Jetting for a cb350 with Mikuni 32mm round slides

So I got these carbs with the bike and I am having a hell of a time getting them tuned.


Here is what I got back from the mechanic.


Main Jet - 85
Pilot Jet - 30
Jet Needle 6dh7
Needle Jet - 159 P5
Air Jet - 2.0
Slide - 3.0
c-clip set at the leanest position
Pod filters

They also advanced the timing to lean it out. (I am not to clear on what the effect of advancing the timing does)

With the original megaphone mufflers and spec plugs I was getting 12 miles to the gallon! You can imagine how smokey and smelly the bike was. Told this to the mechanic and he told me to put in hotter plugs. After I put in hotter plugs I was getting about 22 mpg. Still unacceptable and obviously not tuned. (oh yeah did I tell you how they wanted to solder my jets and drill them out because the right jets weren't "available" through his supplier!? Like he can't order the right jets from any number of places?)


So after paying this mechanic $300+ to not tune my bike properly I decided it was time for me to stop being shy about carb work.


Well I call Sudco who says my main jet is way out of range and sell me a 190 along with a Mikuni 35 pilot jet (the 30 I have doesn't have the official Mikuni stamp). The guy from Sudco also tells me to go to straight pipes to lean it out a little more.


Here is what I have currently:

Main Jet - 190
Pilot Jet - 35
Jet Needle 6dh7
Needle Jet - 159 P5
Air Jet - 2.0
Slide - 3.0
c-clip set at the leanest position
Pod filters
Straight pipes


I do all of this but the bike seems to be way to rich still. When I give it throttle to quickly it sounds like it is filling with gas and bogs down for a a few seconds even after I let off the throttle. It still smokes and the plugs are gunked with carbon. Obviously still to rich (considering the only thing I did to lean it out was straight pipes and everything else richened it... not to surprising).


Anyways, I figured I would go back to the 30 pilot jet, and it still bogs down, so now I am thinking the main jet is to big (because besides that, the only other thing different is the pipes). But what confuses me is that it bogs down at 1/8 throttle and up, way to low for the main jet to matter.


Are these carbs just to big for a cb350? Does anyone have experience tuning these carbs for a 350 or similar bike?


I could sure use some help.


Thanks

What mods does the engine have?
search around here its been talked about alot.

32mm are WAY WAY to big for a stock bike and will never run right. even 30mm is to big for a stocker. really the weak link in the motor isnt the carbs.

These retailers that are selling these carbs really should do a better job of telling people what they are for.

Also straight pipes? i assume just the stock headers with the mufflers cut off?
that also will not help tunning.
and if you go back to stock stay away from pods....they suck...

Your description is classic symptom of bad airflow.
3.0 slide cutaway should make off idle lean, until airflow catches up with throttle position (motor can cut out)
190 main is also way too big for 350.
Sudco should know that?
They are only telling you what stock jetting for that carb is
If your not racing and don't have a modified motor, you WILL be far better off with 28 or 30mm carbs

surffly said:

What mods does the engine have?
search around here its been talked about alot.

32mm are WAY WAY to big for a stock bike and will never run right. even 30mm is to big for a stocker. really the weak link in the motor isnt the carbs.

These retailers that are selling these carbs really should do a better job of telling people what they are for.

Also straight pipes? i assume just the stock headers with the mufflers cut off?
that also will not help tunning.
and if you go back to stock stay away from pods....they suck...

Click to expand...

No mods to the engine that I know of but I have noticed my 73 CB350g has considerably more power than my buddies 69CB350.


These carbs came with the bike but I understand what you are saying about retailers. If you do a search for cb350 carbs all you find are these Mikuni 32mm.


Thanks for the advice I will be going back to stock carbs. Yeah, stock headers no mufflers.

crazypj said:

Your description is classic symptom of bad airflow.
3.0 slide cutaway should make off idle lean, until airflow catches up with throttle position (motor can cut out)
190 main is also way too big for 350.
Sudco should know that?
They are only telling you what stock jetting for that carb is
If your not racing and don't have a modified motor, you WILL be far better off with 28 or 30mm carbs

Click to expand...

