butre
Well-Known Member
did you experiment with plug gap any? I find that plug gap makes a way bigger difference to performance than heat range
China girl 66... Maximum RPM?
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Steve Best
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I did today. I used the Z4 plug because it was the best working overall.
I tried from 0.020" to 0.040"
0.020" started slightly better but detonation at 55-58kph. Unable to go over 60kph.
0.040" sluggish acceleration at very slow speeds. Detonation started at 58-63kph. Top speed 66 today.
These top speeds are all the engine will do, even on a downhill, before a grinding clatter of detonation kills power.
Does that match your experience Butre?
Also tried Champion CJ6 and RCJ6Y plugs in this head today.
New plugs, shorter reach so have 2-3mm threads exposed.
Excellent starting and low speed performance, even in spite of an apparent compression loss due to short length.
Top speed was only 54kph with a high pitched rattle as I approached this speed. Good at low speeds, not high.
Could not tell a difference in performance between these two plugs.
I am surprised by the differing performance of these plugs and gaps and how extended high speed driving varies plug needs from short slow trips. All this was done with my "high speed" cylinder head which gives me about 5kph more at the expense of low speed torque. I will gladly trade that extra 5kph for tractor like power at very low speeds.
Steve
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butre
Well-Known Member
typically high rpm stops detonation. are you sure you're not running lean?
Steve Best
Well-Known Member
True, but 2 strokes tend to build cylinder pressures with rpm, especially with a tuned pipe (I do not have yet).
Not running lean at those speeds, did plug chop to verify. Still quite rich actually.
I suspected it might be spark scatter from the CDI getting beyond rpm range, but it is distinctly plug related.
I still suspect an rpm retard ignition like Jaguar's might solve this.
Too much timing at high rpm would raise cylinder temps and make it prone to this.
I figure explore all the plug angles first, next onto CDI.
Steve
Steve Best
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Swapped heads, less top speed but no detonation now.
The head on the left makes 62-66kph but poor low speed torque and has detonation at max speed.
The head on the right makes 55-58kph but is strong off idle, accelerates better and no detonation.
Note head on left has leaking headgasket again, probably due to detonation and weakness.
Detonation will eventually take out the bearings too.
Used full synthetic oil this time. MUCH cleaner. These picts are before wiping clean.
Steve
The head on the left makes 62-66kph but poor low speed torque and has detonation at max speed.
The head on the right makes 55-58kph but is strong off idle, accelerates better and no detonation.
Note head on left has leaking headgasket again, probably due to detonation and weakness.
Detonation will eventually take out the bearings too.
Used full synthetic oil this time. MUCH cleaner. These picts are before wiping clean.
Steve
Frankfort MB's
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Why don't you use higher octane gas and a colder plug
Steve Best
Well-Known Member
I did try a colder plug and it did work to reduce detonation.
The whole point was to investigate what affects the detonation. It is useful to have it just on the threshold.
I'm not totally convinced it is not a timing issue as the rpm gets over 9000.
I am still going to go back and try a timing curved CDI like Jaguar's.
Also I'd like to try altering the squish angle.
It matches the piston now, ideally there should be some relief angle to it.
The piston has a 3" radius so the squish area should less radius or more of an angle.
And I should have tried higher octane just to gauge the effect, you are right. So much to try!
Do you know how long it takes to burn a tank of fuel through one of these bikes?
About 3 hours of WOT riding! Longer if you go easy.
Steve
The whole point was to investigate what affects the detonation. It is useful to have it just on the threshold.
I'm not totally convinced it is not a timing issue as the rpm gets over 9000.
I am still going to go back and try a timing curved CDI like Jaguar's.
Also I'd like to try altering the squish angle.
It matches the piston now, ideally there should be some relief angle to it.
The piston has a 3" radius so the squish area should less radius or more of an angle.
And I should have tried higher octane just to gauge the effect, you are right. So much to try!
Do you know how long it takes to burn a tank of fuel through one of these bikes?
About 3 hours of WOT riding! Longer if you go easy.
Steve
Frankfort MB's
Well-Known Member
You could just add a bit of octane booster too the fuel that's already in it
Steve Best
Well-Known Member
Most of that octane booster stuff is junk. Raises octane less than a full point, which is insignificant.
I have a case of it in the shop. I should try it to confirm. Good idea.
Now I gotta put the other head back on...
Steve
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