Topic: Which way to go??

[juan_carlito]

Bike: 1979 Honda CB650 running pod filters and a basically open exhaust (does have mini baffles, and some steel wool packing but I don't think they do much lol)
Carbs: Keihin PD50A mechanical slides with drilled out jets.
Question: I need to know which direction to go with the jetting....yes the stock air box would help. Yes proper exhaust would help. But thats not what I have to work with lol. Currently my floats are set to stock height (I think) and the idle adjustment screws are 2 turns out. Slow jet has been drilled to aprx. a #42 (hard to tell exactly) and the mains are aprx. #118. Now, bike starts up really easy needing very little choke which I can shut off very quickly. Idles awesome around 1000rpm or slightly lower. Has decent response from idle till about 1/2 throttle....At which point if I try to crack it open it has major hesitation and stumbles. So....do I need to keep going up in jet size until it makes a difference OR go back to smaller sizes? Also, If it starts super easy with almost no choke, is my idle jet to large and therefore too rich as well??
Thanks

[70CB750]

Did you look at the spark plug color?

What you describe was in my case needle position, the needle was too low.

One notch on the needle is about 1.5 in jet size, my K0 with almost open exhaust (just mechanical baffle in straight pipe) and pod like air filter likes the needle in 4th position the best - I tried all five when I was testing it.

But my experience is with 750 only, I dont even know if your needles have positions.

[70CB750]

This could help.

[juan_carlito]

Did you look at the spark plug color?

What you describe was in my case needle position, the needle was too low.

One notch on the needle is about 1.5 in jet size, my K0 with almost open exhaust (just mechanical baffle in straight pipe) and pod like air filter likes the needle in 4th position the best - I tried all five when I was testing it.

But my experience is with 750 only, I dont even know if your needles have positions.

Spark plug colour showed two cylinders running rich one running ok and one running good. And no my carbs don't have adjustable needles, but that chart might help e out so thanks for that!

[juan_carlito]

 well . . . . I did take it out tonight and try riding and pulling the choke out slowly to see how the bike responded....as soon as I did it started to hesitate and stumble...If I tried to open up the throttle with any more than maybe 1/4 choke it died completely. Does that tell anyone anything as to where I stand?
Also I paid more attention to the rpm rather than throttle position tonight...great response from 1000 to 3000....then bogs down and stumbles from 3000 to 3500ish...then it feels like you hit a power boost till just past 4000....everything after 4000 on was stumbling and/or basically non responsive!
Maybe I need a proper mechanic lol

[Davez134]

My knowledge is pretty much on the 750, so I don't know much about your carbs, but symptoms sound rich. Rpm (mostly) doesn't matter, it's throttle position. When cold, it should not idle easy too quickly with no/little choke. That means it's rich at idle. I'm not sure if you can easily adjust idle air mixture on your carbs. You say you "think" floats are correct. Very important, especially because plugs are different colors. Also, what is the condition of your entire ignition system? If floats are good and ignition system checks out per manual, including resistance in coils and caps, I would start by leaning out main jets.

[juan_carlito]

My knowledge is pretty much on the 750, so I don't know much about your carbs, but symptoms sound rich. Rpm (mostly) doesn't matter, it's throttle position. When cold, it should not idle easy too quickly with no/little choke. That means it's rich at idle. I'm not sure if you can easily adjust idle air mixture on your carbs. You say you "think" floats are correct. Very important, especially because plugs are different colors. Also, what is the condition of your entire ignition system? If floats are good and ignition system checks out per manual, including resistance in coils and caps, I would start by leaning out main jets.

I'll look into it all! thanks for the response

[juan_carlito]

Well.....Took the pods right off and zip tied some fabric over the air intakes to keep out debris and took er for a ride to see if the pods where restricting any ports...ran like #$%*! That was not the problem lol. But then I decided when I got home to drain the carbs individually into glass jars and compare how much fuel came out....They were close at 1 and 2. and 2 was close to 3. But 4 was low and none were actually the exact same...SO I checked the manual and cut out a measuring tool and set the floats to .50 in as suggested.And then I had an idea.... I changed out my modified main jets and put in the stock set again....then I re installed the stock air box and re lubed a sticky throttle cable so the throttle actually snapped back to close instead of sticking slightly open :p I also set the idle adjuster screws to 2 turns out as a starting point. The only thing I can't do that I want to is replace the stock idle jets . . . I don't have another set. . . So in theory everything should be far more balanced now. The idle jet circuit will still be rich clearly but hopefully I will be back at a good base to start tuning from. I did roll it out of the shop and start it up. Still required little to no choke, but was still slightly warm from previous test ride. Still idled well around 1000rpm and if I tried to crack it open really fast it stumbled a little, but if I eased it up in throttle I was able to get up to 5000rpm without an issue. I stopped there as it's 1am and the family is asleep and seeing as I have cut off exhaust I don't want my wife and neighbours planning my accidental death lol
Hoping to get a quick test ride in tomorrow and see where I'm at! Who knows if I am really lucky maybe I can find somewhere or someone who has a set of idle jest I can grab, then someone said I could probably get it tuned with a simple needle shim and float adjustment. Fingers crossed!!!

[scunny]

I'm running the same motor and carbs with pods. 125 mains and stock idle jets. purrs like a kitten.
the 650 is bloody hard to get running right with pods, unless you are going flat out there is always a stumble when riding at about 5000 rpm, this can be overcome by using a bit of choke which leads me to believe this is the transition point on the needle taper. I haven't tried to shim the needles yet but may look into that this winter.