1981 CB650C Carb/Jet Tuning Guidance

[kstylian]

Hi guys. I posted this on the SOHC4 forums hoping to reach a larger audience, but had little luck there.

Here's the scoop. I'm hoping to get some guidance setting up, or rather jetting, the carbs correctly on my 1981 CB650C.

Background:

Engine has ~19K miles. PO installed new rings prior to selling it to me and never got it to run correctly. After ordering/rebuilding a set of 1982 CB650 CV carbs (removable pilot jet) the bike starts up, idles and generally runs pretty well. It pulls strong from low to mid RPM, but feels a bit anemic/sluggish from ~5K up to redline. It does pull to 9K, but it doesn't have me "death gripping" the bars. I just feel like it should have more to give.

OEM 4-4 Exhaust. OEM Air box, cover removed, with K&N filter. 38 Pilots (stock 35) and stock 110 main jets. Pilot screws ~2 turns out. Stock needles jets/position (non adjustable) and float heights are spot on (also non adjustable). Compression 110-120 PSI cold/dry and 120-130 PSI cold/wet (WOT CV carbs). Elevation 2000 ft.

I'm currently going through the tune up procedure again (value lash, timing, cam chain, carb sync, etc) to make sure all is correct before I start trying to dial in these carbs.

Pictures below are from the plugs that have been in since initial test/tuning and have about 200 miles on them. Any thoughts regarding their general appearance? They do look a bit sooty/rich, but don't know what circuit may be contributing to this. I know that specific circuits will need to looked at individually via plug chops.

I have a fresh set of plug in now and have another spare set for the initial plug chops (may need more). What is the general consensus for tuning carb circuits? Do I work from the main jet down; WOT chop (main jet) --> 1/2 throttle chop (needle) --> 1/4 throttle chop (pilot circuit), or vise versa.

Thanks for any help and I'd like to keep this thread going with progress/questions until things feel/look good.

Cheers!

-Charlie

[kstylian]

More pics

[usbear]

1)WOT- (main jet)Take it out and run it for a good run, say a tank of fuel in Mtns and straight stretch.
Stop and cool enough to change to new plugs. Then go to high rpm run and hold full throttle for a short distance and pull clutch and kill switch and coast to side of road. Pull plugs when cool enough and read them. (main jet)
2) Midrange is needle and main jet.
3) idle to 1/4 is slow speed jet and air circuit and slide cutout

overlap but all you have is main jet, needle slide and low speed jet.


Go for good ride and clean it out under load. 76 with techron will help with this. Then see what it looks like and then Jet. First ride and remember you have rings not sealing good yet. When compression comes up it will run cleaner.


The custom model is sensitive to air gap setting at pulser generator on left side of crank. Use non-magnetic feeler gauge to set.

Keep posted!

[cb650]

The custom and standard are the same engine mechanically. pulse generators are on the right side. Alt is on the left.

[usbear]

TY
Got up on the wrong side of the bed today!

[kstylian]

Thanks guys. Compression is still about what it was when I bought the bike, about 120 psi on all cylinders. I've been riding it fairly hard varying throttle positions and loads trying to get the rings to set. Been using Lucas fuel system cleaner in the tank and also decarbonised with water/steam. Changed the oil to Castrol conventional as I don't know what the po used (hopefully not synthetic) to make sure rings bite.


How long should it take for new rings to seal? should I lift the carb pistons and WOT while doing compression test to make sure cylinders are filling fully?

Going to go through tune up procedures again to make sure all is well.

keep it coming......!

[usbear]

Yes, Lift slides, all plugs removed, full charge battery!
What is your plug gap?

[cb650]

TY
Got up on the wrong side of the bed today!

I do that sometimes. I get scared at nite and hide under it.

[kstylian]

Yes, Lift slides, all plugs removed, full charge battery!
What is your plug gap?

Did a lot of work today. Pretty much went through the whole tune-up process to make sure all was within spec.

Adjusted cam chain tensioner. Disassembled/cleaned/lubed timing advance mechanism. Pulser generator gap set to 0.3mm. All valves adjusted to Hondaman's recommendation (.003/.005 inches intake/exhaust). Plug gaps set to 0.6mm. Carbs synched to within 1 in Hg of each other. Ignition timing spot on and reaches full advance at ~3k RPM.

Pulled the carb pistons and did another cold/dry compression (WOT) with a fully charged battery. All cylinders are between 120-125 psi. At least they are consistent. Seems a bit low, but hopefully they'll come up with more running? How long does it take for rings to set? What compression #'s are other folks seeing?

Pulled plug #4 (new plugs) after carb sync and ignition timing. Ran about 1.5 quarts of gas during the process, all at idle and short blips. Plug looked a little dark/rich. Still need to adjust the pilot screws a bit more.

Didn't get a chance to take it out today, but will this week.

[kstylian]

Interesting read regarding compression test result and equipment. Makes sense to me.

http://forums.sohc4.net/index.php?topic=39971.0

and

http://mullengineering.com/rt/files/2_tech_docs/THE%20TRUTH%20ABOUT%20COMPRESSION%20TESTING.pdf

[Volker_P]

Pulled plug #4 (new plugs) after carb sync and ignition timing. Ran about 1.5 quarts of gas during the process, all at idle and short blips. Plug looked a little dark/rich. Still need to adjust the pilot screws a bit more.

Didn't get a chance to take it out today, but will this week.

So this really means you did blow 1.5 quarts of fuel on your main stand to get some questionable plug reading at idle?
I really hope I misunderstood.

BTW, where can I buy these plugs with the removeable threads?

[kstylian]

I really hope I misunderstood.

I think you did...you can sleep easy.

Any comments re: compression results?

[Volker_P]

If you want ot have a useful reading of spark plugs from a CB650 go to a ride of at least a few tens of miles at changing laods and revs. Depending on the history of the engine, you may feel that it will run much better after that.
Then have a look at the plugs immediately (and not after you rode a few slow miles home through town or so).
For the next readings maybe 5-10 miles should be sufficient then.

I would be very surprised if CV carbs with 110 mains (I think stock is 120) should be even slightly rich with a K&N.

Pilot screws are for idle mixture only. Their full adjustment range is in the order of the step to the next idle jet size.