Stumped on Carbs, 2000 ST1100

[Grabcon]

Okay I have run across something I have never had issues with on any four cylinder bike with or without a fuel pump and I need help.

Please read carefully before commenting.
Issue, after carb rebuild I got about 800 miles on the bike and I started to have issues in the midrange power band. No or little power, starting at about 3000 to 5000 rpm. Prior to this is was running great.

I check all of the obvious things before really digging in to this.
Fuel pump flow and pressure
Replace fuel & air filter
The fuel shut off has been removed
Valve clearance with in spec.
Float height is good
yad yad yad.

Found that it was running on cylinders 4 and 3, the back ones.

Did the next obvious thing and pulled the carbs
Went through another complete cleaning and rebuild. Although no new parts on this one.
Put them back on and the same issue.

Pulled them off again and remove the bowls one side at a time and checked for fuel flow. Each carb flows fuel and shuts off fuel when I move the floats by hand. But and here is the big but. Now the carbs are back on the bike and cylinders 1 & 3 are cold, NO fuel to 1 & 3 Carbs. Bike runs on 2 & 4. So now I am left and right instead of front and back.

I have spark on 1 & 3.

Removed the fuel tee between each bank and just hooked the fuel line up to the right hand bank, carbs 1 & 3. No fuels get to those carbs. I can squirt gas in 1 & 3 carbs and it fires on those cylinders.

I have check all lines to make sure they are clean and can flow fuel.

I am stumped.

 

[vinny]

You seem to be very meticulous about troubling shooting the issue.... just wondering if you are overlooking something simple like a kink in the line when mounting the carbs? I know, I know... but sometimes we overlook some simple things. The fact that the issue changed from one combination to another, seems to suggest issues with the fuel lines. Have you check the fuel lines for fuel flow without being hooked up to the carbs? If you had cleaned the carbs because the bike sat for quite a while, and they were dirty... well the lines may be dirty also and moving them around may have dislodged something causing the blockage. Just thinking out loud.

 

[Grabcon]

Vinny, Yes I have checked all lines for being clear and free of kinks. This was all done when the carbs were off. All rubber lines were removed and checked.

Irish. Yes I checked them too as well as the air cut off diaphragms.

I have been through the carbs so many time in the last three week that I am surprised that there are still threads on the screws. LOL.

 

[vinny]

OK, I think there may be an issue with the way the floats in 1 & 3 were re-assembled. You hooked the lines straight to these carbs, no fuel... when fuel squirted into the carbs, they work. Something happened when you installed the bowls on those two carbs. The float seat is the only obstacle for fuel unless the idle or low speed jets are clogged, still. Did you do a drain check on the carbs in question to verify no fuel was getting to the bowls?

 

[Grabcon]

I am having one heck of a time getting to the drain screws on the carbs when the are on the bike. So No I have not tried to drain them, but the only way 1 & 3 will fire is by squirting gas in the carb from the top. I got the how to photos from Adam's write up about how to drain the carbs and I just cannot get them.

Your thought on the issues with the carb bowls and the float seat have also crossed my mind as well. The float seats have always been my thought on the issue.

Saturday is another day. Maybe in the morning I will pull the carbs again. But I got to ride in the afternoon. The wife wants to take her bike out. So I will fire up the 1991 ST and go for a ride. If not resolved by Sunday it will need to wait a week as I need to see my son that has Hodgkin's Lymphoma and spend some time with him.

 

[DeanR]

I am having one heck of a time getting to the drain screws on the carbs when the are on the bike. So No I have not tried to drain them,

It's tough, but do-able with the very long flat blade screwdriver, a flashlight and lots of patience with searching for them.

 

[Grabcon]

Dean I know they are accessible but my patience are running out.

Okay Dean. I pulled the carbs from the boots just to get to the bowl bleeders. No gas carbs 1 & 3. I cycled the fuel pump with the bleeders open and nothing. 2 & 4 are fine.

Keep in mind that this started on 2 & 1 not filling which are the front two. Now it is on the right bank. I have clean and re-clean the carbs, check everything for blockages. The carbs flow gas without the bowls on. So now it is back to Vinny's comment about the bowl installation, although I question it I will go back a 4th time.

If my 1991 wasn't running so good I would try switching crabs, but I don't want to be with out a bike. I am getting a little exasperated at this point.


Later in the day
As mentioned above I already have the carbs of and recheck the floats, needles and seats. The only thing I found was on the #1 carb the little spring on the top or bottom of the needle depending on carb position was reversed from the other three carbs. I rotated 180 and we will see what happens. I doubt the it will change anything.

The floats and seat seem to operate properly at this point. Again I will put gas on the on the bench to see what happens. Fingers are crossed, but honestly I really did change anything so I again am doubtful on success.

 

[Grabcon]

Okay here is what I know for sure with the carbs on the bench.

All fuel passages are clear.
With all carbs hooked up to a fuel source 2 & 4 fill and 1 & 3 do not.
Blowing air from my mouth through a tube connected to 2 & 4 with the bowls on I can hear air passing through. Same process on 1 & 3 bowls on no air pass. Loosen the bowl on the #3 carb and air passes.

