Mid End "Wall"

[jamaha51]

I have had this problem ever since I got my shee a few years ago. I got it in a trade not running, replaced the stator (not oem, but it has been going strong and I have tried another stator with no change) and then it would run on both cylinders. It had a nasty mid end "bog" that I figured was a jetting issue. After trying messing with the jetting on the stock carbs with no difference, I switched to a single 34mm keihin pj carb with exciter intake manifold. It did the exact same thing even after messing with the jetting on that carb as well. After that I figured it was because it had 85psi compression and probably wore out reeds. This past weekend I just got the bike back together after putting in a 4 mil crank and some mildy ported cylinders for it along with brand new reeds from TDR. After doing my heat cycles, I go to take it out and it does the same exact thing except now it is basically unrideable. Before, I could just clutch it and could ride it just fine. With my new setup it seemed to just exaggerate this flat spot. Basically low-mid rpms, you hit this rpm where it will not get past it. It doesn't matter if you vary the throttle, it just is stuck in the same rpm. If you clutch it, you can sometimes get past that stage and it will rev out. I have switched literally everything on this bike from when I get it except the cdi and coil. That leads me to believe it is one of those. It feels like the timing just goes wack and that prevents the rpms from going anywhere, but I have never heard of that. Also, I leak down tested it before and after the rebuild and it holds 6 lbs for 6 minutes so there are no airleaks and I do know how to jet a bike so it is not that. If it helps this is my exact setup: Redline Racing 4 mil play port, 65mm wiseco 795 pistons, hot rods crank, 20cc domes with cool head, single 34mm pj, fmf fatty pipes with stock silencers, Tony Doukas modified reeds. Thanks

[hoppedupandcutdown]

Swap a known good CDI. They can do some crazy stuff when going bad.

[JUSTWANNARACE]

Unscrew your spark plug boots and cut them back about 1/2" and screw boots back on(better to replace boots). If that doesn't help replace the coil(coil is probably the issue, but boots is a cheaper place to start) jmo

Edited April 3, 2018 by JUSTWANNARACE

[Larry's Shee]

Sure exhaust isn't plugged with mouse nest ?

Start ohming the electrical.  Ohm boots and coil separately.

Clean timing nubs on flywheel, set gap.

Refresh all grounds. Try separate ground from engine to coil mount.

[jamaha51]

Ok, I am picking up a coil and cdi and will see what those do for me. I also need to check the pickup coil gap, forgot about that. I did check the silencers and I know that they aren't plugged. I will let y'all know what happens hopefully sometime soon.

Edited April 5, 2018 by jamaha51

[hoppedupandcutdown]

If the electrical doesn't fix it, check the expansion chambers for nesting material. It could run fine until there is enough exhaust pressure to lift it and plug the stinger, then drop when the throttle is let off.

I've seen it with nuts. (The tree kind, not the mate to a bolt kind)

[jamaha51]

I didn't hear anything shaking them, but it also did this with another set of pipes so I am almost positive it's not them. I wish it was something real easy like that.

[Ayesully810]

yeah rats nets could be in the expansion chambers i had that happen before same problem no revs cant just shake it gotta blow it out with compressed air .  Coils are cheap u can find one any where on here 

Edited April 9, 2018 by Ayesully810

[Larry's Shee]

Set of pipes my son got was plugged with pink insulation, makes no sound.

[jamaha51]

Ok, a little update. I picked up an eBay cdi just to test and it did the same thing, so I switched back to the stock cdi. I switched coils out next and it made an improvement. It's actually rideable now and the first time I felt what a 4 mil is like 😀 It still has that mid end big, but it's not too bad to get past it. My next step is to reroute the wiring harness and make sure all grounds and connections are solid.

[sleeper06]

How did you sync the carbs, you could have a bad choke/idle knob letting too much fuel by. What needles you running since you’re problem seems to be in that realm 

[JUSTWANNARACE]

15 minutes ago, sleeper06 said:

How did you sync the carbs, you could have a bad choke/idle knob letting too much fuel by. What needles you running since you’re problem seems to be in that realm 

He has a single 34 pj

[sleeper06]

Single carb that small you might need a dek needle or eek . Any single carb engine the needle is the most important tuning piece. Not sure about a pj but on ported bikes I’ve seen mostly 42-45 pilot 195-210 mains with my porting