CB350 to cafe' from scratch by a (not so much) teen

Alon said:
Can I grind off the rocker arm instead?
Is that something I can do by myself?
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No, this is a precision adjustment required and the valves are, as you'd expect, hardened steel so it has to be done with the proper equipment to both get the right results and for the modification to be reliable. Remember, the rocker arm pushes the tip of the valve thousands of times a minute, each and every minute the engine is running. If not done properly it can lead to excess clearance that you won't be able to adjust (beyond adjustment range, exact opposite problem you have right now) and potential early wear and failure of the tip of the valve stem.
 
The end of the valve stem or the tip needs to be ground off not the rocker arm. I personally would not try to grind it myself unless you have precision machining at your disposal.

Here is a you tube video of a machinist measuring and grinding off the tips. I haven't watched the entire video but it looks like he know what he is doing.

 
Currently, I set the valve at the closest I could get to the measurement.
I took it out for a ride, and it's kind of acting up. It's sputtering on low throttle and the 4-5k rpms. I had to change the plugs twice on the same day. I feel like it's getting too much fuel and burning off my spark plugs. I have Mikuni VM28s from niche cycle, and when purchased, I was told they were dynoed beforehand. The spark plugs come out with a lot of carbon buildup. What should be my next move? I thought maybe checking the float height.
 
Alon said:
Currently, I set the valve at the closest I could get to the measurement.
I took it out for a ride, and it's kind of acting up. It's sputtering on low throttle and the 4-5k rpms. I had to change the plugs twice on the same day. I feel like it's getting too much fuel and burning off my spark plugs. I have Mikuni VM28s from niche cycle, and when purchased, I was told they were dynoed beforehand. The spark plugs come out with a lot of carbon buildup. What should be my next move? I thought maybe checking the float height.
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I just read the ad for their 28mm Mikuni set for the 350 and it seems, if it is to be believed, that they dialed in the carbs in multiple ways that most don't. But, there's never a guarantee that pre-jetted carbs are right for your particular engine based on what the setup was. I can't recall, are you using the air filters sent with the carbs? IIRC they're of the pod variety which usually means pretty high air flow so if true, those carbs must be jetted richer than your bike needs at this point but it also depends on how you're riding it currently. Also IIRC (this thread has gone on for a while), your engine's top end was rebuilt too so maybe you're not running it too hard right now as well.
 
You might want to read the tuning guide from Mikuni, which is available on their website.

The float height for the Mikuni carbs is not related to the float height from the FSM for the original Keihin carburetors, so I would advise leaving that alone until you've made other attempts to tune via the air screw, idle adjustment, and/or small changes to the jetting. It would be worth documenting the current pilot and main jets as well as the current number of turns out on the air screw. How is the idle adjuster screw set currently? You can watch the slide lower as the idle screw is turned out and it would be useful to document the position in terms of the number of turns out before the slide bottoms out. This will allow you to return to the same starting point after fiddling around.
 
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ancientdad said:
I just read the ad for their 28mm Mikuni set for the 350 and it seems, if it is to be believed, that they dialed in the carbs in multiple ways that most don't. But, there's never a guarantee that pre-jetted carbs are right for your particular engine based on what the setup was. I can't recall, are you using the air filters sent with the carbs? IIRC they're of the pod variety which usually means pretty high air flow so if true, those carbs must be jetted richer than your bike needs at this point but it also depends on how you're riding it currently. Also IIRC (this thread has gone on for a while), your engine's top end was rebuilt too so maybe you're not running it too hard right now as well.
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What do you mean by not running it too hard?
 
I can also note that the carbs drip fuel on a very slight tilt to the left. I have the bike almost straight on the side stand with some blocks of wood under it. But once it has a little more angle then they constantly drip
 
Alon said:
I can also note that the carbs drip fuel on a very slight tilt to the left. I have the bike almost straight on the side stand with some blocks of wood under it. But once it has a little more angle then they constantly drip
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The manual I linked has information about the float height, which seems to be 15-17 mm for the VM28 carbs. It's possible that yours are set too low right now.

I believe AD was referring to the RPM range in which you ride. Often people keep RPMs lower after a rebuild and slowly increase the range as things are broken in. Riding at low RPM can mean a less efficient combustion and hence foul plugs more rapidly, depending on the specifics of the setup.
 
Now I started with 0.5 a turn out on both carbs for the mixture screw. I turned it to 1.25 turns and turned the idle screws to get the rpm at around 1300.
After that I set the air mixture screws to 2 turns out. It’s idling but I’m getting some backfire.
Here’s a link to a video:
 
Regarding the main jet and pilot jet, I’m not sure what their sizes are. I left the ones that I got. I also got some extras for changing sizes with the kit.
 
Forgive me for the ugly taillight 😅 I just put it on for it to get licensed.
 
Alon said:
Now I started with 0.5 a turn out on both carbs for the mixture screw. I turned it to 1.25 turns and turned the idle screws to get the rpm at around 1300.
After that I set the air mixture screws to 2 turns out. It’s idling but I’m getting some backfire.
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That's not backfiring, that's cylinders not synced at idle. The one is running "faster" than the other, dragging the cylinder with the lower idle speed adjustment along with it. With the engine off and the pods removed, you can use something under the cutaway in the slides to get them the same height with the idle speed screws, then sort out the mixture screws after it's fully running on both.
 
I've posted multiple times about Mikuni VM, they are set up for a two stroke motor so get richer at high airflow, it isn't possible to make them work 100% with the brass they have.
The needle jet is the main problem, then the slide cut-away.
You need a four-stroke needle jet, easiest way to get then is look for Yamaha RD350/400 parts which are available but not cheap.
What you have is the type one in picture below, it's called a primary choke the 'dam' creates lower pressure so richer mixture that air cooled two strokes need
It's from an early 70's carb manual to teach techs but is pretty much ignored by everyone and deliberately lied about by people selling Mikuni VM (including SUDCO, the American importers)
The carbs can be made to work pretty good but never as perfect as they should work without buying a lot of parts.
I bought new slides for the pair I used on my Yamaha XS650 (bored to 800cc) and made emulsion tubes.
I can't make any more because precision drill is broken, the cross drillings are tiny and difficult to do (0.80mm)
Oh, almost forgot, the Japanese to English translation may be difficult to understand, 'revving type engine' means two stroke and 'piston valve slogging type' is four stroke or two strokes with rotary valve (positive opening and closing or Detroit diesel type two stroke)
primarychoke1jpg.jpg

Primarychokeandairbleedneedlejets.jpg
 
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Yep, I've done this twice using my backyard technology. I know how much money you can throw at tuning these carbs and back when I was running my first 450 drag bike I had a hell of a time getting it jetted until I simply ground off the half-pipe on the needle jet to make #1 look like #2. Never looked into the emulsion holes because I'm not very carb-smart, but it works. Did the same thing on my red 450 and it carburetes nicely for Mikunis on a 4 stroke, still a tad rich but doesn't foul plugs and I'm lazy when it comes to carbs.
 
Over the years I've tried lot of different things with 'primary choke' carbs on 350/360 (and even 250's, both variants)
Shortened the 'hood, removed it, drilled holes in it.
It all 'works' to lean things out but isn't really a great option.
When used good condition AMAL's were available (usually off crashed Bonneville's) they were pretty much a direct replacement (after you made manifold adapters)
Mikuni's off crashed Yamaha's and Suzuki's were much easier to get hold of though, even back in 70's
 
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