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Author Topic: Idle speed change  (Read 1177 times)

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Offline 70CB750

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Idle speed change
« on: March 24, 2014, 08:33:12 am »
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?
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Offline flybox1

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Re: Idle speed change
« Reply #1 on: March 24, 2014, 08:41:39 am »
Why did you chose D7EA's?
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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Offline 70CB750

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Re: Idle speed change
« Reply #2 on: March 24, 2014, 08:50:29 am »
They always worked better than D8 for me.
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Offline flybox1

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Re: Idle speed change
« Reply #3 on: March 24, 2014, 08:59:27 am »
beer always worked better than water for me, but that doesn't mean its best  ;D

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.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline 70CB750

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Re: Idle speed change
« Reply #4 on: March 24, 2014, 09:05:31 am »
I suspect the idle jet, it was not dirty as clogged but I could see through it better after some cleaning.
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Offline Dream750

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Re: Idle speed change
« Reply #5 on: March 24, 2014, 05:37:19 pm »
Quote from: 70CB750 on March 24, 2014, 09:05:31 am
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:
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Offline skidooextreme

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Re: Idle speed change
« Reply #6 on: March 24, 2014, 05:49:59 pm »
Very interesting!
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Offline 70CB750

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Re: Idle speed change
« Reply #7 on: March 24, 2014, 06:09:53 pm »
Thank you, Dream. Very illustrative. I suspected it since I drilled some - and the jet did not work afterwards.
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Offline ekpent

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Re: Idle speed change
« Reply #8 on: March 24, 2014, 07:05:41 pm »
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  :D
PS-Wonder if some of those aftermarket slow jets are machined as well ??
« Last Edit: March 24, 2014, 07:08:15 pm by ekpent »
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Offline trueblue

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Re: Idle speed change
« Reply #9 on: March 25, 2014, 03:30:15 am »
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. ;D
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Offline Dream750

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Re: Idle speed change
« Reply #10 on: March 25, 2014, 03:12:08 pm »
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. :P :P :P 

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:   
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