BST38SS maintenance

[Upshotknothole]

Changing out the emulsion tubes on my carbs, was bogging in mid range and already lowered the needles as far as they’ll go. Been procrastinating for a while because I hate pulling the carbs or doing anything under the tank on this bike. 

One of the previous owners fit an aftermarket petcock with a single outlet, that also needs to all get fixed. 
 

Dynojet really didn’t want to sell me replacement emulsion tubes. Multiple emails and a long phone call finally get me a set of new ones. 
 

 

And something I’ve noticed with these carbs over the years. I think most sets have at least one broken connector for the powerjet on the float bowl. I run dual pods so can’t use them, but they seemed to be a lot of trouble for the level of fine tuning they gave when dyno tuning the bike. 
 

 

It’s gonna be raining the next couple of days here, so should have it all back together with some of that rats nest under the tank cleaned up by Thursday. Then I’ll take it out and test it. I don’t think there’s a single shop with a dyno within an hour of here that’ll touch carbs now. Well, maybe one Harley shop, but would prefer not to go there. 

[DAZ]

Definitely a dying art .... 

[Upshotknothole]

So I've been working on the bike this afternoon. Went to swap out the single outlet aftermarket petcock that was installed when I bought it for an old spare stock one I had. And of course the stock one just pisses out fuel everywhere. Tried pulling it apart and cleaning it up, but I think it's just been sitting for far too long. Gonna try one of those K&L rebuild kits for right now. Always heard mixed results with them, but I don't feel like installing a pingel or paying for an OEM petcock.

 

[Upshotknothole]

Never had a problem with this petcock in regards to flowing enough fuel for the stock carbs, but it is next to impossible to change from “on” to “res” while riding. The lever is just too short to be useable with gloves on, and it’s a different fit to stock ones. 
 

[Upshotknothole]

Stopped wasting my time with the petcock for now. Carbs have the new emulsion tubes in them, needles on the 2nd notch from the top, AFR 2.5 turns out, and it won’t do anything other than idle. 

I’d tried running the fuel lines under the filters this time, thought that might be the issue, and rerouted them vertical again, same thing. If the bike sits, it starts and idles fine, but try and rev it and it dies like it’s out of gas as soon as you hit 4K. 
When I pulled the carbs off originally, the mixture screws were anywhere from 3.5 to 4.5 turns out, I tried adjusting them back to 4 turns out and same thing, just runs richer now before it dies. Gonna let it sit over night while I think about what to try next. I guess I should pull the carbs again and double check the float heights. Just feels like it’s barely getting enough fuel to idle and anything more than that and it dies, and needs to sit for a few minutes before it’ll start again. All new fuel line, but who knows, maybe some dirt got knocked loose and clogged stuff up. 

 

[Upshotknothole]

Maybe I’ll go get fresh gas in the morning as well, not sure how old the stuff in the bike or the stuff in my gas can are right now. 

[Dezza]

How does it behave with a small auxilliary fuel tank instead of the proper tank? If it's the same you can eliminate tank/tap issues. I helped my mate when he had a 750w and lost the will to live when I saw that mess under the tank so I feel your pain .

[Upshotknothole]

4 hours ago, Dezza said:

How does it behave with a small auxilliary fuel tank instead of the proper tank? If it's the same you can eliminate tank/tap issues. I helped my mate when he had a 750w and lost the will to live when I saw that mess under the tank so I feel your pain .

I started with an auxiliary tank, switched to the actual tank to make sure that wasn't the problem, and then switched back to using the auxiliary tank ever since. I really prefer working on naked GSXRs without all the excess crap under the tank, and Ws are a nightmare.

Thought about it some more and it feels like it's dying as soon as it comes on to the needle, might swap the old emulsion tubes back in and see what it does.

[Upshotknothole]

Fired it up a couple times this morning here. Air fuel screws are still way too far out, but it starts right up with no choke, still won’t rev above 4K without bogging and dying. Let it sit for a bit and fired it up with full throttle and it rev’d all the way up, then immediately acted like it ran out of gas and died, so still thinking a fueling issue. 

