TOPIC:

Guys, i done a search & have looked at the manual but i cant see a thread or page on how to balance my BS38's. Can anyone point me in the correct direction please. I know the balance tool (like pressure gauges) connects to the outlets on the carb holders but thats where my knowledge ends.
What do you need to adjust & does it need to be done with the bike at operating temperature?

Looking at some pics & reading a bit about other carbs is the screw inside the red circle the correct thing to adjust? (I know you need a special carb tool for adjusting it)

My carbs were going to get cleaned after this pic by the way.



There is also a screw circled in yellow which seems to have an effect on the same mechanism, what does it do? If looking at the balance adjuster from the right hand side of the bike when this screw is turned clockwise (in) the whole balance adjuster turns anticlockwise


The reason i ask is because at one time i had some throttle play which i sourced to the adjuster in the red circle so i tightened it all up. I could actually turn the throttle in the opposite direction to lower the engine revs. If i slacken it off the same thing will happen again obviously hence the question about the screw circled in yellow

Here's a link to an article in KZr's FILEBASE.

Sync is explained in the last few pages of the article.

www.kzrider.com/filebase/doc_download/50...retorrebuildtutorial

Good Fortune!

Thanks Patton. Seen that & ignored it because it was Suzuki & a bank of 4 carbs..........lesson learned!

I just bought a 1978 kz750 twin. The link doesn't lead to what it used to. What do I adjust to balance the carbs? The service manual only says you need a special tool. Any help would be appreciated.

You need a carb sync. I suggest Motion Pro or Morgan Carbtune. the cheapy Emgo one isn't very good and I have owned all 3. The special tool mentioned in the manual isn't necessary really, all it is is a long screw driver that passes through a hex wrench so you can lock down the adjustment screw once you have it set. You can buy it at z1enterprises but you can usually tighten the nut without the adjuster screw turning too much or you can compensate by turning it too far so when you tighten the lock nut its perfect. www.z1enterprises.com/carb-adjustment-tool-set.html
Getting the carbs synced perfectly really makes these twins purr.
If you want the whole service manual PM me and i can send it your way.

Thanks for the reply. I have the entire service manual, I just don't know where on the carb that the balance adjustment screw is. I took the carbs off and cleaned them, but now it is running really rough and leaks a bit from the overflow nipple on bottom. I'm waiting for a rebuild kit to come, and then I'll do the butterfly and idle adjustments, but I still do not know where the adjustment screw for synchronization is located. Is it one of the circled nuts from the first post in this thread?

Also, when should I think about checking the engine timing? could that have anything to do with the rough firing of cylinders?

Jessmansimon wrote: Thanks for the reply. I have the entire service manual, I just don't know where on the carb that the balance adjustment screw is. I took the carbs off and cleaned them, but now it is running really rough and leaks a bit from the overflow nipple on bottom. I'm waiting for a rebuild kit to come, and then I'll do the butterfly and idle adjustments, but I still do not know where the adjustment screw for synchronization is located. Is it one of the circled nuts from the first post in this thread?

Also, when should I think about checking the engine timing? could that have anything to do with the rough firing of cylinders?

leaks a bit from the overflow nipple on bottom -- For whatever reason, the float valve isn't properly performing its function, and the fuel level rises and stays too high, which results in an excessively rich mixture.

Until the overflowing is corrected so that the fuel level remains at the correct height, attempting to adjust idle, mixture, etc. is wasted effort, imo.

