1200 misfire..

[Fatti]

Hi all, got a real headache goin on with my '97' 1200..
Got a misfire around 3500- 5000rpm. Only when the bike is hot. Open the throttle and bike reves clean right through vboost no worries. Feels like fuel but isn't, I've fitted stage1 dynojet and bike has been on Dyno twice, been told 100% it ain't the carbs. I've ditched the coils (cracked) and got cops, tried new plugs twice and swapped out the ECU to no avail. Misfire only occurs when bike is at running temperature. Anyone had same problem. ? Help and advice appreciated..

 

[Fire-medic]

Stage 1 DJ is a no-no. Stage 7 if you have a complete aftermarket exhaust system, and not a Cobra 4-into-4 [slip-ons canister only don't work for more power]; or OEM jetting, dropping one or two sizes (OEM in USA is usually 152.5 Mikuni, one size down would be main jet 150 Mikuni Main Jet, two sizes down would be 147.5 Mikuni Main Jet) will usually help make more power, and enhance throttle response, with other tuning done. That includes carb synching and air bleed screws below the CV caps being adjusted for either best idle or matching temperature to other cyl's.

 

[desert_max]

I would be taking a close look at both ends of the plug wires. Unscrew the caps on the coil sides and unscrew the caps on the plug side. Make sure you have fresh copper. No green oxidation allowed. They are green, take a quarter of an inch off down to good shiny copper.

<<< Never mind. Missed the cops upgrade. Then again, maybe that's the problem! Could have a bad unit >>>

 

[Parminio]

been told 100% it ain't the carbs.

If it's got a stage one in it, you were told wrong.

Misfire only occurs when bike is at running temperature.

That suggest that it's a fuel problem. I'm wondering if you're just leaning out too much once you're warmed up and that's causing the carbs to starve a bit.

Have you changed when VBoost opens? Is it set to open earlier? That would explain it if you have.

 

[sdt354]

If it's got a stage one in it, you were told wrong.


That suggest that it's a fuel problem. I'm wondering if you're just leaning out too much once you're warmed up and that's causing the carbs to starve a bit.

Have you changed when VBoost opens? Is it set to open earlier? That would explain it if you have.

Imo it IS a carb. issue. Ignition issue should break up still on the top end,, it clears up at 5k, correct? Stage one, my hat is off if you get those needles to work well. If it were me, I'd start with a plug chop and read them. And, "only when the bike is hot" suggest a rich condition. Like F.M. said, no full header, no carb kits. It's like pissing through a cocktail straw.

 

[maleko89]

Agree, it's a needle/transition to needle issue. Do the needle test to set that but good luck getting DJ stage 1 needles to work right. It's just way too aggressive of needle for a stock Vmax.

1) 4th gear, 4K RPMS: WOT until 6K RPMS; let off throttle and if it speeds up, running rich and lower the needle.
2) 4th gear, 4K RPMS: WOT thru 6K RPMS; at 6K, give it some choke; If it speeds up, running lean and raise the needle.

If neither condition happens, needle position is fine.

 

[Fatti]

Thanks all for your opinions and advise. Should have mentioned I've a full Black widow exhaust system fitted. I will however take steps based on your advice.
Thanks guys and I'll keep you posted with any progress ..

 

[MaxMidnight]

Can we assume the carbs have been balanced?

 

[02GF74]

Are you able to borrow a set of original coils? Or fit series resistors?

Generally COPs are 1/2 resistance of coils so the output drivers in the ecu may be intermittently failing once hot.

Most likely not the cause of your problem but it does rule them out.