Topic: WHEN IN DOUBT........ASK FOR AN EXPERT OPINION

[KRONUS0100]

the bike.......1976 CB750F
the motor in bike....1972 CB750K2
the carbs.....stock 1976 with the hourglass restriction smoothed out, and 130 mains/45 pilots
the exhaust....Kerker 4 into 1 without a bafflle
the air filter......stock box and filter slightly modified.....7/8" holes drilled in backside where not visible, old air filter peeled to 1/2 wire mesh showing

the conundrum......this bike used to run 120 mains on the factory F 4 into 1 exhaust with a THRUSH muffler.  used to run 40 pilots and not idle properly..but would run 105 with my 285 lb ass on her.   plugs were a slight grey to tan color....roughly 30 miles to gallon. 

Now....I put in the modified filter and box....as well as the open KERKER......no power to even get out of its own way.  I up the mains to 130.....bike runs like a bat out of Hades to 80mph.......tops out, no more.  Still not idle properly.......so I drill my pilots to 45 and she idles fine, but still no more than 80mph.  My plugs are white to light grey. 

Can I go bigger on the mains?  Like maybe a 140?

oh yeah...taking the Kerker off  IS NOT AN OPTION!!!

[Duke McDukiedook]

Plugs say still lean. Go up and see what 140 does.

[KRONUS0100]

gotta order them unless i can find an old set of 4 to drill out.  thanks.

[NickC]

I'm no help, but do you happen to have the muffler that was on the kerker? 

[Mdub]

If your needle clips are still in the middle slot you still have two notches worth of rich.
Try raising your needles all the way.

Mike

[KCRSXTypeS]

Plugs say still lean. Go up and see what 140 does.

Agreed with Dukiedook...the light grey color makes me think you are lean.  (But Im no expert)

Hey Dukiedook...I see you are in KC.  I'm in Olathe.  Trying to finish getting my bike running at the moment.

[Alan F.]

Wait a tick, where are your air screws at?

[eurban]

Your carbs have an idle system with a jet size and a mix screw position, a slide cutaway shape, a main jet needle with a specific profile and multiple clips positions to adjust it up and down, and a main jet.  Each of these carb "tuning points" overlap somewhat and may individually be rich, lean or just right.  Looking a plug that has been run repeatedly through the various throttle positions where each of these points dominates the mixture to determine how you need to adjust your main jet is pretty much pointless.  I am assuming that your bikes other systems are up to snuff and that your carbs are working properly (float needles shutting on and off fuel, passages clear, no air leaks) and baselined adequately (float levels, carb synch etc)??  If yes, then clearly your bikes performance is off and you have some tuning to be done. . . . . . Main jet selection should be based on performance at wide open throttle (WOT)  Saying the bike tops out at 80mph doesn't really tell us (or you) enough info. I am not really sure why you messed with the "hour glass restriction" or what effect it will have in your carbs abiliity to pull fuel through their jets but 140 mains are pretty damn large.  Keep in mind that the systems in the carbs overlap so that if for instance your needle position is set too lean it is possible to effect it with main jet size.  A too large main may make the bike run better in the throttle positions where the needle is dominant only to fall flat on its face in the widest throttle postions.  Choose your main by how the bike runs at WOT in top gear.  Once you get the jet size close (you might be waaaay off right now) you can test the main by running at WOT in top gear and then just slightly close the throttle noting whether or not you actually pick up speed.  If you do pick up speed you are rich and should jet down.  If the bike slows with the slight throttle closing then you could be just right or lean.  Up the main and try again etc etc.    Bottom line is that carb tuning ain't easy, particularly if you are not methodical and patient.

[Jerry Rxman Griffin aka MuthaF'er]

Perhaps opening the throat has created your issues. Slower air flow through the throats = less vacuum on the main jets (lean) to pull gas out therefore way larger jets than should be necessary to compensate but still slower velocity at the throat with poor mixing and very poor mileage. Poor mixing in my opinion is the problem. I'm running 115 mains with stock carbs, Kerker, and K&N filter on my 75 810F Yoshi at 7000 feet and it's slightly rich but pulls like a mofo above 6500 rpms.

I think you should try a normal set of carbs and start over.   

[BLUE71TURBO]

Put the baffle back in. Less noise and the bike will run better. 

[KRONUS0100]

neddle clip is in 2cd notch from bottom..smoothing the hourglass out roughly does the same thing as a smoothbore carb......better air flow.  The idle  screws are 1 1/2 turns out.  I did have them at 2 turns and had a choppy idle.  By the way...I have not checked....but this motor may have a mild cam in it........I had a "PRO" mechanic tell that it did not sound stock compared to other 750 hondas he has.  I tried rejetting to the smaller 120 mains and the bike would not run enought to get out of its way in 3rd, even worse when went back to the 110 mains.   Just did a compression check..all within 7 psi of each other at 115psi....i know that is a little low compared to book spec, and valves are all proper, ignition is perfect.  this bike will not run with pods...have tried twice with 2 different carbs.  this is also not the first set of carbs i've had this done to, the other bike was almost perfect...the kerker still has the muffler on it...just no baffle, not real loud until above 5500.  Bike is plain mean thru the gears to 7500 , hit 5th and at 80 you can feel her start to hit the limit...no more available power.  I am wondering if i need to drop to a 46 rear sprocket from the 48.....hope this helps.

[Jerry Rxman Griffin aka MuthaF'er]

More air flow but not necessarily better. The hour glass shape causes the velocity to increase at the center of the carb body thus creating a higher venturi effect at the point where it pulls gas from the bowl creating better atomization of the fuel and thus a better mixture. That is the point I was trying to make. 

Have you tied multiple compression tests with a warm engine to validate the one you did? Low compression in higher gears won't allow it to pull strong at higher rpms. It needs that compression pressure to push it stronger with a load. Kinda like an old car with worn out rings or valves going uphill. Just not going to have the power.

The 46 tooth rear sprocket will have the opposite effect unless you have the extra power to push it. Go down in front or up in the back to increase power to the rear wheel. The 46 will decrease engine speed at the same mph and could give more top end but you need more power to push it too.

Keep at it. You'll find it.