1981 GS850glx (BS32SS) Air screw/tuning help needed!

[aflythe]

Hello! I am new to the GSResources so I am very sorry if this question has been asked, I tried to look for info but I didn't find much. I have a 1981 GS850GL that has come a long way since I first got it last year to fix up. I've learned a lot on this bike, like re-wiring it with a moto gadget M-unit since the wiring was totally trashed. Now, I am in need of some help with tuning the bike, as you know I have the BS32SS carbs on this bike, but I did some modifications that would affect the tuning. I've over bored the engine 0.5mm using genuine Suzuki parts, I got lucky finding those, and I have a delkevic 4-1 exhaust now. My issue is that on deceleration, the bike crackles and pops, seems to have good power, and the pops don't bother me, but my worry was that I had it lean with the modifications I did. I do have the stock airbox. I have the air screws 3 turns out on each and it seemed to idle highest there, my question is, is that too far out and I need to go up a jet size or is that ok? Will the popping every go away? If it doesn't hurt the engine, then I'm happy with it. Yes, the carburetors are vacuum synced, and the valves should be in spec. Also, I know the idle jet on these carbs get their fuel through the main jet, so should I go up a main too? I haven't noticed any hesitation in any range of rpm, in fact, on a hard pull it will pick the front end up. I have a Gunson ColorTune and it seems to be good, but I can't see what it looks like under load going down the road. Any help would be much appreciated!
Thank you,
Andrew F!

[Nessism]

You can try one size larger on the mains, and lifting the needle a small amount. Don't mess with the pilot jets. Trialing different positions for the pilot screws is unlikely to have much effect, but you will know for sure if you try. 3.5 turns open is max rich, going down to about 2 turns open. Experiment, but one thing at a time.

BTW, check the carb rebuild tutorial in my signature to see how to lift the needle (hint: it doesn't include washers under the clip)

[aflythe]

OK! Thank you! I will order some main jets, I suppose one size bigger so 117.5? also, where do you buy mikuni jets? I've seen some but not sure if they were quality. Also, I used your rebuild tutorial when doing mine so thanks! For lifting the needle or sanding down the spacer, how much should i take off at a time? .1mm you think?

[Grimly]

When you installed the Delkevic system did you use the red high-temp goo? If not, or not enough, the Delko system will crackle and pop like a bowl of Rice Krispies.
If you need more red high-temp goo, some stove high-temp red silicone will do the job.

Personally, I like the popping.

[aflythe]

No, I did not, I had never even heard of that. Do you mean to seal the exhaust port? they gave me copper crush washer; I figured that was good enough. If the popping is just the exhasut then I am perfectly fine with that. Using the ColorTune it seemed to be tuned pretty well.

[Grimly]

No, I did not, I had never even heard of that. Do you mean to seal the exhaust port? they gave me copper crush washer; I figured that was good enough. If the popping is just the exhasut then I am perfectly fine with that. Using the ColorTune it seemed to be tuned pretty well.

The high-temp goo goes into the downpipes / collector / muffter joints. if theyr'e unsealed, air gets in and causes partial combustion of unwholly burned exhaust gases.

[danny01975]

The high-temp goo goes into the downpipes / collector / muffter joints. if theyr'e unsealed, air gets in and causes partial combustion of unwholly burned exhaust gases.

You mean here?
image.png

Mine pops like crazy as well. And I did not add any red goop. That wasn't included in the instructions. :/

[danny01975]

FWIW, mine has larger mains, and it still pops like crazy. Could be related to the issue listed above that I never knew about.

For a while I was trying to tune it out using the pilot jets. The only way I could get it to stop was to turn them WAY out. But then it would run so rich it would foul the plugs out super quick. Once I dialed the pilots back to a reasonable setting, the decel pop came back, but the plugs are always perfect now. I'm def going to try the red goop!

[aflythe]

Hm ok, i have the screws all at 3 turns out from lightly seated, the plugs are white on top but black everywhere else. so its running rich and lean? I've already ordered 1 size up for both the idle and mains, ill try mains first and see what happens. The gunson colortune still makes it look like it runs fine, weird. I will put in bigger mains and see what happens with the plugs. Im gonna feel like an idiot, but screwing the screws OUT (counter-clockwise) means MORE fuel right? Cause now that i think about it, the screw being on the top and engine side of the carbs means its an air screw doesn't it. I've done this wrong the whole time if that's the case.
https://youtube.com/shorts/F1OQOD3qR1Y?feature=share hopefully that link works, I uploaded a video of what the colortune looks like on my bike.

