Topic: CB-350 Twin Idle FRUSTRATION!

[dmac]

My wife has a 1972 cb350 twin that I cannot, for the life of me, get to idle properly. It is either too high or too low. When I set it even the tiniest bit too high, it will rev itself up to 5000rpm when I'm idling. When it is set too low, it will start out around 3000rpm when I let off the throttle and slowly wind down to 2000rpm. When it hits 2000rpm it quickly drops and stalls.

Here's the work I've done so far to try to remedy this...
-clean carbs
-sync carbs
-adjust timing
-check valve clearances
-Clean and gap points

I'm out of ideas!!! I need help! We have a track day setup for next week and I need to get this thing running right. Any help is much appreciated.
Thanks,
Mac

[750goes]

if it hits 2000rpm then stalls I would think the idle or slow jet circuits are still clogged, or the bike is sucking lots of air from somewhere...

possible really sticky throttle or choke cable may not help, all linkages nice and free ?

also the possibility of a cable hanging up somewhere  - does it do the same thing if bars are turned right or left ??

good luck

[ohiocaferacer]

Did you use an aftermarket rebuild kit for the carbs? Such as Keyster or K&L???

Also....check and clean the mechanical advance behind the points plate. Make sure the unit will advance smoothly and spring back closed.

Greg

[seaweb11]

I did not see a new air filter on your work done.  How is it?

Turned out to be a real problem solver on my CL 350 when getting it right.

[mikedialect]

all of those suggestions are exactly where I started when I had that issue. Also, make sure the throttle cables are set back the same length on the carb.

[dmac]

I did clean and check advance plate behind the points and checked the cables.  I haven't changed the air filter in a while...I'll check that out.

Do you think it's possible that a clogged petcock could cause this? I noticed some rust in the crossover cable on the bottom of the tank. I have inline fuel filters leading to the carbs, so they should not be affected by this.

There is one issue that may be related- The exhaust pipe on one side does not fit securely onto the head. One of the bolts that holds it on is broken off. The bike ran fine for a couple years with this, so I don't think it is the cause of my recent issues.

Thanks for the input.

[mack808]

I was just curious, since the twins have no provision for
vacum gauge or mercury sticks to synchronize the carbs, how exactly did you do this procedure? And from what I could read it will not idle below 2000rpm. either your pilot jets are still blocked or your advancer has springs that are sacked out and it is holding timing at full advance and once bob weights do return to idle position you have the idle set too low now and it wont/can't idle.  When 175/350/450 twins were prevalent we synched by putting a hand between both carbs to touch throttle butterflys and used touch to determine if both were opening simultaneously.

[dmac]

A mechanic friend of mine, who worked in honda shops in the 70's, drilled ports in each carb to make it possible to sync them with gauges.

[GNXFan]

I agree with all of the previous suggestions. Have you checked the routing of your throttle cable. It should go through the triple clamp area between the guages, then around the left(from rider's View) side of the frame head and follow the backbone of the frame and down to the carbs, there should be no sharp twists or bends. An easy way to adjust the idle is to start the engine, pull the left plug wire off, adjust the right carb idle where you want then re-attach the left wire and pull the right ire and adjust the left carb idle where you want. Also check the levers on each carb where the idle screw is to see if they start to pull at exactly the same time. Sorry for the rambling, it's early and I need more coffee!!!

[dmac]

I followed GNXFan's advice and tried adjusting the idle one cylinder at a time by removing one of the spark plug wires. I actually was able to get each cylinder to idle reasonably well on their own. That being said, when I connected both spark plug wires back up and started the bike the idle immediately raced up to 5000rpm.  Arrghh! What gives? 

[HITMAN]

There have been many good and right answers so far, but it really sound like it could be a vacume leak.  Among many ways to find it (carb cleaner, propane, mapp-gas or ether) whatever trick you use, if it will not at least idle at 1K you have too much of something.  Especially if you have already checked the points and advance, lastly would be what condition are the mounts or boots for the carbs? 

[Ibsen]

I agree with Hitman. Check that you don't have an airleak in the intake manifolds or the joints between the manifolds and the carbs and the manifolds and the cylinder head.
As you let the engine idle, spray WD40 on the manifolds and the joints (take one carb at the time), and if the idle drops when you do this you have got an air leak. If the air leak is large enough the engine will stall (it won't run good on WD40 ).

[Raul CB750K1]

My two cents here, as I'm about to go that way because I just installed the carbs the other day in my CB350 resto.

As it has been said, the carbs have no provision for synchonizers. The carbs have basically two adjusters: the idle screw, and the butterfly stop screws, used for synchronization.

First thing I would do would be to loose the throttle cable adjusters so the cable has some slack -to make sure it is not forcing open the butterflies-, and open the butterflies screw to make sure they are in the fully seated position. Set the idle screws to whatever the standard position is -I can't remember, say 1 and a half turns from closed-, and start the bike. It should idle smoothly. Now adjust the throttle cable so they pull at the same time when twisting the grip, making sure there are a couple of mm of slack. The butterfly adjusters are used to synchronize the carbs, and with no provision for gauges, it should have to be done by ear and feel. If the bike stalls when revving, means there is a problem in the transition from idle to low, so you will probably have to use the butterflies adjusters to slightly open and slightly close the idle screw, this way compensating the fuel intake in idle but favouring the butterfly circuit.


My theory is that you could have the butterflies way too open in the fully seated position, but it is just a theory shot miles away from where your carbs are -you know, a long shot..... 

[Gordon]

When did they start putting the vacuum ports on the twins?  My '76 360 has them, but they're larger than the sohc4 ports, so I haven't been able to balance them, yet. 

[menzelg]

Hi all, just found this post. Dmac, did you ever solve the mystery? I am having a similar (and equally frustrating!) issue with my '72 Twin, she didn't run when I picked her up, but after a carb rebuild (all honda parts) new plugs, points, battery and gas, and she fired right up. Now I can't get it to idle right. idles high or low but not consistent, backfires and then dies. Can anyone help with the order of what to do? Carbs, timing, valve clearance, point gap etc.

Thanks