Topic: My CB350K4 twin won't keep the idle

[Raul CB750K1]

Some of you may remember my hardships with my cb350, which after a full resto snapped the camchain. It took me another year to finish another ongoing projects until I had the time to rebuild the engine again.

I was lucky that the camchain broke when the valves were closed! no damage to either pistons or valves! the cylinders even had still the honing marks, as it had been just a few hundred kilometers after the rebuild.

I put the head upside down and poured some alcohol on the combustion chambers, to check for leaks. No leaks.

Rebuilt the engine, installed on the bike, put everything back, but the bike run like crap!

It had correct jet sizes, but the carb bodies were wrong (725A vs 722A). 725A's were for the CB250. I had a spare set of 722 lying around, and for the life of me I couldn't see any difference. Anyway, to err on the safe side, decided to rebuild the correct carbs and ordered new, shining jets and needles. Installed everything back and still run like crap.

Went to check the float height (26mm as per the info I judge most accurate) and notice sloshing inside the floats!!! what??? Never seen it before, probably because it has always been a long time since I park the bike until I work on it. So I put my soldering iron tip on the float hole in order to open it and empty it, and due to the heat I see a tiny, almost microscopic stream of gas getting out!!! The hole is almost imperceptible. Tried to repair it but the weight of the solder is too much, decided to buy new floats too.

I found out that the tip of one of the idle screws was bent! How come? It was not like this when I installed. Checked and found out that a previous owner had broken the tip of another screw, no way to remove it. No wonder it wouldn't idle.

Bought another set of carbs and made sure every tiny passage was surgically clean. I made sure that I understand from where the atmospheric pressure up the diaphragm is taken, etc. Installed all the jets, floats, etc, back, and the bike still run like crap!


It took me ages to realize that I had the ignition wrong!!! After painting the frame, I decided there was no need for the coil bracket to make electrical connection with the frame. Wrong. The condenser is attached to the coil bracket, and without it, there is no ground to the condenser. After removing the paint in the frame, the bike finally run like a champ!!!


But it won't keep the idle. I adjust it when it is at operating temp. 3/4 turn out, and then with the slide stop screw, a similar amount in both carbs. It will idle fine at 1.200 rpm, but then, after 1 minute, it would die. Or I will run a block and then the idle will stay at 2.000 rpm. Tried with 1 1/8 turns out, but it made no difference.

The electrical system is very weak, so I thought about a low-voltage battery that can't keep with the voltage needed when the rpm's are low and the alternator is not giving juice. But I hooked a car battery to the bike and it would do just the same.

Sprayed some WD40 at both ends of the carb insulators but it made no difference. Replaced the auto-advance unit in case it was not returning, no difference.


While I'm writing this it has dawned on me that the carb butterfly rotates on an axle that has some felt washers. I would swear I put everything back in place but it may be a point from where the air comes in. I'll check.


Apart from all this, is there something I could be overlooking? I have found many posts on the internet relating the same problem with the idle, but it seems that people never come back to the forums to tell how everything ended up.....

[Gordon]

Hey Raúl!  It's nice to see a post from you.

Have you synchronized the carbs? 

[grcamna2]

I Know that you are well down the road to solving that erratic idle problem.
I have the exact same model as you & I will be doing my carburetors very soon;
the cycle was sitting for 30 years and I imagine that I will need to do everything
necessary to complete the project.
   I thank you for posting your fixes here on sohc4 forum !

[Raul CB750K1]

Hey Raúl!  It's nice to see a post from you.

Have you synchronized the carbs?

Hey, thanks for the "ú"..... :-)

They way these carbs are synchronized is this: 1. set the air screw in both carbs. 2. remove one spark plug. 2. Start the bike and adjust opposite throttle stop screw until the bike idles on one cylinder at, say, 2krpm. 3. Proceed in the same way with the opposed cylinder. 4. Start the bike with both spark plugs, the bike will rev higher than it did with only one cylinder. 5. Adjust the idle by adjusting both throttle stop screw the same amount, and from that point, always adjust both stop screws the same amount.


That's where I started from, every possible adjustment with the stop screws couldn't make the bike idle at a steady rpm.


BTW, I was unpleasantly surprised to see that I had to switch to reserve at 120 km (something like 75 miles). That makes a gas mileage of around 38 mpg, close to what I have from my CB750!! That could indicate rich running, but the plugs look fine, with a perfect tan. Is that 75 mile range normal for the CB350?

[70CB750]

I was loosing idle setting overnight.  It would idle and the next day it would not - either die or idle high.  Turns out there was some gunk on slides and I would adjust it with slides in weird positions.  Cleaned slides with grey (fine) scitch brite - problem solved.

[Gordon]

What condition are the carb diaphragms in?  When I was working to get my 360 idling well, even though the diaphragms seemed to be in good shape it turns out they weren't sealing well around the edges.  I ran a thin bead of sealer around both of them and it idled perfectly. 

38 mpg does seem a little low, but I never calculated the mileage I was getting on the 360 and I don't own it anymore. 