Conclusion for some reason the floats for 1 & 3 are touching the bowls when the bowls are snugged down. How do I know this, well it is a guess but with the bowls on and lose they pass air and when snugged they do not. I know it is not scientific but from what I am doing it is the only logical reason. I have not a clue why this is happening. The floats are plastic so bending them slightly is not an option.

Float level is set to 7mm, all floats, seats, needles and bowls are on their original carbs.


So after today they will sit for a week. I know they are not self healing so I will still have a project when I get back from the visit with my son.

 

[Brant]

Okay here is what I know for sure with the carbs on the bench.

All fuel passages are clear.
With all carbs hooked up to a fuel source 2 & 4 fill and 1 & 3 do not.
Blowing air from my mouth through a tube connected to 2 & 4 with the bowls on I can hear air passing through. Same process on 1 & 3 bowls on no air pass. Loosen the bowl on the #3 carb and air passes.

Conclusion for some reason the floats for 1 & 3 are touching the bowls when the bowls are snugged down. How do I know this, well it is a guess but with the bowls on and lose they pass air and when snugged they do not. I know it is not scientific but from what I am doing it is the only logical reason. I have not a clue why this is happening. The floats are plastic so bending them slightly is not an option.

Float level is set to 7mm, all floats, seats, needles and bowls are on their original carbs.


So after today they will sit for a week. I know they are not self healing so I will still have a project when I get back from the visit with my son.

Haven't had these Kiehn carbs apart as of yet, but in past on my xs11 Mikuni carb bank, inner edge of bowl gaskets needed trimmin' with an x-acto knife, with gasket in place. Once bowls tightened, 'squished' gasket interfered with float itself. Not that that is the issue, but JAT to consider.

 

[dwalby]

The floats are plastic so bending them slightly is not an option.

I'd try putting them back together without the needle valve and test again. That way you can determine if the passage is actually being blocked by the float/needle itself, or something else is going on with a gasket or ??? Having two floats become inoperable at the same time seems highly unlikely.

Its been an eternity since I worked on carbs, but back then even though the float was plastic, IIRC there was a little metal tab where the valve needle attached. With small needlenose pliers you could bend that metal tab to adjust the float height. Don't know how this is setup on the ST as I've never been in there. If its all plastic and non-adjustable, then disregard message. But, if its metal, is it possible that you inadvertently bumped into the floats with the bowls off and bent this adjustment tab ??

 

[Grabcon]

dwalby - Yep the floats are plastic but there is a metal tab for the float height adjustment. Each time before the bowls gone back on I check that height.

The needles and seat are functioning properly. My last test before I give this a one week break was to put gas on the 1 & 3 bank and while the gas was on open the bleed screw nothing on either carb. NO Gas. Next I slightly loosened one bowl and let it drop ever so slightly gas immediately started to flow on each carb. Snugged the screws back up, drained each carb and turned the gas back on nothing. This confirms my suspicions that the floats and bowls have an interference. The big question is why?

In all of my years working with carbs I have never seen something like this.

 

[dwalby]

what I was suggesting is do your blow test one more time, but without the needles in place, just in case you're getting some weird gasket interference that goes away when you remove the bowl pressure. I think you have the carbs in a place where doing this additional test would only take a couple minutes, right?

 

[Grabcon]

Ah got it. Well it will need to wait until I get back. But it is certainly worth a shot.

 

[Grabcon]

I am back in town and ordered the Float valve sets as Adam has been suggesting in PMs all along. At nearly $200 (parts, & tax) for four it took me a bit to crack the wallet and let the moths out to spring for these little turds. I just hope at this price the issues are solved. I will know Friday or Saturday this week when I get the parts.

 

[hotrod1]

How did it turn out?

 

[Grabcon]

After all of the work I really have not 100% resolved the issue. 6 or 8 weeks ago I bought a set of ignition coils because I had found two bad spark plug caps. I swapped them out and had a minor improvement.

Interestingly enough today I am just going to swap out the coils to see if that makes a difference. I have check the ohm values on the coils I bought and they are with in spec. I have check the values of the coils on the bike and they too are good. But it is a few hours of work and maybe just maybe there is something intermittent that is causing the issue.

Stuff like this drives me crazy. Luckily I have several bikes so it has not been a pressing issue.

 

[ST1100Y]

Obscure, rare issue I'm reading with interest...
I'm pretty sure its most likely some basic, elementary thing, hiding in plain sight...

 

[Grabcon]

Well I did swap out the ignition coils this morning and there is a marked improvement in the overall performance. I find it interesting the although the original coils bench test fine the difference between the before and after the swap is significant. IMO it is time to leave well enough alone.

 

[ST1100Y]

So is it now a lack of fuel issue, or ignition related??

 

[Grabcon]

I believe that it was ignition related as things have improved. That is not to say that the carb work was not need because mot likely it was and it was a good exercise. I am done with this and ready to move on to other projects.