Big bore GS(X) 400

[Chuck78]

AFR tuning info:

http://www.thegsresources.com/_foru...-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.

 

[Chuck78]

If you were doing full throttle at the higher rpm's, then it sounds like your mains should be 135-140 based on the guy running VM32's on a bigger cc twin, and your hunch that it was lean. Lean produces more of a zingy sound, where rich can produce more of a deep muffled throaty sound. Lean is BAD BAD BAD at high speeds at full throttle, be careful. The AFR gauges are awesome for very quick assesment of jetting for that reason and a multitude of others.

If your main jet going from 185 to 130 changed the way your bike idled, then you are running quite lean on the pilot jets (idle jets) I'd venture to say. Bump that up one or two sizes.

I was really suspecting that with this setup, you'd have the hardest trouble tuning the low mid to mid throttle area with the needle jet and jet needle combinations. If you can get it set up to give you a good AFR reading just by adjusting the needle height, you are quite lucky.

You really need to approach it this way:

#1 main jet in the suspected vicinity of size based on similar bikes and your quick test runs
#2 needle probably in middle or higher positions (if 5 nothches to adjust, I'd try #4 for starters)
#3 adjust valve shimming and bench synch carbs, then adjust idle jet size
#4 tune idle perfectly (a Gunson Colortune for $40 used on ebay is AWESOME for this) and vacuum synch carbs
#5 find a remote location (desolate uphill straight road is best) and roll on to full throttle 5000-10000rpm pulls, swapping main jet sizes until 6500/7000rpm+ feels and sounds the best
#5.5 take some extra spark plugs and socket, do plug chops when you get the mains close. do advanced search for "plug chops" under my username Chuck78 to get detailed info - most of the info I posted above in your thread.
#6 now with main jet sizing set, do 5000-8000rpm pulls at a little above and then a little below half throttle, while tweaking the needle height to get it to feel best, and do plug chops to confirm mixture at that range. If you have a lot of trouble at all needle positions, I'd stop here and weld an o2 bung on your exhaust and get an AFR setup, or go visit your friend with a Dyno, as he may likely have an AFR exhaust probe for his Dyno tuning, and he could tell you with that real quick if your mid throttle positions at mid to high rpm's are far too lean or too rich.

If you have trouble tuning the 1/8 to 1/4 throttle range after that, you may need to look into upsizing or downsizing the needle jets slightly, and then re-adjust the needle height (down if larger needle jet, up higher - lower clip position - if smaller needle jet.

I believe this to be the best possible method to attack tuning this setup. Again, I HIGHLY recommend the AEM, Innovate, or similar wideband o2 AFR setups for their incredible ease of getting exact results immediately, as opposed to spending a ton of time with trial and error on jet sizing etc - especially for your setup running non stock carbs that are not tuned at all for this size engine. With a stock carb, you know that your needle jet and needle and other settings will be remotely close, but with XS650 prepped VM34's. you may be far off in tuning specs, and the AFR readings would really help get you going the right direction immediately. I think I paid $140-170 US dollars for mine. Very well worth it. The Gunson Colortune glass spark plug was an awesome tuning tool for the idle as well. With only 2 cylinders to tune, the AFR setup and careful attention to engine rpm;s while tuning as well as careful plug reading,a nd you might be able to tune the idle just as easily with the AFR gauge. On a 4 cylinder with the AFR gauge, tuning the idle is just too complicated with a single o2 sensor in the collector vs individual sniffer in each head pipe. The Gunson is really awesome though, I still recommend it. yellowish flame, too rich. blue with occasional flickers of yellow, slightly rich and slightly fouling. blue flame, perfect. blue-ish white, too lean. Very easy to read.

Excited to hear your progress.

 

[Chuck78]

With the GSX400 4 valve per cylinder, you probably are somewhat limited in cam selection (OEM only!), but I bet MegaCycle can regrind your cams for you no problem, as they will do the big four cylinder bikes, which should be the same as your cam but with the outer two cylinders' lobes whacked off. Just browse their catalog for the GSX750 grinds for something that works well with aftermarket pistons in the 10.25:1 compression range. A lot of their full race cams say they need 11:1 or as high as 13:1 compression. The more modern cam profiles really trap more of the intake charge, as they have a higher lift compared to the duration, vs the old cams that kept the valves open for a longer time to have an easier ramping rate. The OEM cams bleed off more compression and have much lower lift. Big gains jumping to a modern performance grind.

 

[GregT]

thank you for taking an interest Chuck - but we have pretty good cam grinders here in NZ too...Kelford, local to me do all the Cosworth "import car" cams worldwide. I've built motors for three previous owners of the business so have pretty good relations there.
Carb tuning...40 plus years of experience.
We'll get it sorted.

 

[MuzzaB]

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.

According to my 12 year old daughter, whose eyes are way better than mine, the number on the idle jet on the bike is 15, the 2 sets not in the bike are 12.5's.
What do you suggest getting? or can we drill out the 12.5's to something usefull?

 

[MuzzaB]

On a side note, it was the first time I had had the bowls off the carbs, interesting way the floats work, not what I expected at all.

 

[MuzzaB]

I know it doesn't mean anything as not a proper plug chop, but after my quick flick up the street and back both plugs are a nice tan colour, running on 98 octane unleaded. Will most likely get some AvGas for the next track outing.

 

[GregT]

According to my 12 year old daughter, whose eyes are way better than mine, the number on the idle jet on the bike is 15, the 2 sets not in the bike are 12.5's.
What do you suggest getting? or can we drill out the 12.5's to something usefull?

yes we can. Did you say you'd bought a small set of drills ? The 0 -1mm set in a plastic dome by any chance ? Mine has a few gaps now - not bad given it's about 25 years old. I'd suspect that one set is actually 17.5 not 12.5, without a lens they're very hard to read.

 

[MuzzaB]

yes we can. Did you say you'd bought a small set of drills ? The 0 -1mm set in a plastic dome by any chance ? Mine has a few gaps now - not bad given it's about 25 years old. I'd suspect that one set is actually 17.5 not 12.5, without a lens they're very hard to read.

Coming in the post is a 20 Piece Micro HSS Drill Bit Set that has:
0.3. 0.35. 0.4. 0.45. 0.5. 0.55. 0.6. 0.65. 0.7. 0.75. 0.8. 0.85. 0.9. 0.95. 1.0. 1.2. 1.3. 1.4. 1.5 and 1.6mm drill bits

Plugs after run up the street.

 

[GregT]

That's probably still a little lean on our 98. If you see that colour on avgas, it's correct.

 

[MuzzaB]

Hey Greg,

Drill bits arrived. Really sure both sets of pilots not in the bike are all 12.5's. I checked what the carb kit was supposed to come with and it said 2 sets of pilots, so guess they put in an extra set by mistake.

What are you up to on Friday 27th Feb? With racing Saturday I usually head over after work Firday, but could take the day off and come over earlier to see you for a tuning session if it suited.

Cheers
Muzz