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

[aflythe]

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.

Ah thank you! Funny enough I ordered a AFR kit, now i know you are supposed to weld bungs or use a sniffer, and i dont want to drill into my headers, so what can i make a sniffer out of? i do not think my exhaust is air tight after the headers at the collection pipe. I will definitley be check exhaust sealing at the head just to make sure i get an accurate reading on AFR. Also, I've heard mix opinions on whether i can get an AFR reading at the collection pipe or if i need to get a sniffer or bung into each individual header. To me, getting an individual reading on each cylinder makes more sense than at the collection pipe. Thank all of you for the help! Also manual says to torqued exhaust bolts to 10 foot pounds. Does that sound right? i figured I would need to tighten them a little more, normally i would go off of how tight it felt but after extracting 2 broken off exhaust bolts from previous owners i get the torque wrench out. Also in case you didn't see it, the bike has been overbored by .5mm so i've ordered one size up for both the mains and the pilots so 117.5 and 42.5 mikuni jets. I've heard that I should start with the main jet and work down but I've also heard I should start with the pilot jet and work up. I was thinking since the idle jet on these bikes get fuel from the main jet then I should start with a bigger main and see what happens, or should i just go ahead and throw them in at the same time.

[aflythe]

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.

yes i wouldn't think the exhasut alone would mean new jets, but I had the engine overbored by .5mm so that will change some things. on my very cheap harbor freight compression tester i got 120 on a "new" engine however all the cylinders were very close to one another so that is what was more important to me, i dont trust the HF tool that much.

[KiwiAlfa156]

...Also, I've heard mix opinions on whether i can get an AFR reading at the collection pipe or if i need to get a sniffer or bung into each individual header. To me, getting an individual reading on each cylinder makes more sense than at the collection pipe. ...Also in case you didn't see it, the bike has been over-bored by .5mm so i've ordered one size up for both the mains and the pilots so 117.5 and 42.5 mikuni jets. I've heard that I should start with the main jet and work down but I've also heard I should start with the pilot jet and work up. I was thinking since the idle jet on these bikes get fuel from the main jet then I should start with a bigger main and see what happens, or should i just go ahead and throw them in at the same time.

As the cylinders are fueled by carbs with Fast/Main Circuit (mains/needle jet/needle clip/main air jet) is or should be identical for all four carbs, having four sniffers across the headers is redundant. A lambda per cylinder is only required when fueling is EFI (closed-loop) with individually controlled injectors (and coils) varying mixture (and ignition timing) on the fly. So a single 'sniffer' just down stream from the collector will do just fine.

As for the Slow/Pilot circuit (Pilot jet/Pilot air jet/Fuel screw) the circuit takes air via the pilot air jet located in the bell mouth. This air is feed to the pilot jet where the air enters via the holes on the side of the pilot jet, mixing with the fuel that is drawn through the slotted end of the jet. The mixture of air and fuel (referred to as an "emulsion")* is fed to the single pilot and multiple (2 or 3) by-pass orifices located on the top of the BS carb bore, where the closed butterfly makes contact with the bore when the throttle is closed. The pilot screw, mixture screw, or fuel screw (take your pick of names) adjusts the fuel flow though the pilot orifice ONLY, which the one closest to the inlet valve and is always under engine vacuum when the engine is idling. When the throttle is cracked off idle the 'non adjustable' by-pass orifices are exposed and progressively flow pilot mixture in addition to the 'adjustable' pilot orifice.

The point to be made is that the pilot mixture overall is set be the Pilot Air Jet and the Pilot fuel jet, the fuel screw position important at idle and up to about 1/8th throttle. It is the main source of fuel at idle where it is feeding its mixture into the small volume of air passing past the butterfly. But as you open the throttle the by-passes flow more fuel than the fuel screw controlled pilot outlet and as you add more throttle, the needle jet takes over as the prevalent fuel supply. The pilot circuit doesn't stop fueling, it just becomes a tiny percentage of the total fuel flow.

There are lots of methods for setting the pilot mixture, I've tried them all. Find the one that gives you the results you are looking for. For me its the blip tuning. The key is to set a base-line (lightly seated) and adjust each fuel screw identically. If using the AFR, this is straight forward. But at the end of the day its the rideability though good carburation that is important.

Yes you are correct that the pilot jet in the BS32/34 draws its fuel through the main jet. But given the flow rates at idle, a couple of size changes on the main will have negligible effect, and if it does, you adjust. That's carb tuning, baby.

The golden rule is to make one change at a time and evaluate. Don't think that you have to start carb tuning by swapping jets to what you IMAGINE your engine might need. I'd test with the stock jetting first as a baseline and if your testing shows there are performance/response issues, correct these. If you start by changing stuff, your changes might be introducing problems that don't exist. Remember jetting is adding or removing enough fuel to achieve the correct AFR under various conditions. If the AFR is in fact good already your preemptive tuning is taking it out of tune.

With the 0.5mm over-bore, it may not make much of a difference carburation wise. The first generation GS750 and GS1000 (and some of the 16-valve versions) would often have the same bore carbs; VM26 (BS34). The larger displacement engine would run SMALLER main jets that the 750. Which seems counter-intuitive. But the fuel flow through main jet is not only determined by the size of the hole in the jet, but also by the pressure difference between the float bowl and the vacuum being generated at the venturi. Think of a garden hose with a nozzle on the end, if you turn up the pressure at the faucet, the water flow increases although the nozzle is the same size. Higher air flow from the larger engine, creates a higher vacuum in the carb venturi. So main jet size to capacity isn't a direct relationship.

*The pilot jet is miniature version of the needle jet/emulsion tube which mixes air from the main air jet. The 'emulsion' is a fuel/air froth which is more easily stripped off the jet orifice and into the air flow through the carb bore, easily atomizing into a mist that burns faster and cleaner when lit.