Topic: '78 750K first project by a car guy...

[AintNoEasyWay]

Looks rad, but is it just me or is the front tire super close to the fender here?

[madmtnmotors]

Looks rad, but is it just me or is the front tire super close to the fender here?

Yes, it is. Be sure to check the brake line bracket. I've had that bracket rub a tire that had a lot more clearance....

[martin99]

Now I get the BMC part of your name - you've nicked one of our Minis!

Flared arches, strap holding down the bonnet - guessing there's a nice motor in there?

[RandyBMC]

Yep - it is a 1275 with a ported head, roller 1.5 ratio rockers, a long branch header, etc.  I use it as my mostly daily driver (not the last few days here in Houston - need more of a boat!).  I've been a Mini guy for years and years.

I am a little worried about the fender clearance too.  At rest just rolling it, there are no issues, but obviously that may change with centripetal force during rotation at higher speeds.  Waiting for some testing.  The photo does make it look even closer though.

I got some project work done finally (been working a lot, which has interfered with my CB time).  I finished up the wiring, including my DCC gauges.  I used noob's guide, but I used he Br/W wire instead of black to supply power (stock switched hot from what I read on the wiring diagram).  I also finished up all of the stock harness wiring.  I am pretty happy with how it all came out.  The fuseblock I got from Hondaman does have three extra fuses (one each).





I also got the tank and oil lines all mounted up.



I also decided to go a different way then I originally planned with the license plate/taillight.  I welded up a bracket for both.  I measured it about six hundred different ways using string and calipers.  I am digging the way it turned out.









It has an angle that gives it a bit of a rear fender look, with plenty of clearance.

Now on to the carbs, ugh.  Worried about these!  I rebuilt triple throat Webers back in the day, so I'm hoping that may give a similar enough skill set to tackle the PD42As...

[AintNoEasyWay]

Sweet. Will serve as a fender for you.

[RandyBMC]

Took the floats out of the carbs, rejetted to 135 mains and 42s (836cc, 4-1, pods) to start with.  I cleaned everything out while I was there, and reset the float levels, which were off by a couple mm different ways per carb!



What is the apparent vent to?  I don't see it in the shop manual.  It isn't the supply, but smaller.



Is is where it goes into the carb lines:


It is the line under the supply line in this photo:

[RandyBMC]

Is that smaller of the two lines just a vent?  Is there a return of some sort?

I've dug through the two factory manuals, looked at the parts diagram, and I can't see where that line is supposed to go.

Help - I didn't take it apart remember.  This came to me in baskets, and it is my first build!  Sorry if it seems to be a daft question.

Thanks for the help!

[flybox1]

I leave mine vented to the atmosphere.  Its there to help the carbs equalize. 
Some route it to a nipple installed on the airbox to create a small vacuum to raise the fuel level at WOT.
Either is fine.

[RandyBMC]

Thanks a bunch.  I figured the only place it could have gone was to the original air box.

In other news - the electrical is finished!  It all works (need to make sure there is spark still once the carbs are on).  Happy with that, since I rebuilt the controls, the fuse area was rebuilt, put in a new harness, and I installed aftermarket front/rear signals, brake light, tach/speedo.

I ordered new carb holder boots from Vintage CB750. The old ones were just too stiff for my liking, and to rejuvenate with wintergreen oil would have required a purchase and wait as well, so I figured I'd just get a new set and save the old ones for spares.  Once those come, the exhaust is the last bit.

On that front, I'm leaning toward the Delkevic "classic straight".  I've been chatting with Janielle over at Delkevic US, and it sounds like a great system that meets my needs.  Fits the oil filter, center stand can be used, and I can use the passenger foot pegs.  From YouTube videos, it sounds the dog's bollocks too.

[RandyBMC]

So I got the carbs mounted!  The new boots were definitely the way to go.  I mounted them on the head side, then the carbs popped right into place with very little "motivation" needed.  Mounted the choke cable, and today I'm going to put the throttle cables on and set them up.  I have a synchronizer, so once it is up and running, I'll tweak the bench synch I did with a proper vacuum synch.







