mystery misfire

[elliottles1]

folks,

i have a new build finished and have an irregular misfire happening just as it gets to the power band, rev related, not throttle. the misfire is very irregular, like ignition cutting in and out.

the engine is a yz250 cylinder and head grafted onto a dt250 bottom end, all new rebore, crank, brand new carb, correctly set up and jetted, new carbon reeds, intake manifold. also new is the spark plug, cap, coil, cdi box (rex's), the flywheel rotor has been swapped out for one known to be good and the stator plate has been modded to get the correct ignition timing for the yz.  the exhaust has a new silencer and is as new all the way thru.

i just cannot think of anything else but it happens every single time it is ridden. any thoughts will be welcome and thouroughly checked out.

cheers
les

[1976RD400C]

Sounds like the spark is weak and is having trouble when under a load.

[m in sc]

tighten the plug gap to .020. if it gets better, then weak spark.

[elliottles1]

thats a good thought, i just wanted to eliminate all of the simple things before spending out on an inner rotor ignition set up as essentially i am happy with the standard cdi ignition, but not if its failing!

les.

[rodneya]

I had something similar and it turns out I had a broken bullet connector on an earth. The female part was completely cracked off, but the plastic sleeve held it together enough to make a connection. At certain revs the vibrations broke the connection and caused a misfire. Took a while to find that one.

[elliottles1]

thanks, i will check both of these out in the coming days, when the rain stops.

les.

[teazer]

Dt exhaust or the one that matches the YZ top end?  The reason I ask is that it sounds like a resonance problem. It could be a bad connection and at certain revs, vibration causes the parts to momentarily part, but more likely to be a mismatch between pipe and ports or just the way the pipe works at specific revs.

If so there are two possibilities: 1- modify the pipe or ports to eliminate that off-pipe problem or 2, stay in the powerband all the time. It may be be possible to lean out/richen midrange to change exhaust temperature at that point in the rev band but that can lead to other issues.
 

[elliottles1]

teazer,

i had thought along these lines with the exhaust, which is a home made attempt at a yz system.
i may well buy a yz exhaust of that era and modify to fit.

many thanks for your input
les.