wont rev above 4500

[DannyMotor]

So running kn filter aftermaket exhaust 115 mains stock pilots and screws are all standard setting... did carb balance, valves are good and needles are raised 1 ring.

No rev above 4500... just bogs. Rides great otherwise... just wishing I could use the entire rev-range.

Petcock is fine and fresh gas. It revved in the garage OK with 102. I changed to 115 thinking the hesitance to rev up high was in regard to being lean. Am I too rich?

The pilot air screws dont do a thing to the idle unless theyre screwed all the way in... so I left them at 1.25 turns.

 

[BassCliff]

Hi,

What does a plug chop tell you at WOT?



Thank you for your indulgence,

BassCliff

 

[DannyMotor]

Doing that in the am...

New development... its not the timing advance, everything moves freely. Also, got it up to 60 down hil... dropped it into second and let the motor engine break up to 7K rpm. Now the bike will rev straight up to 6K no issue. Its having a crap time on cold starts though. Really hard to get it cranked after cooling down about an hour or two.

 

[DannyMotor]

Also took the plugs out from today... theyre a rusty brown.

 

[tkent02]

If it ran better on 102.5, make a smaller change to see what happens... Maybe 107.5? Wait, are you sure it was even too lean with the 102.5?

 

[DannyMotor]

Well, with the aftermarket pipe and filter... I thought so.

But the bike had these same 'symptoms' where, in the workshop, it didn't want to rev very high, but I was accounting that to the 102.5 jets.

I'm going to do a compression test. I'm starting to get a sinking feeling I may need rings.

I have a set of 112's that I can try.

I tried unscrewing the Fuel screws to 1 full turn, and that didn't help much at all, it's almost like the bike is just running on the pilot circuit. Float levels are set to 26mm... and the carbs have been thoroughly boiled and everytime I take them off, I go through every jet/passage and hole and clean it.

I'm just at a bit of a loss...

I'm going to try the following tomorrow:

- Compression test.
- Install my second set of slides with thinner needles and spring/plastic washers
- Install 112 jets.
- Double-check petcock screens.
- New plugs
- Re-set the fuel and air-screws.
- Re-check carb balance.

If that doesn't work... I'm at a loss... is it normal for the Air screws to have NO affect on idle unless fully closed? There was no idle change when I performed a check on this earlier... but judging by how crappy it's running, it may just be masked by a larger issue.

 

[Dreef1999]

do the jets first.

 

[chuck hahn]

Read the symptoms part on page 18.....

http://www.vintagesnow.com/Carburetors_files/vmmanual.pdf

 

[DannyMotor]

do the jets first.

I just read this: http://www.iwt.com.au/mikunicarb.htm

So it's obviously my needle/main that are the issue. The bike only runs up to 1/4 throttle, hence why it bogs after 4500 rpm, because it IS just running on the pilot circuit.

So, I'm going to swap jets first.

Read the symptoms part on page 18.....

http://www.vintagesnow.com/Carburetors_files/vmmanual.pdf

By that logic, it's running too lean.... however, my above quote about the pilot circuit makes that hard to determine at the moment.

 

[chuck hahn]

Im not diagnosing anything..just happened to see this and passing on whatever may or may not be of value..yours to read and use if you can..

 

[DannyMotor]

I had the wrong needles... and the stock jetting is supposed to be 100. Throwing in the 102.5's and the correct needles.

God I hope this fixes it.

Also... lots of sediment in the bottom of the tank.... God damn it. More vinegar to the rescue.

 

[DannyMotor]

Looks like I have a set of 750 and 850 carbs... same bodies, different jetting and needles.

What fun.

 

[Flyboy]

Once again, here we go, someone asking for help and then ignoring good advice from those who dispense it.
Check Cliffs post, post #2 have you done a pug chop, as suggested.???????

 

[DannyMotor]

Yes... twas not ignored. All plugs were brown @ wot this morning...

just fiddled with the needles and the only thing that works well... not perfect.. is 102.5 jets with 5DL36 needles on the highest notch. Meaning sitting waaay low in the jet. I may try going to the 5F21 needles on the middle setting and see what that does. I was running them on the second clip from the bottom with the 115's and was experiencing the issues not revving over 1/4 throttle.

