MKIII popping a lot when revving

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Pete Komarek

Pete Komarek

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My recently rebuilt 850 pops a lot when revving. It's been too cold to ride so I strobe timed it. So far, timing is correct-28 degrees advance at 5000 rpm. Lucas Rita ignition, new coils and plug wires. Carbs are clean. Exhaust nuts are tight. Seems to be the left side. Plugs are black-it's only been started a few times to time the motor. She starts well with choke and tickle. Air screws 1 1/2 turns out.
I would expect the pilot jet circuit to be the main culprit. Suggestions?
 
When one of my pilot jets got clogged it would run on one cylinder when idling then the other one would kick in when more throttle applied. Also, it would pop on deceleration as if the air screw was too far out.
 
I'm wondering if my carbs are just worn out. Others have looked at the slide wear and said they were OK. Adjusting the air screws doesn't do much.
 
You say while revving. Need to be a little more specific. Put a piece of tape on the handle bars and mark throttle position, 1/8, 1/4, 1/2, 3/4, wide open. See if the popping is confined to one throttle opening. That will help to start sorting out the carb circuit to work on.
 
If adjusting the pilot air screw does not change the revs then the pilot jets and/or circuit are blocked.
 
Also, you can run a bit more advance, like 32 degrees BTDC with an electronic ignition. The 28 degree figure is as far as double points and coil could go accurately, but the e-box provides more precision ignition timing control.
 
My 850 was popping when going off throttle...is that what you mean by popping when rev'ing?
Solution for me was to better seal the intake manifolds...at carb and head sides. I used Permatex grey flange sealant lightly on the sealing surfaces.
 
htown16 said:
You say while revving. Need to be a little more specific. Put a piece of tape on the handle bars and mark throttle position, 1/8, 1/4, 1/2, 3/4, wide open. See if the popping is confined to one throttle opening. That will help to start sorting out the carb circuit to work on.
Click to expand...
not at any steady throttle opening-at all openings
 
I also suspect an air leak-took it outside today and started it. fiddled with the air screw and choke-ran best on the choke, so I'll clean the carbs again. When first cleaned I used solvent and compressed air. I wonder if they are original- 29,000 miles would vibrate them a bit apart.
 
One leak test you can do with carbs on bike...spray some carb-Cleaner, starting fluid, even propane from a nozzle on the manifold joints while at idle...any sudden change in rpm is telling you it's injesting the extra fluid/propane at that area.
 
Plugs are black
==========

This seems like an obvious problem to easily fix with some new plugs.

IF it pops on deceleration then you are too lean, either an air leak OR the idle circuit is too lean as it is on that when throttle shut down decelerating

get some new plugs, if that does not solve the problem then it IS likely carburation
are you still running with old Amals?
 
Hello Pete. You live in one of the coldest spots in the lower 48, try turning the air screws in 1/2 turn/trial. If your idle circuits are indeed clear the popping may go away or may just move up the rev range. Next raise the needles, then one size larger needle jet, then one size larger main jet. When the ambient temp consistently gets over 60 (F) you can start putting the carbs back to what you had them at...

Heated garage??? Doesn't count, unless you have a death wish... At some point you would have to open the garage door or take it outside, As soon as it begins breathing cold air the mixture will become lean (popping lean), if you are stuck with using E-10 matters will become worse (less heat content and more resistance to atomize), if you are lucky enough to have access to non-drunken fuel the mixture will still lean out in the cold and require adjustment/re-calibration, but not to the same extent.

Carbureted aircraft have user adjustable controls for mixture and heat just for this reason.

All the best.
 
If it ran better with the choke on then that means it's running lean. Air leak, plugged jet or maybe very low float level (tho unlikely).
 
Pete Komarek said:
I also suspect an air leak-took it outside today and started it. fiddled with the air screw and choke-ran best on the choke, so I'll clean the carbs again. When first cleaned I used solvent and compressed air. I wonder if they are original- 29,000 miles would vibrate them a bit apart.
Click to expand...

the pilot jets cannot be cleaned just by solvent and air, you must use a #78 or 16 thou drill proxies into a WD40 aerosol tube or brass tube on the jet.
 
1up3down said:
Plugs are black
==========

This seems like an obvious problem to easily fix with some new plugs.

IF it pops on deceleration then you are too lean, either an air leak OR the idle circuit is too lean as it is on that when throttle shut down decelerating

get some new plugs, if that does not solve the problem then it IS likely carburation
are you still running with old Amals?
Click to expand...
 
RoadScholar said:
Hello Pete. You live in one of the coldest spots in the lower 48, try turning the air screws in 1/2 turn/trial. If your idle circuits are indeed clear the popping may go away or may just move up the rev range. Next raise the needles, then one size larger needle jet, then one size larger main jet. When the ambient temp consistently gets over 60 (F) you can start putting the carbs back to what you had them at...

Heated garage??? Doesn't count, unless you have a death wish... At some point you would have to open the garage door or take it outside, As soon as it begins breathing cold air the mixture will become lean (popping lean), if you are stuck with using E-10 matters will become worse (less heat content and more resistance to atomize), if you are lucky enough to have access to non-drunken fuel the mixture will still lean out in the cold and require adjustment/re-calibration, but not to the same extent.

Carbureted aircraft have user adjustable controls for mixture and heat just for this reason.

All the best.
Click to expand...
Originally I stuck the rear end out of the garage so it was getting warm air initially. Screwing the air screws in did not help. I understand re cold dense air. Tomorrow I will check the manifolds and use some solvent spray on the carb joints. Appreciate all the help.

So far I haven't heard anyone suspect the Lucas Rita ignition.
 
A faulty Luca Rita would be bottom of my list of suspects.

I used them for years, road and race, never had one fail.
 
There is a difference between a miss and a pop. If it pops as you shut the throttle, it is probably due to an exhaust leak. If it misses as you accelerate, I'd be looking for a faulty electrical connection or perhaps a spark plug with too cold heat range. If you have got the needle and needle jet slightly too lean or correct, you will get a cough as you open the throttle in cold weather, until the engine warms up.
 
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