VM26 needle swap? modified GS750

[Chuck78]

I was thinking while riding that a needle that was skinnier than the 5F21 but had a taper that started sooner might be my best bet for now. when I do the 920cc build, a 5DL31, 5DL35, or 5DL36 sound like the perfect ticket. for now, I;m uncertain.

[Chuck78]

I should also add that I only did color tune on number one cylinder and replicated the results on the rest. I have a dent in one of my header tubes, and I have figured that I should fix that before trying to vacuum synch the carburetors... I know, I should probably take care of that first, but I thought I could get it in the ballpark even without fixing that! it did run much better before except that the main seemed too small. it is riding acceptably now or at least rideable but still has some bugs obviously.

I should also note that both Vance and Hines and K&N separately say that with their product alone on the bike, it should be upped to 15% larger on jets or to a 115 Main. Stock was 100 to 105 depending on year. So with both, it seems as if 120-125 should be right. May swap the 115's for 120's then. I need to do a WOT 3RD gear highway plug chop tonight I suppose. The stumbling down low is the problem that I need more insight on. Any help and advice is greatly appreciated. Finally getting a decent grasp on tuning the VM's, but I still could use some guidance. thanks!

[Chuck78]

Looking through the needle specs, I am curious about a few others of the same length that may be better for my airflow setup, especially the 5CL10 and 5H5. the 5CN15, 5CN18, 5J6, and 5L1 are also potential candidates to get a taper that starts sooner and get something thinner at the tip than the 5F21. I will try and cut and paste a screenshot soon to compare these all on the same page.

The document lists the measurement points and repeats k3-k7 on the dual taper needles, and on the straight taper needles it just lists k2-k7 as the same because the taper does not change as in the dual taper needles. That's why the confusion arose over how to read that chart and why it listed the same point and diameter multiple times. It's basically just a straight taper between the start and finish points unless it lists a different spec for point k2.

Plug chop seems to indicate that 122.5 mains are a tad too rich for my setup at WOT, making a few adjustments to the pilot and then running out to pick up some jets tonight!

[Chuck78]

Hmmm... Seems as if the needle jets control the mixture more in between the pilot and the needle... I need to find a chart that shows needle jet sizings, or else figure out between the 0
o-2, o-4, and o-6 that GS550-1000's used, the order from richer to leaner... Any help???

[btbarb]

I need to find a chart that shows needle jet sizings, or else figure out between the 0
o-2, o-4, and o-6 that GS550-1000's used, the order from richer to leaner... Any help???

Sudco has a catalog that I think has specific sizes. Check this link out and see if it's what you are looking for

[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...