Cutting out at high rpm...

[dexameth]

Hey guys... Back at my Minarelli build again. Had crud in my float needle seat, so the bike was leaking fuel on the side stand or during bumpy rides so I parked it months ago. A few days ago I hand choked it and it fired on the 4th kick.
Much excite.
As it was warming up I noticed the fuel spilling out the overflow again. Got to work, pulled the carb and with a cotton swab I was able to pull some green gunk out of the float seat and presto no more leaking!

But here's the issue: at high speeds/RPMs it feels like it's misfiring. I can cruise around the neighborhood with zero issues, but as soon as I get on the road marked 45mph and hold WOT it'll cut out right around 10k RPMs or 45-50mph. If I keep the RPMs under 9500 there's no issues.
If I'm riding along wide open and it happens and I continue to hold wide open, it feels like the kill switch is randomly being flipped, but really really quick.

I had these issues right before I parked it. I thought the fuel spillage had something to do with it and after I fixed the float I thought it was solved. Before I had parked it I swapped both coil and CDI. The exact same issue is occuring.

When this build was young, I could drag race myself and get 60mph in a quarter mile, hit 65 a few times on US19 and the bike ran amazing. Pulled wheelies hard, just ran good all around then one day it started acting up and I have not been able to pinpoint it yet.

The only thing I haven't changed is the Hoca Performance Alternator (mini flywheel and stator). It has less than 50 hours of ride time on it and I'd be pretty upset to learn the stator is faulty... not a cheap part to fail so quickly.

To recap: after noticing the issue I swapped coil, CDI, cleaned carb multiple times, new fuel petcock, new fuel lines and filter, fresh premixed fuel, different reed block and then lost my mind.

Lemme know if anyone has any ideas...

[geoffh]

Just to get them crossed off the list,is there a filter in the gas cap and a filter inside the tank it,s part of the petcock on my CS50.

Geoff

[ryan_ott]

Have you swapped out the plug, wire and plug cap?

[dexameth]

Mar 20, 2021 8:53:51 GMT -5 ryan_ott said:

Have you swapped out the plug, wire and plug cap?

Yup, done did that.

[dexameth]

So I noticed I was running a bit lean. Went from 110 to 115 main jet that seemed to bring back some power, but didn't change the issue at all.

Then I went from 4g to 5.5g sliders, to see what the RPM change would be like. Same deal. Get up to around 48ish mph and at WOT it starts cutting out. If I back off slightly and maintain speed it goes away. Hold wide open again and it sputters.

I made a video, let me see about uploading it.

[Happypancake!]

Mar 22, 2021 17:23:36 GMT -5 dexameth said:

So I noticed I was running a bit lean. Went from 110 to 115 main jet that seemed to bring back some power, but didn't change the issue at all.

Then I went from 4g to 5.5g sliders, to see what the RPM change would be like. Same deal. Get up to around 48ish mph and at WOT it starts cutting out. If I back off slightly and maintain speed it goes away. Hold wide open again and it sputters.

I made a video, let me see about uploading it.

Does the new CDI have a limiter?

[dexameth]

Mar 22, 2021 17:28:24 GMT -5 Happypancake! said:

Mar 22, 2021 17:23:36 GMT -5 dexameth said:

So I noticed I was running a bit lean. Went from 110 to 115 main jet that seemed to bring back some power, but didn't change the issue at all.

Then I went from 4g to 5.5g sliders, to see what the RPM change would be like. Same deal. Get up to around 48ish mph and at WOT it starts cutting out. If I back off slightly and maintain speed it goes away. Hold wide open again and it sputters.

I made a video, let me see about uploading it.

Does the new CDI have a limiter?

Nope. The new CDI is the one that was used before I upgraded to the Hoca mini flywheel and stator. So when this started acting up I swapped from the Hoca CDI to the stock Minarelli and same deal.

[dexameth]

I really can't remember when the issue started either, that's the killer. She's been under a cover for months because of the issue and I thought it was the fuel bowl overflow problem. Guess not.

[190mech]

What carb and airbox are you running now?

[dexameth]

Mar 22, 2021 18:15:55 GMT -5 190mech said:

What carb and airbox are you running now?

28mm PWK and just a mesh screen. Been that setup since I built it. The carb mouth is 3mm from the frame, there is no filter that will fit the space I have so I just run open carb but added the mesh screen after I broke a few reed pedals from sucking in debris.

[dexameth]

I'm almost wanting to think reed flutter. Riding home after this video was okay around 35mph, but crack wide open and I heard some weird noise... Almost detonation but different. Idk. Plug looks good:

[dexameth]

Strange. Video isn't showing... youtube.com/shorts/N9YozGxsTyA

[repherence2]

Dexameth, do you have a vacuum operated petcock?

If so, where on your intake path are you drawing vacuum from? And how big is your vacuum port/hose that runs to the petcock?

[captincvmn]

Does anything else electrical flutter when this is happening? Have you ridden at night to see if lights are rock steady or wobbly?

[90GTVert]

That sounds more like a mixture issue than spark to me, but you can run about 1/2 the plug gap if you still suspect the ignition. Usually these are something like 0.024-0.028" gap, so you'd set it to 0.012-0.014" and see if it runs better. That's not to say you should then run the gap that way all of the time, but it's telling you something isn't quite right with the ignition system.

I would try some jetting changes and see if you can clear it up and check float height. If you try a larger main jet and it gets worse or happens more quickly after you go WOT, that may be telling you that it's running down the fuel in the bowl too quickly (supply or float height).

This kind of thing can also happen with piston ring issues, so if all else fails you may want to do a compression test and/or take a look.