Topic: CB350F Adjusting idle stop screw during every start up

[Allen Statu]

Hello there! I just started riding in January. Got my first bike. CB350F. Been coming to this forum for some electrical help and it's been great. I'm new to bikes but will try to contribute to the best I can.

My CB has 28,000 miles on it and I've pasted a few details after my question if you want more info. So the problem I'm having is: Every time I start up my bike cold, I have to adjust the idle stop screw to warm it up, rather than the choke. And I'm wondering how I can avoid using that, since most people I see start their bike up only have to use the choke to warm it up.

My mechanic told me that my engine is not in the best of shape. Says it runs rich BUT if I take care of it should last me a good amount of time. He told me that immediately after I start the bike up, to open the choke fully. I do this and after about 10-20 seconds the bike shuts off. He showed me how to use the idle stop screw to warm the bike up and then find a good spot to leave it at for 1,100 RPM. Usually I turn the screw in 3 full turns, and then slowly turn it out while it's warming up. I ride it only about 8 miles a day back and forth to work and I've been getting used to doing this, but was wondering if there's anything I can do. I usually leave it at about 1,200 to 1,300 RPM, because when I let it at 1,100 the engine sputters and sounds like it's going to die out at a stop light.

thanks for any help.

'73 CB 350F
28K Miles
Mechanic put: new tires (contigo Go), new chain, rebuilt carbs, changed battery,
I've changed: Some wiring for the lights, flash relay, solenoid starter. I even changed the regulator/rectifier to an after market combo, but the battery charge kept dying so I put back the OEM parts.

[D-Ral]

Ride your bike around enough to completely warm it up. Adjust the idle screw so that it idles where it should.

When you cold start with the choke, use the throttle to adjust the idle. Ease the choke off before it kills your engine, and ride away.  Some ride away with a little choke. Depends on your bike. If you have to stop at any red lights before its warmed up, just hold the throttle open so it idles higher.

Funny, I had the same question about 2 years ago... Enjoy your bike

[Allen Statu]

Thanks for the tip/instructions D-Ral! I'll give this a go tomorrow. Gonna take side streets to work just to be careful. I'll reply back as well. Thanks again.

[LesterPiglet]

Yes indeed. Once you set your idle screw for tickover at operating temperature it needs never be touched again. The choke is totally variable between on and off, it's a balancing act between this and the throttle until warmed up.

[D-Ral]

Yeah, I forgot to mention that your choke has an off, an on, and everything in between. Figure out what works for your bike and you've got it made. Until something else breaks.

[DaveBarbier]

My mechanic told me that my engine is not in the best of shape. Says it runs rich BUT if I take care of it should last me a good amount of time. He told me that immediately after I start the bike up, to open the choke fully.

 I even changed the regulator/rectifier to an after market combo, but the battery charge kept dying so I put back the OEM parts.

Strange that he'd say that it's not in the best of shape and that it runs rich. He makes it seem like this is a problem that you have to live with. Like it's just some unfortunate thing that can't be fixed. But the carbs can be adjusted to not run rich. Rejetting is a common thing that people here post about all the time. Might want to look into it.

Also, why would he say to put the choke on right AFTER you start the bike up? Why not put the choke on first, then start it up? On a cold bike, in my mind, his way would make the bike not want to start. It needs more fuel from the beginning if it's cold; that's what the choke is for. (Unless it's running THAT rich that it doesn't need the choke on start up. Then why use the choke at all?)

Lastly, these regulators and rectifier's work very well. Many people keep the factory units unless there's a charging issue or going to a new technology battery like a LiFePO. That being said, if your new one caused charging issues then it seems like it was defective. I hope you tried to get your money back.

Nice bike, LOVE that color!


---
1978 Honda CB550K

[Airborne 82nd]

When you cold start with the choke, use the throttle to adjust the idle. Ease the choke off before it kills your engine, and ride away.  Some ride away with a little choke. Depends on your bike. If you have to stop at any red lights before its warmed up, just hold the throttle open so it idles higher.

