Big bore GS(X) 400

[Chuck78]

Please correct me if I'm wrong, but I would think that VM34's would seem a bit large for that unless you intend to run only at very high race rpm's. Lower & mid rpm's & you wouldn't have enough air velocity for an unrestricted (no CV butterflies) venturi. I would think a pair of VM28's would be more appropriate, or vm29 smoothbores - both would probably need to be custom fit since I only know of factory four cylinder applications, where the #1&#4 carbs would need to be used.
Unless this bike sees only 5500-13000 rpm's, it doesn't seem to me that you'd have enough air velocity to make the jets function as intended. I suppose this could be adjusted to much larger jets to make up for lack of velocity, but the needle & needle jet selection seem like they would take a high degree of experimentation.

Bikes that ran vm22 & vm26 carbs in the 70's jumped up to bs32's in 1980, & that was with basically no other changes to the engines on many models, implying that a 26mm unrestricted venturi is equivalent or thereabouts to a 32mm with a butterfly throttle valve blocking the middle of it.


If other racers run vm34's, then the key is that you need to run a much larger jetting than the much smaller bs32 or bs34 ran. Bigger carb @similar displacement = lower air velocity than smaller carb, therefore less siphoning or venturi effect to pick up fuel from the fuel bowl, hence larger jets needed. The needle jet & jet needle selection is a whole separate challenge, the main & pilot jetting only goes for idle to 1/8 throttle, & 4/4 to full throttle. Mid throttle is all in the needle jet bore & jet needle initial diameter (1/8-1/4) & taper (1/4-3/4).

Also, unless you are running fairly rich, a VM carb bike requires you to do the thinking that the CV carb vacuum diaphragm/spring/slide/orifice did on the CV carb. Just whacking the throttle wide open will kill all air velocity & the bike will bog. You have to roll on the throttle to keep the air velocity at optimal levels to enable the jets to meter properly. Look up carb tuning tips. I know I've re-posted some good charts on here & links to great tuning sources.

Some will tell you that you should be able to hold wide open throttle fully open from a roll on off idle & have the bike run very well to redline. I have read and experienced otherwise, as that way of thinks ng only works for cv's since they raise the diaphragm throughout the rev range to keep optimum air velocity to keel the jets in their proper metering conditions.

[MuzzaB]

Hi Muz - wondered when you'd find this...post one of the horizontal pics from nelson for a giggle.

as owner/user any comments about vibration ?

Just for you Greg,

MuzzNelson1.jpg

[MuzzaB]

MuzzNelson2.jpg

sliding.......

[MuzzaB]

What it looks like when all good....

MuzzNelson3.jpg

[Chuck78]

Ever tried tuning with a wideband o2 sensor and AFR gauge? I use the Gunson Colortune for idle and off idle transitions, and then the WBo2 "UEGO" sensor setup to tune the rest, and verify plug readings now and again in tuning to verify that all cylinders are running the same. The air fuel ratio for various throttle positions and rpm ranges varies from high e11's up to mid 12's. you don't want to touch 14+ on an air cooled engine. I think if I recall (read my posts), low 13's was the leanest you want to go on an air cooled engine. I'd have to reference my previous posts. high 12's to 13.2 was peak hp, and peak torque was high 12's. idle may need to be a bit more rich on these bikes if I recall. It;s been many many months since I had been fluent with this data in using mine to tune.


REVIEW: http://www.thegsresources.com/_forum...rb+tuning+tips

Also, read all of these. One in particular caught my attention when it verified the various rpm's and what throttle range you should and shouldn't be testing for at vertain rpm's when tuning manual slide or direct action slide carbs.

