Really bad cold starting. Ign or fuel?

[Scottish Badger]

Hi folks.  My CB400F2 is giving me grief. When I bought it last year, it was a poor starter but I figured a good service and some new plugs etc would sort it out, how wrong was I!

So far, I've checked the valve clearances, ignition timing, removed the carbs and given them a full overhaul, fitted NGK Iridium plugs, new air filter, oil change and new battery. The carbs were bench-synchronised so can't be that far out.

Once the engine is running and warmed up, it's really smooth and quiet and after tweaking the idle mixture screws (max adjustment about 1/4 turn from factory on one carb, 1/8 on another two) it idles beautifully. When switched off, hot, it will re-start on the button instantly.

My problem is cold starting. No amount of throttle or choke seems to have any effect, after what seems like ages cranking it, it'll fire randomly with the choke almost fully off, repeated attempts will get it to go a bit longer each time until eventually it'll pick up and run. I've tried more and less fast-idle on the adjuster, altering the idle speed screw, and every combination of throttle and choke imaginable! I get nothing at all trying to kickstart it. The carbs have been back off and re-checked, the plugs have a lovely fat spark whilst cranking and are clean.... 

Help, it's doing my head in!!

[McCabe-Thiele (Ted)]

Will it start easily after standing for an hour?

[Nurse Julie]

Ditching the Iridium plugs would be a good start. Fit NGK D8EA.

[Scottish Badger]

Ted, it re-starts just fine until the next day. Fuel flow to the carbs is good, confirmed by checking each one in turn with drain screw undone.

Julie, the Iridiums were fitted to see if they made any difference - they didn't. It was exactly the same with the D8EA's previously fitted.  I've used Iridiums in all my bikes to good effect, my XL500 fires up first kick from cold since fitting one - ok, different ignition system type, but the plug made a big difference.

[K2-K6]

Try misting some e-z-start into the airbox intake before you crank it first time to see if it changes at all.

Have you got any compression figures for it ?

[Scottish Badger]

I haven't, no, but it feels good on the kickstart, nice and even on all 4, and you can feel it turning the crank by spanner. Doesn't sound low when cranking on the starter either.  I'll give it a little squirt of brake cleaner tomorrow whilst cranking it and see what happens.....

[McCabe-Thiele (Ted)]

If it starts readily an hour later I would look at two options poor spark or poor compression. Just my first thought. I'm assuming basic stuff like the choke is operating.

[Oddjob]

Have you checked simple things like jet sizes, clip position on the needle etc. Don’t assume they are correct, people change them to suit the bike and refit old stuff to sell it not thinking to change other stuff back.

[Scottish Badger]

Ted & Oddjob,
Carbs have been off and fully stripped / rebuilt - all jets checked clean and correct, needles checked (one did have an incorrectly positioned circlip originally, causing #1 cyl to run a little rich). Choke functions correctly, all choke plates synchronised properly, spark is fat - checked each cyl individually, cranking on starter with the other three still fitted and yes, you can really hear the difference in compression so as much as I haven't actually done a compression test, I doubt there's any issue there to be honest and it doesn't produce any smoke on startup or when riding.
I'm 57, been working with bikes and engines in general since I was about 14 - everything checks out, yet first start of the day is horrendous!

[SteveW]

My 1980 650 is an absolute bitch to start when cold but once warmed up is perfect.

I found a way to get it to start first time every time no matter how cold it was and it sits there and idles perfectly.

Choke on full, kill switch off. Throttle open fully, turn over engine a couple of times.
Close throttle, kill switch on, press start button and it fires first time and idles perfectly.

Don’t know if this is the proper way to start it but works for me.

[Laverdaroo]

Not sure if it’s of any consequence but have you got good sparks at the plugs? I nipped off the first inch of the old leads and re attached the caps to a bike showing similar symptoms that I dragged out of a barn a couple of years ago, our 400F2.

I had done same as you and found similar results although I never ran it until hot, hot as it was only in the workshop but she was at operating temp. The old leads had just lost the poke and a bit of verdigris had crept up leads causing a crap or weaker spark. If you have the length, it’s a good tip to get a better zap out of the plugs.

Obvs if you’ve already done this then I’m sure somebody that actually knows stuff will be along soon to add something of relevance to your discussion

Hope that’s at least good for thought……

[Scottish Badger]

Steve W, I'll try that in the morning and see if it makes any difference, thanks for the tip.

Roo, I have a big fat blue spark at all of the plugs, checked one at a time with the other three still fitted so the electrics were actually loaded up by the starter working against compression. Thanks for the tip about the leads though, I do have a spare set of new, unused coils that I may try (same as these, no idea of quality..  https://www.ebay.co.uk/itm/144813238373).

[Laverdaroo]

Yeah sorry about that, I thought the post above was the first post. I’m in a travel lodge on a job and the Wi-Fi is awful. My bad

Just seen the rest of the posts as I went to the van for summut and put my hotspot thingy on and got some decent interweave .


Sent from my iPhone using Tapatalk

[Oddjob]

How about plug caps, have they been changed. Once had one that failed when it got hot, possible I suppose for the opposite to happen. Or resistance in the caps may be too high at cold but ok when hot as it needs less spark to fire. Similarly resistance may be to low.

[taysidedragon]

Have you double checked the fuel level in the float bowls. Could cause a problem.