Dominant Carb/Cylinder

[cgguy09]

Knights of the round table,
I'm noticing a condition with my 1971 cb350 (stock) Carbs. My left carb seems to be somewhat anemic, especially in the lower range, and especially on first start up. The right, however, is quite strong. Both Carbs have been rejetted and rebuilt to work with my shorty reverse cone exhaust and Sirius Consolidated Inc. air pods. The engine idles comfortably at about 1000~1100 RPM, and for the most part runs quite well, (a little hesitation on acceleration when she first starts up) and I can tell the left cylinder is dragging when the throttle is 1/4~1/2 open.

As a test, when she was at idle, I'll lift the butterfly on the left cylinder. I get a very delayed or little to no RPM response. When I lift the right, she responds instantly. Both carbs are jetted exactly the same and have been airjetted out in accordance with the scriptures. I've also attempted to play with the airjet on the trouble carb to see if I get any different performance...I don't.

I'm thinking maybe the valve are spaced incorrectly or possible a lack of fuel, although the spark plug suggests she's geting fuel. Let me know what you guys think. below are some basic read-outs that my OB-II reader told me...

Compression test: Right (good) Cylinder: 160 Left (bad) Cylinder: 165
Spark test: Good blue spark on both
Spark plugs: Right: good color (perhaps a little lean) Left: slightly fouled
Air Jet: 3/4 turn out on both. Pilot jet is slightly larger than stock.
Cabrs: Diaphragms are good on both, float bowls and jets seem clean.
Exhaust: Right (good) Exhaust runs a little hotter than the Left (bad). Right exhaust gives a good consistent air flow. Left exhaust feels like a strong air push, with a little variation in consistency.
No back/after firing on either cylinder.

I'll do a little more investigating this week, after I finish law school exams (word of advice, don't go to law school).

Thanks, Magi.

-cFogs out
BT
NNNN

 

[Kanticoy]

Hey Magi....I'm leaning toward a fueling issue coming from what you're describing. Are you still running the stock setup of individual fuel lines for each carburetor? Getting good flow from both of the outlets if so? Even in short bursts, fuel will tan a plug, so even if the fueling is wonky you may still be getting a lack of fuel. Start there and then work down to float height, etc. even an intake leak could cause issues. The higher the rpm, the more lean the leak would make that cylinder. Spark and compression sound decent to me. Consider throwing a timing light on it for giggles just to make sure. The little twins act crazy if a point is off even a little.

 

[crazypj]

Just sounds like out of sync plus cables need adjusting.
May be a good idea to check the nipples are correctly seated in the splitter box under tank
Did I modify carbs?
You don't normally need larger pilot jets, just secondary main jets

 

[cgguy09]

You don't normally need larger pilot jets, just secondary main jets

Yea, the larger pilot jet came in the set...there was no way I WASN'T going to use it.

Thanks for the advice guys, I'll take a round turn on those this week. (I'm going to look into the splitter box, I'm not quite sure what that is)
I'll report back with any progress!

As always, appreciate your sage advice
cFogs out
BT
NNNN

 

[crazypj]

CB350 has a single cable at handlebar and 'splits' into 2 for the carbs.
There is either a nylon or zinc alloy piece connecting all the cables together.
You should be able to unscrew the end and check things inside the tube (later ones were all plastic with a snap on top)

 

[cgguy09]

Hey Magi....I'm leaning toward a fueling issue coming from what you're describing. Are you still running the stock setup of individual fuel lines for each carburetor? Getting good flow from both of the outlets if so? Even in short bursts, fuel will tan a plug, so even if the fueling is wonky you may still be getting a lack of fuel. Start there and then work down to float height, etc. even an intake leak could cause issues. The higher the rpm, the more lean the leak would make that cylinder. Spark and compression sound decent to me. Consider throwing a timing light on it for giggles just to make sure. The little twins act crazy if a point is off even a little.

To answers you questions Kanticoy, I do have the stock setup with the ol' fuel petcock.

