Topic: CB650 Choke Start Issues

[jinx2350]

I recently rebuilt the carbs on my '79 CB650. Prior to the rebuild of the carbs, this bike was cold-blooded, hard to start, and it helped a lot to give FULL choke, then slowly easing off the choke while it warmed up. It was a bit of baby-sitting, but I've been used to it.

I did make 2 small changes to the jetting during the rebuild since it has a 4-into-2 exhaust, and was never rejetted. I updated to 100 mains, and 38 slow jets. Not a huge jump really.

So the issue is that now the bike won't start with the choke on, but is still VERY cold blooded. It requires a lot of coaxing, and endless cranking.

Another catch is the bike really LOVES to start when I cover the air intake (with a shop rag, or even just my open palm). I thought I had somehow jammed up my choke flaps when re-installing the airbox. I removed the airbox side boots enough to see it was working properly, so ruled that out.

The rag method isn't a long term solution as it causes a vacuum of some kind and gas gets sucked into the airbox and pours of out of the overflow at the bottom of the box.

Anyone got any ideas on what could have made such a drastic change in the choke and starting?

[MetroRedneck]

What if you crack the throttle for accelerator pumps a couple of times and then start on full choke?

My 650 is cold natured as well but it will fire off with a bump of the starter every time as long as the battery is hot.  It needs a hot battery, full choke, and the choke needs to be babysat for a few minutes after it starts.  I also prime with my accel pumps a couple of times if it has been sitting.

[Bankerdanny]

Your description of the problem is a bit confusing.

First, full choke is generally always required to start a completely cold bike except for maybe during the hottest times of the year. My bikes have all responded differently, some could come off choke almost immediately, some took longer. Generally the newer the bike the longer the choke time because later model carbureted bikes tend to be tuned leaner due to evolving emissions control rules.

Covering the intake with a rag or your palm is functionally the same as engaging the choke. You are reducing the flow of air and richening the mixture.

If your choke is working properly then covering the intake should make no difference at full choke since the closed butterfly effectively blocks the carb throat and further restricting airflow should not make a difference.

You say that the choke butterflies are working properly, but if that were the case I wouldn't expect blocking the intake to make any difference during a cold start.

[Paintedseat]

Make sure the choke is closing.

[scunny]

+1 on giving a couple of wacks with throttle to prime before starting, when the bike hasn't run for a while.

[Bankerdanny]

+1 on giving a couple of wacks with throttle to prime before starting, when the bike hasn't run for a while.

Just like cars back before fuel injection was standard, a couple of pumps on the gas to squirt some fuel into the manifold.

[MetroRedneck]

Fired up my 650 last night because this thread reminded me it had been a while.  Had to hit the accelerator pumps 6 or so times before it finally popped off.  Then a couple more cranks and another pump or two, then choke on for a few minutes.  I don't remember my jetting specifically, but I am pretty close.  Compared to my choke and kick 550 it is a different process entirely different and seems finicky.

Close the choke and gets some gas in her throat.

[jinx2350]

Your description of the problem is a bit confusing.

First, full choke is generally always required to start a completely cold bike except for maybe during the hottest times of the year. My bikes have all responded differently, some could come off choke almost immediately, some took longer. Generally the newer the bike the longer the choke time because later model carbureted bikes tend to be tuned leaner due to evolving emissions control rules.

Covering the intake with a rag or your palm is functionally the same as engaging the choke. You are reducing the flow of air and richening the mixture.

If your choke is working properly then covering the intake should make no difference at full choke since the closed butterfly effectively blocks the carb throat and further restricting airflow should not make a difference.

You say that the choke butterflies are working properly, but if that were the case I wouldn't expect blocking the intake to make any difference during a cold start.

I understand it sounds confusing, that’s becaus it is confusing. That’s why I came here! Moderate choke seems to be ok, like half shaft pulled out. It sputters a bit and acts like it’s about to fire up. The second I do the rag  over the intake it comes alive and fires. I understand this effectively “chokes” but it acts very different than choking does.

[jinx2350]

What if you crack the throttle for accelerator pumps a couple of times and then start on full choke?

My 650 is cold natured as well but it will fire off with a bump of the starter every time as long as the battery is hot.  It needs a hot battery, full choke, and the choke needs to be babysat for a few minutes after it starts.  I also prime with my accel pumps a couple of times if it has been sitting.

This is exactly how mine acted before I rebuilt the carbs.

[jinx2350]

This is still happening.

Fires and tries to run with no choke, but won’t stay running.
Twists of throttle result make it worse.

Really hates full choke. Like isn’t even firing just turns over forever with no signs of firing.

Rag over the air intake with about 1/4 choke works ok, and then requires a little baby sitting. Combination of throttle and tiny bit of choke works. Turning up the idle screw helps plenty.

I’m not sure what it’s called but I recall there’s some kind of accelerator circuit in the choke. It’s essentially gives turns the throttle a little a full choke. Perhaps I set that wrong and it’s flooding with the choke on??

I’m kind of stumped with how different it’s acting after a carb rebuild.

Oh, and just to reiterate, fuel somehow backs into the air box and is come out the overflow drain right above the front sprocket and foot peg on the left side. Carb over flow tubes do not over flow.

🤷🏻‍♂️



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[jinx2350]

I’m curious now if upgrading to the 40 slow jet has caused this bike to flood when it’s starting? It really hates to get any throttle when starting.

Also, this is kind of normal for these SOHC4 Bikes, but if it sits in the sun and gets nice and warm it starts much easier.

Should I try the stock slow/isle jets again?


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[CB650CPastor]

These carbs can be a bit fiddly. It's easy to see why fuel injection caught on.

My CB650 used to be very hard to start and exhibited several of the symptoms you pointed out with your bike. The CB650 is designed to start without any throttle input (as the owners manual states) and the choke on full. This took me a while to get to. After a complete carb rebuild, for me, it was the high idle setting that needed to be dialed in. (I have the 1980 with the mechanical slide carbs.) If the cold idle is set too high, it will get too much air to start the bike. The start mixture needs to be rich.

Usually, guys resort to pumping the throttle to add more gas to make up for the fact that the engine is getting too much air at cold start. That might work as a band-aid, but it can also flood the bike as it was not designed to start by pumping the throttle. (The pilot screw settings need to be correctly adjusted to get good starting as well.)

The idle/pilot jets in the PD50B carbs on my bike have REALLY small orifices! I needed to use the smallest wire I could find to gently clean them out. Make sure yours are clean and clear. You didn't mention what year CB650 you have. Of course, if you have the CV carbs, this advice may not fit. (They switched to CV in 1981.) I'm just sharing my experiences in case it might help you or someone else.

Ah, I just noticed, by your signature, that you have a 79. This advice should apply to you as you have the mechanical slide carbs too.