MY 1979 CB750 DOHC BUILD

[RodneyCB750K1979]

I'm starting this thread to track my progress rebuilding a 1979 Honda CB750 Limited Edition bike I purchased in Nov 2012 while visiting my son.  I grew up with Honda bikes and had a 1978 SuperSport.  We saw this bike on Craig's list at a price which sounded reasonable and snatched it up. Initial inspection revealed No. 1 cylinder wasn't firing and had no compression, No. 4 carburetor was dripping gas, and the ignition cables appeared spliced and taped.

[seestheday]

Sweet.  What are your general plans?

Mechanical overhaul only?  Cafe?  Chopper/Bobber?

I'm in the middle of a mechanical overhaul on an 81K myself, and I'm doing a few small cosmetic changes at the same time.

[TOOLS1]

I am so glad you did not say "I want to build a Cafe"
TOOLS

[750K8]

Progress?

[RodneyCB750K1979]

Mechanical rebuild and keep it stock.  Anyone know a good source for 4-into-4 exhaust systems?

[RodneyCB750K1979]

No plans for a cafe bike here. I'm a stocker, trying to preserve the past for future generations.

[seestheday]

I have never seen 4-4 exhausts for the DOHC's except on ebay for ridiculous prices, and these stainless ones for $1,200 http://www.benjiescaferacer.com/4-into-4-exhaust.php

It's an area where many opt for a much cheaper 4-1 or 4-2-1 option.  I guess it all depends how bad you want to remain truely stock.

One option might be to try to find a complete bike that still has an exhaust on it and buy that.  Then you get a parts bike plus an exhaust.

Right after I bought my bike I found another almost identical one with a 4-4 for $1K.  I'm still kicking myself for not buying it.

[RodneyCB750K1979]

Part one. I removed the carburetors and disassembled. I got a copy of "Carb Rebuilding Honda DOHC-4s" by MacGergor Carb Cleaning Services. Pretty much following it word by word. Cleaned all the parts in a PineSol solution and polished the tops and bowls.  Discovered a few small ports and jets were plugged. Here are some images.

[RodneyCB750K1979]

Pipes look sweet.  I want to maintain a stock OEM megaphone sound. That $1,200 spend has been wearing on my mind for several months. I may eventually give in and buy.  I once owned a '78 SuperSport with a 4-1 stock exhaust.  The sound was different, almost like a car.

[seestheday]

Carbs look good.

All new o-rings and replaced screws?

I recommend the hex head bolts, but if you are intent on keeping it stock, you can buy new Phillips head bolts.

[RodneyCB750K1979]

Part 2. After running a compression test and some other tests, I knew the engine had to come out.  I couldn't figure out how to remove the engine until I saw a posting to lay the bike on its side. That sounded rediculous at first. But in a last minute act of desperation, I did it.

[RodneyCB750K1979]

Part 3. Engine disassembly.  I was getting 120 to 145 psi compression on all cylinders except No 1 which read zero. Removed the cylinder head, jugs, and pistons. #1 piston was heavily pinged and the top ring groove was compressed. The #1 valve seats were also pinged.  Appears something got into the #1 cylinder.

[RodneyCB750K1979]

Part 4. I bought a used piston on Ebay and cleaned the others up. I took the cylinder head and jugs to my local Honda dealer and they replaced the valve seals, replaced both exhaust valves on No 1 cylinder, cleaned up the valve seats and lapped the valves, and did a light hone on the cylinders. I had a broken exhaust stud which I eventually took to a welding shop and they were able to extract it. I installed NPR piston rings. The end gaps didn't require adjustment. All were within the acceptable ranges. I didn't have instructions with the rings but after some research, I concluded the top compression ring has the shiny "chrome" lightly chamfered edges and the second compression ring was just steel with no chamfered edges.  Hope this is correct. The stamped letter on the two compression rings were installed facing up.

[seestheday]

Do you have the FSM (Factory Service Manual)?  For the DOHC bikes you can just unbolt one side of the frame and the engine slides out (you need a friend to help you).  There are instructions on how to remove the engine in the FSM (download link on this forum - check the stickies on the front page).

For the SOHC's it's much harder as the frame doesn't unbolt like ours does.

Also, I'd be remiss if I didn't let you know that this forum is super friendly, but is more focused on SOHC's than DOHC's.

There is another DOHC specific forum (http://www.cb750c.com) that has guys on it that have forgotten more about DOHC's than I'll likely ever know.  I'll won't say anything else about it other than recommend you do a lot of reading through threads there before you start asking questions to them.  I post on both forums.

[TOOLS1]

I like the way the engines come out out of the DOHC bikes. Just set a jack, or blocking under the engine. Then remove the lower frame section on the right, and slide the engine out, or slide the frame away. Either way I can do it by myself. No need to call a fried over to drink my beer, and watch me work.
TOOLS

[seestheday]

I needed my brother to drop by, but I don’t have as much experience as Tools.

Sent from my BlackBerry device

From: TOOLS1 [via Honda CB750'S]

Sent: Friday, January 25, 2013 5:06 PM

To: seestheday

Subject: Re: MY 1979 CB750 DOHC BUILD

I like the way the engines come out out of the DOHC bikes. Just set a jack, or blocking under the engine. Then remove the lower frame section on the right, and slide the engine out, or slide the frame away. Either way I can do it by myself. No need to call a fried over to drink my beer, and watch me work.
TOOLS

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NAML

[RodneyCB750K1979]

I need a way of putting the engine back in.  My son said there is a posting that describes a hoist method that uses the motorcycle stand like I have from harbor freight and a piece of 2 inch pipe.  I'll definately not install the exhaust studs until the motor is back in.

[RodneyCB750K1979]

Does anyone need a set of piston rings for the 1979 Honda 750 DOHC? I bought a set, thought I had tossed them, and bought a second set. I found the first set last night when I wasn't looking for them.

[seestheday]

Assuming they fit a 1981 dohc I might be un need of some. Haven't gotten to the cylinders yet.

Sent from my BlackBerry device

From: RodneyCB750K1979 [via Honda CB750'S]

Sent: Friday, January 25, 2013 6:36 PM

To: seestheday

Subject: Re: MY 1979 CB750 DOHC BUILD

Does anyone need a set of piston rings for the 1979 Honda 750 DOHC? I bought a set, thought I had tossed them, and bought a second set. I found the first set last night when I wasn't looking for them.

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NAML

[RodneyCB750K1979]

The following is the description of the standard piston rings I purchased:

THESE RINGS FIT THE 1979 TO 1983 CB750 L / K / KA / KB / C

                           AND THE 1983 CB750SC NIGHTHAWK

                   ALSO FITS THE 1978 CB250T- I / 1979 CB250 - II

                   AND THE 1983 CB250NDC SUPER DREAM