VM carbs - pilot fuel screw settings and variances between inner and outer cylinders?

[Chuck78]

Alright, so I've been struggling finding the best settings for my 920cc GS750 engine build, mostly around the pilot circuit (and needle jet size swaps which I just finally got nailed down). I noticed today that with 1+ turns out on my fuel screws and 1.5 turns out on my air screws, I was getting nonstop consistent firing on cylinders 1 and 4, but when I hooked the timing light to 2 and 3 just to monitor misfires, I was getting lots of misfires on 2 & 3 with identical settings to the 1 and 4 carbs. I have not put the colortune on yet since this (since getting a wideband o2 setup actually), but I think I need to do so to both monitor misfires and check the flame color to see what is really a more ideal AFR for the pilot circuit.

My observations today made me think of some newer GSXR and Katana carbs that have a different size main jet for 2&3 vs the outer 1&4 cylinders, and I wondered if anyone here ever payed very close attention to their state of tune to notice that maybe the inner two (hotter running due to lack of outer cooling fins) cylinders requiring a bit different pilot settings vs the outer two??????

I was having easier starts at 3/4-7/8 turns out on all 4 fuel screws, and at 1+ turns, I have to crack the throttle open slightly to get it to fire on the first kick. I am seeing here evidence pointing to the inner two cylinders maybe needing to be slightly leaner than the outer two, as no amount of air screw tweaking in it's current state got rid of the 2&3 misfiring, and it was running much better at the 3/4-7/8 fuel scew settings on all 4 carbs.

I would have thought that the inner/hotter cylinders would have required a richer mixture, but as far as getting it to run and idle smoothly, it seems as if it needs to be leaner...??? Hmmm...thoughts?????

Thanks everyone.

 

[Chuck78]

Katana (GSX600F)
1988-89 (US Models)
Type: BST31SS
Main Jet
Cylinders #1 and #4: 137.5
Cylinders #2 and #3: 135
Main Air Jet: 1.0mm
Jet Needle
California: 4CZ-5-1
Expect California: 4CZ-4-1
Needle Jet: P-8
Pilot Jet
California: 37.5
Except California: 32.5
Pilot Air Jet
California: 155
Except California: 150
Pilot Screw setting: Preset
Starter Jet (choke): 45
Float Height: 14.6mm (+/- 1mm)

Katana 1100 (GSX1100F)
1990
Type: BST34SS
Main Jet
Cylinders #1 and #4: 112.5
Cylinders #2 and #3: 110
Main Air Jet: 0.6mm

These Suzuki specs that I just looked up confirm the inner two cylinders needing a leaner main jet than the outer two, which correlates directly to my pilot settings and findings despite my reasoning that the inner two would need a richer setting. Very interesting! Comments on this oil/air cooled application vs an air cooled GS, and correlations from this main sizing to pilot tuning, anyone??????

 

[Steve]

Just "fuel for the fire" here, but you did invite discussion.

The '83 and newer 550s and the 1150s had different jetting for the inner cylinders, but it was RICHER.

The 550 went up three sizes, from 95 to 102.5, the 1150 went up just one size, from 120 to 122.5. :-k

A richer mixture would tend to run a bit cooler, helping out with that lack of moving air problem on the inner cylinders.

.

 

[Agemax]

the main jets have no influence on the pilot circuits.

 

[Guest]

the main jets have no influence on the pilot circuits.

good point.

 

[Chuck78]

the main jets have no influence on the pilot circuits.

Yes, I am well aware of this, but I thought that the same reasoning might also influence the pilot settings. I understand that at wide open throttle, there is a lot more heat generated than at pilot cruising throttle opening levels, but due to my findings with my pilot settings & the misfiring on the inner two while the outer two fired flawlessly & consistently, it seemed that there was a similar relation of different air/fuel requirements between the outer(cooler) & inner (hotter) cylinders on an inline 4.

I have a wideband o2 meter, but welding 4 bungs into my exhaust instead of one just sounded too complicated if I can tune things easily on the pilots by traditional methods and using the collective afr of all 4 as a rough guide to how things are running without having to pull spark plugs just yet.

The wb o2 afr gauge setup is really awesome however, despite running into this slight issue.

 

[Chuck78]

the main jets have no influence on the pilot circuits.

let me rephrase that, since the main jet requirements are sometimes different between the inner and outer cylinders, I suspected after my findings that potentially in some engines, the pilot settings might also need to be slightly different between the inner and outer cylinders.