Not-Stock CB350 Jetting on Stock Carbs

First post, Google indicates there are folks with knowledge around here, so I thought I'd try.

I've got a '73 CB350 which I've recently rebuilt after an oil starvation incident. Clutch pushrod seal popped out, crankcase emptied on the interstate and the right side cam bushing welded itself to the cam. Decide if I was going to rebuild, I'd try to make it faster as I went. (Here's some unnecessary back story if you're interested: http://advrider.com/index.php?threads/cb350-rebuild-advice-needed.1061807/ )

In any case, I've now got 65mm pistons, Megacycle 12320 cam, a mild port/polish, and the fancy kibblewhite valves and springs, all from Bore-Tech. The bike is back together and running, but poorly. Sadly, I'm not sure it's possible to get it running well with stock carbs. No matter what I do, the bike runs crazy rich now. I'm guessing this is from the increased air velocity from the cam/valves/porting My stock CV carbs are in perfect shape, and were working perfectly prior to the rebuild. I'm using a #35 slow jet, #68 primary jet, and #100 secondary. I cannot find anything smaller online.

Admittedly, I'm at 9,200 feet, which is only going to make things worse. But with a stock setup things were happy.

Now I can only get it to idle with the fuel screw all the way in. Thinking I could maybe force it to run leaner by messing with the float level, I dropped the float from the stock 26mm to 30mm. That did allow me to move the fuel screw out 1 turn for optimum idle, but sadly I can't get it to run very well otherwise. Max power seems to be at about 4,500 rpm at 1/2 throttle. More throttle or higher RPM and it just bogs down. Plugs are soot-covered. I can get it up to about 50mph with some careful clutch manipulation and throttle trickery, but it's not happy.

So, what do I do? I can think of 2 options:
1) find someone who can weld jets and drill them back out smaller. Any suggestions?
2) Install Mikuni's like everyone else just so I can jet the thing properly.

#1 sounds miserable and may or may not work.

#2 will probably work, but seems like a stupid answer. I have a set of beautiful stock CV carbs, which are well designed for street use. The CV design is brilliant, and I'd love to keep using it, but I need smaller jets than I can find. I'd also really like to keep the bike looking stock with stock air filters and side covers. (I know, probably not much sympathy for that here...) I'm not building a cafe racer--I want it to look stock, just have a little bit more power.

Do I just need to switch to Mikunis like the rest of you guys? I'm trying hard to keep the bike stock-looking, and I hate to ditch some perfectly good stock carbs just because I can't find smaller jets. If I do need to switch to Mikunis, should I go with 30 or 32?

there is crazypj here,that really knows his stuff about the 3 jet keihins hopefully he will chime in
if it really comes down to smaller jets and they are indeed unavailable then it is possible to SOLDER them solid and redrill ,pretty simple really use solid solder and flux for copper plumbing pipe not wiring
and it will take a gentle deft touch to not fill the threads with solder rendering the jet useless
re-drilling on dead center is tricky, a lathe is best as spinning the jet, and not the drill is the easy way to get dead center
and the best rpm is very high for that tiny hole
you would want to size what you have,choose a selection of smaller bits, thats just logical i guess

the other thing are you sure the ignition is 100% ? did you set the full advance timing with a strobe and engine running ?

Timing looks good. Checked both sides with a strobe with it running. Really the only way to get it right, anyway. I only static time to get it close enough to run, then fix it up with the strobe later. Anyway, both sides are on the mark at idle, and the advance appears to be working correctly as well.

Also, awesome avatar. Ariel. I've got that whole shoot bookmarked.

HA hotred i call her i just posted a few things for you to ponder over at A D D re engine oiling

You need bigger carbs.

DohcBikes said:

You need bigger carbs.

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But... it's running rich. Bigger carbs? Not smaller?

the OP may be suggesting that with a fresh motor the gas velocity through the carbs would be higher and that would pull more fuel through the jets. Nence teh suggestion that it needs larger carbs. At least that's what I think was his logic.

