carb cleaning

[rwh]

I've got a 1993 Suzuki GSX750F, which according to the Clymer manual has Mikuni BST36SS carbs.

Recently it started running on only 3 cylinders - after a somewhat more spirited than usual blat through the Wairarapa, including the former site of the sprints ...

In my efforts to find something wrong, I've started dismantling the carbs. This may have been a mistake, but never mind.

I've been following the instructions in the Clymer manual, but got stuck on removing the jet block - plastic thing that the piston slides through, and forms part of the main tube the air goes through, with the jet at the bottom. The manual says I should withdraw that after removing the pilot and main jets (I'm slightly confused there; I've removed what the pics show as the main jet, but there's still something left in the jet block. Pity the manual gives very little description of how the damn thing works, and what the bits are for). I'm wondering if the bits sticking out the sides of the top of the jet block are clips that hold it in, but the manual doesn't say anything about that.

In terms of gunk that I'm cleaning out, I'm a little worried by the brown sludge around the top of the piston - if I can successfully attach the pics I've taken, I've shown two pistons, one cleaned and the other not. Is that normal?

I'm going to have to assemble them without replacing all the o-rings the manual tells me I should replace - and in fact, without some of them altogether, since they weren't there. I intend to use Loctite 518 Master Gasket there instead (this is around the little tube that comes up through the top cover, and is then capped on top), as suggested by the mechanic I spoke to this morning, and what appears to have been done by the last person to service it.

Any more suggestions?

Thanks

Richard

[offrd]

Go easy on the locktite or it may get into little places you really do not want it to be and block god knows what up. Little on the finger and light smear
Better i guess than silicone which some when in contact with petrol can swell like you would not believe...

[rwh]

Go easy on the locktite or it may get into little places you really do not want it to be and block god knows what up. Little on the finger and light smear
Better i guess than silicone which some when in contact with petrol can swell like you would not believe...

The only place I'm using it is around the tube that goes through the top cover - you can see the red remains of the previous lot in the 3rd pic. Since that tube is then sealed off with a cap fixed to the top cover, I'm assuming that it doesn't matter if I block it - that it's there for a slightly different application (there's a hose coming off there on one of the carbs on the GSXR750 covered by the same manual). Is that a reasonable assumption?

Richard

[Paul in NZ]

My strong advice would be NOT to use any gasket goo on carbs but its your bike.... Try to find some O rings - you would be amazed what turns up at supercheap or repco... That 'tube' is kinda vital to the carbs function.

I assume you did identify the dead cylinder before diving into the carbs?? If so - did you find anything in that carb?

Paul N

(pricks of things getting the jet blocks out of those things - leave em be I reckon...)

[rwh]

My strong advice would be NOT to use any gasket goo on carbs but its your bike.... Try to find some O rings - you would be amazed what turns up at supercheap or repco... That 'tube' is kinda vital to the carbs function.
Despite being blanked off? Is it a tuned length or something? Hmm, perhaps I should redo the first one ... I think the others were better. And I'll try and get some o-rings next time - which will hopefully be at a better time of year (not that I didn't see some o-rings that looked 'about right' at Schrodoco this morning ...)


I assume you did identify the dead cylinder before diving into the carbs?? If so - did you find anything in that carb?

Well ... not really. At one stage, I fired it up briefly, and found that #4 header was stone cold while the rest were hot, so I figured that was it. Then later, I did the same thing and they were all hot - but maybe I'd run it a bit too long and the heat had transferred. Anyway - didn't find anything particularly bad in #4 carb. I did find differences that looked like they're intentional, though.

In the float bowl, there's a pilot jet that's screwed in down a deep hole in the casting, off centre. In carbs #2 and #4, there's a rubber bung in that hole, that isn't mentioned in the manual - I can't see how any fuel (or anything else) gets to that jet. #3 doesn't have one, and I haven't opened #1 yet (next task)

Thanks for all the help (whether I'm following it or not ...)


(pricks of things getting the jet blocks out of those things - leave em be I reckon...)
Ah ... well, you're probably going to be horrified at this ... but I screwed the main jet back in, then backed it off about half a turn at a time, and tapped it (gently) with a screwdriver handle ... hopefully not hard enough to distort anything :dodge:

Richard

[Paul in NZ]

Tappings probably fine...

