No Power above 4000/5000 rpm

[roloscot]

Hi,

I am not sure if anyone can help but I am extremely frustrated because I cannot seem to fix this recurring problem with my Yamaha xs400D 1977. The problem is that I have no power above 4000/5000 rpm. When I open up the throttle the bike nearly goes backwards and I get uneven power delivery as well; it feels like that there isn't enough fuel. Below 4000/5000 it seems to run fine and idle isn't too bad either.

This is what I have done so far:

- Two into 1 exhaust
- Cleaned the carburetors out (sent away to do this)
- New main and pilot jets. (2 sizes up from standard)
- Float high set to 28 mm
- Air screws set to 3 turns
- Running Pod filters instead of the standard air filter
- New manifold intakes
- Points adjusted and timing adjusted
- Inlet and Exhaust valves gaps adjusted
- New timer advance

I should also mention that there is a backfire when at idle and one of the cylinders seems to running hotter than the other cylinder.

Any help great appreciated!!!!! Ahhhhhh

Mark.

 

[eric123]

Are the carbs sync'd? What do your plugs look like? Can you do a plug chop at 4K/5k RPM's and report the results? Gas tank vent clear/ fuel filter clear?

 

[roloscot]

Hi,

Thanks for the reply. The carbs are sync'd and seem to be ok. I will check the fuel filter. How do I check the fuel filter and what is a plug chop?

I check the plugs and one cylinder seemed fine and the other seemed to be running a little rich.

Mark.

 

[Tricksta229]

Mine did this the other day... and then I realized the choke was on.. ha ha

 

[akronevan2]

I, too, have the same problem. I noticed that my right exhaust pipe is blue and the left seems to be normal.

 

[thlillyr]

I had this same problem. I fixed it by rejetting. Turned out some dork had put 95 jets it. The stock is 125. If you have a k&n filter you should do 135. Now it runs perfectly from 2000 to 10,000

 

[OG-Rider]

These engines seem to work best if you keep the revs up. Putting around at 2000-3000 rpm's a lot causes them to load up. When I first got mine, it was so carboned up that it would fall apart above about 6500. I went out on a favorite back road and ran it above 5000 rpm for about half an hour, and it cleared right up. Been fine since then.

 

[drewpy]

These engines seem to work best if you keep the revs up. Putting around at 2000-3000 rpm's a lot causes them to load up. When I first got mine, it was so carboned up that it would fall apart above about 6500. I went out on a favorite back road and ran it above 5000 rpm for about half an hour, and it cleared right up. Been fine since then.

ha ha its called an Italian service!

 

[Scorpio1963]

I would try turning the air screws to 2 1/2 turns it seemed to help my bike run better.Check and make sure your spark plugs are properly gapped.Also have you rebuilt or cleaned out the petcok on the tank?