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Author Topic: Pilot air screw and carb issues  (Read 1822 times)

Offline 92bandito

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Pilot air screw and carb issues
« on: August 07, 2016, 11:04:11 PM »
Last August I got a 1992 Bandit 400 US version with the 32mm carbs and it was working fine idling and throughout the revs. About 2 months ago I decided it was running a bit rich as there was always some black soot on my turn signal above the exhaust exit. I took the carbs off and turned the pilot air screws 1/2 turn in. Got it back together to try and start it, it turned over but wouldn't start. Spent about 3 weeks trying to get it to start, and in that time I ended up rebuilding the carbs and found out the pilot air screws were set to about 4 turns out to begin with which seems like a lot. It does have a K&N air filter and Devil slip-on exhaust but 4 still seems excessive. I eventually found out about the hidden "prime" position on the petcock and it will start consistently now.

The problem is since I've rebuilt them and messed with the pilot air screws so much it doesn't idle correctly. It will start fine and idle with some choke to warm up, but as soon as I barely touch the throttle it will shoot up to 4,000-5,000 rpm and be extremely slow to return back to a normal idle. I've checked multiple times for air leaks with no luck and tried adjusting the air screws from 1.25 - 4 turns out with no significant change to the hanging idle. I've built a carb sync tool and balanced them each time. A couple times with the air screw between 1.5-2 turns out with the tank off it will idle much better and only be slow to return from 2,000 down to 1,000, but when I put the tank on it gets the same high hanging revs.

I'm not sure if the previous owner got a new carb jet kit for the exhaust and filter, but I know the main jets are 97.5 and the clip is on the middle notch of jet needle. I don't know what size the pilot jets are. Since I've got it in this state right now I'm going to do a valve clearance check, it's due for one anyways. Is it possible they could be out of tolerance causing this? Or maybe the previous owner had the timing advanced which is why the air screws were so far out? I'll put up another post on where I find the valves to be. Luckily the specs valve tolerances are on a metal plaque on the frame so I can use them.

Any ideas on what could be causing this? 

Offline TJS

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Re: Pilot air screw and carb issues
« Reply #1 on: August 07, 2016, 11:13:39 PM »
What do you consider "rebuilding carbs"? New gaskets, jets, float needle...??? Did you compare OEM parts to what you have?..eg.. possible jet kit or some other "home mechanic" stuff?

Offline Squishy

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Re: Pilot air screw and carb issues
« Reply #2 on: August 08, 2016, 02:58:47 AM »
If it shoots up and stays there or doesn't wanna drop in RPM fast enough, the engine must be able to get too much air from somewhere.
Either a leak, bad carb sync, idle set too high (throttle valves) or way too lean mixture.

You mean 4 turns as in 4x 360 degrees?

Offline ventYl

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Re: Pilot air screw and carb issues
« Reply #3 on: August 08, 2016, 05:11:32 AM »
valve clearance might be one of suspects here.

even more if some of retaining screws loosen itself and valve clearance is changing constantly it can cause effects similar to carbs out of ballance. this would also cause significant clicking noise from engine which won't go out after engine heats up.

but i don't think that valves itself would cause such behavior. search for air leak somewhere. are you sure that rubber carb adaptors at cylinder head do not have any ruptures? it is quite a common case that they are leaky.

additionally. if your fuel tank works only in prime position, do you have fuel tap connected and/or is it working properly? if fuel tap lane is not connected it might cause idle RPM increase (even quite significant) or drop based on current fuel mixture.
Bandit 400 1991 - stock except of swap from GK75B to GK75A

Offline 92bandito

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Re: Pilot air screw and carb issues
« Reply #4 on: August 08, 2016, 11:13:29 PM »
Thanks for all of the replies.

For the carb rebuild I took out the floats, main jets, black plugs (couldn't get the pilot jets out), diaphragms and slides and jet needles and then sprayed liberally with carb cleaner. All of the o-rings and gaskets look to be in good shape. I would replace them but I can't find where to buy a set. Not sure how all of the parts compare to oem.

I checked again for an air leak with the tank on before starting on the valves and still no changes in rpm. I sprayed carb clean all around the intake boots, but nothing, even when the rpms were hanging around 3500. All of the rubber intake boots look fine, no dry rotting or cracks and the carbs seat on them nicely. I've got the idle screw turned all the way out and have been syncing them just about every time I adjust the air screws. And yes, 4 x 360 degrees turned out.

The fuel tank and petcock also work fine in the "on" position and I've got all the tubes hooked up correctly.

I got the valve cover off to get a look at the valves. None of the lock nuts were loose. The specs listed on the side of the frame say .13 - .18 mm for the intake and .20 - .25 mm for the exhaust. Most of the valves seem to be in tolerance, but how much force should I be using to get the feeler gauge through? I'm using quite a bit of force and wiggling the gauge back and forth to make the .15 mm and .23mm gauge fit. The sizes on the lower side of the spec ranges are a little easier to fit. One of the intake valves was at about .279 mm though and I was able to get a .254 mm gauge through one of the exhaust valves so I will need to adjust those.

Thanks again

Offline TJS

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Re: Pilot air screw and carb issues
« Reply #5 on: August 08, 2016, 11:58:00 PM »
You are probably wasting your time til you do at least these:
http://www.ebay.com/itm/161913291847?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

and maybe these:http://litetek.co/ReplacementPartsSuzuki.html

page wasn't working last time I checked for the GSF 400 Bandit

Offline ventYl

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Re: Pilot air screw and carb issues
« Reply #6 on: August 09, 2016, 12:32:59 PM »
i think that while he isn't experiencing any external leaks and due to my experience that needle valves are usually tight if they visually look good i think that for now this can wait. what he will really need are the O-rings.

