Carb Jetting Guidence

[jt]

I have a 79 engine with VB64 carbs, I rebuilt the carbs and went with #68 Slow speed jets and mains are stock 105 and Pilot's are 2-1/2 turns out. Stock air box with Stock paper filter. New plugs but they are a little sootie after idling and revving in the garage. I am running a Lazer 6 into 2 into 1 header with a 2in mid pipe and a Cone Engineering muffler. Starts good and revs good up to 3-4 k but after about 4 k it's breaking up some, Hopefully you can tell in the video. So I think I need to up the mains and that's my question, what do I need to order 110's or 115's or ?? Do I need to shim the needles ? Would like to get lucky and just drop the carbs one more time - !

[jt]

video File is to big .

[NobleHops]

Did you verify that the advancer is working properly? I see Dyna coils, assuming also Dyna ignition? 4K is about when the advancer is… advancing. Preston gave out nice mini advancer/timing wheels several years ago, it was installed on the ignition shaft, indexed to the cover, and then checked with a timing light at 4K RPM to verify that the ignition was advancing properly.

N.

[jt]

Hi Niles, I will check. thanks
JT

[daves79x]

If your carbs are VB64, they do not have two main jets. The single 105 is the stock main jet size for the VB64. Do you mean you went with #38 (not #68) low-speed jets? If so, you should be pretty close, but it will be slightly rich in the low/mid range with the #38s.

Also, have you actually ridden this bike? If you are just starting it up in the shop, that won't tell you much of anything, until you get it out and thoroughly warmed up.

No, you do not need to go bigger on the mains. I'd cut the mixture screws back to 1-1/2 turns also.

Dave

[jt]

Dave, Thanks for the reply, Yes I meant #38 on the slow speed, My plan was to try and ride it this past weekend but worked on some wiring and plumb the front brakes but did not get to put in the brake fluid and bleed them. The clutch plates are stuck together so need to get some heat in the oil to free it up. Hopefully this weekend I will.

Mostly I've heard on the site 2-1/2 turns out on the pilot's screws, you are recommending 1-1/2, why is that ?

Thanks again

[bobcat]

Hi jt, When you rebuilt the carbs did you replace the air cut off valve ? (on the side of #1 carb).
If not, check the diaphragm for pin holes or replace it. #38 slow jet is a bit too rich. I have had very
good results drilling the #35s with a no.79 drill bit which makes it a 36.75 and setting the pilot screw
to 2 turns out along with shimming the needle .020". The main jet is actually slightly over rich from
the factory and the last thing that needs changing unless you go down to a 102.5 when you shim the
needles. I would suggest checking the cam timing as they are likely retarded from cam chain stretch.

[daves79x]

I've found that anything over two turns out is just too much, The factory set them at barely 1 turn. As I said, you can't tell ANYTHING about this until you actually get out and ride it. Repeated warm-ups in the garage do far more harm than good.

I've also stopped shimming the needles years ago - just too rich also.

With all due respect Bob, I don't think JT is anywhere near worrying about fine-tuning cam timing. If he gets the carbs right, It'll run just fine.

Dave

[Jeff Bennetts]

I have also stopped the needle shimming, if you’re running the stock airbox and filter, enlarging the idle jet is sufficient adjustment for the crappy fuel we use most of the year, it makes a smoother transition from circuit to circuit.

[bobcat]

I've found that anything over two turns out is just too much, The factory set them at barely 1 turn. As I said, you can't tell ANYTHING about this until you actually get out and ride it. Repeated warm-ups in the garage do far more harm than good.

I've also stopped shimming the needles years ago - just too rich also.

With all due respect Bob, I don't think JT is anywhere near worrying about fine-tuning cam timing. If he gets the carbs right, It'll run just fine.

Dave

Maybe jt isn't ready but have you (Dave) ever tried degreeing the cams to lobe centers in.103, ex. 105 ?
I've done several 81-82s and it makes a world of difference.

I stand by my jetting suggestion with the exh. sys. he is running. True, he does need to ride it.

[jt]

Yes I replaced all the rubber parts in the carbs and cleaned them extremely well , No I've never did cam timing before. Did have issue with the accelerator pump with Randykks kit, after I let the carbs sit for a good while to test for leaks just before I put on the bike I check to make sure I was getting fuel to all the fuel squirters , I was not getting nothing. I pulled the valve out and the rubber had de laminated. So I bought a K&l and installed. I've used the the Randykks kit several times before with no issues. Must have got hold of a bad one.

I am running stock ign with Dynas coils for now. but at one point I switch over to a IgnTech system to check but no improvement. I think I need to get it out for a ride and see what it's doing. Advancer moves freely will try to put a timing light on it this weekend

Thanks everyone.

[daves79x]

Bob: Frankly, I'm way past getting into degreeing cams. I see a ton of CBXs every year and guys just want a solid, reliable CBX that runs pretty much as it should and have all the problems sorted correctly that these things accrue after 40 years. Most guys would be thrilled if their bikes were to that level, whether the cams were a few degrees from perfect or not. Most guys are lucky to put 1000-2000 miles a year on their CBX and that's the way they want it.

As you well know, time is money and most guys want to spend their money on the millions of other things their CBX really needs.

Dave

[Jeff Bennetts]

And unless you have new cam chains and tensioners, the slop from the used chains vary the timing a few degrees here and there so what are the gains in the real world anyway?

[bobcat]

Bob: Frankly, I'm way past getting into degreeing cams. I see a ton of CBXs every year and guys just want a solid, reliable CBX that runs pretty much as it should and have all the problems sorted correctly that these things accrue after 40 years. Most guys would be thrilled if their bikes were to that level, whether the cams were a few degrees from perfect or not. Most guys are lucky to put 1000-2000 miles a year on their CBX and that's the way they want it.

As you well know, time is money and most guys want to spend their money on the millions of other things their CBX really needs.

Dave

So you promote mediocrity and a running CBX is and should be acceptable. Why bother with getting
the carbs, ignition timing or valve clearances just right. Never mind bearing or piston/cyl. clearances
to spec or cam chains properly adjusted etc., etc. If it starts and runs you should be happy and just
polish and paint away.
I thought this was a site that promoted contributions to make your CBX the best it can be, a place
where improvements and precision were welcomed. Maybe I had it right 8 years ago. My suggestions
ripple the establishment pond and must be dismissed as fantasy, disinformation. Beware folks, this
bobcat guy knows nothing of which he speaks and his ideas should not be taken seriously.

[bobcat]

And unless you have new cam chains and tensioners, the slop from the used chains vary the timing a few degrees here and there so what are the gains in the real world anyway?

Ditto my previous post.