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Carbs have me stumped

3K views 15 replies 9 participants last post by  nickr 
#1 ·
Ok,so I have an 1100 Katana motor with 40mm CV carbs, running K&N dual pods and a Muzzy stainless pipe. I was having some issues this summer with the bike stumbling at 3500 to 4500 rpm. Occasionally it would foul the plugs and run like crap.
I took the carbs off a while ago, put in new plugs and checked the settings of the floats. The floats were way out, way too rich, so I set them to 14.6mm as per the specs. I also checked and my mains are 127.5. The needles, I cannot recall the number but when I cross referenced them, they came back to a GSXR750RR. Which may make sense. I got these carbs from a guy who has a RR and they have the bolt on the float bowls to quick change the jets. The needles were set in the middle of the adjustment as well.

Anyhow, I checked the mixture screws and they were all over the place. I reset them all to 2.5 turns. Now the bike does not pop on deceleration like it used to, great. But now the bike has a huge stumble pulling away from a stop. It also is now backfiring a bit on acceleration, which it never used to.
I reset the screws to 2 turns out, with little to no change. The screws were anywhere from 1 to 1.75 turns out before I changed them the first time.

The fouled plugs and sooty muffler make me think it was rich, but now it seems like it is way worse. I will try setting the screws to .5 turns out tomorrow, but if they need to go that far in, doesn't that mean my mains are way too big? The stock jets, I think, for these carbs are 125 so I don't think that they are way too large.

The bike used to pull great anywhere from above 4500, but now its basically running like crap.

Help!
 
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#2 ·
It sounds to me like its all outta whack. Sooo, generally speaking pod filters on cv carbs makes for a headache, but we won't open that can of worms for debate. Pod filters DO however lean out the mixture. The guy you got them from may have over compensated for that with too big of jets. The f/a mixture screws shout be between 2 and 3 turns out. You may want to go back to stock, and put a stage one jet kit in it. Factory pro makes them and they come with 5 position adjustable needles, that should help the tuning process. I think it also comes with extra jets so. You can do some mix n matching to get it right.


You may also want to check the coils/plug wires as you may be bleeding current and getting a weak spark.
 
#5 ·
The factory pro link posted is a great resource for tuning.

Do you have the o rings and washers in the mixture screws? They are small easily lost and will cause some problems if they are not there or are old and dried out.

You might be a little to rich on the mains. Stock gsxr 1100 with 40's uses 125's and I think with the smaller cams in the Kat motor they should be smaller. Not to familiar with the Kat motors so I cant say for sure
 
#15 ·
not to necro-bump, but I wanted to give a shout out to rklontz. Finally changed those pesky o-rings and after setting the pilot screws to where the bike wants to run, it's better than its ever been.

No more stuttering and surging at low rpm, revs easily from idle and just plain feels way better. Pilot screws are at 1 turn out from bottomed, which makes me think its still a bit rich. But right now I am finally able to enjoy this beast.

Thanks again rklontz and GDC
 
#6 ·
I don't think the 40mm carbs are a simple bolt on, they'll need a little work done first.
The 40mm cv carbs are good - BUT - everything needs to be in spec: correct float heights, no wear in the needle jets/emulsion tubes and the inlet ports etc and inlet rubbers matched to the larger sized carbs - turbulence isn't good for carburation. The 1100 M/N heads were slightly different to make use of the big carbs, although the US had the smaller 36mm carbs for emissions reasons.
The GSX1100F engine had small carbs standard and the inlet ports/rubbers need to be matched for the big carbs, with bigger carbs the inlet gas velocity is slightly less, so details make a larger difference to quality of mixture to be ignited, they will work worse than smaller carbs unless the boots/ports are blended in to suit, even before the you get the mixture right.
My 1100N works very well with it's 40mm carbs, clean pick-up right through and it revs right out - but I've learnt more with these carbs than on any other set ! My inlet boots to port union are matched perfectly, no ridges at all, in the press blurb at the time it was said that the ports were hand finished, mine do show evidence of some grinder action, so perhaps it was true, or someone has cleaned my head up before I bought it(it's only a few areas).
The 40mm cv carbs need work for them to work well, they are great for keeping good manners, yet flowing well, decent economy- they suit mildly tuned bikes with big bores that have had a bit of head work done. For many bolting on a set of 36mm flatslides would be easier, because unless you have an 1100M/N, or a 750RR, you will need to alter the inlets slightly to get the best from the 40mm carbs, the mismatched inlet diameters will upset carburation at lower revs in particular.
 
#7 ·
From the sticky on carbs at the top of the page, this information might give you a baseline:

1991-1992 GSXR1100 U.K

Type: BST40SS
Main Jet: 125
Main Air Jet: 1.2mm
Jet Needle: 6ZD13-3
Needle Jet: P-2
Pilot Jet: 40
Pilot Air Jet: Not Specified
Pilot Screw setting: Preset (2 turns out)
Starter Jet (choke): 40
Float Height: 14.7mm (+/- 1mm)

As I couldn't figure out where your needles are set, try dropping them down by putting the clip in the second groove from the top.

As in the carb sticky, bench sync the carbs before you put them on and see if you can line up a set of vacuum gauges to properly sync them once you are up and running again.

