One cylinder running lean - intake leak or ?

[Not Sure]

So here is where I am at, I *think* I have an intake air leak on #2 cylinder causing it to run REALLY lean.

yzf600r inline 4 carburated, jet kit and slip on exhaust... I should mention it sat in a garage for about 14 years until last summer. Since then, new tank, carb tank clean 2 times (very thorough clean of jets/passages), new plugs, and finally a carb sync. The bike ran pretty decent until recently, Ive put about 3k miles on it since the resurrection last year.

Bike runs great between idle (1250rpm) and about 3.5k then develops a slight stutter until 4k. Above that it runs like a bat out of hell! I have adjusted the a/f multiple times to every spec I could find... no matter what I end up with this stutter condition after the bike warms up.

What I have found so far: Using a thermal temperature reader, you know.. one of those laser pointers that show the temp, which are awesome for tire temps and playing with cat at night. Cylinders 3-4 show average of 475-500F when warm but #1 shows about 375-475F and #2 is at 750-800F!! WTH?! It is obviously running lean... but WHY?! Is carb #1 stealing gas from #2 somehow, one runs hotter and one way cooler... and they do share the same fuel intake port.

The other classic scenario: I can spray some carb clean/starter fluid around the #2 carb/intake boot and Vrrooooommmm it speeds up... there is a vacuum leak! Easy fix right? Tell #1 to stop stealing gas, and then fix the intake leak!

I have disassembled everything down to the cylinder head and checked and rechecked each connection and even tested each part with a vacuum gauge, and on top of all that... my final reassembly consisted of slightly greased orings all around and some RTV on the outer edge of the boots both cylinder and airbox. It seems to be running a little better by sound, but alas... the thermometer still shows #2 running 200F+ over the others... AND you guessed it, the idle still speeds up with a little squirt of starter fluid around the carb, specifically the upper half that connects to the airbox but also houses the vacuum bladder slide puller thingys.

There is a little pipe that connects the #1-2 carbs with a t-fitting with the t running up to a check valve system.... wth is that and could that have a bad o-ring drawing in air and also upsetting the fuel balance?

I've done a half assed leak down test of the valves, when they seal they seal so they are not sucking in air, but if somehow they were out of spec.. could that cause one cylinder to run super lean?

Ideas?

At this point, stick a fork in me. I've already began shopping for a better commuter :ride

 

[El Feo]

The ol' carb cleaner spray isn't an ideal litmus for a vacuum leak. The fluid vaporizes quickly and then get sucked into the carb boosting the rpms, especially when the airbox is off.

It would help to identify the how far open the throttle is open when the stutter occurs, not just the rpm range.

Also, check temperatures just at idle. This will narrow down the idle pathway and pilot jet, if they are functioning properly.

The fuel T is positive pressure. So, you'd get a leak before you could accuse number one of being a thief. Ensure #2 float bowl check valve is working. A quick simply way is to prime them both by turning on the key a couple of times. Then drain both by loosening their respective plugs. Compare drained volumes of the two.

I suspect your #2 diaphragm is not sealing/or torn. Lack of vacuum will fail to pull the jet needle away from the needle jet thereby producing a lean condition.

Carbed bikes make great commuters, if you commute more frequently than one every 14 years.

GL

 

[Maddevill]

Could be a torn or misaligned diaphragm but check that the carb boots aren't cracked due to old age and sitting for so long. The fact that your idle changes proves you do have an air leak in that area.

 

[Not Sure]

The ol' carb cleaner spray isn't an ideal litmus for a vacuum leak. The fluid vaporizes quickly and then get sucked into the carb boosting the rpms, especially when the airbox is off.

It would help to identify the how far open the throttle is open when the stutter occurs, not just the rpm range.

Also, check temperatures just at idle. This will narrow down the idle pathway and pilot jet, if they are functioning properly.

The fuel T is positive pressure. So, you'd get a leak before you could accuse number one of being a thief. Ensure #2 float bowl check valve is working. A quick simply way is to prime them both by turning on the key a couple of times. Then drain both by loosening their respective plugs. Compare drained volumes of the two.

I suspect your #2 diaphragm is not sealing/or torn. Lack of vacuum will fail to pull the jet needle away from the needle jet thereby producing a lean condition.

Carbed bikes make great commuters, if you commute more frequently than one every 14 years.

GL

Great commentary!

Airbox is on and sealed while testing, which is why I pulled off each boot and meticulously checked roundness of clamp, clamping tension, carb seating position. Also pulled each boot below and above the carb off and checked for any leaks that were unseen by using a vacuum pump on the carb sync line while plugging both ends of the boot.. all good with no leak at all. From the airbox down to the intake mounts seem to be solid with no leaking.

All temps were taken at idle, no choke and at about 70C (which is basic warm up for the bike).

Throttle position is right about at the 0-1/4 or 1/3 transition. If I throttle up REAL slow, or hammer it off the line then the stutter does not happen. Its during normal everyday type riding, like 25-35mph city streets in 2nd gear is where it is most prevalent, shift to 3rd and roll on an its usually fine.

I checked the diaphragm 3 different times and could not find a single crack/stretch/tear or anything that look out of place, smooth and elastic feeling as ever and laid perfect in its groove.

I have not checked the float level, they *looked* all the same but that will be my first to-do tomorrow... I never really knew a good way to check but doing a volume comparison should be sufficient, thanks for the idea!

Thank you sir! :thumbup

btw, I looove my carb'd bikes... but my FI bikes are so much easier to tune!! (I parked this one with 3k miles in 2002, when I had a baby and switched to dirt bike racing )

 

[afm199]

swap pilot jets between two carbs and see what happens

 

[zanshin]

With apologies if I am belaboring the obvious, but I didn't see this in the problem description -- are the slides responding identically (whoosh sound, resistance level, speed of return) when raised by finger?

 

[afm199]

With apologies if I am belaboring the obvious, but I didn't see this in the problem description -- are the slides responding identically (whoosh sound, resistance level, speed of return) when raised by finger?

Excellent point. They should push up with slight resistance and be forced back down by the spring with slight damping of speed. They should not slam closed.

 

[El Feo]

Okay, since those were idle temps, I think you have two issues.

You're lean on the number 2 pilot circuit (you knew this). After a bikes sits for over a decade, cleaning carbs more than twice is not unheard of.

So, the pilot jet could have replugged, like Ernie said, which is very common. Or, there is something blocking the fuel idle pathway (I think you said you blew it out.) Or, the float bowl is not filling properly.

The second issue is that you're getting a rich stutter (it bogs for a seconds and then takes off, right?). Jet kits usually come with an A/F mixture starting point, like two full turns out. Start there.

Check your slides, like zanshin said. If you still stutter, lower the needle height (and check the jet kit's recommendations) on all the carbs.

A/F affects all rpm ranges. And jet needle height plays a huge part when you crack open the throttle. So, it's often a combination of the two.

But, fix number 2 first. Start swapping parts. Make sure you can see through the pilot jet. I might have an extra laying around if you need it.

 

[Zerox]

It's time.


youtu.be/kHz8ft56lQQ