Honda CM125/250 carb rich/lean or electrical??

[texaschris]

Hey Y'all.

So I am in the final stages of dialing in my bike. It is a custom rebuild with parts from a few different models but is essentially a Honda CM125, with a CM250 motor, and a carb from a Cb250rs.

I have been tweaking the carb, because it has an aftermarket airfilter and the carb wasn't originally set up for this motor. trial and error-ing differnt needle clip postitions and jets sizes, etc.

The bike starts fine and will idle relatively consistently, and I did get it to accelerate pretty good all the way through the power band with one combination (but then tried to dial it in further and lost it, and can't get back to that positition).

The issue seems to be at wide open throttle the bike does nothing, doesn't quite stall but just kinda bogs and stops accelerating. it'll backfire sometimes as well. However with that one magic combination mentioned above, it seemed to pull good at all throttle positions, so its gotta be getting close.

However, now that I've tried a multitude of main jet sizes, and i am still getting similar results at W.O.T and some sputtering and backfires. I am wondering if there is a possibility it is a electrical/spark issue?

Plugs/caps/wires/coils are all new, and I am led to believe that the CDI tends to either send the signal to spark or doesn't. I thought maybe the battery was getting drained from all my starts and stops and short runs up and down the street all week, but pulled that out last night to hook up to a charger and that showed it was still at pretty much full power (so the bikes charging system must be doing its job still as well). I did pull the plugs and looked for the spark outside the cylinder, and they did seem to look 'weak' (but i dont know if my analysis of that is credible)

Anybody had/seen something like this before? Does it sounds like a rich/lean mixture issue, or a electrical/ignition issue? I just dont want to keep driving myself nuts playing with the carb, it the issue lies else where. but the elcetrics seemed to otherwise be doing their job?

The issue did seem to get slightly better last night as i ran the bike for a little longer (a bunch of laps of the block rather than just a couple 'up and downs' of the street)

If you read through all of that, I owe you one. If you can offer up any suggestions than I guess I owe you two. as this bike isnt really stock anymore, experiences/advice from any model would probably be benificial.

Thanks for any help,
Chris

[texaschris]

So either that made no sence or nobodys got any better ideas! haha

I did some more work last night, tried a bigger main jet and it pulled pretty good (well better than it has). However, it shortly went back to the old surging and no wide open power. What I did notice was it seemed to be, after I drained the carb and made an adjustment it would work alot better at first, but then deteriorate. Like the fuel suppy was running out or the jets were getting blocked.

Now I checked flow out of the tank, and it is consitant and solid. and i drained a bunch out the carb (with the petcock ON) and that kept flowing strong too. Its fresh gas and I've drained some into a containor and can't see any sign of debris or water drops.

So today, I have had a new thought...could a blocked/restricted air vent to the carbs float chamber cause some kind of pressure that would restrict fuel suppy? Does the tall brass tube connected to the drain tube play a part in venting the float chamber during normal use? cause there is a really long legnth of tube on the drain nipple, that could possibly effect air flow up into the chamber. failing that, Iam going to double check for any other vents to make sure there are no obstructions this evening.

Sound resonable? or does it still sound more likely to be ignition/electrical related?

Thanks again,
Chris