Topic: Problem revving out in 5th

[Retro Rocket]

three teeth smaller on the rear can cause a bit more load,what is the stock gearing meant to be?

I got him to put a 17 on the front {was 18}yesterday Dave and it quickened up the bike down low but did the same thing in 5th...

[dave500]

check if the rear is 45 but should be a 48 maybe?im not familiar with the various stock 750 gearings.

[scondon]

he's pretty switched on and has measured and tried a couple of different heights for the floats, i will suggest the clear line method and see how that goes. Wouldn't that effect revs in all gears though.? Or would the extra fuel needed for the load in 5th allow this to happen.?

The engine has to work hardest to spin the rear wheel in 5th gear, just like a pedal bike. One low fuel level won't be as apparent in lower gears because the engine doesn't have to work as hard to spin up the revs and suck fuel/air mixture relative to engine speed and throttle position. Same goes for needle height and main jet choice

His trouble might not be carb related, but I was just throwing that out there because I've had my share of carb tunes that pulled awesome through the first 4 gears, but hit the wall in 5th.

[Retro Rocket]

Thanks Scondon, i will get him to give that a try....

[TwoTired]

Hondaman reported that aftermarket float valves often had stronger spring pins, which ruin correct fuel level in the bowls even though the floats height were mechanically set correctly.

Has he replaced those float valves with aftermarket?

[BIGRUSS58]

 thanks for your help still running all standard floats

[BIGRUSS58]

Thanks Scondon, i will get him to give that a try....

thanks for your help still running all standard floats

hi guys russ here mick retro rocket been helping me out , i did replace the needle and seat with after market ones , is this what your are referring to as  float valves

[BIGRUSS58]

Confirm Sprockets as a 18/45 is taller than any OEM stock. Dropping the front to a 17 just makes it equivalent to OEM 18/48. But this can't be the root problem.

The bike in stock trim should have no problem pulling redline in 5th, so before applying bandaids I'd return it to pure stock, check the compression, timing, then cry, pull hair out etc.

Oh, I see you've done compression. 130 is a little low IMO. 160-170 would be better, but still 130 shouldn't be bad enough to limit top end. Just not crisp. I wouldn't expect your symptoms till 110 or lower.

From what I've read if all else is stock as a board, I have to suspect the Dyna. When they go bad they can be very fickle, no logic to the failure symptoms. Get a points plate and try it then.

did it with points before i went to dyna ignition

[BIGRUSS58]

In all seriousness, how's the leakdown?  Compression is only half of the equation.

Cam gear could be off a tooth.  That would be a pretty extreme case, but if the cam was that far advanced he'd have no power at higher rpm's.  Speaking of the cam, if a lobe went bad he might be down on enough power to not be able to wind it out in top-gear.  Are all the exhaust pipes reading the same temp with an infrared thermometer?

Leak down hasn't been done as far as i know, bike runs well in lower gears so i doubt the cam is out a tooth, not sure about exhaust temps. One thing i do know is that the bike has been like this since he bought it, he has been right through the carbs multiple times, checked out the breather in the tank, checked fuel flow and replaced most of the electrical components with new, still has the same problem. I suppose the top end may need closer inspection..
Thanks for the input Dave...

went and bought a leak down tester today as soon as it gets here i will post the results , thanks for the hint  russ

[TwoTired]

Thanks Scondon, i will get him to give that a try....

thanks for your help still running all standard floats

hi guys russ here mick retro rocket been helping me out , i did replace the needle and seat with after market ones , is this what your are referring to as  float valves

Yes, that is what I was referring to.  Do you still have the old ones to compare the spring pin rebound strengh?  Or, do a bowl fill level test with a clear tube run up the outside

[dave500]

ill get flamed here,the float height isnt dead critical,,what happens inside the bowl of fuel when your accellerating or cornering or going up or down steep hills and rough roads,,its all over the place inside like a washing machine.

[BIGRUSS58]

ill get flamed here,the float height isnt dead critical,,what happens inside the bowl of fuel when your accelerating or cornering or going up or down steep hills and rough roads,,its all over the place inside like a washing machine.

 hi mate yeh Ive got a few sets of carbies it runs smooth all the time, that's the part that's  annoying,Ive done the plastic tube bit and set the floats at 24mm and 26 mm and still no difference, im starting to think it may have a worn cam lobe with the infra red temp gauge one does run slightly hotter than the rest by about 5 degrees

Russ

[BIGRUSS58]

Hondaman reported that aftermarket float valves often had stronger spring pins, which ruin correct fuel level in the bowls even though the floats height were mechanically set correctly.

