CB750F3 trouble starting in cold weather

adam750

adam750

New Member
I have a '78 CB750F Super Sport, it is in extremely clean condition, no rust, no old cables, etc... and it does not seem to want to start in temps below ~52 degrees. Once it's warm it idles great, and it runs great in all gears. The previous owner put a Dyna system in, and I'm wondering if that may be the cause of the problem. Battery is strong, carbs have been recently cleaned/synced, new spark plugs.

I have to hold down the elec start for a long time (when it's cold) while the engine is trying to start. One trick that seems to help is to adjust the idle speed to much faster than it should be. I do this just when I'm trying to start the bike, I have to slow down the idle speed once it warms up(otherwise it would be idling at ~2500RPMs).

I'm pretty new to maintenance/adjustments... but I was planning on trying to diagnose the problem by taking the carbs apart, checking the timing, float levels, etc.. I was just wondering if anyone has advice in where to start?

Thanks in advance
 
Do you give it a little gas when you try to start it? Old bikes (cars, trucks) usually need a little finesse and figuring out to find the sweet spot for starting.
 
No, actually it hinders the process a bit (It kills it!). The manual says the accelerator pump makes it unnecessary to use the throttle when starting anyway.
 
Does it not have a choke? That should be all you need, all my honda bikes I have or have owned need a choke on cold days and cold engine to start. Process is, pull choke, fire up, let idle till revs go up, push choke in until idles normal if all the way, ride, if not, wait for revs to climb and push choke rest of way in.

Edit, your bike came with a choke, if it isn't there, buy one. Bike bandit has them. Will solve your problem, it's what it is for.
 
Has a choke. The choke doesn't seem to help much until the bike is warm. Maybe a problem with the choke adjustment?
 
adam750 said:
Has a choke. The choke doesn't seem to help much until the bike is warm. Maybe a problem with the choke adjustment?
Click to expand...

OK so something is out of whack with your fueling. Choke helping when bike is warm means you are jetted way lean or something is clogged. If you crank the idle adjuster up to get it going you are opening the slides to allow more air/fuel in, choke blocks air and increases fuel?? Something is not adjusted right. Your bike should stall and run like crap if the choke is pulled after it is warmed up.
 
The bike runs great actually. The only noticeable problem is difficulty starting in colder weather. Maybe I'm explaining the choke problem incorrectly, it helps once the bike has started, while it is warming up. Once the bike is warm and running, I do not need the choke, nor do I use it then. It does seem like a fuel issue. Where should I start to look for a problem with fuel? the carbs?
 
adam750 said:
The bike runs great actually. The only noticeable problem is difficulty starting in colder weather. Maybe I'm explaining the choke problem incorrectly, it helps once the bike has started, while it is warming up. Once the bike is warm and running, I do not need the choke, nor do I use it then. It does seem like a fuel issue. Where should I start to look for a problem with fuel? the carbs?
Click to expand...

Ok this makes more sense. Old hondas are all cold blooded and it may not be anything abnormal. My 79 GL has to have choke and cranks 4-5 times before it even thinks about starting when at 50 or lower temps. If turning your idle control up helps it start, that would be the same as cracking your throttle open slightly, but doing both wouldn't work. My old air cooled CB400 used to be the same, if cold motor and cold outside it would take a few cranks to get it thinking about starting, then a good warm up for it to be ridable at all. after warm it was perfect. I think your bike is probably fine. These old bikes are not like a modern fuel injected bike or car where they fire right up and you can start driving. They need to warm up at least a bit and can be hard starting when cold out.
 
I crank the bike a couple of times with the enrichment circuit (choke) on, kill switch off, just to prime with fuel. Then put the kill switch on and she starts.... The 3 Hoanda's I have owned are all cold blooded that way....
 
Thanks for the help guys. Pretty sure it is a fuel issue, going to check the fast idle, choke adjustment, and maybe get into the carbs this weekend. I'll report back.
 
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