CB360 Mikuni 32 VM tuning

[Si123]

I have a 1974 CB360, the original carbs diaphragms were shot, so I invested in some Mikunis. After doing a little more research I wish I would have went with some 30 VMs. It seems as if it is running way to lean right now. It has a 190 main jet, and a 35 pilot jet. If it is backfiring into the air filter does that indicate lean? It also seems to take a little bit to idle down after you rev it up. I took the filters off and used my friends and I hands to restrict some air and it seem to run pretty good. It idles but if you give it too much throttle it dies. I found one guy with the same carbs and bike and he switch to a 150 main, and a 30 pilot. So I went out and found the 150 mains and when I put them in it seem to run way way lean.

 

[crazypj]

It should be real rich with 190 mains.
They are sealed up properly? (sounds like you have an air leak?
What other 'modifications' have you done?
Is the ignition timing set properly?
Does the advancer work?

 

[Si123]

I haven't done any other modifications.

I checked the timing once but i'll check it again.

How do you check the spark advance?

I have a feeling I have a leak somewhere. It's kinda difficult to get the POD filters on, so there is some stress on the carb boots. I'll try moving the battery out of the way today.

I bought some 150 main jets, do you think they should be more appropriate?

Do you think a 35 pilot is too big? I had a place order some 30s

 

[skatetheame]

The spark advance is behind the points plate. It just slides off the cam. If it doesn't spring back to fully closed take it apart and clean it, and grease it up.

 

[Si123]

It springs back.

I think I figured out my problem though te carb farthest from the pet cock isn't getting very much fuel so it's running Way lean. Fuel flows out of both ends of the pet cock nicely so im thinking it has something to do with my plumbing. These are top fed carbs the originals Were bottom fed. Should I run the line for the right one under the left?

 

[teazer]

Should make no difference. Remove the float bowl drain screws (the big brass 17mm nut) and turn on the gas - one carb at a time with a container to catch fuel.

Do they both flow the same? No need to measure actual flow rates - it's fairly obvious if it's OK or not.

 

[Si123]

I feel stupid one of my inline filters was slightly clogged and I think it was sucking air through the line. I took the filters off and filled the line with gas and it ran okay. Still way to rich, I can actually see the gas being sucked through the line fairly quickly when I rev it. I'm hoping going from a 35 pilot to a 30 will help. I'm still thinking the 150 mains are a little much though. Man I wish I would of done some more research before buying the 32s. I still have the stock keihins but the rubber diaphragm things are bad. I guess its a good excuse to eventually invest in better exhaust.


What kind of fuel line would you recommend the clear stuff or the thick rubber stuff?

 

[crazypj]

Clear fuel line goes 'yellow' pretty quick and hardens with almost all fuel (even quicker withE10)
Pretty sure you will be taking tank off multiple times?
I normally use grey fuel line.
Quite a bit more expensive but stays flexible.
Personally, I don't like the 'clear' (didn't like it 35 yrs ago, don't like it today)
You can get new diaphragms for $19.00 each with fitting instructions
Should be multiple links as a few people have switched them out by now

 

[Greenbush]

NEED HELP!!! I have a set of VM32s I'm trying to tune to my CJ360T Tracker. I kinda jumped before researching. Yeah, the 32s can be a bit large. But I got the pair for $110, nearly brand new.
I'm having a horrible time trying to tune these to my bike. I'm running pods and a stubby pipe coming off the stock 2 into 1 exhaust headers. I was given these these as the in the ball park starting point.
Pilot 30-35
Slide 2.5-3
Needle Jet P4
Needle 6DP17
Main Jet 180-2002

If these are off, please drop your 2 cents.

Now, I used this and was able to get her to idle well. But when I went to ride her there was no off the line power. I ended up having to run the bike with the Choke circuits on to get some balls in low mid to top. Then I tried blocking air flow a bit. But really not too successful.
I tried to adjust thinking the it needed more fuel. But now it's all messed up. I can't get any off idle rev. I've lowered and raised the Pilot jet size and the needle height.
As much research as I could do all I found was a setting for a CB360 with a set of VM30s, all the VM32s didn't list all the different jets.
Here is the VM30 settings; Pilot 25, Needle Jet P5; Needle 6F4 and Main 200.
This is what is in the carbs now; Pilot 25; Needle Jet P4, Needle 6DP17, Main 180 and Needle clip on the lowest(needle up) slot.
Having trouble getting her to rev at all.

