Topic: Why do my RPMs drop when choked?

[JoeP]

I have a 1975 CB550F with Keihin carbs, velocity stacks with screens, 4-into-2 exhaust with no baffles (not stock, from PO).  The plugs are gapped correctly, the timing is set, valves are set, carbs are synced, the air screws are 1.25 turns out (just a bit richer than stock), stock jets (i think). I have the bike idling well at 1000 rpm without the choke with the engine warm. The throttle response is great (bike in neutral, haven't taken it for a test drive yet). But when I apply the choke, the rpms drop and the bike starts to stutter. I thought the rpms should go up if choked? What's going on here?

[bjbuchanan]

You are choking the bike, that is supposed to happen. You can prove it to yourself on a cold engine in cooler weather. Start it up, snap the choke right off and watch the revs shoot up

[MiGhost]

When the engine is cold. The air entering the carbs is cold making the fuel\air mixture leaner, and harder to ignite. The choke reduces the amount of air entering the carb making the fuel\air mixture richer, and easier to ignite.

If you apply the choke to a warmed up engine where the fuel\air mixture is set for proper running. You create an over rich condition, and the engine will bog, and stumble.

On a set of properly tuned carbs. You should not have to use the choke once the engine is running.

[DavePhipps]

My question is why are you applying choke to an already running engine?
You do know that it's there to aid in the starting of the bike?

[ToryII]

To start my 92 cb750, I wait until after it starts to apply choke, FOR THE PURPOSE OF RAISING RPM. If I keep rpm's up until engine warms then it drives perfect. It's warm lately here, so I can't say about cold temps yet.

I hope nothing is wrong.

[dave500]

its got two cams that's whats wrong.

[Deltarider]

its got two cams that's whats wrong.

Yeah, GET OUT OF HEREEEEE!!!

[JoeP]

When the engine is cold. The air entering the carbs is cold making the fuel\air mixture leaner, and harder to ignite. The choke reduces the amount of air entering the carb making the fuel\air mixture richer, and easier to ignite.

If you apply the choke to a warmed up engine where the fuel\air mixture is set for proper running. You create an over rich condition, and the engine will bog, and stumble.

On a set of properly tuned carbs. You should not have to use the choke once the engine is running.

Thanks, that makes sense. I was comparing with my other fuel injected bike for which applying the "choke" always raised the revs. It turns out that what I thought was a choke lever (on my FI bike) was actually a fast-idle lever that did the exact same thing as opening the throttle.

[JoeP]

To start my 92 cb750, I wait until after it starts to apply choke, FOR THE PURPOSE OF RAISING RPM. If I keep rpm's up until engine warms then it drives perfect. It's warm lately here, so I can't say about cold temps yet.

I hope nothing is wrong.

It's warm here too, so I'll try again in colder weather to see if the choke does what you say and raises the rpm. Doesn't seem like I need to use it at all to start the bike.

[JoeP]

My question is why are you applying choke to an already running engine?
You do know that it's there to aid in the starting of the bike?

Yes, I know. I was just playing around to see what would happen. Then I got mixed up and thought choking resulted in a lean condition. Beer. MiGhost's post cleared things up though.

[JoeP]

its got two cams that's whats wrong.

I don't get it. 

[Deltarider]

I don't get it.

This is SOHC site. Single Over Head Cam

[JoeP]

I don't get it.

This is SOHC site. Single Over Head Cam

You mean I spent 3 hours last night tearing my bike apart looking for that second cam for nothing?!

[bjbuchanan]

The newer sport bikes call it a choke but it's not a traditional choke like on most sohc and older bikes. The newer bikes and certain sohc like pd carb bikes have a fast idle feature to raise revs when choked.

[Bailgang]

its got two cams that's whats wrong.

I was at a Honda shop in Bloomington, IN and the know it all behind the service desk was trying to tell me mine was a twin cam, nope not until 79, then tried to tell me my primary chain was a hyvo chain, hondamatic maybe but not my 77 750. He didn't believe me till he looked it up in a parts fiche himself then got pissed at being wrong and wouldn't talk to me.

[Bankerdanny]

The 77-78 550K models have carb linkages that include a cam to raise the idle speed when the choke is on. The 74-76k bikes and all years of the F models do not.

So on a cold bike you need to choke the bike and give it a bit of throttle to start and idle. On a warm bike you should not need either, but a small amount of throttle may help.

If you use choke on a warm bike all you are doing is starving it of air and since combustion requires fuel, air, and spark removing or reducing any of those 3 will cause poor running or no running at all.

[Bailgang]

and all years of the F models do not.

  Huh? The PD41B's on my 77 750F have the fast idle cam. Forget what I said, this is about 550's not 750's

[TwoTired]

and all years of the F models do not.

Huh? The PD41B's on my 77 750F have the fast idle cam.

The 75-77 Cb550Fs do not.  Want to raise the idle speed, use the idle knob or twist the throttle.  Choke is just and air inlet block.

Why does the o.p. need to know about 750 details?  He has a 75 CB550F.

[Bailgang]

and all years of the F models do not.

Huh? The PD41B's on my 77 750F have the fast idle cam.

The 75-77 Cb550Fs do not.  Want to raise the idle speed, use the idle knob or twist the throttle.  Choke is just and air inlet block.

Why does the o.p. need to know about 750 details?  He has a 75 CB550F.

DOH!! My apologies, for some reason I blurred the lines on that one because I assumed he was referring to 750 F's as well. My error.