Topic: Tweeking advice. Carbs ain't quite right.

[few-regrets]

Beautiful day in Tucson so...I took a break-in ride on my latest project. It's the blue '77 that I use as an icon (look left). New first-over bore and fresh head. 100% mechanical and cosmetic do-over. Really happy with the look and the motor is..is...MAKING ME NUTS! Sounds sweet but I can't get the carbs right. I've synced them puppies till they think with a single mind BUT I have that weird "hits a wall at 95mph but accelerates again when ya back off the throttle" thingy goin. I remember the same grief with my first CB750 back in '76 (it was a k2) and remember that we tried all kindsa crap to fix it - and finally did - but I can't remember what did the trick! It's got k5 carbs on it (not k7s) with 110 main jets and the needle's dead center. I'm at about 2500 ft here in the Tucson and that sounds about right to me. Timing, valve lash - all that other stuff is spot-on and it's a VERY quite motor for a CB. Has stock airbox ('77) and stock exhaust (k5 HM341's) with baffles. Purrs like a hooker with a full wallet  - everywhere except balls-out on a nice flat desert road. Bike had 15 miles on the top end (9200 total on the clock) at the start of the ride. Gotta 100 on it now.

So do ya think that's a lean-out problem since it pulls again with a partial throttle roll back? Plugs looked a little sooty like maybe a hair too rich. ARGGHH!! Gas lines not pinched, float bowls clean, petcock clear. Absolutely no unusual noises at any speed. Just no pull way up thar! My other CB's are both 836's with lotsa mods so maybe I'm just spoiled? Any and all thoughts would be welcomed and when I get it sorted out I'll share whatever it was that cured it right here. Thanks guys!

[HondaMan]

First, presuming the carb hoses are not hard and have a mild vacuum leak or two...

...try a 17 tooth front sprocket. You didn't mention the rear tire size (or oversize) that you might have: the bikes after the K1 were overgeared by installing an 18 tooth front sprocket, largely to reduce honda's warranty costs and quiet the bikes for touring, which they dominated in those days, with fairings.

Next time you have it apart: advance the cam timing about 3 degrees (early). You have to cut slots in the cam sprocket for this one. The later engines had late cams for EPA reasons, which make a nasty flat spot at about 6500 RPM.

Is the head on this one from a K2-K6 era engine? Those are notorious for poor passageways and large valve shrouding areas that can be improved upon considerably with a Dremel tool and some time. They also tend to have a lot of flash in the cooling passages, which makes hot spots on (in particular) the center cylinders, reducing power.

[few-regrets]

Thanks HondaMan.

Carb tubes (fore and aft) are all NOS and premo. I'm runnin' a huge (620?) chain and sprockets (sliced a few cases in half already) but the gearing's good. Cuts about 6900 rpm at 90 or so. Pretty much inline with my other ones. That's "about" 120 at 8500 rpm (if it'd pull it). Head's a stock '77 with the bigger valves and such. Stock cam. I DO have an adjustable cam sproket fitted. I got 8 of 'em on ebay for a song a few years ago so I always use one - 'least till they're gone. I HAVE NOT messed with the cam timing though. I'll give that a try when I re-torque the head in another hundred miles or so. Since this is a stock-bore bike and not a hot-rod I didn't opt for HD cyl studs so I'm gonna pull and re-torque just outta paranoia. I do remember hearing that advice years ago (now that you've reminded me). Still have a gut feelin' that's it's a carb thing 'cause of the surge as the throttle gets closed a bit but man...if I had a dime for every gut feeling that made me look stupid.. Well, I'd be able to pay somebody else to type for me!

Thanks for your thoughtful insights. I'll try that cam advance tomorrow and letcha know! (And thank GOD that the motor's easier to pull on a '77 frame than on my older ones!)

[Rob M]

It sounds a little like a fuel starvation, to me, are you running in-line filters?  I had a problem with the fuel flow through in line filters that was the same symptoms once.  Other idea is it may be float level.  I'd check just to rule it out at least.  Hope it helps.

[Spanner 1]

I think I'd be looking at fuel delivery too before I took apart the head  ... slow fuel to carb bowls would lean-out the mid-range throttle response for sure with low fuel level unable to keep up with demand... my 2cents......... is the carb venting A.O.K. ?

[few-regrets]

Hmmm...I'm using the same in-line filters that I use on my other (non-problem) 750s so I can probably rule that out. They're open and runnin' freely - just checked. But raising the bowl levels sounds like a easy thing to do before I tear it back outta the frame. No leaks and the vents are clear. I'll have a look at the float levels. Letcha know. Thanks, guys.

[GammaFlat]

It's got k5 carbs on it (not k7s).....  Has stock airbox ('77)....

hmmm.  The airhorns (rubber part between the carb and airbox) are different between the K1-K6 and the 77 and later K and F setups.  If you've got K5 carbs, the airhorns that would fit the K5 carbs would not mate up with a K7 airbox.  The openings in the K7 airbox are too large.  Maybe you have a K5 airbox?  In any case, when you run K1-K6 carbs, you almost have to run K1-K6 manifolds, airhorns and airbox as a set just as a K7 set of carbs requires the manifolds, airhorns and airbox from a K7 or K8. 

A glance at the carb differences:
The PD carbs are 100mm deep while the K5 carbs are 87mm
The PD "airbox side" throat OD is 50mm while the K5 is 40mm
The PD "engine side" throat OD is 39mm while the K5 is 37mm

If you are running the parts you say your are, maybe it's not a great fit?  I may have some of this wrong as I'm not too familiar with the F's but I always thought the 77 and 78 F bikes had PD carbs just like the K7/K8... at least dimensionally. 

[few-regrets]

GammaFlat! Wow! Damn good reasoning. I'm sorry to have led ya down the wrong road. It's also a k5-ish airbox. I remember trial fitting before paint and discovering that it wouldn't seal with the rubber tubes that I'd already bought. So I swapped it out with an old-style one (k1-k6) that I had lay'n around and promptly forgot about it. I got the bike as a roller with a motor and some carbs but not much else. I'm callin' 'em k5 carbs 'cause they're a dead ringers for the carbs on my unmolested '75. Same rebuild kits for both and no diff in any detail. Oh and it's a K model - I made the tail section to mate with the old style tank. And I'm pretty sure that you're right about the 77/78 K/F models sharing the same carbs. Well, before the PO got hold of mine.

Anyway...you oughta write mystery novels or solve crimes. That was some IMPRESSIVE deduction. Much appreciated.

[8 Track]

I'm going with fuel starvation, too, although all of the other ideas are very difficult to ignore.  I feel this way because of the surge you talk about.  Once, my petcock screen was dirty with that tan dust bad fuel makes and I had similar problems.  Ooh, put it on a dyno, run it up to that cut off speed, reach down, shut the fuel off and do a bowl reading.  I'd never do that on the street!