Not reaching top speed

[Bjorn]

Never realised I wasn't reaching top speed until i used my GPS last year. Turns out that my bike for the last several years has not been able to reach what i believe is its top speed. It tops out at 125 kmh (77 mph). I always though my tacho was off and thought the needle just didn't pass 5k RPM. However now I realised the bike isn't going over 5k RPM

I dont want to set new land speed records but i do want the bike to operate as it should. Below are some details about my set-up and all the things i have tried so far to figure it out.

PS I have been using the awesome checklist posted in this thread : http://www.xs650.com/threads/low-top-speed.46070/

My bike:

• 1980, however not original.
• 650cc
  
• TCI ignition
• 18'' rear wheel
• 2 in 2 high pipes (scrambler style), Heiden tuning megaphones
• bs34 carbs, large K&N filters
• main jet 145
• pilot jet 45
• I live at sea-level

Things I have tried so far:

1. new plugs
2. checked the cam chain tension
3. set the valve play
4. changed the oil and filter
   
5. compression check cold/no oil/ WOT: left and right around 125 psi.
   
6. checked the brakes if they were dragging, they are not.
7. pumped up the tires to >32 psi
8. checked if the drive chain was not to tight, its not
9. throttle cable is fully opening
10. put some good fuel in the tank
11. checked for holes in the diaphragm by putting my finger over the oval hole in the carb, the go down very slowly.
    
12. While stationary, if I rev the engine it zips up to the redline.
    
13. I removed the air filters to see if the slides fully raise when stationary, they do.

Things im not sure about, I've got a 17t front sprocket and a 31t rear. I know that the rear one is not original. Could this be robbing me of power? I see this combination rather often at the forum, not sure if its geared too tall?

something else i worked on is the carb settings. I red the carb guide again, and realized that >5k RPM is mainly the main jet. I changed form 145 to 140 and did a test run. Now i was able to get up 132 kmh (82 mph) @ 6k RPM. is was in 4th gear. if i go to 5th i cant go faster.
Today i tested some WOT runs in 3rd gear. from 5 to 6k RPM takes some time, then i got it very slowely up to 7k RPM. maybe try a smaller main like 137,5?

sorry for the long read but maybe you guys have an idea where to take it from here

 

[Jim]

My guess is it's a combination of compression that's on the low side and too tall gearing.

Also... if you're running the pleated K&N filters... that ain't helping. They don't play nice with the CV carbs.

 

[Bjorn]

I just redid the compression test since the bike was still warm.

RHS: 140 psi
LHS: 140 psi

I don't really have an reference, what numbers should I be aiming for?

 

[Jim]

140 is pretty good. I think 150 is optimal. I think you'd see a big improvement if you bumped the rear sprocket to a 33T.

 

[TwoManyXS1Bs]

Bjorn, about 1/3 into your video, when the engine revs up and the slides rise, you'll see a white fuel-laden fog that spits back out of both carburetors. Classic sign of exhaust reversion traveling thru the intake valves during overlap.

Wrong exhaust pipe geometry.

Post #12.
http://www.xs650.com/threads/lets-t...-bogging-and-back-pressure.27927/#post-278859

Do a forum search on "reversion", read them all...

 

[Jim]

Guess I shoulda watched the video....

 

[650Skull]

 

[Bjorn]

Lol that's my bike from when I just build it. I lost the login for my old YT account. Ill make a new video about the current exhaust design.

Btw great info on that reversion. Ill dive into that the next couple a days.

 

[650Skull]

Yes, taken from your old thread and a still to show the size of the header pipes, for a visual as a guide for reversion inserts as per 2m's suggestion.........what is the ID

 

[Bjorn]

Reporting live from the shed. I just measured the length and diameter of the pipes.


It's not close to the mm but you'll get a rough idea.

• RHS pipe incl. Muffler: 137cm (54")
• RHS pipe ex. Muffler: 90cm (35")
• LHS pipe incl. : 140cm (55")
• LHS pipe ex. 93cm (37")

The inner diameter measured at the header: 37,50mm (1,476")

The pipe is completely welded with backing gas, so no colllyfowers on the inside hehe.

I was thinking about the air filters, would a test run w/o filters give me an indication of they are part of the problem?

 

[Bjorn]

Btw these are the baffles I'm running inside the megaphones. Bought these from Jerry vd Heiden from Heiden Tuning. He recommended these since they should work well with this engine.

 

[TwoManyXS1Bs]

See what happens if you remove the megaphones...

 

[Bjorn]

They are welded to the pipes...

I can try to take the baffles out of the megaphones

 

[joebgd]

I am over 200 pds n I run a 30 tooth in the rear no problem. She would go up to red line straight away while riding if I wanted. I think it is something else.

 

[TwoManyXS1Bs]

Well, I'm fixated on the "hovering fog".
Air/fuel should always be going *into* the engine.
So, something is causing it to be spit back out...

 

[Bjorn]

Totally agree thats not how it should be haha. I'm gonna try the easiest first. See what happens.

@Joe, good to know the bike should be able to move my 170lbs

 

[Ceed Klumper]

Can you alter the ignition timing on this TCI?
The fact that it easily goes to redline in neutral has me puzzled. Then I wonder about the fuel delivery. If you can try with a set of stock exhausts that would be great.

Succes ermee!

 

[Bjorn]

As far as I know the TCI can't be changed. If I still had the original pipes I would try them! But I threw those away like 8 years ago

 

[5twins]

Even though your headpipes are only 1.5" I.D., AR (anti reversion) inserts may still help. A larger diameter pipe works OK down away from the head but up at the head to pipe connection, these motors like a smaller pipe diameter. Too bad you threw away your old stock pipes. AR inserts can be made from the ends .....

They improved the way mine revved out.

 

[Bjorn]

I got a lathe hehe. I'll make some new ones