Personally - I would not have worried too much about removing them unless the bike was running rich etc Sounds like you are having fun - If you get stuck, I have a book here someplace that covers carb basics but it does not go as late as those mikunis.

[The Pastor]

carbs are very simple things, you cant go too wrong with them. just take them apart (one at a time) and check that all the jet holes arnt blocked up. wack em back together and then balance the carbs (i get the shop to do mine as i cant do it / dont have the equipment).

Once its all done nicely go to trademe, list the bike and buy one with FI ;)

[rwh]

Well, they're all back together, and the bike is running no worse than before ... in fact I thought it was running better to start with, then it deteriorated again during my test ride. I've got a feeling it's somehow related to heat.


Sounds like you are having fun - If you get stuck, I have a book here someplace that covers carb basics but it does not go as late as those mikunis.
The fun part is a bit mixed - it would be better if a) there were more shops open to buy bits from when I need them (though then I probably wouldn't be doing it at all), and b) I hadn't planned to be heading to Auckland on it yesterday (though again, if there wasn't the hurry I'd be leaving it to the shop).

I could be keen to borrow that book at some point - or just get enough info so I can find it in the library or something.



Once its all done nicely go to trademe, list the bike and buy one with FI ;)

LOL - tempting, though I don't have the dosh at the moment. And the more I've played with it, the easier it gets. I could now dismantle the carbs, starting from a fully faired bike, without looking at the manual every step, so that probably saves about 90% of my time :)

Richard

[awayatc]

Well, they're all back together, and the bike is running no worse than before ... in fact I thought it was running better to start with, then it deteriorated again during my test ride. I've got a feeling it's somehow related to heat.

Richard

Well done on the carbs, but if you think its related to heat....did you check/replace your sparkplugs....?

[rwh]

Well done on the carbs, but if you think its related to heat....did you check/replace your sparkplugs....?

That was my next step, though without real knowledge to correlate heat with plugs.

#4, which is the cleanest (best?) looking, is the one I thought was dead in the first place. #2 is the darkest, with #1 and #3 not that far behind (pic 1). However, when I pulled the lead for #2, that's all I got (pic 2) ... the cap (pic 3) was left behind. Does that mean it was probably faulty already, or is it just a bad move to pull on the leads rather than the caps? I assume I can't fix it, so I'm waiting till the shops open to get a replacement :( Pic 4 shows a coil - do I fit a new lead to that? I hope I'm not up for a new coil as well ...

Do those plugs give useful clues as to the source of my problem?

Sorry about my inability to focus properly ...

Thanks,

Richard

[Pixie]

I've seen the strainers on each individual carb become blocked with debris.This causes one or more carb to be starved of fuel
The strainers can be accessed with the bowls removed.Just follow the path of the fuel to each carb.
This was the cause of an RF 900 running on three cylinders.
The outer carbs are more susceptable than the inner ones.

[Paul in NZ]

If I pulled those 4 plugs out of any bike i owned I'd be concerned regarding the differences between them. Somethings not working at all.. In the unleaded era and given the carbs on your bike I'd expect em to be a bit dark so the super clean one would worry me most.

I can't help with the HT lead as I can't see how it connects - ie on my old bikes they are easy to replace from repco or shite I have laying about the shed... Many jap ones are not designed to come to bits and you need to replace it as a unit. I doubt the usual bandits will have one in stock either - not a good thing to break.

[offrd]

Swap the clean as plug with one of the others and see what the plugs do colour wise, the white plug would look to be either very new, like almost out of the box no km or running very lean, lean is not a good thing..The plug next to the super white one is not too bad, the others are too rich. Of course you will have to fix the plug lead sone how..:doh:. It may screw back on, have a look if it has what looks like a screw up inside that cap that goes into the lead..
With the carbs check that the needle comes of the seat when the float drops in the bowl and that fuel can pass thru into the bowl. If the carbs are back in and a prick to get out it may have fuel drains on each bowl, see if fuel comes out.

Many other things to try like swapping coils between cylinders, not just the leads:no:

Engine must have 3 basics to run... Air, Fuel and spark.

