1991 GSXR 750 BST38 Super Rich.

[alocker]

 

Greetings All from the USA!  I'm looking for some help as I'm stumped by these BST carburfuckingettors. 

1991 GSX-R750 with very low miles. Bike is spotless but idle/low RPM is an issue. It runs very rich down low and cleans up around 4000 RPM. I rode it about a mile after the purchase and it pulls fine and smooth up top. Once fully warm, it chugs down low and will die when after pulling the clutch in. The bike sat a lot so I just assumed it need a carb clean.

I pulled the carbs and they were spotless. Not a single spot off oxidation and zero gumming. The previous owner was meticulous and ran ethanol free gas exclusively. They were 100% factory stock with the brass pilot plugs still installed. I disassembled everything for inspection and everything looked perfect. No wear on the needle/emulsion tubes. I also installed a Factory Pro stage 1.5 jet/needle kit along with activating the Power Jet circuit and extended pilot screws. New stock OEM Suzuki air filter back in the airbox. 117.5 main jet. Stock pilot jet. Float set at 15mm

I go to fire it and the same thing. Fired right up on choke and runs smooth at 4k. Choke off and chugging down low. Idles for a bit but eventually fouls the plugs. They are wet and black. I cleaned them to mess with the pilot screw settings but get the same result. I thought coils at first but after troubleshooting, all 4 holes have good spark.

Carbs back out. I installed a full set of K&L supply o-ring kits along with new slide guide orings. Inspect again to make sure everything is perfect. I did suspect maybe the slide guide orings could be an issues, they seemed flattened and wet. I also thought maybe I had some float valve leak hence the new kits.

I took an extra step to verify float height and found some interesting results. I used 4 small pieces of clear tubing and put them on the newly activated Power Jet fittings and filled the carbs on the bench. I noticed 2 of the bowls had a significantly higher fuel level. 2 carbs were even with the carb body/bowl and 2 of them were 2-3mm higher. I drained and re-filled to confirm and got the same result. I decided to lower the floats to 16mm in attempt to lean things up. Same result, 2 bowls higher.

I went ahead and re-installed thinking in a running situation it would be fine. Same result, very rich and fouling plugs. I did check the floats, I did not see any fuel in them but maybe I missed something??

Should I set the floats based on actual fuel level? Any suggestions on what that level should be? It seems Mikuni's are supposed to be around or slightly below the carb body split???

Sorry for the novel, I just wanted to be clear. It's amazing how many carb issue posts seem to never get resolved. I'd like to make I share my resolution even if it's something stupid I did.

Part 2

Put a new set of plugs in and ran it a bit more.  

First, I can screw the idle screws all the way in and it results in highest idle. When I adjust the idle down it still runs rough. I also found it will hang at about 3K for a bit after revving. It eventually falls back to normal idle. Also when revved hard, thick black smoke comes out of the tail pipe. If it pull a cap off the top of the diaphragms creating a leak, it idles up a decent jump. When I pull the petcock hose, it idles up a lot, like to 5K.

I used an IR temp gun to check each header pipe at the first bend. I was able to get right on the pipe so the readings are fairly accurate. They are all within 50 Degrees F but often closer. This tells me I don't have a dead miss which it doesn't sound like it does, it just doesn't sound right...It kind of lopes like it has cams but doesn't really fully clean up.

Current conclusion.
--Still pig rich.
--Occasional hang indicates vacuum leak.-INTAKE BOOT O-RINGS???
--Idle screws all the way in for best idle-A interesting fact, when I drilled out the brass plugs before factory screw removal, I noted that all 4 were fully seated. I thought this was odd but maybe they are all like this.
--I'm wondering if I need to check the valves???

Edited April 2, 2022 by alocker
Added intro

[TonyGee]

sounds like float valve leaking, did you replace the needles ? 

[alocker]

1 hour ago, TonyGee said:

sounds like float valve leaking, did you replace the needles ? 

I would tend to agree but...The second time I had the carbs out I put a full K&L kit in with new viton needles, brass seats, and made sure all the other O-rings were new.  I bench tested the float valves and they hold fine, does not seem like they overflow into the engine.  Oil height has been steady.

