RPM troubles around 4000

[dwaynemcfarlane]

Mikuni BS38 xs650D 77

RPM troubles.
I had problems with my rpm in idle and when driving and pressing the clutch.
In idle the carbs more or less had its own life. So i changed the jet pilot, went from 25 to 50. It seemed to help. But when driving in any gear and pressing the clutch the rpm had a tendency to rise.
The clutch had been a problem for a while so I kind a fixed the clutch. Loosened the nut and adjusted the screw etc. well that worked out fine. The RPM started to drop nicely when pressing the clutch. ( it still can be better i recon ).
Now to the new problem and the main reason for my cry for help.
When driving and i hit ca 4000 rpm the engine rev ( is that the right term ?) and wont continue to “accelerate”. Is this because i have to change the main jet or?
Im a newbie so be kind

Sincerly Dwayne

PS
I think ive ruled out air leaks.
Fully cleaned carbs adjusted floats to 24mm etc etc.

 

[5twins]

First of all, changing the pilot from a #25 to a #50 is a HUGE increase. You shouldn't need that even with mods. Usually all that's needed is a one or two size increase. In your case, that would be a 27.5 or 30. You shouldn't need to re-jet at all unless the bike has been modded. Has it been? Do you have different air filters and exhaust?

If the bike is still stock then the issue may not be carb related. It could be your ignition advance rod sticking. That would cause the RPMs to hang high because the ignition is sticking in the advanced position. The advance rod has to be kept lubricated so it can turn freely between the retarded and advanced positions.

 

[grizld1]

According to a 2018 poll conducted by ASS (American Society of Shadetreemechanics), 80% of assumed carburetor problems were caused by ignition defects, 5% were caused by valve train defects, 6% were caused by inadequate compression, 7% were caused by bad fuel, and the remainder were caused by defective carburetors.

 

[dwaynemcfarlane]

Aahh THX ! Ill lubricate and try 30s tomorrow!!!
If that wont work could it be that main jets are the problem when hitting 4000 Rpm?


I bought the bike in very bad condition. I had to disassemble the piston head and change the rings etc. I have powerfilters and standard exhaust i think.

 

[joebgd]

I think these guys and I agree are saying look elsewhere than carbs 1st, are you riding or just revving while on stand? Are both plugs getting spark, are both cylinders fireing?

 

[dwaynemcfarlane]

Im riding. Both plugs are getting spark and both cylinders are fireing!
But i installed boyer bransden ignition not that long ago and i never thought that it would be the problem. Tomorrow ill recheck retarded and advance position and grease the advance rod.
What grease would you guys recommend?

 

[grizld1]

No advance rod is used with any Boyer ignition. The OE rod is replaced by a length of allthread that mounts the trigger magnets and is locked in place in the camshaft, no lube required or recommended. Spark advance is controlled electronically. After installation timing at full advance should be checked with a strobe.

 

[dwaynemcfarlane]

Ok so the advance rod is ruled out!
I guess i have to use 80% of my troubleshooting on getting my timing ignition right? And ofc install some smaller jets.

 

[joebgd]

Just trying to get an idea of what's going on.
When you are riding going threw the gears once you get to 3rd or 4th gear lets say, it stops accelerating at 4000 rpm? The one thing I might check on the carbs is float height maybe. Changing jets isn't the answer it would rev right up to red line w stock jets no problem. Unless the carbs are dirty maybe semi clogged jet. I'm not saying never change jets it might run better with tweeking jet sizes and mix screw but let's get it running properly first. I think I would maybe stick with close to stick size jets until this is sorted out. The only time I had a similar problem I was running off one cylinder, it my case it was due to a carb not getting fuel.

When it is time to tweak the carbs for max performance a size or two at a time is plenty when changing jets.

 

[JRay77]

I agree. A lot of issues are in ignition and charging. Back to the carbs. The only 100% way to tune them, AFR gauges. Making slip ons with welded in sensors bungs. Then ride thru the gears and make sure the ratio is close or stays in the neighborhood. Idle doesn't dictate shit.

 

[joebgd]

I agree. A lot of issues are in ignition and charging. Back to the carbs. The only 100% way to tune them, AFR gauges. Making slip ons with welded in sensors bungs. Then ride thru the gears and make sure the ratio is close or stays in the neighborhood. Idle doesn't dictate shit.

That sound interesting, where do the sensors read from? Can you post a pic or maybe start a thread on AFR s.

 

[5twins]

You haven't told us if the bike is modded. Did you change to a freer flowing exhaust and air filters? If yes then yes, the main jet could be a problem if it's still the stock size. The '77 carb set comes with very small mains stock. They're fine if the bike remains stock but are quickly outpaced if you start doing mods.

 

[dwaynemcfarlane]

Installed new air filter pods and i just changed the exhaust also. The bike is “modded” since i bought it as a piece of junk. Ive reassembled the cylinder head replaced the oilrings and new caskets etc etc.
Its i mostly in 3rd 4th and especially in 5th gear it acts up around 4000 rpm.
I did a full carb cleanup. I cant imagine any dirt surviving that

I really appreciate the help!!

 

[hovel]

In the higher gears, the engine is under more load and is experiencing a system failure as a result. If it were mine, I'd put fresh clean plugs in it. Then run it up to 4 grand in 3rd or 4th where it starts to crap out. Pull in the clutch, cut the throttle, kill the ignition and pull the plugs right there on the spot. This is called a "plug chop". Look at the color of the porcelain around the center electrode. Tan to chocolate brown is good, white is lean and black is rich. Report back.

 

[5twins]

I would check your main jet size. Like I said, the '77 carb set came with quite small mains, 122.5's, and in Europe it could be as small as a 117.5. These are quickly out-paced by mods and need to be increased. Now, on most of the 650 carb sets, usually all you need for "pipes and pods" is one to three sizes bigger, but your set may require a bit more, like 3 to 4 sizes up. I would get some 130's and 132.5's. I did up a set of '76-'77 carbs for use on my '78 and that's where I ended up. I have basically the same "pipes and pods" mods like you. My final settings were 132.5 mains, 27.5 pilots, and the needles leaned a step to clip slot #2 (from the top).