wont rev above 4500

[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

[DannyMotor]

Nope. Have not fiddled with chain tensioner or cams. The timing advance is working, but I dont think its advancing enough... could explain why so many different jetting combos are leading to the same result.

[platinum2]

sounds like a kinked fuel line to though its letting in just enough to keep her going until you "step on it"
maybe check the fuel for kinks, and dirty lines, also!- under the needle and seat in carbs- there is a screen they ALWAYS collect dirt and rust from the tanks! mine did do this! and fuel wouldnt keep up, hope this helps

[DannyMotor]

Tank and screens have been gone through. fule line is new and unkinked with an inline filter and needles and seats are new.

[allojohn]

You probably don't want to hear this but are you sure the carbs are clean. I know you boiled them but maybe 24 hours in Berrymans is what they really need.
Edit - I've seen plenty of post about inline filters restricting fuel flow.

[Steve]

One other thing that I noted.

Float levels are set to 26mm...

Stock float height for the '78 750 is 23.0 mm +/- 1.0 mm.

That higher number actually lowers the fuel level, making everything run leaner than it should.
You will be chasing your jetting all over the place until you get the float level set correctly.

.

[BassCliff]

Hi,

Somehow I missed the float height spec and inline filter. Good call, guys.

Remove the inline fuel filter and set the proper float height. Then let's see what happens.


Thank you for your indulgence,

BassCliff

[chuck hahn]

Just got home and yup I agree with the ditching the inline fliter..if the tank screen is clean and good then that really is all you should need. And they are spot on about the float hghts too. And be sure that you measure WITHOUT the bowl gasket installed. Bare metal from the little raise edge to the top of the FLOATS..as in BOTH of them in each carb. Such as this...

[DannyMotor]

Crap crap crap crap crap.....

Specs:

Float Heights = 23mm perfectly
Main Jet = 102.5
Needle = (the fat original one) 2nd grove (from bottom)
Fuel line = new clear tube, no filter
Fuel screw = 3/4 turns out from soft stop
Air screw = 1.25 turns out
530 chain, 110 links, 42 rear, 17 front, 2.47 final gearing

Double checked tank and petcock... all's well in operation. Fuel flows freely on all settings (with vacuum on RES and ON of course).

Timing is set dead-nuts .012" points gap... timing is set to F on 1&4 and 2&3 perfectly. (This was off from before... it's slightly more advanced than previously, and the gap is down from .016".)

Bench synced the carbs 100%.

---------------

The motor was cold (well, cool) and I re-installed everything... airbox, yadda-yadda.. turned it to prime, gas over-flowed from the carb tubes (which hadn't happened at all before) I put it on choke and it fired right up... so we're at a good place there.

---------------

Took it for a ride.

MUCH smoother than before... however, 4000rpm, it bogs still...

Through the gears: It'll hover at 4000rpm, try to crack the throttle more and it just bogs, not decelerating, just not accelerating (but it's acting like it wants to). Once you get into 5th, it'll hit 70mph at about 4200rpm, just cruising at about 1/4 throttle... try to go more and you can feel it want to, but it still stumbles and won't accelerate.

-----

SO, I tried this.... Because I wanted to see what would happen.

When it started to bog, I pulled in the clutch... VROOOM... it revved right up to redline no issue, just like in neutral in the garage.

Let the clutch back out... and it was back to bog-ville. Prior to all the above changes it WOULD NOT rev up like that at all... not even in the garage.... not that smoothly anyway.

-----

Another test: I took it down a hill.... 5th gear, 65-70mph and just cracked the throttle to WOT. It maintained, but didn't enjoy it.... once the ground leveled out the bike bogged ALL THE WAY to 3000rpm, throttle input did NOTHING, once it hit about 2900rpm it picked back up to 4000rpm and continued on it's bogging ways.

-----

So in closing, it's still too rich. I'm going to drop the needle to the middle position and see if it improves. It feels like it's RIGHT THERE...

---

Current concerns still bugging me:

1- Could the super high gearing be causing this? (the 530 42 rear is about 2.5 inches SMALLER in diameter from the 630 41 tooth stock)
2- I'm using the same spark plugs through all of this. Would a new set help?
3- I got a mild zing from touching one of the plug boots while running, could this cause a loss in power like this?
4- What was the stock position of the needle? Middle? or 1 up from middle?

If I say anything that gives any of you an "ah-ha!" moment... please let me know. At this point, I'm going to continue fiddling with the needle height... and THAT'S IT. If that doesn't fix this, I don't know what will.

The fact that it even revs to redline that easily has me thinking I'm on the right track...

[kjsamm]

That sounds like a Nettle Jet problem. Not Jet Needle.

[DannyMotor]

Wait... what? You think I should up-jet, or down-jet?

Or are you correcting my terminology?