Bandit 400 variable valve, won't rev past 10k

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Jones5563
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Bandit 400 variable valve, won't rev past 10k

Post by Jones5563 » Tue Dec 01, 2015 8:06 pm

Hi, I have a 1991 bandit 400 VC variable valve (red rocker cover). I recently bought it and had a major service and repairs, including carbs balanced and adjusted, new coils, new throttle position sensor etc etc. It now runs beautifully up to 10k revs, however it then makes a nasty mechanical noise (similar to engaging the starter motor when it's already running) and won't rev any higher, however Im aware it should rev upto 14k! I imagine this has something to do with the dreaded variable valves, however I have no idea where to start! Any help very gratefully received! Cheers, Andy.

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Re: Bandit 400 variable valve, won't rev past 10k

Post by SevenThreeSeven » Wed Dec 02, 2015 12:11 am

Have you seen this video on YouTube? https://www.youtube.com/watch?v=7IIlplh-Yt4

Even if you don't speak Japanese there's a lot of information in this video. It can help you visualize what's going on inside your variable valve motor.

Generally speaking your description of the problem sounds like an engine "running out of breath", which means it's running into the limit of its volumetric capability. Internal combustion engines are basically air pumps. The amount of horsepower you can make is absolutely limited by the amount of air that can move through the engine.

The purpose of the variable valves was to produce more low and mid-range horsepower, not more top-end horsepower. The valve profile on your variable valve engine that makes better horsepower/torque up to about 9,500 RPM falls flat on its face (volumetrically speaking) at 10,000 RPM. If your engine is unable to shift out of the low and mid-range valve profile it will be unable to rev out to the redline. It just won't be able to move enough air through the cylinders to continue accelerating.

I don't own a variable valve B400, mine is a 1993 U.S. model, it's the first year of the "de-tuned" Bandit 400 (less valve lift and duration, resulting in 54hp rather than 59hp). Do you have the Suzuki Service Manual for your Bandit? I understand that the Variable Valve Bandit Service Manual has a procedure for testing/troubleshooting the variable valve system. If you don't have the Service Manual here's an online SlideShare version of the chapter you need: http://es.slideshare.net/utena8/manual- ... ndit-vv-97

If you look at page 11-47 you'll find the variable valve system troubleshooting matrix for "Not Reaching High Speed and Power Insufficient".

And on page 11-48 you'll find the troubleshooting matrix for "Abnormal Switching Sound Occurs During Running".

Good luck getting your engine sorted.

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Re: Bandit 400 variable valve, won't rev past 10k

Post by Jones5563 » Wed Dec 02, 2015 1:32 pm

Wow, fantastic! Thankyou for such a comprehensive reply.

I will do a bit of investigating and let you know what I find out.

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Re: Bandit 400 variable valve, won't rev past 10k

Post by Jones5563 » Fri Dec 04, 2015 5:23 pm

OK so here's the latest: I went through the diagnostics and listened for a click at 9k revs which would indicate the actuator engaging, and there was none, which pointed to an electrical system failure. I then traced all the wires to check for broken/loose wires and found the connection to the ignitor was loose. I gave the connection a tighten and hey presto, it worked! while sat stationary the bike revs happily to the 14k redline! fantastic.

Unfortunately that is not the end of the story, as I then of course took it for a test ride, and found that under load (particularly under sharp acceleration) it seems to bog down at around 8-9k and will go no higher, certainly nowhere near the redline. The valves still seem to be working because if I then hold in the clutch and rev the throttle it will happily go to the redline. This suggests to me it may be a separate issue? or perhaps the bike needs to get used to the valves working again? I'm aware the bike has been sat for a while without a decent run, so I'm thinking perhaps once its been run for a while it'll sort itself out?

If anyone has any better ideas (and I imagine that's likely!) then any help would be gratefully received.

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Re: Bandit 400 variable valve, won't rev past 10k

Post by SevenThreeSeven » Fri Dec 04, 2015 7:31 pm

I was re-reading your posts on this thread, trying to get as much information as possible so I could think about the situation and maybe help out a bit. I noticed something you said in your original post:

In your listing of items replaced and the maintenance you had done on the bike after purchasing it you mention,
new throttle position sensor etc etc.
None of the Bandit 400 models have a throttle position sensor on their Mikuni carbs. I went back and looked at the GSF400VV wiring diagram and the Ignitor Box pin-out listing just to be sure and couldn't find any indication of a throttle position sensor.

(Some of the GSX-R400 models had a throttle position sensor on their Mikuni carbs. This throttle position sensor was wired into the GSX-R400's Ignitor Box. The GSX-R400 also had an extra coolant temperature sensor mounted into the bottom of the thermostat housing. This extra temperature sensor was also wired into the GSX-R400's Ignitor Box.)

Do your carbs actually have a throttle position sensor mounted on them?