[Upshotknothole]

Swapping out emulsion tubes has turned into almost a full tuneup now. Floats were all off, and I was lazy and didn’t bench test the float valves before putting the carbs back on. Flooded and fouled the plugs. Finally pulled them tonight and they’re shot. 
 

New plugs are on order, gonna pull the carbs off again tomorrow and double check the floats and bench test the valves this time. It’s gonna be really nice here by the end of the week, need to get this back on the road. 

[2scars]

Try raising the needles up one position when they go back in.  DO you have new needles as well or just the tubes?  Sounds like to me the needles are the culprit.

 

Good luck!

[2scars]

Changing needle height is easy since you don't have to pull the carbs to adjust them.

[Upshotknothole]

Needles are Ti, I'd lowered them previously because the emulsion tubes were warn, put them back to the recommended setting with the new emulsion tubes. I need to pull them and check the float heights again, part of why I haven't touched them in over a week. Ordered a set of replacement O rings for the cages that hold the floats in place, and been dealing with home repairs and decided I needed to swap the forks and front brakes . Might try working on them again today otherwise it'll be next week.

[Upshotknothole]

Got my front end swapped and a new brake setup, so I guess I should start working on these carbs again.

Pulled them apart tonight, took the caps off and everything looked normal, drained the bowls and pulled them off and the floats, all still normal. Started pulling the main jets and after struggling for a bit, finally was able to read them with the help of my phone. According to Dynojet, no air box and full exhaust should be a DJ160 main. When I first got the bike it had individual pod filters and an obnoxiously loud chopped down Yosh full system. The jets in the carbs are DJ124 which is what's called for with full exhaust and air box.

Dug through my box of random jets and found a set of DJ130s that I'll try tomorrow to see if they make any difference. I've also got a spare set of CV 38mm carbs that I'll open up to see what they have in them. They're nicer looking carbs as well, with the plugs in the float bowls to access the main jets on the bike. Not really sure if there's any difference between the two sets of carbs other than that, they both have the external hose for the power jet. I'll post up photos tomorrow.

I looked on the Dynojet website and I can order the correct (according to the instructions) jets for my setup, but the shipping will cost more than the jets and won't be here for at least a week. At this point though I'm half tempted to just order a Factory Pro kit and start over with a clean slate. Or pull this 750 engine and swap in a spare 1100 engine and jet the 38s for the 1100. Not the 40s that most places got, but still better than the 36s America got.

[Upshotknothole]

Opened up a spare set of 750L carbs and it looks like they have a factory pro jet kit with a more appropriate main jet size. Gonna pull everything out and see what shape they're in.

[Upshotknothole]

This has turned into such a headache. The spare L carbs are pretty trashed on the outside, but I’m pulling all the internals and throwing them onto my N carbs. Lots of weird things going on with the jetting on both. L carbs had pretty big main jets, #147.5, but also had #7.5 power jets. For the life of me I can’t remember where I got these carbs. Maybe they were on my 1100M that I bought back in ‘06 for a 7/11 donor bike, but not sure if it had an air box or not. Might have been given to me as well. Getting old sucks. The N carbs had Dynojet jets and emulsion tubes, but stalled as soon as they came on to the needles after I swapped the tubes. 

Gonna throw the what I’m guessing is a Factory Pro setup into my N carbs with new Mikuni emulsion tubes and see how it runs. If it’s still off I’ll throw the Dynojet stuff back in with the worn emulsion tubes and call it good enough until I can decide what to do. 

Any reason why I can’t run the L float bowls on the M/N carbs? The Ls had access plugs for the main jets, M/N lost them and the shapes are a little different. I’ll double check float height between the two. 

[Upshotknothole]

Carbs are all back together. Currently sitting in a vice on the work bench with a fuel bottle and the floats are all working as intended.

