Stumbly Bumbly

[ninskrillz]

Hey guys

Hoping to get some help with an issue with my latest xs650 project.

I just got my bike on the road after working on it for about 8 months. Starts easily, idles great and pulls strong! Up until about 4-5k rpm that is! Then it stumbles and sputters until eventually gets through it or I upshift and its smooth again.

Although I don’t know the year of the motor, I bought it from a fellow who had it built with a 277 rephased crank, shell #1 cam, and 700cc pistons. Included with the motor was a set of later model BS38 carbs and a Vape/powerdynamo ignition. I have since replaced the carbs with some VM34s.

I feel like I’ve done everything I can to solve this high rpm stumble to no avail. I’ve hosed the thing down with WD40 to check for air/exhaust leaks, changed jets, sync’d and set carbs, changed plug wires, resistor caps with regular plugs, regular caps with resistor plugs, changed carbs, adjusted timing and nothing seems to have taken care of the issue.

Compression was 148/150 and other than the high rpm stumble I’m happy with the performance. I’ve never used a CDI and I’m pretty sure that this is an electrical/ignition issue.

Any help is greatly appreciated!!

 

[5twins]

Well, it sounds like the upper midrange is too rich and the bike is struggling to make the midrange to main transition. The usual "fix" is to lean the needles a step and/or reduce the main jet size.

 

[ninskrillz]

Well, it sounds like the upper midrange is too rich and the bike is struggling to make the midrange to main transition. The usual "fix" is to lean the needles a step and/or reduce the main jet size.

right on - I’ll give that a try.

I’ve only changed the pilots and mains haven’t messed with needle at all yet.

One of the reasons I lean towards it being electrical is that it persisted with new carbs.

 

[Jan_P]

I would check the ignition installation wring try to contact Vape/powerdynamo
This can be a case of electromagnetic interference spikes into the ignition Brushes sparking ... and then difficult to find.
Shielding / Separating

 

[grizld1]

Can't comment on VM34 carburetion without a complete jet list, but you're probably right that the problem is electrical. Sorry, can't help with that, no experience with Vape ignition and alternator.

 

[5twins]

Well, it's common to have to lean the needle setting on just about any carb once you start increasing mains and get more than a few sizes up. If your BS38's had larger mains installed then they probably should have had their needles leaned a step too.

 

[JRay77]

Isn’t there a wire on the Vape system that is critical to keep within a certain length for voltage drop? If memory serves I had to move components closer for this reason. It’s in the directions. Vape is a pain to time. Reason it’s a pain, taking off the rotor every time. I was wondering if it’s worth the money. Maybe I’m used to the banshee swap charging system with the Boyer or Pamco ignition.

 

[ninskrillz]

I would check the ignition installation wring try to contact Vape/powerdynamo
This can be a case of electromagnetic interference spikes into the ignition Brushes sparking ... and then difficult to find.
Shielding / Separating

Not a terrible suggestion - I usually prefer to figure it out on my own or have some help from others who have similar experiences. Getting support from the manufacturer will be a nice change of pace!

Can't comment on VM34 carburetion without a complete jet list, but you're probably right that the problem is electrical. Sorry, can't help with that, no experience with Vape ignition and alternator.

No worries man - the carb guide you usually refer people too does a good job at handling carb stuff and I’ve used both the VM and BS versions many times over the years!

Well, it's common to have to lean the needle setting on just about any carb once you start increasing mains and get more than a few sizes up. If your BS38's had larger mains installed then they probably should have had their needles leaned a step too.

The 38s did in fact have the needles leaned out slightly - although they had a washer so it was only about a half step.

Isn’t there a wire on the Vape system that is critical to keep within a certain length for voltage drop? If memory serves I had to move components closer for this reason. It’s in the directions. Vape is a pain to time. Reason it’s a pain, taking off the rotor every time. I was wondering if it’s worth the money. Maybe I’m used to the banshee swap charging system with the Boyer or Pamco ignition.