Anyone have experience in this area?

 

[Sonreir]

The VM32s are WAY too big. You're probably never going to get them to work on the street.

Some racers use them, but they don't care about idling.

You stand a chance of getting these to work if you raise your float height up to 20mm or so and up the main jet to 200. It's gonna run like shit though. You're just using the wrong part for the job. I'm not trying to be a dick, but I think you should just sell them and get some VM28s or VM30s.

 

[crazypj]

Drop needle to 2nd groove from top.
It may help.

 

[Greenbush]

Drop needle to 2nd groove from top.
It may help.

Thank you guys. I wish I had the funds to switch out carb set to 28-30s. Tight on play time funds. Missed the group buy.

PJ- I've lowered it, raised it..... My next suggested step(from an Old Timer CB350 Racer) is to get a p-5 or p-6 Needle Jet. I really can't get the bike
to rev. I can't figure if it's too much air or gas. I've been told that tuning these have made grown men cry. Sold off my very clean CV stock set. Short of buying a smaller set, I think I may try the larger NJ and some type of velocity
tubes. I have some nice velocity stacks but they're a bit small to fit on the outside. Obviously this is playing around but what about fitting velocity stacks down the throats to cut down on the bell size of the intake side. I'll pause for laughter.....It's ok....... But stop for a sec and ponder it. Say an insert that fits tight inside just before the pilot and choke jet ports. The mouth would then wrap around the carb inlet mouth. The rubber boot would then slide over and past it to clamp down. So in effect it would be cutting down on ease of air flow.
Ok, after you guys stop laughing and choking on your beers, tell me what you think.

Here's the bike. You can see the stubby pipe. I put a small baffle insert in it. Pretty free flowing.

 

[teazer]

It took me a moment to realize we changed bikes in this thread. OK. So this one with the tracker seat and a short stubby pipe is the one we're talking about. OK. Fist off, what's in that pipe? That's the first potential issue in terms of messed up jetting.

How did it run before the carb swap? Same problems, different problems, don't remember? What have you done to check all the usual starting points like valves, ignition timing, compression etc? We could spend a long time chasing jet issues only to find that the thing has no compression, that's why we need to know a bit more history.

Are the new inlet mounts sealed or are they too tight and bowed?

Is the tank flowing enough gas to both carbs? Ball park is close enough.

Let's get that squared away and then we can focus in on jetting one circuit at a time. First is idle. It has to idle.

 

[Greenbush]

It took me a moment to realize we changed bikes in this thread. OK. So this one with the tracker seat and a short stubby pipe is the one we're talking about. OK. Fist off, what's in that pipe? That's the first potential issue in terms of messed up jetting.

How did it run before the carb swap? Same problems, different problems, don't remember? What have you done to check all the usual starting points like valves, ignition timing, compression etc? We could spend a long time chasing jet issues only to find that the thing has no compression, that's why we need to know a bit more history.

Are the new inlet mounts sealed or are they too tight and bowed?

Is the tank flowing enough gas to both carbs? Ball park is close enough.

Let's get that squared away and then we can focus in on jetting one circuit at a time. First is idle. It has to idle.

The bike and motor has only 7,200 Original miles. Solid, solid. Ran great with stock CM carbs. Just knew she had more potential. I checked all during build. Valve clearance, timing, compression.....
The stubby has a 5" baffle insert. Pretty loud still.
Insulators match up great and are in very good condition.
Brand new petcock. Lines are filled with solid fuel.
When I first started with Pilot 30, Needle J P-4, Needle 6DP17, Main 190 it idled and reved well. But it was when I went to ride her that the power was not there.
Now, I became busy with a life stuff that kept me from continuing at that time. So the work has been off and on.
But to get he to pull with some power I had to put both choke circuits on. So it would run like poop on the bottom end but when she got moving she would pull.

 

[Greenbush]

It took me a moment to realize we changed bikes in this thread. OK. So this one with the tracker seat and a short stubby pipe is the one we're talking about. OK. Fist off, what's in that pipe? That's the first potential issue in terms of messed up jetting.

