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Nov 24, 2011 17:54:10 GMT
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Hi guys, I must be honest, this is not for a car, but for my Suzuki motorcycle. But since there are a lot of knowledgeable people here, I'm still hoping you could help me. This is also something people might encounter when mounting bike carbs to car engines, so someone may already have the answer for me. So I have a Suzuki GT750, which is a large 2 stroke triple, with open air filters instead of the original airbox. This is not by choice but by necessity due to fitment of a newer gen engine in an older gen frame. I'm having quite a lot of trouble getting the jetting right, it's either too lean up top (which is obviously dangerous on a 2 stroke) or too rich down low (which rides terrible). Now for the interesting theorem I came up with with my friends; the original airbox had a certain air flow resistance, which in effect was some kind of "choke" making the top part of the rpm richer. Now with the open filters it no longer has this resistance and also a much higher air velocity inside the carbs. And we think this is causing this "hard-to-jet" situation I'm in now. So, how do I solve this? Does the higher air flow necessitate larger main jets (making it undriveable at low rpms)? Do I need smaller jets because the higher air flow will create a bigger venturi effect and actually draw in more fuel? (I know these 2 things go against each other) Perhaps I need to adjust the CV spring? Does the increase in air flow have any effect at all on the CV slides? I was arguing about this last night with my engineering buddies and my head is still spinning  |
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Nov 24, 2011 18:16:56 GMT
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CV carbed bikes are very sensitive to airflow, and tend to run very badly rich without the correct "resistance" - I've had this problem many, many times  with customer's bikes, they really don't like cone filters. The slide rises to fast, and you end up with the midrange over rich. The solution is often to deflect the airflow away from the CV port at the top of the carb mouth - you see this in some OEM arangements and it seems to work well. |
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To get a standard A40 this low, you'd have to dig a hole to put it in |
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Nov 24, 2011 19:19:16 GMT
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If the main problem is that the CV slides rise too fast, can't I just stick in some stiffer springs? This is a picture of said carbs (sadly mine aren't half as shiny  )  Can you explain how I go about deflecting the airflow away from the CV port? Do you perhaps have an example? |
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Mr S
Posted a lot
10-4 Good buddy.
Posts: 2,654
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Nov 24, 2011 20:30:06 GMT
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Last Edit: Nov 24, 2011 20:30:22 GMT by Mr S
Suzuki GSXR1000 K2
BMW R1150GS
BMW K1200RS
Chevy K5 Blazer
Chevy Suburban LT
Jaguar XKR
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Well, I had a look around on those websites but couldn't really find what I was looking for, they're mostly 4stroke oriented anyway..
I was hoping scaryoldcortina could tell me some more about deflecting the air flow? And also, I have a second set of carbs, couldn't I double up on the CV slide springs?
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sparkyt
Posted a lot
selling stuff
Posts: 1,767
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Is it more to do with the taper on the needle in the carbs as air flows through the carbs it draws the slide up and off the needle and in turn up the taper of the needle giving a richer mixture .. like an Su
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Nov 30, 2011 15:40:28 GMT
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My old RD350 was horrible to ride on K&N's. Trouble starting, poor economy etc despite changing jets to those that came with the kit. Reversion back to original airbox (and jets) cured all the problems !
Can't you simply fit the new engine type airbox ?
Paul H
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mikeymk
Part of things
'85 Polo Coupe S 1.6 16v
Posts: 931
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Nov 30, 2011 21:43:03 GMT
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They're not efficient for multiple carbs. Much better for the carbs to share one large filter than a small one each.
This is because only one carb is sucking air at any time to any great degree, so each carb is pulsing. With individual filters you have standing air, then all the demand is drawn at one time.
But with a shared filter, you have stable airflow.
On top of that, and working with it, the large filter on a box is less restrictive than a small filter on each carb - again, because the air speed through the large filter is no faster than the air speed through just one small one when it's moving.
If you wanna do away with the box as you have to, two oval filters which each cater for two carbs are better for a 4cyl set than individual ones, with one full length foam one being better still. You only have a choice of the latter, which might be a bit wide. I've heard socks are less restrictive than cones, i don't know. If you need a little restriction up top then it may be a good thing, but i'd rather get them running right with as little restriction as possible! Maybe this would require a different needle profile, i dunno.
I run them filterless myself, but mine are in a car - on a bike I don't suggest it!
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Nov 30, 2011 22:42:36 GMT
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If they are the k&n filters with the chrome caps on the end these could be causing your problems.Apparentley they cause a reverb pulse from the end cap and it causes problem with the flow.
Had terrible problems with my lc getting it jetted and cured it by putting the normal foam filters on.
Lc's don't run cv carbs though so i don't know if it would help your kettle.
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With a bike like a Kettle I'd definitely spend the £s and dyno it. With the extent of the running problems it has to be setup more than purely down to choice of filter. Not just jets, maybe springs, air bleeds, needle height etc need mods.
You might come into issues with fine-tuning because of the filters, but got to get somewhere close first.
Could try the foam filters instead of the mesh/capped ones, although I've had both types with no problems after fettling. Are you using a Dynojet kit? Although they often need tuning, it might help. Dyno session has to be worthwhile vs holed pistons and unusable low revs etc.
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'66 Amazon <-> '94 LS400 <-> '86 Suzuki 1135 EFE
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Thanks for all the replies! It's good to see there are still intelligent people on the internet:)
Sadly I'm stuck with these filters, the old style frame doesn't have the space required for the new style airbox. And since the airbox is metal with chrome end pieces (and rare) I'm not going to cut it up.
I'm aware of the fact that almost every aspect of the carbs need adjustment, but I was just going to start with full throttle and pig rich and work my way back/down. I can always send it to a pro with a dyno if I don't get it sorted myself. Getting the fueling right is also part of the "fun" in this project though, so I'm not giving up yet.
I'm under the impression my main problem right now is the CV slides going to high, especially since the main jet size has influence on the mid range, which it shouldn't; needle jet and jet needle diameter difference should be smaller than the main jet diameter in mid range meaning the main jet has no effect (that's my understanding anyway)
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i think you need to get it on a rolling road mate to get it right.and go on oldskoolsuzuki theres a two stroke section.
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97 volvo 940 turbo wagon
87 fiat strada abarth
78 gs1000
82 katana 1100
84 gsx1100 efe
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with customer's bikes, they really don't like cone filters. The slide rises to fast, and you end up with the midrange over rich. 
)