I have the rack out again and confirmed again, everything look perfect.  My #1 carb float is set at 16mm which is lower than parallel, my fuel level on my clear power jet tube and it's about 2-3 mm above the bowl/body split and holding steady on the bench.  

[TonyGee]

I take it the air filter and airbox are ok ?   did you check the small air jets at the airbox side of the carbs ?  one of my plugs was running rich (black plug) turned out to be a blocked air jet. 

[alocker]

18 hours ago, TonyGee said:

I take it the air filter and airbox are ok ?   did you check the small air jets at the airbox side of the carbs ?  one of my plugs was running rich (black plug) turned out to be a blocked air jet. 

Factory airbox in great shape along with a new OEM Suzuki filter.  I am actually in the process of checking the idle air bleeds now.  They seemed clear and free last time I checked but admittedly I did not fully removed the bleeds and brush.  I started with the #1 and cracked the bleed screw slot trying to remove it.  I think they are big enough to leave in place and run a small orifice brush through again just to make sure.  

[alocker]

Did some more work inspection.  All pilot air bleed circuits are perfectly clean, zero dirt or gunk in them from the bleed jet all the way back to the adjuster.  I set up my clear tube float test again and they are actually all the same now that I have set up a more stable carb stand.  

All bowl fuel levels are 2-3 MM above the body/bowl split.  If anyone can confirm if this level is correct I can move on to something else.  There is zero fuel level info on BST carbs that I can find on the internet.  Only float height info.  

I also pulled the intake boots to inspect the o-rings.  They all look fine along with the boots.  I'm going to replace the orings before re-assembly.  

[Blubber]

I do not think your fuel level is correct as you describe it:

Classics usually set their level below the split 

This picture is not of a BST carb but is does show it is below the split. 

 

Hth

[alocker]

4 minutes ago, Blubber said:

I do not think your fuel level is correct as you describe it:

Classics usually set their level below the split 

This picture is not of a BST carb but is does show it is below the split. 

 

Hth

Blubber, Thanks for that pic.  I did some digging and found a ton of specs that state the level of most Mikuni's in that same range below the bowl.  However this spec is mostly for older round slide VM series types. 

There are 2 things making me second guess the BST38 series though.

First, why are my float height settings so far off.  The levels were high at the factory spec of 14.7mm (I set them at 15mm).  The current setting is 16mm.  I'd have to drop them to 17 or 18mm to get 2-4 mm below the bowl/body surface.  Even 16mm you can see they are not parallel to the mating surface.

The second part messing with me is the cutaway of a Mikuni BST40 you can see I've attached below.  You can see the fuel level does in fact show its above the body/bowl mating surface.  FYI I have the carbs tilted to simulate them installed in the bike. i.e. the tops of the bowls are flat unlike the picture below.  

I feel like I'm missing something simple but the bike ran this way when I purchased it.  Is it strange that Suzuki set the pilot screws all the way in (full lean) from the factory?  I have about 2 decades of messing with 4 cylinder Japanese carburated bikes but this is my first with BST carbs.  They seem finicky from what I read.

[TonyGee]

yeah i agree, the level does seem too high !!!  it might be worth raising the float height (lowering the fuel level in the bowl)  just to see if it makes a difference.  a high fuel lever will make it run richer.   

Edited April 3, 2022 by TonyGee

[alocker]

16 minutes ago, TonyGee said:

yeah i agree, the level does seem too high !!!  it might be worth raising the float height (lowering the fuel level in the bowl)  just to see if it makes a difference.  a high fuel lever will make it run richer.   

I might try it but sick of installing these things already:)  Current level shown in pic attached.

I found a few BST guru's here in the US and I'm going to make a few phone calls before I drop the level and re- install.  I'll update as soon as I can. 

Thanks again for everyone's help!

[Upshotknothole]

Try giving Factory Pro a call or shoot them an email? They have a little bit of tuning info on their website. http://www.factorypro.com/Prod_Pages/prods06.html Also check your intake boots the next time you have the carbs on and the bike running. Just spray some starter fluid or similar around the intake boots and see if the throttle jumps. These bikes also don't have a ton of power below 3K, but you shouldn't be fouling plugs. How many miles on the bike? Usually the emulsion tubes are the first thing to wear out.

