CB350, Not Quite Perfect...

cjdisaffected

cjdisaffected

New Member
My 73CB350 just turned over 7k miles (1000 of which are mine). Overall runs pretty well, but it's still not perfect. Last weekend I had it out on a ride about 130 miles. No matter what I did, I couldn't get the bike to turn over 8k RPM in 5th gear, so I was topping out about 80MPH.

Additionally, when I really crank to WOT, sometimes I feel a little shudder. Motor is close to stock with shorty mufflers (stock airbox etc.) Carbs are rebuilt with stock 35/68/105 jets, slides were polished, etc. Compression is perfect, and while I have had some gremlins in the past it currently idles very well, and starts every time without any choke. Attached are some shots of the plugs.
 

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Probably need bigger secondary mains with the short exhaust.
May need new points?
Return (closing) springs go weak with age and allow them to bounce at less than 10,500~11,000rpm
 
Thanks PJ!

I'll hop up to 110s and see how it feels. I will take a look at the points but they have less than 1k miles on them so I assume (hope) they are all good.
 
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if it burbles/stutters then accelerates, it is too rich.

if it simply hesitates for a second then goes, it is too lean.

those plugs look pretty decent except the top one looks like it has some moist goo around the threads . . if so, it is burning oil . . if it is dry, then it is too rich.

install one size smaller pilot jets.

try 2 sizes down on the main . . if it gets worse, try to sizes up.

you might need to lower the needle 1 position.
 
Quick update, I went up to 110 and that exacerbated the condition. Some 100s are making their way towards me as we speak. Hopefully that will sort me out.
 
barnett468 said:
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if it burbles/stutters then accelerates, it is too rich.

if it simply hesitates for a second then goes, it is too lean.

those plugs look pretty decent except the top one looks like it has some moist goo around the threads . . if so, it is burning oil . . if it is dry, then it is too rich.

install one size smaller pilot jets.

try 2 sizes down on the main . . if it gets worse, try to sizes up.

you might need to lower the needle 1 position.
Click to expand...
Unfortunately the needles can't be lowered without machining throttle slides to get a lower seating position.
If it's good to 8K the pilot and primary main jet sound about right so smaller secondary jet will be correct action (particularly as you tried large and it made things worse ;) )
 
It may be worth checking, CB350 comes with either 38 or 35 pilot.
I've found the 360 only needs a 35 plus stock primary main (#68 in 350 and 360 but different attachment)
The rubber plugs over pilots are not missing?
 
For what it's worth, the stock jets for my my carb (Keihin 722) are 35/68/105. My current setup, which I mentioned is running very well is 35/68/100 all NOS Keihin jets.

I am kinda confused as to why I would size down my slow jet if I am not having any issues there. The only issue I was ever having was at WOT, which is only affected by the secondary main. I have had a Colortune on on the bike as well, and the burn at idle and just above is flawless.

barnett, you seemed concerned about starting without a choke, is that not a GOOD thing?

PJ, all the carb rubber (plugs, o-rings, etc.) is in place and brand new.

I would consider trying a #98 secondary, just for fun I suppose.
 
The primary main still supplies fuel all the way to max rpm.
The discharge will be through All the drilling's, (pilot and primary) giving a pretty good dispersal
W.O.T. is 'variable' when you have CV carbs ;)
 
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if it starts easily without a choke the pilot is too big, thats all i can tell you . . yoiur bike should not stat and idle perfectly without the choke, . . if it does it is rich irregardless of what the colortune says . . the colortune will not change much by going down in the pilot..

the pilot contributes to fuel sat wot also . . reducing the pilot size will reduce fuel at wot slightly.

going down 1 size on the pilot will not cause a hesitation . . there is no point in burning more gas than necessary . . it is a small change.
 
Reducing pilot jet size won't affect top end fuel delivery but will mess up starting, idle and pick-up as it's fed from primary main jet.
Both main jets determine total fuel discharge, the pilot is only an outlet connected to primary main
The rubber bung prevents pilot getting fuel direct from float chamber
Hope that makes sense?
 
That makes perfect sense. Thanks for the breakdown.

I guess I will look around for a #32 slow jet. That is not a size associated with the CB350 (only #35 and #38 were), so there is no Honda part number associated with it. I will have to track down another Keihin carb that uses the same slow jet but has #32 as an option.
 
Yes, if you can find one, I think 350 is easier to find primary jets for than 360 ;D
 
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for the second time, changing the pilot will change the amount of fuel delivery at ALL throttle positions and anyone one that says anything different has very little, to absolutely no idea, how a carburetor works and therefore should not be giving jetting advice.
 
Yeah PJ, I think it is :)

Barnett, believe me I understand the distinction (after the very clear explanations given by you and PJ) between changing the pilot and changing both of the mains.

That said, if the pilot size in question doesn't exist and for the sake of this discussion we'll say that I genuinely don't care if my bike is a tiny bit rich at idle as long as it's perfect from 1/2 to WOT, then I believe that correcting the size of the mains would address my specific concerns, wouldn't it?
 
CJDisaffected said:
Yeah PJ, I think it is :)

Barnett, believe me I understand the distinction (after the very clear explanations given by you and PJ) between changing the pilot and changing both of the mains.

That said, if the pilot size in question doesn't exist and for the sake of this discussion we'll say that I genuinely don't care if my bike is a tiny bit rich at idle as long as it's perfect from 1/2 to WOT, then I believe that correcting the size of the mains would address my specific concerns, wouldn't it?
Click to expand...

i am the type that likes things right so im a bit more anal than others, but if your bike runs perfectly off the bottom then obviously the jetting is close enough per se, and yes, jetting the mains will have far more affect on your problem.

the percentage of fuel the pilot adds decreases with greater throttle opening, however, there is no valve that shuts off the fuel from the pilot once the throttle is opened a certain amount, which is why it does add fuel at all throttle positions . . its just that the amount it adds at the greater throttle openings becomes less significant to wot tuning.

this being said, your bike will require more fuel when the weather gets cold, therefore, if you are a hair rich now, it will be closer to ideal in the winter.
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