VM26 needle swap? modified GS750

[Chuck78]

I have been getting closer on the main jets now, 122.5 bogged a lot, plug chop bottom half was sooted. 120 was tan-grey for the bottom 2/3rd's with no soot ring, may try for 117.5 as I think that will be the cutoff. 115 may put me too close to detonation, and I run it hard in the hot summer months, have to be careful.

I still have not settled on a needle or position yet. I wasnt happy with the 5DL35's, and swapped the 5F21's back in into the 4th position. Now I am thinking that since the 5F21 starts it's taper 3.7mm further down than the 5DL31/35/36, I may be best off running the slightly less slender 5DL31 needle in the 2nd clip position once I get the best main jet installed. I will swap the 5DL35's back in and try this with them first to get an idea of the 3/8 to 5/8 throttle response, and who knows, maybe with the correct mains, I won't be overly rich at 3/4 throttle! Crossing my fingers! The incredibly thick tip of the 5F21's has me really thinking that I could do better with a less drastic jump from 1.6mm to nothing.

I am still really curious about a few other needles, especially the 5CL10 and 5H5. the 5CN15, 5CN18, 5J6, and 5L1 may do the trick as well to get a taper that starts sooner and get something thinner at the tip than the 5F21

 

[Chuck78]

Test rode the bike again tonight and noted that at lower speeds, I can do 1/4 to 3/4 throttle and have "ok" acceleration, not as good as it should, but "ok." Then at higher loads such as 3500rpm at 50mph-ish, much past 3/16th throttle, it starts to bog and act up severely (no power at all at these throttle positions at sub-5500 rpm under a harder load). at 6500-10000 rpm at these throttle positions, it accelerates pretty darn fast.

Again this is with a 5F21 in the 4th position. 78 GS750C and 79 GS750N used the 5DL36-2 (2nd position). Even running these two different combos, with the 5F21 starting it's taper 3.7mm later (relies on pilot more at lower openings), a 5F21-4 still starts it's taper 2mm later than a 5DL35 or 5DL36. the 35 and 36 do have a very mild taper at the beginning, which has to do a lot with the earlier start of the taper.

If doing a plug chop for the needle positions, what am I looking for in the middle portion of the insulator? and should I ignore it if it's black below the middle section? Plug chop articles only go into detail about the lowest portion of the insulator for the main jet sizing.

 

[silverbullet132]

This post made me realize how fk'd I am lol, my 78 750 has a 910 kit in it and I have to jet it too. 6Sigma makes a kit specific to each customer/bike, maybe try them?

 

[Chuck78]

This post made me realize how fk'd I am lol, my 78 750 has a 910 kit in it and I have to jet it too. 6Sigma makes a kit specific to each customer/bike, maybe try them?

Don't trust a company like Sigma that makes a kit specific to each customer/bike unless you bring the bike to them and they put it on a dyno and try out main jet/needle/pilot jet combos and sell you a combo package of all three of those in the proper size. I suppose they could mail you a kit with 4 Mikuni 110 mains, 4 112.5 mains, 4 115 mains, 4 117.5 mains, 4 5DL36 needles, 4 5DL31 needles, and 4 17.5 pilot jets... Heck, at that, you might as well have already ordered the OEM equivalent jets from JetsRUs.com for far less!

at 910cc and high compression I assume, with VM26 carbs, I would try out some 117.5's or maybe 115's and see how rich the plugs look on a WOT plug chop. I suspect you may be right on by running 112.5's, from talking to a dyno tuner on here that built an 890cc or so GS850 engine with some custom JE pistons or similar. I think for his setup, he was at 107.5 or 110 but real close to the leaner limit of a great mixture on the border of detonation potential (max power if you're really squeezing it is to be had just one size richer than detonation-inducing lean mixture) I think he said he found the best needle combo to be a 5DL36-2.5 (shimmed a half clip position. So for yours, you should try a little rich and do plug chops to drop down a jet size or two or three, and try those GS850/1000 5DL36 needles. GS550's have an almost identical needle as I talked about above. the EBC/RD or jetsRus "OEM equivalent" jets are I believe around $2.60 each, Mikuni's are about $6 or $7 often...