Jetting and Throttle Response

[Shifty2932]

I am trying to dial in my carbs to make them operate better.

Bike idles great! No issues there. Bike even runs and rides down the road good.

Current settings. PJ 32.5, MJ 140, NJ position 3. Running UNI pod filters and straight pipes.

Current issue is bike bogs down at 1/2-WOT when RPMs are low. Of bike is above mid range on RPM throttle response is good at 1/2 but still bogs at WOT.

it seems like the NJ may not be lifting and I understand that it won't until it reaches a certain RPM. But is there a way to correct or help it by adjusting the NJ position or the MJ?

For sluggish throttle response and engine bog down at 1/2-WOT am I looking at this the correct way? The issue is mid to high range, I.e. Needle and/or main jet.

I had the same problem with NJ at position 2 and MJ was 142.5.

I know this cannot be fixed with one switch or flip of the magic wand and I will have to switch some things around and have to deal with the response of read the carb manual and figure it out. All I am looking for are suggestion or opinions which way to proceed.

 

[XSLeo]

Ok, I assume your running 77 carbs. Stock the 77 carbs used 25 pilots, 122.5 mains, with a Z8 needle jet, 4M1 needle in the third slot.
I have run a similar set up but with a free flowing straight through muffler. Mine ran great with one up on the pilots and two up on the mains. Your up 3 on the pilots and 7 on the mains. I might back down a bit on both. Try 27.5 on the pilots and 127.5 or 130 on the mains.
Set the idle mix screws on 1.5 turns out. Needle in slot three then tune from there. In the carb guide there is a section, "Tuning for Mods" It may help you.
Leo

 

[Shifty2932]

Leo,

Your assumption is correct. '77 carbs. I know I have a bunch of extra jets in my carb kit. I will have toave to see what I have on hand before ordering. Surely will not go to the Yamaha store to buy jets as 5Twins stated that is only for fools. So I will have to use whatever I have that is close to your recommendations until the other jets arrive in the mail.

Thank you for your input! I will post again once I've had time to look and see what I have and switch some things up to see how it runs.

Thanks!

 

[fredintoon]

Hi shifty,
street bikes need back pressure. Not much but some. Your open pipes ain't got any at all.
A pair of el-cheapo slip-ons should fix you up.

 

[5twins]

You might be able to get the jets at a Yamaha shop, or a Kawasaki or Suzuki shop, as long as they're selling them as Mikuni jets and not the Yamaha/Kawasaki/Suzuki branded Mikuni jet. The retail for a Mikuni main is about $3.50 or $4. Yamaha sells the stock 132.5 main for the BS34s for near $6. Same jet with a mark-up because it comes in a "Yamaha" parts bag. I've gotten jets locally from several shops. I look for ones that deal in a lot of dirt bikes, ATVs, and snowmobiles. Many times they have a giant box of assorted jets you can pick out of and buy individually. The trick is knowing which Mikuni jet type you need. They make 5 or 6 different main jet types and several pilots as well. Your '77 BS38s use what's called the Mikuni small round main, the type N102/221 .....

Yes, you're jetted way rich for a '76-'77 carb set. Mains in the mid 130s are usually as big as you can go because of that very rich Z-8 needle jet. By NJ, I think you're referring to the needle itself. You'll need to lean that a step with these larger mains.

 

[Shifty2932]

Fredintoon,

Thanks for your reply. I know they need some back pressure. I saw an improvement when I put a few thumbscrews in each pipe. So I do have some. Maybe not the best setup, but is something. Not really looking at putting baffles or a slip on muffler on it.

 

[Shifty2932]

5Twins,

Thanks for your input!


So here is where I am at currently. I searched my carb kit and I have 27.5 and 30 PJ's, but only have a 135 MJ. So I installed the 30 PJ and 135 MJ, moved the needle to position 3 and mixture screws are at 3 - 3.5. The bike now runs worse. It still idles good and throttle response in first gear is ok. But in second gear or any other gear I get sputters and blips at 1/2 throttle and total bog or flat spot anything behind 1/2 throttle to WOT. Maybe adjusting the needle to position 3 with still a large MJ wasn't the best idea.

Any thoughts??

 

[5twins]

Position 3 is the middle and stock setting for your needle. With larger mains you need to put it in position 2, making it leaner. That should help clean up the upper midrange and midrange to main transition area (say 3.5 to 5.5K rpms).

You may still be too big on the mains. The '76-'77 carb set differs from the other BS38 sets. It had a much smaller main (122.5) as stock. Combined with that, it had a very rich needle jet (Z-8), the richest of any 650 carb set. That rich needle jet is going to limit the size of your main. While many of the other 650 carb sets may require mains up into the low 140s, you'll never get that big on a '76-'77 set with that rich needle jet.

I would suggest just one up on the pilot to 27.5. Again, you're dealing with the influence of that rich needle jet. It bleeds over and affects both the upper part of the idle circuit and the lower part of the main circuit. Stock setting for your carb set on the mix screw is 1.5 turns out. It should dial in around there, maybe even slightly less with the larger pilots.

 

[Shifty2932]

5twins,

Thank agin for your continued assistance!

So I will put the needle back into position 2, reduce the PJ to 27.5 and reduce the MJ to 130, reset the air mixture screw and see what happens. I wont be able to make the changes for about a week, but I will post an update once I get it done and have a chance to ride the bike again.

Thanks again!

 

[Shifty2932]

Update.

So I ran the bike with needle in position 2, 30 PJ, 135 MJ bike ran like crap. Would not run in gear. Switch the needle back to position 3 with 130 MJ and 30 PJ bike ran okay minute screw was at 2. Still got flat spots at 1/2 throttle. A little sputtering until the RPMs rose through the mid range then it would slowly exile rate until it reach night RPMs.
Next I swapped the PJ and put in 27.5 with 130 MJ and mixture screw at 2.5. Bike still has flat spot at 1/2 throttle. Felt a little worse than previous setup with 30 PJ. When accelerating bike sounded flat but still pulled power and when I let off the throttle got lots of back fire.

Any thoughts?