Any good history of that working on a thoroughly modified CB350? At this point, I just want it to work, and I think the tuning options available for Mikuni are my best bet. Experimenting on something no one else has done (with stock carbs, even!) is becoming a lot less fun now that I've got many many hours into tuning this carb without it working well.
Not-Stock CB350 Jetting on Stock Carbs
- Thread starter berck
- Start date
it was not uncommon to see them used back in the day on hotted up 350's or even 305's, i got the idea from a fella in australia that used them on a 350 with good success
but no i dont have any tuning experience with that carb on a 350 and you are better off with a known good base for jetting and a pair of kunis
i threw it out there but it is not practical for you so...NEVER MIND
but no i dont have any tuning experience with that carb on a 350 and you are better off with a known good base for jetting and a pair of kunis
i threw it out there but it is not practical for you so...NEVER MIND
crazypj
Split personality, I fake being smart
berck said:
It isn't possible to just jet a Kei-Hin '3 jet' carb, it needs other work
If I already did them you should be able to run stock jetting with just a slightly larger secondary main jet up to 3,00ft elevation.
At 9,000+ you will need smaller jets but shouldn't need to go that much smaller.
The pilot shouldn't need to be smaller than 35 as the slide doesn't completely close
What is the percentage change in air density from 3,000 to 9,000 ft? (I can't remember off hand)
you could probably go close to the percentage change, slightly larger won't harm anything.
I've made the secondary mains several times as they are the same as CB500/550.
Primary mains would probably be just as easy to make
I think you biggest issue is the cam shaft. You just don't have enough air at your elevation to be able to use it until motor is spinning around 15,000rpm or more
And now it's the end of June...
So, Bore-tech recommended VM32's for better power. I've now got at least $200 invested in brass, and have spent at least 50 hours. I'm exhausted, tired of spending money, and have basically nothing to show for it. I do have a half-dozen needles and a half-dozen needle jets... plus a bunch of pilot jets. Only a few mains because I can't get far enough to tune the top end.
The bike idles beautifully. It seems happiest idling with #20 pilots, air screw 1.5 turns out. And that's the end of the good news.
Twisting the throttle makes it go completely lean and die. If I snap the throttle to, say, 1/4, it stops firing. Let go quickly enough, it'll idle again. I know it's lean because if I richen it up with the richest needle I can find set as high as it'll go, I can twist the throttle normally. Only, at that point, it's so rich that it'll barely run. I'm down to a 2.0 slide, which is the smallest I can find for the VM32. (At least, both sides. I can find smaller for the left side.) Way better than the 3.0 it came with, but still not good.
But wait, there's more! Opening the throttle completely at low RPM (below about 4,000 rpm) also results in a nearly completely-lean condition such that there's no combustion. If bay the throttle (1/8-1/4 open) and get it over 6,000 rpm, I can open it the rest of the way and it works great, and it'll pull to redline easily.
My current theory is this: at low RPM this engine just doesn't pull hard enough to make the carburetors work at more than 1/4 throttle. Maybe racers don't care since they can spend all their time between 6,000 and 9,000 rpm?
Also, reversion with this cam is a bit crazy. Spits gas like crazy around 3,500 rpm. That can't help.
So now I'm thinking about buying a set of VM30s as a last-ditch effort. If that doesn't work, I'm out of ideas. Sell the engine and go back to stock?
So, Bore-tech recommended VM32's for better power. I've now got at least $200 invested in brass, and have spent at least 50 hours. I'm exhausted, tired of spending money, and have basically nothing to show for it. I do have a half-dozen needles and a half-dozen needle jets... plus a bunch of pilot jets. Only a few mains because I can't get far enough to tune the top end.
The bike idles beautifully. It seems happiest idling with #20 pilots, air screw 1.5 turns out. And that's the end of the good news.
Twisting the throttle makes it go completely lean and die. If I snap the throttle to, say, 1/4, it stops firing. Let go quickly enough, it'll idle again. I know it's lean because if I richen it up with the richest needle I can find set as high as it'll go, I can twist the throttle normally. Only, at that point, it's so rich that it'll barely run. I'm down to a 2.0 slide, which is the smallest I can find for the VM32. (At least, both sides. I can find smaller for the left side.) Way better than the 3.0 it came with, but still not good.
But wait, there's more! Opening the throttle completely at low RPM (below about 4,000 rpm) also results in a nearly completely-lean condition such that there's no combustion. If bay the throttle (1/8-1/4 open) and get it over 6,000 rpm, I can open it the rest of the way and it works great, and it'll pull to redline easily.
