Topic: 1973 HONDA CB350F CARB SYNC

[jetthugo]

Im a newbie to the whole world of bikes. I just finished doing a vacuum carb sync on my 350f. Prior to doing the carb sync it was idling at 1,200rpm, but after the sync (all for are almost tuned exact) the bike will idle at around 3,000rpm. What do I need to do to fix this? Decrease all 4 equally until they get back to 1200rpm is what I am assuming.

[flybox1]

turn the idle sync screw out to bring the idle down.
you should be doing the vacuum sync with the lowest comfortable idle for your bike. 
vacuum adjustment and a blip of the throttle should be followed by adjusting the idle lower or to keep it in the 1100-1400 rpm range.

[jetthugo]

turn the idle sync screw out to bring the idle down.
you should be doing the vacuum sync with the lowest comfortable idle for your bike. 
vacuum adjustment and a blip of the throttle should be followed by adjusting the idle lower or to keep it in the 1100-1400 rpm range.

thanks for the feedback flybox. So I adjusted the idle screws to bring down the rpm to around 1200. When I turn the throttle it hangs around 2000-3000 and takes about 30 seconds to go back to 1200. When I  rode it up and down he street, it jolts and then will kick in. So it's not riding smooth. As you mentioned earlier, since I have the rpm lowered, would I need to resync the carbs once again? Thanks for your help. Here's a video of it hanging around 2500 rpm


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[flybox1]

The hanging idle can be caused by a few things:  take your time.  eliminate each one.
vacuum leaks at the carb boots (check all 8 - probably the most common cause)
incorrect fuel level in the bowls (use the clear tube method to check http://forums.sohc4.net/index.php/topic,151263.0.html )
incorrect A/F mixture at idle (tell us what you are using for intake and exhaust, and what jet#s and mixture screw settings are)
slides sticking/not closing when throttle is released
incorrect vacuum sync

[jetthugo]

The hanging idle can be caused by a few things:  take your time.  eliminate each one.
vacuum leaks at the carb boots (check all 8 - probably the most common cause)
incorrect fuel level in the bowls (use the clear tube method to check http://forums.sohc4.net/index.php/topic,151263.0.html )
incorrect A/F mixture at idle (tell us what you are using for intake and exhaust, and what jet#s and mixture screw settings are)
slides sticking/not closing when throttle is released
incorrect vacuum sync

Thanks flybox, I'll do these and get back to you to let you know what happened. I think I'll start off with readjusting the carbs once again.


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[flybox1]

if youve used any aftermarket carb rebuild parts, jets and float needles especially, tuning can be difficult.
Aftermarket float needles - often differe in length or internal spring rate.  use the clear tube method to verify float level @3-4mm below bowl gasket seam.  you can be anywhere from the (book)21mm float height up to 24mm.  bowl fuel level matters.
Aftermarket jets - can differ in length and throughput from stamped size.  use OEM Keihin for ease of tuning.

[harisuluv]

hanging idle is usually a lean symptom.  are you running the air box or pods?  exhaust?  what mods have you got?

[jetthugo]

The bike is all stock with no mods. The bike was running fine before I tried to sync the carbs. Almost perfectly. I screwed it up when trying to sync the carbs "perfectly" but now its all packed out. All I'm trying to do is put it back to where it was prior to trying to sync the carbs. I never took the carbs apart or anything like that because it was running fine. Definitely a beginners mistake of trying to fix something thats not broken. Learned my lesson!

[jonda500]

Im a newbie to the whole world of bikes. I just finished doing a vacuum carb sync on my 350f. Prior to doing the carb sync it was idling at 1,200rpm, but after the sync (all for are almost tuned exact) the bike will idle at around 3,000rpm. What do I need to do to fix this? Decrease all 4 equally until they get back to 1200rpm is what I am assuming.

You would only have to use the four sync adjusters to drop the idle speed back to 1200rpm if you can't do it using the idle speed screw - there is only ONE of these!
John

[jonda500]

So I adjusted the idle screws to bring down the rpm to around 1200.

I am confused - which screws do you mean?

[jetthugo]

So I adjusted the idle screws to bring down the rpm to around 1200.

I am confused - which screws do you mean?

Not in front of my bike at the moment so I grabbed this image of the Internet, but it's the one on the side that has the tip like this I hope this is the idle screw...


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[jonda500]

That is a picture of a float bowl drain screw, you have 4 of these, one on each float bowl,
John

[jetthugo]

That is a picture of a float bowl drain screw, you have 4 of these, one on each float bowl,
John

That was probably the wrong image, this is the one I was talking about. I adjusted both of these to bring the rpm down.


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[harisuluv]

Yeah man... that is the air screw.  If you were trying to vacuum sync your carbs you are way off.  Those mixture screws will only affect the rpm because you are changing the air fuel mixture.  Turn them all in til snug then turn them out 1.5 turns, then you should be roughly back to normal.

As you said, if it was running fine before, then you don't need to mess with anything else.

[jonda500]

this is the one I was talking about. I adjusted both of these to bring the rpm down.

the inner two carbs have these idle mixture screws too, set all four to 1.5 turns out from lightly turned all the way in. Take note how many turns out the inner two are set if you haven't turned them yet so you'll know where they were set before...
John

[jetthugo]

Yeah man... that is the air screw.  If you were trying to vacuum sync your carbs you are way off.  Those mixture screws will only affect the rpm because you are changing the air fuel mixture.  Turn them all in til snug then turn them out 1.5 turns, then you should be roughly back to normal.

