Help reading plug chops & jetting - '77 GS750 pipe + K&N

[49er]

You're over analysing the pilot circuit on your VM26 carbs.

Those carbs will run well with either #15 or #17.5 pilot jets. Both can be set in the 0.75 - 1.25 turns out, and still perform ok. The trick is setting them to give steady idle with clean/strong transition onto the needle. And that is where the fun begins.

For a road machine to run great, you must balance the pilot fuel setting with the needle shape and transition starting position. Fortunately, Mr Zuki did a lot of research and found a pretty good performance package. With SMALL tweaks, it can be improved on, but not by much.

Suggestion........fit #15 pilots, one turn on the fuel screws, 2 turns on the airs. This will give a good starting point. Try a set of 5DL36 needles in the middle clip position. This is half a position richer than on my 900cc engine, running a modified airbox with a free flowing filter. Your mains should probably be around 115 -117.5. Mine are still 105's. On the dyno, I dropped 2 hp when running 108 mains.

FWIW, I tried 17.5 pilots and they went well, but I have the cleanest/strongest transition on #15's.

Spend more time doing the basics right, and you'll save yourself time and grief!! Just tuning to one cylinder is courting gross inconsistency.

I tried setting my pilot circuit with a Colortune too, but found the best performance was achieved on the richer side of their ideal colour. I discarded it all together for my final tuning.

Have fun.

[Chuck78]

I'm thinking that a 5DL31 needle might be the best bet for me. its skinnier than the stock 5F21 & thicker than the 5DL35 and 5DL36. with that or the stock 5 F 21, with that,I might be able to get away with running in notch 3 a lot better & have the taper start sooner than with the 35 or 36, so that I don't have to have the pilot screw set as rich. does that make sense?the 35 or 36 will need to be set a little lower because they have such an aggressive rapid taper, but that means the taper won't start as soon inalower setting, relying on the pilots more. the needle jets come into play in this range as well but they are outrageously expensive. I have a set from some junk GS 1000 carbs, but the carbs are in rough shape and had water in I'm not sure if they will be usable

[Chuck78]

http://www.dragstuff.com/techarticles/reading-spark-plugs.html

New engines or engines that pump a little oil may show a thin oily line way down inside on the porcelain where the porcelain meets the metal wall of the plug. This oil line has nothing to do with the air/fuel mixture but may be confused with the fuel ring you are looking for.If you are having a hard time figuring out if what you are reading is correct or because you are not sure if the plug heat range is correct then tow the car back to the pits and drop the headers and look inside the pipes. If they are black then you are too rich, if they are light gray or white then you are too lean. The pipes should be a medium to dark gray or tan color.

How does one differentiate between an oil line coloration and the fuel ring on the insulator? Tkent I think recommended just trying the 125's as they sound just about right to him and some other pod&pipe gs750 owners. I am pretty sure I am on the rich side, but just wondering about visible plug differences between an engine that burns a little bit of oil, versus too rich of fuel mixture.

My 4-1 header has been black on the insides, not tan, but I have never pulled it after a wide open throttle run like a plug chop to check the color, as referenced by the automotive drag racing website link that I included in the quote above.

[Chuck78]

Rain today, but tomorrow I think I may throw in the 120's and one brand new B8ES plug and do a chop on that. Unfortunately I don't know what to expect, as I have gone from a Mikuni 105 main originally (was way too lean with stock airbox and free flowing 4-1) to K&N free flowing open element dual oval filters and an EBC/RD Mikuni style 122.5 mains (supposed to be exacting OEM tolerances) and now the 120's I have to install are JetsRUs's "OEM equivalent" which is half the price as Mikuni and according to them, the exact spec of the Mikuni's. The 117.5's are EBC's, and the local shop has Mikuni 115's in stock...