I did a real quick test today to see if we could be on the right path with fuel. I simply switched the lines coming out of the petcock. and it had a large impact...not day and night but day & twilight. I was able (with some consistency) to lift the butterflies on the carbs individually and get a response on the engine. The left cylinder/carb (the trouble one) still would slightly hesitate, but it was far less frequent and less noticeable. The Right carb still was very strong all throughout the range.

I have two battle plans. First, I'll remove the fuel lines from the carbs and have them drain into a bowl to see if there is a dominant "nipple" on the petcock. Next, I plan on taking off the left carb & adjusting the float to permit more fuel. Given the results I received, I would like ot think one of those two will help...However, (I have to ask) could there be an issue with the fact the fuel has to travel further being on the left side & the petcock being on the right? Logic would suggest 2-3 inches would not make a difference, but would shortening up the fuel line have any impact?

I also checked the throttle pullies ( a new one I picked up from DCC after the original broke 5 months ago) it appears to be pulling both carbs exactly at the same time and the height of the butterflies matches up perfectly.

I didn't get a chance to ride her, but from the little testing I've done I'm sure it would be significantly noticeable.

I'll report back.

As always, thanks for the assistance, champions.

cFogs out
BT
NNNN

 

[cgguy09]

CB350 has a single cable at handlebar and 'splits' into 2 for the carbs.
There is either a nylon or zinc alloy piece connecting all the cables together.
You should be able to unscrew the end and check things inside the tube (later ones were all plastic with a snap on top)

Appreciate the explanation...Just didn't know the terminology
(I did check this & it looked good...I'll double back & verify when I take off the tank to clean the petcock)

 

[Redliner]

aaaaaand when you put your hands at the exhausts, the pulses are coming out with even force, right? Put your hands at the exhaust tips, and if one feels stronger, swap hands to confirm it. This is just a basic way to get the idles dialed in close.

 

[cgguy09]

aaaaaand when you put your hands at the exhausts, the pulses are coming out with even force, right? Put your hands at the exhaust tips, and if one feels stronger, swap hands to confirm it. This is just a basic way to get the idles dialed in close.

This gets to the crux of my "night v. twilight" comparison. The right exhaust is pushing air strongly and consistently, you could set your watch by it. The left exhaust is not as consistent or as strong. It hits good about 6 times out of 10, but the other 4 it seems slightly weaker or timed slightly wrong. (Not something you would notice unless you had your hands right there)

I know there are number of factors that can change this, including: idle screw, pilot jet, good fuel level. Based on what I've seen thus far, I'm guessing she's just not getting enough fuel. The Pilot Jet has been adjusted and without positive result and the Idle Screw wouldn't account for the inconsistency in timing or the occasional strong/weaker exhaust push.

But I'll take another look at all those factors & see what we got going on.

Thanks guys!

-cFogs out
BT
NNNN

 

[Redliner]

the Idle Screw wouldn't account for the inconsistency in timing or the occasional strong/weaker exhaust push.

Sounds like ignition. Breaker point gap and fouling, ignition timing, and coils. Even a weak battery can cause this. You should have about 12.5-12.8 volts.

Be very careful and find a guide for swapping the two coils and see if the problem follows them. Do you have any wax leaking from them?

I think all you have to do is trade positions of the yellow and blue leads coming from the coils, then swap spark plug leads. I wouldn't do more than that. Two thin leads and then the spark cables

 

[cgguy09]

Sounds like ignition. Breaker point gap and fouling, ignition timing, and coils. Even a weak battery can cause this. You should have about 12.5-12.8 volts.

Be very careful and find a guide for swapping the two coils and see if the problem follows them. Do you have any wax leaking from them?I think all you have to do is trade positions of the yellow and blue leads coming from the coils, then swap spark plug leads. I wouldn't do more than that. Two thin leads and then the spark cables

I'll be sure to check all of the above. battery is brand-new, and always on a tender. I was pretty meticulous when it came to the points/gaps when I timed the bike, but of course those things can change.

Wax leaking out of the coils.....ccough cough, um, maybe, just once. just a little. cough cough( I try not to think/talk about it because coils arn't cheap. ) Probably something I should look into... But the sparks are bright, blue and strong.