If nothing else you are learning your bike i guess....

Good luck

[rwh]

It may screw back on, have a look if it has what looks like a screw up inside that cap that goes into the lead..

Aha! Just had a nosy, and it does indeed appear to have a screw in there, so that gives me something to look forward to in the morning - I'm not stuck after all. Then I can have a look back over all the rest of these wonderful tips.


If nothing else you are learning your bike i guess....


Yes - I played with cars as a teenager, but never dared dismantle carbs ... though I'm guessing the Stromberg (I think) on my Hunter might have been simpler ... and of course there was only the one.

Thanks all

Richard

[Motig]

OK I'm not terribly mechanically minded but I'll have a try at most things. The very last thing I would have done is start pulling the carbs to pieces without first checking that the dead cylinder was getting a spark or fuel, it pays to try the easy things first and by the process of elmination find out whats causing the problem. If you've already done that my apologies but you dont mention anything else but the carbs in your post.:no:

[rwh]

OK I'm not terribly mechanically minded but I'll have a try at most things. The very last thing I would have done is start pulling the carbs to pieces without first checking that the dead cylinder was getting a spark or fuel, it pays to try the easy things first and by the process of elmination find out whats causing the problem. If you've already done that my apologies but you dont mention anything else but the carbs in your post.:no:

You're right, I haven't been following a very ... structured fault diagnosis plan :( I'm a bit lazy, not thinking straight, and somewhat annoyed that I'm not up north riding, which all mix up together to produce bad results. I just felt I needed to do something.

Actually I had tried the #4 plug lead; with the help of a nail stuck in the cap, I was able to produce a spark against the cam cover. I'm not sure, however, how to detect whether fuel is getting there, which certainly seems to be a good next step. I guess I can undo the drain screw to see if there's any in the float bowl - there was when this all started; draining it to remove any sludge was one of the very first tips I followed. Unfortunately I also haven't been able to reproduce the '#4 header is cold while the rest are hot' test result, so I'm not sure that it's still the one giving me grief, or what sort of grief it's giving me.

Richard

[Paul in NZ]

Since you have gone this far I'd suggest a compression test (leak down is better but...) is essential - maybe you have a valve clearance issue?? Have they ever been checked?

[rwh]

Since you have gone this far I'd suggest a compression test (leak down is better but...) is essential - maybe you have a valve clearance issue?? Have they ever been checked?

Hmm. I'd need special tools for a compression test, wouldn't I? I guess I could dive in and check valve clearances - are shims easy to get/fit? I've only done valve clearances on a pushrod car engine, with screws on the rockers.

Richard

[Max Preload]

Try to find some O rings - you would be amazed what turns up at supercheap or repco...

The trouble being that the muppets at Ripco won't know you shouldn't use nitrile o-rings for a petrol contact application especially in something as critical as a carburettor. See here (http://www.kiwibiker.co.nz/forums/showpost.php?p=1233858&postcount=4).

[Max Preload]

When does the bike run on 3 - constantly, even at idle or under light medium or heavy load (throttle opening dependant)? Or does it seem to 'clear' if you thrash it?

Did you fill up shortly before it happened? I filled up at Turangi on the way to the 'Cold Kiwi' this year and my bike started running on 3 under light load throught the first series of tight corners on the Desert Road. I thought the plug hole seals on the rocker cover were leaking again (I'd replaced them after it developed exactly the same problem when the oil went down the spark plug supressor caps - the seals were rock hard instead of being supple) but it cleared shortly after and I put it down to watery fuel at Turangi - it didn't do it again on the rest of the trip.

[rwh]

When does the bike run on 3 - constantly, even at idle or under light medium or heavy load (throttle opening dependant)? Or does it seem to 'clear' if you thrash it?
My mind is starting to play tricks on me, and I'm starting to doubt whether I can tell if it's running on 3 or 4. However, the rough sound is most noticeable under load, and at lower revs - either leaving an intersection or through the twisties. Oddly, while it seems to be down a little on power when taking off, it seems to have plenty through the twisties. Though again that's hard to tell as it varied through my test ride the other day.


