Nighthawk 750 high RPM miss - experts required - step right up!

mrwolf

mrwolf

live fast die young
hi all,

working on my latest project. before I tore into the bike I noticed a "miss" or "bog" above 5000 rpm, a sudden loss of power and a constant stutter or sputter, regular in its intervals. now i know these bikes are notoriously lean from the factory, so i put it down to the mains being too lean and got working on the bike.

a few months down the line, i finally got her up and running again. carbs have been rebuilt, synched & re-jetted, 132.5 on the outside cylinders and 135 on the inside. 40 pilots needle raised 2 shims and slide holes drilled.

bike runs much better, more responsive with more power and smooth through the rpm's. however, once i reach the 5-6k rpm mark, the same thing: significant loss of power and the infamous sputter.

i know this is not a fuel or carburation issue as I have run it with the choke, cleaned the fuel lines, got an in-line filter, ran it without the gas cap (vacuum) etc etc.

as electrics are not my strong-point, my only ideas are an issue with the pick-up, valve clearance, or spark plugs. i'm wondering if anyone can chime in here and give me some advice.

much appreciated and thanks in advance.

setup:
1992 Nighthawk cb750
4x K&N pods
Mac exhaust with Supertrapp header
Engine otherwise stock

http://www.dotheton.com/forum/index.php?topic=42280.30

please don't tell me to put my airbox back on! i'm not 12.
 
same thing happened when I worked on a kids nighthawk 900f I got everything running perfectly and it sputtered really bad around 5 and 6k He ended up moving away and taking the bike with him before I could fix that still wondering what it was. I'd assume something in the charging system or coils?
 
thanks for the reply. i'm going to start with some iridium plugs, hopefully that will atke care of it.

if not i'm going to get a new pick-up and see if that solves it. could also be timing, but since this bike doesn't have adjustable timing or points, i'm not sure if that could be it.

maybe even a rectifier/regulator issue. i suck at electrics..

anyone else have any ideas?

thanks
 
mrwolf said:
thanks for the reply. i'm going to start with some iridium plugs, hopefully that will atke care of it.

if not i'm going to get a new pick-up and see if that solves it. could also be timing, but since this bike doesn't have adjustable timing or points, i'm not sure if that could be it.

maybe even a rectifier/regulator issue. i suck at electrics..

anyone else have any ideas?

thanks
Click to expand...

Have you tried playing around with spark plug gapping?
 
I don't wish to seem insensitive but in your other thread you boast about your ability to set up CV carbs with pods. Did you try it with the stock air box just to see if that cleaned it out or was it just as bad before you started?

If you did and you have pulled the plugs to see what they look like, do you have access to a spare igniter box to swap out and see if that's the issue. Did you check battery voltage as revs rise to see if it goes up or down?
 
teazer said:
I don't wish to seem insensitive but in your other thread you boast about your ability to set up CV carbs with pods. Did you try it with the stock air box just to see if that cleaned it out or was it just as bad before you started?

If you did and you have pulled the plugs to see what they look like, do you have access to a spare igniter box to swap out and see if that's the issue. Did you check battery voltage as revs rise to see if it goes up or down?
Click to expand...

the bike was doing the same thing with the stock airbox as I had mentioned above. it was doing it when i bought it from the PO. it still does it with the stock airbox back on. as I also said, it runs A LOT better than it did before, rebuilding carbs and jetting. i guess so many people would love to believe its the pods. i'm replacing plugs this weekend and then going to check pick-up and voltage next.
 
Try a timing light (clamp on style) on one lead and rev the motor to say 5500 and see if the light breaks up ie is interuppted.
 
teazer said:
Try a timing light (clamp on style) on one lead and rev the motor to say 5500 and see if the light breaks up ie is interuppted.
Click to expand...

thanks for the feedback, will try that next. its a bit of a ball-ache to check timing on the newer nighthawks though as it spews oil all over the place.

dropped in the iridium plugs and it made no difference at all. also thinking the issue might lie with the pick-up, if thats faulty surely it would be more noticeable at higher rpm?

electrics=my nemesis
 
How did you check valve clearance? The engine has hydraulic self adjusters. There is no way to adjust the clearance.

The bike has electronic ignition. Usually they fail, not bad in one spot

So that leaves coils, plugs, and wires as a possible issue.

Have you done a plug chop?

When was the last plug change?




Sent from planet Earth using mysterious electronic devices and Tapatalk
 
mydlyfkryzis said:
How did you check valve clearance? The engine has hydraulic self adjusters. There is no way to adjust the clearance.