I found a REALLY REALLY USEFUL diagram and some more supporting info that spelled out something that the other 10+ tutorials and personal instructions hadn't - you are looking for a mere 2mm or so coloration ring on the WOT plug chops, less is obviously lean, more is rich, 2mm hershey bar brown is too rich, tan-brown is good. This diagram really was something I thought belonged on here for reference:




I also read on a Pontiac musclecar website (that failed to mention the importance of plug chops!) that you can read the jetting to some degree from the base ring of the spark plug, as shown in the above diagram as well. It said however that the jetting cannot be read from the insulator! Well, maybe that should have read "cannot be read from the insulator without cutting the threads off that shroud the insulator base."

Also, I read a CV carb tuning page on the FactoryPro website that had some useful info that I had never learned. It was geared more towards the newer carbs like the Mikuni BST GSXR style, and the float height actually is the last thing they adjust on those type of CV carbs,and is considered part of the idle or low speed adjustment tuning?!? I was boggled at first, but then read more on gixxer.com about these carbs and that is to some extent true for the later CV's. I also realized from this that on CV's, rpm range is really critical more so than your throttle grip position, because the true throttle position is controlled by the engine's air velocity through the carb which can only be manipulated by your wrist movement. So it says for main jet tuning on CV's, you need to be at 8000+ rpm's. I think there is some merit to that, and my interpretation of that is - "for WOT mains plug chops, ride it up a big long hill like a drag racer and only shift at redline, and then kill it and do the chop" for any CV carb. Some people here would say to do a WOT chop starting at low speed at 2000 rpm at WOT and hit the kill switch at redline. I don't think that's long enough in the actual range we are testing for to give a good reading in my opinion.

Here are some very useful links that I thought I'd add to this post that I have been dropping elsewhere in a thread or two:

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/techarticles/reading-spark-plugs.html

http://www.factorypro.com/tech_tunin...m_engines.html (for newer cv's, but lacks the plug chop method)

I'm not sure if the exact link I was referring to is in those, that may be more for plug chops, but I will try and dig up the chart that I was referring to for VM style carbs that mentioned certain rpm ranges at certain throttle positions where the chart specifically said "do not tune for this rpm range at this throttle position."

[Chuck78]

AFR tuning info:

http://www.thegsresources.com/_forum/showthread.php?225082-proper-AFR-s-and-carb-tuning-on-new-big-bore-GS750-920cc-build&highlight=carb+tuning+tips

[GregT]

Yes, ideally i'd have liked to see 32's on it but we'll get it sorted on 34's. Primarily, they were fairly cheap... It's a racebike with a 6 speed box so there's no problem keeping it in the right rpm range.
The 4V heads give a very good signal to the carbs so you can get away with carbs a bit bigger than the equivalent 2V.

[MuzzaB]

Please correct me if I'm wrong, but I would think that VM34's would seem a bit large for that unless you intend to run only at very high race rpm's. Lower & mid rpm's & you wouldn't have enough air velocity for an unrestricted (no CV butterflies) venturi. I would think a pair of VM28's would be more appropriate, or vm29 smoothbores - both would probably need to be custom fit since I only know of factory four cylinder applications, where the #1&#4 carbs would need to be used.
Unless this bike sees only 5500-13000 rpm's, it doesn't seem to me that you'd have enough air velocity to make the jets function as intended. I suppose this could be adjusted to much larger jets to make up for lack of velocity, but the needle & needle jet selection seem like they would take a high degree of experimentation.

Bikes that ran vm22 & vm26 carbs in the 70's jumped up to bs32's in 1980, & that was with basically no other changes to the engines on many models, implying that a 26mm unrestricted venturi is equivalent or thereabouts to a 32mm with a butterfly throttle valve blocking the middle of it.


If other racers run vm34's, then the key is that you need to run a much larger jetting than the much smaller bs32 or bs34 ran. Bigger carb @similar displacement = lower air velocity than smaller carb, therefore less siphoning or venturi effect to pick up fuel from the fuel bowl, hence larger jets needed. The needle jet & jet needle selection is a whole separate challenge, the main & pilot jetting only goes for idle to 1/8 throttle, & 4/4 to full throttle. Mid throttle is all in the needle jet bore & jet needle initial diameter (1/8-1/4) & taper (1/4-3/4).

