Carbs issues low to mid rpm when engine warmed up

Hello everyone,

I'm having an issue, tried to solve it by my self, but second season without any luck...

So, I have Yamaha XJ600 1998 year model, by default it was 25kW, I found, that there was a limiter in carbs, plunges had higher plastic spacer to limit the power, to overcome that I bought new set of them. Originally they are for Yamaha vigaro xv250, but by the looks and dimensions they are the same, that's when my problems started...

When I'm driving on 1800-2000rpm and give some gas, it starts to shutter (only on hot engine), but if I give full gas it sputters a bit, and then goes like crazy, it's really hard to drive like that in city or just cruise.

I've cleaned carbs in ultrasonic bath, made full carb sync, measured valves, measured compression, played with pilot screw, checked ignition circuit, played with needles height, checked inlet rubbers. Long story short, tried everything from internet...

Could you guys helm me with that? What could be the issue? Maybe too stiff springs? Maybe someone already tried that and knows solution?

 

Don't know, but have you checked you have the correct needles?

 

Tried original ones and new, with original it's a bit better, but still, everything is the same

 

So maybe they did change the springs - measure the diameter with a digital vernier, and the number. I have a set of 600 carbs here, I can check.

 

Could you measure springs on free time? I will be able to compare them, because now I have no idea what length they should be. That would help me a lot

 

i dont get this question,
xj600 is a 4 cylinder in line engine
xv virago is a v twin , is it not,
how did you get the carbs to fit let alone ,work

 

I haven't replaced carbs on bike, only vacuum sliders inside of carbs.
For both models sliders goes the same, because both bikes has almost identical carbs, only few parts are different and count of them of course.

 

One issue is the slide needles could be different size and the other soaking carbs without a full tear down can destroy the butterfly seals these should always be replaced , Xj4ever should have the replacement seals last time I bought then they were around 39$ yea not cheap but they fit perfectly and eliminate chance of vacuum leak .

 

Original which? the ones from the restricted slides? Do they have a number? post it here.
New? from where? generic ebay? again post numbers, I've come to learn to mistrust anything sold that's not genuine or from a trusted dealer.
The slides - are the holes in the bottom identical?
And this stuff about vacuum leaks from throttle seals - yes, it could cause uneven idle, wrong idle screw settings etc, but once the throttle is open those issues dissapere don't be talked into changing them yet.
Measure your springs wire diameter, number of coils, free length, post here. I can do likewise...
Oh, one last thing, when you have issues with hot missfire, it's almost guaranteed to be a rich mixture - so your slides are not lifting fast enough, hence the questions about springs and holes in slides...

 

Restricted ones were originals, unfortunately without any part number.
New ones, generic from AliExpress, by dimensions they are identical, but I will check holes on the bottom and will compare them.
Also I will check springs on free time, probably next to the weekend.
I thought, that they are opening too fast and I'm getting too rich mixture, but as I understood from you, if they are not opening, the mixture is too rich too? I'm a bit confused at this moment. As I understand, if needles aren't opened, fuel comes from pilot jet, is it possible, that pilot jet gives more fuel than main jet at that case? Just curious, want to learn as much as possible from this case

 

Here we go then.
Your carbs are constant vacuum - often referred to as constant velocity, but that misses the point.
As air flows under the slide it creates a partial vacuum (bernoullis theorem, reason why airplanes fly), which is what draws the fuel out of the jet past the needle.
More engine rpm and more opening the throttle equals more air, which would increase the vacuum at the jet - but, and this is the important bit, the vacuum under the slide is fed through the hole in the bottom of the slide, all the way up to the top of the slide (which is hopefully sealed by the diaphragm), and this lifts the slide, against the spring, until voila - the pressure under the slide settles back to where it was before. Hence constant vacuum - CV, or as the old Zenith carbs were termed, CD -constant depression.
So the jet and the needle can now meter fuel exactly as required for the airflow, because the hole varies in size, but the pressure diff across the jet stays the same! Neat eh?
Well what about acelleration/tip in richening fuel I hear you ask? This is where the hole under the piston comes in, or more accurately, the size of the hole. A large hole allows the vacuum above the piston to equallize with that underneath quickly, a small one takes more time. So if you snap open the throttle, a small hole will take time to lift the slide, which means a higher vacuum exists under it (the balancing hadn't had time to work yet). Hence the jet sees this low pressure, and given the engine is sucking pulses of air and fuel in, mixture goes rich, bike off like a rocket.
That's about it really. I didn't mention the springs, which are there to make sure the slide doesn't hit the top stop until the engine runs out of the need for more air - if it did the fuel metering and CV functions would be lost - likely a lean mixture, tuning into a rich mixture would result.So springs are important.
One last point about low restriction/no air filters - the slide has the vacuum under it on one side lifting it, on the other side it has the upstream, air filter pressure (could well be a vacuum also). Removing the filter, airbox etc changes the pressure across the slide, throwing it's callibration right outthe window..
Bit long that....

