Topic: 1972 CB750 Fixer Upper

[mrkaylor]

I got a new toy a few weeks ago.  The title said it was a 1973, but when I checked the engine and frame numbers it turned out to actually be a 1972 CB750K2.  She ran when I got her, I was even able to ride her home in the dark.  She didn't run very well however.  The last registration sticker was from 2000 so I'm assuming it had been sitting until the PO decided it was time to get rid of her.  My plan is turn her into a decent running cafe racer.  My immediate goal however is just to get everything running properly.  After she sat in my garage a few days I figured out that the carbs were overflowing and draining on the engine case and if I shut the petcock off then it leaked also, so she got to sit outside for a few more days while I decided were to start.  I figured either the floats were set wrong or something was stuck open with rust particles from the gas tank.  So, I decided to clean out the rust and fix the petcock first.  I removed the petcock and disassembled it and thoroughly cleaned it.  By oiling the rubber gasket under the petcock I was able to stop it from leaking.  Next I had to clean the tank.  I decided to use electrolysis to remove the rust since it's free to try (other then the electricity to run the battery charger.)  It took a couple days to fully complete this but it turned out very well, no more rust.

Here is what she looked like the day I got her:


And Here:

[mrkaylor]

I ran a compression test today and I got 150lbs in all four cylinders, so I don't think I'll have to tear the top end apart just yet.  Carbs are next.

[mrkaylor]

I took the bowls off and found this:


Since this is a problem I bought this set off fleeBay:


I didn't think to ask the seller what casting number they were so they may not be right for me.  I'm not sure if I can mix casting numbers on carbs.  My casting is 657A. 
I guess I'll just have to wait and see.

[KB02]

What the heck was in the carb? I think you made a good choice with getting a new set.

[HavocTurbo]

I have seen race gas - if left for a very long time - eat away things in the carbs....

But that???

Some sort of aluminum eating bacteria? Or gremlins?

[Artfrombama]

Nice looking bike, congratulations and welcome to the best source of Honda SOHC information in the world!
I too am restoring a 1972 K2 750.
Check out Hondaman's replies to my carburetor problems. With his advice and others I've finally got a reliable, good running old bike. Loving it!
http://forums.sohc4.net/index.php?topic=38712.45

EDIT=I've used a 50% vinegar/water solution to soak the carburetors in to dissolve the oxide deposits. Don't leave the carbs in there too long!

EDIT II= 1972 was a changeover year. be careful ordering parts. Especially front forks/brakes.

[kghost]

Welcome.

I love the 72's....Have two myself.

That carb is a victim of corrosion...plain and simple...someone just cleaned it at some point before you got it.

Nice complete bike there minus the exhaust.

[smpls70]

i also have a 72,title says 73

[mrkaylor]

I got my carbs from ebay yesterday.  Although the picture from the auction doesn't look all that well the carbs are in pretty good condition.  The casting #'s are 657b.  My old ones were 657a's.  Does anyone know the differences?(other then the smaller main jet).   Also can they be mixed, in other words what happens If I use two 657a's and two 657b's?  And if that's possible do I make all the jets the same or do I leave the jets different?

anyway,
I went to Walmart thinking I would be able to find some Berryman's chem-dip since Berryman's website says walmart is a dealer, but like 90% of the time I go there I was disappointed.  I bought a jug of Super Clean instead, although it's not even close to real carb cleaner I thought it couldn't hurt to give them a bath in it anyway.  Well the stuff doesn't like aluminum at all, it started to corrode my float bowls and now they are much darker than before.  Super Clean did work on the rest of the carb and carb parts very well however.  Since I've only cleaned two of the carbs so far I'll either have to corrode the second set also or polish the other ones to match.   Or paint them...glossy black carbs would look sweet. 

I also ordered a few parts from z1 enterprises...and $220 later I should be good for a while now.

[mrbreeze]

You should be able to buy Berrymans at NAPA.......its the way to go unless you can find the elusive Yamaha carb clean.Man.....those carbs you got are looking pretty sad!!!!!!!!(boat anchors fer sure!!).I don't know the difference between the #'s you mentioned but you could probably just visually compare them.If they look different...I wouldn't mix parts.Don't get Keyster rebuild kits!!! Good luck with it and post away if you get stuck.

[Artfrombama]

The Castrol Super clean is loaded with caustic soda. Will corrode aluminum and other soft metals quickly.
I buy the carb dip at Auto Zone.

[mrkaylor]

I found some Berrymans at O'Reily's.  Napa only had napa brand carb dip and they wanted like $49 for a gallon, and it was sold out (stupid people).  I got the berrymans for $20.