For the coils, I'll look to your recomendation about switching them. It shouldn't be that hard, but i'll take a little bit of thinking just to ensure I don't mix myself up. It should be as easy as switching the blue & yellow wires & switching the sides the plug wires lead to. If I'm not mistaken.

Thanks guys.

cFogs out
BT
NNNN

 

[Rich Ard]

Wax leaking out of the coils.....ccough cough, um, maybe, just once. just a little. cough cough( I try not to think/talk about it because coils arn't cheap. ) Probably something I should look into... But the sparks are bright, blue and strong.

Are you fucking kidding me

 

[cgguy09]

Are you fucking kidding me

Nah, I'm just fucking. I replaced them. hahahaha

I did replace them with an unused OEM set I got off a buddy. But I will be pissed if I take off the tank & there is wax.

 

[cgguy09]

But I was serious when I said I hope its not my "new" coils...because if it is, I'll be fed up enough to buy nice (non-1970) coils which will be expensive.

 

[crazypj]

If the ignition was left on it's possible to heat up a coil and insulation will fail.
The points can also heat up and weaken the spring
Brendon had a coil fail, may have been due to Dyna ignition failure. It was OK when cold but missifres when warm
http://www.dotheton.com/forum/index.php?topic=11736.725

Towards bottom of page is pic of coil

 

[cgguy09]

If the ignition was left on it's possible to heat up a coil and insulation will fail. The points can also heat up and weaken the spring
Brendon had a coil fail, may have been due to Dyna ignition failure. It was OK when cold but missifres when warm
http://www.dotheton.com/forum/index.php?topic=11736.725

Towards bottom of page is pic of coil

Shit. Leaving on the ignition was something I did recently (about 2 weeks ago) it also killed my gel battery. I would like to think the coil would be able to stand up to it, being its only been used for about the last 5 months, although it was built around 1970. I'll give a look to that as well. But I know a busted coil is as dangerous as a WMD, so I'll check that today.

I still want to say its the fuel level though. The fact the preformance greatly improved by switching the fuel lines tells me the problem likely lies within that realm.

Something else I should have noted was the exhaust consistency and flow rate was perfect when I lifted the butterfly above anything other than idle. That is to say when I lifted the left carb's butteryfly independtly of the throttle the air rate was strong and on a consistent hum...(sorry I should have said that earlier....that's what I get for trying to covertly post during work hours.)

Thanks gents!

cFogs

 

[crazypj]

Use the cable adjuster between the carbs to set left side so it opens the same time and amount as right side.
The screw is only to set butterflies the same for idle and doesn't sync carbs

 

[Redliner]

RPM can effect the strength of your ignition. We'll wait until you swap coil-sides.

*sits*

 

[cgguy09]

RPM can effect the strength of your ignition. We'll wait until you swap coil-sides.

*sits*

Done. No wax & the same symptoms during coil swap. (wipes sweat off forehead)

(Redliner, we were right on the coil switch procedures...btw)

I'm guessing it may still have something to do with Fuel flow. I plan on taking the carb off on Friday and adjusting the float...I'll have to figure out if there is an exact guide for how to do it with pods & a more free flowing exhaust. Otherwise, I'll just do a little trial & error.

But I'm set to take a 24 hour law school exam beginning tonight at 2000. (meaning it must be started by 8:00 and turned it by tomorrow by 8:00) But after that non-sense, I'm get to turning some wrenches.

I also took a look at the carb cables as PJ mentioned. They are solid. I remember tuning them with the throttle during my rebuild. I actually had a helper if I recall.

I thought I was going to eat a crow from the karma gods after making the joke about the coil...thankfully no gremlins have gotten up there yet.

Thanks gents, when this is all over (& you're in st. Louis) I'll buy beer...and I'll get the good stuff too! (Buuuuuuscccchh!)

 

[Redliner]

Yeah...about that, I forgot you said the left one was the bad one. Since that is farthest from the petcock, the host can get a little pinched or kinked on the way over. You're certain it's completely free and mostly straight?