Backfire through carb when accelerating

[Bill Hancock]

Hi Everybody<br />I posted this question on 1 other board for good luck, so I apologise if you saw it there too.<br /><br />I have a 1985 2.5 litre OMC stringer with 2 barrel rochester carb.<br />When I accelerate, either hard for water skiing, or really lightly, as in a marina, the thing backfires and sometimes stalls. This happens whether the engine is warm or cold, and usually after it has been idling for a short while. In other words, if we are cruising and I slow down for a no wake zone for a brief period, it seems to accelerate fine, but if we have an extended period slow rpm's, it always wants to backfire through the carb when I call for a bit more speed.<br />I have cleaned the carb and adjusted the idle/mixture screws.<br />What else should I do?<br /><br />Thanks, Bill

 

[Hog Wild]

Re: Backfire through carb when accelerating

This sounds like an ignition issue. What is the timing at idle (should be like 8 BTDC). Then does it advance? Should go up into the upper 20's BTDC roughly at about 3000 rpms. Could just need an ignition tune up, might have a bad coil (but doubt it). Most likely not a carb. issue. so many people run to the carb. first and so rare that it is the problem. If all that is in spec. then I'm guessing next that you have a leaking valve or a bad cam shaft or lifter/spring. Do a compression check and post back results to a new posting. It should have compression over about 120 to 150 psi. with no two cylinders off by more then about 20% difference.

 

[Bill Hancock]

Re: Backfire through carb when accelerating

Thanks for the suggestions.<br />I will try checking the timing first, although I have to go out and buy a timing light tomorrow.<br />Bill

 

[crazy charlie]

Re: Backfire through carb when accelerating

When it happens,shut down and pull the dist. cap and check the inside for moisture.Charlie

 

[ron7000]

Re: Backfire through carb when accelerating

what do you mean by cleaned the carb? did you pull it apart and clean the bowl and internal passages, or did you leave it assembled and only cleaned the choke plates and throat?<br /><br />I will get the argument going and disagree with hog A backfire is usually caused by a lean condition. And I would say the most common cause is the carb. In this case, since it's a 1985 motor (when was carb last serviced?) I would start there. If it were the ignition crapping out, you would get more of a stutter and a miss not a backfire through the carb. If you did go through the carb internals, don't be suprised if you have to do it again. Another big cause would be the float level too low. Raise the float level. Other things to check would be for a vacuum leak, fuel pump output pressure (should be 4-7 psi) and check for clogged fuel filter, fuel screen in carb, fuel line or pickup tube in tank, and fuel tank vent. If these are the case, it's starving for fuel on acceleration which would definitely cause the lean condition and a backfire.

 

[deputydawg]

Re: Backfire through carb when accelerating

I would agree with the lean fuel condition. Bad power valve if equipped, plugged ports or something. Check ignition first because that is cheaper, then get a carb kit and try that. Could also be a deterioated gasket causing an air leak at acceleration into the throttle body.<br />Spraying carb cleaner will not usually fix the problem. Take it apart and soak everything.

 

[Flames]

Re: Backfire through carb when accelerating

sounds like a flat lobe on the cam to me or a push rod has come loose.<br /><br /><br />Rudy

 

[deputydawg]

Re: Backfire through carb when accelerating

I think the key to look at is it does it at acceleration, both hard and light, and more after it idles for a bit. I would lean more to a fuel condition. Take the carb apart and check the float level and clean everything. Also check all fuel filters. Valve train trouble should do it at WOT as well.

 

[jimmythekid]

Re: Backfire through carb when accelerating

id dissagree with flames, if it was a flat lobe it would backfire consantly and never "clear out" probably a vacuum leak or worn, dried out seals in carb

 

[Hog Wild]

Re: Backfire through carb when accelerating

Hog Wild has no argument with Ron7000, as matter of fact I think I agree and 2nd his thought on the lean condition. Timing just popped to my mind but it would have to be very far off to cause the mentioned condition. With the mention of the carb. cleaning I really now agree that something is wrong with it causing this.

 

[deputydawg]

Re: Backfire through carb when accelerating

Because the trouble only happens during acceleration it has to be a lean fuel condition. Small possibility it could be the spark advance, but probably not. <br />Adjusting the idle mixture screws won't help, they only control fuel flow during idle. The float level is where most of the fuel flow is controlled. <br />I think the fix here is minor. Fuel filters and carb kit at the most.<br />If this carb is made with an accelerator pump, when it backfires through the carb, the backfire usually ruins the accelerator pump. This will be replaced in the carb kit.

 

[Flames]

Re: Backfire through carb when accelerating

I guess i misread, i thought it was backfiring all the time. i agree if its not back firing at WOT then it is probaly a lean condition.<br /><br />Sorry,Rudy