Also, unless you are running fairly rich, a VM carb bike requires you to do the thinking that the CV carb vacuum diaphragm/spring/slide/orifice did on the CV carb. Just whacking the throttle wide open will kill all air velocity & the bike will bog. You have to roll on the throttle to keep the air velocity at optimal levels to enable the jets to meter properly. Look up carb tuning tips. I know I've re-posted some good charts on here & links to great tuning sources.

Some will tell you that you should be able to hold wide open throttle fully open from a roll on off idle & have the bike run very well to redline. I have read and experienced otherwise, as that way of thinks ng only works for cv's since they raise the diaphragm throughout the rev range to keep optimum air velocity to keel the jets in their proper metering conditions.

Hi, Thanks for the input, had I known more I would have brought some VM32's, my the logic at the time was the stock carbs are BS34's, so I figured a more efficient 34mm carb was the way to go, and bingo on ebay there was a kit for a XS650 with 2 x carbs, air filters, throttle cable, mainfolds, and a range of jets. In the shed during the week I tried a range of main jets to see what worked best when opening the throttle, you are right in that it definately likes bigger jets. Although in saying that, I have only recently learnt that mikuni round jet size numbers are different to the hexs, so the 185 hex jet I have in the VM34's now, is not that different in flow to the round 127.5 in my BS34's. I had previously run the BS34's with 140's which = 230's in the hex's. I don't expect to be able to snap the throttle open withoput it bogging, and it is a race bike once up and running it doesn't drop below 5,000 - 6,000 revs, we'll get it there. cheers Muzz

[Chuck78]

Muzz, I highly highly recommend the Innovate or AEM wideband O2 air fuel ratio setup for optimal tuning. Nearly as good as paying someone to dyno tune it, but you can do it yourself (but no power output #'s). If you read the threads that myself and others discussed what AFR's to look for, you will be right on track. Mid and high rpm & throttle openings are easy to tune with the WBo2 setup. Start with WOT 7000rpm+, get your mains to around 12.6-12.8, then see where you are at 5000-6500 rpm at half throttle & 3/4 throttle. Get that tuned to 12.8-13 or so AFR. Then get the pilots set for a good start & steady idle once warmed up 5-10 minutes. Make sure to synch the carbs well at this point, & re-check valve clearances cold (overnight bike-off not in hot sun temperature, before doing a vacuum synch). If they were for an XS650, your needle jet & jet needle are probably way way too lean on a 436cc setup. You gained only 18cc per cylinder, & if those vm34's were really set up for an xs650, they are set up for the air velocitythat 107cc more per cylinder would create (much more air velocity).
I had previously arrived at these basic benchmarks to shoot for to try out:
12.6-12.8 @ WOT (peak power range, don't want to go much higher because that will mean more heat buildup at high rpm/high loads)
12.8-13.0 @ 1/2 - 3/4 throttle for great power, and slightly less worries of heat at less than WOT)
13.7-13.0 @ 1/8 - 1/4 throttle range trying to get more fuel mileage but may have to richen slightly if I have stumbles
12.7-ish @ operating temp idle (don't want to have to go below 12.2 as to keep it good for cold weather/high altitude richening up to 1 point richer)



Needle jet diameter vs jet needle initial (larger end) size and taper have control over most of 1/8-3/4 throttle mixture on VM's.
I captured this helpful screenshot Motorcyclecarbs.com, no idea what size you will end up needing, or if your needle jet size range wil be listed in this chart, but this gives you an idea. 0-3 was a bit on the lean side for me, but an o-6 was a bit rich, and I couldn't get my of idle and cruise AFR above 11's. With an o-3, I was at the high 13's range. I think o-4 would be ideal if I can ever find a set of GS850 1979 carbs to rob them out of. The needle jet diameter plays hand in hand with the nedle taper and diameter. The needle basically plugs the needle jet opening artially, and the taper alows more fuel to atomize into the carb as the throttle slide is raised with the rpm's increasing.

to get in the ballpark of needle jet and needle combo, you need to get the idle set good, and then get a good AFR reading at 1/4 throttle or so. Once you have settled at that, you can mess around with needle clip height positions and if needed, different tapers.