EBC's youtube video on their jets makes it sound like they are made to very precise tolerances as OEM, but who knows, jumping around brand names, I might get surprised from the inconsistencies in flow ratings and diameters even when 2 different manufacturers/distributors claim exacting tolerances. I have a machinist's drill index with bits from .039" and up in about .003" increments, so I typically check the jet sizings with those. The jet sizes vary approximately .002-.004" per size jump in the Mikuni range I am shopping in. Mikuni's are "flow rated," Dynojet and RD jets are hole diameter size rated. Therefor the sizes do not cross over from one brand to the next. Keihin carb jets use a completely different rating system of unknown measurement.


Great links on this subject:

http://jetsrus.com/FAQs/FAQ_spark_plugs.htm

http://www.xs11.com/xs11-info/tech-tips/maintenance/carburators/68-tuning-carbs-by-reading-spark-plugsthrottle-chop.html

http://jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm

http://www.strappe.com/plugs.html

http://www.dragstuff.com/techarticle...ark-plugs.html

[Chuck78]

Pulled this from a thread on sohc4.net, thought there was some good info there

>>>Typical lean conditions:
*Poor acceleration - feels flat
*Engine doesn't respond when throttle is snapped open - picks up speed as *throttle is closed
*Engine runs hot, knocks, pings and overheats (end result- hole in piston)
*Engine surges or "hunts" when cruising at part throttle
*Popping/ spitting through carb when throttle is opened, or popping and *spitting through pipe on acceleration with a closed throttle (classic lean *pilot circuit symptoms)
*Engine runs better in warm weather, worse in cool
*Performance gets worse when the air filter is removed

>>>Typical Rich Conditions
*Acceleration is flat, uneven
*Engine will "8 stroke" as it loads up and skips combustion cycles
*Throttle needs to be opened continuously to maintain acceleration
*Engine works better when cold
*Black smoke from the tail pipe
*Poor fuel economy
*Engine performance improves when air cleaner is removed
*If the pilot screw is overly rich, idle is rough and the engine won't return to idle without blipping the throttle
*Black sooty plugs, sooty exhaust pipe

[Chuck78]

120 and 122.5 plug chops

Well I put the 120's in today, and it instantly felt like a fair improvement over the 122.5's as the last plug chop would have indicated would happen.
I tried a plug chop on the freeway on a long uphill entrance ramp, but as I get closer to the correct mains, it gets harder to maintain WOT for anything more than 10 seconds on public roads! had to ditch out and try a long stretch out of an uphill turn entrance ramp to a looonnggg straight downhill exit ramp.

Hitting near top speed very very rapidly after coming out of the 35mph uphill turn onto the highway happened pretty darn fast and had me flustered as I pulled in the clutch a split second before the kill switch, which revved the engine to 10500rpm+ with no load, which I hope didn't taint the readings.
I now have cut back ALL THE THREADS on the previous 122.5 chop, and realized I was running richer than I had thought as I hadn't cut to the bottom of the insulator. I'd think 117.5 may be the ticket. What do you all think of the 120 plug? Best mixture, or still too rich?
Here's two plugs, the same 120 plug 3 times with different lighting. the 122.5 and first 120 were from the same photo/lighting.






JetsRus says the middle plug here is perfect Air/Fuel mixture, looks like I went from between 1&2 to maybe plug 2 or 3, need to look like plug 4 supposedly:


Did I taint the readings by riding 70 miles on it first, knowing that the midrange may be a half clip position or so too rich??? Maybe I should have put the plug in 2 miles before this chop run????
Or maybe WOT blasts for 10 seconds will rapidly burn off all previous reasonable coloration? The feel of the bike at 3/4 throttle vs full throttle is better now, but I still feel like it could go faster. Not a massive increase in acceleration going from 3/4 to WOT currently, but not bogging as it did with 122.5's. The feel of it and reading the plugs would make me think that 117.5's would be the ticket.

[Chuck78]

Keep in mind that "the motor's been bored out" (previous owner said of bike's history) and it looks to have GS850 pistons, which may be why some people with pods and pipe run up to a 125 and I am running rich with a 122.5 and 120. More displacement drawing more air through the same carb = more air velocity = more fuel being siphoned through the same size mains. Hence a displacement increase with the same head and carb as before will need smaller jets if no other changes had been made! Look at '77-'79 GS750 (100-105 mains) vs '79 GS850 (102.5 mains) vs '78-'79 GS1000 (95 mains), all on VM26SS carbs.

