Engine cuts out at high rpm, comes back and keeps revving

[Faolan]

212 predator, 22mm mikuni 265 cam boosted compression, gravity fed fuel tank above carb. Plug gapped .035.

Runs great from low to top, but when full throttle once it hits ~7k sometimes it will cut out and start revving down and pop a little bit if I stay on it it’ll recover after a second and continue revving clean, full power. Plug looks great. Gas tank vented well, new plug, new coil. would really like to figure this out. Any help is appreciated

 

[ole4]

Sounds like valve float what springs were you using and what was the spring seat pressure or if you did not measure it what is your installed height? What valves are you using? In some cases, before the springs just give up the ghost you can hit a point where the resonant frequency of the opening and closing of the springs can cause them to resonate and lose control, then above or below that continue to function but they won't go much higher before losing control altogether of the valve train.

 

[Faolan]

Sounds like valve float what springs were you using and what was the spring seat pressure or if you did not measure it what is your installed height? What valves are you using? In some cases, before the springs just give up the ghost you can hit a point where the resonant frequency of the opening and closing of the springs can cause them to resonate and lose control, then above or below that continue to function but they won't go much higher before losing control altogether of the valve train.

They are stock valves, predator non hemi, on 26# springs. Unsure on installed height but I did use the shim that came with them. I’m about to go for a ride now, I’ll see if I can catch the audio of it happening and post it up

 

[Faolan]

Okay so it did it when I was in the way to the straight away, then I rode 5 minutes without it happening, stopped, started another video (this one) and it did what I’m trying to fix

http://imgur.com/a/78bchcf

sometimes it is worse though, sometimes it doesn’t do it at all,

 

[ole4]

Kind of hard to tell from that audio. 26lb springs and stock rocker ratio with stock retainers and valves should have enough pressure with a dyno 265 cam to not float the valves. Wondering if you could be fuel starved? post a pic of your fuel tank setup.

 

[Faolan]

Kind of hard to tell from that audio. 26lb springs and stock rocker ratio with stock retainers and valves should have enough pressure with a dyno 265 cam to not float the valves. Wondering if you could be fuel starved? post a pic of your fuel tank setup.

In the audio clip, the sound is at 22 seconds, I never let off the throttle when it happened

 

[ole4]

What main jet and pilot are you running? Does your chikuni use Mikuni or Kehin jets?

 

[Faolan]

What main jet and pilot are you running? Does your chikuni use Mikuni or Kehin jets?

It uses mikuni, it has a 110 main and a 17? Pilot. Came with a 130 but seemed very rich so I worked down in sizes

 

[Faolan]

Does it seem lean or something?

 

[ole4]

I use them on a number of my bikes and for the modified clones run a 130. My tecumseh runs a 120. I also use a stock 15 pilot and leave needle in middle position just for reference. DId it still cut out with the richer jets? When you say it seemed very rich what observation are you basing that on?

 

[Faolan]

I use them on a number of my bikes and for the modified clones run a 130. My tecumseh runs a 120. I also use a stock 15 pilot and leave needle in middle position just for reference. DId it still cut out with the richer jets? When you say it seemed very rich what observation are you basing that on?

Well, the plug was black, and it would pop and sputter when I’d give it too much gas, sort of like the issue now but much worse and would happen a lot more with anything past 1/4 throttle, my jet is in the middle position

 

[Faolan]

Well, the plug was black, and it would pop and sputter when I’d give it too much gas, sort of like the issue now but much worse and would happen a lot more with anything past 1/4 throttle, my jet is in the middle position

But the weird thing is, it used to run great with the 130, then it started acting up and ran better as the jets got smaller.
what should I try? Going back up to a 115 and stepping up from there? I can’t tell if it’s rich or lean right now, I think the plugs look ok, maybe a little lean but I can’t read plugs well

 

[Faolan]

Well, threw a 125 jet in there, feels the same as the 110 with maybe a tad more acceleration.. didn’t get it to recreate the issue yet but I only had time for a 10 minute ride. Letting it cool off to check the plug again.
How can I tell if it’s lean or rich wot?

