High RPM Stutter (Custom Bike)

I recently swapped a CRF450X motor into a RS125 road racing chassis. Call it... an RSF450! The bike is super fast but I am experiencing an intermittent stutter / miss in the higher RPM range (above 6-7K) when at steady throttle. First of all, here's a couple pics of the bike for reference:





Here are the specs that may be relevant to the issue:

Motor:
- '14 CRF450X base motor (low miles, near new condition)
- '08 CRF450R head, valves, and cam (brand new)
- JE Pro Series 12.5:1 piston (#261965)

Intake / Carb:
- '14 CRF450X carb w/ JD jet kit
- #170 main jet
- #45 pilot jet
- NO AIRBOX (just a clamp on cone filter)

Exhaust:
- Custom SS system with 1-3/4" header into universal glasspack w/ 2" exit

I'm running the unmodified CRF450X wiring harness and all OE electronics.

If you watch the video below starting around 0:20 you can hear the miss. I'm holding it steady right at about 7K RPM as I go by.



Things I've already tried:

- replaced spark plug
- inspected spark plug (looks perfect, #170 main jet seems good)
- bumped pilot jet down from #48 to #45 (fuel screw now in optimal adjustment range)
- raised needle clip one groove
- lowered needle clip one groove
- swapped out coil and wire with another known good unit
- swapped out CDI with another known good unit
- checked pulse generator and flywheel was clean of debris
- checked all electrical connectors
- checked cam timing
- checked valve lash
- checked tank vent

None of the above changes really made any difference.

Any ideas on what else might be causing this 'stutter'? It runs great and pulls like a raped ape otherwise. It's only when cruising at steady throttle in the upper RPM range (above 6K) where I'm experiencing this issue. You can hear it and feel it (there is a momentary loss of power).

 

I know the man who painted those wheels

 

Probably not the case but worth checking - I had a bike with an intermittent ground that stuttered at a certain RPM just due to vibrations. Easy enough to double-check all power & ground connections (and connectors) just in case.

 

possible intake manifold leak?

 

I don't see where you messed with the Main jet. Go up a size and down a size on the main. Also try wrapping something around the filter to slow down the air velocity as that might contribute.

 

I'd first try what pscook suggests (if you haven't tried that already) and see if changing main jet helps. How does the bike do at 3/4 to full throttle?

One other thought: does the '14 CRF450x run a fuel pump? If so, and you're not using it on the conversion, the float bowls might be running dry.

 

It sounds like it's not getting enough fuel at high RPMs. If your throttle is WO at this condition the needle ought to be mostly free of the main , lowering the clip or a thinner size needle or larger main jet might help. Screwing around with mains and needles is a lost art. Make sure it's got enough fuel to the bowl, check any filters and perhaps the fuel line needs to be bigger (assuming not pumped)? Another thought is how much vibration is there? At certain frequencies the gas in the bowl may be getting frothy.....If it pukes gas right after one of these episodes that would indicate the bowl is getting over filled.....OK, I'm done guessing.

 

Couple thoughts: You probably have MX/Trail riding jetting. RR jetting is probably different than MX due to throttle and position. I would think that MX is mostly roll-on/roll-off whereas RR is 1/2 to WOT, so you will probably want to mess with the main as the pilot, the idle fuel mixture screw, and needle only overlap below 3/4 throttle. If it is rolling on to your liking, and only badness at WOT-ish, then main.
Float height: Is the carb at the exact same angle in the 125 chassis as it would be in the MX frame? Angle of carb defines float height. A steeper angle of the carb (forward canted engine) will allow more fuel in the bowl, but that usually affects idle up to half throttle.
Spark plug: New? Plug wire good?
Needle: If you are at or above 3/4 throttle, the needle is out of the way of the main, so you can raise it if you want but likely that you won't see a change. Dropping it might change it if you have the clip on the top groove.

As crazy as it sounds, I would give it a 160 main OR a 180 to see if there's a difference. I know that on my WR450 that I noticed a difference between 165 and 167 on the road, so a big jump should make a difference. If no change, then it isn't the carb. Or try a 150 or 190. But make it a noticeable step and see what changes.

 

Thanks for the ideas everyone. I just tried several of your suggestions, and have good and bad news. The good news is that I was able to change the characteristics of the stutter; the bad news is it only changed for the worse.

What made the most dramatic difference was when I put the needle clip in the furthest most grooves (max rich and max lean on the needle). The bike stuttered much worse than before, especially on the max lean groove setting. It basically sounded like it was hitting the rev limiter. I've tried every groove setting and the middle position is still where it stutters the least (as seen in video above). I also tried a different needle.

At least this seems to indicate that the issue is in fact in the carburation and not electrical / ignition?

I am only at 1/4 throttle or so when cruising at 7K RPM - nowhere near WOT. It seems to run great at WOT.

 

Is the carb gravity fed?? Is the petcock able to flow enough fuel for the carb?

 

Sounds like way undersized main to me.

 

I found this awhile back.

 

Same opinion here. Pop in a 190 for fun. With a carb road race bike you should have a wide selection of pilot and main jets ready to go. Buy a box of each and start to jettin'.

 

If you're trucking around at 1/4 throttle when this happens, and it gets worse at lowest clip / highest needle position, then you need a richer needle or a richer nozzle (ex-TZ driver).

 

I'll try swapping in a super rich main jet. I've tried #175 (with no change) but will go extreme and see what happens.

 

Might be irrelevant in this situation, but I have heard of the end caps on air filters like that K&N causing issues like you describe.

A foam UNI filter or similar with no end cap might be worth a try.

 

Thanks for the idea. I've tried various intake shapes, filters, and restrictions, yet none have had any dramatic effect on the stutter!

 

I may be crazy but that video run is 7k rpm? Sounds nowhere near that to me. Where are you getting that number from? Again, i may be crazy but just throwing it out there. It does sound like its on a rev limiter though and that's why i mention it as a possibility.

 

Yea, the vid is right around 7K RPM, as confirmed by a Trail Tech computer I have hooked up as well as just owning three 450X bikes and kinda knowing their rev range. It does sound like its on a limiter, but it ain't. I can go full throttle from this stuttering 7K RPM state and it'll haul ass up to the actual limiter!

I'm starting to think maybe I need to get a hold of a 450R carb and try that. The 450R carb is 41mm as compared to the 450X carb at 40mm. Since I've bolted on all R head, valves, and cam, maybe it just wants the R carb to run right. But this is definitely a hail mary attempt...

 

Finally got a brand new OEM 2008 CRF450R carb to try. Installed as is - still stutters exactly as it did before.