Topic: Jetting fun! 77 CB750F Pd42a carbs.

[CB750R]

I'm running a cb750f2

The jets are
137.5 mains
35 pilots
Needles are f2051f shimmed with a washer underneath and at lowest clip position.

Dude says there's a huge lean spot than corrects and may be a tad rich on he top end.

I'm already maxed out on the needle. And seem to be running very large jets.

Wtf is going on???   It pulled 55 hp

My only mods are a Tintop air cleaner dyna s ignition and coils.  And some very mild clean up of ports when I rebuild my engine.

Dyno guy said its Waaaay off bike runs pretty nice on the road...

So new needles?? Or more main jets? 

[scottly]

I'm running a cb750f2


35 pilots

Dude says there's a huge lean spot than corrects and may be a tad rich on he top end.

Sounds like you need bigger pilots. Got a printout showing Air/fuel ratios? BTW, 55 RWHP ain't bad for a mostly stock 750.

[bjbuchanan]

What RPM and throttle positioning are you misjetted? There isn't anyway to help, it is too vague. I would say try 38 pilots and make sure that the bike's accel pump is giving a healthy shot of fuel, no dribble. When correct it really shoots. The main jet seems large, even with the intake mod and slight work. 130's could be closer, that is pretty big.

You could be overcompensating with the needle to hide the pilot jet leanness, it will mask it a little bit but make it richer than it needs all around. What is the air:fuel ratios being measured, like what is his definition of lean and rich?

[CB750R]

So I've been searching without much luck. Where are folks sourcing said press fit style pilot jets.

Sirius inc is out of stock.
Dynoman has nothing on site.
Vintagecb750 no dice
Z1 ... even ebay is a strike out.

What's even more funny I have a nearly identical post from 2008
http://forums.sohc4.net/index.php?topic=34882.0

I figured now that I have a dyno guy in the shop behind my house it may be worth sorting it out.

[CB750R]

Sorry about the vague info from the dyno, he's a bit of a crusty guy, and said it was so far off on the air fuel it wasn't worth me spending any more time on the dyno till I got the needles corrected

It had a huge lean spike at 4000 k way above the line where 13 afr is. The. Dips low corrects around 12 for the remainder.

[Jerry Rxman Griffin aka MuthaF'er]

Put a stock airbox on and see what happens.

[CB750R]

Put a stock airbox on and see what happens.

I don't hate life that much yet.  I hate wrestling with that dam thing  waaaaay more than I hate fiddling with carbs!!

Plus tintops air cleaner is just too cool!

[Parkwood60]

I must admit, I haven't been inside a set of those later carbs, but this guy seems to have jets, needles and washers for EVERYTHING http://www.jetsrus.com/a_jets_by_carburetor_type/a_jet_kits_keihin_selection.htm

Pilot Jet=Slow Jet, right?

[scottly]

Just a thought: when I put my FT 500 on the dyno with the stock muffler, it flat-lined the A/F graph at 18:1. The muffler prevented getting an accurate reading. After removing the muffler and inserting the probe into the collector, I was able to get more accurate data, with an A/F within the meter's range.

[754]

Is the dyno guy Rod M?

[Don R]

 I hate it but they're made to run with an airbox restriction helping the carbs work. I'd try the bigger pilots, I like to make a big change to see what the trend is then work back from there. Sometimes a small change is hard to see.

[CB750R]

Jets r us doesn't list the pd, carb series. So looks like another strike out.

It's Murray, I believe he used to work down in Vancouver.

I've ordered up a .45mm drill and pin vise. I have two sets of pilot jets #35 so will drill up to the next size. And see.

Or start saving pennies for some cr29.

[Djfob]

http://www.jetsrus.com/a_jet_kit_street/honda_750_CB750F_1978.html

[jamesbekman]

Like the Dyno guy said...  Look into needles, not jets...   A little rich on the top end isnt too bad, if anything get a half step down and a full step down on the jet size. 

You need a different taper on the needle.  There should be a chart that shows your needle and you can referece it to the needles that are made that are a next step richer etc...