[Grimly]

This is what I used when the original Delko goop ended, with no problems.

https://www.amazon.com/dp/B086Q5MXZX

Not exactly that make, but in a different place different suppliers of the same thing exist.

[KiwiAlfa156]

Hi There, I've used the colortune, and had good results (setting up air screws on VMs)... and not so good results... I think the colour variation is too blunt a tool for fine tuning BS fuel screws.

The most accurate way is to fit an AFR (lambda) sensor and an ARF gauge... Or go old fashioned.

I have got my BS34 (Katana 750) running sweet on a engine running a 4 into one, a large foam pod in place of the rear airbox, squish clearance closed up to 0.75mm (0.030") port/inlet bowl/short side radius clean-up and slightly higher compression ratio, multi-angle valve seat recut. I did this the old school way and here's what I've learned.

If you need to re-jet it's because whatever you have changed has caused more (or less) air to flow through your engine compared to stock at particular throttle opening/rpm. Re-jetting is adding more (or less) fuel to that changed air volume to obtain the correct AFR. I know that seems self-evident, but it's surprising how many folks think 'more gas = more power', when it's actually 'more air = more power (when the AFR is correct)'.

Tune the carbs on your particular bike from the evidence you see/feel. Especially if you have made any changes. If someone gives you advice like 'go up 2 jet sizes and 3 turns out' and doesn't know your bike, take it with a grain of salt.

Most common mods have minimal effect on the idle/slow circuit. Pods and pipes can effect mid-throttle jetting and pod-induced leanness is best corrected using an air correction kit. Trying to compensate for pods with bigger pilots sacrifices idle and off-idle performance and is a poor way of getting acceptable mid throttle performance and makes mid-corner roll on from a closed throttle *@%^$*@%^$*@%^$*@%^$ty experience.


Best way I have found to set the fuel mixture is to set all screws at the same number of turns out (whatever the manual says for your bike). If there isn't a spec, start at 1½ turns out. With the engine fully warmed up 'flick/blip' the throttle grip letting the throttle slam shut under it's return springs. Do it so it revs pick to about 3000 rpm. If the revs hang or drop slowly after you've released the throttle grip, the fuel screws are lean. Turn all the screws out ⅛ turn and repeat the blip test and turn out until the revs drop quickly back to idle. If you blip and revs drop quickly but drop below the initial idle rpm before coming back to the set idle (undershoots) or stalls you're rich and the screws need to be turned in. Again ⅛ turn and repeat until the revs pick up and drop swiftly to a steady idle.

Main jet tuning by road testing can be difficult, because you need to be WFO, close to redline and under load for long enough to do plug chops. License threatening stuff. Also having plugs fresh enough to give accurate readings is necessary.

Forget the old plug reading photo montages from the 1970s, a lot of them were for two-strokes and modern unleaded fuels don't leave the same deposits. The correct main jet mixture will give a white insulator with a carbon ring close to where the porcelain joins the metal body of the plug, which requires either cutting away the metal or an appropriate magnifier to observe.

Another thing that I learned during the tuning process was 'roll off tuning' for the main jet. I got plug chop color which looked good for the main jet selection and was happy enough with the performance on the odd occasion I gave it full throttle at speed. But what I did notice was a momentary but very noticable surge of power when I rolled off the throttle from WFO. The bike went faster for a second (like a turbo kicked in) when the throttle was being closed. A little research and the explanation was that the main jet was too lean.

When you roll off the throttle from WFO, the volume of air passing through the carb is reduced immediately, but the flow of fuel which is denser and has momentum due to its speed though the main jet reacts slower to the change than the air. This causes a momentary enrichment of the mixture. Which if the main jet is too small results in a mixture closer to correct with a resulting increase in torque. I went up 1 size and then another in response to the feedback from the engine in this situation.

With the changes I made to my engine, the nett result was bigger main jets which made sense as all the mods made were made to improve high rpm air flow. Apart from the squish and compression which is about increasing combustion turbulence and speeding up the burn.