This thread contains a picture of the GSX-R400 carbs with the Throttle Position Sensor: viewtopic.php?f=53&t=23128 (as you can see it mounts on the Cylinder #1 carb body)

I'm not trying to irritate you or be pedantic with this questioning, I'm trying to rule out possibilities.

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Re: Bandit 400 variable valve, won't rev past 10k

Post by SevenThreeSeven » Fri Dec 04, 2015 9:23 pm

Your description of the acceleration problem leaves me with questions: In your first post on this thread you said,
It now runs beautifully up to 10k revs,
Does this mean that when you rode it the Bandit would accelerate HARD under full throttle, but upon reaching about 10,000 RPM it stumbles and goes flat, unable to continue accelerating?

Then in your most recent post to this thread you said,
I then of course took it for a test ride, and found that under load (particularly under sharp acceleration) it seems to bog down at around 8-9k and will go no higher, certainly nowhere near the redline.
Does this mean the bike will accelerate HARD under full throttle, but goes weak at about 8-9k RPM?

Or... is the engine just generally weak in acceleration? Or... if it accelerates HARD under full throttle what does the stumble/goes-flat moment feel and sound like? I'm trying to imagine if the failure is fuel-related or ignition-related.

You're in Europe somewhere, right? Do you know if your Bandit had (or has) some sort of horsepower restrictor kit/modification on it?

How much of do you know about your Bandit's history? Is the seller available to contact and ask questions?

Does your Bandit's Ignition Control Box still have its factory identification sticker on it? What does it say? My research tells me that a Variable Valve Bandit should have an Ignition Control Box marked: 32900-33D00 BP7242. I ask because there appears to be another Ignition Control Box that has the same connector (a single 12-pin connector plug, just like your Variable Valve Bandit's Ignition Control Box plug) but was for the GSX-R400 GK76A, this box is marked: 32900-10D30. (Sometimes people replace a bad Ignition Control Box with the wrong one)

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Re: Bandit 400 variable valve, won't rev past 10k

Post by Variablevalves suck » Fri Dec 18, 2015 9:55 pm

You could have a number of problems together but for sure the. VVC system is not working correctly.
Its at 10000rpm that the VC swaps from low to high and the reason it goes flat is because when it does not swap cams the low cam goes past its optimum power point and the power drops off same as it would at 14000rpm om the high cam.

You would clearly hear the engine note change if the cam swapped, you get a much louder and higher pitched sound at the cams.
Gsxr gk76sp 12 pin box will work with a VVC but only runs in high setting so you definitely have not got a gsxr box fitted.

You need to download the vvc manual from this site and run through the vc checks, make sure the cam sensors are working, oil servo on the top of the cambox is switching ,
Can't remember when sat here but it might be possible to plug the tps into the cam sensor by accident as they all run three pin connections so check that, general connections and wiring condition.
Also check the vc relay under the side panel, its got 4 pins and is very easily affected by water.

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Re: Bandit 400 variable valve, won't rev past 10k

Post by Jones5563 » Tue Jun 28, 2016 6:16 pm

Hi Guys

Wow, thanks for the replies in my absence! Truth be told the bike was put away for the winter and I've spent the last couple of months trying to get the bugger started! But she now lives again (spark plugs were knackered, even though I only replied them last year, strange!), but I'm still having the same problem as before.

I've been giving it some thought and I'm pretty confident I've been barking up the wrong tree. The variable valves kick in at 10k, but as the bike won't get to 10k when under load (when the clutch is pulled in it revs fully to the redline at 14k) then I'm confident the problem must lie elsewhere. I don't think it's restricted as I'm told the bandits were only restricted in 5th & 6th gear (correct me if I'm wrong!) and this one struggles to rev past 8k in any gear (again, only when under load).

My current thoughts are therefore that it's running lean and not getting enough fuel through to power through when moving. My first plan is therefore to mess about with the petcock (it's totally knackered and only lets fuel through in the off position) as I'm told if the diaphragm is broken it won't let enough fuel through.

If anyone has any better suggestions or has encountered this problem then I'm all ears! If I fix it in the meantime however I will let you all know (but don't hold your breath!)

Cheers, Andy

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Re: Bandit 400 variable valve, won't rev past 10k

Post by Hopefuldave » Sun Jul 24, 2016 1:33 pm

Just a thought, my boy's VV has a 33 BHP restrictor kit fitted (allegedly) to make it legal for the old "limited" UK licence, if it's similar to other restrictors it may be a spacer ring in each carb top that prevents the slides opening fully - that could limit the revs as it wouldn't allow full throttle (ever!), so under load you'd run out of power - out of gear it takes a hell of a lot less power to spin up to the rev limit!
We're about to tear his apart, he bought it 5 years ago and only just got around to looking at it...

Dave H. (the other one)


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