I went through my spare carbs, and they did have a factory pro stage 3 kit in them, and the factory pro kit that I had kept checking for spare parts was actually the right box for that jet kit. Helps if you actually read the box once in a while. So yesterday I removed all the dynojet parts and modifications from my carbs, moved all the factory pro parts over, installed my last set of new Mikuni emulsion tubes. I even called up Factory Pro to ask them about the power jets they include with their jet kit, because in the archives it says to block them off if not running an air box, but they said to run them and only Dynojet likes to plug holes on carbs. I'll try them with the recommended jets, 67.5, and I have one set smaller, 62.5, as well as the 0s if they cause problems.

As a little tangent here, earlier this year I emailed Dynojet about replacement emulsion tubes, as you can't order them off their website. First they asked me to email them photos of my current ones, which didn't make any sense to me since you can't use them with anything but Dynojet needles. Eventually they told me I had to call them to order them. Call their number, tell them I need replacement emulsion tubes, they transfer me to the dyno department, the dyno deparment, confused as I am, transfers me to the right department, and I finally order a set of their tubes. I think the whole process took a couple months. Yesterday I called up Factory Pro, told them I had one of their old jet kits and had a question, they asked me what bike and were super helpful. I even mentioned to them that the date on this jet kit was 1994, they didn't care, said it should still be the same kit they sell today. Told me to give them a call back if I had any issues tuning the bike.

And that right there is why I will always happily give my money to Factory Pro.

[Upshotknothole]

Had a couple hours to kill in the garage today, spent some time going back and tightening all the fasteners I knew I had touched, then said screw it and reluctantly threw some new plugs in the engine and got the carbs installed with their fuel bottle still attached. An important reminder here, remember to double check your A/F screws before you have a fuel line routed through the frame to a bottle on a stand and you’re holding the carbs with one hand and screwing the mixture screws in and back out with the other. Anyway, got the carbs adjusted and installed in the bike. Don’t even hook up the throttle, either it works or it doesn’t. 
 

it fired right up! Killed the choke, adjusted the idle, noticed I’d forgotten to clamp the vacuum line for the petcock, starts and idles nicely now. Pulled the throttle cable by hand and it hit 7-8K nicely without any hesitation. 
 

it’s about 90 here(33C) and I’m tired of sweating my ass off so calling it a day. Next step is fully reassembling the bike and taking it on some test rides when it’s not so hot out. Hoping all this work accomplished something, and those stage 1 dynojet main jets could not have been helping it. More to come. 
 

 

[Upshotknothole]

Took it out for a quick shake down ride this afternoon. Pulls real nicely all the way up to 13K. Getting a little bit of surging around 4-5K when cruising, I still need to sync them and I'll see if that clears it up at all. But over all pretty damn impressed with using the recommended settings from Factory Pro.

[Dezza]

How do the fancy front brakes perform relative to a stock set-up?

[Upshotknothole]

9 hours ago, Dezza said:

How do the fancy front brakes perform relative to a stock set-up?

My stock brakes were pretty worn out, in comparison these are feeling really good. Yesterday I was testing new jetting, brakes, and forks, and mainly focusing on the jetting. Things I did notice, these rotors are noisy when moving the bike around. They've got a fair amount of side to side play in the holders. Other people I've talked to that ran them back in the day said this was pretty normal for them. The next time I take it out I'll put more effort into really heating them up and getting all the surface rust off those rotors and seeing how they do.

I will also get new pads for them and I just ordered a rebuild kit for my M/C.

Edited July 16 by Upshotknothole
spelling

[Dezza]

Sounds normal for fully floating race discs. My Spondon discs are the same.

[Upshotknothole]

Finally got it out for a proper ride today. About 100 miles of twisty mountain roads and some highways. Carbs feel good, might still order a set of the next size larger mains to double check the top end, but I can swap those out without even pulling the carbs. Otherwise, my butt dyno says the bike is running pretty damn good right now, and got a respectable 40 miles per gallon today. When I first got the bike, it was only doing 25-30 MPG when cruising on the freeway.