'80 GS250t- carb rebuild, now idles great, Boggs w/any throttle

[Guest]

Stripped and rebuilt these carbs, new o-rings throughout, new diaphragms, new boots and o-rings on the head, new boots on the air box side. Cleaned and inspected everything inside. Cleaned and polished the fuel valve seats. Had to cut the tops of the air screws w/ a Dremel , but then new o-rings, and cleaned up nice. These are a little different than the carbs on my wife's 550L. There are no pilot jet plugs and the choke linkage is different. Using the tutorials as reminders, it went pretty smoothly.
Got the engine to fire with minor effort. It idles quite well. When I give it any throttle at all, it bogs down and will die if I don't let off. I can VERY slowly and gently get the RPMs to come up, but it will soon begin to stumble. It usually can't be saved by letting off the throttle.

I did verify that all jets are stock. The air box is intact, sealing well, filter in great shape.

What at am I missing?

Thanks!

[John Park]

Is it possible to get the slides in in a different orientation? It's been ages since I've had one apart, but if you've replaced the diaphragms, did you get it all back together so that the hole in the bottom of the slide is at the front? It goes forward. Plus there's usually a washer under the needle clip and then the plastic thingy the spring rests on.

I've been really lucky and haven't had to take a CV apart for a long time, so my memory is dim, but it sounds like you have some sort of slide related problem. That or the needle jets are [still?]plugged. If you can slide the air box back or get the boots off and verify that the slides are indeed rising as they should...

[MatBirch]

I was careful installing the new diaphragms to keep the holes in the right place. This symptom rears it's head before the slides would even need to start sliding though, I think... Doesn't that happen at a little higher rpm with a lot more vacuum?

[Ric]

That's what I'm thinking, too. I just put the slides back in the carbs on our 300(same carbs as yours) after readjusting the needles, and I noticed the diaphragms have a spot on the outer ring that's molded to fit in a fat spot on the carb - it's how they're "keyed", for lack of a better term. As long as you got that right then your slides are where they need to be.

If they weren't opening, it seems like the engine would just sputter at a little bit higher rpm instead of stall and die altogether. Only thing I could think is gonna sound painfully obvious and I'm certainly not trying to make you look dumb - but did you put the slide springs back in? That'd open the slides way too soon and stall it out.

[MatBirch]

Definitely have the springs in. The JMB diaphragms don't have the little molded key, so they are very clear the need to align them properly. I have no doubts in that part of the system. I believe that the way carbs work, a specific range of throttle opening is governed by one jet, then another, then the needle., as the throttle is opened.... I think... I believe that the slides /needles lifting are also aided by vacuum, which on our bikes is necessitated by load also??? I don't think they lift much, if at all, in neutral do they??

[Ric]

Pretty much. On most any Mikuni BS carb the main players in fuel delivery - from idle to wide open - go in this order:

Idle - pilot screw/jet
Off-idle to light throttle - pilot jet
Light to mid throttle - throttle valve(the profile machined into your slide)... this is also when the slides should start to rise.
Mid to heavy throttle - needle/needle jet
Wide open, 249CCs of fury - main jet

Yes, vacuum lifts your slides - the butterfly valves control that vacuum signal. There is a small hole in the bottom of the slide that allows the intake vacuum to suck the slide open. Like John Park said, that hole needs to be forward of the needle. That'll give the strongest vacuum signal, and it'll properly orient the throttle valve on the slide as well. Idling in neutral, the slides should be all the way closed. At idle the butterfly valve is all but closed, which keeps the vacuum from reaching the slide. That vacuum is what sucks the fuel through the tiny port controlled by the pilot screw.

[Ric]

Have you set your float levels?

[jdvorchak]

Have you run it long enough to see if the problem exists when the engine is warm/hot?

My guess is that you don't have the enrichener (choke) circuits clean. On my 550 the enrichener circuit starts in the float bowl. Small hole in the bottom and that leads to a channel up to the carb body. That channel has a very small "jet" inside. If you can't squirt fluid from the bowl up or from that channel down to the float bowl it will need to be cleaned/opened up. It's pretty easy to clean that jet. Just takes a very thin wire, I use a welding tip cleaning tool and smallest wire to probe that jet. Doing this from the top of the float bowl down. Stick the wire in and probe it up and down. When you think you have it clean, mark the depth of that wire and confirm that it goes all the way down below the level where that little hole in the bottom of the float bowl. Confirm it's clean by squirting carb cleaner through both holes. From the bottom up and from the top down. Should get a nice flow of carb cleaner.

[MatBirch]

Well, I'll know more once I'm able to ride it, but I think maybe I got it?? I pulled the carbs back out, confirmed the choke circuit, readjusted the floats, (one was out on the not shutting off side), and just generally confirmed that everything was clean and clear, slides are functioning, etc. It started right up again, and let it warm a bit. It was still bogging when I'd snap the throttle. Once it was nice and warm, I revved it slowly and just revved the snot out of it a few times. It seems to have cleared up a bit. I'll get some new plugs and get the carbs synced, and go from there...

[MatBirch]

Evidently, I have an intake air leak. If I choke off the top of the air box with my hand, it runs great. I pulled the filter, and re oiled it but it didn't help at all. It acts just like trying to run with pods. I'll do more digging, but first, I need to re-rig my temp fuel tank, as it leaks constantly onto my floor. Not the safest, or the least stinky...

[jdvorchak]

I really hope you find the leak. Check the air box boots for a tight fit in the air box and make sure they completely cover the air intake. May have to use a mirror to make sure they are installed on the bottom of the carbs. Look for a hose left off or loose like from the crankcase vent.

If nothing is jumping out at you with the above checks, then I have to reiterate about the enrichener circuit. Your bike is exhibiting the exact same symptoms as my KZ550 including putting your hand over the intake and it runs great. Good idle but won't accelerate.

Good luck

[MatBirch]

Well crap! now I appear to have lost idle as well! I warmed it up well, re-checked everything, again and again... It started same as it has been fiddled a little with the air screws, but never got any better. Then it just kinda slowed and stopped. I have confirmed fuel in the bowls and flow from my temp tank. It will start with the choke on, but barely. It won't keep running at all without it.

[John Park]

Have you checked the valves recently? I've seen some really erratic carb symptoms from too tight valves, and tighten is what they always do.

[MatBirch]

Have you checked the valves recently? I've seen some really erratic carb symptoms from too tight valves, and tighten is what they always do.

Ha, that was one of the first things I tackled. I've learned the hard way not to try to screw with the carbs before adjusting the valves.

[MatBirch]

Ok, got some ideas-
the two ports beneath the diaphragm do not allow air to pass. I pulled a pretty good sized plug of crap out of one side the air screw side. The horizontal bore is evidently still plugged. Is there a way to clean the cross drilled passages? They all have the gold colored ball bearing looking plugs on the outside.



if I could pull these, life would be a dream! If not, just more time in the dip??

Any other tricks??

thanks for the help guys!