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For Royal Enfield owners and enthusiasts
- Tue Aug 30, 2016 1:57 pm
#61918
Of course other factors may cause pre-ignition, and of course there must be factors that influence the plug-tip temperature causing it to become excessive, that is incandescent, but a plug that becomes excessively hot, that is incandescent, will without doubt cause pre-ignition. (That's me finished on this oen!)
- Tue Aug 30, 2016 6:15 pm
#61922
Hi Stephen, increase the main jet size some more - try a 135, I have one this size in a 28mm Mikcarb on a slightly tuned 500 and I had to open that out by a tiny amount to get rid of some pinking in that. I also slimmed down an old needle from a MK 1 concentric and used that, to get a bit more fuel in where it was required. I doubt a change of spark plug type or grade will change anything much, though. Regards, Paul.
- Tue Aug 30, 2016 8:18 pm
#61925
Air temp and humidity effects relative air density, so there will be minor variations; worthy or 1 or 2 main-jet size? A RAD chart is so handy to have...
Running ExGas probes and CHT gauges are a very quick way of confirming if a mixture is over rich, or otherwise. Or if the head/plug is too hot? Detonation..?
Many Bullets (mostly 500's) I see that which have been rejetted, (main jet) are more frequently too large to the extent petrol is often contaminating the oil (Bore washing) and piston crowns are thick with carbon in under 5000km. Too many also show evidence of chronically 'out' timing; over-advanced and/or retarded.
My experiences with the 350 and 30mm Carb, is that with the stock cams, 180-200 main with
Amals, is a good start, which is on the rich side. Bouncy cams and an open exhaust etc +20% larger. And with the 28 Mikcarb, 120 is the largest I have ever needed to fit.
The Mikcab ‘reverse’ main jets are marked by hole dia.. Double the size, quadruple the flow.
Where as ‘Amal’ jets are calibrated for actual flow.
The rejetting formula for CALIBRATED main jets is: Larger carb? New jet ~ Old jet New carb size/Old carb size, squared. And that starting point is erring to the larger size.
IF in doubt you will do less damage with too large ajet...
Running ExGas probes and CHT gauges are a very quick way of confirming if a mixture is over rich, or otherwise. Or if the head/plug is too hot? Detonation..?
Many Bullets (mostly 500's) I see that which have been rejetted, (main jet) are more frequently too large to the extent petrol is often contaminating the oil (Bore washing) and piston crowns are thick with carbon in under 5000km. Too many also show evidence of chronically 'out' timing; over-advanced and/or retarded.
My experiences with the 350 and 30mm Carb, is that with the stock cams, 180-200 main with
Amals, is a good start, which is on the rich side. Bouncy cams and an open exhaust etc +20% larger. And with the 28 Mikcarb, 120 is the largest I have ever needed to fit.
The Mikcab ‘reverse’ main jets are marked by hole dia.. Double the size, quadruple the flow.
Where as ‘Amal’ jets are calibrated for actual flow.
The rejetting formula for CALIBRATED main jets is: Larger carb? New jet ~ Old jet New carb size/Old carb size, squared. And that starting point is erring to the larger size.
IF in doubt you will do less damage with too large ajet...
- Tue Aug 30, 2016 8:30 pm
#61927
Be very wary of modifying jets/needles?
0.0005 difference is MAJOR and represent a jump of 1/2 a size in needle. And the average Joe has no way of accurately measuring/calibrating a 'home drilled' main jet.
Modified jets? Mark it, try it and if it works, get the right jet and throw away the modified item!
0.0005 difference is MAJOR and represent a jump of 1/2 a size in needle. And the average Joe has no way of accurately measuring/calibrating a 'home drilled' main jet.
Modified jets? Mark it, try it and if it works, get the right jet and throw away the modified item!
- Tue Aug 30, 2016 8:34 pm
#61928
"if in doubt you will do less damage with too large a jet", that has always been the way to go, start rich and reduce accordingly, unfortunately I never had the benefit of modern gas analysers rolling roads etc, it was seat of the pants tuning back then and playing safe was the order of the day, how things change, a couple of hours on a dyno covers work which took a week.
- Wed Aug 31, 2016 7:10 am
#61931
In some cases shutting off the throttle does not cause an instant drop to the normal tick-over speed, the engine continuing to ‘run on’ at a higher speed. If the throttle slide spring has returned the throttle slide to its lowest position, then a possible cause is a weak pilot fuel/air mixture.
Just as an engine will often ‘race’ immediately before running out of fuel, due to a momentary weak mixture, so it may ‘run on’ for a similar reason.
It may be that the tick-over has been set by too high, and by an incorrect method, with the pilot air screw set further out than would be ideal, and the throttle stop screw screwed further in than ideal – a combination that results in a weak pilot mixture. The problem may be remedied by following the correct sequence for setting the tick-over. If that does not cure the problem then maybe a larger pilot jet may be required.
‘Running on’ may in theory be caused by an engine that has over-heated, and/or developed an incandescent hot spot in the combustion chamber - or a plug that is too 'soft'. Although rare this may cause the fuel mixture to ignite independently of the spark plug. The engine will continue to run after the kill switch is operated, the ignition turned off, or the plug lead disconnected.
Just as an engine will often ‘race’ immediately before running out of fuel, due to a momentary weak mixture, so it may ‘run on’ for a similar reason.
It may be that the tick-over has been set by too high, and by an incorrect method, with the pilot air screw set further out than would be ideal, and the throttle stop screw screwed further in than ideal – a combination that results in a weak pilot mixture. The problem may be remedied by following the correct sequence for setting the tick-over. If that does not cure the problem then maybe a larger pilot jet may be required.
‘Running on’ may in theory be caused by an engine that has over-heated, and/or developed an incandescent hot spot in the combustion chamber - or a plug that is too 'soft'. Although rare this may cause the fuel mixture to ignite independently of the spark plug. The engine will continue to run after the kill switch is operated, the ignition turned off, or the plug lead disconnected.
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