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Author Topic: Carb questions  (Read 2412 times)

Offline yuckhil

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Carb questions
« on: January 09, 2014, 08:32:33 AM »
Happy new year everybody!  Kudos and much gratitude to those who resurrected the forum, and also, thanks heaps to those who maintained it all these years.

FYI, Canberra Motorcycle Centre and Mick Hone Motorcycles have published heaps of OEM parts lists and schematics.  Enjoy.

So, with much info from this topic, I pulled the carbs for the first time, dismantled the top and bottom, and by the end of the day, all I achieved was cleaning out 3 partially blocked pilot jets and general varnish.  I reassembled it and found that it starts easier and idles smoother than before, but still bogs down noticeably around 2,000-3,500 rpm.  The float heights were a few mm more than the recommended 14.6 mm, so they could be flooding.  Fuel economy was once as high as 24 km/l, but has since decreased from 21 km/l to 18 km/l after this operation.

I will soon have the time to build a 4-bottle balancer rig and open it up again, so I was hoping to first get some advice on the following issues; refer this image for item numbers.

1. One of the float assemblies (18) came off it's mounting socket and allowed me to check the needle valve (19), but the other three float assemblies just would not budge.  Should I grab the central pedestal above the o-ring (21) with pliers and pull, or is that likely to damage them?  I pulled them as hard as I could grip with my fingers...

2. The GSF250VY service data pdf recommends pilot screw pre-set = 1.5 turns out.  Does this refer to the air screw (30)?  After pre-set all four to 1.5 turns out during cleaning, are we expected to then tune these screws for optimal fuel/air mixture when reassembled on the bike?

3. The pilot jets don't have a plug on them, as seen here, and neither do they in the schematic.  What is the purpose of this plug in the 400cc Bandit carbs?

4. Do these parts come off only after the float assembly is removed?  Hook a finger from the top and pull, or insert a screwdriver from the bottom (main jet removed) and push?

5. Since the bike is now 14 years old, and some of the o-rings in the carbs were worn/squished, should I just replace all rubber in all 4 carbs?

6. How much should we tighten the ring clamps around the intakes and outlets?  The screws kept turning until the clamp ends met, but I am worried about the clamps cutting into the intake and outlet pipes.

Thanks very much!
2000 GSF250VY

Offline ventYl

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Re: Carb questions
« Reply #1 on: January 11, 2014, 12:14:40 PM »
0. Don't make ballancing job until you have all four carbs serviced. After full service, you'll need to do it again.

1. Try no to pull directly up, but instead push to sides. First of all do it easy. Wasting hour with float assembly removal is less wasted time than waiting for their delivery from eBay. Maybe you should inject some oil into holes or pour carbs into oil.

2. Pilot screw is screw placed between two mounting screws (outside of float bowl) which mount carb onto rail. Default preset is "you should crank it up with this setup", you can further adjust it, but you'll need emission test device or at least CO sensor to adjust it properly.

3. The purpose is anti-tamper safety. If the plug is replaced or damaged, you'll not pass your MOT or any local equivalent of such certification. Not all jurisdictions require such plug. If they are removed, they were either not used or were previously removed and pilot screws were already readjusted.

4. You don't need to remove float assembly to remove slider holders, but you need to screw main jet. I don't recall right now if it is possible to screw it out without float assembly removal. DON'T use screwdriver! You'll damage internal thread for main jet and render emulsion tube useless. Usually this thing can be pushed out with finger from air-cleaner intake port of carb. In the base of slider holder, there's O-ring, thus this parts holds quite well and may need a lot of power to remove. If by any change you manage to separate emulsion tube and slider holder, note that carb body contains nut which limits position in which emulsion tube can be inserted in body. There's lock on bottom end of emulsion tube which fits the nut in carb body.

5. Replace all O-rings. This count two O-rings under float assembly (both are important), O-ring under top cover and O-ring under slider holder. You can buy generic FPV (Viton) O-ring for cca 4 Euro, or waste your money and buy OEM O-ring kit.

6. I think that you will damage thread on clamp screw sooner than clamp will have chance to damage something. Are clamps of correct size?

After removing float assemblies, do complete carb body cleanup by some fuel system cleaners. You may also remove rubber parts which left (if you removed carbs from rails, there should be no parts leftover) and pour whole bodies in cleaner and then use pressurized air or cleaner to clean up all channel you manage to put pressure in. Usually chamber around emulsion tube and emulsion tube itself are most clogged parts of body.
« Last Edit: January 11, 2014, 12:18:19 PM by ventYl »
Bandit 400 1991 - stock except of swap from GK75B to GK75A

Offline yuckhil

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Re: Carb questions
« Reply #2 on: January 12, 2014, 04:38:36 AM »
Wow, :welcome: to the forum, and :thanks: for such a detailed reply!

0. Thanks, yes I know that.

1. They were REALLY tight, didn't budge at all; I even tried prying the little tab with a screwdriver.  Let's see what happens next time.

2. I was wondering how we are expected to access these vertical screws, to tune the mixture while the bike is running.  Right-angle screwdriver?

3. I guess the GSF250V does not have plugs over the pilot jets.

4. Right, I'll try again.

5. Four sets of 5 OEM o-rings costs A$65 = 43 EUR, plus A$30 = 20 EUR for the float chamber gaskets.  Is that expensive?  I don't see Viton o-ring kits much cheaper on eBay.

6. Yes, 100% OEM.  I was worried about the ring clamps biting into the plastic inlet hoses.

Right, I must get to those emulsion tubes this time.  Thanks!
2000 GSF250VY

Offline ventYl

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Re: Carb questions
« Reply #3 on: January 12, 2014, 06:38:17 AM »
1. Yes that means they are really old and/or received huge amount of heat. If there's at least some space, try to inject oil / WD40 under tab.

