1992 GSF400 Carb Conundrum

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lknbandit400
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1992 GSF400 Carb Conundrum

Post by lknbandit400 » Tue Dec 15, 2015 2:52 pm

Hi all, first post here. I'm in NC over in the US and I picked up a 1992 GSF400 Bandit a couple of weeks ago. It is a real garage queen with only 3800 miles on it. The previous owner had it running (showed me a video) but let the bike sit. Pilot jets got clogged and wouldn't start the day I went to pick it up...so that's the background. I should also mention the bike apparently has a Dynojet Stage 1 kit.

Fast forward a couple weeks, I have stripped the carbs of all internals for a thorough cleaning. I have also replaced the idle air jet and the pilot fuel jet because the head was stripped, so those are brand spanking new. :grin: I have also cleaned all ports and verified that both air and carb cleaner is adequately making its way through all circuits of the carb.

The only thing I am thinking I should have replaced was the O-rings, but to foreshadow a bit, I have just purchased a set from Lite Tek. No idea when they will be here.

Now fast forward a little more to last night. Got the bike buttoned back up and had a terrible time getting it to start, after some coaxing, turning of the idle jet, playing with the choke lever, she fired up....and went right to 9k. :roll:

I immediately backed the choke off, and the RPM's dropped to about 5k and held there. Any time I would apply the choke, the revs would climb again. Before I go further I should also say I'm confident the throttle is installed correctly....however, when I applied throttle, the RPMs would actually fall, like it was bogging.

I did notice that the float assemblies were a little difficult to install, making me think I should need new o-rings there. The o-rings on the pilot screws looked OK, maybe a little dry - hard to tell from looks. O-ring under the top cap felt and looked good.

So to get started, I have a few generic questions:
1. Is there a standard setting for the throttle stop screw? I found the manual and the only comment it has in there the following, which confuses me: "Set each throttle valve in such a way that its top end meets the foremost bypass" What?

2. I have the pilot air screw set at 3 turns out because that's what dynojet's website says - sound right?

3. I have read a lot about an o-ring at the emulsifier tube? Is that just for a different carb model? My carb does not look like the following, but there definitely isn't an o-ring around my emulsifier tube. There may be around the slide holder (white thing). Am I missing something?

To make my symptoms more clear, and to give you guys the best information as possible, I took a video. You can't quite hear it, but the diaphragm slides are also chattering quite a bit as well. I did check for holes in the diaphragms, but didn't notice anything.

Thanks in advance.....

https://www.youtube.com/watch?v=tAV4Dv5HFVU

lknbandit400
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Re: 1992 GSF400 Carb Conundrum

Post by lknbandit400 » Tue Dec 15, 2015 3:06 pm

I should also post up that I ONLY took the airbox off to try to video what the diaphram slides were doing. I'm aware of how the airbox can impact CV carbs!

SevenThreeSeven
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Re: 1992 GSF400 Carb Conundrum

Post by SevenThreeSeven » Tue Dec 15, 2015 4:25 pm

Hello fellow U.S.A. B4 owner! There aren't that many of us.

Judging from your write up on this Bandit and from your short video I would advise you to just stop trying to run the engine and wait for the Litetek kit to arrive so you can get the carbs sorted properly. It looks and sounds like your air/fuel ratios are really messed up, all over the place and nearly uncontrollable at idle. I just don't think you're at the point where test-running the engine is going to be of any help to you.

I think your next move for this bike should be a very careful, thorough "by the numbers" carb inspection/inventory and refurbishment (with the Litetek kit), making sure that every rubber item is where it is supposed to be. Then, and only then, remount the carbs very carefully so there are no air leaks around the carb-to-engine connectors and try running it again.

(Further detective work on your post): You mention that your B4 was a "garage queen" and you also mention that the carbs appear to have been subjected to modification with a Dynojet Stage 1 kit. This phenomena: being pushed to the back of a garage and forgotten, is actually a common situation for B4s .

The reason behind the abandonment is usually that the B4 somehow became an untunable/unrideable beast due to over-rich fuel mixtures which often results from carb tuning efforts by people who know nearly nothing about carbs (there is an almost universal assumption among the uninformed that "richer is better" or "richer means more power"). A secondary (but closely related) contributing issue that pushes the bike into abandonment is the idiotic pursuit of lightning-quick throttle response. This fixation leads unskilled people to do things like trimming length (coils) off of the carb CV diaphragm springs to reduce their hold-down force. This results in carb CV diaphragms that immediately snap open at the first hint of intake vacuum which makes the just-off-idle throttle control very difficult. The fact that you said your slides visibly "rattled" during your initial start up trials makes me suspicious that the springs in your Mikunis have been messed with and possibly modified (i.e. cut).

The root cause of this sort of abuse (the psychology that motivates it) is probably a product of the B4's basic nature, being a high-revving small displacement creature in a world of larger displacement motorcycles. In case you're expecting your new B4 to have anything going on below about 6,000 RPMs I want to straighten you out right now. Your B4 has to rev to produce its power, there is no low-end grunt in this motor. Scenario: a guy gets his hands on a B4, he's impressed with the bike's looks and sound but it doesn't have any low-end torque (like his buddy's 600 or 1000 supersport does) so he digs into the carbs hunting for power that the B4 cannot, will not, deliver. Humiliation and disaster follow...