Agree 100% Your hand is right there on the throttle. Why on earth would he tell you to move the idle screw back and forth all the time? Oh wait a minute I know why he doesn't know what the hell he is talking about. Find another guy. All you are doing is paying for his education. This is just the way your carbs work and one reason Honda went to the PD carbs with a pump. Leave a little early and get the bike up to full temp. You want to burn off all the condensate that builds up over night your pipes and other parts will last a lot longer if you get them to full operating temp. Some of that water will end up in your oil you don't want that. So get her hot.

[Bodi]

I read it is as he was told to fully open the choke right after starting. Still wrong.
Once the engine has warmed up enough to run at all without full choke, you open the choke a bit and use the throttle to keep revs up until it's warm enough to stay running at idle.
I think the 350-Four has the choke idle advance (the 400F is a close relative and has this), that's there to free your throttle hand for other things as the engine warms. At a click or two off full idle, the choke isn't "choking" the idling engine - the choke butterfly opening is many times larger than the throttle slide idle opening so the vacuum at the slide is not increased much if at all. At less that full choke, the choke lever is really just a throttle advancer at idle. It does choke the carbs at increased throttle of course, and helps tame a sputtering cool engine once you start riding.
So use full choke as needed for starting, reduce choke and feather the throttle grip until it's running fairly smoothly at ~2000rpm, and then do your pre-ride check and get your helmet on etc. The revs should steadily rise, just nudge the choke open more to lower rpm. After a couple of minutes it should idle at 12-1500 with choke close to or fully open. In really cold weather you may need more choke to eliminate missing and surging on acceleration; withing a few minutes of moving off you should be able to open the choke all the way. Forgetting and leaving it partly closed for spirited riding will foul the plugs.
Needing constant idle screw adjustments suggests the carbs need attention. Only occasional tweaks should be needed.

[vames]

There are a lot of good answers here, but just to be clear:

These bikes were not built with a mechanism that allows you to start it up cold and walk away while it warms up and you put on your helmet and gloves. (There are much later CB750 models that I believe do have a fast idle cam -- a heat or vacuum controlled mechanism that automatically does essentially what you are doing manually with the idle screw but your 350 definitely does not have that.)

Likewise, the function of the choke is not to keep the idle higher during warmup. It is simply to restrict air and richen up the mixture to allow it to start. In my experience with five SOHC4s, I've never had to leave the choke on longer than it takes to get it to fire. If you need to leave the choke on half or full or quarter for any amount of time, your bike is tuned wrong.

Startup should look something like this.

1) Put on your gloves and helmet and get ready to ride.
2) Turn on petcock
3) Turn on full choke
4) Make sure the key is on and the kill switch is on 'run'
5) Press button or kick, giving it about 1/4 turn of throttle.
6) When it fires, keep giving it about 1/8 - 1/4 throttle to keep it running.
7) Turn off the choke immediately
Keep feathering the throttle to keep it running for about 45 seconds.
9) Ride away being gentle on the motor.
10) By the time you go about a mile, it will be idling on its own and warm enough to start winding and riding like normal.

[DaveBarbier]

I read it is as he was told to fully open the choke right after starting.

Yes, you're 100% right. I mis-read it, oops.


---
1978 Honda CB550K

[vames]

I read it is as he was told to fully open the choke right after starting.

Yes, you're 100% right. I mis-read it, oops.


---
1978 Honda CB550K

I'm not sure if I'm right or wrong (and sorry if I'm confusing things for the OP) but on my 1973 CB350, 1975 CB400F, 1975 CB550, 1975 CB750F I never needed to leave the choke closed for longer than it took to fire it up. None of them had any running issues -- sputtering or otherwise. Never had to have it in mid-choke positions or anything like that -- just choke - start - unchoke - and warm it up.

The factory owners manuals are all vague -- they just tell you to choke it, start it, and warm it around 2000 RPM until it will run without the choke. I guess that leaves us all to our own interpretations.

[HondanutRider]

Sorry to say guys, but in my opinion and experience the advice given so far hasn't really been accurate.