And here is a chart from a german pdf file Mikuni jet needle spec catalog. The examples are for my setup with vm26's, but here to show you what you are looking for. Need to see what needle length your carb takes, and when the taper starts on it, and start experimenting from there. A word of advice on what someone else in the 400-500cc class runs on VM34's would be a very very helpful starting point, as needles and needle jets are expensive!
Also, on this chart, you will see missing numbers as it skips from d2 to d7, which just means the diameter is a straight taper between those two points. Some needles have a slow taper at first to keep it leaner at lower throttle, and then switch to an aggressive taper after that for more fuel requirements.


Why didn't you just go with a GS500 carb? Seems that would be a lot more in the ballpark of tune, less guessing and buying mains/needles/needle jets.

[MuzzaB]

Hi Chuck78,

Yes in hindsight there were probably better options for carbs, but I'm not writing the VM34's off just yet. Even though it was running badly at the track, which may have been due to a loose electrical plug to the coil making a missfire and dropping a cylinder, making me think it was the carbs. It showed glimpses of what it will be like when running right, I did manage to coax it to 100mph as it was. It can only get better. I do have an aquaintance with a dyno so plan to get it run there for a proper tune up once I get it
in the ball park. Half the fun of racing is tinkering around and learning how things work and what happens if you change something. Cheers Muzz

[GregT]

GS500 carbs are not an option as it runs in pre 82 class for which the late flatslides are illegal - either CV's or RS/FCR flatslides.<BR>i'm confident it can work well.

[MuzzaB]

Bit of seat of the pants tuning today.

185 main, needle full rich, started easly and idles well. Accepts throttle.
Out on the street, gently let out clutch and roll on the trottle, big bog at 4,000rpms.
Change main to 165, same result. wondering if it is a lean bog or a rich bog. Only have leaner jets.
Change main to 130, doesn't idle as well, doesn't like opening the throttle in the shed.
Out on the street, gently let clutch out and roll on the throttle, takes off like a rocket, change into second at 7,000rpms,
heaps of power, into third and button off, turn back to home at the corner and repeat back to home, ...yay....progress.
It may give up after 7,000rpms, will have to get somewhere to give it a full throttle test.
At least now I have a good running point to work from. My gut feeling is that the 130 main could be lean.
I could try leaning the needle with a richer main to see if it responds to that. Time will tell.


Cheers
Muzza

[GregT]

Muz, what idle jets are in it ? Typically, these will work with quite big idle jets but smaller mains. The big idles get you midrange response but because they bleed fuel right through the range, smaller mains than you'd think are required.
Are you coming over for the CAMS meeting at the end of the month ? We could do a little drilling....

[MuzzaB]

Hi Greg, yes coming over at the end of the month.

I'm not sure what the pilots are, the carbs came with 3 sets, the ones not in the bike all look to be the same size to me.
(I'll check what they are tonght)
I haven't pulled the ones from the bike to look because it starts easily and idles well, so I thought best not to muck with it.

The main jets I have are 120, 125, 130, 165, 170, 175, 180, 185.
Actually have ordered a micro drill bit set as was thinking the 120-130's would be too small and was going to drill them out bigger
than the 185's. But now think I will get a pair of 140's and 150's to fill in the gap in the range.

I know Chris runs 135's in his VM32's on his GS450(500). His has velocity stacks with just a gauze in them to keep the bugs out.

Cheers
Muzz

[GregT]

Those 135's without filters will equal 125's with filter. The old rule is down 10 on the main with a filter.
See if you can read the numbers on the idle jets.