1992 GSF400 Carb Conundrum

lknbandit400 · Post by **lknbandit400** » Tue Dec 15, 2015 9:25 pm

I've been searching for a B4 for over half a year and ran across this one. Talked the guy into holding it for me while I saved up the money and liquidated some stuff, and here we are.

As far as the tank goes, it very well could be a gradual leak that caused it. It's not wet and it isn't visibly seeping at least. I have done some research and it appears that a SP370 petcock will bolt right on, so I may have to give that a try. There's some generic petcocks out there to eliminate the vacuum but the outlet is always smaller than the 5/16 fuel line ID, so it would take some creativity to step up the ID through a universal filter, then back up to 5/16 for the carb bank inlet. I'll see what happens. Replacement vacuum versions are still available from my local dealer.

As for the repair - I think I'm going to do the same as you and just hunt a nice tank. It's a shame because the paint is incredible on the rest of it. Who knows, I might just leave it alone until my pockets are a little deeper. As long as it's flowing fuel into the carbs and not onto the floor, it's good with me for now.

I'll also assume the diaphragms are OK for now too. They all feel like they are falling slow enough to make me think there's a vacuum behind it, so that's good. I'll keep an eye on it.

As for the throttle stop screw, is there a setting from the factory that I should be shooting for? Is the butterfly supposed to be more or less closed line-on-line with the throat? I'm confident that the position of the throttle stop screw isn't correct, because I watched the PO try to adjust it trying to get the bike started for me, with no luck. Do I measure the distance from the bottom of the butterfly to the carb throat floor to get a reading?

SevenThreeSeven · Post by **SevenThreeSeven** » Tue Dec 15, 2015 10:43 pm

Congrats again, especially now that I know about your persistence in the search.

I also searched for quite a while. I have to admit that my first choice was actually the Honda CB1, so I was looking for that bike primarily with the GSF400 as my close second choice and as a third choice the Yamaha FZR400 (or any one of a whole pack of various Non-U.S.-imported grey-market Japanese 400cc 4-cylinder bikes that regularly turn up for sale in Vancouver Canada).

In the end I didn't have the kind of luck that you did with finding such a perfectly preserved example. I try to be philosophical about my outcome because my plan was to really change the bike from stock anyway, so maybe it was meant to be that I would only find a really scruffy B4, something I could tear into without feeling bad about destroying its well preserved history. Of course, in the time since I purchased my B4 I've had to watch people (forum members, you included now) pull several amazing bikes out of nowhere all across the U.S.

As for the throttle stop screw, is there a setting from the factory that I should be shooting for? Is the butterfly supposed to be more or less closed line-on-line with the throat? I'm confident that the position of the throttle stop screw isn't correct, because I watched the PO try to adjust it trying to get the bike started for me, with no luck. Do I measure the distance from the bottom of the butterfly to the carb throat floor to get a reading?

Okay, now you're revealing the actual depth of your knowledge with regard to carbs. Synchronizing or "balancing" a bank of carbs means equalizing the level of intake vacuum produced by each one (essential step to insure each cylinder gets fed an equal air/fuel ratio, preventing one or two from running too rich or lean).

Synchronizing a bank of carbs (i.e. 2 or more) has nothing to do with the throttle stop screw.

If you look at the B4's bank of Mikunis you'll see that there are spring-tensioned adjusters in the linkage between each carb and its immediate neighbor. To synchronize a bank of four carbs you set up a vacuum reference tool so you can monitor all 4 cylinders at the same time, then with the engine running at a Suzuki prescribed RPM you adjust the outboard linkages then the inboard linkages until you have all 4 vacuum references lined up together. There are plenty of "how-to" videos on YouTube and the Suzuki GSF400 Service Manual has a very good step-by-step procedure.

Regarding the throttle stop screw: once you do get your B4 up and running again you'll need to set it to idle the bike at about 1,300 RPM which is Suzuki spec. Less than 1,300 RPM and the engine doesn't come off-idle with enough enthusiasm and more than 1,300 RPM will make the bike run hot or overheat at stoplights in the summertime.

lknbandit400 · Post by **lknbandit400** » Tue Dec 15, 2015 10:56 pm

Ha - did I reveal too much? I think I actually more or less phrased the question incorrectly. I'm aware of what balancing carbs means with regards to flow. My question is that just like the throttle cable pulls on the sprung linkage opening the slide to increase air intake, the throttle stop screw does the same thing but with a finer adjustment.