First of all, the carbs on these models had a linkage via cam from the choke to the throttle (assuming everything is properly set-up and adjusted), that advances the throttle when the choke is engaged.  You can verify this operation with the bike turned off/not running - by engaging the choke and observing the throttle grip slightly turning.  Before you start-up a cold motor, you engage the choke which raises the throttle slightly so that it has more idle speed when starting.  On earlier models they didn't have this choke-to-throttle linkage, so you needed to manually advance the throttle slightly while starting it and then feather/fiddle it for some time while the motor warmed  The CB350F models (and probably the other in-line fours) didn't need the throttle to be touched nor advanced during cold start-up; by applying the choke the idle speed was raised.  As the motor warms up, the idle speed climbs and the choke can be backed-off in stages to keep the motor warming at a reasonable speed. 

Only experienced riders should consider riding off with the choke still on.  It is better to wait until the motor is fully warmed up and idling smoothly with no choke applied.  Otherwise you run the risk of stalling while in traffic, or being distracted by having to keep fiddling the throttle and choke.

That Idle adjustment screw was only intended to be adjusted on a warm engine to set/tune the idle speed - and then be left until needing adjustment for a rather permanent setting.

[LesterPiglet]

(and probably the other in-line fours) didn't need the throttle to be touched nor advanced during cold start-up; by applying the choke the idle speed was raised.

I disagree. There is no way to warm my bike without using both hands, choke and throttle. It doesn't take very long but there is no way I could start my bike and it keep running without my input.

[Allen Statu]

Needing constant idle screw adjustments suggests the carbs need attention. Only occasional tweaks should be needed.

Thanks for the instructions. The mechanic rebuilt the carbs, and I know it's possible it wasn't done right, but do you think I should skip worrying about the engine for now, and just concentrate on the carbs?

Leave a little early and get the bike up to full temp. You want to burn off all the condensate that builds up over night your pipes and other parts will last a lot longer if you get them to full operating temp. Some of that water will end up in your oil you don't want that. So get her hot.

Thanks for the info. I'll do this as well. I felt that I shouldn't have to touch the idle stop screw, so I'll look into a different mechanic. He works on CB's often and was recommended but it's worth it for me to look around, like you mentioned.

[LesterPiglet]

Takes a wee bit longer in colder weather.

[Allen Statu]

Nice bike, LOVE that color!

Thanks! I was going to go for a black tank but I keep getting compliments on the color, so I'm gonna leave it.

[Allen Statu]

Takes a wee bit longer in colder weather.

Thank you for that. I don't normally ride off that quick, but I will do exactly what the video shows and go from there.

[HondanutRider]

Takes a wee bit longer in colder weather.

Thank you for that. I don't normally ride off that quick, but I will do exactly what the video shows and go from there.

After starting I'd try leaving the choke on without touching the throttle.  When the idle starts to rise, back off slightly on the choke to keep the idle in check, as this will decrease the amount of throttle the choke progressively applies when it is engaged.  When it's really cold, you may need to compensate for decreasing the choke by applying some more throttle manually.  A cold motor will warm-up quicker with a lean mixture (ie less choke) and/or under load (ie if ridden) but it probably needs more throttle (ie higher speed/fiddled throttle) to keep from stalling.  As the motor warms up and the idle keeps climbing, continue to back-off the choke until it is fully off and the motor is warm and idling on its on.  Several blips of the throttle should confirm fully warm, when it doesn't stumble or stall but rather responds normally. 

[vames]

First of all, the carbs on these models had a linkage via cam from the choke to the throttle (assuming everything is properly set-up and adjusted), that advances the throttle when the choke is engaged.  You can verify this operation with the bike turned off/not running - by engaging the choke and observing the throttle grip slightly turning.  Before you start-up a cold motor, you engage the choke which raises the throttle slightly so that it has more idle speed when starting.  On earlier models they didn't have this choke-to-throttle linkage, so you needed to manually advance the throttle slightly while starting it and then feather/fiddle it for some time while the motor warmed  The CB350F models (and probably the other in-line fours) didn't need the throttle to be touched nor advanced during cold start-up; by applying the choke the idle speed was raised.  As the motor warms up, the idle speed climbs and the choke can be backed-off in stages to keep the motor warming at a reasonable speed. 

