Topic: 1978 cb750K PD42A Carbs - accelorator pump spring

[SKTP]

I would like to try a new Accelerator pump spring...but I cannot find a part number for just that spring

Any help?

Thanks

http://forums.sohc4.net/index.php?topic=146893.0

As seen here:


Adding this link to this:

http://forums.sohc4.net/index.php/topic,173302.0.html

[flybox1]

that functions as a return spring, so i doubt another will help 
but....you could find another set of carbs, or someone with parts.
harisuluv?
I might have one as well.

[trenchgrinder750]

I might have one. I’ll look tomorrow afternoon

[SKTP]

Thank you for the replies and potential springs...I hope to have a better look at everything this weekend and install Dyan S electronic ignition...the bike runs and starts well...but if I really hammer on the gas it sputters...I have to roll it smoothly...figure either Points, pump or jets?

[flybox1]

...the bike runs and starts well...but if I really hammer on the gas it sputters...I have to roll it smoothly...

...thats kinda how a carb'd bike runs 

[Erny]

SKTP - I think I observed the same issue on my 1977 CB750K PD 41A Carbs. (I still consider it is not just standard behaviour)
I assume accelerator pump is the same or very similar. I did not find many information about it (compare to lot of info about pre K7)

I'm not native english speaker, but try to explain the issue - typical test case:
Engine runs on idle, I twist trottle quick, engine first tends to stop (revs drops) and just then takes higher revs. I normally expect revs goes up quickly without any drop
If I do the same smootly all is OK, revs are increasing as they should.

I tried to investigate with my brother, and it looks to us as it is caused by accel pump behaviour - there is spring connection between trottle shaft (connecting all 4 carbs) and accel pump shoulder, when I tried to twist quick, spring first enlarges and just short moment later moves accel pump shoulder rod to spry fuel. So there is kind of delay inserted by spring, depending how fast trottle is twisted
When doing it smootly, no issue occurs, as there is no delay and spring moves with accel shoulder with no delay.
So looks like that accel pump reaction is not fast enough to cope with trottle opening and in that period engine runs too lean, until accel pump wakes up

I tried to make video to better explain that. it is 8x slowed down, for comparison, we tried to simulate direct link between trottle shaft and accel pump shoulder and no issue observed (see from 1:08 - last try just before end of the video) 

So, I started to think to swap current spring by "stronger" one to eliminate that effect and keep trottle more linked to accel pump...unless that is normal behaviour?

=> Is there somebody able to explain-confirm this is normal behaviour?

[BobbyR]

I would like to try a new Accelerator pump spring...but I cannot find a part number for just that spring

Any help?

Thanks

http://forums.sohc4.net/index.php?topic=146893.0

As seen here:


Adding this link to this:

http://forums.sohc4.net/index.php/topic,173302.0.html

Looks like a fairly generic spring. I have 78K but it has been awhile.  You can adjust the accel pump to come in earlier by adjusting the tab that pushes on the pump shaft. If you have not cleaned the carbs at this point you may want to do that fairly soon. I do not have to roll on the gas on my 78K.

[SKTP]

Thanks for all this info...I will try to see how you adjust the tab that pushes on the pump shaft.

Thanks

[trenchgrinder750]

I checked this afternoon and do not have an extra spring. Sorry

[flybox1]

make note of where you are 'hammering' the throttle.
if cruising at anywhere above roughly 1/8th throttle travel, the accel pump is not active when you get on it.
If from rest, you hammer it and it gasps...you might need to lean out the idle circuit a bit.  at the moment your accel pump squirts, your mixture is probably over rich until you get into the meat of the pilot jet, and the accel circuit is spent....

[Erny]

In my case it is from the rest... Bu as I tried to explain, if I simulate hard link between trottle shaft and accel pump its works as I would expect, engine revs immidiately w/o any drop.
So, it seems to me opposite, idle is already lean and needs help from accel pump to rev quickly.

[flybox1]

Yes, with PD carb'd bikes, the idle circuit needs to be lean, so when throttle is applied, the accel pump circuit gets the mixture right, for a smooth holeshot.

[Erny]

Small update on this topic - yesterday carb sync done, engine now runs much smoother in idle, but that effect with accel pump remains...

Isn't here anybody who knows well behaviour of K7/K8 carbs who can advice if it is normal or not?

[flybox1]

Small update on this topic - yesterday carb sync done, engine now runs much smoother in idle, but that effect with accel pump remains...

Isn't here anybody who knows well behaviour of K7/K8 carbs who can advice if it is normal or not?

This thread was started by SKTP.  Please start a separate thread based on your issues and we will answer you there.
SKTP will appreciate us staying on topic.

[Erny]

Yes, I appologize... I'll do so. Sorry!