RPM'S MOVING UP AND DOWN AND BACKFIRING

[imported_FRANK1]

I HAVE A 1987 225HP JOHNSON LOOPER, ON MY 22 FT CHRISCRAFT, THAT I USE FOR OFF SHORE FISHING.
MY PROBLEM IS THAT IT IS REVING UP TO 2000 RPM AND THEN IDLING BACK DOWN TO 850 RPM WITH THE THROTTLE IN THE IDLE POSITION. AS IT DOES THIS, IT WILL BACKFIRE THROUGH THE MIDDLE CARBURETOR ON THE PORT SIDE. (IT HAS 6 CARBS)
THE LAST TIME I RAN IT, IN SEPTEMBER OF 06, IT RAN GREAT. I BROUGHT IT HOME AND ADDED MY FUEL CONDITIONER AND PUT IT UP FOR THE WINTER.
I AM NOW GETTING READY FOR THE SUMMER AND UP POPS THIS BUGGER.
I THOUGHT THAT I MAY HAVE A CLOGGED JET IN THE CARBS, SO I ORDERED 6 KITS AND INSTALLED THEM.
THE CARBS WERE ALL CLEAN AND CLEAR BUT I STILL HAVE THE SAME PROBLEM.

HELP, HELP, HELP !!!!!

 

[Dhadley]

Re: RPM'S MOVING UP AND DOWN AND BACKFIRING

Actually you have 4 carb assemblies. 6 carb bodies, 2, two barrel throttle bodies and 2 single barrel throttle bodies. You have 2, two barrel carb assemblies and 2, single barrel carb assemblies.

As confusing as that is, the port carb assemblies feed the startboard cylinders and the starboard carb assemblies feed the port cylinders.

If it's firing back thru the middle carb body on the port side, it's affecting the middle starboard cylinder. It sounds like it's sucking air somewhere. If it is, the middle starboard spark plug should look dry and sooty compared to the rest.

The most common places for them to suck air is around the side plate on the carb body, reed gasket and carb body to throttle body O ring. Since you just had the carb bodies off we'll assume the O ring is OK.

Without the motor running, pull the airbox cover off and put the throttle at wide open. Look down the throat with a flashlight. There might be a piece of the reed gasket out of place and protruding down into the intake passage.

BTW -- never, never run the motor under a load with that cover off. It'll lean out and burn a piston in a heartbeat.

If that's not it, spray something like Engine Tune on that carb plate while it's ideling. If it's leaking air, it'll straighten up with the Tuner in it sealing the leak.

This is all assuming two things -- the compression is good and the plug wires are not crossed on two cylinders.

Another place it can suck air is by the throttle butterflies. Loosen all the linkages so the butterflies snap completely shut. This probably isn't it because if the middle one is off the seat the top one will be too. The bottom one is connected by a coupler under the middle throttle body.

 

[imported_FRANK1]

Re: RPM'S MOVING UP AND DOWN AND BACKFIRING

I UNDERSTAND WHAT YOU ARE SAYING, ABOUT THE PORT SIDE CARBS FEEDING THE STARBOARD SIDE CYLINDERS.
BUT I THOUGHT THAT WAS ONLY ON A CROSSFIRE ENGINE. THIS IS A LOOPER.
ANYWAYS, I APPRECIATE THE INFO AND I WILL CHECK OUT YOUR SUGGESTIONS IN THE MORNING, THANKS A MILLION.

 

[tashasdaddy]

Re: RPM'S MOVING UP AND DOWN AND BACKFIRING

frank please loose the caps, hard to read, and considered SHOUTING, in computer languages.

 

[imported_FRANK1]

Re: RPM'S MOVING UP AND DOWN AND BACKFIRING

frank please loose the caps, hard to read, and considered SHOUTING, in computer languages.

 

[imported_FRANK1]

Re: RPM'S MOVING UP AND DOWN AND BACKFIRING

sorry tashasdaddy about the caps. at 60 year old i still haven't mastered typing. however, if you think caps is like shouting, wait till i get this motor running and i will show you some shouting. like from the tallest mountain !!!! thanks for correcting my grammer, i hope that i didn't offend anyone.

 

[tashasdaddy]

Re: RPM'S MOVING UP AND DOWN AND BACKFIRING

no problem Frank, i'm 58, check your throttle linkage for any worn spots, there are little nylon bearings, that could be worn, allowing the timing and carbs to fluctuate.

 

[imported_FRANK1]

Re: RPM'S MOVING UP AND DOWN AND BACKFIRING

YAHOO !!!!!!! IT IS RUNNING AGAIN. THANKS DHADLEY.
I DON'T KNOW HOW OR WHY, BUT THE BUTTERFLYS IN THE THROTTLE BODY WERE OUT OF ADJUSTMENT. WHEN I LOOKED AT THEM THIS MORNING I COULD CLEARLY SEE THAT THEY WERE NOT CLOSED COMPLETELY . IT PROBABLY SHOULD HAVE BEEN BACKFIRING OUT OF ALL THREE CARBS BUT I ONLY NOTICED IT ON THE MIDDLE ONE. I LOOSENED THE SCREW ON TOP OF THE THROTTLE ROD AND THEY SNAPPED CLOSED AND ENGINE CLEARED RIGHT UP. also thanks to tashasdaddy, you were definately headed in the right direction, with your suggestion about the linkage. thanks guys !!!!!!!

 

[tashasdaddy]

Re: RPM'S MOVING UP AND DOWN AND BACKFIRING

CONGRATS!!!!!! another happy ending.