I also pulled the trigger on the Delkevic system.  The packing was awesome, and I'm really happy with the product.  It arrived the same day as my new exhaust bolts did (yesterday), so that was good timing.  I went ahead and refinished my exhaust clamps and mounting stubs by wire wheeling and clear VHT.   My CB came with a bunch of stuff in big bins, and that included the drag pipes and early style mounting bits.  I ordered the Delkevic for the '76 style accordingly.  I'm hoping to get that mounted up today too.





After filling it with oil the other day to check for oil pressure (which it had!), after a couple more days I found an oil leak through the center of the neutral switch.  I'm waiting on a new one to get here.

I will probably go ahead and try to fire it (getting super excited about that) and synch everything with the oil leak.  I used regular Valvoline dinosaur based oil that I had (10W40) but then read I shouldn't use auto oil in the bike because of the clutch.  So I ordered some Honda oil from Amazon.  When I switch out the neutral switch, I'll change the oil.

Thanks for all of the help to Stoli for the carb info, SKTP for general help, HondaMan for guidance, Steve-o for encouragement and a tank, and all of the other members here for making this such a helpful community.  I'm really enjoying learning.

[Stev-o]

It's getting to a very exciting time! 

Hearing it fire for the first time will be a moment you will not soon forget.

Confirm you have spark on all four before trying to fire and once running, use a fan on the cylinders to prevent overheating.

Good luck!

[RandyBMC]

Woohooo!!!!

Not only does she start, but I rode her around the neighborhood!  Man, this thing is awesome!  Having never ridden a CB750 before, I was shocked at how quick it is - without revving it over 4500RPM or proper timing/carb synch yet.  It sounds great too.

It is a little tough to get started when it is cold (choke full and a couple twists of the throttle - correct technique?), but once it is warm it barely takes a bump of the starter and it fires right up and idles at 1000RPM with the choke off.  2500ish with the choke pulled.

Paint, timing and carb synch next - then riding!!

[Stev-o]

WooHoo!  Congrats.....nice job!

Yes, for cold start, full choke two twists of the throttle and crank it. Reduce choke to half once running and let it warm up two min...

[SOHC4 Cafe Racer Fan]

for cold start, full choke two twists of the throttle and crank it. Reduce choke to half once running and let it warm up two min...

Tried and true start up routine.

[RandyBMC]

Thanks guys.  If it doesn't start after about 20 seconds on the starter, then what?  Two MORE squirts of throttle, leave the choke pulled full out?  I'm hoping once I do dynamic timing setting it may be easier to start...

[Stev-o]

I would not run the starter for 20 seconds.  After 5 seconds stop, two more shirts and crank again for 5. 
Yes, leave choke pulled out til she fires.   

These ol girls are cold blooded. 

[RandyBMC]

Perfect - thanks (again)!!

[RandyBMC]

Ok, I pulled the rack off the bike again.  I reset the float height based on the pilot jet level method and then checked them using the clear tube method.

Also, the accel pump, which worked great on the bench, had a diaphragm that was apparently not made for fuel!  This was a brand new rebuild kit.  It was soft and gooey and had torn at an edge too.  I cleaned up my old one and straightened the bent rod, and presto, it worked again.

Once on, the bike fires right up with choke pulled and a couple throttle twists!

The only thing left is a slightly sticky choke.  I have to figure that one out.  I'm thinking the plates themselves may just have to be loosened on the shaft and adjusted left to right.  Does that make sense?  I'm not sure if that works on these or if there is another known problem.  I didn't break the rack apart, so I didn't remove the shaft or butterflies.  If I pull the cable hard, the butterflies close and the cable works the choke plates normally.  If I lightly or even normally pull the cable, the plates don't close (but they will by hand without much resistance at all).

[SierraCB]

WooHoo!  Congrats.....nice job!

Yes, for cold start, full choke two twists of the throttle and crank it. Reduce choke to half once running and let it warm up two min...

