Author Topic: 1975 CB750F Help with Choke and Idle (Read 4270 times)

izzyryder · « **on:** March 30, 2015, 05:33:23 AM »

Here is what's going on:

When I go to start the bike, if the choke lever is up the idle slows (like it's starving for gas) and dies out. If the lever is left down, the idle speeds up. The the only way i can keep the revs up during startup is by turning the throttle stop above the carbs (instead of holding the throttle slightly open manually). Then once the bike warms up, I have to manually turn the throttle stop back (other wise it will be idling at like 2k).

I'd like to be able to start the bike and have the choke work correctly. If I have to make adjustments to anything I will, but need to know what a good place to start would be. Carbs are in good shape and cleaned. The idle screws are backed out 1 full turn (if that's not right please let me know). Any help would be appreciated.

izzyryder · « **Reply #1 on:** March 30, 2015, 05:38:26 AM »

And just to ask.. because this would explain a lot.. which way is "ON" for the choke lever? When it is Vertical to the ground or Horizontal?

DH · « **Reply #2 on:** March 30, 2015, 06:02:41 AM »

Choke is fully on when lever is pulled up and pointing outward. Choke off when the lever is straight down. Idle speed set with engine at operating temp, fully warmed up. Hondas are cold blooded, and the choke is usually needed (in various states of closure) during warm up. This is normal. Turn the choke off when engine is warm, and runs normally.

Stev-o · « **Reply #3 on:** March 30, 2015, 06:04:04 AM »

I'm not sure what carbs are on the 75 F, round top? But I'll try to help.

First off, set your idle when the bike is fully warmed up, around 1400 is good (I like it a little higher than some).
You state the idle screws are backed out 1 turn, do you mean the air screws?
When the choke is on and is "starving for gas", it most like wants more air, reduce the amount of choke.

ekpent · « **Reply #4 on:** March 30, 2015, 06:13:03 AM »

The round top carbs used on your bike do not have a fast idle circuit built into them like more modern carbs. So fiddling with the choke lever, holding throttle or upping idle when cold is a pretty normal routine with these.

izzyryder · « **Reply #5 on:** March 30, 2015, 06:13:29 AM »

Thanks for the quick response guys..

Am I am mis-using the choke lever? Pull up is "off" and "down (vertical to the ground) is "on". Seems like it should be the opposite. Is something wrong wtih the lever? Maybe that's where my problem is. Because the engine idle speeds up when the lever is down (vertical to the ground).

Yes I did mean air screws --back out 1 full turn. Is that correct? I know I've read some differences on 1 turn, 1 1/2 turns, etc.

ekpent · « **Reply #6 on:** March 30, 2015, 06:55:15 AM »

Lever up is choke on and down if off.

izzyryder · « **Reply #7 on:** March 30, 2015, 07:16:25 AM »

Ok got it.. so my question is.. why does the engine idle slow down and die when the lever is up, but when I put the lever down it speeds up?

flybox1 · « **Reply #8 on:** March 30, 2015, 07:23:23 AM »

Quote from: izzyryder on March 30, 2015, 07:16:25 AM

Ok got it.. so my question is.. why does the engine idle slow down and die when the lever is up, but when I put the lever down it speeds up?

In the HORIZONTAL position, you are choking off air to give the carbs a richer mixture for easier start-ups. When warm, you will choke out the engine, as you are describing. when your bikes idle slows, its needing MORE AIR, so its telling you to push the choke lever down.

In the DOWN position, the choke butterflies are open. This is where it should be set for normal operation. Allows max air through the carbs.

When your bike is all warmed up, and your choke lever is in the DOWN position, set the idle with the idle screw.
1000-1500. somewhere where it idles smoothly. You really shoudn't need to change this idle screw anymore.

Put the choke lever into the HORIZONTAL position for start-ups....the bike will tell you when it needs this lever pushed down. Mine bike usually needs small incremental changes of the choke over the first minute or two, and then its ready to go..

izzyryder · « **Reply #9 on:** March 30, 2015, 07:48:09 AM »

Quote from: flybox1 on March 30, 2015, 07:23:23 AM

Quote from: izzyryder on March 30, 2015, 07:16:25 AM
Ok got it.. so my question is.. why does the engine idle slow down and die when the lever is up, but when I put the lever down it speeds up?
In the HORIZONTAL position, you are choking off air to give the carbs a richer mixture for easier start-ups. When warm, you will choke out the engine, as you are describing. when your bikes idle slows, its needing MORE AIR, so its telling you to push the choke lever down.

