Stuttering bings at 1/4 throttle ???

[patanga]

Hello brains trust. I have a pair of 32mm Bings that are causing some grief at present. Symptoms: Stuttering/hunting or surging off idle under load between 1/8th to 1/4 throttle. Throughout the rest of the range things seem pretty normal. The condition only exists when riding and at rest it idles and throttles up perfectly. Left side carb tends to lightly pop on occasion at start up. Right side cylinder is also showing rich (black electrode) but mixture adjustment & running indicate all is normal. After clearing out a blockage in one of the pilot jet carb throat passages there was an improvement in running but it was not a complete fix. Since that remedy I am now also noticing a light flat spot under load/throttle. I expect that the latter symptom was preexisting but would have been less noticeable under the rougher running conditions.

The condition is definately carb related as I have fitted another set of bings to compare and with those in place the engine runs perfectly. The problem carbs have just been fully overhauled including: Full strip down, ultrasonic clean, new throttle bar O rings, diaphrams, jet O rings & choke gasket. The butterfies have been correctly oriented and aligned. Floats are set to level with carb base, mixture set to run well at half a turn above lean dial in falter. Chokes are correctly oriented, throttle balance is set, cables have been adjusted to have slight play at rest. Hoses to heads are also new. Timing, valve clearances and compressions are all good. In my efforts to find the cause I've also swapped out the diaphrams just in case. All carb body passage ways (pilot, idle, vents and enricher) have been confirmed to be clear. Pilot jets = 45, Needle jets = 2.68, Main jets = 135 are confirmed to be correct for the model. Main jet needle is set to # 3 position from top, floats have been set to level with edge of carb base and with float needle at first seat contact point (i.e. not pressed on to seat)

Slowly going mad in the process Any suggestions?

 

[Kenny]

Best to measure the the depth of fuel on the float bowl. Turn on taps as usual to fill bowls; turn off fuel taps and gently remove the float bowls...depth of petrol from base of 'well' in float bowl to height of petrol should be 24mm. If they are correct....phase two of your mission commences....

Left carb sounds a tad weak...unscrew the mixture out about a 1/4 turn ( maybe even 1/2 turn ) and see if that helps ( you can obviously go back to your standard settings if doesn't work ).
Your right carb maybe needs the opposite; i.e. screw in the mixture screw 1/4 ( or again 1/2 ) turn etc.

 

[patanga]

Thanks Kenny. Floats are within the range you mention and I've been shifting mixtures around trying to achieve an acceptable outcome. That said the problem is now solved. The fix in the end was in adjusting the main jet needle height. Spec for the model is at the # 3 and at as I thought the symptoms more closely resembled a lean condition, at that stage of throttle, I adjusted the e clip to the 4th slot without any notable change. From there I set it to the second groove and bingo a good result. A quick tickle of the mixture screws and all is good once again. Cheers.

 

[Rob Farmer]

If you've replaced the butterfly rubbers have the throttle discs been centred properly?

The other thing to do is set the tick over to around 1200 - 1300 rpm and then turn the mixture screws one at a time to get peak rpm, once they've both been adjusted like this turn the tickover down to around 950. This assumes you have balanced the carbs and the engines at normal temperature.

There are three tiny holes in the throttle body near the butterfly on the pilot mixture screw side of the body. These are transition jets and prevent a stumble as the throttle opens, a bugger to clean out but make a difference. I poke these through with a single strand if copper wire.