Poor performance above 4,700 rpm

[roloscot]

Hi,

Well the bike is back in one piece and running. The bike runs really well below 4,700 rpm but once you get above 4,700 rpm the engine runs rough and the engine loses power. However once you get about 5,800 the engine picks up again. I should also mention that it takes a while for the bike to start but it does eventually and it does not idle very evenly.

I sent the carbs away to be cleaned, new manifold rubbers, and the petcock is new and the line to the manifold looks like a tight seal. I am running stock air filters and 2-1 exhaust system. Points cleaned and static timing done.

Any ideas would be greatly appreciated,

Mark.

 

[reed]

this guy more or less tells for beginners but he has a carb section that explains what jets-needles powers what rpm. do a search for dans motorcyle repair I cant seem to post it here right now.

 

[roloscot]

Is this the reference you are referring too. I would guess that the needle jet needs to be cleaned would be my guess from reading the guy's description.

Mark.

http://www.dansmc.com/carbs2.htm

 

[bcware]

Your 2-1 exhaust is causing you to run lean I would guess. Is that the only modification you've done? It sounds like the transition from slide needle to full-on main jet is not smooth. Trouble at idle could also mean your pilot jets are too small.

http://www.factorypro.com/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

 

[roloscot]

Hi,

I think that you are right. I going to put back on the original pipes and see how the bike performs. If this smooths out the acceleration then I will proceed with experimenting with the carbs.

Thanks for the help,

Mark.

 

[drewpy]

don't think the 2 in 1 will be the problem, check fuel flow from the pet cock first.

 

[bcware]

Would fuel flow trouble only affect mid range rpms? I thought it ran well at low and very high rpms. Still, I am sure it is worth a look. Drewpy has more experience than me!

 

[drewpy]

Would fuel flow trouble only affect mid range rpms? I thought it ran well at low and very high rpms. Still, I am sure it is worth a look. Drewpy has more experience than me!

if fuel got low and picked up becaise of the time factor (the time to re-fill the bowls)

i had an issue similar and it ended up being the a bit of crud semi restricting the flow. I could rev and then it bogged then picked up when the revs went down.

Another thing my be the diaphram in the carb maybe stiff or torn or the person who cleaned the carb put the needle in the wrong position giving a flat spot.

 

[roloscot]

By the way, when I pull of the vacuum tube to the petcock when in Idle the Engine stops dead. Is this normal or is a clue to the problem?

Mark.

 

[drewpy]

probably not normal. the petcock it needs the vacuum to operate and you also weaken the intake at the same time.
mine came away whilst riding and took a few mins to falter and die as the remaining fuel was used up!

 

[bcware]

Yeah, even if you pull the vacuum tube off and stop flow there should be enough fuel left in the bowls to run for a little bit.

 

[ahale2772]

^ agreed, sounds like a float problem to me

 

[roloscot]

Before dismantling the carb, I decided to give the bike one more chance to behave and took it out for a ride. You know what it ran fairly well without a flat spot. So I have decided to employ the old saying, "if ain't broke don't fix it". So I will see how things progress over the next week. I have embarking on a 1000 mile round trip to West Virginia on the Yamaha, so we will see how things work out. I know for sure that my backside is going to be sore!!!!

Thanks for all the advice and guidance,

Mark.