High idle and no power when hot

[Perthbiker]

Hi there, I have the problem as per the description.

The bike runs really well when cool and warm, but is noisy (detonation?) at high RPMs and down on power when hot. Supporting symptoms when the engine is hot are a high, lumpy idle when pulling in the clutch, and the kill switch doesn't stop the motor (which it normally does no problem). The motor just runs on and eventually stops after about 10 seconds. I think this is called dieseling?

I can't accurately read the plug under these conditions as I can't kill the motor quickly enough but the plug is generally brown as opposed to white (too lean) or black (too rich) when running for extended periods at mid throttle and revs.

Details are:
RSE 70cc engine with high compression head and SBP expansion pipe.
Speed carb with circlip set on second notch from top, i.e. second leanest setting.
B6HS plug with standard magneto and CDI, but replaced plug lead and boot.
Standard 44 tooth rear sprocket.
High octane fuel mixed 40:1 with synthetic 2-stroke oil.
Sea level at about 25C or 80F.
Rider is 70kg / 150lbs.

Any tips / fixes / thoughts?

 

[Deleted member 12676]

the standard CDI causes excess engine heat by having a very advanced spark ignition at high revs. if you have been on this forum long then you know what you gotta do to get a real 2 stroke CDI on it that retards the ignition at high revs.
If you chose to be stubborn and keep that POS on it then lower the compression and use a colder plug.

 

[DeathProof]

hehe he said retards

 

[Deleted member 12676]

yes, a retarded ignition spark is one that happens later and a retarded human matures and learns later than what is normal.

 

[DeathProof]

sounds like my girlfriend high idle no power when shes hot i gotta do all the work

 

[HeadSmess]

i never had an issue with my retarded cdi and i was hitting 80km/h for a while... then i made my own unretarded cdi and noticed absolutely no difference so went back to the retard


what jets in the carb?

it gets hot cus its too lean.

it gets lean cus the jets too small.

it idles too fast cus the needles too low cus youre trying to adjust the overall mixture with the needle that only affects the first 3/4 of the throttle...

raise needle, drill jet, do a plug chop... check for airleaks first...

 

[Perthbiker]

Thanks for the input guys.
Glad I was on to the right track when fingering the CDI and possibly the mixture, as opposed to the high compression head. I have a RSE HP CDI Ignition Box and Reed Valve Kit on order so they should arrive this week. I'll fit the new CDI first to see how that changes matters.
I've read and need confirmation that fitting a SBP expansion pipe means that I need to richen the mixture? If this is this correct, then I'll definately tweak the main jet to something bigger than the standard (#70?) on the Speed carb. This should also help with the overheating as the mixture will help cool the combustion chamber.
Would I need to further richen the mixture with the reed valve kit or doesn't that have an effect? I'm guessing that the low to mid range revs that the reed valve affects can be managed by tweaking the needle once the main jet is sorted?
PS: I'm taking for granted that there are no air leaks at this time as the bike works well in all conditions except when worked at high revs for a while which brings on the overheating. After it has cooled somewhat it settles right down and behaves as it should. New B8HS plug burning a lovely 'chocolate milk' brown after a good run in cool weather yesterday.

 

[Perthbiker]

An update on this case is that I've fitted the new CDI (which looks much the same as the original except for an MSD boot and lead) without any real improvement.
Even swapped the spark plug out for a much cooler B8HS in the hopes of 'drawing out' some heat from the combustion chamber. No change.
Also fitted the reed valve kit which seems to work well although it only affects the bottom to midrange performance which was working fine already.
Guessing I'll need to read up some more on the ignition aspects of these engines. I'm sure that with a bit of tweaking with the ignition and perhaps the mixture, they should be able to run at 40kph / 6,000rpm for extended periods without drama.

 

[Al.Fisherman]

Ummm all that work and expense, my stock HT will do every bit of 25mph (clocked on speed readout). BP6HS, NT carburetor, milled head, OEM 44 tooth sprocket, and 210# zooooom zooooom, although I ride mostly about 15mph.

 

[Deleted member 12676]

Does the new CDI retard teh spark at high revs? (If not then you wasted your money and should of bought the Jaguar CDI)

Some high compression heads bring the compression way too high for these motors. get a compression tester and make sure you dont have more than 130psi.