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Discussion Starter #1
I know this subject has been beat to death but I need help with 30mm Mikuni carbs on my cl350. I know basically how the jetting should be done. Short history: got the carbs jetted like this: needle jet 159P5, jet needle 6F5 clip in center slot, slide 2.5, main jet 135, secondary jet 27.5, air jet 2.0. Bike is stock bore, electronic ign, cl pipes with shorty open mufflers, pod filters. When I put the carbs on, the bike would not run on main jet. Jetted up in steps to 180 mains and now is a little rich on top end but running pretty well, however it is missing and running lousy right off idle to about 1/3 throttle. I have raised the needle and changed the secondary jets in steps all the way up to 40's and still runs lousy (cutting out and spitting). I don't know what else to do except changing the needle jet and needle, but I don't have cubic dollars to throw at this project and would appreciate any advice.
 

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did you do a compression test? You can't make the carbs work if the pistons and valves don't want to play, compression test is the quickest easiest way to see if things are behaving as they should inside the motor.
 

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Discussion Starter #3
Yea, I put new rings in to start with along with a new timing chain set up. The compression is about 180 on right side and 175 on left side but I am about ready to believe the trouble is somewhere else but don 't know where it could be.
 

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does it still have points ignition? "cutting out and spitting" sounds spark related. Points and condenser are highly suspect if the firing is hit and miss.
... trust you already realize CV carbs and pod filters is a poor combination with vacuum operated carbs. No mention of year model so I assume you have CV carbs as most do.

Good battery is also very important on a battery/coil ignition, others here might chime in on that.
Spark plugs, ignition wires and plug cap, all good? I've bought brand new plugs in packs of 2 where 1 was DOA, crack in a plug insulator can sometimes result in the spark intermittently happening up inside instead of at the gap, that will also make the bike sputter and fart like crazy. Water in fuel also produces similar symptoms.
 

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Rule out all those things and you might simply be experiencing CV carb combined with shorty open muffler & pod filter syndrome.
Did it run well with the stock air box and an exhaust system that has some reasonable back pressure in place, or was that ever an option?
 

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Discussion Starter #6
Mikuni carbs on CL350

The carbs are Mikuni 30mm and ignition is electronic. What baffles me is the trouble is only in the lower throttle position. Most mechanical troubles would not occur only in this range. Bike runs very well at high RPM's.
 

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The carbs are Mikuni 30mm and ignition is electronic. What baffles me is the trouble is only in the lower throttle position. Most mechanical troubles would not occur only in this range. Bike runs very well at high RPM's.

You say Mikuni carbs like that is supposed to mean a type. Mikuni is a brand, not a type of carb. They made several types of carbs. BS (CV), or VM (mechanical slide), etc.. What type do you have.
 

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put some cb750ko carbs on it and it will run perfect straight away
that said you do need to make sure all is well with the ignition
what brand e ign do you have
process of elimination dictates testing with points ignition back on the system because they work perfect without issue this is a known
the mikuni vm carb while it can be set up to run quite well at wot and higher rrevs
well it was never designed as a four stroke carb and it is simply impossible to get them to work anywhere near as well as an oval venturi keihin as far as ridability and throttle response
what kind of exhaust have you got on that turd
you need loooong pipes baby shorties will only work on a nitro motor running wot all the way up the hill
short pipes rob every bit of exhaust scavanging untill up near redline and then it doesn't loose as much but short pipes create the same effect as having too much back pressure
 

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Discussion Starter #9
Okay. VM carbs, and I have used this muffler set up on CL pipes before with stock CV carbs and they work great. No reason they shouldn't work with the MIKUNI CARBS.
 

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Okay. VM carbs, and I have used this muffler set up on CL pipes before with stock CV carbs and they work great. No reason they shouldn't work with the MIKUNI CARBS.
In very general terms, VM is good, CV can get evil without an air box ymmv.

? maybe a problem in one of the float bowls ? if fuel level is off the venturi effect don't work so great.
... that would do the same symptoms you described.
? fuel level too high maybe, on Some carbs you can accidentally install floats upside down and it's a bitch to spot that.
 

