4 stroking at 1/2 throttle upwards

[Lambr62]

Hi out there,
I have a recently restored a SX150 which remains pretty standard other than I changed to GP crank and added electronic ignition set at 19 deg. The problem is when I rev up on the stand to half throttle and above the engine sounds like its 4 stroking almost flat not crisper sounding like below half throttle. Although when I first start it up it sounds better around half throttle but soon changes.

when I ride it I get the same thing in each gear around the same throttle positions ie at half throttle start getting problems. First thought is carburation problem or possibly electrical? Thanks for any help in advance.

[Digger]

I think you have answered your own question.

[Fast n Furious]

It may not be carburation.
Sometimes with electronic ignition they can generate a "rev limiter".
Often it is a result of a misaligned pickup or even a worn mag side main bearing.
This happened to my TS1 earlier this year coming back from scarboro.
No matter what gear I was in it would limit out at 4800 revs. So it was 50mph all the way home for me.
I was cringing as the 125's crawled passed me.

[coaster]

If it was running fine before I would be looking at the ignition side first. Change the plug first and then try another CDI. A weak spark can give similar symptoms. If that doesn't work, look at the pickup height alignment as F&T suggested and use shims to get the gap tight.

[Hornbyfrog123]

I had a similar problem, looked at all electrical possibilities and it turned out to be a faulty HT coil. Low rev spark but nothing at high end.