Funny, mine starts right up, but will always seem to need adjustment of the choke to be happy for a while.  I need to go a few miles before it will idle normally without the choke on.  Maybe just my bike?

[Stev-o]

WooHoo!  Congrats.....nice job!

Yes, for cold start, full choke two twists of the throttle and crank it. Reduce choke to half once running and let it warm up two min...

Funny, mine starts right up, but will always seem to need adjustment of the choke to be happy for a while.  I need to go a few miles before it will idle normally without the choke on.  Maybe just my bike?

Not all bikes have the exact starting procedure or behave identically once running, might have to do with jetting.

[cb750tr]

WooHoo!  Congrats.....nice job!

Yes, for cold start, full choke two twists of the throttle and crank it. Reduce choke to half once running and let it warm up two min...

Funny, mine starts right up, but will always seem to need adjustment of the choke to be happy for a while.  I need to go a few miles before it will idle normally without the choke on.  Maybe just my bike?

Mine is the same. Starts as if it was warm without any hassles at all. Won't idle well until it has been ridden a bit though, but then it purrs as a kitten. K7.

EDIT: Need to correct myself. I had quite a bit of junk in the carbs, once I cleaned it properly it is definitely not start normally.

[RandyBMC]

I figured out the choke was from my own doing.  I had changed the location of the original pull on the triple tree, so an adjustment fixed that.  It starts well but doesn't happily idle until it's run for a few minutes.  Seems par for the course with these.

I've been riding a lot trying to get through the break in.  It runs great!  I got paint done, but I'm still waits on stripes.  I'm thinking of black with a white or silver pinstripe outlining the black supersport style swoosh.  For now, I like it "clean".  I ended up going with "Moonwalk Grey" which is a new MINI color.  I figured it was appropriate.  It's grey but has a metal in it in the sun.  I'm happy with it.







I even rode it to work a couple of times.





It runs well, but it does run higher RPM in top gear than I'm used to.  It has stock sprockets on it, so I'm assuming that's just the way Honda set it up.  75mph is close to 5000RPM.

I had been riding around with my Snell SA helmet from the racing days in cars, so I finally got a proper helmet to riding my CB.  I like the bubble visor, and it is the newer shell Bell 500 which I like a lot.





Then it got pretty hot here in Texas - like heat indexes over 100F.  I was running break in oil using 10W40, and at idle after running in traffic, it started getting a flicker of an oil light.  It stays pretty consistently after the ambient temps stayed over 90F and I would get stuck at lights for any length of time.  I changed to 20W50, but the problematic oil light still reared its ugly head at long waits at idle.  So after reading up on the forum (thank you), I changed the oil pump o-rings and pulled the cover on the shifter side to change the small o-ring.  The oil pump o-rings looked fine (I may have already replaced them - I couldn't remember), but the 5x2.5mm seal on the shifter side was pancaked and looked like it had seen better days.  None of the local stores had the right width, so I'm currently waiting for that part to get here.  I figure I will put it back together and try it before pulling the clutch apart to add or chamfer holes there.  I took a lot of photos of the shifter linkage for myself and figured I would post them here if it might help someone else.

















Hopefully the o-ring will take care of the oil light.  I have another sender too, but the symptoms make sense that it actually was low oil pressure - really hot, low airflow, low RPM, and a bad o-ring.  It shifts great and neutral is super easy to find, so I don't want to touch the clutch if I don't have to!

Thanks again to everyone who contributes on this forum.  It's great to have such a go to resource!

[Stev-o]

Looks great, I'm nominating you for BOTM next month!

Life begins at 5K RPM with these bikes!  [once break-in is complete - 100 miles]

[jaycox]

Really nice and clean.  I like it a lot.

[RandyBMC]

Wow - thanks!  Huge honor.  I wish I had some better photos of it, and I'm stuck waiting for that o-ring so I can't ride it anywhere to shoot some.  I really appreciate the nomination though.  It makes all of that sweat, blood and tears worth it (that and riding it).