 

[DannyMotor]

Oh and did a second chop with new plugs after installing the 36 needles on the middle clip and they were soot black.

 

[Flyboy]

Ok then, sorry, didn't see any reference to the chops or feedback from them.
Sounds like you were almost there with the 36's on the top clip, and moving them to the middle clip gave you a rich cut.... mmmmmm.
What altitude do you live at? You know if you live high, like I do then it is not always necessary to rejet for an after market pipe, as the bikes a re jetted for sea level from the factory, which will cause a rich run at altitude, putting on a 4-1 brings it to where it should be with the stock jetting.
My bike, Jennifer has lived with a 4-1 and stock jetting for 25 years of her life and runs like a swiss watch, I live at 5500ft above sea level.
And to all those who are about to wave their fingers at me, tellig me what evils will eventually befall my bike, no, not she has done the better part of 375000Km like that, if it was going to kill her, it would have happened by now.
Danny, you may just need to find that sweet spot between the top and next groove, maybe try a thin washer as a shim under the clip to raise the needle a tad but not so much as all the way to the next groove.
I would suggest returning the carbs to bone stock jetting and configuration, do plug chops and work from there, small adjustments at a time, otherwise you are just bouncing around not really getting anywhere fast. My bet is it only takes a main jet change of one maybe two sizes bigger to get it screaming around the rev counter. Nothing more than that.

 

[platinum2]

what bike are you working on?, check the compression, does it bog through the air box? mains ,and pilots 2 small, if plugs are black /wet jets to big, to much fuel
you sure its not electrical?
did it just start doing this, or after messing with it?

 

[DannyMotor]

Welp... compression test looked grim. 85, 90, 82, 88... warm motor. Then I realized, I am a moron... and didn't open the friggen throttle during testing. Trying that again tomorrow. So, knowing that, hopefully they're in the +110~120 range.

After the bike warmed up using the 36's on the high ring with 102.5's... it went back to it's old habits... this time, rev'd up quick to about 5k... and then bogged.

Bogging through the air-box? No idea what that means...

Here's the exact description of what I'm experiencing..

- Drive around shifting before 4000rpm... it drives completely normal. Peppy, shifts nice... plenty of torque. Try to crack it past 1/4 throttle (up past 4000rpm) and it bogs... rip it all the way to full throttle, and it'll maintain the same bogging... not slowing down, but not accelerating.

Here's the exact setup: 78' GS750EC
KnN oiled drop-in, aftermarket exhaust w/baffle, gearing is 17/42 w/530 chain @ 2.47 final drive ratio.

Here's a run-down of BEST to WORST combos of jets/needle/needle settings:

102.5 / 36 / top ring : still bogs at 4500rpm (plugs brown)
112.5 / 21 / middle ring : almost identical to above in behavior (plugs brown)
115 / 21 / second ring from bottom : bogs at 4000rpm (plugs DARK brown)
115 / 21 / top ring : bogs at 4500 rpm (plugs DARK brown)
102.5 / 36 / 1 ring from bottom : ran horrible, barely made it home (plugs soot black)

Tomorrow, I'm trying the following:
102.5 / 21 / 2nd ring from top. This should be MUCH leaner than any of the previous combos...

I'm just surprised it's running SO lean with the aftermarket do-dads. Stock jetting is said to be a 100 main, so I guess 102.5 is the next logical size... I just wouldn't think I'd have to lower the needle at all.


-- And I just noticed (on my freshly painted motor) that my head gasket is weeping.

So, if my jetting debacle isn't fixed by the 102.5 / 21 / 4th grove... then I'm going to invest in a head gasket, base gasket, and rings.

... sigh...

 

[DannyMotor]

Heres a thought... could my timing be off??

Its very hard to start... choke helps... but it takes a lot of cranking and prayer.

 

[BassCliff]

Hi,

Yes, it could be ignition timing. It could also be valve timing. Have you had the cam chain tensioner off?



Thank you for your indulgence,

BassCliff