1992 GSF400 Bandit Carb Issues

I'll try to give as much background as I can here. Picked up this bike - 1992 GSF400 with a stage 1 jet kit. Fat spark on all 4 cylinders, and just cleaned the heck out of the carbs. I also replaced the pilot air jets and the pilot fuel jets because the heads we buggered.

So, here's what I'm dealing with:

Cleaned carbs (didn't disassemble bank)
New pilot and pilot air jets
Nice spark on all 4
Pilot screw turned 3 out to Dynojet spec

I got it fired up for the first time tonight and was surprised when it ramped right up to 9k RPM. I had the choke on so I backed it off and the RPMs dropped to about 5k and held there. That's where the video below pics up.

I tried both with airbox on and off without much change.

Symptoms:

Opening throttle actually drops the RPMs to about 2500

Opening choke (enrichener) actually brings RPMs back up to crazy high.

Video doesn't show this, but the diaphragm slides were chattering really bad. Tried to get this in the video but you can hear its impact more than see it.

Wasn't really smoking except for right when I cranked it up. Smelled like steam actually, maybe from water in the pipe or something? Cleared up after that.

Bike only has 3k miles on it.

Here's a video to show what I'm dealing with. I'm really baffled that opening the throttle drops RPM. I'll pull the plugs tomorrow to see what I find. Any ideas?

http://youtu.be/tAV4Dv5HFVU


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I should also mention that I tried to adjust the throttle stop screw while it was running but it didn't have much impact until it rapidly died - I can't attribute that to me turning it out too far or if I ran out of fuel from my umbilical tank:


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Bump - I just ordered a new carb o-ring kit from these guys:

http://www.litetek.co/contact.html

I have heard that if the o-rings start to go bad, major problems start to happen which causes some crazy symptoms. Any insight or experience with that?

What is it that people STILL don't understand about needing an airbox with cv carbs?

I mean really, not to be a prick here, but did it cross your mind that since these carbs are designed to work on specific amounts of stable air at a specific velocity, that completely depriving them of these conditions, could possibly cause them to malfunction?

Why did you put a jet kit in it?

Sorry 1sttimer but it HURTS MY HEAD to watch someone start and attempt to tune a bike with zero plenum, airbox, or filters. Especially a CV carbed bike. How could it POSSIBLY run right with none of this?

Don't feel bad, it's an honest mistake, but maybe not one a DTT member, with all this reading material, should make.

If you actually read the post, you'll find that I tried both with and without the airbox to see the difference.

My intention was to take the video with the airbox off so you can see the diaphragm slides actuating. That's why the airbox is off. I did it on purpose.


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Actually, I did everything I could to ENSURE the bike had such stable conditions when I first cranked it up. I had the bike running, yanked the airbox with no resultant change and took a video. That is what you see above.


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It certainly will make a difference, unless your carbs are WAY out of tune or your ignition is malfunctioning. Did you balance them while the airbox was on.

Showing us what happens to the slides when the airbox is disconnected is irrellevant.

May sound silly but... have you checked the throttle cable routing? are all the slides sat at the bottom of their travel with just a 1mm or so air gap?

you said you're using an "umbilical petrol source" does your bike use a vacuum tap.. if so did you block up the vacuum pipe from the carbs??

by the sound of your problem it has all the hallmarks of air leaks... ie: carbs drawing too much air in. could be perished inlet rubbers?

GreyDay said:

May sound silly but... have you checked the throttle cable routing? are all the slides sat at the bottom of their travel with just a 1mm or so air gap?

you said you're using an "umbilical petrol source" does your bike use a vacuum tap.. if so did you block up the vacuum pipe from the carbs??

by the sound of your problem it has all the hallmarks of air leaks... ie: carbs drawing too much air in. could be perished inlet rubbers?

Click to expand...

This is right where my head is at, man.

To answer your questions, cable routing is good. I have a feeling that the idle stop screw setting may be off, however and the manual is absolutely terrible at describing the setting. Sounds like "get it close and then turn to achieve 1400 rpm. Here's the manual on that:



Shooting for 1 mm is what I'll try.

As for the fuel tap, it is indeed vacuum. I have the tank off because I know the fuel tap needs to be replaced. I do have a silicone plug in the hose that attaches to the fuel tap to block that off.

I am thinking it's an air leak too, but for goodness sake I don't think the thing has ever seen rain or bad weather. Hard for me to believe that if everything else is in such great condition, the rubbers would be bad but it's possible. I'll get it running and shoot carb cleaner around the intake tonight to see what happens.