I really like my B4 and I'm okay with its rev-happy nature and the lack of anything below about 6,000 RPM. I've discovered that I have to be in the right mood to enjoy riding it, I have to be in the mood for DRAMA! Revs, noise, intensity. There's not a "laid back" bone in its whole body. When I'm in the mood for a calming, relaxed riding experience I take out a different bike.

So, on the bad side, the carbs on this new-to-you B4 may very well be in a weird state of tune. This situation could take some effort to overcome but is completely correctable. On the good side, if the "boy racer" prior owner hadn't messed with the carbs and made the bike into a hateful, unrideable over-rich pig it probably wouldn't have been abandoned in the back of a garage to wait 20 years for you to come along.

Do you have the PDF file of the Bandit 400 Service Manual? If not I can email it to you if you want (PM me). Also, congratulations on the "barn-find", the bike looks very nice in the short video you posted.

Greg

p.s. My 1993 Bandit 400 is my "tinker-toy" project. I have modified it heavily, including a fuel injection + ignition control conversion (nothing done to increase its absolute horsepower, just a thorough modernization of the bike). My forum build thread for the project is over on BanditAlley at this address: http://forums.banditalley.net/index.php?topic=13747.0

lknbandit400
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Re: 1992 GSF400 Carb Conundrum

Post by lknbandit400 » Tue Dec 15, 2015 4:58 pm

Hi Greg! Thank you so much for your comments. There's tons of information there. I'll start by saying that I did try to subscribe to Bandit Alley but my registration authorization is still pending. Strange. I've never had it take more than a day.

I will heed your advice and push pause until I get the o-ring kit in house. I'll take the carbs back down and get some pictures posted of what I'm dealing with.

As for the nature of the bike, I'll be totally content with the rev happy inline 4. It's precisely the reason I got rid of my CX500. I'm just not a huge fan of low grunt. I'm happy to play at moderate speed and high RPM. My other bike is a CB175 and my commute is short enough to have fun on twisties without spending time on the highway.

To your "boy racer" comments, is there a way I can tell if I have a Dynojet kit installed? I looked on their website but I can't find a BOM of what the kit comes with, only a picture of the box. Thy would probably help me get a baseline. I also think the spring is stock and not modified. If you have the free length of one I will verify.

Hang in there with me, I'll need your help! What side of the country are you on?

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Re: 1992 GSF400 Carb Conundrum

Post by lknbandit400 » Tue Dec 15, 2015 5:03 pm

FYI here's a picture for reference.

[img]//images.tapatalk-cdn.com/15/12/15 ... 452d9d.jpg[/img]

SevenThreeSeven
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Re: 1992 GSF400 Carb Conundrum

Post by SevenThreeSeven » Tue Dec 15, 2015 5:22 pm

I'm in Spokane, WA (Spokanada, Spokanistan, Spokompton, Spoklahoma... I'm a Spokandinista)

Are you absolutely sure your CV springs are stock? I found that my B4's previous-owners, obviously a boy-racer type, had "trimmed" my B4's springs. I could tell that the springs had been clipped because they didn't have the usual "finish" turn to one end (you know, where a normal spring flattens out and loops around to touch itself).

Here's a picture of my springs (as they looked when I opened up my carbs):
Image

You can see in the picture that one end of each spring (the end oriented upward in this picture) has been clipped. I did some research and discovered that stock B4 CV springs (U.S. B4s have Mikuni BST32 carbs) have 38 or 39 "turns" to their windings. Each of my CV springs had been clipped down to exactly 34 turns. This modification resulted in very abrupt "off-idle" throttle response.

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Re: 1992 GSF400 Carb Conundrum

Post by lknbandit400 » Tue Dec 15, 2015 5:27 pm

That gives me something very concrete to check for sure. I will pull and check - and I understand the spring finish taper you reference (like the bottom side of your spring pic above).

How about the diaphragm itself? Is there a test to determine the condition of them?

I will admit that in my ignorance I got some carb cleaner on them which caused them to shrivel - I added a little lubricant and let them sit for a day in an unfixtured position and they came back to normal shape. I didn't have any issue installing. When you manually pull the slide up and let it fall, all slides fall at about the same rate. I'm not sure how fast they should fall, though. I can't find a spec on it in the manual other than checking for pin holes.

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Re: 1992 GSF400 Carb Conundrum

Post by SevenThreeSeven » Tue Dec 15, 2015 5:52 pm

Just saw your picture of the B4 on the trailer. Wow, a beautiful time-capsule bike. That is some real luck finding something like that. Hard to believe there are bikes like that hiding in garages and basements.

Don't be put off by slow movement over at the BanditAlley web site. The Bandit world is very small now, getting smaller all the time as the bike ages so there's not much going on (compared to other bike sites), this tends to make the BanditAlley forum a relatively "low-energy" experience, but there's still a small core group of active and engaged owners and we're a good, knowledgable, helpful bunch so come join us.

