VM26 needle swap? modified GS750

[Chuck78]

I am rejetting the VM26's on my GS750, as it was already running too lean at WOT with a 105 main, and I have now added some very free flowing open element K&N RC2222 dual oval filters. I will eventually be doing a 920cc upgrade with some old stock 72mm GS750/850 10:1 pistons, and I believe the bike already has 844cc pistons in it currently. I chatted with a guy on here who had done a 890-ish cc engine with these carbs, and he ended up using the 5DL36 needles with the best results according to his air fuel ratio wideband o2 setup. I have a set of 5F21 needles in the carbs (stock) which are really fat, and a spare set of 5DL35 needles which are nearly identical to the 5DL36 - much skinnier taper at end, about 2% leaner at lower speeds, much more fuel beyond light cruising throttle positions. These allegedly were used to give real serious performance at wider throttle openings, but easily pass emissions standards at lower throttle openings. I have read on here that the Dynojet needles have a much more drastic taper than the stock needles as well, but maybe they start a little earlier. This may be the closest thing I can find in a stock Mikuni needle, and I happen to have a set of 4 nice ones here from some parts carbs.


The 5F21 that was stock is a much fatter diameter (leaner mixture) needle...
The taper on the 5F21 goes from 2.515mm to 1.688mm,
taper on the 5DL35/36 goes from 2.522mm to 0.878mm.

Think this would be a good upgrade for my bike, over the stock needle raised one notch, or will it be too rich? I was going to try a 122.5 main jet, but I am thinking with this skinny taper needle, maybe I could do really great with a 115 main jet due to the needle jet being pretty unrestricted at 3/4-full throttle vs a larger jet with a fatter needle restricting it. Does this make sense? The GS850 VM26 used a 5DL36 in clip position 2, GS1000 VM26's 5DL36 in clip position 3. My carbs ('79 engine #, '79 carbs?) have the same cutaway at least as the 79 GS850, a 1.5mm slide cutaway. stock '77 carbs had a 2.5 cutaway and larger mains I'm sure this has to do with midrange throttle effective venturi area in relation to air velocity - less velocity needs a larger jet (main and pilot).

[Chuck78]

more info from an old post:

The GS750B came with a # 100 main jet, 0-6 needle, 5F21-3 jet needle, and # 15 pilot jet. The GS750C (1978) and GS750N (1979) carburetors were changed to a #102.5 main jet, 0-4 needle, 5DL36-2 jet needle and #15 pilot jet.
What that all means is that the GS750N is leaner at the small throttle openings usually encountered in EPA emissions test modes. The dual-taper 5DL36-2 jet needle is initially leaner, secondarily richer to smooth the transition from idle to off idle, (or from small, steady-state carb openings at low speeds to gentle acceleration, as encountered in traffic). The float level was also lowered between the GS750B and GS750N, from 26mm to 24mm.

[KEITH KRAUSE]

If I understand correctly, if you have a '79 motor and you believe '79 carbs, I would use the jet needles that come with the '79 carbs.
I wouldn't try to address main jet issues by using a thinner jet needle.
A well tuned 750 with quality pipe and quality pod filters can generally run a 125 main. For the jet needles, if the needle jets are the correct ones for the '79 carbs, I'd probably try lowering the e-clip 2 positions (to the bottom) but if it's rich then placing an approx' .022" jetting spacer on top the e-clip in position 5 should work (position 4 1/2). Position 4 would be too lean in my experience.
Probably need one size up on the pilot jets, 17.5's.
Initially set the pilot fuel screws to 1 turn out. Set the side air screws to 2 turns out, then warm up and adjust for highest idle.
Remove the floatbowl vent lines and leave the ports open.
Test.

[Chuck78]

Still tempted to try the skinnier needles on the middle clip position. GS850's stock ran those in the 2nd from top. GS1000's ran them in the middle (3rd down) position. On the carbs no spacers and springs holding the needles into the slides, is it possible still to shim the needle to 1/2 clip positions??? looks like only enough room to compress the c-clip between the linkage and the slide.

Better to start rich and go lean if necessary, maybe you have convinced me to try the 122.5 and 125 mains first.

Remove the float bowl vent lines permanently? or just while tuning?

[KEITH KRAUSE]

You should have the nylon ring/spacer/spring assembly. If not, then I don't know what you have.
If you still have the stock needle jets that are used in '79 carbs, then running the thinner jet needles, not designed for the needle jets, will probably give poor results.
Remove the floatbowl lines permanently. More often than not, running pod filters will create venting issues (fuel starvation) if the stock vent lines are left on. Freeway speeds/crosswinds worsen the problem. Dynojet tells you to remove them when running their stage 3 jet kits.

