CB550F2 '77 slow to rev when starting

[alexdecker]

Hey all

So my bike starts up pretty fast. But when it starts, it doesn't come up in revs by itself. It kind of halts a little bit, and then needs a small twist of the throttle. When doing that, it settles in on a nice 1200RPM after that. But if I don't rev it a little, the engine dies again.

Is this the normal nature of the beast, or can I optimise something to prevent it?

[Johnwebley]

When the motor is cold, its normal,
These are 50years old carbs and bikes

Once warm,it should run nicely just over 1,100 rpm

Sent from my SM-A546E using Tapatalk

[Oddjob]

If it's set up well John you should be able to press the button and walk away and it will only need the choke closing a little after a minute or two. A good tickover is 1000 revs, anything over that is compensating for noise and wear in the engine, a really good engine will go as low as 600 before it starts to splutter. That's in an ideal world though, modern fuels don't help. Try vaccing the carbs Alex and see if anything has moved..

[alexdecker]

Wow, 600 reps! When mine is warm, I can get it to 1000rpm when idling, but 600! Has anyone done that?

[McCabe-Thiele (Ted)]

Mine seemed happy at 1200  (once warm) before I took her to bits.

[Oddjob]

My L reg just after being rebuilt used to idle at 600rpm, below that it used to start to knock a little, it would go as low as 400 before it stalled out. Must say though it didn't really sound that nice that low down, lots of knocks and bangs going on.

I've just done a members bike on here recently that was a bitch to start, I did a full rebuild on his spare bank of carbs and it now starts on the button everytime, I must ask him if it will start JUST on the button, no throttle movement at all. From all accounts it's now running extremely well and touch wood no flooding. He intends to post a few pics in a short time when he's finished doing a few other bits.

1200 is normal these days for a four to tickover at, at 1000 it can sound loud, at 1200 most of that noise seems to go, that's because the engine loads up more of the components that are making the noise, primary chain especially on the 500/550, clutches on the 400. If the engine sounds better at 1200 I'd leave it there, 1000 is just an aspirational figure, if it ticksover at that speed and it's quiet then all is well inside that engine IMO. The lower it can go the better the engine is inside.

[robvangulik]

The lower it can go the better the engine is inside.

True, but as these engines need oil pressure to run, it's wise to only try this once. The lower your revs go, the lower your oil pressure goes....

[Oddjob]

I'm not advocating a tickover that low Rob, just that's it's a sign of a well set up engine IF it can go that low. I only did it occasionally to see how the engine was faring, I'd say 1000 was as low as I'd go normally, when they were new the tickover was always set at 1000, it's only as they grew older and more worn that the tickover seemed to creep up as well. Saying that, those were the days of decent petrol.

[K2-K6]

The lower it can go the better the engine is inside.

True, but as these engines need oil pressure to run, it's wise to only try this once. The lower your revs go, the lower your oil pressure goes....

The oil light can be out just from kicking them over on the kick-start, that can't be more than about 150 rpm. I dont feel thats an issue in pump delivery terms.

I don’t ordinarily chase a particularly low idle speed, not for any real reason but just dont mind them running a little faster.

Older engine than these normally had heavier flywheel to keep them going, also now with injection they can measure crankshaft pulse to smooth out low load idoe speed.
Generally much new stuff has a cutoff of about 500 rpm with target more around 6/700 minimum in ecu. Saves a little fuel and heat Generally if low.
As noted Ken, when hearing a new one originally they always sounded so smooth naturally, along with dead steady tacho needle. I think one of the things that most impressed buyers coming from brit bikes etc.

[McCabe-Thiele (Ted)]

I've never understood the variation of standard oil pressure amongst different brands of vehicles. In the days when oil pressure gauges were fitted to cars Mercs would have a maximum of 45, Minis liked 75 plus, Jaguars were around 40.iirc

[deltarider]

Once I managed a low idle of 800 rpm after a perfect sync I guess. Long time ago a CB Four mechanic warned me not to strive for that. Camshafts wear most at idle. I forgot to ask why this is. I'd rather stay on the safe side. Mine idles fine at 1200 rpm and there's nothing against it. Within a minute after a cold start chokes are fully open again. BTW, these bikes with the oldstyle carbs are not supposed to ride with the chokes still partially closed. Just have a somewhat raised idle for the first mile or so, which is also what the book recommends.

[alexdecker]

Thanks all for replies. Will go through usual maintenance next week, ending with sync. See if that helps things.

Does any of you have a video of a well set up bike and motor, so I can hear the sound of the idling for reference?

[Oddjob]

Doesn't really work Alex, the video makes the bike sound like a bag of nails usually.

[alexdecker]

Yeah I figured. Just like to hear engine sounds from the CB bikes 😂

[McCabe-Thiele (Ted)]

This is my 400 before strip down - bag of nails!

https://www.youtube.com/watch?v=3CpAYjNHggI

Someones 500 https://www.youtube.com/shorts/a4CVm8PAAXE