I tend to think that since you are not having a problem at high revs but from the bottom end up, that it suggests a different problem and a different solution.

Assuming that ignition timing is correct, and I am not convinced of that just yet, there are several possibilities. I'd start by checking cam timing just to be sure.

After that I would check fuel level in the carbs - not float level ad set it at 3-4mm below the gasket surface and check that they are not flooding.

Then I would check all the air and fuel passages again in the carbs as well as the jets to determine what's going on. A trick I use is to use a can of carb cleaner to blow through a circuit in one carb and then repeat on the other carb to see if they are the same. Then repeat on every air or fuel circuit.

Then check carb synch to be 110% sure that both carbs open at exactly the same time. Then let us know what you find.

I'm very much having a problem at higher revs. Or I would be if it would rev that high. Idle is good, 2-3.5k is terrible, 4.5k is okay, but can't go higher than 5k.

Timing is good, or at least as good as its ever been. Don't know I can verify better than with a timing light on both sides.

Carbs are very good. Perfect before rebuild, but I went through them again. Everything looks good. The float levels were stock and correct, but I moved them lower to make it run leaner... It did, but not enough.

Can timing could be wrong. How can I verify without pulling the engine?

the timing card should have the spec of when intake or exhaust opens sometimes they consider open .010 or even a mm of lift off seat you would find this by observing a valve through the lash cap,with a dial indicator positioned correctly
ideally this is done after finding true tdc and with a degree wheel mounted to the crank at the mag rotor bolt

You need bigger carbs.

How big and which ones?

DohcBikes said:

You need bigger carbs.

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You may be right, but how do the issues at 2.5k up to 5k suggest he needs bigger carbs? Not sure I follow the logic. If the motor ran fine but lacked top end and felt strangled, I would get it, but at lower revs and smaller throttle openings, it shouldn't be an issue.

I'm not sure that it's even an air jet issue either. Is it possible that the jets have been opened up by a PO? Are the slides rising or is it possible that one or both have sticky slides or torn diaphragms?

I would have to research before suggesting a particular carb for your bike, whereas many others have probably been through this and should be making suggestions already. Honestly it sounds as though you already know you need different carbs but just don't want to accept it. Sounds like you've done some research.

Here is something that doesn't make sense to me as a hot rodder.

"I bought bigger pistons, better valves, a performance cam, paid for a port and polish, but I think I'll leave these stock turd carbs on it."

No offense but I just don't get it.

No, your carbs aren't jetted correctly right now, but there's no point to jetting carbs that are just too small for your application. Some here may be attempting to expose other issues and that's great advice. However, your initial post may deter many from suggesting larger carbs. I am attempting to save you the time and hassle of tuning or buying parts for carbs that in the long run aren't correct anyway.

Yes, a bigger carb with the same jetting will generally run the engine leaner. More air/same amount of fuel availability= leaner.

Mild port and polish eh? Does he have lots of experience with this particular engine? What were the flow numbers? He does have a flow bench right? It is easy to lose power when blindly porting.

You installed larger pistons, larger valves (which may or may not have hurt power on this engine) and a cam with higher lift and more duration than stock. You need more air available at any given throttle position. The higher revs is where this will be the most evident.

teazer said:

I'm not sure that it's even an air jet issue either. Is it possible that the jets have been opened up by a PO? Are the slides rising or is it possible that one or both have sticky slides or torn diaphragms?

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I do not believe the jets were modified. These carbs worked perfectly on this bike until the rebuild. I completely disassembled and reassembled the carbs this weekend, and they look perfect. Diaphrams are intact, no holes.

I'm certainly willing to believe it's not a jetting issue--I just don't know where else to look. It's running crazy rich. I believe the reason it won't rev higher than 5k is because it's too rich....

DohcBikes said:

Here is something that doesn't make sense to me as a hot rodder.

