Suzuki/Mikuni Carb Help

[Mike Lawless]

I seek knowledge!
1982 GS300. It had been sitting for a while and ran "OK"... for a while. It began to run poorly, bogging, sputtering missing, etc. So I assumed some varnish had worked loose and had begun to muck up the works. I decided to clean the fuel system, including disassembling the carbs, soaking them and blowing out all the passages. Simple.
First, please keep in mind that I have been around the block a few times, and know my way around a carburetor.....as long as it's a Holley, Rochester, Weber, etc. For the four wheel variety of hot rod.

However, it seems somewhere along the line, I have screwed the pooch on these little Mikuni's. They seem simple enough, but after the cleaning and reassembling, one leaks fuel out of the bowl vent. (See photo). These were very clean to begin with. Now I have had them apart at least a half dozen times, checked and rechecked float levels, cleaned and re-cleaned the needle valves, even swapped floats and needle valves side for side. Still leaks out the same side.

What is really interesting, when draining the bowls, the side that leaks has significantly LESS fuel in it that the side that's good. Logic dictates there must be something unique to that carb body that is causing the problem. I didn't have any leftover parts, (many times I do), but it's obvious I have done something wrong.

Is there a check ball that may have inadvertently fallen out of the main well? Does it have something to do with the slides and needles? I'm stuck here guys. Help!

 

[ILoveThumpers]

Since there's no overflow on that model of BS, fuel out the bowl vent means your needle isn't seating or your float is taking on fuel.

-Deek

 

[Mike Lawless]

Issue resolved. Chalk it up to both an unfamiliarity with these carbs or a brain fart. On a positive note, I learned a lot.

 

[AlphaDogChoppers]

Care to share with the rest of us what the problem was?

 

[Mike Lawless]

Well, I feel a little foolish, (Chalk it up to a "senior moment," or not being familiar with these particular carbs) but on a positive note, I learned far more from this than I would have otherwise. What need to be kept in mind, is that when it comes to automotive carbs such as race prepped Holley and Quadrajet, and Weber IDA and IDF's, I have a wealth of knowledge gleaned from close to 40 years of working with them.

When I went to re-assemble the carbs, there was a brass tube stickin' out of the right side carb. Yep, the one that was giving me trouble. I THOUGHT it was venturi vacuum, because on automotive carbs, that's where they are. This one was right in line with the venturi, and not really appearing to be near the float bowl. The left side carb doesn't have this. I capped it. After all, we don't want any vacuum leaks now do we?

Last night, I'd been through everything and I knew for certain the needle and seat were good. So, I have the carbs chucked up in my bench vise to hold them while I filled them with a small funnel in the fuel line. Little by little, it starts to fill, then it started bubbling out the air bleed. I got the bright idea that perhaps it was air locked and it would be OK once it filled. After all, whenever I took it apart again after encountering a problem, there was only a small amount of fuel in the bowl. It couldn't possible be overflowing. Anyway, To help it along, I blew air into the air bleeds. When I did that, it stopped leaking. So I says to me-self, "Self, we're onto something."
So, I put my thumb over one air bleed and blew lightly into the other. As I did that, the cap flies off that brass tube!
The carb then fills normally with no leaks.
The brass tube was the vapor recovery tube for the fuel bowls and not a venturi vacuum port!
OK. Lesson learned. I also learned a lot about what makes these carbs work in the meantime. So that is a good thing.

But the question I ask is, why only a vent tube on the right side and not the left? A mystery for the ages, I reckon!

So, I put 'em back on, it fired right up, and fell into a nice idle without having to touch the mixture screws that I bench set at 2 1/2 turns from being seated.

The good thing is, no sledge hammers needed to be used to pound the carbs into submission, and I gotta tell ya, I was close to doing that! And I didn't have to spend any money.

 

[AlphaDogChoppers]

Blocked vent tubes can call all kinds of suffering. I've seen them get plugged with mud dauber wasp nests and completely disable a bike.

 

[Mike Lawless]

That brings up an amusing memory from the early 80s.
Re-assembling a small block Mopar race motor from an off-season freshening. Everything went together as smooth as glass. I had stored the carb and intake manifold inside a cabinet. I reckon about six or eight weeks.

I fire the thing up and it sounds great. Responds sharply when I whack the throttle. Pleased with myself, and confident that it would run well, off to the track we go when the next event comes along. All is well, until I get fully into the throttle and the thing falls flat on it's face. Back to the pits, checking this, that and everything. Then I pull the sight plug off the fuel bowl on the secondary side of the carb to check the float level. No fuel. I loosen the needle and seat (externally adjustable while the pump is running on race Holley carbs. A VERY convenient feature). No matter how much I screw it out to increase the flow into the bowl, nothing happens. I shut the pump off, and unscrew the seat assembly.

Wedged tightly within the assembly was a grub worm. So tightly, no fuel could pass.

So, the moral is to bag up your fuel system components while off the vehicle and not being used, even if the are stored in a cabinet. Other wise one can get undone by the ol' grubworm in the needle and seat trick....or the mud dauber in the bowl vent!

 

[ILoveThumpers]

Woah. I NEVER would have guessed that one right. Well done for tracking that gremlin down.

-Deek