Has he replaced those float valves with aftermarket?

yes i did replace them with aftermarket ones

[Doctor_D]

ill get flamed here,the float height isnt dead critical,,what happens inside the bowl of fuel when your accelerating or cornering or going up or down steep hills and rough roads,,its all over the place inside like a washing machine.

 hi mate yeh Ive got a few sets of carbies it runs smooth all the time, that's the part that's  annoying,Ive done the plastic tube bit and set the floats at 24mm and 26 mm and still no difference, im starting to think it may have a worn cam lobe with the infra red temp gauge one does run slightly hotter than the rest by about 5 degrees

Russ

5 deg F isn't that big a deal and could be a minor synch or mixture issue on that cylinder, but it's hard to know if it's done at idle.

Its better if you can check the temp under load.  This is best done on a dyno.  The poor man's way to it is to run the bike in high-gear on the center-stand, and drag the rear brake a little. Keep the rev's at 3k as you apply a bit of foot brake.  Have someone else run the thermometer while you're doing it.  I want to stress that you don't need (or want) to do drag the brake for long - 15-30 seconds is sufficient to get a temp rise at the exhaust.

As for a cam lobe, look at how many threads are showing on each of the valve adjustment screws.  If there's significant wear to a lobe, AND it's not making a ton of racket, one of them will be screwed in quite a bit further.

[Spanner 1]

I think it's a slow fuel supply..... normally you don't spend much time in 2, 3, and 4 th. gears. Just run up thru' them to get to 5th. and so don't run the fuel levels down far enough to effect acc. in those gears.
 I guess the test would be to hold the bike in, say, 4th. @ 4-5K rpm for some distance and see if the fuel levels/fill rate can keep up.....

[MCRider]

I've read most/all of this thread, but may have missed the answer to this question: Has the bike, in its current trim, ever been able to pull redline in 5th gear?

If so, what has changed immediately before the symptom?

[754]

 And was it apart before you got it, and are you checking it in RESERVE position..or have you infact checked fuel flow LEAVING THE TANK?

[MCRider]

You know Spanner and 754, you may be on to something. happened to me many moons ago. Fuel tap was clogged. ran fine around town, get it up on the interstate and ask for some top end wasn't there. Frustrating.

Or could be the Dyna... 

[TwoTired]

ill get flamed here,the float height isnt dead critical,,what happens inside the bowl of fuel when your accellerating or cornering or going up or down steep hills and rough roads,,its all over the place inside like a washing machine.

I'm not flaming or insisting that the bowl fuel level is the problem.  I will explain that the fuel must be lifted to the carb throat from the bowl, and the longer the lift the more force is needed to lift it.  Further, the amount of force needed, relates to the volume of fuel that can be lifted.
This is why setting fuel level in the bowls has an optimum fuel metering.

I'd still like to see pics of the spark plugs after a plug chop during the exhibition of the problem.  A lean mixture should show if fuel starvation is the fault.
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It could also be fuel tap restriction. Or, fuel line restriction.

One way to tell, is to do a variation of the plug chop, only with the fuel valve.  Get the machine to exhibit negative symptoms, turn off the fuel valve, then hit the kill switch, and pull in the clutch.  Coast to a stop and pull off the fuel bowls to check fill level.  If the fuel level is below the point where the pilot jet tube extends, you've found the problem and can chase down the source.

Which reminds me...
Some pilot jet aftermarket parts distribute a pilot jet physically shorter than what was stock length (28mm O.A.L., I think.)
The bowl fuel level could be low under high power demand situations, and that would surface as a fuel deficiency, even though such a small pilot orifice is providing little mixture augmentation.

The pilot jet tube inlet is normally positioned high in the fuel supply than the main jet.  But, how high is significant.