2. Probably some kind of that. I've never seen such tool nor did it myself. That's probably reason why most service manuals say you don't have to adjust these screws on your own. But basic setup described in service manual works.

3. Maybe lower displacements didn't have such a scrict policy back then.

4. I looked at image you posted and it clearly shows you don't need to remove float assembly in order to get access to main jet. Under rubber plug, there's another jet hidden but you probably already know it. Don't lose those rubber plugs, you'll get rough idle and low-RPM without them.

5. Here we have shop which sells generic O-rings of nearly any size and type you imagine. You just need to know right size and you'll get one O-ring for around 0,05 - 0,5 EUR. FPV (Viton) ones are little bit more expensive (upper end of interval), but work well like replacement for most OEM O-rings for fuel lines. Mine GSF400 is fitted with these generic O-rings and after carb cleanup cranked on first try. My cas uses them too. Only thing i did wrong was size guess, some of O-rings are little bit thick so I had to push float assembly stronger to get it in it's place, which might be also your problem because O-rings are of metric size.

Float chamber gaskets are unique for this type of carbs so you need to buy OEM or OEM-like quality. 20E for 4 gaskets is good price at least when comparing to what I managed to find.

6. If you are worried, don't tight it further. You can check air-tightness of carb connection by spraying some flammable right onto clamps. Usually all types of quides will advice you not to do that but with brake cleaner it's safe to spray it near running engine (even on hot engine it won't burn but vapour out) and you'll hear RPM increase if it gets inside throught clamps. Of course you must ensure that cleaner won't be sucked throught airbox air-inlet probably by some hose long enough to get clean air in. If after that you'll hear RPM increase, something isn't air-tight (check airbox itself first) and you maybe need to tight them further. Aren't they bent already?
Bandit 400 1991 - stock except of swap from GK75B to GK75A

Offline yuckhil

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Re: Carb questions
« Reply #4 on: January 25, 2014, 11:43:13 PM »
Man, you are a genius.  Well, I took her apart again and extracted the emulsion tubes this time.  They were quite clean, so that was a waste of effort.  However, I replaced the squashed o-rings under the white plastic sections.

I squirted Pro Honda carb cleaner over one of the slider diaphragms and watched in horror as the rubber wilted.  Realised that the fingers on the vinyl glove on my hand had also stretched.  I even got some carb cleaner in my eyes.  Not pleasant.  Flushed the diaphragm with water and eventually it returned to the original shape, and although I put it back in for now, I think I should replace it soon.  The diaphragm and slider are available separately from Suzuki.  If I buy just the diaphragm, does it just insert on to the slider, or do I have to glue it on?

This time I found the mixture screws.  3 of them were 2.5 turns out, and the fourth had the washer missing and was about 5 turns out.  Replaced the o-rings, which were a bit flattened, and set them all at 1.5 turns.  Voila, she idles and revs noticeably smoother now, and the mid-range flat spot is barely noticeable.  Need to buy a washer for #4 and see whether that helps further.

Float assemblies: the pedestal portion that the float is mounted on, sits over a brass tube.  What is that tube for, when the float assembly mounting o-ring seals it off?

Hey, thanks for your guidance.  I am happy with how she runs now, and I will check for vacuum leaks.  Did you recommend spraying brake cleaner on the inlet + outlet hoses?  Is it recommended/necessary to replace all the transfer hoses after X years, or just when cracks appear in the material?

Thanks heaps!
2000 GSF250VY

Offline ventYl

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Re: Carb questions
« Reply #5 on: January 26, 2014, 04:22:54 AM »
Extraction of emulsion tubes is never waste of time, at least the O-ring under white plastic slider holder is often worn out and causes vacuum leaks.

I never saw slider and diaphragm took apart so i don't know how diapraghm is fit to slider. I always thought that these part are sold as one part. I cleaned my carbs with high doses of VIF additived gasoline. Removed all rubber / plastic parts from carbs which I was able to remove without unmounting carbs from frame and dipped them into additived gasoline for about two hours. To help remove heavy dirt I placed the tank with carbs on top of table-mounted sander with bent axe and formed something like hungry programmer's ultrasonic cleaner.

Washer under mixture screw makes pressure to O-ring even, so the missing washer probably causes vacuum leak on idle-mixture fuel circuit, bike may idle even smoother. Check also needle position and main jet hole. Needle runs throught this jet and mine jets have holes malformed by needle.

The secondary tube contains pilot jet under rubber plug. If you remove this plug, you can notice small hole in wall of tube which connects pilot jet tube with tube of main jet. Tube is sealed because this engine has so small level of vacuum when idling that big hole would cause rough idle due to vacuum pulses. So tube is sealed and connected only by small hole from main jet tube. If you are asking about tube to which other end of float assembly is inserted, float assembly contains tubing and one jet. This is choke fuel circuit and little jet in float assembly tubing loves to collect dirt.

Yes, if brake cleaner gets into engine, you'll hear RPM increase. So if you can spray brake cleaner in way that it won't be sucked by airbox air-inlet and you get increased RPM, you have vacuum leaks somewhere. IIRC, these transfer hoses are needed to be replaced only when cracks of certain size appear. Smaller cracks are not problem if they don't go all the way down to inner wall. But if you can buy them I think it's good idea to change them now while bike is in garage rather than patch them on the road.
Bandit 400 1991 - stock except of swap from GK75B to GK75A

 

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