Looking at the butterfly, you can twist the throttle stop screw and watch it gradually move. What I'm asking (granted not effectively) is if it keep turning the throttle stop screw to "bottom out" or fully close the butterfly, # 1 will the throttle stop screw stop turning once that happens and #2, how many turns from full closed in the opposite direction is a good starting point for the stop screw?

For the sync, the manual describes a process in which you grab a measurement at full throttle of the top of the butterfly (which is more or less horizontal at that point) to the carb throat. Then adjustments are made to equalize thy across carbs. As far as balance goes, from the measurements I have taken, they appear to be balanced across one another. A glance at plug color doesn't reveal anything way out of whack either. Might not be an excellent indicator but none are wetter or drier than the others.

Sorry if I came across ignorant there. It's a safe bet to assume that and let me clarify after the fact :)

SevenThreeSeven · Post by **SevenThreeSeven** » Wed Dec 16, 2015 12:22 am

What I'm asking (granted not effectively) is if it keep turning the throttle stop screw to "bottom out" or fully close the butterfly, # 1 will the throttle stop screw stop turning once that happens and #2, how many turns from full closed in the opposite direction is a good starting point for the stop screw?

#1 no, the throttle stop screw will keep turning (retracting) well after the throttle plates have come to rest on the metal of the carb bores. In fact, the throttle stop screw will continue backing out until it falls out in your hand.

#2 I've never heard of any particular setting for the throttle stop screw. If you look closely at the threads on the throttle stop screw you'll see it has very, very fine threads on it which means lots of turns for just a little movement (I really can't remember ever seeing anything on a vehicle, either car or motorcycle, with finer threading on it). So I would simply run it in until it just begins to lift the throttle plates. Then when you start the bike you can jockey the choke lever and maybe the throttle grip as necessary to get the bike fully warmed up. Then, with the bike fully warmed up, you set the throttle stop screw so the bike idles at 1,300 RPM. With that done the next start up from cold should just be a matter of using the proper amount of choke until the engine warms and settles into the 1,300 RPM idle you set.

I'm aware of what balancing carbs means with regards to flow. My question is that just like the throttle cable pulls on the sprung linkage opening the slide to increase air intake,

This is where my personality gets me in trouble... the absolutely compulsive need for clear communication (it's a positive in aviation but causes problems almost everywhere else). Can't... help... it... have to clarify: the throttle cable doesn't have any linkage to the slides, it moves the throttle plates.

SevenThreeSeven · Post by **SevenThreeSeven** » Wed Dec 16, 2015 12:36 am

I have done some research and it appears that a SP370 petcock will bolt right on, so I may have to give that a try. There's some generic petcocks out there to eliminate the vacuum but the outlet is always smaller than the 5/16 fuel line ID, so it would take some creativity to step up the ID through a universal filter, then back up to 5/16 for the carb bank inlet. I'll see what happens. Replacement vacuum versions are still available from my local dealer.

My recommendation would be to stick with a vacuum-actuated petcock. An old-school "remember to turn it off yourself" type of petcock is a long term gamble: you vs. inevitability. If you forget/neglect to turn it off and even one of your carbs has a float needle or float needle seat that allows gradual fuel leakage you'll end up with fuel in your crankcase oil (and its all downhill from there).

lknbandit400 · Post by **lknbandit400** » Wed Dec 16, 2015 12:44 am

Ah, no your personality to get things clear make forums easier - I appreciate that.

Also you'll have to forgive me. This is my first set of butterfly plate carbs (or CV carbs at all) that I've worked on. All carbs I've worked on have actually been throttle actuated slide carbs (round slide) so my terminology is out of whack. I'll use throttle plates and slides from now on. That should make things easier.

What you said in your answer to both # 1 and #2 makes perfect sense. Thanks for that.

lknbandit400 · Post by **lknbandit400** » Wed Dec 16, 2015 12:47 am

SevenThreeSeven wrote:
I have done some research and it appears that a SP370 petcock will bolt right on, so I may have to give that a try. There's some generic petcocks out there to eliminate the vacuum but the outlet is always smaller than the 5/16 fuel line ID, so it would take some creativity to step up the ID through a universal filter, then back up to 5/16 for the carb bank inlet. I'll see what happens. Replacement vacuum versions are still available from my local dealer.
My recommendation would be to stick with a vacuum-actuated petcock. An old-school "remember to turn it off yourself" type of petcock is a long term gamble: you vs. inevitability. If you forget/neglect to turn it off and even one of your carbs has a float needle or float needle seat that allows gradual fuel leakage you'll end up with fuel in your crankcase oil (and its all downhill from there).