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HaHa... that manual looks a bit like the manual i had when i rebuilt my VFR400NC24... except mine was in japanese

Dont open the butterflies (the round bit pictured) as they're usually fully closed at idle.. what you need to see is if the slides are open, If you open the throttlevalve so the butterflies are fully open you will see the throttle slides(tubes/Pistons) that have the needles protruding from the bottom. these should be open just a little, not the butterflies.. my NC24 had small deposits of dirt in the carb bodies that were holding the slides partly open and this caused a fast idle... to sort it i had to take the carb tops off and clean the slides and carb bores so they didn't hang up.

it's an easy check as long as the carb top screws come out easy, just be careful not to damage the diaphragms when taking them out.

it's not uncommon for the butterflies to stick, the SV650 carbs were terrible for sticking butterflies, you can flick the pistons up with your finger and they should each return back at the same rate. if they feel a bit sticky or rough when you slide them up then they'll need cleaning. they will feel like they're oil damped which is the diaphragm and spring doing their thing. but overall the slide movement should feel smooth.

just had a quick search around the interweb thingy, and found this.... https://rideapart.com/articles/fix-5-common-bike-issues may help..

if you still have the standart jet kit.. i'd try that as well

False Idles

If your bike isn’t idling right, warm it up and take note of if adding choke (making the mix richer) or cracking the throttle ever so slightly (making leaner) makes the bike change behavior. Even Electronicly Fuel Injected motorcycles reveal a lot by “blipping” the throttle. Rich bikes are getting too much gas, so they idle high but lose power when given initial throttle because they can’t burn all the gas. For carburetors that suddenly show signs of being in a rich condition, gently whacking the bottom of the carbs to make sure the floats (tiny little pieces that control gas flow like the rubber piece in the back of your toilet) are free. Also check that the choke cable works. After that, solving a rich condition gets more involve such as checking the spark plug gap or fuel bowl levels.

Lean running engines tend to “hang up” at high rpms when the throttle is let go, and they also rev down slowly after the throttle is closed. A key clue is that the issue gets better as the bike warms up or that you are running hot. Extremely lean motors can also stall out completely when given some light throttle. Another signs of lean running is the “hunting idle” where the motorcycle revs up randomly, changes RPMs or takes forever to come down to idle.

First, start simple and look for a loose or open vacuum hose. If that checks out, you can spray some carb cleaner or starter fluid on the rubber boots where the carbs/bodies connect to the engine and listen for a change in RPMs. Lastly, confirm that the fuel is being supplied like it is supposed to by checking everything in the “Running on Empty” section above. If it is, lean running is the number one sign you need carb work but it can also indicate you’ve really gone a long ways between tune-ups.

1sttimer said:

I am thinking it's an air leak too, but for goodness sake I don't think the thing has ever seen rain or bad weather. Hard for me to believe that if everything else is in such great condition, the rubbers would be bad but it's possible.

Click to expand...

Rubber deteriorates just with age as well as being weathered. Your intake manifolds are 23 years old.

hillsy said:

Rubber deteriorates just with age as well as being weathered. Your intake manifolds are 23 years old.

Click to expand...

Yeah again it's possible, I agree. It's even harder to me to believe it's been 15 years since the millennium


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Just realized your manifolds have O-rings, so it's probably them rather than the whole manifold.


BTW - didn't you change out the air jets for different sized ones??

hillsy said:

Just realized your manifolds have O-rings, so it's probably them rather than the whole manifold.


BTW - didn't you change out the air jets for different sized ones??

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Well, I had to because I couldn't find 1.45 sized air jets anywhere. I went to 1.5 so yes, technically. But 1.5 is spec for California (no idea why there's a different spec - emissions?) specifically so I figured it would get me in the ballpark at a minimum. If I had the choice of going to 1.4 or 1.5, which I did, I chose 1.5. I wouldn't say that it would have no impact, but if nothing else is changed between the California model except this, I wouldn't think it's my only problem I'm struggling with.


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I agree - there's something much more "wrong" than just a step up in air jet size for your bike to be idling at 9K rpm.

BTW - looks like there's 2 air jets per carb?





Did you remove both? Put em back in the right spots?

hillsy said:

BTW - looks like there's 2 air jets per carb?

Did you remove both? Put em back in the right spots?

Click to expand...

Good eye. One is removable and the other is "pressed" in (just a brass hole). I did clean both passages.

I'm still leaning toward o-rings and air leak. I've got the carbs back off now and the boots look nice and pliable. Maybe the junction between the boot and head like you mentioned.


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Did you make sure you didn't lose the tiny O ring under each of the carb diaphragm caps?


Also, bench sync your butterflies as best you can.

hillsy said:

Did you make sure you didn't lose the tiny O ring under each of the carb diaphragm caps?


Also, bench sync your butterflies as best you can.

Click to expand...

Check. As hard as it was to keep track of those little things, they are all in place. And the diaphragms are in the groove properly. The butterflies are pretty dang spot on. Visually checked and caliper measured to the throat.

Thanks for all the double checking hillsy. These are all important things. Reason is say o-ring kit is needed is because the float assembly was hard to install because of the o-rings. If those are crusty, all the others are as well probably, most notably the o-ring under the idle air screw...


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