Topic: Hard cold starts

[GurgleMurgle]

1975 CB750F in poor to fair condition...

I’ve found that for a real cold start - say 45 degrees F, bike hasn’t run in several days, and bowls empty, this bike is very slow to start.  My procedures as been this - fuel on, full choke, wait maybe 10 seconds, switch and ignition on, headlight off, CRANK for several seconds.   No signs of life.  Wait a few more seconds, repeat. 2 or 3 more of these and one cylinder will start to pop off.  Repeat a couple more and I can keep it running by nursing the throttle.    By the end, the starter is spinning the engine over noticeably slower. 

If it has only been a day or so since the last run, it’s much easier to start.  It may take a couple of tries but it’s not really a problem.


With the engine even remotely warm I can start it with my hand gently and slowly pushing down the kicker. Not that you’d want to but it starts very easily.  Which makes me think spark, fuel, and timing are all OK otherwise.



So I’m thinking it’s just taking longer than I want it to to fill up the bowls. About how long should no wait after turning on the fuel assuming dry bowls?  A minute? 5? 10?    I’m afraid I’m going to wear out my starter and I think this battery is already on its way out. 

[newday777]

What oil weight are you using at 45 degrees? If you still have 20w50 in it, that will make turning over hard in cool temps and will eat battery amps down.

Are you keeping the battery on a tender? I find it is a good practice to keep the tender on after every ride so the battery is at full charge the next time I ride as a lot of times, as even in the summer,  I ride it once a week, maybe more. Having the battery at full capacity is good insurance.

[newday777]

But....
Are you running points or electronic ignition? Points cleaned?
When is the last time you did a 3,000 mile service?
The hard starting is also indicating you need to clean your carbs as the poor running at fire up you are experiencing is a sure sign.

[GurgleMurgle]

I’m running 15W-40 in it, and it is fresh.   So it could be a bit thick in the cold. 

And yes I keep that battery up with a tender.  It will hold a standing voltage of about 12.4 which makes me think it’s a little long in the tooth.  I won’t trash it until it’s really toast. 


I’m running points and original coils which seem to provide good spark.  The points have been cleaned and gapped and ignition timing verified very recently.  Same for the valve clearance adjustment.  Spark plugs are new Densos.  Plug caps are in good condition with proper resistance, very even.   

The carburetors are squeaky clean but maybe not perfectly tuned.   It runs a little rich, which if anything should make it start a little easier.  It doesn’t bog under power, just a little sooty on the plugs.  So I don’t think it’s extreme.   

[newday777]

I’m running 15W-40 in it, and it is fresh.   So it could be a bit thick in the cold. 

And yes I keep that battery up with a tender.  It will hold a standing voltage of about 12.4 which makes me think it’s a little long in the tooth.  I won’t trash it until it’s really toast.

The 15w40 is fine at 45 degrees.

Points or electronic ignition?
3,000 mile service done? Points cleaned and adjusted, timing, timing chain and valves adjusted, carbs synced?
Are carbs clean?
Is the tank rust free inside? If not that'll clog up the carbs.

[GurgleMurgle]

I’m running points and original coils which seem to provide good spark.  The points have been cleaned and gapped and ignition timing verified very recently.  Same for the valve clearance adjustment and cam chain tension.  Spark plugs are new Densos.  Plug caps are in good condition with proper resistance, very even.

Timing verified with a strobe -  the advance mechanism works properly though I don’t think that would affect starting. 

Im using a k&n air filter in the original airbox in good clean condition, properly oiled - not dripping just a good spritz.

The carburetors are squeaky clean but maybe not perfectly tuned.   It runs a little rich, which if anything should make it start a little easier.  It doesn’t bog under power and pulls nicely at all engine speeds, just a little sooty on the plugs.  So I don’t think it’s an extreme.

All 8 carb rubbers are new (they sure think a lot of those things...$$$$).  Spraying flammable things around has not revealed any vacuum leaks. 

Fuel lines are new and the proper Honda stuff ($$$ again).

The Carburetors have been vacuum balanced recently. 

At some point this winter I’m sure I’ll have them off again.   What fun. 


