Topic: Potential 1970 CB750 K0/K1 Hybrid

[Don R]

I never got a set of K0 carbs that had the top gasket already in them. The biggest problem I've had is starting the top nut threads. They are easy to cross thread and after the gaskets are in there's that many fewer threads to hold the top on. I've found holding the top plate down against the spring and perfectly centered is key to avoiding issues.
 I also sharpened a piece of tubing to re-punch the holes in newer top gaskets.

[Mikey G.]

Thanks for the clarification.  When I ordered mine I did it by part number, and so far everything fits smoothly, so I'm hoping I got the correct set.  I didn't have the gaskets before this, and with as many problems as I seem to be having, I figured it can't hurt to have them.

[BenelliSEI]

Thanks for the clarification.  When I ordered mine I did it by part number, and so far everything fits smoothly, so I'm hoping I got the correct set.  I didn't have the gaskets before this, and with as many problems as I seem to be having, I figured it can't hurt to have them.

Sounds like you did it right. I ever have the correct ones when I need them

[Mikey G.]

Did a bit more picking at the bike tonight.  The carbs are off again, but I have an ultrasonic cleaner this time, so I'm going to give each carb a round in it.  Way I figure, it can't hurt.  I also noticed a few of the choke arms are starting to re-rust, so I'm giving all them another round of zinc plating.  They started out REALLY rough, so I'm not surprised.

While the carbs were off, I did some experiments with the bowls.  I've been chasing a persistent overflow tube leak on several of the carbs, and while I thought it was float-valve related, this was the perfect chance to check the tubes themselves.  So I set up just the bowls, filled them with enough fuel to at least cover the bottom of the overflow tubes, and waited. 

Sure enough, I have two that leak, and a third one that's possible.  So I get to go digging in threads to see the best way of sealing those up.  Never a dull moment.

[Finnigan]

I had the same issue with my 4 cable KO carbs, I replaced them with cb450 bowl tubes (they press in) and they were an exact fit.  Have plenty of extras if you find you want to go that route.

[newday777]

I replaced them with cb450 bowl tubes (they press in) and they were an exact fit.  Have plenty of extras if you find you want to go that route.

Are those bowl tubes a part numbered piece(got the part number??)or just a removed from used bowls?

[cfairweather]

I can fix your carb bowls and will do it for free.  Just pay shipping and I will take care of it for you.  I use Formlabs Rigid 10K resin to fix leaky tubes.  After the resin cures, it is similar to glass and it will not leak again.  Best fix I have found for this common issue.

[Finnigan]

I replaced them with cb450 bowl tubes (they press in) and they were an exact fit.  Have plenty of extras if you find you want to go that route.

Are those bowl tubes a part numbered piece(got the part number??)or just a removed from used bowls?

I removed them from cb450 carb bowls, I have about a dozen spares laying around.  You'd be surprised how many parts those two models share.

[newday777]

I replaced them with cb450 bowl tubes (they press in) and they were an exact fit.  Have plenty of extras if you find you want to go that route.

Are those bowl tubes a part numbered piece(got the part number??)or just a removed from used bowls?

I removed them from cb450 carb bowls, I have about a dozen spares laying around.  You'd be surprised how many parts those two models share.

Yes indeed they do. Having been a parts manager in the 70s and I'm still using the parts fiches to get NOS parts, and verify used parts I have run across many shared part numbers.

[Mikey G.]

I can fix your carb bowls and will do it for free.  Just pay shipping and I will take care of it for you.  I use Formlabs Rigid 10K resin to fix leaky tubes.  After the resin cures, it is similar to glass and it will not leak again.  Best fix I have found for this common issue.

I think I'll take you up on that offer.  PM sent.

[BenelliSEI]

Curious….. are the tubes split or are they leaking through the space between the bowl and the tube?

[Mikey G.]

Best I can figure it's between the bowl and the tubes, because I can't find a split on any of them.  I do have at least a couple that leak from the drain screw as well, but that's something a little more easily solved.

[Mikey G.]

Hit a bit of a speedbump, but work continues slowly.  I did take cfairweather's offer to look at my carbs, and long story short, most of my problems were leaky overflow tubes.  Both between the tube and the bowl, and the tubes themselves.  The tubes got sealed up, the carbs got ultrasonically cleaned, and sent back.  THEN UPS got their hands on them.





The carbs were actually packaged quite well.  But from the looks of the box, this thing was dropped on it's corner, HARD.  On the plus side, I got lucky with replacement parts- Yamiya offers K0 carb brackets, and somebody on ebay just happened to be selling a K0 bowl for the correct side, for not stupid amounts of money.  But still- it's a lot of expense I could've done without.

But we're back together, and while I need to dial in a proper idle sync, the bench sync beforehand allowed me to start the bike easy, on all 4 cylinders, running well enough to give me an excellent place to start.  I'm hoping to have it ready to take it to the Gilmore Car Museum show in a month, so here's crossing my fingers.

[BenelliSEI]

Congrats! You’re getting close!

[seanbarney41]

MikeyG, am I trippin'/ or did your forum handle change?  No big deal, sometimes I just start to question my sanity...

