The old cold start issue......

[G9DUB]

The thing with these CB650 is they seem to be terrible at cold starting and totally hate being left for a period of time. Depending what you read people say it's down to the head flow being to efficient, incorrect carb jetting, poor quality parts etc, but if they were like this from new how the hell did they ever sell them???????

If I went to a dealership to pick up a new bike and they starting telling me a complete process of how to start the bike ie draining carbs, twist the throttle numerous times, I'll need to crank it over three or fours time etc and hopefully fingers crossed it should start, I would just walk out laughing and tell them to keep the bike.

So why is it so many people seem to have nothing but issues with starting them. Is it really a combination of the things above???????????

Mine is a complete cow to start from cold or if it's been left, but fine if it's used every other day. What is the real problem with these bikes, I just can't believe Honda would produce a bike that bad or even be able to sell them.

I welcome your thoughts and onions ..........

[cb650]

Actually they are not that bad if kept up. They are lean from honda for emissions purposes but they do have accelerator pumps. 35 year old bike that have been mistreated needs help. Can be any number of problems. ACC pump not working dirty tank bad plugs low compression. Most all of mine have started pretty good cold. Sometimes very cold.

[G9DUB]

I see your in the USA which is a lot warmer then the UK so this may be of help. Mines all up together, plugs, leads, coils, clean tank, all correct adjustments, spotless carbs and really good compression but it just hates to be left more than 1 week. And I see the same questions always popping up "cold start problems". Just wondering if there might be an obvious answer...

[Folsoml]

Denver is both high altitude and pretty cold. I'm not sure his climate is the difference.

[cb650]

I have started a few of mine at 15F (-5C?) after setting out over nite. For about 10 years my bikes was my only transport unless it was icy I would use a company truck. On the sohc4 site in the first relay rally ride thread there is pics of a few of us in the snow. 2 of the bikes are mine.

[MEHall]

This cold start issue can be remedied a bit if you use an extra battery.
There is Special power pak (note spelling) that is excellent to use with cold starts.
I had this cold start problem so my battery went weak and i could'nt start my bike so i bought a power pak and viola it started with no problem.
And i used it the last time i started my bike when it had sat for a month, and it worked like a charm even if it took a bit longer before it started.

[Volker_P]

Here may be one part of the answer: viewtopic.php?f=4&t=3142&p=26905&#p26908
Another part is how much of the lightweight parts of the fuel may evaporate during standing from the carb bowls.
And I would guess there are still some more.

[MEHall]

Thanks Volker_P. That is also an issue, fresh fuel always makes it easier to start. Here in Sweden i know there is 10 % ethanol mixed in the gas/petrol and that can make it hard to do a cold start if the bike have sat for long.

A tip is to prime a cylinder with fresh fuel.

So do a check at your closest gas/petrol station and see if the cheapest gas/petrol do have ethanol in it!

[janasonic]

My bike ('80 CB650C) has this very problem. I drained the battery trying to start it for my last ride before my insurance ran out in December. I just charged it and fueled it up the other day and same thing.

The bike had been sitting for years and I had a guy do the carbs last fall (utrasonic cleaning, synching, everything "proper"). Ran fine for the first 100-200 kms but then it became quite hard to start. It also does this thing where, at least when it's still kind of cold, you give it throttle quickly and it dies. Which is very scary cause it's stalled mid intersection on me.

I suspect it's the accelerator pump... Before I take it back to my guy I'm thinking about having a look myself. Wondering: can I get an adequate look in there without taking the carbs off? I assume the main things to look for would be crud or a torn diaphram? Will the carbs need any adjustment after?

Thanks!

[Volker_P]

Welcome here!

The bike had been sitting for years and I had a guy do the carbs last fall (utrasonic cleaning, synching, everything "proper"). Ran fine for the first 100-200 kms but then it became quite hard to start.

Did you clean out the tank? Very essential issue for a CB650.

It also does this thing where, at least when it's still kind of cold, you give it throttle quickly and it dies. Which is very scary cause it's stalled mid intersection on me.

This is more or less normal. The CB650 needs the choke for some time (up to a few miles) to avoid this.
But this should not happen at all any more at warm engine.

I suspect it's the accelerator pump...

I would not think so, rather new gunk from tank or so.
Anyway you may check if the accelerator pump circuit works.
Pull back the airbox a bit to have a look into the carbs from the rear. The rubber boots between airbox and carbs are not straight but carry a mark in case they come out during that by chance or by intention.
If you twist the throttle, there should be a slight slay of fuel from the bottom on the position of the connection/distribution tubes for the accelerator pump (for position of the "accelerator jet" see the nice pictures of jcasey007 in the Accelerator pump issues thread).
I would check this only for one outer carb (#4).

[janasonic]

Thanks... I did this and yes, I see fuel in there when I twist the throttle, so I guess the acc pump is ok.

I cleaned the tank with Seafoam when I got the bike. I pulled the petcock & filter today and it looked fine.

Hmm... what's next?

[Volker_P]

From your first post I assume you did already read this: viewtopic.php?f=4&t=727&start=9

Another issue is that idle must be set very low. Turn out the black idle knob completely. Then turn it back to contact to the lever and another 1/2 to 3/4 turn in. If your black idle knob is further out, things will become more annoying. If it needs to be further out for a good idle when warm, your carbs might be out of sync.

You may check carb synchronization visually (with the carbs off): If you turn out the black idle knob completely and then in again, all four sliders should open simultaneously to the same tiny gap.