[alocker]

29 minutes ago, Upshotknothole said:

Try giving Factory Pro a call or shoot them an email? They have a little bit of tuning info on their website. http://www.factorypro.com/Prod_Pages/prods06.html Also check your intake boots the next time you have the carbs on and the bike running. Just spray some starter fluid or similar around the intake boots and see if the throttle jumps. These bikes also don't have a ton of power below 3K, but you shouldn't be fouling plugs. How many miles on the bike? Usually the emulsion tubes are the first thing to wear out.

I did speak with Factory Pro and he didn't really have any other suggestions.  

I did remove the intake boots to inspect the orings.  I am going to replace them.  The boots themselves are in very good condition.  Still nice and soft and no cracks or deformations to be found.  The bike only has 2500 miles on it.  It was stored in a climate controlled shop and is in unbelievably nice shape for an all original 31 year old bike.  I 100% plan to test more thouroghly for intake leaks the next time I run it.  

I appreciate the info on how these bikes run under 3k.  My closest experience with these motors is a GEN I B12 which is a tractor off idle especially with a pipe.  I did think that maybe this is just the way these things run considering at the time it really was basically a superstock race bike with headlights.  Idle screws all the way closed from the factory is suspect though, something seems not quite right.  

Right now I'm re-inspecting everything and trying to get my fuel levels correct.  I confirmed from Derek at Motolab that 1.5mm above the flange is the correct level but can be changed for tuning purposes.  I need to get to at least that level before moving forward since some are 3.5mm-4mm right now.  I'm exploring my emulsion tubes and pilot jets to make sure everything is perfect before the next test.  

[Upshotknothole]

What part of the country are you in? With only 2500 miles and what were stock carbs, nothing should be worn out in them yet. Sounds like you're on the right track with the float levels, and just accept the fact that these engines like being revved out and were never really designed to be useful below 3K. As long as they idle well, no hanging idle or anything, and it doesn't stall when rolling around parking lots, they should be fine. All the real fun is up above 9K. The tachs on the 88/89 750s didn't even start until 3K.

 

I've got a (US)92 750 with the same carbs, but they have a warn out dynojet kit in them and they run like shit below 5K. I was going to pull them and stick on a set of Mikuni RS36s, but I've got a spare 1127 engine, carbs, and exhaust, so I'll be going that route instead. Post up some photos of the bike, don't see a lot of super clean low mileage stock bikes around here.

[alocker]

4 hours ago, Upshotknothole said:

What part of the country are you in? With only 2500 miles and what were stock carbs, nothing should be worn out in them yet. Sounds like you're on the right track with the float levels, and just accept the fact that these engines like being revved out and were never really designed to be useful below 3K. As long as they idle well, no hanging idle or anything, and it doesn't stall when rolling around parking lots, they should be fine. All the real fun is up above 9K. The tachs on the 88/89 750s didn't even start until 3K.

 

I've got a (US)92 750 with the same carbs, but they have a warn out dynojet kit in them and they run like shit below 5K. I was going to pull them and stick on a set of Mikuni RS36s, but I've got a spare 1127 engine, carbs, and exhaust, so I'll be going that route instead. Post up some photos of the bike, don't see a lot of super clean low mileage stock bikes around here.

I'm NW suburbs of Chicago.  How about you?

There are some issues like a hanging idle occasionally, still working on it.  I'm pretty sure I can get it to idle better.  I'm going to set my levels which should lean it out a bit more.  I want to be able to get the pilot screws functional.  Pics attached.  Part of bring the bike back was a set of spare rims with new tires to preserve the originals.  They aren't even flat in the middle.  Everything else is 100% original Suzuki.  I just put the factory pipe back on also just to keep my neighbors happy.

[Upshotknothole]

I'm out in Portland, OR. That bike is really clean, definitely hold on to that full yosh system. Aftermarket exhausts are pretty much impossible to find for our gen of 750s at this point. I was running an 1100 exhaust on mine, gave up on trying to find a used exhaust anywhere.

 

Here's mine. I rode it from LA to PDX last fall and haven't really touched it since. I need to go through the whole bike again now that it's finally starting to warm up out here. I sure do love GSXRs with colored wheels.