Anyway I got my new carbs cleaned up and of course I found a hairline crack right next to the main jet on the left carb.  I sealed it with some JB Weld for now while I find another one.  I'll take a pic of the JB'd carb when I get a chance.  I found this stuff for pot metal also, I wonder if it will work on a carburetor: http://www.caswellplating.com/aids/solderit.htm   When I order their aluminum polishing kit I'll probably try it on the old carb that's really screwed up.

[HondaMan]

I got my carbs from ebay yesterday.  Although the picture from the auction doesn't look all that well the carbs are in pretty good condition.  The casting #'s are 657b.  My old ones were 657a's.  Does anyone know the differences?(other then the smaller main jet).   Also can they be mixed, in other words what happens If I use two 657a's and two 657b's?  And if that's possible do I make all the jets the same or do I leave the jets different?

anyway,
I went to Walmart thinking I would be able to find some Berryman's chem-dip since Berryman's website says walmart is a dealer, but like 90% of the time I go there I was disappointed.  I bought a jug of Super Clean instead, although it's not even close to real carb cleaner I thought it couldn't hurt to give them a bath in it anyway.  Well the stuff doesn't like aluminum at all, it started to corrode my float bowls and now they are much darker than before.  Super Clean did work on the rest of the carb and carb parts very well however.  Since I've only cleaned two of the carbs so far I'll either have to corrode the second set also or polish the other ones to match.   Or paint them...glossy black carbs would look sweet. 

I also ordered a few parts from z1 enterprises...and $220 later I should be good for a while now.

Mixing 657A and 657B is OK, since the 'A' and 'B' designated different mainjets in production, nothing more: the 'B' came with 110 mains until early 1973, then they were 105. The 'A', if on a K1, had 115 mains, and on a later (6/72 onward or so) K2 had 105. The needles are the same, the bodies the same, and the idle jetting-mixing is the same, between the models.

Be sure to check Art's post about his carbs, as yours are almost sure to have the air emulsifier passage blocked, or partially blocked. Those corroded carbs could also have suffered from the MTBE fuels we had in the 1980s, if the PO also added the "104 Octane Boost" stuff you can buy at Checker Auto, then left it sitting. I've seen that before. Usually, if you replace the brass parts, they work fine, just look ugly, so long as the float bowls still seal up.

[mrkaylor]

OK,  I finished cleaning out the carbs installed them and fired the bike up.. and it runs.  It runs much better than it did before.  It sounded like it was missed a little on the 3rd cylinder, but considering I hadn't adjusted the valves or synced the carbs or checked the timing it was running pretty well.

Next I thought I would adjust the valve clearances.  After reading about it on HondaHobby, the Haynes manual, the Honda cb750 manual, the Owners Manual,  salocal and various forum posts I adjusted the valves.  Most of the valves were already very close.

I Then adjusted the cam according to this http://forums.sohc4.net/index.php?topic=38561.msg437589 but I didn't feel the tensioner release at all.  Sure enough when I started the bike the engine is making way more noise then before.  I'm assuming it's the cam chain so either I did it wrong or the tensioner is stuck.  I'll pull it off tomorrow and either get it working or order a new one.

I also got all the stuff I ordered from z1 and one of the items was their allen head bolt set so I replaced all the bolts on the alternator cover and the gear shift cover.  I had to use my hand impactor for most of them and I stripped on so I had to drill the head off and remove the alternator cover to remove the bolt, but that all worked out.

Well that's all for now, tomorrow I'll tackle the cam chain tensioner.

[mrkaylor]

Sure enough the chain tensioner was stuck.  I took some engine degreaser and a wire brush to it and it's working well now and the bike is quite again, other then the exhaust I only hear a little clicking from the valves.

My right carb started leaking again, a piece of something was stuck in the float valve.

DO NOT BUY NAPCO CARB KITS.  The float gasket swelled up terribly.  I let it dry out and I was able to get it back in but I noticed tonight the carb is leaking again, so I'll have to figure out if it's the valve again or what.  I had already bought the kit before mrbreeze had warned me about the keyster kits which are nearly identical to the napco ones, but i thought if I just used the gaskets everything would be hunky dory, but I was wrong again.

It also snowed about a foot here in ND today so I won't be making any test runs anytime soon.

Deer hunting this weekend so I won't have time to do much else on the bike.

[Hush]

Good start, shame about the carbs which seem to be the Achillies heal of these old bikes.

[HondaMan]

Bigger shame about the hunting season taking over the bike's time! 
But then again, fishing time fights against my K2, too.