CB750.... hmmmm

[Arne]

Got the CB1000 engine split up and the CB750 engine out of the frame.

Just need to pull apart the 750, alter the top cases a bit to receive the liter jugs, get my gasket set, clean stuff up, and reassemble.

This should be a pretty fast project. (these 1980's four stroke engines just come apart so easily compared to dried out crusty 70's two strokes that I'm used to working on : )

[Arne]

Alright, so here's a new one for me: lapping valves.

It appears that you do this everytime you overhaul a 4 stroke?

Do you use the socket and hammer tap trick to remove the valves?

Do you use a drill with the lapping compound or do it by hand?

Inquiring minds want to know. Thanks - Arne

[tz375]

Arne,

You remove and replace valves with a valve spring compressor. Good ones are only about 40 buck.

If teh valves are spotless and no signs of pitting on the seats, they can be lapped by hand with fine lapping paste and a hand held device that looks like a miniature toilte plunger. You spin it back and forth between your palms.

If there's any pitting, take the head and valves to your local Auto machine shop with a Serdi machine and have them grind all 16 valves and all 16 seats (three angle please).

If you plan on cleaning up the ports to remove any obvious small lumps, do it before you see the Serdi man.

It is all quite eas, but it takes time.

[Arne]

Arne,

You remove and replace valves with a valve spring compressor. Good ones are only about 40 buck.

If teh valves are spotless and no signs of pitting on the seats, they can be lapped by hand with fine lapping paste and a hand held device that looks like a miniature toilte plunger. You spin it back and forth between your palms.

If there's any pitting, take the head and valves to your local Auto machine shop with a Serdi machine and have them grind all 16 valves and all 16 seats (three angle please).

If you plan on cleaning up the ports to remove any obvious small lumps, do it before you see the Serdi man.

It is all quite eas, but it takes time.

Thanks! I've been asking around a bit on some other forums...

I think I've got most of it figured out. Valves are out, I didn't use the valve remover to do that, though I will use it to put them back in.

(to get the valves out you can just ball a cloth into the head combustion chamber, set it on a flat table valve stem sides up, stick a bit of cloth in a socket and then give it one good hit and it all kindof just falls apart.)



Apparently you can't grind these honda valves because they have a thin hardened surface on them (you can only grind the seats)

This head is off of a bike with only 15000 miles on it, so everything looks pretty good.

It appears that most people run the valve into the hole, put some fuel line on the valve stem, and then pull and spin with fine valve lapping compound. Then they put it all together and fill the head combustion chamber with gas and if any of them drip anything you relap those valves. I'm going to try this as I don't have the little suction cup valve deal.

I'm not quite sure what if anything I should match or clean up in the intake ports (the small lumps you mentioned?) I'm probably just going to leave well enough alone in there as this is my first valve service job that I've done.

Thanks - Arne

[tz375]

I'll pretend i didn't see that last picture.

The fuel tube is a good idea, and yes, I had forgotten that Mr H had hardened valves.

if the seats are pitted you will wear away that hardened "skin" by grinding. It's better to have them cut with a Serdi and then just finish lap teh valves for a perfect (thin) seat.

You are looking for a thin gray line about 1-1.5mm wide on the intakes and say 2-2.5 on the exhausts.

After they are lapped, do not mix the valves up. they must stay in the right holes. but they must be 110% clean. I use an old piece of cardboard market from left to right intake and exhaust and push the valves through in2 lines.

That way they don't get all mixed up.

And then the valve clearance shimming starts and that can be enough to send you mad if you aren't methodical. Just take your time and it will all be fine.

[tz375]

One I prepared earlier (about two decades earlier). It's a CB900 with a color scheme that took me forever to work out. It's a n adaptation of a CB1100R and the stock CB750.


Just for inspiration - it even tells you where I err borrowed it from.

[Arne]

Got the CB750 top case back from the machinist today with a bit bigger holes for the 1000 piston sleeves, a little more cleaning and I should have the engine back together for timing and valve clearance checks this weekend.

[Arne]

Been out of touch for a little bit working on house stuff, truck stuff, and work stuff.

Thought I'd share some pics of the paint by my good buddy Dave, should be on the road this weekend:

http://www.thegsresources.com/_forum/sh ... p?t=140344

[TLRam1]

Hey Arne, love the new paint!!!

[Arne]

Just an update, its been a busy few years - not so much with the bikes though. I did get the CB750 / 1000 project on the road last year, and have about 4,000 miles on the build. Turned out pretty nice:



I should probably take a picture of it with the rear wheel on though : )

I got the GT750 project out of the mothballs tonight. Hoping to ride it this fall yet... I guess we'll see.

[Arne]

Here's it with the tire on. Good bike.

1000cc's, 36mm carbs, and a 4 into one do make the old CB750 go right down the road.

[tz375]

Looks good Arne. What were the carbs off? They look almost like a set of Europe only GSXR Slabbie flatslides.

[Arne]

Looks good Arne. What were the carbs off? They look almost like a set of Europe only GSXR Slabbie flatslides.

Thanks, they are 36mm Mikuni aftermarket RS series flatslides.

They are a bit big for the 1000cc with stock valves, but after 5000 rpm they start breathing right.