[alocker]

I'm kind of at a loss here.

I spent a bunch of time setting fuel levels on the bench and triple checking everything.  I put new o-rings on the head/carb boots and made sure they where tight.  Started it up and the same thing.  Seems rich, runs weird, like a triple maybe, but all 4 header pipes are hot.  The #4 pipe is a little cooler but could be due to pilot screw tolerance.  This time I didn't fully install the airbox boots to save time.  I notice it seems like you can see and feel them spitting out of the inlets.

I decide to check valve clearances.  Everything is well within spec.  Not a single tight valve.  Purchased a compression tester.  I have between 90-92 PSI across all cylinders.  Now I know this is a very low numbers but the engine was cold and throttles where closed.  I would think if they are all that close I wouldn't have an issue BUT.....maybe I do.  The bike does not smoke by the way.  It spews black smoke when I rev it, still looks super rich.  I don't see white or blue smoke which you would think would be present if all 4 holds had bad rings???  

[beardo]

You know, I hesitate to even say this, but are you positive this is a rack of carbs issue, and not an electrical/ignition issue? 

You've been through the rack so many times, but you keep having the same issue. 

I can feel your frustrations just reading your posts. Just wondering if there isn't something else going that is giving deception of being a carb/fuel issue.

[TonyGee]

when i put a GSX1100 engine in my 750ET i checked the compression and wasn't great but all where equal, it even chuffed a bit of smoke out on cold starts now and then, I know the engine had been stood for a long time, but after a few 100 miles the compression has increased to over 140psi and it seems to of stopped chuffing oil out.   it also runs better than it did when i first got it. 

food for thought. 

[edhenderson]

No wear on the needle/emulsion tubes

Longish post, so I might have missed it but- how many miles on the needle jets?

Once they start going obround (and even barely visibly so) you'll be chasing your tail forever.

.

Edited April 10, 2022 by edhenderson

[alocker]

3 hours ago, beardo said:

You know, I hesitate to even say this, but are you positive this is a rack of carbs issue, and not an electrical/ignition issue? 

You've been through the rack so many times, but you keep having the same issue. 

I can feel your frustrations just reading your posts. Just wondering if there isn't something else going that is giving deception of being a carb/fuel issue.

Nope, at this point I'm not positive its the carbs.  They have 2500 miles on them and look brand new inside and out.  Strange thing is the pilot screws were set all the way in under the brass plugs from Suzuki.  Why would they do that?  That would indicate a overly rich issue, right???  

I thought coils at first.  I checked windings and they were in spec.  Did wire pulls while running, all are firing.  Did visual plug fire and they did seem a little weak/orange but that's just my opinion.  

2 hours ago, TonyGee said:

when i put a GSX1100 engine in my 750ET i checked the compression and wasn't great but all where equal, it even chuffed a bit of smoke out on cold starts now and then, I know the engine had been stood for a long time, but after a few 100 miles the compression has increased to over 140psi and it seems to of stopped chuffing oil out.   it also runs better than it did when i first got it. 

food for thought. 

I have seen that before but the still run decent at first.  Thing thing sounds like a tractor and gets worse as it warms up.  One theory would be from above, if this thing was rich from day one, maybe it washed down the cylinders and never broke it.  That could explain all 4 being equally low but still hard to believe.  

2 hours ago, edhenderson said:

No wear on the needle/emulsion tubes

Longish post, so I might have missed it but- how many miles on the needle jets?

Once they start going obround (and even barely visibly so) you'll be chasing your tail forever.

.

2500 miles and stock needles had very little wear.  No ovalling on the tubes but anything is possible.  Maybe it has the wrong tubes in it from the factory???  Would too big of tube make it idle so rich??

[alocker]

[TonyGee]

I think the next step in this saga would be to go down a size on the pilot jets and see how it runs 

[beardo]

A couple of things. 

The mixture screws under the brass caps. Mine were all in different states. All between 2 turns and just a 3/4 turn from seated.

I've never seen all closed from the factory. That said, nothing would surprise me. 

As you probably know, from before, or at least, know now, from reading, one pulls those brass plugs and turns them OUT from where they are supposed to be set at the factory. As in, they are set lean. 