I always forget to write down dimensions of O-rings I use but the numbers should be written here in some thread. I am sure that some GSX-R / GS500 or similar forum contains the data for Mikuni BST33SS carbs which are valid for Bandit 400 (maybe for 1200 as well) too.

You should be able to buy these O-rings in some local store. They have to be Viton (or FPV - Fuel/Lubricants resistant). Whole kit will be (far) less than 10 bucks.

There are four in each carb - one on fuel needle valve seat, another under choke fuel line (both on float assembly) secret one under emulsion tube after it is released from slide holder and fourth one under top cap. If these are missing carbs can provide additional air for increased idle RPM. After years O-ring at needle valve tends to shrink and leak fuel into float bowl causing various problems.

Next, 4 (x360) turns out is simply far TOO much to me. Basic setup is about 1+1/4 to 1+1/2 for standard can and from 2+1/2 to 3 turns out with aftermarket can + high flow filter. Rule of thumb says that if you are above 3 turns you need larger jet. Having the mixture so rich can provide enough fuel for unmetered air leaking somewhere thus RPM is so high (and needs initial throttle impulse to rise).
Bandit 400 1991 - stock except of swap from GK75B to GK75A

Offline greg737

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Re: Pilot air screw and carb issues
« Reply #7 on: August 09, 2016, 04:01:27 PM »
Quote
I got the valve cover off to get a look at the valves. None of the lock nuts were loose. The specs listed on the side of the frame say .13 - .18 mm for the intake and .20 - .25 mm for the exhaust.

Are you sure that your Bandit 400 is a 1992 model?  I ask because those valve clearance specifications you quoted (from a frame sticker on the bike) are for the 1993 Bandit 400 engine.

On the 1993 Bandit 400 Suzuki de-tuned the engine slightly, with changes made to the camshaft profiles that reduced valve lift on both intake and exhaust.  These changes resulted in horsepower being reduced from 59hp to 53hp.

With the reduction in valve lift the Suzuki engineers were able to remove (delete) a shim from underneath each of the exhaust valve spring sets because the reduced valve lift of the 1993 Bandit engine didn't require as much valve spring force to insure valve closure at high RPMs.  Not enough spring force can result in "valve float" at high RPM which could result in all sorts of bad things like overheated valves or valves hitting the piston top. 

Here's a couple of screenshots from the 1993 model year supplement to the Suzuki GSF400 Bandit Service Manual.

This shot shows the deletion of the shim under the exhaust valve springs:


This shot shows (with asterisks to show which values had changed from the previous year model) some of the valve train specifications that were changed from the 1992 year model engine to the 1993 year model engine (on the 1993 engine the valve lift was reduced on the intake from 7.6mm to 6.2mm and the exhaust valve lift was reduced from 7.0mm to 6.2mm):


The above screenshot also shows the changed valve clearance specifications for the 1993 year model engine.  The reduction in valve lift on the 53hp engines lessened the rather critical nature of the valve adjustment on the 59hp engine.  The valve clearance setting specifications on the 1991-1992 engines are like a highly tuned racing engine.



And... just in case you're saying to yourself, "But my Bandit 400's VIN sticker says it was manufactured in 1992!" I have this picture to show you:  This is a picture of the VIN sticker of my 1993 Bandit 400 which indicates it was assembled in August of 1992 but I can prove that it was without a doubt built as a 1993 model (the bike's engine has all of the 1993 differences from the 1991-1992 engines).

It's a bit hard to read but the date in the upper right hand corner of my VIN sticker shows "8/92".

So... if your engine serial number is later than my engine's number you can be sure yours is also a 1993 engine:


« Last Edit: August 09, 2016, 04:49:33 PM by greg737 »

Offline ventYl

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Re: Pilot air screw and carb issues
« Reply #8 on: August 09, 2016, 05:36:53 PM »
it is not uncommon that vehicle manufacturers start to make new model year after summer break. it was the case for both of my cars (one even has VIN for year of manufacturing 1994 but due to it's paperwork and various dates on internal parts it was manufactured, sold and registered at the end of year 1993).

Also, back then Suzuki used full 14 digit VIN format only for US. Particularly in Europe this started to be mandatory for motorcycles after year 2000. Before year 2000 Suzuki (and most of motorcycle manufacturers except of Honda to my knowledge) was using shortened VIN format (e.g. GK75B xxx xxx) which does not allow to decode model year.
Bandit 400 1991 - stock except of swap from GK75B to GK75A

Offline greg737

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Re: Pilot air screw and carb issues
« Reply #9 on: August 09, 2016, 05:48:55 PM »
The Original Poster on this thread is in the U.S. (as far as I can tell) so I'm guessing he'll have a standard 14 digit VIN to look at on his bike. 

On my Bandit 400's VIN tag you can see a manufactured date that is well inside the year 1992 (it's stamped with 8/92) and you can see that digit number 10 in the VIN is a "P" which is the Suzuki year code for 1993 (a 1991 bike would have an "M" in the 10th digit position and a 1992 bike would have an "N" in the 10th digit position).

Offline 92bandito

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Re: Pilot air screw and carb issues
« Reply #10 on: August 09, 2016, 09:43:02 PM »
I checked the VIN and it does have a P so it's a 1993. Thanks for pointing that out. I'll get some new o-rings to replace the old ones.

 

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