1972 Bitza Build thread...

[Trionda]

I am in the process of getting a friend's 1972 CB750 back on the road.  He bought it a year ago from a guy who "specialized" in flipping bikes "professionally" and until recently had a shop, but bad business decisions and sketchy workmanship put him under.  That seems for the best, really, as this bike, in the condition delivered to me, was dangerous as hell.

Suspension, chassis and engine bolts had all been hodge-podged, most of them were loose, the brakes were shot, and the front master cylinder was leaking and taking in air, nothing was adjusted correctly, etc.

Amazingly, while my friend actually felt he paid a good deal more than he should have ($3000.00), the bike is in fantastic condition, and a few weeks going through the chassis, replacing bearings, seals, and the like have brought the bike back to a safe road-going condition.  The frame is laser straight, the engine either has ridiculously low miles or has been rebuilt, the carbs have not been attacked by a ham-fisted shadetree mechanic, and the finish on aluminium parts is very nice.  The shop stripped the bike down, and rattle canned the frame (I'd have appreciated it if they had simply powder-coated the darn thing, as it's only about $300.00 for a nice professional PC job in my area of Virginia).

The reason he spent so much was due to the custom fabbing and high end parts he had the shop install.
The bike has ace bars, Joker rear sets, custom up-swept Mac pipes, new café seat, new tyres, and a host of other bits that make this a desirable machine.

I have replaced most of the fasteners with SS, and have polished the cases some, installed new braided SS lines, a real oil filter, and other things.
The engine cranks freely, and while I've not yet brought it to life, I'm close.

At this point, I'm really liking this machine.  This is my first CB750 build, but I'm no stranger to Hondas, and have had a few of them over the years, including a 1971 CL350, a 1978 Goldwing GL 1000, and a 1982 CB900.

As this is a 1972 bike, my friend has retained the stock 1972 tank, and had it powdercoated (no dings, dents or bodywork), with the inside epoxy coated for use with ethanol.  The rear sets don't really work for me, and the kick starter hits the brake lever, but my friend wants to keep them, so I either need to mount the rear sets even further back (uncomfortable seating position), or bring the kicker mechanism out an inch or two.  At this point, that's not a priority, but I'd like to hear from any of you who have installed Joker rear sets, and what you did to accommodate the kicker.

At this point, I am fighting for real estate regarding mounting the fuel line(s), and the carb inlet is not much lower than the fuel petcock hose barbs.  Since this is a 1972 tank, it uses a twin feed petcock, but the carbs he has mounted are 1978 models with the accelerator pump (nice score, as far as I'm concerned), which only have a single fuel inlet on the 2nd carb from the left.

I've procured a 2-into-1 hosebarb, so all is well there, and as he's running pod filters, we've no airbox to worry about (somewhat problematic, I understand, as Kehein Vacuum carbs really NEED some vacuum from the airbox).

My questions for today are:
1) Whether I can run the longer intake rubbers from earlier model CB750s with these 1978 carbs.
I realize I might lose HP, but I should gain torque with longer runners, and I really need the room to get the fuel lines away from the back of the head, and get a nice bend in the lines for adequate sumping.

2) Whether I can purchase larger pilot jets for the 1978 carbs (press-fit), or whether I have to drill them out by hand?
Currently the bike is equipped with 35 pilots and 110 mains, and I'd like to step up to 42 pilots and 120 mains.
I have new Dyna coils and an electronic ignition that I'll be installing shortly, as well.

Thank you for any and all advice you might be able to provide.

[TOOLS1]

If it were me, I would go back to the round top carbs, but if you really want to use the keyhole carbs, and change the jetting, you can drill out the pilots to 0.45mm. This makes the bike start, and idle a lot better. Then you can shim the needles up to 1.5mm. If you need to go any further, you will need needles out of 77 carbs or A model carbs.
TOOLS

[Trionda]

I'm not opposed to going with round tops.  That's what this 72 would have had originally, and they are a little easier to tune, from what I've been reading on this forum.

The owner is also not opposed to going back to an airbox if need be; he wants a reliable bike that will go anywhere he points it.  I could always modify a stock airbox for a little more intake charge.  I've had good luck opening things up in other airboxes, while still retaining enough vacuum for the carbs to act right throughout the entire usable RPM band.

At this point, my priority is to get the bike running, tuned and balanced.  The brake lines were original and retain the original date codes (yikes!) complete with cracks through the outer jackets and all.  The intake rubbers (carb holders) are also original, and hard as bricks.  They leak like a sieve, and must be replaced before I make any further attempts at bringing this old gal back to life.  I'm just hoping that I can use the longer intakes with these later carbs.  It seems to me like I should be able to.

I failed to mention that the engine equipped in this 1972 model is a 1977 K engine.  