Did you fill up shortly before it happened? I filled up at Turangi on the way to the 'Cold Kiwi' this year and my bike started running on 3 under light load throught the first series of tight corners on teh Desert Road. I thought the plug hole seals on the rocker cover were leaking again (I'd replaced them after it developed exactly the same problem when the oil went down the spark plug supressor caps - the seals were rock hard instead of being supple) but it cleared shortly after and I put it down to watery fuel at Turangi - it didn't do it again on the rest of the trip.

I guess I did - filled up just before heading over the Rimutakas - but I've also topped up again since then, when I thought part of the problem might just be lack of fuel. I guess it could be water in there ... maybe I'll do a drain and refill.

My other plan is to stick the tank back on and ride down to see if Boyle's are open and have time to take it on ...

Richard

[Paul in NZ]

Or you could just synch your carbs...

[rwh]

Or you could just synch your carbs...

Isn't that what you posted pics of you doing with all sorts of fancy equipment?

It's in the shop now; hopefully I'll get a little bit of my holiday.

Thanks for all the help.

Richard

[Pixie]

If I pulled those 4 plugs out of any bike i owned I'd be concerned regarding the differences between them. Somethings not working at all.. In the unleaded era and given the carbs on your bike I'd expect em to be a bit dark so the super clean one would worry me most.

I can't help with the HT lead as I can't see how it connects - ie on my old bikes they are easy to replace from repco or shite I have laying about the shed... Many jap ones are not designed to come to bits and you need to replace it as a unit. I doubt the usual bandits will have one in stock either - not a good thing to break.

This does not necessarily indicate an electrical problem.If the fueling to that cylinder is wrong (starved for instance ) you'll see it reflected in the plug colour.

On fours it is often difficult to tell when one cylinder dies intermittently.
The symptoms you described are exactly the ones my brother had on his RF 900.
I strongly suggest you check those strainers before you do anything else.

[Paul in NZ]

This does not necessarily indicate an electrical problem.If the fueling to that cylinder is wrong (starved for instance ) you'll see it reflected in the plug colour.

On fours it is often difficult to tell when one cylinder dies intermittently.
The symptoms you described are exactly the ones my brother had on his RF 900.
I strongly suggest you check those strainers before you do anything else.


Two different problems refered to -

Those carbs are the same as used on RF900s and FZR's - they all slog out their needle / needle jets making the mixture richer with time and combind that with unleaded gas not depositing the nice biscuit coloured lead on the electrode when correct (like the old days) so you expect to see a darker plug. I'd be assured something is dreadfully wrong if I pulled those 4 plugs out of any engine I owned!

I doubt it had a problem with the leads before it got pulled apart - lessons are only learnt by doing things - no biggie - done worse myself.

I'll file that diagnostic tip for 4's away as yet another reason I like twins :shifty:

[Pixie]

Two different problems refered to -

Those carbs are the same as used on RF900s and FZR's - they all slog out their needle / needle jets making the mixture richer with time and combind that with unleaded gas not depositing the nice biscuit coloured lead on the electrode when correct (like the old days) so you expect to see a darker plug. I'd be assured something is dreadfully wrong if I pulled those 4 plugs out of any engine I owned!

I doubt it had a problem with the leads before it got pulled apart - lessons are only learnt by doing things - no biggie - done worse myself.

I'll file that diagnostic tip for 4's away as yet another reason I like twins :shifty:
He doesn't have a problem with the leads - he said he had spark.



Fuel starvation in one cylinder will give a lean mixture indicated by the white insulator deposit on the plug
if the mixture is lean enough it won't fire.

If you loose one cylinder on a twin you're rooted at least a four has 3 left to carry on with

[Pixie]

This is a useful thread:

http://www.gixxer.com/forums/showthread.php?p=2374074

[rwh]

My carb looked rather different from that - I don't think there were any plugs left that a strainer could have been hiding under. The pivot and float valve were all part of the orange plastic assembly on mine, there is no mid jet, and neither of the two brass plugs down the bottom were there either.

Richard

[F5 Dave]

oh dear, this is all looking depressingly familiar.

Have a search on RF900 on this site. I suspect your emulsion tubes have worn on the LHS cylinders. Don't ask why they do more than the RH ones but they do. Take out the plastic blocks & emulsion jet & swap into the RH side ie: 1 & 2 to 3 & 4.