The bike has electronic ignition. Usually they fail, not bad in one spot

So that leaves coils, plugs, and wires as a possible issue.

Have you done a plug chop?



When was the last plug change?




Sent from planet Earth using mysterious electronic devices and Tapatalk
Click to expand...


thanks for the reply. yes by valve clearance is good i mean, it is what it is. so not really part of my troubleshooting process due to the hydraulic adjustment.

so you are saying its not possible for the alternator pulse generator to go bad during high revs? this was going to be my next hurdle to tackle, but if you suggest its either good or bad i might do something else.

will check the coils next. plugs are a nice tan on all four, changed to iridium yesterday, with no change to the high rpm bog.
 
kopcicle said:
you are 12

get some real carburettors and quit screwing around


~kop
Click to expand...

thanks for your comment. very constructive.

the issue is electrical and in no way related to the carbs.

also, i shuddered to even mention the pods as it always hijacks any thread. i have run pods on all my builds and with some patience and know how its always possible to make them work like a charm.

the very same carbs (and pods) were put on a friends bike, synched and it ran like a champ.

i'm replacing the pick-up next, hoping that will be it. timing is also in spec.

running out of things to diagnose, but thanks to all who have made suggestions please keep them coming.
 
teazer said:
I don't wish to seem insensitive but in your other thread you boast about your ability to set up CV carbs with pods. Did you try it with the stock air box just to see if that cleaned it out or was it just as bad before you started?

If you did and you have pulled the plugs to see what they look like, do you have access to a spare igniter box to swap out and see if that's the issue. Did you check battery voltage as revs rise to see if it goes up or down?
Click to expand...

what is your definition of boasting? i'm pretty humble to be honest. i only ever said that i got pods/stacks to work on every bike i have built.

i'm busy trying out a friends ignition control module or CDI to see if that clears up the issue. voltage is also good, this has me stumped. thank you for your help to date.
 
Didn't says pods don't work.

They require extra effort to work as well as they can.
They have less filter surface area then even the stock airbox, so higher restriction and lower performance.

So they work if you are wanting to look cool, but not if you want better performance.

They essentially say, I prefer "looks nice" to "works better"

Which always baffle me why people would do all that work for lower performance.

I also like to make sure that new people reading this understand that pods are more complicated than just tightening the clamps and replacing a jet or 2....

I would love to see a dyno run of a stock bike, stock airbox compared to the same stock bike with pods. My money is on "No one wants to know that pods decrease intake pumping efficiency"
 
It's not just that most pods hurt overall performance, but that they really hurt midrange riding because they don't track as well between circuits.

My bike has pods and CV carbs and it's pretty responsive and if i I didn't know better I would say it's fine. When I check the Air Fuel data it is clearly a long way from where it needs to be and after changing needles and needle jets and main jets etc, I'm just one step closer to getting it right and smooth across the power band/rev range.

Some carbs are close to impossible to jet with pods and others are quite easy. GSXRs run pods without much trouble, early Hondas not so much. On the RS100 race kit, Honda blanked off the Primary main jets completely to get them to run right. Their banking plugs were called #0 jets.

That said, Dynojet sells ( or used to sell) a kit for the early DOHC bikes kit 1127.002 so maybe they worked out a way to make them work. I have no idea what they are like at part throttle though.
 
teazer said:
It's not just that most pods hurt overall performance, but that they really hurt midrange riding because they don't track as well between circuits.

My bike has pods and CV carbs and it's pretty responsive and if i I didn't know better I would say it's fine. When I check the Air Fuel data it is clearly a long way from where it needs to be and after changing needles and needle jets and main jets etc, I'm just one step closer to getting it right and smooth across the power band/rev range.

Some carbs are close to impossible to jet with pods and others are quite easy. GSXRs run pods without much trouble, early Hondas not so much. On the RS100 race kit, Honda blanked off the Primary main jets completely to get them to run right. Their banking plugs were called #0 jets.

That said, Dynojet sells ( or used to sell) a kit for the early DOHC bikes kit 1127.002 so maybe they worked out a way to make them work. I have no idea what they are like at part throttle though.
Click to expand...

Ok it seems I have found the problem! Now on how to fix it. The timing is spot on, but it's advancing clockwise instead of anti clockwise! The PO must have fiddled with it somehow.

Any suggestions? Could it be that a damaged CDI (ignition control module) is causing this?

Reversed polarity on the pickup?

Wiring harness checks out otherwise..

Thanks
 
OK Sorted!

Runs beautifully now.

CDI (ICM) was faulty.

Will post pictures when she's done.
 
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