Warm up time

[Simon46]

My 78 k8 takes about 6 mins to settle itself down before its rideable.is that normal? The idle speed has to be lowered as its idling at 2500 rpm when it's hot. Any ideas?

[Bryanj]

That one has the lean running PD carbs i think so that sounds about right

[K2-K6]

As above,  they do take a while to get warmed to a normal throttle response. 

The cold running on full choke and fast idle is listed by Honda at 2750rpm plus or minus 750rpm.

The hot idle you may have a problem with setup though. The routine will never adjust the no2 carb when synchronising the slides.  This one stays in the same relative position in regard to the throttle lift mechanism and could be where your problem lays.
Normally you'd adjust 1,3 and 4 slides to bring parity with no2 during setup,  but if the no2  carb has been set incorrectly  it will give an unrequired offset to the whole carb rack.

To get out of it, you'd have to reset no2 so that when the main idle screw is moved it will have the range to bring that no2 carb slide fully closed and up to a reasonable idle.  Only then can you bring the other three to match it.
This should give you normal hot tickover ability and range.

Bryan has a really good routine to set the carbs on a bench and get them on track for a good start point,  may have to search on here for it.

[Simon46]

Thank you. When I got the bike all jets were blocked with brown crud and the idle screw was wound in fully. I've had tank and carbs cleaned from the1990s petrol which took 2 weeks so adjustments are required I expect.

[Erny]

...

Bryan has a really good routine to set the carbs on a bench and get them on track for a good start point,  may have to search on here for it.

Can you advice where to find routine you mentioned? My K7 has the same sympthoms

[Bryanj]

Just posted it again in tips

[royhall]

My F2 (same carbs) does exactly what you described from cold. But once warmed up it runs superbly. You didn't say how yours ran after the six minutes. Mine won't tick over reliably until it has done around 5 miles of running and is fully warm.  It worried me a bit at first but the PD carb models all appear to do that. If it runs well after it's warm set your tickover at around 1200rpm and just leave a very small amount of choke on until your free of traffic or its warmed enough to tickover.

Sent from my Nexus 6 using Tapatalk

[K2-K6]

As Roy has said,  they take a while to get warm and respond normally to throttle.

A few friends had F2s from new and they where exactly as described.  You'd not find it easy to pull out from a T junction with it cold and the choke pushed in 

Stone cold starting,  two twist of throttle to pump fuel from accelerator pump,  set choke,  start and fast idle.  As Roy says,  pull away with some choke on until it's properly warm.

Even left to cool down abit when parked for a while you'd still then need to choke it a bit when starting again, especially if air temperature is lower.

[Erny]

You perfectly described how my 750 K7 behaves...

I'm wondering if this cannot be improved by rejetting idle jets one step up to have bit reacher idle? Someone tried?

[royhall]

Idle jets don't just control idle, they would be better called slow running jets. Upping the size will change the mixture to around third throttle.

[Erny]

Yes, that's correct, I was not precise enough

But still question remains, if somebody tried to change slow jets to improve this behaviour

I'm quite sure I've seen somewhere info (maybe SOHC us forum) about changing slowjets from 35 to 40, but cannot find it quickly

Attached specs of PD41A / PD41B

[Bryanj]

Problem you have is getting the pressed in pilot jets in different sizes, only place i know of that produces any of them is Siriusonic in Canada and they do not do many sizes.

[Erny]

Thanks for tip, yes they sell them, not very cheap, but I understand it
https://www.siriusconinc.com/pro-detail.php?pid=&product_id=1807

Note: link is accessible via PC only

Now, the question is if there is anyone who did that change, what is result and if something else has to be changed / replaced too?

[K2-K6]

I've been mulling it over for a few days and feel that you can try to move it a little richer without having to change the jets.

Nearly all carbs have got some adjustment available to take into account temp and altitude variations,  they are supplied to general markets like ours sitting more or less in the middle of that range.

To bring up the slow speed mixture using airscrews to adjust. If you turn them in a quarter turn it should bring that mixture toward more fuel through those low response rpm.

The main jets. You can drop the circlip/raising the needle to overall bring the mixture up across the whole range from about 1/4 throttle upwards.

If you try it with the idle set first to see if you get a discernable change,  then you may want to add the mains to that. This should preserve the mixture parity across the whole rpm and avoid making significant steps in the fueling gradient (ie avoiding flat spots).

[CR750]

I've been mulling it over for a few days and feel that you can try to move it a little richer without having to change the jets.

Nearly all carbs have got some adjustment available to take into account temp and altitude variations,  they are supplied to general markets like ours sitting more or less in the middle of that range.

To bring up the slow speed mixture using airscrews to adjust. If you turn them in a quarter turn it should bring that mixture toward more fuel through those low response rpm.

The main jets. You can drop the circlip/raising the needle to overall bring the mixture up across the whole range from about 1/4 throttle upwards.

If you try it with the idle set first to see if you get a discernable change,  then you may want to add the mains to that. This should preserve the mixture parity across the whole rpm and avoid making significant steps in the fueling gradient (ie avoiding flat spots).

On the PD carbs to make the mixture richer [more fuel] you turn the screw out not in, turning the screws in weakens the mixture on the PD carbs.