Author Topic: 400 stuttering/bogging from 6000 rpm [SOLVED]  (Read 33132 times)

Offline ventYl

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #15 on: April 14, 2015, 06:35:20 AM »
In such cases I use some thin long barrel instead of fuel tank. spares a lot of time.
Bandit 400 1991 - stock except of swap from GK75B to GK75A

Offline Squishy

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #16 on: April 14, 2015, 07:56:25 AM »
In such cases I use some thin long barrel instead of fuel tank. spares a lot of time.
Yeah me too, when I'm working on the bike..
But everytime you wanna look at the filter, change needles or do ANYTHING basically, gotta remove that tank and disconnect them hoses :(

Offline GSF

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Re: 400 stuttering/bogging from 6000 rpm
« Reply #17 on: March 01, 2016, 10:35:41 PM »
Well I hoped you removed the toiletpaper from the carbs before starting it  :rofl:

Anyway without any filters I'm surprised it even ran OK to 6000rpm. (well, it should go to 6000rpm in neutral but probably not when actually driving it)

ps. airbox and especially air filter is NOT "extra bits". It's an essential part.

Okay, I think you've just pointed me in right direction to solving the problem with my B4. The airbox, well, I can tell its not the original one because someone had to cut it in half to make it fit under the tank and covered the hole they'd made with gorilla tape. The filter also looks like it came out of a vacuum cleaner and doesn't fit snugly in the airbox either. I'm starting to think it's a bit leaky.

Offline Squishy

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Re: 400 stuttering/bogging from 6000 rpm
« Reply #18 on: March 02, 2016, 11:31:07 AM »
Well I hoped you removed the toiletpaper from the carbs before starting it  :rofl:

Anyway without any filters I'm surprised it even ran OK to 6000rpm. (well, it should go to 6000rpm in neutral but probably not when actually driving it)

ps. airbox and especially air filter is NOT "extra bits". It's an essential part.

Okay, I think you've just pointed me in right direction to solving the problem with my B4. The airbox, well, I can tell its not the original one because someone had to cut it in half to make it fit under the tank and covered the hole they'd made with gorilla tape. The filter also looks like it came out of a vacuum cleaner and doesn't fit snugly in the airbox either. I'm starting to think it's a bit leaky.
Sounds like it. Just google for Bandit 400 airbox pictures and you'll see the original one.
I'd suggest finding a 2nd hand one.

Offline GSF

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #19 on: March 02, 2016, 11:56:55 AM »
Already scouring feebay to find one.

She also suffers from the problem of bogging down when you throttle her a little bit just after starting her up. After running for about five minutes it tends to settle down and you can give her some juice, but she can still lose power when you get her in gear and try to pull away. She does not like starting the first time each day either, but once she's warmed up completely she'll start easily for the rest of the day and power feels okay, until you get up into the high revs when she'll start sputtering. Sounds a lot like what you've been describing, which would mean a leaky airbox and given the state of the mine it's a good place to start. I didn't realise the airbox was that important, just thought it was to stop crap getting in the engine.

Offline ventYl

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #20 on: March 02, 2016, 09:38:56 PM »
First find the airbox and filter. Original is foam-type. There's also K&N aftermarket but considering the price of foam I don't think K&N makes any sense at this stage.

Then we will see if you need to rebuild your carbs (but most probably you will need to do that too).
Bandit 400 1991 - stock except of swap from GK75B to GK75A

Offline GSF

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #21 on: March 02, 2016, 10:14:35 PM »
First find the airbox and filter. Original is foam-type. There's also K&N aftermarket but considering the price of foam I don't think K&N makes any sense at this stage.

Then we will see if you need to rebuild your carbs (but most probably you will need to do that too).

Yeah, I saw the K&N one and it's just crazy prices.

I already know I'll need to rebuild the carbs, but that did make me laugh. Almost like it's inevitable.

When I got her she was doing about twenty miles to a gallon, which is not great. I've already dismantled the rear-end, cleaned everything up, put some actual grease in the bearings and put it all back together without tightening everything up to breaking point (I don't think the previous owner understood torque settings) and just from that alone I'm now up to 35mpg. I would expect the bike to be getting at least fifty though, so I'll see how much closer a new airbox and filter gets me before hitting the carbs.

Offline Gouraami

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #22 on: March 04, 2016, 07:48:39 AM »
I am glad my thread is helping you out, don't be scared of those carbs, they are relatively straight forward
Project: Suzuki Bandit 400 Limited Edition, either a '90 or '91 needs a lot of TLC

Offline GSF

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #23 on: March 22, 2016, 10:20:03 AM »
So yeah, I got a new airbox, air filter and petcock, plus a new fuel-line. Had to jump start her because the battery was dead and after remembering the new petcock is manually operated got the bike running easy enough. She idled at about 5k rpm! Okay, I figured at least the carbs are being properly fed now, I just need to make some adjustments.

