Backfire....

[jason9080]

27foot omc 460. crusing along and every 30 seconds or so i get a backfire. if im sitting at 3600 rps at about 32mph ill get a bang every once in a while. the more i pull the throttle back the better it gets and usually if i leave it at 3200 rpms its good but as soon as i get it up to 35 and over i get the backfire. i have rebuilt the carb, new plugs, cap, rotor what else can i do. is my timing off. also my rps are not getting as high as i think they should. does this have something to do with it??

 

[Purduebarry]

Re: Backfire....

I think you answered your own question. You mentioned the tune up stuff. First thing, check the wires and make sure you have the firing order correct. Then check the timing. If it seems to run smooth at idle I think you'll find your problem is with the timing. It may seem obvious (backfire every 30 seconds) but don't run this thing until you have this corrected. Bad things can happen. Let us know if you still have a problem after checking both.

 

[marunr]

Re: Backfire....

You previously had a problem with water coming in and it was suggested it may be coming in through the bellows. Did you find out? The reason I'm asking is I wonder if any got in the drive. And the reason for this is are you sure it's a backfire through the carb and not a "skip" in the outdrive? When the dog clutch goes out and slips on the gears it can be a very rythmic thing at cruising speed and almost seems like the engine is skipping or cutting out.

 

[Purduebarry]

Re: Backfire....

Wow, marunr...talk about a 180! Good catch on the game of Concentration. I certainly hope it's the timing your diagnosis sounds much more expensive!

 

[calwldlif]

Re: Backfire....

Rpm suggests sticky valve

 

[marunr]

Re: Backfire....

Unfortunately I know this situation all too well. Hopefully it's the timing or the sticky valve.

 

[jason9080]

Re: Backfire....

thanks for all the help. you said a skip in the outdrive. I dont feel a skip in the outdrive at all. im almost positive its coming from the engine. i have thru hull exhaust, if it was coming from the outdrive it would prob be hard to hear or would be muffeld. i have the boat out of the water now. is there any tests i can do to see??

 

[Blk-n-Blu]

Re: Backfire....

Jason, You may be able to pinpoint the problem using a vacuum gauge. Have a look here:
http://users.bigpond.net.au/ergoff/vac1.htm

 

[Purduebarry]

Re: Backfire....

I still say do the easy stuff first. A timing check is quick and easy. Others suggested the other dasterdly offender, sticking valve. Good luck!

 

[Bondo]

Re: Backfire....

After you do the Tune Up,........

Change the Fuel Filter,+ Check the Fuel Pressure at the Carb,...........

It sounds like it's going Lean when you're really Working it.......

 

[searay3]

Re: Backfire....

RPM not high enough, backfire at upper range, I think Bondo is on. Sticky valves, timing issues (advance) are all possible, but fuel delivery would be the place to start. Tank, filters, anti-siphon etc etc....

 

[JasonR]

Re: Backfire....

I had this same issue in my motorhome when loaded down. I just fixed it. It had new caps, rotor wires on it when I bought it several months ago. I put a new fuel filter on it, redid the timing with no success. Was about to take it in to have the fuel pressure tested and injectors checked when I decided to replace the cap and rotor again. I found that the "new" cap was a cheapo and the center stud for the coil was loose. The rotor also had some weird wear patterns. Problem fixed.

 

[jason9080]

Re: Backfire....

finially got the time to take the valve covers off. turns out two of my rocker arms are way to tight. I tried to losen them but as soon as i loosed them just a touch they get to loose and sure enough they rattled out within 5 min of running. i think sticky valve is the problem as some of you said. question is will a compression test tell me that answer? and should i try a can of mysery oil first to try to losen it up before i start taking the motor apart?? any experience with this. thanks

 

[calwldlif]

Re: Backfire....

Sorry to hear:%
I didn't see any history of the motor.
I Like MMO and have seen it do good things.
If the valves aren't bent, springs not broke
or weak, I would definately try to free the
sticking stems.
I would try any and all Snake oils before
pulling heads.
If the motor has been stored or at end of life
a head job before any seriuos damage
has merit also.