How did it run before the carb swap? Same problems, different problems, don't remember? What have you done to check all the usual starting points like valves, ignition timing, compression etc? We could spend a long time chasing jet issues only to find that the thing has no compression, that's why we need to know a bit more history.

Are the new inlet mounts sealed or are they too tight and bowed?

Is the tank flowing enough gas to both carbs? Ball park is close enough.

Let's get that squared away and then we can focus in on jetting one circuit at a time. First is idle. It has to idle.

PS, I think I have her idling well.

 

[crazypj]

That's a CJ not a CB
You would be far better off with modified CV carbs.
Major issue with the VM is they are jetted for a two-stroke and have the 'wrong type' of needle jet

You have the top type and need the lower type.
You can get something from Mikes XS to fit or use Sudco.
Yamaha RD350 used '4 stroke' emulsion tube/needle jet ( I gthink the Banshee 350 does as well?)
I've used RD carbs on my 'modified' CJ250 and bike ran fantastic (they are only 26~28mm)
Scroll down, there are two different emulsion tubes listed (listed as needle jets)
I've got a couple of 366-Y0 but I'm using a pair I made to get a different fuel flow
http://www.mikesxs.net/products-37.html#products

 

[Greenbush]

That's a CJ not a CB
You would be far better off with modified CV carbs.
Major issue with the VM is they are jetted for a two-stroke and have the 'wrong type' of needle jet

You have the top type and need the lower type.
You can get something from Mikes XS to fit or use Sudco.
Yamaha RD350 used '4 stroke' emulsion tube/needle jet ( I gthink the Banshee 350 does as well?)
I've used RD carbs on my 'modified' CJ250 and bike ran fantastic (they are only 26~28mm)
Scroll down, there are two different emulsion tubes listed (listed as needle jets)
I've got a couple of 366-Y0 but I'm using a pair I made to get a different fuel flow
http://www.mikesxs.net/products-37.html#products

I know it's a CJ. I have been told that the CJ is the same as the CB360, minus starter and dif gears.

HOLY CRAP. Really? I don't need the 159 NJ style? These carbs came off a Bonnie that had been ran over.
They came from CPW. I was told by an Old School Racer of CB350s and 360s that I should use the 159 NJ style.
Looking at the schematic it would make since that when I put the chokes on that it ran a bit better. Seems that the other style gets more fuel with the bypass circuit, per the arrows.

What is the other style's model number?

 

[teazer]

Everyone uses 159 series NJ and in some cases they have to because the "right" jet isn't available in larger bores.

You want a 175 series. 2 stroke (primary) needle jets are richer at the top end and lean out at the bottom end to compensate for the fact that piston port 2 smokes have a real problem with reversion at low revs. Bleed types are not. So if you use a 2 smoke NJ on a 4 stroke it tends to be lean at the bottom end or if that's correct, it's rich at the top end.

If it runs better with choke that's making it richer, so it must be lean without choke. That could be the needle jet (follows the logic). It would be fine at idle and then too lean up to about 1/2 throttle or more and after that it might come good again.

#175 series were fitted to RD400C&D models as PJ pointed out. RD400 were P-2 according to my notes and RD350 were O-8

 

[Greenbush]

Everyone uses 159 series NJ and in some cases they have to because the "right" jet isn't available in larger bores.

You want a 175 series. 2 stroke (primary) needle jets are richer at the top end and lean out at the bottom end to compensate for the fact that piston port 2 smokes have a real problem with reversion at low revs. Bleed types are not. So if you use a 2 smoke NJ on a 4 stroke it tends to be lean at the bottom end or if that's correct, it's rich at the top end.

If it runs better with choke that's making it richer, so it must be lean without choke. That could be the needle jet (follows the logic). It would be fine at idle and then too lean up to about 1/2 throttle or more and after that it might come good again.

#175 series were fitted to RD400C&D models as PJ pointed out. RD400 were P-2 according to my notes and RD350 were O-8

WOOOHOOOO. Getting somewhere with LOGIC. Speaking my language. OKAY, so what size 175 NJ do we think I should get? I feel richer side is better, for safety reasons, right? Can always pop in a hotter plug.

 

[crazypj]

Quick and dirty.
Drill a1/16" hole through the 'hood' sticking up
It changes flow rates
It's possible to make the 'primary choke' needle jet work better than it does now but will never be as good as the 'correct' needle jet/emulsion tube