[Chuck78]

The GS550 and GS850 running this size of needle both use the O-6 needle jet, as do my carbs. the GS 1000 that uses this needle size uses a slightly different needle jet, I believe an O-4 or O-2. I'm not going to Loctite anything in the carbs for now so that I can play around with needle positions and swap the needles back and forth.

[Chuck78]

looking at those specs for that anomaly quoted above, it looks like I should probably swap to the O-4

[Chuck78]

Okay the document that I found has some kinda funny and obscure specs, and some measurements are not really listed (only gives 3 points on the taper but repeats a few of them). A member here with some different literature seems to also have some inaccurate info at least on the needle lengths quoted in the needle chart document he has, but he tells me the taper on the 5F21 and 5D35 (not the L? he quoted back to me that instead of 5DL35) start at the same point, that they do not have 5 notches and they are differing lengths (both statements are incorrect from what the Mikuni needles I have actually are, and the german link that says the 35&36 are the same length), BUT he tells me the taper on the 5DL36 does start sooner than that on the 5D35 and 5F21. This means that you don't have to raise this skinny needle as much to get the main/needle dumping in more fuel sooner. I was afraid that it may need the taper sooner, but that raising the needle much more would just simply richen up a lean spot but then create a big range of 1/2-3/4 throttle that would then be too rich. All just speculation however! I have the 5DL35's installed now on the bench, I think I will be running them for a test run on clip position 3. I slightly sanded them with emory cloth all at once by hand to try and get a slight bit of taper started sooner as I suspected I would need, but I only rolled them three times while uniformly sanding them in the exact same position while rolling them, so it may have had only a very negligible effect.

[Chuck78]

Ahhhh... There is in fact a 5D35, not the same at all as a 5DL35!

The german link I realize gives 7 reference points of measurement on the taper, but lists the 3rd through 7th all at 54mm from the top of a 57mm needle (lists the thickness at this point as 5 seperate points), and lists them all obviously as the same diameter, so given three points of reference only, the 5DL35 and 5DL36 are the exact same, although I bet the taper from the 2nd point to the tip is very slightly different (the missing measurement areas).
There is a 5DL37 that is almost identical as well, but it shows to be slightly thicker by a few thousandths at the 2nd measured point on the taper but by the tip it is slightly thicker. Sounds like one of these three needles could be the one that I want to run with the 920c engine rebuild, and I may start trying them out now. I have a feeling I will be swapping needles and clip positions many times, as well as main jets and maybe even a 17.5 pilot going in.

If I need the main/needle to start feeding fuel sooner to help the transition off of the pilot circuit, it sounds like maybe a Dynojet Stage 3 needle would be the best bet of all. taking the stock fat lean needle and raising to the 4th or 5th clip will have a similar effect but with less gains in the "go fast" range of throttle position. The 5DL3# needles will give a lot more fuel at the higher end than the stock 5F21. I have A WHOLE LOT of experimentation to do!

[Chuck78]

Here ya go, I did some screenshots, cut and pasted pertinent info from 3 pages together, and deleted all duplicate measurements from points K3 through K6, as they were all listed as the exact same point at 54mm down from the top which is what K7 is as well. Basically they were missing info from K3-K6, so they just copied info from the K7/D7/A7 (reference point/diameter/area) and listed it in a way to confuse us!

Anyone got a chart that references Needle Jet specs???!?!? I am wondering about o-4 versus o-6.



this info is in the middle somewhere on this very long document, around page 43 I think. http://www.docstoc.com/docs/14789169...--mintelonline

[Chuck78]

Dynojet does not have a kit for 77 or 78-79 GS750's, but they do offer a 78-79 GS1000 kit (stage 3 only), but $95 for a set of needles that come with some jets that are several sizes larger than I can use on my bike (130 & 132.5 Mikuni flow-rated sizing)... not happenin. Can I buy these needles separately I wonder??? Seems as if I can get good results with the 5DL3# needles, but I bet the Dynojet needles would have a more powerful transition off the pilot circuit for sure.

http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Dynojet+Kit+GS1000+1978-79&item=DJ3304

[saucebox]

Sudco, maybe? (http://www.sudco.com/).

Interested, though...in a similar situation, so I'll watch this thread. Damn good job on your homework...