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here ya go you must have the bleed type needle jet for good response
if it was me i would copy this entire setup
including the 2.5 cutaway slide
your welcome aLL I DID WEAS A GOOGLE SEARCH
https://www.caferacer.net/forum/technical/15910-cb350-mikuni-vm30-jetting-results.html
I EVEN COPPIED THE TEXT FOR YOU OUT OF THIS LINK
Well it's been a long road, but well worth it. This old girl is finally running great after 4 weeks of testing, tuning, and waiting to receive jets a few times over. I've got some jetting numbers to share to feed the hungry baby birds out there who seek to run a bleed type needle jet. Keep in mind, these numbers work for MY bike in MY conditions. Perhaps it will get you in the ballpark and save you some cash.

Bike:
1972 Honda CB350 twin
Generic 12" shorty mufflers, freshly repacked, with additional baffle inserted before muffler</u> for a bit of backpressure and less raspy pop
NEW Mikuni VM30's and K&N pod filters
Fresh points at proper gap, condenser, plugs, valves checked, correct timing, clean and smooth timing advancer, good compression
87 octane fuel w/10% ethanol


Conditions:
Typical hot summer, sea level.

Jetting:
Main Jet:Large Hex 200
Emulsion tube/Needle jet: BLEED TYPE 176 series P-8
Slide: 2.5
Pilot jet:30
Needle: 5F3, lowest (richest) clip setting
Air Jet: 0.5
Air Screw: 2.0 turns out

So far I've got a few hours of spirited riding on this combo and it feels great from idle to redline at all throttle positions. Plug has a slight cardboard color so far and I'll keep an eye on it. The bike can idle for an extended period and still have a clean off-idle response. I'll eventually try an even richer needle to get the clip settings closer to the middle, but let's enjoy it for now. Hope this helps.




 

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... if it was ignition timing related and everything seems perfect I'd retard the ignition to make it work better at low speed, advance it for wot.
But thats usually just a performance rule of thumb thing, not really a trouble-shoot :|

Just throwing it out there, possibly you already tried all these things.
 

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ONE VARIABLE IS THE CAM INVOLVED ALTHOUGH IT WONT MAKE MUCH DIFFERENCE IN THE JETTING
1971 or so honda ditched the higher revving cam for a slightly milder grind to make the bikes more easy to lug down like a harley
for sure 1970 or earlier is what is called the fat cam which is only fatter in the way the casting has a thicker center section
the cam lobes are actually generated from a significantly smaller dia base circle and hence the need for matching rocker arms with more reach
i am using the fat cam grind and love it,it has lots of torque from 4000-on up but i turned 2.2 lbs of iron of the crankshaft
 

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Discussion Starter #15
Well the float arms are level in both carbs. That is the way they came. I don't have any measurements to check them with, but I suspect they are right.
 

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Agree that BLEED type needle jets are better on a 4 stroke for two reasons. First is that they flow less fuel than the same sized primary type and as revs rise, that difference gets larger, so that leans things out and secondly a BLEED type does a better job of atomizing fuel droplets so they burn better.
 

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i use a clear hose attached to the float bowel drain to observe the actual level
in fact i ride with them on the carbs
i route the hose up the side of the carb u bend it straight down so water cant get in
makes for a quick diafgnosis check if out riding and having fueling issues
on the vm mikuni as well as the keihins you want the level to be at least 6mm below gasket
if you have a pamco in there yank it off and put the points back in you have to or you could go mad chasing a carb problem that is really an ignition issue
the pamco is good unit but they fry easy from simple little wiring snafus
 

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... quick and easy test, elevate your front wheel.

needle and seat getting stuck closed is another possibility to watch for.
stuck open would normally have it puking fuel out of the overflow
 

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Discussion Starter #19
Well I appreciate all the help. I did copy all the jetting info and will start over from there and yes it is a PAMCO ign which I will remove and put the original points ign back in.
 

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Pamco says: "You must use 5 ohm resistor type spark plug caps or resistor type plugs (not both)" ... whats the resistance between your coil and plug now?
 
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