I have not had any rust issues from the tank but I have run several gallons through since I last had the petcock off.  I should take it out and inspect the screen again. 



Overall it runs pretty good and warm starts perfectly. 

[newday777]

How did you clean your carbs?
I looked through your posts and didn't find your replies to your other posts on the carbs and questions given you on parts you got for your carbs. Do they have the Keihin brass in them still?

[GurgleMurgle]

Carburetors were cleaned by spraying copious amounts of carb cleaner and compressed air through every orifice.  The o-rings and little pieces of fuel line on the tees between 1&2 and 3&4 were replaced.  The jets were replaced with no name parts (yeah I know...).  I do have the original keihin jets that have been absolutely butchered by someone trying to get them out (not me this time, for once), pilots and mains.   Also the float heights were all over the place.  So someone even more hamfisted than I has been in there.  The emulsion tubes look OK. The carbs weren’t what would describe as “squeaky clean” at that time but nothing was completely blocked. 

The needles are original, and I think the needle jets are too - I did not attempt to remove the needle jets. 

It ran before I did all of this it ran extremely rich at all throttle positions including idle.  Soot clouds out the tailpipe rich.   I’ve gone down a size on the pilots and lowered the needles one notch. They did NOT* appear worn to me, but I suspect I’m compensating for some wear in doing that. 

These changes were made 1 at a time with improvements noted with each.

I have no idea if the jet “sizes” are true to what they claim, but they do appear uniform and smaller than the originals  for what that’s worth (not much).

[newday777]

Good replies. 
Where do you live?(that isn't in your profile)
I'm in southern NH so my K5 is in hibernation because of salt on the roads and temps are chilly 19 degrees this morning, snow on the ground is crusty.
Do you have a stock motor or is it built?

You should probably do an ultrasonic clean of your carbs to ensure the internal passages are clear, you may have some blockage in the supply ports?? Thus the hard starting. Where did you get the Denso plugs from?

[jpdevol]

I'm wondering if the oil in the tank is leaking - over a period of several days - into the sump and causing the varied hard starting symptoms you describe? Easy enough to verify level in the tank after sitting short period vs. longer period.

[carnivorous chicken]

1975 CB750F in poor to fair condition...
So I’m thinking it’s just taking longer than I want it to to fill up the bowls. About how long should no wait after turning on the fuel assuming dry bowls?  A minute? 5? 10?    I’m afraid I’m going to wear out my starter and I think this battery is already on its way out.

This. Give it more time instead of wasting battery power and trying to start it too soon. 30 seconds? Trial and error. If my 350F sits for a week I give it 30 seconds or so and that does the trick.

Obviously want to get other running issues sorted, but it seems to me you answered your own question in the first post.

[GurgleMurgle]

Located in the southeast where temperatures in the teens are rare.  It does happen occasionally though, likely this weekend. 

The densos were sourced from rock auto which I reckon is a reputable vendor.   


I’ve considered an ultrasonic cleaner but keep talking myself out of it.   I just can’t get over the thought of how many zillions of carburetors have been perfectly done without one.   I’m sure I’d find plenty of other stuff to put in it if I had one. 

[newday777]

Well if you have an old Coleman camp stove you could get a disposable pan and boil them clear in a simple green and distilled water mixture for 20 minutes.

[HondaMan]

At some point this winter I’m sure I’ll have them off again.   What fun. 

When you take them off, replace the O-rings in the faces of the 4 intake castings. They are likely hard now. When hard and dry they leak vacuum (which often shows up as dark sparkplugs), and when wetted by a few starting attempts they tend to seal a little better. This matches your bike's symptoms.

These O-rings are the same size as those found in the valve caps on the engine.

Also: if the mainjets were changed, they probably now have the wrong O-rings on them (all the current vendors but JetsRus are supplying the wrong ones, even in carb kits) and they are too skinny, letting extra fuel seep past them. This makes the bike both rich running, and hard to start cold because the extra fuel is drowning the plugs. I have the correct size O-rings, if your carbs are that type. The 550F bike used PD carbs with screw-in mainjets to solve this problem (which the first 550 inherited from the CB350 twin carbs!).