I think my timing may be advanced slightly too far, can anyone confirm this from my mangled ground strap (after annihilating the plug with the bandsaw to read the insulator)? I put an arrow on where it had previously shown me the change in color, indicates electrode strap getting pretty hot I think:



Also still experiencing bogging when I am under higher loads and try to go 1/2 through WOT at 3000-5000 rpm's, may need to drop the needle I suspect? or maybe changing the main will affect this throttle area slightly? Running it like I'm drag racing, it NEVER bogs, but cruising steady and then going out of the pilot circuit makes it bog a lot at lower rpm's.

[Chuck78]

117.5 plug chops

Alright, I finally got around to installing the 117.5 mains today and messing around with the Gunson Colortune. The bike is running better and better every time I make an hour or two to experiment with it!

Here is the photo of the plug chop that I just did on plugs with only 8 miles or so on them vs the other chops with larger jets.
9

Gettin' better! The last plug chops were done in 80-ish degree weather, but this one was done in about 66 degree weather. I was afraid it might mess up my comparison, but it appears that I may still be slightly rich. I have no speckles or spots on the plug insulator indicating detonation, but I am not sure exactly what I want to be looking for as far as coloration. The Jets R Us photo of the 7 plugs is confusing when you get past the "way too rich" plugs, as they all look the same from good to way too lean in that tiny photo!

Looks like I would probably be safe to get some 115's and try them out, what do you guys think? The JetsRUs photo makes it look like very little color is a perfect mixture. I would figure maybe just a little lower on the coloration ring and I would be near perfect for an air cooled high performance engine? I'm afraid to go too much more lean due to excess heat with a leaner mixture on an air cooled engine. Advice on this vs 115 vs 112.5 mains???

[posplayr]

Ever consider getting a WB02 sensor

[Chuck78]

Yes I did! Big money compared to a little gasoline and some $2.49 spark plugs! Wideband 02 AFR setup would be nice, but then you have to know exactly what AFR to shoot for based on throttle position, load, and needing to be a little on the rich side due to it being an air cooled engine.

The Gunson colortune and plug chops seems to my logic to be a little more precise, but lots of trial and error however.

I think I am going to order some 115's and 112.5's from Jets R Us in their "OEM Equivalent" spec brand. they are half the price of Mikuni's and supposedly just as accurate. Locally I can get EBC Mikuni replacement jets (made by RD) or genuine Mikuni's, but neither place has a complete selection typically.

[Chuck78]

what do you guys think of the coloration on my 117.5 plug? Think 115 will be best for all conditions? If I could get away with running a 112.5 I may potentially still stay with a 115 because I don't know if I want to have to put larger jet in or late fall riding in 50 degree temperatures when the air is much more dense, and if a 115 will work good but a 112.5 is maximum power before detonation, with running a 115 it will cool better with a mixture slightly on the rich side.

I would like to say that even though this is very time consuming, I am very glad to be able to learn all of these tuning tricks all of you tech wizards here on GSR!

[Chuck78]

Well I swapped in 112.5 mains, but at thd same time I tried to address the bog at higher loads below 5000rpm at anything above 1/4 throttle (thought maybe was too rich) by loweri g the needles to the middle clip position.

WOW as I wrong! Bike runs TERRIBLY,acts like it's running out of gas at anything more than mild throttle openings! I had heard that lowest of second lowest clip position is where I'd want to be, but this tells me DEFINITELY raise the needle height up all the way with the stock 5F21 needles in the lowest clip slot! I am contemplating trying the skinny gs550/850/1000 typle 5DL35 needles again, as I tried then before in the 2nd lowest position (2nd highest clip slot) thinking thst I was compensating for a very aggressive needle, but it ran like crap. I think I would have been great on that one in the 1 spot down from the middle position. I will try the highest needle height with stock needles first, but should I just go for the skinny needles????? All the other bikes used them in 79, even one model gs750ec or gs750n I think, but not all 750's.