'91 Bandit VC - Summer carb teardown

[T2098]

After trying unsuccessfully to get my poor '91 to run correctly in the low-end, I've finally torn the carbs off and am working on rebuilding them, along with installing:

1) Factory Pro CRB-S41-3.0 carb kit
2) K&N RU-1824 Pod Filters
3) Complete Viton o-ring kit from Thailand (http://www.litetek.co/Carb_Kit_Suzuki_GSF400.html)

Symptoms previously were:

- It was always very hard to start.
- Once started, it idled alternatingly on 2-3 or 4 cylinders unless the idle was turned up to ~2000rpm, or the choke was all the way out (even when warm)
- Snapping the throttle open below 6000rpm would give a significant bog, which was worse when cold.
- It wouldn't accept full throttle below 4000rpm (even gently!) in high gear.

Since I'm currently without a choke cable, my temporary workaround before tearing the carbs off was to take a twist tie looped around the choke plunger rod, tie it to my clutch cable, and keep twisting until the choke was propped about 1/3 of the way open.   This actually made it run decently well when warm and greatly reduced the fueling issues below 6000rpm.  Clearly I was running lean.

Before pulling the carbs off, I found that all 4 pilot screws were about 5 turns (!!!) out.  After resetting them to factory spec of 1 5/8, the bike wouldn't even start with full choke.  At that point I gave up and decided the carbs were coming off for a rebuild.

[T2098]

With the float bowls off, so far I can see that the previous owner put the main jets in the wrong spot.  The Japanese-spec bike is supposed to have a 102.5 in cylinders 2&3, and 105 in 1&4.  Currently the bike had the 102.5 on 1&3, and 105 on 2&4.  Clearly the last person working on the carbs wasn't paying much attention.

[Squishy]

Sounds like it wasn't getting enough fuel. I think it's just cogged up pilot jets.

The bog sounds like a problem with your needles or diaphragm
Depending on mileage, they could be worn..

What did you find when you opened them up?

[Squishy]

With the float bowls off, so far I can see that the previous owner put the main jets in the wrong spot.  The Japanese-spec bike is supposed to have a 102.5 in cylinders 2&3, and 105 in 1&4.  Currently the bike had the 102.5 on 1&3, and 105 on 2&4.  Clearly the last person working on the carbs wasn't paying much attention.

Are you sure?
Usually the inner carbs have bigger jets to cool them more, not the outer ones.

[T2098]

100%.   I bought a copy of the JDM Bandit service manual in hardcopy for when I finally have the dreaded task of doing my valve clearances, and it's very clear that 105 is for cylinders 1&4, 102.5 for 2&3.

Tough to say why - from the service manual it seems to be a quirk of the VC models only - the later 53HP 'restricted' version was set up the same way, whereas even the JDM versions of the non-VC engine had #100 main jets in all 4 cylinders on the same 33mm carbs.

[T2098]

At least my OEM needles don't appear to be appreciably worn, and the emulsion tubes (needle jets) don't look visibly ovaled either.   Crazy to see just how much bigger the jets in the Factory Pro 3.0 kit are than stock for the USA-spec bikes.   (the kit comes with 117.5 and 120 mains)

Part of the secret sauce might be in the needles though, which are very visibly different than the OEM ones - the Factory Pro ones have a smooth taper all the way down rather than a very abrupt step - I wonder how much this is responsible for the fact that the main jets are so much bigger (the Factory Pro needles looks like they'd block more of the flow from the main jet, even wide open):

[T2098]

Managed to get everything all clean now, and remarkably, even at 25,500km, my needle jets definitely still look round, so I'll be re-using them.   For anyone who was ever curious, the stock markings on them are "659 P-0" (P-zero) although I've yet to find anyone other than Suzuki who sells 659-series Mikuni needle jets.

[T2098]

Hooray, it runs again.   Now to button everything back up.
https://youtu.be/yysZGXn8OEY

Initial setting of 2.5 turns out on the mixture screws with the stock #32.5 pilots looks to be just about right.  Even without the pod filters on, off-idle pick up is way better than before, even with the choke off and the bike still rather cold.  With the previous mixture screw settings of 5 (!) turns out, the bike would only accept throttle slowly from idle with the choke off, even when completely warm.

[ventYl]

I've had such a crazy pilot screw setup too. it was maybe 3.5 turns out. The bike started on first crank but the mixture was so rich that everything other than slow throttle caused it to heavy misfire and slowdown. Then black huge smoke flown out of silencer and engine began to accelerate. Now I'm on stock 1 1/4 turns and it is maybe slightly leaner that it should but definitely better response to throttle than before. I guess I am on opposite side of mixture hill.

[Chris H]

Good to hear you got it running.
I use the Dynojet needles on my VVC and run same jets across all 4 cylinders, its been on the dyno and fuel is spot on.
Be carful comparing jets from different companies as the rate differently so a mikuni 110 can be different to factorypro etc, if you need to change them slightly get same make.

One of the problems with the b4 is the pilots are linked to the main emulsion tube and then rely on the rubber plug to make this happen, this eventually goes hard or worse is lost by previous owners and causes major headaches with fueling.

A little fine tuning and you should be there.

[Rich_T]

Sorry to jump on this thread, the little rubber plugs, are they still available do you know, one of mine was lost by a previous owner and the others arent great. Cheers

[Chris H]

13357-34C00 is the part number.
You in the US, UK?

[Rich_T]

Thanks, I can be in either, I live in the UK but my Mum lives in the states.

[Chris H]

Speak to Robinsons then they supply most of the shops anyway.

[T2098]

Thank-you for the advice Chris and everyone else. :)

It's running like a champ now - after swapping to the 195F thermostat, it idles much happier and off-idle pickup is definitely better.   It's funny, I'd read some threads before about people not wanting to run individual/pod filters on the Bandit 400 due to intake resonance, noise, etc. being too unbearable, even when wearing a helmet.

Some of this might be due to the very sedate cams the VC model uses in the low RPMs, but if I didn't know the pod filters were there because I installed them, there'd be no way I'd know the difference.  I've run other bikes with individual filters before, and my VTR250 Interceptor (V-twin) and GSX400e (parallel twin) both got almost unbearably noisy with intake resonance/"honk" without the airbox - the Bandit does not.

It is noticeably noisier/angrier above 9k or so when the VC switchover happens, but I consider that a good thing - if I'm revving it above 9k, I'm obviously trying to give it the business, and so a little extra intake noise is no biggie.   When cruising around though on the low cam though, it's quiet as can be.

Fun facts from the JDM Service Manual:

Advertised duration on the 'high' intake cam is 292 degrees on the '91 VC model, same as the '91 USA 49-state or Japanese non-VC model, with 7.5mm lift

Advertised duration on the 'low' intake cam is only 232 degrees with 5.8mm lift though, which is part of why it's so gentle and tractable at low RPMs.