[Raul CB750K1]

I was loosing idle setting overnight.  It would idle and the next day it would not - either die or idle high.  Turns out there was some gunk on slides and I would adjust it with slides in weird positions.  Cleaned slides with grey (fine) scitch brite - problem solved.

The CB350 has CV carbs. When the throttle is closed, the butterfly is shut, therefore it doesn't matter much where the slide is, or so I think. Are you talking about CB350 carbs?

What condition are the carb diaphragms in?  When I was working to get my 360 idling well, even though the diaphragms seemed to be in good shape it turns out they weren't sealing well around the edges.  I ran a thin bead of sealer around both of them and it idled perfectly. 

That would make some sense, but from what I understand, when the butterfly is shut, the vacuum suctions mixture from the tiny hole in the carb throat. The gas is provided by the slow jet, mixed with air taken from the diaphragm chamber, below the diaphragm. In that point there is atmospheric pressure, so would it make any difference if the diaphragm edge is not sealing? It would take atmospheric pressure in any case, I can't see how would it affect idle....

[Gordon]

That would make some sense, but from what I understand, when the butterfly is shut, the vacuum suctions mixture from the tiny hole in the carb throat. The gas is provided by the slow jet, mixed with air taken from the diaphragm chamber, below the diaphragm. In that point there is atmospheric pressure, so would it make any difference if the diaphragm edge is not sealing? It would take atmospheric pressure in any case, I can't see how would it affect idle....

All I can say is it made all the difference in getting my 360 to idle reliably.

[ohiocaferacer]

Raul,

Sorry to hear you still having issues with those carbs.....but it sounds like you know them pretty well know. As Gordon stated.....sounds like maybe its with your slides....either they are sticking or the diaphragms are leaking. Its sooo hard to help diagnose over the internet.

The only thing I would do is maybe take some 0000 steel wool and rub the slide down real good, along with slide throat in the carb body. Maybe check for small pin-holes in the rubber diaphragms.

One other thing maybe to check is the mechanical advance behind the points plate....sometimes they stick. Make sure they are real clean, move freely and the return springs are good.

Hope you find the solution....keep us updated.

[70CB750]

OK, I didnt know hat carbs you have - just sharing my (very limited) experience with CB750

[Raul CB750K1]

All I can say is it made all the difference in getting my 360 to idle reliably.

Don't take me wrong Gordon, I just wanted to understand how it could affect the idle. In fact, I can do what you say without removing the carbs -which is a big PITA- as just by loosening the clamps and slightly rotating the carbs it is possible to remove the top cover. I think I'll give it a try.

Thanks for the tips Greg, I owe you the info about jet sizes! I have replica mufflers and thought that maybe I should increase the slow jet to the next -and only- available size, if only because I'm running out of ideas. Once you think you know something, suddenly you discover you don't know a thing....


And thanks for your help 70CB750 too, I have a CB750 too and you never know when somebody other's experience will be useful to you.

[Gordon]

All I can say is it made all the difference in getting my 360 to idle reliably.

Don't take me wrong Gordon, I just wanted to understand how it could affect the idle. In fact, I can do what you say without removing the carbs -which is a big PITA- as just by loosening the clamps and slightly rotating the carbs it is possible to remove the top cover. I think I'll give it a try.

Not at all, Raúl.  I can definitely appreciate your desire to learn exactly how your carbs work.  The time I had to work on my 360 wasn't much overall, and was in short sessions spread out over a few years, so I wasn't able to really get to know these carbs nearly as well as I know those on my sohc4's. 

From what I do know about working on these carbs, it seems that if there isn't a good seal at the top of the diaphragm, additional air can be sucked in to the throat of the carb, leaning out the mixture.  It's just another location that can cause an intake leak if it's not sealing correctly, just like with the carb boots. 

I wish I had more than anecdotal evidence to share with you.  The only reason I came across this fix for my 360 is that I was going crazy trying to figure out what was wrong, and had very little time to spend on it.  I had gone over the carbs, ignition, valves, etc. multiple times with a fine-tooth comb, and it all came down to the diaphragms.     

[lordmoonpie]

Junk them standard carbs Raul - get a pair of Mikuni VM32s and you'll never look back....

[Raul CB750K1]

Junk them standard carbs Raul - get a pair of Mikuni VM32s and you'll never look back....

I have already considered that route. I don't mind at all at the $300 or so that they may cost - I have spent more in time and frustration.

My only concern is whether they fit right in with the stock air filters and carb insulators, and the right jet and needle choice. If I have to fiddle again with the mikunis, I rather fiddle with the Keihins. It is cheaper and they are original after all....

[Raul CB750K1]

The inconsistent idle didn't keep my CB350 to be awarded the "best restoration" trophy yesterday at a local rally among another 50 or so bikes....   this kind of things gives you some encouragement after you have put your money, sweat and blood with a resto.

[Gordon]

Congratulations!  That is definitely a fantastic looking bike!