for a little diagnostic work. Three tests come to mind... vacuum test, compression test and leak down test. I would do them in that order.<br /><br />Without the results of this testing, I would go out on a limb and say you are in need of a valve job. Engine should run fine with the 2 barrel. Throwing parts at it is and expensive and unsatisfying approach.<br /><br />BT

 

[oldboat1]

Re: OMC '82 305: carb or engine problem?

sounds like diagnostic work would be appropriate -- marine mechanic if you are not up to it. I would sure rule out things like ignition problems before getting into any heavy lifting -- could be something as simple as plug wires (or a coil, although probably more unlikely). You could check the timing in the meantime to see if it needs readjustment. Maybe the clamp securing the distributor wasn't snugged down tight enough after you set the timing.

 

[magster65]

Re: OMC '82 305: carb or engine problem?

Check your fuel tank vent, it sounds like it might be leaning out.

 

[hondon]

Re: OMC '82 305: carb or engine problem?

Take a fuel sample out of that tank .You might be amazed at how much water you get outta there.If that looks good,then first check your state of tune which will include a compression test.If everything mechanical and ignition is savy, then you are right back at that carb.

 

[Boatist]

Re: OMC '82 305: carb or engine problem?

Gerryp<br />Backfires at high speed likely to be weak spark. Plugs gapped too wide or bad, bad wires, bad dist cap. Do you still have old points and condenser, if so may want to put them back in. I would pull each wire and see how far spark will jump. Should get at least one inch. Too lean a fuel can cause back fires also. 13 degrees BTDC is correct but make sure you read degree wheel right.<br /><br />If your manifolds had lots of rust and scale two years ago and you run in salt water I would check them very carefully. When the manifolds rust thru will get water to the exhaust valve and into the engine cylinder and cause big damage so make sure they are good. This will not be your high speed problem as would show up as a low speed miss.<br /><br />Start with a spark test on all Cylinders, do a compression check, check your fuel pressure. Make sure carb adjusted correctly but again this is usually a low speed problem.

 

[gpane]

Re: OMC '82 305: carb or engine problem?

thanks for all the recommendations. Will do the tests once it gets a little warmer and see what happens.<br />thanks