The idle adjustment screw is bang in the middle and after playing with it for a few turns either way I found that turning it in made the engine run faster (which I'm guessing means the mix is richer) and turning it out made it run slower. I ended completely removing the adjustment screw and she still idles at 4k rpm!

Offline ventYl

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #24 on: March 22, 2016, 10:39:10 AM »
Did you do basic carb setup? It sounds to me like the carb rack is completely desynchronized. Especially if engine is slowing down after being revved in extremely lazy fashion.
Bandit 400 1991 - stock except of swap from GK75B to GK75A

Offline GSF

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #25 on: March 22, 2016, 02:04:48 PM »
Did you do basic carb setup? It sounds to me like the carb rack is completely desynchronized. Especially if engine is slowing down after being revved in extremely lazy fashion.

I've just fixed it and taken her for the first proper run in I can't remember how long. I am so ridiculously happy to be mobile again!

The problem was the new fuel line was pushing against the throttle adjuster on the carbs, which meant the throttle cable was taut enough to be engaged as if the throttle itself was about a quarter turn. I had an inkling this was the reason and when I turned the handlebars the engine would rev higher turning it to the right and then drop when I turned it to the left, which backed up the theory. I just took the tank off, loosened the throttle adjuster nut to give more play on the cable and after putting her back together I got the expected stalling when trying to start her. Put the idle adjuster screw back in, set it midway and after some tinkering got the idle down to around fifteen hundred. She is running almost perfectly for the first time since I got her and after taking her for a spin to charge the battery now starts on the button first time. I'll see how she starts tomorrow from cold, but I'm hopeful I might have finally sorted out that particular problem.

I'm guessing a little, but the airbox and filter are not just there to stop crap getting in the engine but also to regulate just how much air is getting to the carbs. A leaky airbox can't control the airflow effectively across the range, so you get all manner of weird problems happening. I also replaced the petcock because the previous owner had chopped the filters on the old one and put in an inline filter in the fuel-line instead. That actually ran uphill, so I think the flow of fuel to the carbs was also causing problems, probably because it was only trickling down to the carbs.

I'm going to see what the mpg is like over the next couple of weeks before deciding if I want to rebuild the carbs just yet. I've got a hundred other things to sort out (no headlights, no indicators, retainer missing from rear wheel, no rear brake...) and at least with an engine that runs my time might be better spent sorting those out before taking her off the road for a few weeks to clean up the engine itself. I just want to be able to ride in truth, I don't drive so the bike is my only means of transport and it sucks when I can't wander into town for weeks and months at a time (I live in the middle of nowhere).

I know at some point I'll have to rebuild the carbs and swap out some parts, maybe the needle or main jet because the idle screw is turned out a lot further than it should be to get the bike idling at a reasonable rpm (if I turn it fully in she idles almost on the redline!), but I'm not overly concerned right this minute to start playing with it.
« Last Edit: March 22, 2016, 02:08:23 PM by GSF »

Offline Gouraami

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #26 on: March 27, 2016, 05:40:01 AM »
I'm glad things are going  well, it is a good feeling when you are able to fix something yourself  :thumb:
Project: Suzuki Bandit 400 Limited Edition, either a '90 or '91 needs a lot of TLC

Offline Thief400

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #27 on: June 26, 2016, 10:44:06 AM »
The 35mpg your getting is about as good as a 400 will ever get. It's engine is in a very high state of tune it put out more horse power per liter than the formitable GSXR1100 of the same period. It was never intended to be a fuel sqeazer it is a little hotrod. Want gas mileage get a CB125 lmao  :stickpoke:

Offline Squishy

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Re: 400 stuttering/bogging from 6000 rpm [SOLVED]
« Reply #28 on: June 26, 2016, 11:34:09 AM »
The 35mpg your getting is about as good as a 400 will ever get. It's engine is in a very high state of tune it put out more horse power per liter than the formitable GSXR1100 of the same period. It was never intended to be a fuel sqeazer it is a little hotrod. Want gas mileage get a CB125 lmao  :stickpoke:

Uhmmm what?   :stickpoke:

My GSF400 averages 1 liter per ~20KM (=55 MPG) for about 45000 kms and 6 years. Cruising, highway, or very sporty riding in the mountains. Always between 1:18 and 1:22.

And I have the 59BHP version, even with bigger main jet... and it does 215km/h on the speedo.
« Last Edit: June 26, 2016, 11:38:03 AM by Squishy »

Offline TJS

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More Bandit 400 stuff on my youtube:
https://youtube.com/c/StanleysGarage369