Help! Not running on back two cylinders!!

[kx250gs1100]

Giving you a little background on me. I have around 17 years experience as a motorcycle tech. Mainly working on metric bikes and ATVs. Well I finally got my hands on my first vmax believe it or not. Customer complained that it wouldn’t idle. Would have to hold the choke on to keep it running which 9 outta 10 times points to clogged up carbs.

Got it to the house, pulled off the carbs and started tearing them apart. One carb had a clogged secondary jet, another one had a pinched slide diaphragm, and another was dry as a bone as if it wasn’t getting gas to it. Fast forward a couple days later, carbs are back together and installed on the bike. First thing I do is drain the bowls to make sure they are all getting fuel to them. Which they are. Fired up the bike did some tweaking on the air screws then synchronized the carbs. Had it idleing and checked the exhaust temps, front two were hot and the back cylinders were cold!!

I pulled the plugs to see if they were getting fire and they were getting fire on the back cylinders. The owner had champion plugs in it so I went ahead and replaced them with ngk plugs. Still not firing on rears. I hooked up a timing light to see if maybe it’s loosing fire after it starts but the timing light shows that they’re firing.

Next thought is a fuel issue. With the bike running I can literally look down the carb venturies and see gas coming out into the intake as the bike is running. So it’s getting fuel for sure. Just for the hell of it I sprayed brake cleaner down the carb while the bike was running and it still didn’t pick up that cylinder.

I’m kinda at a loss here. I don’t know the ins and outs of these vmax’s. Is there something I’m over looking? Really need yalls help!!

 

[MaxMidnight]

If you have (the correct amount) fuel and you have a spark then that only leaves a compression issue?

 

[kx250gs1100]

I forgot to mention I checked that also. Both back cylinders were around the 160 range.

 

[cwkerr007]

is it just at an idle ??Does it start hitting after you 1/4 open throttle (and lets call a cyl hitting cause i get confused cause if i ask is the cyl fireing i mean is the plug firing ok)what did the plugs look like that came out ?? got any pics

 

[one2dmax]

If it's an 85-89 the most likely cause is a bad CDI box (provided the carb pilot jets have been cleaned)

 

[cwkerr007]

is it just at an idle ??Does it start hitting after you 1/4 open throttle (and lets call a cyl hitting cause i get confused cause if i ask is the cyl fireing i mean is the plug firing ok)what did the plugs look like that came out ?? got any pics

I meant pics of the spark plugs sorry

 

[kx250gs1100]

Sorry for the late reply
To answer cwkerr007
It’s both at idle and with it revved. I’ve pulled all the plugs out (that I just replaced) and the front two are brown and the back ones are brand new looking still. Sorry no pics.

To answer one2dmax
It’s actually a 97 model. And I ended up pulling the carbs off a couple more times double checking everything and all the jets are completely clear

Now on to some of the other things I’ve tried recently. One thing I forgot to mention was a slight but loud popping coming from the front right cylinder. I tried playing with the air screw to fine tune it but wasn’t able to clear the popping up. The more I leaned it out the more it popped. Figured the float level might have been off so I pulled them back off, and readjusted the levels to what I assumed would be right from what I knew about other styles of carbs. I put the carbs back on and it seemed to have picked up the back cylinders (they were hitting) thought before I could put my hand on the exhaust pipe right behind the head and they were cold but now they were a lil warm but nowhere near as hot as the front ones. Popping still remained. Pulled the plug for the front right cylinder and it was a very light tan like it was way too lean . Pulled the carbs back off installed a bigger jet in that carb. Went from a 170 to a 180. Fired the bike up and the popping was gone (take note it has an aftermarket exhaust and I called the customer and he said it always had a slight pop but not bad). Problem is the back cylinders still didn’t seem to be hitting like they should. So I went online and found the actual float height specs which was 18mm (if I remember correctly) from the bottom of the float to the edge of the inside of the bowl. I fine tuned all of them to exactly that. Put the crabs back on. Fired it up and it’s popping like a MF!! Didn’t let it run long at all. Didn’t wanna piss my neighbors off and was about to pull my hair out!!
This bike is eating my lunch!!!!!!!!!

 

[sdt354]

The jet blocks were removed, disassembled, and cleaned? Replaced with new gaskets?

 

[kx250gs1100]

Jet blocks were removed disassembled soaked in carb cleaner solution with the carbs overnight then washed out and blowed through with air. All passage ways were clear. All jets were free of blockage and run through with a torch tip cleaner. Only thing I didn’t do was replace the gasket. It looked to be in good shape. No tears or pieces missing.

 

[one2dmax]

The little 37.5 main is common to get blockage and usually 90% of the reasons these bikes don't run on a cylinder (when it's not the 85-89's)

 

[kx250gs1100]

Double and triple checked those jets every time I had the carbs off and they were always clear.

I might try and double check the float levels.
I set the float height at 28.5 mm. That’s with the bowls off measuring from the bottom of the float to the edge of the inside of the bowl (seen that done in some pic online). But I see that the Yamaha service manual says to use a clear hose attached to the drain hose and measure a fuel height of 15mm +- 1mm. Might try that next.

 

[kx250gs1100]

I don’t have a manual for the bike, just going by what I find online.

 

[MaxMidnight]

I don’t have a manual for the bike, just going by what I find online.

Get it here

 

[kx250gs1100]

Awesome!!! Thanks!!!

 

[dannymax]

The fuel level is 15mm - 17mm below the tick mark on the side of the carb body. Wet level check to be certain.

Seeing light thru the pfj's could indicate only a partial opening they need to be fully open.

 

[kx250gs1100]

What do you mean by pfj’s

 

[dannymax]

pilot fuel jets

 

[kx250gs1100]

Ok well I did the float level test with a clear hose and the measurements were

Front right ( popping cylinder) 13mm
Back right (dead cylinder) 17mm
Front left (good cylinder) 16mm
Back left (dead cylinder) 14.5mm

 

[kx250gs1100]

Just out of curiosity, the jet block gasket should be reusable correct? I know it’s a paper gasket, but when I removed these they were in great shape. No tears or damages. Or should I have replaced them?

 

[sdt354]

Just out of curiosity, the jet block gasket should be reusable correct? I know it’s a paper gasket, but when I removed these they were in great shape. No tears or damages. Or should I have replaced them?

I don't reuse them, it's not worth a problem. They aren't very expensive. I always end up having to clean some gasket material off when removing the jet blocks. The pilot jets themselves, It's sometimes plugged with crud, so hard, I have to push a fine stainless guitar wire through. Then a .0375 drill bit through, twisted by hand. If it opens up a little more than .0375, even better.