My current theory is this: at low RPM this engine just doesn't pull hard enough to make the carburetors work at more than 1/4 throttle. Maybe racers don't care since they can spend all their time between 6,000 and 9,000 rpm?
Also, reversion with this cam is a bit crazy. Spits gas like crazy around 3,500 rpm. That can't help.
So now I'm thinking about buying a set of VM30s as a last-ditch effort. If that doesn't work, I'm out of ideas. Sell the engine and go back to stock?
crazypj
Split personality, I fake being smart
I could have modified 350 stock CV carbs for about $60.00.
You would still need to check secondary main jet but it would pull full throttle
As for reversion, are you 100% sure cam timing is right? I've never heard of 350 cam being ground 'wrong' but it is very common to get installed 1 tooth 'out'
Mechanical slide carbs will always do what your describing, you can't just whack it to full throttle at low rpm as there will always be a pause before rpm picks up (unless you use accelerator pump and/or too small venturi)
It's a PITA to modify slide down to 1.5 but that's probably what it needs.
The 3.0 cut-away is useless, particularly at your altitude.
A major overbore would compensate for the oversize intake port (same as it does on CB360)
I forget the biggest piston that will fit, I think around 71mm
I know 75mm is too big, even with offset bores, got the 'dead' parts to prove it
You would still need to check secondary main jet but it would pull full throttle
As for reversion, are you 100% sure cam timing is right? I've never heard of 350 cam being ground 'wrong' but it is very common to get installed 1 tooth 'out'
Mechanical slide carbs will always do what your describing, you can't just whack it to full throttle at low rpm as there will always be a pause before rpm picks up (unless you use accelerator pump and/or too small venturi)
It's a PITA to modify slide down to 1.5 but that's probably what it needs.
The 3.0 cut-away is useless, particularly at your altitude.
A major overbore would compensate for the oversize intake port (same as it does on CB360)
I forget the biggest piston that will fit, I think around 71mm
I know 75mm is too big, even with offset bores, got the 'dead' parts to prove it
A couple of thoughts in no particular order.
Opening the throttle fast on a large slide carb, the motor will die. My RZ350 with tiny 26mm crabs will just make a waaaaah noise and slow down on the street if I bang it open and it will simply die if I do it at idle. Regular slide carbs need to be opened smoothly and progressively.
Sounds like the top end is OK but off idle transition is not so good. Try a larger slow jet - no other changes and see if it improves throttle roll on (ignore what it does to idle for the moment). If it is even slightly better, it suggests the need for a richer slide. Needles and needle jets apply here also but it's mainly the slide cutaway.
Needle jets can be a PIA on non standard motors too because that motor should run a bleed type and not a primary type. The reason for that is that at low engine speeds fuel is not broken up into fine droplets. Droplets are large and do not burn fast enough and much of the fuel drawn in is not burned at all leading to a super lean mixture and lots of fuel in the pipe.
Symptoms of that are very high HC levels on an exhaust gas analyzer on a dyno at the same time as lean A:F ratios.
What are the settings on those carbs at the moment?
MAin Jet:
Slow Jet:
Needle Jet:
Needle:
Air screw setting:
Fuel height:
or Float height
Opening the throttle fast on a large slide carb, the motor will die. My RZ350 with tiny 26mm crabs will just make a waaaaah noise and slow down on the street if I bang it open and it will simply die if I do it at idle. Regular slide carbs need to be opened smoothly and progressively.
Sounds like the top end is OK but off idle transition is not so good. Try a larger slow jet - no other changes and see if it improves throttle roll on (ignore what it does to idle for the moment). If it is even slightly better, it suggests the need for a richer slide. Needles and needle jets apply here also but it's mainly the slide cutaway.
Needle jets can be a PIA on non standard motors too because that motor should run a bleed type and not a primary type. The reason for that is that at low engine speeds fuel is not broken up into fine droplets. Droplets are large and do not burn fast enough and much of the fuel drawn in is not burned at all leading to a super lean mixture and lots of fuel in the pipe.
Symptoms of that are very high HC levels on an exhaust gas analyzer on a dyno at the same time as lean A:F ratios.
What are the settings on those carbs at the moment?