This is wrong. There is no fast idle cam or similar mechanism on any SOHC4 I have ever owned -- from a 1973 CB350F to a 1976 CB750F and a few in between. I've never seen a later F or K7-8 model, so I wouldn't know about those. But for most years, your hand on the throttle is what keeps the slides open enough for the bike to warm up.

[HondanutRider]

First of all, the carbs on these models had a linkage via cam from the choke to the throttle (assuming everything is properly set-up and adjusted), that advances the throttle when the choke is engaged.  You can verify this operation with the bike turned off/not running - by engaging the choke and observing the throttle grip slightly turning.  Before you start-up a cold motor, you engage the choke which raises the throttle slightly so that it has more idle speed when starting.  On earlier models they didn't have this choke-to-throttle linkage, so you needed to manually advance the throttle slightly while starting it and then feather/fiddle it for some time while the motor warmed  The CB350F models (and probably the other in-line fours) didn't need the throttle to be touched nor advanced during cold start-up; by applying the choke the idle speed was raised.  As the motor warms up, the idle speed climbs and the choke can be backed-off in stages to keep the motor warming at a reasonable speed. 

This is wrong. There is no fast idle cam or similar mechanism on any SOHC4 I have ever owned -- from a 1973 CB350F to a 1976 CB750F and a few in between. I've never seen a later F or K7-8 model, so I wouldn't know about those. But for most years, your hand on the throttle is what keeps the slides open enough for the bike to warm up.

I don't know about the other SOHC4 models, but here is the description in the Honda Shop Manual for the CB350F/CB400F.  My 1974 CB350F is exactly like this.

[Airborne 82nd]

The 77-78 750 K&F also have the fast idle cam along with its own fast idle speed a really nice feature in cold weather. Our choke knob is up in the center of the handle bars. Plus we have a fuel pump to prime the carb for cold start. No matter what system the bike has they are a little like women they all like to be treated a little different even if they are the same age. Just learn what yours likes 

[magnashopper]

I am 100% sure there is a fast idle cam on the choke linkage for 72-74 cb350f-400f. The shop manual covers this adjustment in section 1. It is difficult to adjust with the carbs in the bike and air box attached but you can get to it with a small, thin flathead.

[vames]

Apologies for being wrong. I've still never experienced the function on any of my bikes, but now I'm going to have to go out and tinker on the 400f and try to get it to work.

[Bodi]

It is not unusual for the choke idle advance to be useless after carb work. The adjustment is easy to screw up when the carbs stack has been disassembled. Since the 350 and 400 have no fixed reference carb you're free to set the lifters anywhere. You tend to notice when your idle adjuster is not usable afterwards because of a maladjustment but the choke cam setting will also be way off. I think the 350-Four was the first to have this, but many other models afterwards do.

[Duanob]

The mechanic rebuilt the carbs, and I know it's possible it wasn't done right, but do you think I should skip worrying about the engine for now, and just concentrate on the carbs?

He rebuilt the carbs and then says they run rich? Of course he didn't do it right. Not to get too off topic but, as like most of us it's time to get rid of the mechanic and become your own wrench turner. That's why 99% of us are here. You can do it yourself and probably do a better job with some experience. My first and last experience with a vintage cycle mechanic, they had my bike for over a month just to come back and tell it will cost $450 just to rebuild the carbs and that will not include any other "unforeseen" issues. I told them thanks but no thanks and did all the work myself. It took a while but probably a lot less time than letting the bike sit at that mechanic's shop. But I got it dialed in and running great.

BTW you do have to fiddle with the idle screw a bit when starting cold. And another problem i read is your commute is only 8 miles one way, I see other issues like lack of charging the battery fully, moisture in the exhaust, etc. On your way home I would take it out for a good spirited 15 mile ride just to get the condensation up and blow the carbon out. These bikes like to be ridden not commuted on.