[Trionda]

Thanks Tools, for the good info on carb adjustment.
With these bikes as collectable as they've become, I'm surprised nobody has stepped up to make various sizes of press fit pilots.  I DO like the fact that the earlier carbs have screw in pilots, and seemingly lots of sizes to choose from.
Regards-

[Re-run]

I prefer the newer carbs too. They are not vacuum carbs though, merely typical piston slide carbs. Pods can still be an issue but nothing like the cv carbs used in 79 and on.

The fuel screws are much better than the old air screws. If you do need new juets, I advies against drilling as that destroys the venturi effect the jets deliver. Instead get new pressin jets, yes they are available. A goodle brings this as the first hit: http://www.power-barn.com/servlet/the-1916/Press-Fit-Pilot-Jet/Detail

The ONLY thing I feel is better about the old carbs is there are more parts availability. I love the accel pump.

[Trionda]

Thanks Re-run.  That's good information.
As someone who drag raced cars for a long time, I like accelerator pumps too.
I'll use anything that makes my friend's bike reliable and generally a better runner, but I'd like to see if I can get these Keyholes going first.
I *have* drilled out jets when in a bind in the past, but I prefer to buy quality machined pieces for the exact reason you mentioned.  
Regards-

[Trionda]

I followed the link you provided (thanks), and bought new mains and pilots, and bowl bleed screws.
She fired right up and ran beautifully.  I took her for a spin, and couldn't have been happier with her performance.  Just a perfect first run.

...AND THEN I RAN INTO SOME TROUBLES
I replaced the original 5ohm coils with Dyna 3ohm coils (in preparation for the swap from points to electronic ignition), and she still ran fine.
I then swapped out the points for the electronic distributor, and she still ran fine...until the following day.  She started right up, the first time I fired her over, but throttle response was a little sluggish and idle had become a little rough.  When I fired her again, she ran very rough.  After that, she would crank and fire instantly, but not stay running.  A twist to WOT would make her fire and rev to 5500RPM, but then she would sputter and die.

I replaced everything in increments, and made sure each modification "took" before moving onto the next one.  I did this so I could isolate problems if they occurred.

I suspect the carbs.  I noticed the floats are original phenolic floats, and I wonder if it just took 2 days for the fuel to fully saturate them, and prevent the engine from getting fuel to the combustion chambers.  

For the record, the plugs are dry and sooty now (they were clean before), I have excellent white/blue spark , I have excellent fuel flow and a clean filter, and when I peg the throttle wide open, floods of fuel pour out of the carb mouths where the filters are mounted.
All of these conditions lead me to believe I have a carb problem.

I even swapped the electronic ignition out for the points ignition that worked so well, and the bike acted no differently.

Any ideas?

[shinyribs]

You mentioned sinking floats. Are you fuel levels low in your bowls?

Revving like you describe but not idling is textbook symptoms of a pilot circuit not delivering fuel. There's obviously fuel in the bowl,but it's not present for idling. It does this because the pilot jet is the first to starve for fuel as the level drops. It's mounted the highest compared to the main.

 Whether it's a clogged jet, low fuel level in the bowl or just running out of fuel altogether ( fuel delivery issue-filter,petcock...) is something you'll just have to check on yourself.

[Trionda]

Exactly, and thank you not only for responding, but echoing my concerns.  I know my symptoms are also caused by the needles sticking in the seats, but there is no way they are all sticking.
I suspected the floats being overloaded with fuel, because as you know, if the floats cannot rise, the necessary fuel cannot enter the bowls and feed the engine.
I had not considered the pilot jets, though.  I will pull the carbs and examine them closely.
Again--thanks.

[TOOLS1]

Actually the floats rise, and shut off the fuel from entering the float bowels,  where the fuel is sucked up through the jets, and then atomized by the air stream going into the combustion chamber.
TOOLS

[Trionda]

But if the floats are full of fuel, preventing them from rising, the float bowls will not fill, and the fuel will not be sucked into the combustion chambers and will run back out the carb mouths.  At least that has been my experience/understanding.  If there is a steady supply of fuel and the fuel cannot combust, it gets "returned", so to speak.

Always up for a better understanding, though.
Regards-

[TOOLS1]

If the floats themselves fill with gas, and do not rise, they will never push the needle valve into the seat and shut the gas off from coming into the float bowel. This would cause the float bowel to over fill, and the gas would start running out the overflow tubes in the bottom of the carbs. However if the overflow tubes are plugged, the gas will run out through the airbox. As for unspent gas in the engine, that goes out the exhaust.
TOOLS

[Trionda]

Great info, Tools.
It's amazing how carbs work; despite their differences, they all work pretty much the same, and I appreciate you helping me troubleshooting my issue.

So your recommendation is to check the overflow tubes first?
Admittedly, work keeps me very busy, and as sweet as thing ran for me for 2 days, I know I can't be far off from a solution.
This weekend, I intend to tear into the carbs again, and see if I can identify the issue.
I can't think it's the coils or the ignition, because I timed the ignition perfectly (very easy to do), and the coils, themselves are no brainers.  The contacts are good, and the HT leads are making good contact, spark is strong, and fuel flow is too.
Again, thank you for your input.

