91 VFR misfiring?

[Moki]

Mate of mine has a '91 VFR750 (Jap import) that misfires under load above 5000rpm. Carbs have been off, cleaned and even had new stock jets in, but same-just misfires better. Anybody know it these are restricted? PO said it's done that during his time owning it. It's been to 3 bike shops and all are scratching their heads.

It's gotta be simple....and me thinks electric and not heat related. Stator? Is there a winding that pushes more current through when engine hits 5000rpm?

Runs sweet all over except when pushing hard above 5000rpm.

-stock carbs/jets
-stock pipe
-valves done
-all filters replaced.

Any ideas?

Thanks

[jellywrestler]

what sort of misfiring? how many cylinders? which cylinders? is that mr roddicks one?

[Moki]

Yep - Gav's it is.

[vifferman]

Some things to check:
- Plug boots and leads. Sometimes they will arc across to the engine if there's a small tear in one of them.
- Coils (if it's only one cylinder misfiring, find out which one it is, swap coils, see if it fixes it).
- Starter clutch; it is involved with the pickups for the ignition, and if the starter clutch bolts are loose, then as the revs pick up, the ignition timing can be a 'off'. Here's a post on a UK forum about this:

"When looking at the starter clutch casing the centre bolt on mine had worked loose putting to much strain on the 3 other bolts. This caused the casing to crack allowing ( i think ) the casing to wobble. Thus at high revs the connections for the 2 coils were not being made causing the misfire. If you do take the starter clutch apart to check, LOCKTITE the centre bolt to the correct torque setting. One other point to check are the yellow wires to the reg/rec, mine had started to get warm and discolour. Whether this was do to the misfire or not i do not know. Since replacing the clutch everything has been ok."

[Warr]

I'd go with electrical. Find another CDI unit, its plug & play from there on.

[Taz]

Check that the speed limiter inside the speedo housing hasn't been unplugged. Thus is the sensor that is activated by a disc beneath the speedo face (so you need to pull the speedo housing apart to check) and limits the bikes speed to 190? odd kms. You can move this sensor and zip tie it to the speedo housing but you cant just unplug it as it will result in the symptoms you've described.

Just a possibility.

Andy.

[_STAIN_]

lift the rocker cover and check the trip piece on the cam, for the cam position sensor. These fracture and eventually fall off.

[Moki]

Cheers everyone. I'll work through everyone's suggestions.

[roadracingoldfart]

Mate of mine has a '91 VFR750 (Jap import) that misfires under load above 5000rpm. Carbs have been off, cleaned and even had new stock jets in, but same-just misfires better. Anybody know it these are restricted? PO said it's done that during his time owning it. It's been to 3 bike shops and all are scratching their heads.

It's gotta be simple....and me thinks electric and not heat related. Stator? Is there a winding that pushes more current through when engine hits 5000rpm?

Runs sweet all over except when pushing hard above 5000rpm.

-stock carbs/jets
-stock pipe
-valves done
-all filters replaced.

Any ideas?

Thanks

Can you confirm its above 5000 rpm in every gear ?
If so i would as already suggested , try another CDI from a donor bike to assess any change.

Also , you say all components are original . Do you know this or assuming it as they all fit well and look like Honda parts, but , are they std to your bike / model ???

Paul.

[jellywrestler]

the problem isn't misfiring under load at high rpm, its actually cutting out on the rear to cylinders at moderate rpm, timing light shows this as clear as day, in the dark of course.
FYI he's already tried another CDI

[Warr]

CDI already checked .. mm
Coils, Pickups, Wiring be next then

[kwaka_crasher]

Both rear missing? Are they on one coil? (most 4-pot 4-strokes have one coil per pair of cylinders and fire every 360 deg. of the crank like 2-strokes to eliminate the need for a cam angle sensor). If so check the firing order - if the rear pair are opposite (like a firing order of 1-3-4-2 would fire 1 & 4 and 3 & 2 as pairs) and they're misfiring as a pair, start with the coil. If it's one cylinder only, work from the HT lead toward the suppressor cap & spark plug.

[kwaka_crasher]

Both rear missing? Are they on one coil? (most 4-pot 4-strokes have one coil per pair of cylinders and fire every 360 deg. of the crank like 2-strokes to eliminate the need for a cam angle sensor). If so check the firing order - if the rear pair are opposite (like a firing order of 1-3-4-2 would fire 1 & 4 and 3 & 2 as pairs) and they're misfiring as a pair, start with the coil. If it's one cylinder only, work from the HT lead toward the suppressor cap & spark plug.

Cancel that - I was away with the fairys and realised my mistake when I looked it up - it has a 180 deg. crank with shared crank pins on a VFR to make the irregular firing order, so has to be 4 coils. Firing order is 1-3-2-4 (#1 is left rear cylinder, #3 is right rear, #4 is right front and #2 is left front). It also has 2 pulse generators (hall effect sensors/triggers) for the ICM.

You really need to establish what cylinder/cylinders are misfiring and go from there.

If you're dropping two cylinders, chances are a faulty trigger or wiring to the ICM from the trigger - highly unlikely to be ICM or high tension side. Pulse generator resistance is 450 - 550 ohms @ 20 deg. C across each pulse generator wire pair with no continuity between the pairs. The pairs are either yellow & white/yellow and yellow & white/blue OR yellow & white/yellow and blue & white/blue but you can establish this by the lack of continuity anyway.

If you're only dropping one cylinder it likely won't be the low tension side, especially if you've already tried a different (known good) ICM. So work your way from the coil to the plug, inclusive. Swap bits about one at a time to see if the misfire moves with the change. Coil primary resistances are 2 - 4 Ohms @ 20 deg. C.

This is all assuming someone's checked compressions etc.

[Swoop]

...post...

Do you run a workshop or something? Wish I knew all that a few years back when a recalcitrant bike would just shut down after 5-6km's or so. It was one of the pulse generators. Took ages to find the problem.

[kwaka_crasher]

Do you run a workshop or something? Wish I knew all that a few years back when a recalcitrant bike would just shut down after 5-6km's or so. It was one of the pulse generators. Took ages to find the problem.

Ex car mechanic.