A 3.0 cutaway is big right? Meaning there should be good airflow and subsequently a leaner air/fuel mix?
Those guys at Sudco were not very helpful, the kept saying their jetting was proprietary for business purposes, as if I am going to drop $300 to buy a new set of carbs from them. Well it sounds like the 32mm's are just too big. Thanks for the help.

Somewhat off topic, but your friends 69 Type One motor should have a little more pep in stock trim than your 73. He may want to dig into that.

3.0 slide is too lean and so is the 159 series needle jet. They are designed for two strokes and lean out the mixture as it comes off idle to around 1/2 open. By then the too large main jet is starting to have an effect.

As PJ mentioned, the carbs are a bit large for a stock street CB350. Grown men have been reduced to tears trying to jet them.

I'd start with the 30 pilot jets and adjust the slow running first. If the air screws are adjusted less than say 1 turn to make it run at idle, go up to 35s. If the air screws are more than say 2 turns out, go down another size.

Once it idles, you need to get the main jet close to right for all revs. The quickest and easiest way is on a dyno with gas analysis. If all your local dyno guy has is Air:fuel, that will have to do, but you really want 5 gas analysis. Then start to play with teh 1/4 to 3/4 throttle settings with needles and needle jets.

My preference is 176 series needle jets in P-6 which is as large as they come.

I'm guessing here, but start with say a 160 main jet. If you do it on the dyno, they dyno guys should have a big box of jets and you should only have to pay for the set that it leaves the building with. If they don;t ave jets, you will have to take a selection along.

Teazer, I'm going to look for those cam caps right now so I don't forget again :

Thanks Teazer.


I have the air screw 3 turns out on the 30 to get it idling, so I may move down a step or two. I know you were are just guessing with the main jet but any sort of ball park helps to someone with zero experience such as me.


Any suggestion for the slide size? Also, have you tuned these for a similar size bike?


One last question. If I understand this correctly, adjusting the float height can change the fuel/air mixture for mostly the mid range, but if the tolerance for these carbs is 24mm - 22mm shouldn't I adjust to 23mm and tune around it? In other words, should I be adjusting the floats to tune or should I just put them in range and concentrate on the jet, needles and slides?


I hate defeat but it seems like all signs point to using smaller carbs.

I agree about the carbs being too big. To put it in perspective, a lot of guys put vm34's on xs650's. So you have a 34mm carb feeding a (roughly) 325cc cylinder. And being that a CB350 is actually about 325cc's total, a smaller carb just kinda makes sense. Also, a smaller carb actually increases velocity into the combustion chamber.

Looks like that was jetting for a Race 350 motor?
http://www.eurospares.com/engine.htm

It's for a motor operating around redline pretty much allthe time.
I know one person who tried it
He was so far off the pace it was embarrassing (almost stock motor, he needed it to pull 'clean' lower in rpm range and it wouldn't)
Changing float level adjusts fuel 'everywhere'
set it at 24mm to lean out mid range and top end, you can always jet up slightly for max rpm and drop needle to lean mid range
I would use a 2.0 slide, but, you will have to experiment (and that gets expensive - fast)

3.0 is what they supply and it make the bike really lean as the throttle is opened, so it drops in a hole. Try a pair of 2.0 slides.

The reason they use 3.0 is an attempt to lean out the excess fuel from using a 159 series needle jet. That sorta works but still leaves a hole off idle where you need it to be clean on the street.

It will never take a big handful of throttle with carbs that large and slide carbs never like to be opened too fast at the best of times.

Teazer, I found the GSX cam caps, still looking for GS : (I could have probably made some quicker than this ;D )

PJ, it's for a 16 valve 1100 motor so that's a GS in the US and GSX in the ROW yes?

Yep, never could find out why they kept the GS designation in US?
the 'X' was for 4 valve/cyl everywhere but here ? ? ?
Should have said GSX SACS motor :-[

Hate to eavesdrop, but what are you two talking about? Hopefully putting a GSX head on a CB motor to help it breath?

Sorry for the threadjack Rich. PJ offered a set of cam caps for a GS1100E that I'm helping a buddy with. he managed to destroy all the caps on the intake cams (C and D IIRC).