1978 CB750F2 - Poor Starting

[blackeagles]

ok, almost finished this rather easy 'restore' but not been at it for some time - too cold and too old.... well mainly the latter.

still seem to have a problem, when its hot the bike ticks over at 1200 and revs up without hesitation to settle back to 1200 again BUT getting it to start is something of an art form.  I checked the jets and sprayed them with cleaner and even the pilots were clear! (in fact they all look new), they are the correct size, pilot screw is now 2 3/4 out and all the pump jets fire a jet of petrol into the venture.  press the button and it fires up after a few tries (twisting the throttle to get the pumps to jet petrol into the venture helps) with the choke on and revs at about 4k (I know I have set the grub screw a bit too high at the moment) and I can rev it as much as I like but as soon as I let go of the throttle the revs start to die and the bike stalls.

any suggestions?

[royhall]

They all do that until fully warmed up. Mine starts on the first touch of the button but takes about 5 miles to tick over correctly. I keep a touch of choke on until it's warm, just enough to raise the revs but leaves the butterflies open.

[K2-K6]

As Roy says,  sounds about normal.

Right from new they were like this,  in weather that's cold like now and from stone cold,  then two twist of throttle to use the pump,  pull choke fully out and fire it without touching throttle as you do.

It'll tickover fast and be reluctant to accept no choke until it's good and happy at its normal running temperature.

A friend with one was always dismayed that on his way to work he'd stop off to get a motorcycle news and in winter he'd need to choke it again to pull away.

 

[UK Pete]

Yep that is classic f2 behaviour, the carbs on these bikes are shite.

Sent from my SM-G950F using Tapatalk

[blackeagles]

ok, thanks for the info.

i get most of it but i'm still a bit unsure why when its cold and on full choke I can start it and I rev it up and it initially settles to about 3k but (with the choke still on) I let go of the throttle and the revs drop back and the bike stalls. as I said its ok when it starts to warm up and good when hot - I guess its one of those things you get used to.

thanks again

[royhall]

If its good when hot you have hit the sweet spot. Those carbs are a bit of a compromise in setup so most of us accept the cold running issues as part of getting the hot running spot on. Does that make sense. Rubbish lean burn carbs are the issue.

[K2-K6]

Have you set the "fast idle" adjusting screw?  As there is a routine for that which should hold the revs for you without having to play with the throttle.

The absolute peak rpm when on full choke is 3,500 rpm too,  which with UK temperature is probably needed, not necessarily to run all the time at that,  but more to set the relationship of ambient temp/ mixture/ engine warmth so that it works correctly.

[royhall]

Got mine set to run 2800rpm maximum and works just fine. The setting keeps some fast idle with the choke just on but the butterflies open.

[UK Pete]

Has anyone put the k series carbs on f2, that would be interesting if possible

Sent from my SM-G950F using Tapatalk

[royhall]

K series carbs are different diameter and slightly different spacing. However, someone on the USA forum has achieved it.  Can't find it now as it was a while ago and I'm not a member on there so can't search properly.

[UK Pete]

I would of thought spacing would've easy as when you rotate the rubber you can achieve enough movement to accommodate  this, I think I am going to have play around I have lots of f1 and I stuff so I am going on a mission to see what I can achieve cause I really hate the pd carbs

Sent from my SM-G950F using Tapatalk

[hairygit]

Yep, just fit earlier K or F1 inlet rubbers, and it goes straight on. Someone on here was asking about electronic ignition to use on his bike, which happened to be an F2 with F1 carbs and intake rubbers.  This thread  http://www.sohc.co.uk/index.php/topic,20611.0.html 

[royhall]

When I tried the conversion I found that the inlet stubs on the F2 head were a larger diameter than the F1 so the rubbers wouldn't go on. I was backed up on this by others that had tried it. So I don't really know how this works for some and not others. I must have been missing a step somewhere.

[UK Pete]

Perhaps a fair bit of heat is needed to supple up rubber

Sent from my SM-G950F using Tapatalk

[blackeagles]

If its good when hot you have hit the sweet spot. Those carbs are a bit of a compromise in setup so most of us accept the cold running issues as part of getting the hot running spot on. Does that make sense. Rubbish lean burn carbs are the issue.

I have to say I was 'smugly' happy that it likes running when hot - but this cold aggravation is a bit of a let down

add: well, having had a look at it i have to say i don't have any screwdrivers that will get into the limited space so I will have to cut something down and try again.