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Back firing Thru carb

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EG120

New Member
I am Helping a friend who traded for an 02 883. The bike had been sitting for awhile and the previous owner rode it a considerable distance with it back firing thru the carb. When I looked at it the First time the Carb was gummed up. It has been gone thru several times and is in good shape at this point. The intake gaskets were also changed.

The Problem: the bike will start and idle perfectly, even for extended periods. When you give it any throttle the accelerator pump will shoot the steam of fuel, the slide starts to rise and the RPM starts to come up. At this point the engine will not come up on RPMs and the carb slide will start to rise and fall a small amount but rapidly. THEN the engine back fires thru the Carb, Flames coming out of the carb.

The Compression is strong and every this else seems to be working good, The engine will not come up on RPMs without the severe back fire and then it will eventually die.

Any thoughts on what it could be would be greatly appreciated.

Thanks,

Al
 
EG120 said:
I am Helping a friend who traded for an 02 883. The bike had been sitting for awhile and the previous owner rode it a considerable distance with it back firing thru the carb. When I looked at it the First time the Carb was gummed up. It has been gone thru several times and is in good shape at this point. The intake gaskets were also changed.

The Problem: the bike will start and idle perfectly, even for extended periods. When you give it any throttle the accelerator pump will shoot the steam of fuel, the slide starts to rise and the RPM starts to come up. At this point the engine will not come up on RPMs and the carb slide will start to rise and fall a small amount but rapidly. THEN the engine back fires thru the Carb, Flames coming out of the carb.

The Compression is strong and every this else seems to be working good, The engine will not come up on RPMs without the severe back fire and then it will eventually die.

Any thoughts on what it could be would be greatly appreciated.

Thanks,

Al
Click to expand...

Does that model have a VOES switch on it?
 
If there is flame shooting out of the carb then i would have though that perhaps one of the inlet valves is not fully closed at the time the spark plugs fire
Could be that a valve is not seating correctly at higher revs
ignition timing too far advanced and the valves have not yet closed when spark occurs

these are the steps i would take in trying to eliminate the issue
check ignition timing
Try running without the voes connected ensure the vacuum connection is plugged
Check the operation of the voes as per the manual
strip off the cylinder heads and re grind the valve seats
check the valve gear timing in the gear case

Brian
 
What kind of carb? could be a tear in the diaphram, if it is so equipped. General rule of thumb, backfiring through the carb = lean condition...... backfiring trough the exhaust = rich condition. At what RPM does it begin its backfiring? At what throttle position does it start backfiring?
 
MR - the Diaphragm looks good, but i might replace it as a matter of course. it starts to back fire as soon as any throttle is given. when you try and rev the engine the rpms start to come up, the slide flutters and it backfires thru the carb.

Brian - I will look at the timing as well as and take a step by step approach.

Thanks all for the tips. i will update when i know more,

Happy Thanksgiving!
 
EG120 said:
Hogrotten, Yes it does have a VOES switch.

Al
Click to expand...

Trouble shoot that Voes switch, just one less thing to consider if it is good. You can either remove the vacuum line while it's idling and idleing and listen for a change in the motor or plug the line going to the intake and hook a line to the switch then apply vacuum to it and listen for a change. I wouldn't suspect a valve problem seeing how it only starts acting up when you apply throttle.

Good luck keep us posted whatever you find, the results could benefit another member some day.
 
My best guess is ignition timing is too far advanced and once the throttle is opened the voes tells the ignition module to advance the timing further giving spark just before the inlet valves are fully closed and the burn of the fuel air mix shoots back through the still open valve and into the carb

Brian
 
I'm kinda leaning with Fin, might be a timing issue. But when you inspect the diaphram, use a small led flashlight if ou can get one, out of direct light, put the flashlight against the backside of the diaphram, any pinholes or small tears will let the light shine through.
 
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