If the fuel valve needle spring pins are stiffer than stock, the bowl fuel level will be low.  If the pilot jet tube is shorter as well, you have a recipe for fuel starvation under high fuel flow demand situations.
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Or, it could be that the the coils are not recharging fast enough to provide reliable spark.  But, then I'd expect sputtering/engine misses near the transition point between running well and a power level plateau limitation.   Spark gaps in high pressures under max load in the engine cylinder do require more spark voltage.  Have the stock coils been tried?
Just an idea...

[dave500]

has it got a slotted cam gear and its slipped maybe?years ago i had an old side valve briggs and stratton mower that had soggy power which got worse to the point it would hardly rev above idle,turned out the inlet lobe had worn down so much it had about 1 mil lift,it started first time and idled great.

[MCRider]

has it got a slotted cam gear and its slipped maybe?years ago i had an old side valve briggs and stratton mower that had soggy power which got worse to the point it would hardly rev above idle,turned out the inlet lobe had worn down so much it had about 1 mil lift,it started first time and idled great.

The slottted gear slip is a possible if it's there. Gotta ask, butdoesn't soundlike it.

AS to the lobe wearing, never seen one wear that hadn't already chewed up the aluminum cam bearing, much softer and vulnerable. Usually resulting in a heck of a racket and /or seizure.

Still, gotta ask i realize.

[knowsnothing]

I may have dreamed this up (so be nice), but I think i might have read somewhere on here about a bike that had a similar weird problem.

Because of the position of the overflow tubes,  above a certain speed the pressure of the air flowing up the overflow tubes was causing a back pressure that was preventing the bike from reving past a certain point.  I think all they did was move the tubes a little or something.

Again, be nice 

[BIGRUSS58]

And was it apart before you got it, and are you checking it in RESERVE position..or have you in fact checked fuel flow LEAVING THE TANK?

hi mate yes i did that i answered an another guy just before about the flow rate . thanks for the input
russ

[BIGRUSS58]

And was it apart before you got it, and are you checking it in RESERVE position..or have you in fact checked fuel flow LEAVING THE TANK?

hi mate yes i did that i answered an another guy just before about the flow rate . thanks for the input
russ

i will move the pipes to a different location .you are right they should always face backwards, im pretty sure mine face the right way but i will look at it again

[BIGRUSS58]

ill get flamed here,the float height isnt dead critical,,what happens inside the bowl of fuel when your accellerating or cornering or going up or down steep hills and rough roads,,its all over the place inside like a washing machine.

I'm not flaming or insisting that the bowl fuel level is the problem.  I will explain that the fuel must be lifted to the carb throat from the bowl, and the longer the lift the more force is needed to lift it.  Further, the amount of force needed, relates to the volume of fuel that can be lifted.
This is why setting fuel level in the bowls has an optimum fuel metering.

I'd still like to see pics of the spark plugs after a plug chop during the exhibition of the problem.  A lean mixture should show if fuel starvation is the fault.
...
...
...
It could also be fuel tap restriction. Or, fuel line restriction.

One way to tell, is to do a variation of the plug chop, only with the fuel valve.  Get the machine to exhibit negative symptoms, turn off the fuel valve, then hit the kill switch, and pull in the clutch.  Coast to a stop and pull off the fuel bowls to check fill level.  If the fuel level is below the point where the pilot jet tube extends, you've found the problem and can chase down the source.

Which reminds me...
Some pilot jet aftermarket parts distribute a pilot jet physically shorter than what was stock length (28mm O.A.L., I think.)
The bowl fuel level could be low under high power demand situations, and that would surface as a fuel deficiency, even though such a small pilot orifice is providing little mixture augmentation.

The pilot jet tube inlet is normally positioned high in the fuel supply than the main jet.  But, how high is significant.

If the fuel valve needle spring pins are stiffer than stock, the bowl fuel level will be low.  If the pilot jet tube is shorter as well, you have a recipe for fuel starvation under high fuel flow demand situations.
...
...
...
Or, it could be that the the coils are not recharging fast enough to provide reliable spark.  But, then I'd expect sputtering/engine misses near the transition point between running well and a power level plateau limitation.   Spark gaps in high pressures under max load in the engine cylinder do require more spark voltage.  Have the stock coils been tried?
Just an idea...

thanks for your help . i will try all the things you have suggested and get back to you and send some pics of plugs , i have done it but not exactly the way you told me the plug was black on the threads and white on the electrode part and porcilin was light brownish

cheers russ