MAin Jet:
Slow Jet:
Needle Jet:
Needle:
Air screw setting:
Fuel height:
or Float height
crazypj
Split personality, I fake being smart
I've use earlier RD350 emulsion tubes (pre RD/RZ YPVS)
or preferably 26mm RD carbs on 250/360/390 motors
Yamaha switched from 'primary choke' to 'bleed type' when they fitted reed valves to motor
I've posted information about it multiple time over the last several years
or preferably 26mm RD carbs on 250/360/390 motors
Yamaha switched from 'primary choke' to 'bleed type' when they fitted reed valves to motor
I've posted information about it multiple time over the last several years
Hey, if you're willing to modify my stock carbs for $60, I'm more than happy to give that a shot. I've still got them, and they're in great shape. I also have basically every jet available for them. I was planning to just sell them on ebay as I figure they've probably got reasonable value as working stock carbs are getting harder to come by. Was keeping them in case I give up and sell this engine for a stocker so I can ride my bike again
So you guys are saying you can't go full throttle at low RPM, even slowly? It seems like I should be able to slowly open the throttle all the way at 2,000rpm? I can understand that without an accelerator pump I can't snap it open, but it should be able to pull hard enough that I get a decent mixture if I open it slowly, no? Having to wait until I'm over 4,500RPM to go full throttle (even slowly) doesn't sound very rideable for a street bike.
Even that aside, I can't even open the throttle past 1/4 at idle, except very very slowly if I set the needle/needle jet lean enough so that it's not running crazy rich once the throttle is open. Currently, I've got it rich enough that I can open it to 1/4-1/2, but it's very rich once it's there.
Here's my current setup. This is the "obviously too rich, trying to make it so I can turn the throttle *at all* without instant death setup. I can make it run better constant-state with leaner needle/needle jet, but at the cost of only being able to rev it past idle by just barely breathing on the throttle grip until it gets over about 3,000 rpm. Constant state seems happier with a 6F17 needle in the richest clip, Q2 needle jet.
Main Jet: 170 (possibly too rich with the needle I'm running, but I'm trying to get the mid-range sorted well enough I can ride it to jet the mains)
Slow Jet: 25 (runs a bit rich with a 25, it'd prefer a 20, but richer slow jet compensates for the fact that I can't go smaller than a 2.0 slide)
Air screw setting: 1.5 turns out (also, a bit rich, it'll run with the air screw removed and a #25 pilot)
Needle Jet: 159-P5 (I've got a bunch richer that will work with leaner needles. I've also tried the fattest 176 I can find (176-Q4), and it's way too lean even with my richest needle.)
Needle: 5L1 (Richest I could find, and pretty much the only one that will let me twist the throttle at all without it immediately dying)
Needle clip: on the richest, throttle response is best, in the middle, throttle response isn't as good, but it's slightly less rich in the 1/4-1/2 throttle position.
Fuel height:
or Float height: Stock
I've also played with both 2.0 air jets, and with the air jets removed (no real difference in the mid range, as to be expected...)
I'm leaning toward the VM30s. I figure they can't be worse, and my brass is interchangeable. Only, the VM30s appear to only come in left-only configuration, so I guess that means I can try 1.5 slides, too.
I'm pretty sure the cam timing is on. If it were a tooth off, I don't think the valve tappets would be loose at the right marks on the on the stator (they are). Also doubt it would run so well above 8,000rpm (it purrs up there), as well as idle beautifully, which it also does. From what I understand mid-range reversion is to be expected with large valves and a long-duration cam, which is what I've got.
So you guys are saying you can't go full throttle at low RPM, even slowly? It seems like I should be able to slowly open the throttle all the way at 2,000rpm? I can understand that without an accelerator pump I can't snap it open, but it should be able to pull hard enough that I get a decent mixture if I open it slowly, no? Having to wait until I'm over 4,500RPM to go full throttle (even slowly) doesn't sound very rideable for a street bike.
Even that aside, I can't even open the throttle past 1/4 at idle, except very very slowly if I set the needle/needle jet lean enough so that it's not running crazy rich once the throttle is open. Currently, I've got it rich enough that I can open it to 1/4-1/2, but it's very rich once it's there.
Here's my current setup. This is the "obviously too rich, trying to make it so I can turn the throttle *at all* without instant death setup. I can make it run better constant-state with leaner needle/needle jet, but at the cost of only being able to rev it past idle by just barely breathing on the throttle grip until it gets over about 3,000 rpm. Constant state seems happier with a 6F17 needle in the richest clip, Q2 needle jet.
Main Jet: 170 (possibly too rich with the needle I'm running, but I'm trying to get the mid-range sorted well enough I can ride it to jet the mains)
Slow Jet: 25 (runs a bit rich with a 25, it'd prefer a 20, but richer slow jet compensates for the fact that I can't go smaller than a 2.0 slide)
Air screw setting: 1.5 turns out (also, a bit rich, it'll run with the air screw removed and a #25 pilot)
Needle Jet: 159-P5 (I've got a bunch richer that will work with leaner needles. I've also tried the fattest 176 I can find (176-Q4), and it's way too lean even with my richest needle.)