 

[ole4]

Spark plug color. Now I had a problem once where my CDI was going bad, did not suspect it as I had just made a number of changes to the engine (HS50) and it would start and run OK for about a minute then start missing. On the clone engines they are pretty cheap and it may come down to changing it just to eliminate it as an issue. Are you running a stock or aftermarket flywheel. When you said it ran great with a 130 then it changed maybe its not the carb. They just dont start running richer by themselves unless the fuel is bad or has changed.

 

[Faolan]

That is something I tried as well when it started running poorly with the 130 jet, a new coil ran the same. It does have an arc billet flywheel with 8* timing built in.

I honestly have no idea, I’m thinking you were right about it being starved for fuel, with the new plug it was firing correctly and burning all the fuel at high load/rpm but running so lean that sometimes the mixture wouldn’t ignite.
With the new plug gapped .035 and a 125 jet I haven’t had the issue come up again in the 30 minutes I’ve put on it. And the plug isn’t black with the bigger jet this time. Could be gone.. or not.. :

Still at a loss on what made it run crap with the 130, I chalked it up to being the change in compression (lower now cause the thin gaskets didn’t last long) but clearly that’s not the case as it’s running as good if not better than it did with the .010 gaskets, .040 fire ring now.

Thanks for brainstorming with me, I enjoy these motors and making them run like they should and I’m glad there’s forums around for this kind of thing

 

[ole4]

So the arc flywheel has a minimum gap which if closer will in time fry the coil because of the much stronger rare earth magnets. I think the minimum gap is .040. just verify that you have at least that and we can probably eliminate the coil. By the way the main jet has almost no effect till the throttle is open more than 3/4 throttle, under that the needle jet and emulsion tube control the mixture.

 

[RCB19]

I had the same problem you are describing on one of my motors and sounded very similar to the sound you posted. In short the problem was the newer version Chikuni carbs that are being sold now that come with the Kehin style pilot jets and a Mikuni style main. Could never get those to run right would cut out on the top end like you describe. Didn't matter what changes I made to carb tuning (Jets, float levels, Jet needle height etc) had same outcome. Ran VERY rich at idle too. Bolted on 2 different older version Chikunis that came with ALL Mikuni brand metering inside and runs like a champ. I use 130 main and 17.5 pilot in those. Hard to find those older versions anymore. If someone here has a tip on where to get those it would be appreciated.

 

[bikesisbikes]

"But the weird thing is, it used to run great with the 130, then it started acting up and ran better as the jets got smaller.
what should I try? Going back up to a 115 and stepping up from there? I can’t tell if it’s rich or lean right now, I think the plugs look ok, maybe a little lean but I can’t read plugs well "

The highlighted part, to me, indicates a progressive decrease in airflow. Have you checked, cleaned the air filter?

 

[Faolan]

So the arc flywheel has a minimum gap which if closer will in time fry the coil because of the much stronger rare earth magnets. I think the minimum gap is .040. just verify that you have at least that and we can probably eliminate the coil. By the way the main jet has almost no effect till the throttle is open more than 3/4 throttle, under that the needle jet and emulsion tube control the mixture.

Will check after work

"But the weird thing is, it used to run great with the 130, then it started acting up and ran better as the jets got smaller.
what should I try? Going back up to a 115 and stepping up from there? I can’t tell if it’s rich or lean right now, I think the plugs look ok, maybe a little lean but I can’t read plugs well "

The highlighted part, to me, indicates a progressive decrease in airflow. Have you checked, cleaned the air filter?

I have just recently replaced the filter that came with the chikuni with a k&n, I do ride off-road 90% so it was a little dirty but 130 to 110 dirty? I removed the 1” rlv muffler too. Loved how quiet it was but man, it’s night and day with and without through the entire rpm range.

 

[Faolan]

I had the same problem you are describing on one of my motors and sounded very similar to the sound you posted. In short the problem was the newer version Chikuni carbs that are being sold now that come with the Kehin style pilot jets and a Mikuni style main. Could never get those to run right would cut out on the top end like you describe. Didn't matter what changes I made to carb tuning (Jets, float levels, Jet needle height etc) had same outcome. Ran VERY rich at idle too. Bolted on 2 different older version Chikunis that came with ALL Mikuni brand metering inside and runs like a champ. I use 130 main and 17.5 pilot in those. Hard to find those older versions anymore. If someone here has a tip on where to get those it would be appreciated.

Hmm interesting. Why would conflicting jet patterns cause such a thing?