[danny01975]

Hm ok, i have the screws all at 3 turns out from lightly seated, the plugs are white on top but black everywhere else. so its running rich and lean? I've already ordered 1 size up for both the idle and mains, ill try mains first and see what happens. The gunson colortune still makes it look like it runs fine, weird. I will put in bigger mains and see what happens with the plugs. Im gonna feel like an idiot, but screwing the screws OUT (counter-clockwise) means MORE fuel right? Cause now that i think about it, the screw being on the top and engine side of the carbs means its an air screw doesn't it. I've done this wrong the whole time if that's the case.
https://youtube.com/shorts/F1OQOD3qR1Y?feature=share hopefully that link works, I uploaded a video of what the colortune looks like on my bike.

When the carbs are on the bike, clockwise is less fuel, counterclockwise is more fuel. Black around with white in the center on the plugs is ok. Depends on how white. If they're kind of grey or brown that's perfect. You just dont want them to be all black, or have a stark white electrode. Reading plugs is not an exact science. If I have some time later, I can send you a pic of what mine look like currently.

I had mixed results with the color tune. As someone mentioned below, it just doesn't have enough detail to really dial the carbs in the way you need to. 1/2 - 1/4 turn can make a big performance difference but no visible difference on the colortune.

Per Delkevik, the delkevic 4 into 1 is not supposed to require any carb re-jetting. If you do add one size up on the mains and pilots, like I have, you'll probably just settle on slightly fewer turns on the pilot screw. I'm at 2 turns out now. The stock configuration is 2.5 turns out. I found the "highest lean idle method" worked best for me to find the right mixture.

Lastly, from everything I read, decel pop is normal and doesn't hurt anything. It's just a matter of preference. Some love it, some dont. The engine is likely not running lean based on what you're describing. If it's popping at times other than decel then I'd say there's a tuning issue.

I just learned to live with it since the bike runs great. But I will def try the red goop trick and see if that gets rid of it.

[Grimly]

I had mixed results with the color tune.

The Colortune was designed for stock carbs with stock jetting, so that when you set the idle mix, it's assumed the jetting further up will be correct.
That all goes out of the window when anything is changed, of course.
It's an imprecise method, but good enough for cooking engines in wheezy old cars of the 60s and 70s, which weren't putting out 100bhp per litre or anything like it.
However, once a jetting setting is settled upon, the Colortune can come back into play, if nothing is fundamentally altered after the main jetting process.

[danny01975]

The timing of this post is uncanny. I said my delkevik exhaust pops allot. I don't remember if it's always been like that or for how long exactly. But this morning, I noticed a rattle when I started it up. I didn't think much of it as the bike makes occasional weird sounds. By the time I got where I was going it was gone. About 2 hrs later, I started it up and the rattle was louder this time. After some hunting, turns out my #3 exhaust flange was loose. Who knows how long it's been loose. I guess it finally got loose enough for me to hear it rattle.

I got home, re-tightend the bolts, with a little loc-tite added for good measure, and took it out for a test ride. Wouldn't you know ALL OF THE POPPING WAS GONE! The only thing it does now is make pleasant gurgling sounds on decel. Coincidentally over the last few days, I noticed the popping was getting more pronounced, then it started popping in between gears, which I thought was weird. Then I happened to comment on this post and learned about how the header connections can suck air and cause popping.

Apparently, not only can it suck air from the header to muffler connection, but also from the exhaust flanges. Check your exhaust flanges!

[Grimly]

The timing of this post is uncanny. I said my delkevik exhaust pops allot. I don't remember if it's always been like that or for how long exactly. But this morning, I noticed a rattle when I started it up. I didn't think much of it as the bike makes occasional weird sounds. By the time I got where I was going it was gone. About 2 hrs later, I started it up and the rattle was louder this time. After some hunting, turns out my #3 exhaust flange was loose. Who knows how long it's been loose. I guess it finally got loose enough for me to hear it rattle.

I got home, re-tightend the bolts, with a little loc-tite added for good measure, and took it out for a test ride. Wouldn't you know ALL OF THE POPPING WAS GONE! The only thing it does now is make pleasant gurgling sounds on decel. Coincidentally over the last few days, I noticed the popping was getting more pronounced, then it started popping in between gears, which I thought was weird. Then I happened to comment on this post and learned about how the header connections can suck air and cause popping.

Apparently, not only can it suck air from the header to muffler connection, but also from the exhaust flanges. Check your exhaust flanges!

Yep, that too.
An exhaust leak at the flange can sound like mechanical mayhem is about to explode in the engine, too. Caused quite a few head-scratchings over the years.