Hmm I dunno about the voltage drop wire - I went through the instructions when I retimed it and nothing jumped out at me. As far as weather it’s worth it goes - lookin like a not really from my end. The process of timing it has really turned me off to it. The amount of components I’ve had to stuff into the electrical box is another downfall. I’ll report back once I’ve got some trouble free miles with it.

 

[ninskrillz]

Well I dropped the needle to the second from the top and it made it worse - popping and sputtering a lot sooner than before. So I dropped the clip to second from the bottom to see if that might have an effect and it doesn’t seem like it.

I did try rolling the throttle gently through the problem zone and that lessened the intensity of the stumbly bumbly but it definitely wasn’t smooth. I’ll try bumping the main jet to a 185 and go from there.

According to the carb guide for a bike at sea level I’m still on the leaner side of main jets so maybe that’s the problem.

 

[Jan_P]

What is the rationale working with the carburetors ?? In solution finding ....

One of the reasons I lean towards it being electrical is that it persisted with new carbs.

2 sets of carburetors in all 4 same problem ..One then needs to find an explanation what is so special with that engine which perhaps is possible being a very modified one.
A member Bjorn had to give up on one ignition system replacing it I believed to Vape perhaps he has insights.

 

[RustiePyles]

Are sure your ignition is advancing? Put a light on it and see if it advances as you increase RPM.

 

[ninskrillz]

What is the rationale working with the carburetors ?? In solution finding ....

One of the reasons I lean towards it being electrical is that it persisted with new carbs.

2 sets of carburetors in all 4 same problem ..One then needs to find an explanation what is so special with that engine which perhaps is possible being a very modified one.
A member Bjorn had to give up on one ignition system replacing it I believed to Vape perhaps he has insights.

I guess the rationale is that I have the means to fiddle with the carbs now and don’t have to order parts to do so lol. Maybe I’m just hoping I’ll change one last jet and itll come to life!

Are sure your ignition is advancing? Put a light on it and see if it advances as you increase RPM.

Im fairly certain that when I put a light to it it was indeed advancing. I’ll verify tonight.

I’ll give Vape a call today and see if they have any insights and I think I’ll rewire it to make it cleaner and verify everything is wired correctly.

 

[RustiePyles]

@ninskrillz Where did you get your VM34's from? Not all VM34's are created equal. They are setup from Sudco/Mikuni for 2stroke generally, there are only a few places that sell them set up properly with the right emulsion tube/needle jet combo for 4stroke. 2Stroke emulsion tubes and needles are vastly different than their 4stroke counterparts. Also, what do your plugs look like after a few miles?

 

[ninskrillz]

@ninskrillz Where did you get your VM34's from? Not all VM34's are created equal. They are setup from Sudco/Mikuni for 2stroke generally, there are only a few places that sell them set up properly with the right emulsion tube/needle jet combo for 4stroke. 2Stroke emulsion tubes and needles are vastly different than their 4stroke counterparts. Also, what do your plugs look like after a few miles?

I got them secondhand from someone who got them from Mikes.

6f9 needles, unsure of needle jets, 30 pilots and 180 mains currently, 2.0 slides.

Plugs look black-ish with silver ground electrodes with small amount of whitish deposits on them. Seems too hot or lean. Been that way basically every time I’ve checked them even when the previous carb seemed overly rich. Could have been different issues causing the same results though with the new carb? Dunno.

 

[grizld1]

No wonder the PO sold the carbs, he didn't know what he was doing. Out-of-the-box 2.5 slides usually require 25 PJs, plus or minus a step. 2.0 slides are richer, and usually require much leaner PJs; usually 17.5, plus or minus a step. The NJ is the heart of the carburetor, be sure to look at what you have. Series 159 P-5 or P-6 work equally well. If you find anything else, replace it. If the little brass air jet in the intake bell is still in place, remove it.

BTW the out-of-the-box setup is from the Mikuni factory; you'll get the same thing from any Mikuni distributor worldwide. Sudco's application specific jetting for the XS650 is good but expensive, using 2.0 slides instead of the "factory" 2.5s, which most of us find work well.

