Topic: 1979 CB650 hard cold start

[BVCB650]

Hi Folks. I have been rebuilding a 650 and have run into an issue. First, a littl history. I live at 8000 feet. Rebuilt carbs, new brass, pilot screws out to 2 1/2 turns, stock air box, new plugs, wires, CDI, Valves properly set, runs like a scalded dog. Only issue is it is real hard to start in the morning. Starts with just a bump of the switch when warm. Sounds like a lean issue, but choke works fine and it has a big fat blue spark at each of the plugs. Starts just fine with a 1 second spray of starting fluid. Any ideas? Thanks.

[Goatman]

I also have a 650, 82' CB650SC. Mine is also hard to start when cold. Once warmed up it starts just fine. Seems like the longer it sits the harder it is to start.

Have you solved your hard starting issue??

Thanks

[MJL]

Same thing on my '81. Maybe not hard to start, but it definitely takes longer than it does later in the day.

Now in the springtime, bringing it out of winter hibernation, that is a true battle of will.

[Goatman]

Here's one out of the blue, I have a friend who's outboard motor mechanic told him to totally run the engine out of gas after each use. He tried it and it actually worked. The engine now starts right up, even after sitting for a couple weeks. His outboard is the same vintage as our bikes. I am not equating old mercury outboards to our Honda's but it is interesting. The last time I rode I ran the engine out of gas B4 parking. I plan to ride this weekend and will let you know what happens.

[Hush]

My 79 650Z had been a #$%* to start but since I get to ride it more regularly I don't seem to have that problem, still have to start it then leave it with the idles turned up while I get into my riding gear then drop the idles down again before taking off.
I suspect it is something to do with our carbs, if I ride the bike once then leave it all day it will still fire right up so maybe getting that initial gas injection from the carbs is a flaw in the old 650.
My starting regime is: hold out the knob to get choke with left hand, slight twist of throttle, hit button.   BBBBBBBRRRRRRRRRRRRRRRRROOOOOOOOOOOOOOMMMMMMMMMMMMM

[redschwa]

My cb650sc does the exact same thing. My strategy is three pumps of the throttle followed by 3/4 choke and it only seems to start when it's straight up, not on the kick stand, all of this and i usually get it on the second or third push.

[scunny]

when cold, 650Z, turn petcock on, couple of quick throttle pumps, hit starter, usually catches on 2 or 3 cylinders, the other/s then chime in.
when warm, just look at it and it will start smooth as pie

[redschwa]

when warm, just look at it and it will start smooth as pie

too true

[Hush]

Damn, if I ever win Lotto I'm gonna fly all the 650 guys and their bikes down here for a ride around New Zealand, what a hoot that would be.

[Goatman]

Well the 650 doesn't respond like the Mercury outboard, oh well. I disconnected the little vacuum fuel switch thinking that might be part of the hard start issue. Doesn't seem to help much. Full choke, a bump on the starter then a few throttle twists done a few times in a row seem to be the trick on my bike, once warm starts right up.

[k12techman]

Thought I’d try bumping this before asking the same question again……

I just picked up a ‘79 CB650 that is as cold hearted as I’ve ever seen. First start of the day is brutal. Every start thereafter is a simple bump of the switch.

Anyone ever come up with something concrete on why?

[Flyin900]

Went through this recently with a buddies 1979 CB 650 PD Series carbs, plus he also had running issues too. I am a pretty familiar with all styles of Honda carbs over the years and models.

These carbs use an accelerator pump circuit which consists of a pump diaphragm located on the bottom of carb #2 and a separate fuel rail to carry fuel to small brass nozzles in each carb throat close to the choke plate side. The fix can be as simple as the mechanical lever that actuates the rod and diaphragm of the pump assembly isn't making enough contact for a full stroke of the pump. There is a spec for this gap in the Honda service manual.

The accelerator pump assists in both starting the bike by shooting a stream of fuel into the motor when the throttle is twisted. It also assists when you crack the throttle instantly and want the instant response.

Two issues I found on buddies carbs were someone had been in there previously and the accelerator pump rod on these PD Series carbs is much longer than a regular accelerator pump unit. There is actually a small section bumped out on the side of the carb with a hole drilled through for the rod to pass through. The purpose is to support the rod as it travels up to the linkage area where it is activated.

Someone has missed the hole so to speak in the carb body and the rod was not aligned or working, as it was no where near the linkage mechanism for activation. Hence the poor starting and other running issues once underway.

The second issue I found in general on many accelerator pump circuits is while you can test the accelerator pump circuit with the carbs on the bench with a gas bottle and actually confirm the brass nozzles are working by rotating the bell crank assembly. What you don't see or know is how well these brass nozzles are actually spraying; 100% vs say 30-50% and so appearing clear to a casual observer. That missing 50-70% spray makes for some other funky running issues along with continued hard starting.

I can't make any recommendations beyond really.. really .. cleaning all the circuits and especially those brass nozzles in the carb throat. I almost always replace the accelerator pump diaphragm, as there are many aftermarket kits out there, so your starting with a fresh not 40 year old diaphragm. All of the small circuits and passages in the complete assembly need to be cleaned and verified clear.

[fizzlebottom]

The same exact accelerator pump issues apply to the later CV carbs too. Replacing the diaphragm is easy, but you need to cut the rod to the correct length. Cleaning the passages requires blasting carb cleaner through the tiny holes as much as possible and hoping you got all of it. After all that, there's a tab that presses the rod at nearly full throttle which can be adjusted by bending towards or away from the rod tip according to specs in the FSM.

So as much as design changed between PD and CV carbs, the same principals mostly apply.

[Scotiabiker79]

So I rebuilt a 650 in past few months.  Had problems with cold start.  The trick of putting on centre stand 3 pumps of throttle and choke work like a charm.  Thanks to all members on here for the knowledge you offer