limited top end

[skids]

I have an 81H with an 80G engine in it. The bike has an XS750 final drive. Chrome megaphone-type Jardine 4-1 pipes, like new. Carbs are from a 78E. Airbox has a K&N filter, breathers hooked up to hole in box, box not drilled. I just went through the carbs although I didn't need to. The carbs are adjusted to 1.000 inches average height, 150 mains, 45 pilots, 210 air jets, needles on the center slot (soggy if raised one slot). Plugs are white on the porcelin and maybe just a slight carbon dusting on the ring - read lean. WHY will this bike only do 96 mph and is running out of poop with 150 mains? This bike will push 8000rpms in lower gears. PS, 6000 rpm is about 100 mph on this bike. WHY will this bike only do 96 mph and is running out of poop with 150 mains?
I could put in some 155 mains, but I think that is about as big as they go. What is up with that??? Something must be wrong.

[pggg]

Too rich, go to 140's, plugs always read lean at high revs

[mro]

Have little experience with jetting mods on a stock motor.
When's the last time you checked the valves or compression?

If not the carbs/motor then???
Would ride it for about 5 minuets and check if final drive is getting "hot".


mro

[DiverRay]

Skids,
If you have the '79 carbs on the later air box, did you drill out the carb breather tubes on the box? The '79 carbs vent through the top "T", and if they don't vent, you will run out of fuel as you try to work the engine.

[skids]

Yes, I have the breather tubes to a T and into the crankcase breather hole. They are vented.

Originally posted by DiverRay
Skids,
If you have the '79 carbs on the later air box, did you drill out the carb breather tubes on the box? The '79 carbs vent through the top "T", and if they don't vent, you will run out of fuel as you try to work the engine.

[skids]

I should check that final drive again... I think the compression is good, but I will be checking it. The carbs all react the same and are clean. I already tried lower number jets with the same loss of poop at the high end. I tried 145's. Maybe I need a higher float setting?

Originally posted by mro
Have little experience with jetting mods on a stock motor.
When's the last time you checked the valves or compression?

If not the carbs/motor then???
Would ride it for about 5 minuets and check if final drive is getting "hot".


mro

[DiverRay]

Float may do the trick. IIRC, mine is just over an inch, about 27mm?? I'm to far from home, and nine hours ahead of my normal time zone, so the brain is slightly fogged. ( could also be the 3,800 meter elevation I'm working at.)

[xschop]

elevation

hey skids, what is your elevation there?

[skids]

Its 5500 ft. elevation here.

[xschop]

kleen K & N

Check your air filter, but I would 2nd Pggg on richness. Your elevation is almost 4 times mine in OKC so your getting way less O2 and probably rich. My 2 pennies.

[skids]

Re: kleen K & N

I cleaned the filter just prior to the test rides, but I will check the compression and perhaps tweek the floats a little higher, then move to smaller mains. If all of that fails, I am blaiming the 4-1's!

Originally posted by xschop
Check your air filter, but I would 2nd Pggg on richness. Your elevation is almost 4 times mine in OKC so your getting way less O2 and probably rich. My 2 pennies.

[pggg]

My first 78E 1102cc, stock motor apart from drilled airbox, K&N, 4-1 - ran really well, screaming top end, good response and power right thru the rev range, carbs set with floats at 25mm, pilot air jets 180, emulsion tubes X2 266, needles 5GZ6 at centre notch, pilot jets 45, main jets 140 - 142.5

[skids]

Yeah, I have a 78 (the Beast) with the same get-up excpt that the mains are 147.5 and the pilot air jets are 210. I have only seen the 180's used in the later model carbs.

[oseaghdha]

Sum ting wong

Check your advance.
You can add 2 degrees for every thousand feet on a car, don't know why it wouldn't work for an XS to dial in some more.

What did it do before the 750 mod?

Stock will pull 90 in third, so you have 2 overdrive gears I guess.

[pggg]

These jet needles, both from XS11 carbs, 5GZ6 on left, other is 5Z1, notched the same, 5Z1 can flow more gas at WOT, worked better with smaller jets, I even looked in the XS11 manual, where it mentions 51Z7's and 5GL16's, the 51Z7 sounds close to that 5Z1