"I bought bigger pistons, better valves, a performance cam, paid for a port and polish, but I think I'll leave these stock turd carbs on it."

No offense but I just don't get it.

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This is a very reasonable point. Part of it is that I (possibly mistakenly) don't believe the stock carbs are turds. They worked great on the bike when it was stock, and there are tons of people very unhappy with Mikunis on stock bikes. The CV design is clever and well-adapted for a street bike which is what this is. My goal in building this bike was simply: if I'm going to have to pull it apart and replace parts, I might as well replace parts with better ones. Thus, valves, pistons and cam.

No, your carbs aren't jetted correctly right now, but there's no point to jetting carbs that are just too small for your application. Some here may be attempting to expose other issues and that's great advice. However, your initial post may deter many from suggesting larger carbs. I am attempting to save you the time and hassle of tuning or buying parts for carbs that in the long run aren't correct anyway.

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And I appreciate that. If the stock carbs are absolutely not going to work with my setup, I'm perfectly willing to swap them out. If they're going to work with my setup but cost me 1% of total available power, I'm willing to live with that. I've been under the impression that for a street bike the stock carbs are the best bet. Maybe I'm just flat wrong about that and I'll be happy with Mikunis.

Mild port and polish eh? Does he have lots of experience with this particular engine? What were the flow numbers? He does have a flow bench right? It is easy to lose power when blindly porting.

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These guys: http://bore-tech.com/ did the port & polish. They have tons of experience with this engine and were highly recommended by everyone building CB350s. Bill said, "Minor porting on the head to reshape and smooth the ports should be all that is needed. The ports are plenty big enough as is and do not need to be enlarged." So I have no reason to think he made it worse.

You installed larger pistons, larger valves (which may or may not have hurt power on this engine) and a cam with higher lift and more duration than stock. You need more air available at any given throttle position. The higher revs is where this will be the most evident.

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Absolutely. I'd assumed that in drawing more air, the bike would tend to run lean. I'm rather surprised by the opposite, but apparently the increased velocity can cause this.

So, bigger carbs. Back to my original question: VM30 or VM32? Both are widely used on the CB350. The VM32 has a reputation for being impossible to tune on a stock CB350. But I don't have a stock CB350, and I haven't done some of the things that the folks who run VM32s tend to do. I do not have an all-out race cam, I do not have 12:1 pistons, and I do not have a modified exhaust. It also looks like the VM30 may be easier to mount. So, assuming that my carb is too small, which is the right carb and how do I figure it out?

Honestly I would ask your porter which carbs he thinks best, because he has experience with tuning these engines.

That being said, VM30s should be excellent with your application.

Remember that carb sizing is not the same from cv to mechanical slide carbs. For any given application it generally takes larger cv carbs than mechanical slides to get similar results.

if you can get the air fuel metering correct the stock carbs may very well be fine for street use, sure at wot and up near max revs you may loose a cupl hp over perfectly jetted afterrmarket carbs
the roadrace guys are running aroung at wot up near redline most all the time so yeah out there every bit of top end hp is needed
i suggest dont give up on the stock carbs till crazypj puts in his 2 cents as i know he has built an oversize big bore cb360 with stock keihin cv carbs and it runs really well
the thing about getting vm mikahoonies or any other aftermarket carbs is none of them are plug and play and you still have a bunch of test and tuning to get them right
and without accel pumps it can be difficult to get good throttle response i think its makes good sense to, if possible, dial in these carbs,ride the bike get used to it and check that cam timing make sure it is dead on the specs then if you feel like you want the extra pop go ahead and go with something in a 30mm slide valve
getting back to the cam timing you could be a bit off the spec that boretec wants it to be,but all aftermarket cams that i know of are designed to be "close" using the stock setting,that said it is worth the trouble to verify and even worth it to dial it in if needed
if you were a tooth off on the timing chain sprocket then in order for you to get the ignition timing on the marks the points backing plate would be pretty far out of rotation from where it normally has sat in the past.