In the DOWN position, the choke butterflies are open. This is where it should be set for normal operation. Allows max air through the carbs.

When your bike is all warmed up, and your choke lever is in the DOWN position, set the idle with the idle screw.
1000-1500. somewhere where it idles smoothly. You really shoudn't need to change this idle screw anymore.

Put the choke lever into the HORIZONTAL position for start-ups....the bike will tell you when it needs this lever pushed down. Mine bike usually needs small incremental changes of the choke over the first minute or two, and then its ready to go..

Understood. The problem i'm having is that with the choke up, the bike will die out unless i adjust the idle screw to raise the idle.. after the bike is warmed up, and i lower the choke completely, the bike will rev at 2k to 3k RPM (so I have to lower the idle screw back down).

Could this be indicative that the throttle or idle cable is not properly adjusted?

DH · « **Reply #10 on:** March 30, 2015, 07:53:46 AM »

With the choke on, air to the engine is cut off. Raising the throttle by the idle screw or otherwise overcomes this.
It's better to use part throttle/choke combination to keep the engine running till its totally warm, then it should idle on its own, and the choke at that point, will not be needed.
Edit, flybox's last paragraph says it best.

flybox1 · « **Reply #11 on:** March 30, 2015, 08:20:03 AM »

Quote from: izzyryder on March 30, 2015, 07:48:09 AM

Understood. The problem i'm having is that with the choke up, the bike will die out unless i adjust the idle screw to raise the idle.. after the bike is warmed up, and i lower the choke completely, the bike will rev at 2k to 3k RPM (so I have to lower the idle screw back down).

Could this be indicative that the throttle or idle cable is not properly adjusted?

I see where you're coming from...your choke lever operation is fine, your bike just needs the slides lifted(idle screw in) a bit more on a cold start....and then when its warm, choke lever slid down and idle screw backed out ...

Please include the model # carbs you have on this bike. Usually stamped on the side of #4 , just above the carb bowl.

izzyryder · « **Reply #12 on:** March 30, 2015, 08:24:27 AM »

Quote from: flybox1 on March 30, 2015, 08:20:03 AM

Quote from: izzyryder on March 30, 2015, 07:48:09 AM

Understood. The problem i'm having is that with the choke up, the bike will die out unless i adjust the idle screw to raise the idle.. after the bike is warmed up, and i lower the choke completely, the bike will rev at 2k to 3k RPM (so I have to lower the idle screw back down).

Could this be indicative that the throttle or idle cable is not properly adjusted?
so, on a cold start, with the choke lever HORIZONTAL....your bike dies out immediately? or after it warms a bit?

When cold-If the choke is horizontal the bike will not start at all unless I increase the idle stop to hold the throttle open.

flybox1 · « **Reply #13 on:** March 30, 2015, 08:28:07 AM »

I see where you're coming from...your choke lever operation is fine, your bike just needs the slides lifted(idle screw in) a bit more on a cold start....and then when its warm, choke lever slid down and idle screw backed out ...

what model carbs do you have?

evanphi · « **Reply #14 on:** March 30, 2015, 08:31:43 AM »

Quote from: izzyryder on March 30, 2015, 07:48:09 AM

Quote from: flybox1 on March 30, 2015, 07:23:23 AM
Quote from: izzyryder on March 30, 2015, 07:16:25 AM
Ok got it.. so my question is.. why does the engine idle slow down and die when the lever is up, but when I put the lever down it speeds up?
In the HORIZONTAL position, you are choking off air to give the carbs a richer mixture for easier start-ups. When warm, you will choke out the engine, as you are describing. when your bikes idle slows, its needing MORE AIR, so its telling you to push the choke lever down.

In the DOWN position, the choke butterflies are open. This is where it should be set for normal operation. Allows max air through the carbs.

When your bike is all warmed up, and your choke lever is in the DOWN position, set the idle with the idle screw.
1000-1500. somewhere where it idles smoothly. You really shoudn't need to change this idle screw anymore.

Put the choke lever into the HORIZONTAL position for start-ups....the bike will tell you when it needs this lever pushed down. Mine bike usually needs small incremental changes of the choke over the first minute or two, and then its ready to go..

Understood. The problem i'm having is that with the choke up, the bike will die out unless i adjust the idle screw to raise the idle.. after the bike is warmed up, and i lower the choke completely, the bike will rev at 2k to 3k RPM (so I have to lower the idle screw back down).

Could this be indicative that the throttle or idle cable is not properly adjusted?