 

Thank you so much for this explanation, it's really interesting to read things like these, and helps a lot to understand this type of carbs. The one thing I didn't check was holes sizes on the bottom of slides.
One thing to mention, I have notices, that on original ones, there are holes not only in the bottom, bun in the top side too, maybe that have something to do with power restriction? It looks a bit strange, to have additional holes on top, as I understand, in that case vacuum just leaks fastest and makes it harder to reach top for the sliders.
Also, I was one video, where guy drilled additional holes in the bottom of these sliders, to make accelerometer more reactive (it was done on xv250) but I guess that's quite risky game if you don't know how far you can go.
Also I will try different needles, I have original ones and ones that came with a kit, there is a huge difference between them.
Original ones are more straight, tip is thicker, and new ones are more pointy, and that cutt off starts way sooner. I guess that new ones can give too much fuel, and then I would need to check main jets, but at the same time, old needles (if there are difference in original ones) could not remove restriction fully.
Although, thanks a lot for your comment.

 

if you have needles from aliexpress bin them. your slides must be identical in all respects, holes are important, but no reason why you can't drill out the hole. This is your best starting point - can you not just buy a used set of carbs from ebay? The issue being you don't know what an unrestricted slide looks like. And, original mikuni needles ARE numbered.
Note that different 600s used different size Mikunis - which ones do you have?

 

I'm not sure what code is on my original needles, but as I see from Haynes manual, there should be the same needles for these carbs (4BC14) by the pictures they look the same as mine, of course every millimeter can make huge difference.

 

It might help if you were to post pictures. I'm feeling in any event, your going to have to pull the carbs anyway.
Maybe show multiple views/angles of the slides/diaphragms from old to new will help more experts out.

 

To eliminate more issues it may pay you to buy a full set of unrestricted carbs. Such as..

https://www.ebay.co.uk/itm/145576902533?

 

I will check everything on weekend, will add some photos

 

Don't buy those - they are Diversion carbs...

 

AFAIK 1998 model year makes it the Diversion (aka Seca 2) version.

 

You're correct. In which case I too would recomend buying some used carbs.
I do have an incomplete manky set here if the op wants some cross checks.

 

Finally managed to get to the bike, I'll attach some photos of differences.
But one major thing I noticed, that rubber part was not punctured on new ones on that small Buble (don't know how to call it) on first photo that should be visible

 

without the hole in the lip you don't get vacuum above the slide to lift it, assuming there is a port in the carb that directs the vacuum upwards - better pics of the carbs please - do you have difficulty posting pics, three is a bit stingy?
Also, can you provide pics of the bottom of the slides - the holes at the top may or may not be important, but any at the bottom are. (see my explanation how the CV carb works).
I will dig out some of my 600 carbs, but I'm sure the slides on mine aren't round like yours.

 

This is a 600 diversion slide - at least I think that's what they came off...
note that it doesn't have an "ear" perforated or not, there are two holes to feed the vacuum up, and the spring is great long thing. My best suggestion would be to buy a used set, as @a100man suggested.
some pics. This is a Hitachi from 650 carb.

 

Sorry, I'll post more pics. I guess your carbs are a bit different.

 

Today I had a small ride, and it feels like it's driving normal again, I wasn't able to test max speed, but in city it drives perfect, I guess these small holes on membranes were issue for a whole time...