BUT, you state that ALL were seated closed under the brass plugs. I honestly don't recall you stating what size pilot jets you started with. Might the rack you started with have had too large a size just to have gotten to run, from new??

Also, you stated the spark you got was 'weak/a little orange'.  Ignition spark should be a nice snapping blue.

Weak spark, coupled with low compression, as in, rings stuck/in from sitting, would make for an overly rich running condition.

I kind of have to go with TonyGee on this for a spell. Maybe the bike really just needs running back in before preceding/head smashing anymore. 

Motors/engines, and overall machines, in general, don't take to being run, then sat. 

Maybe the solution in this case is to run the bike. 

That's what I would do. Otherwise, without having gone through a complete rebuild, shrugging shoulders.

Edited April 10, 2022 by beardo

[alocker]

15 hours ago, TonyGee said:

I think the next step in this saga would be to go down a size on the pilot jets and see how it runs 

I have been thinking about this.  It currently has the factory spec 32.5.  There is only one genuine Mikuni size smaller at a 30.  I'm not sure if that would be enough to make a difference but I might try it.  

14 hours ago, beardo said:

A couple of things. 

The mixture screws under the brass caps. Mine were all in different states. All between 2 turns and just a 3/4 turn from seated.

I've never seen all closed from the factory. That said, nothing would surprise me. 

As you probably know, from before, or at least, know now, from reading, one pulls those brass plugs and turns them OUT from where they are supposed to be set at the factory. As in, they are set lean. 

BUT, you state that ALL were seated closed under the brass plugs. I honestly don't recall you stating what size pilot jets you started with. Might the rack you started with have had too large a size just to have gotten to run, from new??

Also, you stated the spark you got was 'weak/a little orange'.  Ignition spark should be a nice snapping blue.

Weak spark, coupled with low compression, as in, rings stuck/in from sitting, would make for an overly rich running condition.

I kind of have to go with TonyGee on this for a spell. Maybe the bike really just needs running back in before preceding/head smashing anymore. 

Motors/engines, and overall machines, in general, don't take to being run, then sat. 

Maybe the solution in this case is to run the bike. 

That's what I would do. Otherwise, without having gone through a complete rebuild, shrugging shoulders.

They have the stock 32.5 pilot jet.  These carbs looks bone stock inside and out not to mention clean as a whistle.  They were perfectly in line with the condition of the rest of the bike.  I was very surprised to find the brass plugs still there.  The previous owner said he did pull the carbs at one point to clean them.  You would think you would do something to them while they were out like I did as we all know factory carb settings on Japanese bikes can be less than ideal (always lean).  I guess the guy could have cleaned the pilots with a guitar string but who the hell knows.  

In regards to spark, it's hard to say if my opinion is accurate.  I might have to try again.  What would cause a weak spark?  Specifically coils.  I should probably check to make sure I have full voltage at the coil leads.  Not sure if that's an issue on these bikes.  

I'm not against your opinion of just running the bike, I'm just hesitant to put everything back together to just have to pull the jugs (I know, wrenching is wrenching).  I once brough back a 78 GS1000 that had been sitting for many years, possibly decades.  The thing fired right up and idled great and had way more miles on it than this thing.  Something just seems off.  I've never had a GSXR 750 of this era but I have owned a Bandit 1200.  I know they are not identical but I know how a boiler sounds at idle, and this isnt it.  It's like there is a ton of cam overlap.  I even check the cam timing marks and chain pins to verify it's not a tooth off.  Thing is, everything looks untouched as I go through it.  I don't think that valve cover has ever been off the bike until now.  

[beardo]

I was got my current Bandit back at the turn of the year. Original owner had the carbs off, cleaned them. Didn't pull the brass plugs. So, I get where you are coming from.

I was in the same boat as you with your GS1000. I put the bike back together. Fired right up. The bike had been sitting for many years prior. I have to admit though, the more miles I get to put on, the more 'broken in' it feels.

As for what could cause weak spark. Lots of things. From the battery itself, to bad grounds (earth) to faulty components. Maybe time to break out the multi-meter and start going through the basics. 

Hopefully someone with more knowledge of the particular bike itself can make some suggestions.