Needle: 5L1 (Richest I could find, and pretty much the only one that will let me twist the throttle at all without it immediately dying)
Needle clip: on the richest, throttle response is best, in the middle, throttle response isn't as good, but it's slightly less rich in the 1/4-1/2 throttle position.
Fuel height:
or Float height: Stock
I've also played with both 2.0 air jets, and with the air jets removed (no real difference in the mid range, as to be expected...)
I'm leaning toward the VM30s. I figure they can't be worse, and my brass is interchangeable. Only, the VM30s appear to only come in left-only configuration, so I guess that means I can try 1.5 slides, too.
I'm pretty sure the cam timing is on. If it were a tooth off, I don't think the valve tappets would be loose at the right marks on the on the stator (they are). Also doubt it would run so well above 8,000rpm (it purrs up there), as well as idle beautifully, which it also does. From what I understand mid-range reversion is to be expected with large valves and a long-duration cam, which is what I've got.
Those numbers are all over the place. I feel your pain.
Couple of points, with a 176 Bleed series you will probably need to go down to say 0.5mm air jets. With a 2.0, it's sucking way too much air in and that air is displacing fuel through the needle jet orifice.
5L1 is a skinny little needle used in RD400 C and D models with 175 P-2 needle jets and 0.5 air jets. the taper starts later than a 6F needle so it will initially be leaner than a 6f and then change to richer, just because of the profile.
Main jets must be set first and then use needle and needle jet to trim part throttle. If it runs too rich on a needle/needle jet combo, then the main jet should be reduced first and vice versa. Main jets must be right when wide open and at that throttle setting Needle and needle jet make basically zero difference.
You should be able to roll on the throttle gently to coax revs up. If you have heavy reversion, that will make it super rich from just below that point to just above that and if it's not the cam, then make intakes longer or the exhaust system. Very short exhausts can cause that but usually it's the intake length and valve event timing. What cam is in it and what did it time up at?
Couple of points, with a 176 Bleed series you will probably need to go down to say 0.5mm air jets. With a 2.0, it's sucking way too much air in and that air is displacing fuel through the needle jet orifice.
5L1 is a skinny little needle used in RD400 C and D models with 175 P-2 needle jets and 0.5 air jets. the taper starts later than a 6F needle so it will initially be leaner than a 6f and then change to richer, just because of the profile.
Main jets must be set first and then use needle and needle jet to trim part throttle. If it runs too rich on a needle/needle jet combo, then the main jet should be reduced first and vice versa. Main jets must be right when wide open and at that throttle setting Needle and needle jet make basically zero difference.
You should be able to roll on the throttle gently to coax revs up. If you have heavy reversion, that will make it super rich from just below that point to just above that and if it's not the cam, then make intakes longer or the exhaust system. Very short exhausts can cause that but usually it's the intake length and valve event timing. What cam is in it and what did it time up at?
I didn't realize that about the 176. Maybe I'll try just plugging the air jet with the 176 and see what that does?
The 5L1 actually starts out just slightly richer than the 6F. 2.42 at 10mm for the 5L1, vs 2.515 at 10mm for the 6F5. It tapers pretty hard and fast, and I can absolutely see that there's no way it's the right needle. I was just trying to do anything to get enough fuel at throttle tip-in to keep it from dying. And the 5L1 does that, with the penalty that it's too rich for anything else.
I really need to be able to ride it to set the main jet. If it dies as soon as I open the throttle, I can't really get moving to jet the mains. I can try by doing things like revving to 6,000, feather the clutch until I get moving fast enough, but that's a miserable process.
The reversion doesn't seem as bad when it's not already running rich. If I run a 6F5 with the 159P5, I have to just barely coax the revs up, but it's not as rich and it doesn't spit gas everywhere. I'm running stock replica exhaust. Longer intakes would be nice, but the air filters already hit the battery box as is.....
The 5L1 actually starts out just slightly richer than the 6F. 2.42 at 10mm for the 5L1, vs 2.515 at 10mm for the 6F5. It tapers pretty hard and fast, and I can absolutely see that there's no way it's the right needle. I was just trying to do anything to get enough fuel at throttle tip-in to keep it from dying. And the 5L1 does that, with the penalty that it's too rich for anything else.
I really need to be able to ride it to set the main jet. If it dies as soon as I open the throttle, I can't really get moving to jet the mains. I can try by doing things like revving to 6,000, feather the clutch until I get moving fast enough, but that's a miserable process.
The reversion doesn't seem as bad when it's not already running rich. If I run a 6F5 with the 159P5, I have to just barely coax the revs up, but it's not as rich and it doesn't spit gas everywhere. I'm running stock replica exhaust. Longer intakes would be nice, but the air filters already hit the battery box as is.....