It's a good recommendation. Problem could be solved with clear fuel lines and a filter to ensure fuel is flowing when troubleshooting.

lknbandit400 · Post by **lknbandit400** » Wed Dec 16, 2015 1:50 pm

Ironically enough, I pulled the slide springs and they are 34 turns and one end (bottom) appears cut. Interested to know what the stock spring looks like. I will reach out to Dynojet to understand how this impacts performance and why they include this in their kit...assuming the PO didn't do this himself....

Edit: just saw the post above where stock is closer to 38/39.

SevenThreeSeven · Post by **SevenThreeSeven** » Wed Dec 16, 2015 3:48 pm

Ironically enough, I pulled the slide springs and they are 34 turns and one end (bottom) appears cut. Interested to know what the stock spring looks like. I will reach out to Dynojet to understand how this impacts performance and why they include this in their kit...assuming the PO didn't do this himself....

Edit: just saw the post above where stock is closer to 38/39.

Well, that confirms it, both you and I bought B4s that had been "boy racer" modified.

The question now is "How far did he go?" Sometimes you find that the prior owner wasn't really into "wrenching" on the bike and for that reason he would only touch things that could be accessed without any real disassembly. On my B4 I found the CV springs had been clipped to 34 but there were no other modifications inside the carbs.

This sort of ignorant, dabbling, half-assed approach to carb modification was common when these bikes were new. Word of mouth and mechanically ignorant internet know-it-alls spread false information (myth and convincing sounding half-truths) about how to make your Mikuni carbs "better".

I think that the better aftermarket carb kits usually included a replacement CV spring rather than requiring you to clip the stock springs shorter. I think Dynojet kits provided new springs.

So the fact that you found clipped springs in your CV slides makes me wonder if your B4 was treated just like mine was: no real carb modifications other than clipped springs and (maybe) the prior owner went one step further and drilled out the U.S. EPA-mandated plastic blocking covers over the Fuel Mixture Screw so they can be adjusted.

But in spite of the "clipped spring" indication of mere dabbling, it is worth your time to investigate and see for certain what was done (or not). You may find there are aftermarket Dynojet needles installed in the CV slides, this would also mean that your Bandit's Suzuki stock needle spacers are gone too. Dynojet needles can be identified by the multiple "E-clip" grooves in their upper end (the Suzuki stock needles don't have multiple grooves at the top end because they weren't designed to be adjustable).

If you find Dynojet needles in the CV slides you can assume that a full Dynojet (stage whatever) carb modification was carried out on the bike.

And, as I mentioned earlier, this type of modification (carried out in search of power/torque the B4 could never deliver) often turned the bike into a hard-to-start, hard-to-ride, harsh-throttled, over-rich-running pig. The end result was a bike that wasn't much fun to ride or deal with and a lot of them ended up stuffed away and ignored in the back of a garage. The good side of that story is guys like you get to eventually find them and make them rideable again.

lknbandit400 · Post by **lknbandit400** » Wed Dec 16, 2015 4:24 pm

I've posted the BOM for the jet kit from Dynojet below - from what I remember when I first stripped the carbs, all of this is in place. The EPA plug you mention was pulled because I was able to access the idle air screw. I'll look for the jet needle tonight and that should tell me.

All main jets are marked with "100" which makes me think they are Dynojet jets.

4 Main Jet DJ096
4 Main Jet DJ100
4 Fuel Needle DNO208
4 Adjusting Washer DW0001
4 E-clips DE0001
4 Slide Springs DSP007M
1 Spacer Drill DD #32
1 Plug Drill DD #5/32
1 Screw DS0001

I certainly agree that it was hard to start - it took me about 4 battery cycles and two carb cleans to get the thing fired up to the point of the video I posted earlier. I should also check the plugs to make sure they are heat level 8 and not 9.

One more thing that I am thinking could cause the high rev is an air leak. The boots are nice and pliable so unless the carbs aren't properly seated, I have a hard time believing there's a crack. However, there ARE o-rings I have discovered under the boot/intake that could be bad. Do you think these are worth replacing while I'm waiting for the carb o-ring kit from Thailand?

It's number 19/21 in the fiche at Partzilla:

http://www.partzilla.com/parts/search/S ... parts.html