Topic: Idle speed change

[70CB750]

On Saturday I was hunting a gremlin.  Turned out to be electrical but in the process I cleaned up idle jets too. 

Long story short, she used to idle at 1250, steady as a rock, now she idles exactly at one thousand.  I may bring out vacuum gauges and set her higher again, but now I wonder where did the change came from.

This is what I did:

- cleaned idle jets
- installed new D7EA plugs gapped @ 0.032" (old plugs were D7EA at original factory gap)
- refreshed cable ends in coils and in boots

What could cause the idle speed drop?  Opinions?

[flybox1]

Why did you chose D7EA's?

[70CB750]

They always worked better than D8 for me.

[flybox1]

beer always worked better than water for me, but that doesn't mean its best 

chalk it up to your tuning/rebuild skills.  my K8 idled best about the same (1250-1300)
it would idle at less than that, but it just sounded laborious.

[70CB750]

I suspect the idle jet, it was not dirty as clogged but I could see through it better after some cleaning.

[Dream750]

I suspect the idle jet, it was not dirty as clogged but I could see through it better after some cleaning.

Those stock 750 Keihin idle jets are almost impossible to properly clean to “as new” condition.
 
The cutaway pic below shows why it’s so difficult:

[skidooextreme]

Very interesting!

[70CB750]

Thank you, Dream. Very illustrative. I suspected it since I drilled some - and the jet did not work afterwards.

[ekpent]

Nice pic Dream. Those type of cutaways are very informative to see how things work. The little idle jet looks quite complicated and lots of places for spooge to form. Now find a cutaway of a no number, an HM300 and a HM341 exhaust pipe and you will be my hero forever. I would like to see a clean look at the differences for kicks     Heck cut up some of yours 
PS-Wonder if some of those aftermarket slow jets are machined as well ??

[trueblue]

70CB750, if you cleaned the idle jets you are probably running slightly richer at idle now which will slow the engine down a touch.  Remember a lean mix is a fast mix, hence why if you are too lean you will get detonation.

[Dream750]

Thanks guys. I’m also a fan of cutaway parts, I wish there were more examples available ...   
 
Eric, you will have to forgive me as I won’t be cutting up my minty original 2/70 exhaust pipe set any time soon.  

I don’t know about the degree of precision machining from the aftermarket jet manufactures, but I suspect the Devil is in the details. In the meantime, here’s a poor quality pic of a stock Keihin 750 main jet: