1980 CB750K won't fire in cold weather

This 1980 model CB750K that i just rebuilt carbs on won't fire below 50 degree's. I've ran the battey down completely trying to get it to fire. pulled wires and getting nice blue spark and can smell gas but won't try to hit even when jumped off car battery. I am at a loss. i've pulled choke all the way out, half way, no choke, half throttle, no throttle, full throttle and it doesn't even try. It ran like a scalded dog 2 weeks ago when temps were in 70's so i'm wondering if i need to change plugs. Any advice other than wait til spring?

[brcree]

   Well, it could be a few things.......what weight oil are you runnen?  With my 76 k....I pull in the clutch and it starts like a breeze even in sub 30 deg. weather. Hope this helps.....

I run 10W30 Amsoil full synthetic. I think it needs a good tune-up and i looked at the air filter and it looks like it's original. 18 degree's with snow so i'll wait a couple of weeks and install new battery and give it a good tune-up with NGK plugs and a new air filter and see what happens.

[woodsrider250-2]

Mine was a pain to start as well.  These things are pretty lean stock in most cases.  Cold makes this worse generally.  The vacuum safely petcock is part of the problem.  Motor has to 'pop' to get the vacuum petcock to start working (to let gas through).  You can remove this, and just use the manual petcock on the tank.  Aftermarket coils are a good thing on these.  The stock ones suck.  Mike took a shot of ether to get running in the cold before doing the above.

[brcree]

  Yes, I agree......up to date coils would be better. I'm getten a set of Dyna coils soon. I hate the stock set up....the wires dont make good contact.

Repaced the stock petcock as it had sat since 1983 so didn't take any chances with a fuel problem. Honda tech ( who happens to live next door) suggested a coil change as the 1979- 1982 models had weak ignitions so he's going to pick up a set for me and we'll try that.

[woodsrider250-2]

Which petcock was changed?  There are two.  One you turn by hand, and one that operates on engine vacuum and it's under the tank.  This is the one that can cause problems...esp in cold weather.  There is a vent hose you can blow on as well, and this will 'prime' the system.  I just eliminated the dang vacuum one.

When i took the tank off to inspect i only found the one line from the tank to the carbs. Where would i look for the other line?

[woodsrider250-2]

It looks like this:

http://www.yamahaoftroy.com/fiche_section_detail.asp?section=125908&category=MOTORCYCLES&make=HONDA&year=1982&fveh=2997

Took tank back off bike and could'nt find that valve. This is an 80 model and the repair manual does'nt show it either. Wonder if this is something they added to later models?

[woodsrider250-2]

Yeah it could be 81-82.  Definitely look into a coil swap.

[slash360-2]

I have a 81 750C, runs great but also hard to start when cold. I started putting a small space heater in front of crankcase for about 10 mins., starts on 1st turn. Saves wear on battery and starter.

[SILVERMAN181241]

Hi mate I have the same problem even with electronic ignition and new plugs but I found a shot of easy start
into the carbs soon encourage it to fire up.  Best of luck   Bob

[cbRider]

An often overlooked item is the plug caps. There is a 5000 ohm resistor in each cap. Check it with an ohm meter to be sure they are not reading open.

[shagnasty]

I'm not sure if the DOHC cb750 has a wet clutch or not but I have always been told not to run synthetic oil it will cause the clutch to slip, this is totally unrelated to your problem but for future reference if the clutch starts to slip

Using synthetic on advice from Honda dealership and a mechanic that raced the older 80s Honda's and Yamaha's. Both told me synthetic wouldn't cause problems.

I've got a small heater i use for oil changes on the old Harley and the Honda so oil drains easily in cool weather. Never thought of using it to try to start the old girl. Thanks, will have to buy a new battery for it when it warms up ( 12 degree's this A.M.) but will definately give it a try.

[woodsrider250-2]

Synthetic is OK, it's firction modifiers that can trash cork clutch discs.  IE anything that says 'energy conserving' on the back.  Pick up a bottle of high $ synthetic or dino MOTORCYCLE oil and I guarantee that it doesn't say this on the back.  Neither does cheap stuff.