[Chuck78]

Well darnit, I discovered a more common problem that I never knew about, although I had seen it before on some VM22 choke pickup tubes (aka starter jets) that are brass pressed into the aluminum carb body). The fuel bowl overflow tubes had a couple of cracks running from where the tube in the bowl comes up out of the aluminum. three out of 4, one bad, one looking like it could get worse, and one that was barely noticeable. I had 3 spares from extra VM22's, 2 of them cracked also!

Anyhow, the good news is i got a set of 78-79 GS1000 #49000 VM26's in the junk bin at the VJMC yard for $20 to try and salvage these grimy oxidized fuel bowls off of (may have to break out the sandblaster or just solder the cracked ones if possible), If I can get these slides freed up out of the oxidized carb bodies, I will have a set of 5DL36 needles to compare and potentially run! Pulled the main jets and needle jets, they are definitely the very slender jet needles that would come on a GS1000.

Starting out with 122.5 mains, I initially bought 115's, knowing now that I don't have high compression wiseco's in my "motor's been bored out" bike and most likely standard bore GS850 pistons made me think that I will need larger jets due to not drawing quite as much air (less siphon effect) from the lack of 10.25:1 compression.

[chuck hahn]

Take a propane torch and heat the carb bodies around the slide towers and add some oil. Use pliers to grip the linkages and see if they will slide out.

Coat hanger wire thru the holes in the linkages works well to get a good pull also.

[Chuck78]

Well I have started messing around with jetting and needles today. I first installed the 5DL35's in the middle clip position. A little too rich, dropped the needles one clip notch. Still a bit too rich now but tons of power, but had a terrible transition from the pilot circuit to the needle into the midrange of throttle opening. I figured with the aggressive needle, keeping it lower to keep the higher throttle opening positions in the sweet spot was hurting the pilot to needle transition a lot, as I needed the taper to start sooner, and lowering the needle started the taper later in the throttle opening.

So I swapped in the 5F21 needles in the 4th clip notch down. Doing better on that transition now, but there are still some bogging areas, and I was thinking I need to swap to a 17.5 pilot jet. My pilot screws are 1-3/8 turns out, tuned with a Gunson ColorTune, with the air screws at 1-7/8 out. I think after running about 70 miles I realized maybe I was a bit rich at idle, as after 45 seconds of me looking at google maps on my phone on the roadside, the idle would start dropping and stumbling (plugs sooting up?). Seemed like maybe I needed more fuel at 3000-ish rpm tho, unless maybe I am running too rich where i thought I was too lean... engine does sound kind of boggy. Exhaust note sounds different for sure.

I still think I am a bit off in the midrange and mains, as it only runs like a bat outta hell if I take off from a stop and never go past half throttle on my way up to the 6000rpm+ range, and then shifting only at redline. If I an in 4th or 5th gear and give it any past 3/8 throttle up to WOT from 4000-ish rpm, the bike just bogs real bad, sounds bad, and has no power. 122.5 mains and 5F21 in the 4th position.Trying to decipher the differences in exhaust sound to determine air/fuel conditions, but my muffler baffle design has changed since half of the baffle got run over by my buddy and then a car or two! 18" straight through baffle turned into an 8" dB Killer type baffle setup. I'm hearing unfamiliar exhaust notes, but not sure if I can use the exhaust sound to determine rich or lean due to changing baffle design.

The bike will still hit 90mph in a big hurry, but I have these few flat spots in the ranges I mentioned above. 3/4 throttle doesn't seem too much different, power-wise than full throttle, and I am trying to determine how to get more, as midrange acceleration at speeds above 65mph is pretty dismal now. downshifting, the bike is real happy, but like I said, 3.4 throttle or full throttle, not much difference in power, slight difference in engine pitch. Mains off? I have 115's, 125's, and 122.5 (installed currently). Before riding, I was thinking maybe 117.5 or probably 120 was going to be my best bet based on intake and exhaust mods and some time spent researching and thinking.

Any advice? Most say around 122.5 or 125 mains on a modded GS750. 122.5 seems off to me, but I keep thinking going down to 120 or so will be better. I have yet to try the 115's. I was thinking of swapping them out for 120's before I try them. I still have some bogging down low, doesn't happen at easy take-offs from a stop, nor does it with hard launches, more like rolling the throttle on at a mid-paced rate from a stop and then trying to get on it. and wider throttle openings at the lower end of rpm's like I said don't fare so well, flat spot. HELP!