[Trionda]

The fuel in my bowls is very low.  Needles move freely in the seats.  Idle jets seem clean and unrestricted, but I have not yet removed them, as they are press-in, and I have to remove the carbs to pull them carefully and not bugger them up.  I'm going to do it, anyway, to be certain.

Tools--you mentioned:  
"...if the floats themselves fill with gas, and do not rise, they will never push the needle valve into the seat and shut the gas off from coming into the float bowel. This would cause the float bowel to over fill, and the gas would start running out the overflow tubes in the bottom of the carbs. However if the overflow tubes are plugged, the gas will run out through the airbox."

I'll be checking the overflow tubes for restriction next.
I will also check to make sure the needle rubbers are okay.  The fact that they were dry for so long, and then saturated with fuel (after the 2 days of running well) might mean they are breaking down, and need replacement.  I suspect as much, though I don't know if that would cause the issue I'm having.  It seems possible though, since there seems to be no mechanical failure, I've not dropped a jet, everything is tight, etc., and the needle rubbers are something that would all be affected similarly as a result of exposure to fuel.

UPDATE:
I just drifted the float pins out to check the floats for fuel saturation, and they were dry.  Good.  I also checked the needle tips, and while they are *okay*, there is definitely a fine pressure line where they make contact in the seats, so I will be replacing them.  If they cannot seal tightly in the seat, they will cause problems, and I know they are old.  It seems we're back to the idle (pilot) jets.  An "in-place inspection" is not sufficient, so I'll be pulling the carbs and removing them to clear them, (or perhaps the circuit above them), of debris.

[Trionda]

UPDATE:
The 72 is running like a top.  A little rich, but otherwise very, very good.

I tore the carbs entirely down to their cores (6 hours yesterday) and cleaned everything,  The battery was flat, and one of the petcock taps had pulled out of the body when I disconnected the fuel lines, so I put the battery on a trickle charge, and fixed the tap with Marine JB Weld, and waited until today for the epoxy to dry.

Here's what I found in the carbs.
The 1st and 2nd carbs (the one with the accelerator pump) had needle valve seats that were almost completely clogged with sludge.  I must have passed this sludge through at some point in the 2 days I had it running.  Anyway, some straight ether and some compressed air got everything clean, and I set everything back up this evening.  The idle circuits looked very clean, and it's my fault that fuel came out of the carb throats (I mistook the overflow tubes in the carbs for bowl drains) and capped them with tight fitting vacuum caps), so I buttoned everything back up, and I'm pleased to say that whilst I cannot precisely understand why the seats were clogged, or why they caused my symptoms, I'm definitely glad the bike is running well again.

I NEED SOME ADVICE:
I am NOT happy with the twin petcock the 1972 tank is fitted with, because the lines I have run are not ideally placed, and the rear fuel line is pinched, and flow is compromised by the lack of space at the rear of the heads.
Does anyone have a good solution for this, or know whether I can run a single outlet petcock?  Remember, I have no experience with these bikes, so I don't know whether the 1972 carbs had dual inlets or not.  any better solution is always welcomed, and thank you all for your sound advice on the carbs.
Regards-

[Trionda]

The bike is running very strong and the engine seems very happy.  Thanks for your help.

I am now having an electrical issue that I hope you may have some advice for:
After being unable to find a viable electrical schematic for this bike (Clymer, Haynes, internet) and deciding I did not want to cut up the wiring harness to address an electrical short to the tail lamp for the stop lamp, I ran a separate line from the front brake switch (Green/Yellow) straight to the rear lamp harness and then pigtailed to the rear brake switch.  I did a clean job, and now both work 90% of the time.

My issue is this:
1) Sometimes the tail lamp won't light, but when either brake is actuated, the brake light works.
2) Sometimes the tail lamp will light, but when I actuate the brakes (front or rear) the brake light won't light up.  
When the engine is off, all lights work as they should.  It's only when the bike is idling that I occasionally get these issues.

I suspect a weak ground, based upon the infrequent sporadic behavior.  It may be that I am robbing too much current by running both the front and rear brake off of the same lead, but that's not what my gut tells me.

What are your thoughts?

Thanks again.

[Truck]

I did a google search for a wiring diagram and had plenty of hits. Now have a nice 11X17 laminated sheet.

[Trionda]

Oh, I have plenty of wiring schematics; just none that match what I'm working with.
I have no idea what year this harness is from, but it does not correspond with either the frame year (1972) the engine year (1977) or the carb year (1978).
That said, I'm not interested in reinventing the wheel, so I've bypassed the erroneous wire, altogether, and it works...most of the time.
Regards-

[Trionda]

A closer inspection of my Clymer manual tells me that the wiring harness installed on this bike is from a 1976 K bike.