Re-read what flybox1 said.

Forget about idle speed for now. Use the throttle at the bars to keep the engine speed up as the bike warms up with the choke on (horizontal lever). Once it is warmed up (take it for a spin around the block), and the choke is OFF (vertical lever), set your idle speed using the adjustment knob on the carbs to 1K-1.5K rpm (personally mine is set best at 1100). THEN DON'T TOUCH IT. When these bikes are cold you have to sit on the bike and hold the throttle open with your hand, unless you have the fancy pinch bolt in the right controls.

theslayedsaint · « **Reply #15 on:** March 30, 2015, 08:43:59 AM »

YAY its not just me!

Jerry Rxman Griffin aka MuthaF'er · « **Reply #16 on:** March 30, 2015, 08:46:46 AM »

Then don't fully engage the choke. Partial only. It's going to depend on the temperature. You may not need to choke it or choke it much.

izzyryder · « **Reply #17 on:** March 30, 2015, 08:59:27 AM »

Quote from: flybox1 on March 30, 2015, 08:28:07 AM

I see where you're coming from...your choke lever operation is fine, your bike just needs the slides lifted(idle screw in) a bit more on a cold start....and then when its warm, choke lever slid down and idle screw backed out ...

what model carbs do you have?

Keihin is the model of the carbs.

It sounds like I need to change up the procedure in how I'm warming the bike up. It's just weird because take the other day for example.. it was cold out..the bike was cold.. the choke did nothing. it's almost like it really does nothing except completely cut the air off to the carbs.

evanphi · « **Reply #18 on:** March 30, 2015, 09:02:13 AM »

Quote from: izzyryder on March 30, 2015, 08:59:27 AM

Quote from: flybox1 on March 30, 2015, 08:28:07 AM
I see where you're coming from...your choke lever operation is fine, your bike just needs the slides lifted(idle screw in) a bit more on a cold start....and then when its warm, choke lever slid down and idle screw backed out ...

what model carbs do you have?

Keihin is the model of the carbs.

It sounds like I need to change up the procedure in how I'm warming the bike up. It's just weird because take the other day for example.. it was cold out..the bike was cold.. the choke did nothing. it's almost like it really does nothing except completely cut the air off to the carbs.

Your constant adjusting of the idle screw may have it too low to begin with.

ekpent · « **Reply #19 on:** March 30, 2015, 09:03:48 AM »

The whole choke deal on these earlier 750's are pretty primitive--Think lawnmower

evanphi · « **Reply #20 on:** March 30, 2015, 09:05:39 AM »

Quote from: ekpent on March 30, 2015, 09:03:48 AM

The whole choke deal on these earlier 750's are pretty primitive--Think lawnmower

Or snowblower for us still in a freeze.

izzyryder · « **Reply #21 on:** March 30, 2015, 09:18:14 AM »

yeah.. looks like i have to readjust my mindset on some things

for a '75F does 1 full turn for the air mixture seem right?

flybox1 · « **Reply #22 on:** March 30, 2015, 09:20:16 AM »

Quote from: izzyryder on March 30, 2015, 08:59:27 AM

Keihin is the model of the carbs.

keihin what?

looked for the stamped alpha-numeric part #

what exhaust and are filter systems do you have installed? These, in part, help determine mixture screw setting..

izzyryder · « **Reply #23 on:** March 30, 2015, 09:35:50 AM »

Quote from: flybox1 on March 30, 2015, 09:20:16 AM

Quote from: izzyryder on March 30, 2015, 08:59:27 AM
Keihin is the model of the carbs.
keihin what?
looked for the stamped alpha-numeric part #

what exhaust and are filter systems do you have installed? These, in part, help determine mixture screw setting..

Sorry i'm at work so don't know the alpha-numeric part #.

The exhaust is a 4 into 1 header with (from what I can tell) a Mac Muffler. The air filter is stock.

mattmac · « **Reply #24 on:** March 30, 2015, 12:13:36 PM »

Hey guys, Im new to this form and new to being a cb750 owner. I've been reading and educating myself as much as possible. huge fan of these bike.
I have a 73 750 and I got a great deal on it. it runs pretty decent gotta sync them carbs and a few odd and ends. my question is on the carbs you have the 2 fuel feed nipples, above them more so in the middle theres 2 more nipples, what are these??? should something be hooked up to them? Im sorry if im putting this on the wrong page.
any info would be great guys!!
matt

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Author Topic: 1975 CB750F Help with Choke and Idle (Read 4270 times)

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