Tuning Carbs based on a Air/Fuel Ratio reading

[DKGS850]

Dear All,

I am tuning the carbs on my Suzuki GS850 from 1979. They are being tuned as the final part of an end to end rebuild, with lots new parts, and parts from donor bikes, so the bike has never ran in its current configuration.
It is however running more or less now, but with the following hickups. The revs hang at 3000-4000 rpm for 10 secs and after that drops slowly down to a somewhat unstable idle.
I have podfilters and a 4 in 1 exhaust on the bike and I am currently equipped with a 20 Pilot jet and a 107,5 Main jets (compared to the stock 15 pilot and 102,5 main jets)

I have bought an Innovate LM2 Air Fuel Ratio meter, to help me shed some light (and data) over this black magic that is carb tuning, but I am finding it difficult to enterprit the data that I am getting.

Do any of you have any experience in reading AFR charts and perhaps can help me translate the readings into new jet sizes or new needle clip positions?
Is there any where else where on the internet where such expertise is available? Preferably related to bikes. I know the the Innovate LM2 is frequently used by hot rodders, but I am not sure that similar problems would result in similar fixes in the classic MC vs the hot rod world.

I have attached a screenshot of a part of one of my tuning trips.

Any input will be appreciated. Thanks in advance

Best regards

Michael

 

[Big T]

The 20 pilot is too big. The stock 15 is generally fine, a 17.5 is sometimes used

 

[chuck hahn]

the problem i see is that 2 cylinders dump into each side..and if theres a crossover tube in there who knows whats actually being read? now if you could isolate each cylinder i think the readings would be more believable. the 79 should have VM carbs so my thinking is that a colortune would be a better tool to see the burn color in each cylinder because it screws into the spark plug hole and theres no way to contaminate the resul

go to youtube and search colortuning a motorcycle. Note that on VM carbs you DO NOT DO NOT adjust with the bottom pilot screws. you adjust the air screws on the side of the venturi. moving the screws IN will INrichen the burn and turning screws out will lean OUT ​the burn

 

[salty_monk]

Posplayr has played about with that tech and can probably help you.

To me it sounds like your pilot jet is too big and your main jet is too small... any change to the air corrector jets or needle?

you might find the easiest way to get in the ballpark will be a dynajet kit and then use the af to tweak from there.....

 

[DKGS850]

Dear all,
Thanks for the tips.
I actually sized up from a 17,5 to a 20 pilot jet, cause my AFR meter said that I was very lean at 1500 RPM, but maybe that was due to lack of Air fuel adjustment
I actually already own a colortune kit, but i didnt find it very usefull. Perhaps its good for the idle circuit though. Ill give it a whirl.
Thanks again all

BR Michael

 

[chuck hahn]

Run it up to around 4500 RPMs and hold it there for 4 or 5 seconds and see what colortune says. AT 4500 youll easily be on the main circuit...which is like when riding. Even though its nice and blue vat idle you may need to tweak the air screws for the main circuit. What is your exhaust set up?

 

[DKGS850]

Hi Chuck,

Thanks your reply. I have a Sebring 4 in 1 with a Megaton muffler. It comes with a db killer, but it is currently not being used.

Ill be trying the color tune tonight.

BR Michael

 

[Steve]

I agree that your pilot fuel jet is too large, the stock 15 should be just fine.

I also agree that your main jet is likely too small.

The stock setup is jetted for the airflow available from the airbox and stock exhaust. The stock exhaust flows remarkably well, there is not much of a jetting change needed, depending on the muffler used. If it is on the quieter side (street baffle, dB killer, etc.), you may not need to change anything.

However, the pods are a different matter. They do flow a LOT more than the stock airbox, so you need to add more fuel to compensate. One thing to remember is that the airbox is mainly restrictive from about half-throttle on up, so there is no need for much jetting correction below that. Your stock 15 jets are fine.

Lift your needles a notch or two. Try 112.5 or 115 main jets. Turn the pilot screw (the one on the bottom) about 7/8 to 1 full turn out from LIGHTLY seated. Start with the air screw (the one on the side) about 1.5 turns out. That will allow a slightly-rich mixture for starting your tuning session. Adjust ONLY the air screws for desired results.

It would be interesting to compare your meter with the Colortune results, if you don't mind.

.

 

[DKGS850]

Dear Steve,

Thanks for your reply.

I changed from a 20 pilot jet to a 17,5 pilot jet yesterday, and the reading were still all over the place. I only tried the colortune with the new jet setup, and depending on the level of choke it went from rich yellow to not firing at all.
I will try your suggested adjustments now, give that a whirl and I will be happy to share any of my findings from both Colortune and AFR reading.
The AFR is easy. There is log-files to be shared there. The colortune results are a bit more intangible. How can I share them?

Best Regards

Michael

 

[chuck hahn]

If the colortune shows no flame at idle you got problems inside the csarb body passages or broke pilot screw tips plugging the hole in the venturi. And yes adding chokes gonna give a nice yellow color. Heres a tutorial on removal of broke pilot tips.

http://members.dslextreme.com/users/cbsaunders/gs/pilot_screw_removal.pdf

 

[DKGS850]

Hi Chuck,

I dont think that the Pilot screws are broken off, but I will for sure check that. I they indeed are broken of, I am sure that the tutorial will come in handy.

BR Michael

 

[DKGS850]

Dear All,

I have followed Steves jetting advice, but the sunday weather was awfull, so I havent had the chance to try out the new carb adjustments.

Ill keep you posted.

Br Michael

 

[DKGS850]

Dear all,

Turned out that a 15 was the correct pilot jets for my setup. The jury is still out on main jet and needle position.

BR Michael

 

[Steve]

Nice to hear the confirmation, thanks. :encouragement:

Have you verified your fuel level before playing with the needle and main?

.

 

[DKGS850]

I have checked the float height. I tried fabricating a Fuel Level Guage, but it didnt really work out as intended.
Float height was good though.