Topic: CB650 Custom Carb Tuning/Jetting problem (Jet needle adjustment?)

[ABikeNamedTrouble]

Hey folks,

So here's the deal:  I'm in the process of re-jetting and tuning the carbs on my 1980 CB650 Custom (with PD series slide carbs).  The stock airbox was degraded and crumbly when I got it from the PO, so I decided to put in K&N RC-2362 pod filters, knowing I would have to re-jet everything.

Stock slow jets are #35 and mains are #90 so far as I can tell (neither were marked).  I installed #40 slow jets and they seem to be performing perfectly.  The main jets, however, were a little more trouble; I started with #95s, then moved to #108s, then to 112s, 115s, then 118s trying to get it tuned and I'm at a dead end.

The differences between the latter three jet sizes (112, 115, 118) are marginal at best.  With any of them, the bike idles perfectly but needs the choke pulled out to go above 3-4k RPM.  With the choke open, laying on the throttle makes the bike hesitate and nearly stall (I haven't actually made it stall out but I have no doubt it would if I didn't immediately release the throttle). 

With the choke closed about 75%, she revs up just fine and takes off; however, the amount of choke needed seems to be dependent on throttle position and how warm the bike is.

What's the next step for me?  I'm already running larger jets than many with pod filters (even those with modified exhaust, and mine is stock 4-4), but should I keep going up until it gets better?  Or should I tear open my carb rack and replace/raise my jet needles?  What other options do I have?

Thanks in advance for the help.

[dave500]

find the standard air box.

[ABikeNamedTrouble]

I know everyone's sick of pod filter questions, but it would be helpful to read my post before writing it off so discourteously.

I'm not asking someone to fix the problem for me (as other "What jets size for ___ setup?" threads do).  I'm asking what the next step is when changing the jet size has no effect.  It seems to me that the ensuing discussion would benefit those with pod filters as well as those trying to restore/tune bikes with the stock air chamber.

So there.  I've defended myself 

Does anyone have any thoughts on the original question(s)?

[Pinhead]

The only real option for the PD carbs in the '79 and '80 CB650 is the factory airbox; the needles aren't adjustable, and carb kits are about $40 per carb.

I've noticed that even with the stock airbox with no "lid" over the filter and 102.5 mains (and a MUCH more powerful ignition), the bike still hesitates off idle until the engine is good and hot. The HEI ignition conversion helped a lot, but the slight hesitation is still there.

Your best bet if you absolutely need to run pods, is to grab some '76 CB750 carbs. They'll snap right onto the '79 and '80 CB650 head.

[cb650PK]

Shimming the stock needles made bid difference to of idle and low throttle opening on my bike. It can be made with out unracking the carbs. I think the mid range is too lean even with stock exhaust and intake. Going bigger on main jet will not fix the hole in the mid range.

[ABikeNamedTrouble]

Thanks for the replies.  I may eventually track down a stock airbox that's in good enough shape, but for now I'm sticking with the K&N pods. 

When I first got the bike and was removing the unholy amount of silicone that covered the (naturally still leaky) carbs, I bought Keyster rebuild kits for each one, and the kits came with a replacement needle that had grooves for adjustment.  I just finished installing them on the richest setting and MAN what a difference.  Even running cold I barely needed any choke to rev through the gears like normal.  Once warmed up, I was able to open it up all the way.  It still feels a little lean wide open so I may end up going to 120 mains.

Current setup:

40 slow jets set at 2 1/8  turns out
118 main jets
Replacement jet needles set two notches richer
K&N pods and stock exhaust.
(I guess it's worth noting I just finished a top end rebuild, so everything should be nice and smooth).

Bike runs a little rich on the idle circuit, and a little lean wide open.  Nothing a little tinkering this weekend won't fix


Sidenote to Pinhead:  I wrestled with the ignition system refusing to charge or hot start for about a week, finally fixed it by replacing the melted connection between the stator and R/R and replacing the melted CDI podding compound with some JB Weld.  The otherwise stock ignition still isn't the biggest fan of hot starting, but she's fired up twice now with <12 V charge on the battery .  I guess the only reason I'm telling you this is because I know you like ignition systems, and I've benefited immeasurably from reading your posts on this forum.  So, thanks!

[Pinhead]

It's good to see another 650 getting the chance to cover some miles though! The good thing about the 650's ignition is, it's set and forget. Not much else you'll need to do (unless you want to upgrade ).

You may want to verify the resistance of your spark plug boots. Mine had a bad resistor, at ~10k ohms instead of 5k like the rest. I could also see a "burn" mark and carbon tracking where it looked like the spark was jumping around the side of the resistor (this is with the HEI ignition; the stock ignition will likely just misfire).

Sorry for the thread hijack. I guess I just never considered carb kits an option for the 650 since it was, what, about $160 for the set of four?

[ABikeNamedTrouble]

I think it was about $120.  Plus another nearly $100 for a set of air cutoff diaphragms.

Incidentally, the first time I took the bike out after saying the electrical system was OK, it died on me and refused to start.  Turns out I have a nasty intermittent short in my rotor when the bike is warm.  Probably should've taken people's advise and replaced the rotor as soon as I got the bike...