BST38s no midrange power

[Upshotknothole]

When I first got the bike, it pulled great through the midrange, recently it started bogging pretty bad with around half throttle through the midrange. Gotta roll off and baby it to upper RPM or whack it open and hope it doesn’t bog. I already hit up the local shop with a dyno and they can’t be bothered to get back to me about tuning it. It’s a GSXR750N, same as an M everywhere outside of America. I wouldn’t be in a big hurry to fix it, but I am going on an 800 mile ride in a week and a half. 

 

I pulled the top off one of the carbs and pulled the needle, I’m guessing someone stuck a dynojet kit in it, the needles taper down super thin for the bottom third, doesn’t look like any of the factory pro or mikuni needles I’ve ever seen. I’ve got a spare set of BST38s that I have had in a box for years that I don’t know the jetting on that I could try swapping over, I’ve got a spare set of RS38s that will pretty much eliminate the entire bottom end for power, or I could order a set of RS36s. I could also just say fuck it and pull the engine and stick an unknown 1127 in it with the RS38s I have, none of which will be perfect jetting. 

 

The bike also gets shit mileage, I just changed out the gearing, it was -1 +2 and I was switching to reserve after around 120 miles of freeway riding, which is bullshit with how big these tanks are. It’s improved a little with stock gearing, but haven’t had time to do a long ride to check mileage yet. 

 

Try and fix the stock BST38s with dynojets?

Swap another set of BST38s that hopefully don’t have dynojets?

Buy a set of RS36s?

Say fuck it and swap in an 1127 or a bandit engine with RS38s?

[no class]

one thing I have noticed in the past about the bst38’s is the abnormal needle wear …… up near the c clip region and where the needles rub on the emulsion tubes …… this may be your issue as well . Dynojet needles and jets are tuned as a kit so just swaping needles to originals will throw it out of tune ….. factor in the drilled hole in the carb slides and softer spring . 

[Upshotknothole]

7 hours ago, no class said:

one thing I have noticed in the past about the bst38’s is the abnormal needle wear …… up near the c clip region and where the needles rub on the emulsion tubes …… this may be your issue as well . Dynojet needles and jets are tuned as a kit so just swaping needles to originals will throw it out of tune ….. factor in the drilled hole in the carb slides and softer spring . 

The needles didn't have any grooves in them, but they did like kind of warn. I'm almost positive these have a dynojet kit in them so I'm gonna check my spare set of BST38s and hope that they don't. I should also clean out these carbs and see if it's anything obvious like clogged jets or anything.

[fab]

what have you changed recently on the bike? air cleaner setup, exhaust system.

you said it was working right so something has changed for it not to work now

or maybe you need to service the fuel system ,clean carburfuckingettors and fuel tank 

[Upshotknothole]

12 minutes ago, fab said:

what have you changed recently on the bike? air cleaner setup, exhaust system.

you said it was working right so something has changed for it not to work now

or maybe you need to service the fuel system ,clean carburfuckingettors and fuel tank 

When the mid range started to bog I hadn’t changed anything. So it very well could be some crap in the carbs. I’d only had the bike a few months and I’ve probably been riding it a lot more than the last guy. 
 

The tank is pretty spotless, but I have yet to crack open a float bowl to see how much varnish is in them. All I’ve actually done with these carbs is pull the cap off one to check the needle and noticed they have a dynojet kit. Gonna pull the carbs tomorrow and go through them. The bike pulled pretty strong through all the revs when I first picked it up earlier this year. 

[manden]

How do the plugs look?

[Upshotknothole]

On 9/5/2021 at 12:58 PM, manden said:

How do the plugs look?

3 out of 4 leads pulled right off the spark plug caps, and the plugs are far from uniform. Gonna swap out the leads, caps, and plugs. 

[Upshotknothole]

Went through the carbs, the tips of a couple of the needles were slightly bent, the dynojet needles are really thin at the tips, straightened them a little, emulsion tubes look a little oval, but the float bowls were really clean, just a tiny bit of dirt in the bottoms. Gonna put in fuel filters, ditch the aftermarket single outlet petcock and go back to a dual outlet stock unit. A couple of the single pod filters were starting to come apart, putting dual K&Ns on. Replaced all the plugs, I think 2 or 3 of the caps, and one of the coils as the leads on the 2-3 coil had been trimmed to the point that it was stretched tight over the breather cover going to #2. 

 

Whichever previous owner of this bike used anti-seize on the coil screws, I could fucking kiss them. I was expecting to be cussing all evening trying to get those horribly soft screws out. 

 

Should have it back together tomorrow and hopefully it runs a little better. I haven’t dealt with CVs in a long time and I don’t miss them. I think this bike is getting RS36s later this year. 

[Dezza]

I want someone to invent an anti-seize threadlock, if that makes sense

[Oilyspanner]

If you can see that the emulsion tubes are oval, they'll be pretty far gone. You can drop the needles down to offset the richness caused - I've done this a couple of times, even gone down a pilot jet size because it caused problems there too. 

[manden]

8 hours ago, Dezza said:

I want someone to invent an anti-seize threadlock, if that makes sense

Haha

[Upshotknothole]

9 hours ago, Oilyspanner said:

If you can see that the emulsion tubes are oval, they'll be pretty far gone. You can drop the needles down to offset the richness caused - I've done this a couple of times, even gone down a pilot jet size because it caused problems there too. 

The needles are already lowered all the way or close to it. This whole jet kit looks pretty old and tired, if I had time I’d throw a factory pro kit in there and replace the emulsion tubes with stock, but leaving for a road trip next week. I think I’m also going to switch to RS36s after I get back and not bother with these carbs anymore.

[Oilyspanner]

Makes sense, standard parts are expensive, not renewing them goes some way to paying for the rs carbs. The rs36 carbs are easier to set up and parts are readily available 

[Upshotknothole]

7 hours ago, Oilyspanner said:

Makes sense, standard parts are expensive, not renewing them goes some way to paying for the rs carbs. The rs36 carbs are easier to set up and parts are readily available 

Yup, a new factory pro jet kit and stock emulsion tubes and I'm already looking at almost half the cost of a set of RS carbs. Some of the rubber bits were also showing their age and the carbs could use a rebuild kit. Easier to just throw new carbs on. Plus my first slingshot had them when I got it and I always loved the way the top end hit on that bike.

[Upshotknothole]

Took it out for a ride, still bogging, but it feels like it's moved lower in the power band now. Made the bottom end even more gutless. What's annoying is I have a spare set of RS38s sitting on a shelf right now that part of me really wants to try putting on, but I know they're way too big for this engine and will make it horrible for riding around town...though I may still try one day while waiting on RS36s.