Clean the plugs & reinstall & run the bike. If the problem follows then I'm right & you need some new tubes. You can also look at the tubes & see if they are ovalised, need a good light & compare them. A little oval is stuffed. Factorypro should do replacements.

[rwh]

oh dear, this is all looking depressingly familiar.

Have a search on RF900 on this site. I suspect your emulsion tubes have worn on the LHS cylinders. Don't ask why they do more than the RH ones but they do. Take out the plastic blocks & emulsion jet & swap into the RH side ie: 1 & 2 to 3 & 4.

Clean the plugs & reinstall & run the bike. If the problem follows then I'm right & you need some new tubes. You can also look at the tubes & see if they are ovalised, need a good light & compare them. A little oval is stuffed. Factorypro should do replacements.

Interesting, thanks.

However, things have progressed somewhat. I took it in to Boyles, and they declared excessive blowby - loads of smoke coming through the crankcase breather. I'm not sure whether that and the varied plugs and the missing when hot are related or not, but I'll leave it to them for the time being - they say it needs head and barrels off to have a look, and I'm not prepared to embark on that at home (gargre isn't tidy enough for me to keep track of that many removed bits, among other things).

Thanks,

Richard

[F5 Dave]

Hmm, they did a compression test presumably? Not saying they are wrong, but I'm naturally suspicious, sometimes it is easy to hone down on a suspected cause (which I might well be doing now) but if it is fouling plugs I wonder how much blowby would be produced as a result?

[rwh]

Hmm, they did a compression test presumably?
Not yet; they've only had it for a brief check. It goes back for real on Monday.

Not saying they are wrong, but I'm naturally suspicious, sometimes it is easy to hone down on a suspected cause (which I might well be doing now) but if it is fouling plugs I wonder how much blowby would be produced as a result?

I'm obviously inexperienced in this area, but doesn't blowby mean that more is getting past the rings than it should, implying that there's something wrong with the rings, bores or pistons? Anything that's causing plugs to foul (incorrect mixture or lack of spark is it, right?) couldn't cause that, could it? Though perhaps the other way round - lack of compression could cause bad combustion?

Richard

[F5 Dave]

Yeah it could, but that would tend to indicate that 2 cylinders had ring problems & the others didn't. Comp test should show that up. Interestingly my RF showed better compression on the cylinders that were fouling, but I think that is irrelevant.

If there was oil getting on the rings then it would leave an oily residue & blow blue smoke. If it blows a bit of black smoke when revved hard then that can be normal or tending rich.

All I'm saying is if you have the bike I'd swap the carb plastic slides & emulsion tubes to the good sides try the bike for a couple of days to see what happens to the plugs (swap plugs as well would be good). If it still fouls then forget what I've said & entrust in the shop. Also look to see if the plastic slides (runners that the slides slide on) are worn near the bottom.

I'd also check the float valves are ok if haven't done already. (Can't remember rest of thread)

[tri boy]

Has anyone checked the spark plug cap resistance value?
Quite a lot of plug fouling/cylinder "drop out" can be traced back to this fault.
Plug cap resistance should be around 5-8k ohms.
Quite a few suzi's were diagnosed with oil fouling leading to top end overhauls, when the real fault was a $20 plug cap.
Worth probing with a multi meter.:cool:

[F5 Dave]

Std they are ~12kohm, but this is still ok, when they fault they are about 40-100k. Of course they are double coil so measuring from cap to cap will net about 40k from saggy memory. Take them off if in doubt.

I found one on mine like this & thought ahhh! But didn’t help, just a red herring that wasn’t helping the cause either.

[SILVER SUZI]

I may be a bit late with this. I have an 89 Gixxer 750 (it has the same model carbs) and had a similar problem. The cause was a very small tear in the diaphram. It requires replacing the diaphram. I'm waiting on a price for new as this has happened before so I'm guessing four new ones is the way to go. It's easy to check, you don't need the carbs out, just the tank off and take the tops off the carbs. Take out the diaphrams and work your way round them streching them slightly so any holes show up. It's quite amazing how similar your symtoms sound, like a miss that you can't put your finger on but you know it's there.
Hope this comes in time and hope it helps.