 

[ninskrillz]

No wonder the PO sold the carbs, he didn't know what he was doing. Out-of-the-box 2.5 slides usually require 25 PJs, plus or minus a step. 2.0 slides are richer, and usually require much leaner PJs; usually 17.5, plus or minus a step. The NJ is the heart of the carburetor, be sure to look at what you have. Series 159 P-5 or P-6 work equally well. If you find anything else, replace it. If the little brass air jet in the intake bell is still in place, remove it.

BTW the out-of-the-box setup is from the Mikuni factory; you'll get the same thing from any Mikuni distributor worldwide. Sudco's application specific jetting for the XS650 is good but expensive, using 2.0 slides instead of the "factory" 2.5s, which most of us find work well.

The guy said he only mocked them up it didn’t seem like their had even been fuel in them at all. They did have a 20 pilot but wouldn’t run without choke. The 25 idled but stumbled - and the 30 idles well. I have in between sizes but haven’t gotten that far in the tuning process. I did remove the air jets first thing before installing the carbs.

Based on your comment about the slides generally calling for leaner jetting I think the issue I’m facing is being masked throughout the rest of the rpm range by overly rich jetting. This is just a thought tho.

Fattening the main and adjusting the curve settings on the advance boxes did nothing for the issue and I’ll take some readings of the ignition components that are testable in the morning.

 

[Jan_P]

Dont know about carburetors
And have not done the reading here ,But if I Get this right there is a rev band power band with Stumbling .. Poor running
If one are on the Carburetors .. working with them
How about go out on the road gloves Spark plug wrench with you enter that powerband with the stumbling.
Hitting the problem zone ride it there a little Kill the engine ..Take out the plugs Take a Photo and put here of the spark plugs
Mr 5T and Mr Grizid perhaps then is kind enough + can tell if it is a lean or rich condition.
A video of bike running can also help with throttle response

 

[grizld1]

If the carbs need PJs fatter than 20 with 2.0 slides, something is very wrong with the setup; the jetting is far out of range. Check fuel, fuel flow, float level, and needle jet size. (What's the NJ got to do with PJ selection, you ask? Plenty--secondary effects of component changes are very strong in VM carburetors, so much so that changing needle position by a single step can sometimes force a change of pilot jets.) Finally, check float level. Measure from the bare gasket surface on the carb body to the points on the float lever arms that contact the horizontal pins on the floats. Level should be 23 mm. +/- 1 mm. If needed, adjustment is made by moving the float lever arms, never the tang.

Plug reading, if done right, can be useful in fine tuning, but on performance issues like the one at hand, you might as well gaze into a crystal ball as into a spark plug. For a guide to plug reading that debunks a host of urban myths, go to www.strappe.com, click the tech button, click the spark plug icon, and read Gordon Jennings' classic article on the subject.

 

[gggGary]

Add to your list check fuel (float) levels
Tore out much of my remaining hair finally finding a poorly sealing float valve in a VM34............

PS: madness has been getting grumpier about idle lately, need to get back into those VM34's, rode 3+ years without touching them.

 

[Jan_P]

Hmmm A rephase means two firing points How does the vape do that
An even 180 / 360 engine a waste spark works since one is at the right position
Is there a possibility timing the two cylinders independently which is rare on other electronic ignitions ??
And how to ascertain it gets right ?
If I am right here ArtcicXS and Bjorn are into these ignitions but not heard of on a Rephased engine.

t it from a fellow who had it built with a 277 rephased crank, shell #1 cam, and 700cc pistons. Included with the motor was a set of later model BS38 carbs and a Vape/powerdynamo ignition. I have since replaced the carbs with some VM34s.

WHAT IS A REPHASE?
A rephase is when you change the configuration of a crank or camshaft to change the time an engine fires.
Eg: a stock XS650 has a 360º crank (when the firing rotation is 360º apart from each other, that means that both rods are at the same position on the crank. 180º would mean they are at opposite sides). A rephase on an XS650 is usually at 277º or 270º so that one piston rod is at 12 O’clock and the other at 9 O’clock (depending on which way you hold it).