Hoping to see that at Gilmore.  I will be in the swap meet this year.

[Mikey G.]

Yeah, I did.  The old handle was a throwback to something I came up with in high school, and I figured it was time to swap to something a bit more accurate.

This bike making Gilmore is entirely dependant on how the idle sync goes.  The gauge needles will still probably be flopping all over the place, but I can live with that.  I'll be at Gilmore regardless.  My yellow GL1000 will be in the show for sure, and my Valkyrie too if the 750 falls through.

[MauiK3]

We would all love to see photos of the show.

[Mikey G.]

I'll take some when I go.  I've went the last 6-7 years in a row, so the photo taking has gradually turned into just walking the show, but there's usually a lot of good stuff.

Small update (because I've been sick the last couple days)- I hooked up my fuel IV bottle to start tuning, and several things happened:

-Gas started puking out of several overflow tubes (this has to be stuck floats, so minor annoyance).

-Discovered that installing the carbs top gaskets had taken up all the slack in my 4 cables, so the bike revved high and the idle stop screws did nothing.

-While checking to see which carbs were leaking, I burned the heck out of my wrist on a muffler. 

Fun times all around.   

[MauiK3]

Some days are like that. It will improve.

[Mikey G.]

I did a little bit of carb work the past couple days.  Carbs stopped leaking, so the floats sorted themselves out.  I'm 99% certain the cables are synced correctly, the idle is still proving elusive.  We'll get there.

I haven't touched idle mixtures yet- I'll do that once I have a better baseline.  1 is quite rich.  2-4 are rich as well, but at least the plugs are dry.  And it's still running on 4 cylinders consistently, so I'm ahead of where I was.

[MRieck]

Such a beautiful bike!

I always wrap cardboard around the frame (with Ty-wraps holding it in place) when R&R-ing the engines. This prevents the scratching on the new frame paint.

Linoleum works great too as it pliable and very tough. Tried it after having left over pieces from my first house. Leave it in the sun or use a heat gun and it hugs the frame very well.

[Mikey G.]

Over the last couple days of tuning, this bike has started to behave suspiciously like a proper motorcycle.  Enough I was able to button it up and move it from the barn back to the garage.  While I haven't taken it super-far yet, a trip up and down the road felt like a proper machine.  So who knows- we might be close.

[BenelliSEI]

Over the last couple days of tuning, this bike has started to behave suspiciously like a proper motorcycle.  Enough I was able to button it up and move it from the barn back to the garage.  While I haven't taken it super-far yet, a trip up and down the road felt like a proper machine.  So who knows- we might be close.

Congratulations for persistence and success.

[Mikey G.]

First extended ride, and I'll say this- it could've been a LOT worse.  Off-ilde on up is quite good.  A small hitch here and there later on, but I have a theory (later on that).  Starting out things were quite good- idle settled nicely, takeoff was smooth, etc.  I think it started getting a bit iffy once I got some heat in to the engine.

After a trip around several (country) blocks, idle crept up to about 2500.  However, it was hard to tell if it was a "set too high" high idle, or a "not synced' idle.  Before I dove into that, I realized that cylinders 3 and 4 were popping out the exhaust quite a bit.

So here's my theory- I wonder if I have a fuel flow issue.  Despite the tank AND petcock being new, last time I had the tank off I decided to use it to get some fuel in my text IV tank.  I had the tank cocked at a weird angle, but still, when I opened the lever I was predominantly getting fuel only out of the rear barb.  Which, surprise surprise, is the one feeding the two properly-functioning carbs.

Now, I'm not ruling out having to dial in the idle sync still.  But if this is a fueling issue, I need to get that sorted first.  Still, we're close. Closer than I've been for a WHILE.

EDIT:  Okay, looks like I got a pair of problems.  First off, I AM only getting fuel out of one outlet.  I'm getting a tiny amount out of the second one, but not nearly enough.  And second (this is new to me), my gas cap vent hole is pretty plugged.  It works, but not NEARLY well enough.

Not insurmountable, but odd, seeing as how both of these are Yamiya-new.  I mean, it did sit for a while, but not THAT long....

[Mikey G.]

Okay, did a bit more putzing around in the barn tonight, and learned a couple new things.

-I definitely need the vent cap drilled out on the gas cap.  It exists, but is small enough that looking at it funny can plug it.  The fact it whistles when the bike is warm and off (heating the gas, causing it to expand, then pushing air out the hole) is one more reason to bore it out a bit.

-The petcock fueling issues may have been a red herring.  While I was only getting gas out one pipe, I think the other was acting as an air intake- I could plug the one with fuel coming out, and it would switch sides to the other outlet.  Odd, but not plugged.  Besides, when I ran the bike up again, I switched swapped the lines on the bike, with no apparent change- 1-2 ran fine, 3-4 didn't.

Ultimately?  I think my sync is still off.  I honestly believe I set the idle sync when the bike was still too cold, simple as that.  Turning the idle mix screws a half turn one way or the other did nothing helpful, and (funny enough) the rough running swapped sides at about 2500 RPM, so my cables may need adjustment as well.

It's fine.  The high idle needs addressed anyway, so we're tearing back in regardless.