Good Fortune!

if it was running well before you cleaned the carbs its likely not something else, however if its new to you and it hasn't been running well i would check the ignition timing, the valve adjustment, and then move onto the carbs. If the rubber manifold boots that hold the carbs are cracked or damaged in anyway I would get new ones. the cracks can be super small and only readily seen once the boots are removed from the bike, these bikes will never run well if there is a vacuum leak from these boots. If the vacuum sync nipples are cracked and leaking this could be a source of your problem as well. Are you running the airbox? this is the screw and locknut. I would follow Pattons advice as well

Thanks a ton for the advice DoctoRot and Patton. Once I get the carb rebuild kit in the mail and I fix the leak, I now know where to adjust for the carb balance. When I get in there again, I'll be sure to look really closely for any cracks in the rubber boots. (When I took the carbs out last, they looked in great shape)
I'm 95% positive that the ROUGH ride is due to the carbs, but now that I'm starting to enjoy reading the service manual, it probably couldn't hurt to check the timing.
Yes I am running the airbox, and the filter is a little brownish, and a new one is almost $100. Is it ok to clean the filter for now?
I'll keep you updated on how the carb sync is going. I'm really looking forward to getting this thing purring like it's meant to!!

Oh, one more question to any who might know: It says in the service manual that the carb needle piston should take 10 or so seconds to fully come down when lifted up and let go with the air intake covered up with a finger. (the upside down u-shaped hole?)
I did this and it comes down just about as fast as if I did not cover the hole. I am just wondering if the diaphragm and needle device needs replacing. There are no visible tears at all, and all looks well. This test result made me wonder, though.

Bend the carb boots to see if there are any cracks. If your bike still has points ignition I would definitely check to make sure they are in good shape. Once you conquer these carbs I would suggest checking the valve clearance. You can get a brand new K&N filter for $56 at z1enterprises.com but cleaning it for now is probably fine. If the finger over the hole produces no change then the diaphragm is leaking. and the bike will run poorly. you can get new ones from JBM industries. jbmindustries.com/Yamaha650.html They work great, i have them in my carbs

When you get your carb kit don't use the jets or air-screws provided, they are inferior to the OEM ones and unless there is damage there is no reason to not use the original jets. make sure you check the float height while you're at it. These BS38s are super weird carbs, i know them pretty intimately because the 1980 year had some unique changes, and finding parts for them was a hurdle for me.

After all I did, the bike runs well, UNTIL I get up to about 60mph. It just loses power. Doesn't stutter or backfire, but just loses power, til I have to back off to about 55mph, and it stays running fine. What is going on?

I did the carb rebuild, using the original jets and needle (no damage). Then I replaced the ignition coil because i was getting fouled plugs and a weak spark. I checked the points and they look really good, like someone must have replaced with capacitor. I did do a multimeter reading for timing and it seems it is just opposite of what service manual says. Manual says the needle should jump when timing mark comes around, but the needle goes swiftly to 0 when the mark comes around. Is that correct? or should the needle be doing just the opposite?

Could this loss of power at high speed be from this timing? I have also not yet checked valve clearance. Could that have something to do with loss of power?

I am pretty certain the carbs are good. I have balanced them and dropped the needle down a mark bc of rich condition. I AM still feeling some vibration like maybe backfiring from 3k rpm to 4k on the exhaust side footpeg but cannot be sure it is backfiring. How can I lean out the mixture even further? Drop the needle another notch? Idle is perfect, low speed too, its just the vibrating at moderate, and the total loss of power at 60mph+

Any input would be greatly appreciated! and thanks for the help so far.

I read a forum about maybe the gas cap not venting properly at high speed causing loss of power, so I will try to leave it open tomorrow after I get the timing correct, check valve clearance, and really take a good look at the timing advancer.

The thing is, when i first got this bike (not too long ago) I drove it over 100 miles straight without this problem and without the vibrating right foot peg. The problem it did have was the idling that is now fixed. It makes me think that it is STILL the carbs because it is really the only thing that has changed, besides completely cleaning out the fuel system from tank to intake, but it doesn't feel like it could be. I have taken them out and readjusted about 10 times now. I KNOW its all gravy besides maybe being a bit rich.
The coils were so bad, they had crud coming out. I replaced them with a basic EMGO replacement. I'm wondering if I should shell out the $ for a dyna coil? Could that really make the difference I'm after?