You say you're in NC. I'm from The South too, Birmingham AL. Ended up in Spokane, WA during my U.S. Air Force days. I go to Raliegh-Durham occasionally (I fly for Southwest Airlines and get the occasional overnight there).

Back to the CV spring issue. I'm pretty sure there were 2 ways to change the CV springs: One is the obvious, crude clipping/shortening of the springs, and the other more subtle way was new CV springs that are part of a "stage" kit. It would be easy to spot clipped springs but pretty difficult to determine if new, softer springs had been substituted (because who really knows what normal Mikuni CV springs look like vs. something like "Stage whatever" Dynojet springs?).

The idea behind the soft-CV spring thing is a racing thing (or boy-racer thing). In racing applications you're going to be using just two throttle settings 98% of the time: Fully closed throttle or wide-f*ing-open. So wanting the CV slide to quickly get up and out of the airflow pathway is a plus. But if you're trying to enjoy a daily drive commute to work this modification is a pain in the butt.

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Re: 1992 GSF400 Carb Conundrum

Post by lknbandit400 » Tue Dec 15, 2015 6:11 pm

"SevenThreeSeven" makes sense in that context then! Ironically, this bike came out of Alabama. Birmingham in fact.

Apart from the carb issue, the only thing wrong with the bike is a spot on the bottom right side of the tank near the fuel tap. Looks like the bike sat for years and developed a pin hole leak. Someone took some sort of filler and tried to fill it (succesfully) but didn't finish the job with paint. So there's a bare metal spot with filler which is killing me inside just looking at it. While I deal with the carbs, I may send that out for repair. Have any good bodywork guys you like to use? I'm happy to ship it for the right person to fix it.

I'll dig into what the DynoJet kit comes with.

As far as the CV, I was more referring to the actual diaphragm - if it was possible to determine it's condition based on looks. I can't see any pin holes and the rubber is nice and soft, so I'm assuming those are OK? I'll take a pic some time soon..

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Re: 1992 GSF400 Carb Conundrum

Post by SevenThreeSeven » Tue Dec 15, 2015 7:57 pm

Ironically, this bike came out of Alabama. Birmingham in fact.
Man, that hurts. I visit Huntsville and Birmingham all the time and having a B4 (stored in one of my brother's garage) to ride in Alabama would be great. And there you go, poaching on my hunting grounds.
Apart from the carb issue, the only thing wrong with the bike is a spot on the bottom right side of the tank near the fuel tap. Looks like the bike sat for years and developed a pin hole leak. Someone took some sort of filler and tried to fill it (succesfully) but didn't finish the job with paint.
Fuel tank problems are a pain. Is this hole spot on the bottom of the tank where it's hidden (i.e. on the bottom, inside the bike's frame perimeter)? Or is it normally visible? Properly fixing a hole in a fuel tank is an art, I don't have any go-to people. I actually had to purchase a good-condition used tank off of Ebay to replace the badly dented and rusted tank on my project B4.

The rubber gasket that seals the petcock to the bottom of the B4 fuel tank will harden with age (your '92 is 24 years old, that'll definitely do it) and then it leaks/seeps fuel along the bottom of the tank. Over time you'll find an area of softened or removed paint on the bottom of the fuel tank. Tightening of the petcock bolts won't help the situation at all.

As far as the CV diaphragm rubber pieces are concerned, yes, a careful visual/tactile inspection is the best you can do to determine their individual condition and long term viability (keep the carb cleaning chemicals away from them). After very careful and precise installation of the diaphragms performing the raise-and-drop with your finger test is useful in that it does help you to verify you've got the diaphragm flanges properly seated and secured in the tops of the carbs (a diaphragm that isn't properly seated and secured will drop very rapidly, all spring-action with no evidence of suction, while properly installed diaphragms will come down more slowly with a bit of suction sound. Also, the vacuum ports on top of the black plastic CV diaphragm caps have to have their little block-off caps in place). Then you'll have to "operation test" them when you get to the point of reinstalling your refurbished rack of carbs to see if they react to engine vacuum in a reasonable way: progressive rise to top with gentle throttle application, then moderate falling back in place with throttle release.

Another thing you can do to prepare for your eventual idle test runs is to "garage bench sync" the rack of carbs. All this entails is visually checking to see if the throttle plates of your rack of carbs are all adjusted to exactly the same angle. I've heard of guys using feeler gauges for this or just shining light from behind the throttle plates and eyeball-comparing the gaps. If you have a carb sync vacuum testing tool for a 4 cylinder bike you can pass on this if you choose although it might help for initial startup as just one really out-of-sync carb can run the engine's idle off the scale.

(Thinking long-term for your carb refurbishment) If all else fails there's a guy who goes by the screen-name "Ducatiman" on the Kawasaki EX250 website www.ninjette.org He's a real artisan/craftsman type of guy and he does beautiful work refurbishing carbs of all types. https://www.ninjette.org/forums/showthr ... =ducatiman


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