As you said, if it was running fine before, then you don't need to mess with anything else.

When I was vacuum syncing the carbs, this Emgo set was attached and I was turning these screws circled in red. After they were tuned as close as I can get them, I then played around with the air screw to bring the rpm down to about 1,200. I'll do what you said with them snug at 1.5 turns after.


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[jetthugo]

this is the one I was talking about. I adjusted both of these to bring the rpm down.

the inner two carbs have these idle mixture screws too, set all four to 1.5 turns out from lightly turned all the way in. Take note how many turns out the inner two are set if you haven't turned them yet so you'll know where they were set before...
John

Damn! I didn't realize there were 4 of these. Doing all 4 would help!


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[harisuluv]

If the rpm is high after syncing the carbs, lower the idle with the "idle set screw."  As Jonda said, it's ONE big screw on the linkage.  This screw adjusts all four up or down at once. 

[jetthugo]

If the rpm is high after syncing the carbs, lower the idle with the "idle set screw."  As Jonda said, it's ONE big screw on the linkage.  This screw adjusts all four up or down at once.

This one correct?


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[harisuluv]

yes

[jetthugo]

You guys rock! I turned all 4 idle mixture screws until they were snug. Then I decreased them by 1 full turn. The bike started beautifully, but it would still hang for a bit at 2000 rpm. So I turned the idle set screw slightly and now it stays at 1200 rpm. Took it for a ride to make sure all was good and it was! I've had the bike for about 6 months now, and I knew it was not running to it's full potential. But now, it rides like it should. It purrs and rides smooth now. I appreciate everyones help and thank you all! 

[Scott S]

 I'm wondering now what you used to do the sync? Did you use screws on the linkage, with the locknut, to sync them? You have to remove the tank to get to them.

[jetthugo]

I'm wondering now what you used to do the sync? Did you use screws on the linkage, with the locknut, to sync them? You have to remove the tank to get to them.

I took the tank off to use the EMGO carb set to sync the carbs. After that I turned the 4 idle mixture screws clockwise until they were snug (actually did not have to take the tank off for this part) then I decreased all 4 by 1 turn. Then I turned the idle set screw very slightly until the rpm was staying around 1200 rpm and not hanging. That was basically all I did.

[flybox1]

the 4 air screws meter air into the carb throats during idle.
IN, cuts off air at idle, and makes your idle mixture RICHER
OUT increases air allowed into the carbs at idle, makes your idle mixture LEANER

for a stock bike, 7/8ths +/- turns out from seated is correct.
they should be set, and left alone unless you make changes to the intake or exhaust.

The idle set screw outside the #4 carb can be set, once your bike is at running temps, and left alone.
no need to adjust or mess with it.
your carbs have a idle cam connected to the choke lever, which should be used to control idle speed during start and warm-up.

[jetthugo]

the 4 air screws meter air into the carb throats during idle.
IN, cuts off air at idle, and makes your idle mixture RICHER
OUT increases air allowed into the carbs at idle, makes your idle mixture LEANER

for a stock bike, 7/8ths +/- turns out from seated is correct.
they should be set, and left alone unless you make changes to the intake or exhaust.

The idle set screw outside the #4 carb can be set, once your bike is at running temps, and left alone.
no need to adjust or mess with it.
your carbs have a idle cam connected to the choke lever, which should be used to control idle speed during start and warm-up.

This is awesome information. Just to clarify, when you say IN, you mean clockwise and OUT as counter clockwise right?


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[harisuluv]

no YOUR carbs:

Clockwise = Richer
Counterclockwise = Leaner

[jetthugo]

no YOUR carbs:

Clockwise = Richer
Counterclockwise = Leaner

Got it! Thanks


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[Scott S]

 You're using the idle mixture screws to sync. That's wrong. There are screws with lock nuts on the linkage that you use.

[Scott S]

 This is a CB550, but if you skip to ~3:45, you can see the screws I'm talking about. They're similar on your bike.

[jetthugo]

You're using the idle mixture screws to sync. That's wrong. There are screws with lock nuts on the linkage that you use.

Yea those screws in the video are the screws I turned to sync the carbs. Two on each side.  Loosened the locknut, turned the screw and once the carbs were matched, tightened the locknuts back up. Isn't that how it's done for the vacuum carb sync?

[Scott S]

 OK.... you kept saying the idle screws (also called IMS or idle mixture screws, or air screws), the ones on the body of the carbs. You DID use the ones on the linkage, right?

[flybox1]

IN Clockwise = Richer
OUT Counterclockwise = Leaner

yes, this is what I said.
its like RONCO Rotisserie ....Set it and forget it!

you have many carb screws
Idle set screw (1)....controls slide height for all carbs at the same time - finger control of idle speed
mixture screws (4) -  depending on your carb, underneath or the outer top edge of the carbs, they meter air or fuel and assist in achieving proper A/F mixture at idle.
vacuum sync screws (4)- depending on carb model, under top caps or between carb tops.  they make micro-height adjustments of the individual carb slide height, to control (sync) carb throat vacuum

[jetthugo]

OK.... you kept saying the idle screws (also called IMS or idle mixture screws, or air screws), the ones on the body of the carbs. You DID use the ones on the linkage, right?

Not sure what you mean by linkage, but the screws I adjusted were these..

[flybox1]

^^ correct.  those are the screws used to vacuum sync the carbs.

[jetthugo]

My bike has been running great these past few days. Thank you everyone for sharing your knowledge and tips!


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