[jamesbekman]

Here is a link for ya.  http://www.salocal.com/sohc/tech/carb/asmpg_mgs/jetgraph.htm

[KeithB]

Put a stock airbox on and see what happens.

I don't hate life that much yet.  I hate wrestling with that dam thing  waaaaay more than I hate fiddling with carbs!!

Plus tintops air cleaner is just too cool!

Compared to a CB550, stock air boxes on the 750 are easy!
Try a stock box. There is a reason they are designed the way they are.

[CB750R]

I found this chart on the site and hope to use his data to get me in the ballpark.

It seems to strongly indicate that with larger pilot jets it will have a major effect on all of the other jetting circuits.

Notice he was up in the higher 120's with stock pilots but down into the 112 with a larger pilot.

Will report my findings when my micro drill bits arrive.

Again I'm NOT ready to restrict my air intake to correct jetting.   More are and fuel in the right ratio usually equals more power, I've got a tuner in my back yard and time to make it work. Just very little in parts seem to be available.

[Thamuz]

Have you gone to http://www.power-barn.com/servlet/the-Intake-and-Fuel-cln-Jetting/Categories

they might have what ya need.

[flybox1]

Jetsrus DOES have PD carb pilots. Keihins, too.
I had 40 and just got a set of 42's from them.

http://www.jetsrus.com/a_jet_kit_street/honda_750_CB750F_1978.html

[CB750R]

Wow thanks I couldn't find that last night.

[Parkwood60]

I sound like a broken record on forums lately, but my CB750 (1972 frame, ? early K motor & carbs, screened velocity stacks Triple A 4-1 header connected to an unknown megaphone and baffle) has always run just fine with stock 1972 K2 jetting in it. No matter what I did to it. Sometime is ran better, sometimes worse, but always pretty good. At the moment, having just got it back on the road with the opened stacks, I'm going slightly lean around 1/2 throttle, maybe 2/3. Most riding is done on the needle, not the main jet (unless you are drag racing). I think your mains are WAY too rich for a mildly modified bike, especially one with an accelerator pump (or am I not thinking of the right carbs?). Fit a richer slow jet, find a few richer needles and go back down to about a 125 main and I think you'll be in the ball park. Are you at sea level?

[KayOne]

Pretty sure options for different needles for the PD 42a/b carbs is very limited. The F2/3 PD's needle weren't even adjustable (except for very limited shimming). Maybe someone can chime in on what model/year PD's had adjustable needles and/or different profiles. Larger slow jet, shimmed needle and properly sized main jet is your only option.

[CB750R]

Parkwood, your running completely different carbs so hard to get a baseline off that, the PD carbs seem to be very lean stock to accomodate the smog era standards in the late 70's.  but your correct, it seems that others have found that by enrichening the pilot circuit they have been able to reduce the main jet, and achieve better over all AFR

I have 77 F2 carbs and needles (the ones with Adjustable clips) I am currently at the bottom, and shimmed as stated in earlier posts,  I'm awaiting some .40 and .45mm drills to upsize my spare #35 pilots, to help with the situation.

So I'm in holding pattern untill tools arrive!

will report my findings when I try these steps.

[martin99]

Enrichening the pilot circuit will have no effect on the main jet.

My 77F2 -
S&B pods, Marshall Deeptone 4 into 1.
Mains #130
Needles in mid-position (third groove, one lower than stock)
Pilots standard
IMS screws two turns out

Runs well, though isn't as smooth through the carb circuit transitions as a standard bike. Definitely more power from 3/4 - WOT than a standard set-up delivers.

Are those tin-top filters really freer-flowing than pods? I can use my bike with no filters at all on current settings with very little difference. I can't understand why you are having to go so rich.

[scottly]

Enrichening the pilot circuit will have no effect on the main jet.


Pilots standard
IMS screws two turns out

Runs well, though isn't as smooth through the carb circuit transitions as a standard bike.

Perhaps a larger pilot would help?