80s cb650 c tuning and potential rejeting?

If it's broken or just needs tweaked

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Jr102893
Posts: 8
Joined: Sat Aug 13, 2016 1:17 pm

80s cb650 c tuning and potential rejeting?

Postby Jr102893 » Thu Sep 01, 2016 4:43 pm

Let's start with the basics
1980 honda cb650 with pd carbs and 17k miles
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When I got the bike it ran hot and smoked out of cylinders 1 and 2 but revved evenly

So far I have removed and reinstalled the carbs fixing an air leak between the carb and engine. After fixing the leak I tuned the carbs, the outer two Cylinders were extremely high compared to the center two. Once tuned I noticed the bike was running extremely lean and would only rev if I covered the air filter with my hand. I've set the mixture screws to between 3 and 3.5 turns out (I don't have a tachometer to set them properly where do I buy one?)
Finaly I reset the cam chain tentioner.

*note: I believe the carbs were rebuilt by PO they look very clean and there is an in-line filter for the tank

It runs and idles very quietly though it takes maybe 30 seconds and a rev or two for cylinder 1 to start firing and heat up noted by a slight backfire pop on cylinder one as the throttle is letting off. It also smokes less now.

The only problem I have right now is the dip in power I have between 1k and 2.5k ifor i rev very fast I risk stalling it. It pulls great after 3k. Any ideas on that issue?


My Plans for the bike!!
I am modeling it after this but I'm not changing the seat structure
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I would like to run the k&n rc-2362 duel pod filters

I am unsure on the rejeting that will be needed on such a lean bike.

Also questions on engen paint what type is safe??

Finally what do you think of my spark plugs???
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Thanks any help is greatly appreciated!!!

JR

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Volker_P
Posts: 5508
Joined: Mon Mar 06, 2006 1:53 am
Location: southern Germany

Re: 80s cb650 c tuning and potential rejeting?

Postby Volker_P » Fri Sep 02, 2016 1:03 am

Jr102893 wrote:It runs and idles very quietly though it takes maybe 30 seconds and a rev or two for cylinder 1 to start firing and heat up noted by a slight backfire pop on cylinder one as the throttle is letting off. It also smokes less now.


30s until it runs at all four is all right. Do not worry.

Jr102893 wrote:The only problem I have right now is the dip in power I have between 1k and 2.5k ifor i rev very fast I risk stalling it. It pulls great after 3k.

As long as it is cold again quite normal. It easily dies on the throttle without choke when cold.
If this still happens at operation temperature check valve settings and carb synchronization.
If these are all right, have a look at the accelerator pump setting and diaphragm.
BTW, this is more or less for idle, you should not expect a CB650 to pull straight from 1k on!


Jr102893 wrote:I would like to run the k&n rc-2362 duel pod filters
I am unsure on the rejeting that will be needed on such a lean bike.

The PD carbs are quite sensitive to the air side so you probably will need considerably larger mains.
Unfortunately this is no plug and play, here is a hint where to start: viewtopic.php?f=4&t=773
If you can, stay with stock airbox and just be happy. Exhaust is less problematic.

Jr102893 wrote:Finally what do you think of my spark plugs???

If they look like that just after en extended turn at some load you would be even on the rich side (for a CB650).
If they look like that after idle or after a turn around the block you can't say much.
Unless they were black before, then it runs certainly too lean now as they typically get black if you play around for a time well below operating temperature. But maybe your picture still just shows how they get black from playing around. :wink:
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum


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