Also, as mentioned by nickr, it wouldn't hurt to make sure your ports and manifolds are match ported. If it was me, I'd pack the ports with paper towel first and then do a bit of grinding to clean them up. Just blend to match the manifold at the start of the port and taper it back into the throat of the port. Remember that less is more here and just go steady steady. Use a vacuum cleaner to remove the grindings. When you have as much of it out of there as you can get, before you pull the paper towel, give the port a quick hose with CRC or WD40. This will push any left over grinding dust into the paper towel and will drag it out with the paper towel. Gently wipe the ports out with another clean paper towel or shop rag with soem more CRC or WD40 on it until it comes out clean and then wipe out with a dry paper towel or clean shop rag.

Another thing to check is valve clearances. Go through and rest them all back to factory spec. Worse case, it can't hurt. Best case, you might find a valve hanging open or not opening enough. It also removes one possible issue from the problem. Check all the valve springs are good too.

The other thing I seem to remember from somewhere is that pods tend to foul the air feeds on the inlet side of the carbs so have a close look at where the pods are sitting when they are installed and you may have to do some trimming and mods to the pods flanges. Mount them on the carbs and have a look. It might even help to fit one side of the dual pod to the outside carb intake and use a small mirror through the other flange to see how it sits.

All of this is just ideas I have had that I would look at doing if it was handed to me and I was asked to sort it out. Some may or may not work but you'll only know by trying. It could be something totally different too like coils breaking down, an ignition pickup moving, the cam tensioner needing adjustment, who knows.

Best best thing you can probably do is grab a manual from the download section and go through the service section to make sure everything is back to baseline so you can focus solely on the carbs instead of trying to chase what might be a problem that is not in the carbs at all.

At the end of the day, a session on the dyno might help too but make sure all the baseline service work is done first.

Keep us in the loop as to how you go.
 
#10 ·
Also, as mentioned by nickr, it wouldn't hurt to make sure your ports and manifolds are match ported. If it was me, I'd pack the ports with paper towel first and then do a bit of grinding to clean them up. Just blend to match the manifold at the start of the port and taper it back into the throat of the port. Remember that less is more here and just go steady steady. Use a vacuum cleaner to remove the grindings. When you have as much of it out of there as you can get, before you pull the paper towel, give the port a quick hose with CRC or WD40. This will push any left over grinding dust into the paper towel and will drag it out with the paper towel. Gently wipe the ports out with another clean paper towel or shop rag with soem more CRC or WD40 on it until it comes out clean and then wipe out with a dry paper towel or clean shop rag.

^^^ That's pretty much exactly how I do that... :)

As noted previously, make sure the fundamentals first. Some of what you describe could be as simple as a breach on the seal between carb-throat and manifold boot or manifold to head. That o-ring can get flattened and hard and fuel damaged to where it is sucking air and screw everything up, even with perfect carbs. I always like to slap a "known set of carbs" on to see if it is in the carb setup or not, and my spare RS38s do a good job as a "litmus test". My last use of them for recent issue on an '86 oiler revealed exactly the issue of manifold seal to head, and caused my good carbs to run as bad as the "suspect" set. The "suspect" set ended up "okay tuning and setup", but leaking fuel from #1 past the float valve. That trickle had also breached the boot at some point upon closer inspection.

At the age of these dinosaurs, regardless of how many miles and especially those that weren't run much or often, one of the first things I check is bandclamp and seal on carb-to-boot and boot(manifold)-to-head.

Otherwise, what everyone else is saying above. :D

Auto part


^^ Old/flat o-ring -vs- fresh one. ;)

-crisp
 
#8 ·
Great responses, thanks.

The ports are not matched to the inlet rubbers, so that might be playing a big part in this. I was going to pull the carbs and do a rebuild on them to ensure that the o rings etc are good. The needles are adjustable and are set in the second groove from the top.

Honestly reading all of the above, it looks like I am over carbed and might go back to my 36mm CV's. I could do the port matching etc to get these carbs to work, but this seems like a pile of work when going back to the factory size carbs may be a better choice.
 
#13 ·
Well the carbs are 40mm, I have measured the carb ID to confirm this. The float bowls are the same as on the RK carbs, but the tops are regular CV tops. The needles are adjustable and as I said the part number on them is from an RK.

The carb boots were brand new last year, and the O rings are not flattened. I have checked for air leaks at the boots with WD-40 while the engine is running and no issues were found.


I think that the turbulence caused by the large step from the boots to the intake port is causing some of the issues. The stumbling at 3500-4500, I took that for richness due to the float height, but now think that could be from the port/boot mismatch. I think that the mixture screws were turned way in to adjust for the overall richness of these carbs.


Now that I have turned out the mixture screws, if there was any damage to the mixture screw o rings, or they were hardened, I think that may be why I cannot get it back to where it was running (albeit not well).


I think that I am going to rebuild my 36mm carbs back to factory settings and put them on over the winter. I still have the old boots from those carbs so I can switch them fairly easily. I would love to go to flatslides, but I have not been able to find any that work with my setup. I would have to get some custom throttle cables made up to use flatslides. As I have a 750 frame and an 1100 front end, the 750 RS cables are about an inch too short.


I could do all the work to port match the 40s to the bike, but I really just want to ride it and enjoy it. With all of the work I have done on it over the last 2 years, I don't really feel like taking the engine out of it again if I don't need to.
 
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