Introduction and first questions - family relic, first start in 30+ years

[efair]

Greetings,
I've just become the caretaker of a (mostly complete)  71 CB750 which has sat unused for 30+ years.  I'm going to drain the oil, replace the oil filter, drop the pan and clear the screen, squirt some oil through the plug holes and tappet covers today.

Do you think it wise/necessary to add a flushing agent to the first oil change, run for a few minutes, and change the oil again?

Any other suggestions to avoid damage on the first start?

The engine turns freely but no spark, after the oil change I will investigate that issue.

Also the throttle mechanism is seized, I can't tell if its the grip, the cable, the shaft, or the carb pistons that are seized.  Is there a common troublemaker here?

Thanks for having me, and links/rtfms/ramblings are always welcome!

Ed, looking for his old tools in GA

[efair]

Oil change complete, the old oil was still amazingly viscous, so I assume the passages are not clogged.  

A little carborundum on the points had positive effect on the spark.

The throttle linkage shaft wouldn't cooperate so the carbs came out for further examination.

 A little ether and BusterTF woke the Neighborhood.   I think he's gonna need new pipes.

Where do y'all buy parts?

[TOOLS1]

Hey Ed. Sounds like you have found a pretty good bike. These bikes are bulletproof and easy to work on. For parts, my first stop in my local Honda dealer. You would be surprised how many parts are still available. I just picked up 4 new carb packing kits the other day for these old roundtop carbs. You rarely need the brass parts and should never replace them with aftermarket parts. My next stop for parts is vintagecb750.com followed by cycleX, and then eBay.
Is there anything in particular you need? I have a big stash of parts.
TOOLS

[efair]

Thanks Tools,
The pistons in the carbs are seized, and the previous owner said he thinks one of the floats was sticking the last time he tried to start it many years ago. These are the keikhin (keihin?) carbs... so I'll start with a basic cleaning and replace the rubber parts, plus the floats.  Any parts you want to sell would be welcomed, as would advice on the carb cleaning.  

This list will probably grow as I progress:

Pan-head screw for carb shaft
Oil Drain seals (pan, oil tank)
Tappet cover seals
Points and condensers (working but replacing anyway)
Air filter
Rear tank rubber mount (existing one is split)
Rubber boots for top of carbs
Battery hold-down strap
Tool bag
Front fork seals and wipers
Front brake lever

I'm glad I found this community and appreciate the support!

[TOOLS1]

For cleaning the carbs, I like to use the chem dip. Ultrasonic cleaning will remove the anodizing from the steel parts and they will rust quickly.
The screws on these bikes are NOT Phillips, they are JIS (Japanese industrial standard). Phillips screwdrivers will destroy the screws and make you hate life. I recommended that you get some JIS screwdrivers. I like the Vessel Impacta screwdrivers. You will need a #2 and a #3 to work on your bike. Amazon has them for around $20.00. You should also get an impact screwdriver. The kind you hit with a hammer, Harbor Freight has them for $10.00. I get new longer bits for mine at vintagecb750.com.
For your parts, your local Honda dealer should be able to get almost everything on your list. vintagecb750 will have anything that your dealer cannot get.
On your ignition parts. I really doubt that they need to be replaced, and the factory original parts are way better than any aftermarket parts. The points in my daily rider have over 100K miles and are still going strong.
TOOLS

[efair]

Does this little brass bit in the venturi come out?

[efair]

Also - here are the photos of the brass components I could remove, along with my attempt to name them:

Idle air screw and fuel rail tee:


test port screw, high speed jet and mixture tube, low speed jet,  float valve seat, float valve and float (barely visible):


There are some drain plugs in the float bowl I haven't removed yet thus not shown in the picture.

I'm wondering if I've found them all - the rebuild kits on vintagecb750.com are confusing.

These are the "7A" carbs, with two "7A IH" and two "7A II" stampings.  The only difference I can tell is the IH and II are symmetrical to each other.

[TOOLS1]

Yes, it can come out but does not need to. There are not any passages in it other than the big one in the middle.
TOOLS

[TOOLS1]

You have found everything. Do not overthink this project. The carbs all use the same parts. Do NOT replace any of the brass parts unless yours are damaged beyond use.
TOOLS

[efair]

Thanks, again, for your spot-on advice.  A 1/4" punch to start, followed by a 3/16" punch, got that last bit out without damage.  chem-dipping now, more as it develops.

[efair]

Anybody know a source, or the specs, for this tiny screw that fixes the shaft to the plate?


I will try the hardware store later today, but I'm not optimistic.  It's ISO threads, but the head size and the length are critical.

[efair]

Amazingly, I found this screw at the hardware store - M3 x 8MM  is just a little too long but can work with some grinding/filing.

[TOOLS1]

I have never had a need to remove that shaft. Was there some damage to it?
TOOLS

[efair]

It was pretty corroded, and I thought seized.  Had I known it was actually the pistons seized I probably wouldn't have disassembled it.  The good news is it's nearly impossible to reassemble it wrong.  That little screw is really the key to correct reassembly -  it registers the shaft position relative to the plate, which should be the first step of reassembly.  It's hidden under a Welch plug though, and if you try to disassemble without finding and removing it first, it will snap (as I have proven).

[TOOLS1]

I have vapor blasted several of these racks while doing rebuilds, it makes them look like new.
TOOLS

[efair]

OK - new bowl gaskets, o-rings for the fuel rail, and float gauge arrived today.  All is back together.  A couple of questions:

1) Is exact float height critical, or is it more important that they all be "close" and consistent?  I found my existing float heights fairly consistent but with a slight gap shown when using the vintagecb750 gauge.  The float/pin assembly isn't exactly precise, I'm wondering if "consistently within 2-3 mm is OK.

2) The o-ring kit included four small o-rings as well - I can't find where these might be used.  The air screws don't have grooves for these rings.  None of the other fittings seem to make use of o-rings besides the fuel-rail tees.

3) what exactly is/how exactly does the "overcross" work?  I.E. the slight amount of "negative throttle" that can be applied when the throttle is at rest?

[TOOLS1]

On the float height. As long as the jets are submerged and the fuel is not overflowing you are good. With the ethanol gas, we have today you can throw the float height specs out. The ethanol does not float objects as high as non-ethanol gas. What I do is set the floats close to spec and then hook up my IV tank and put gas to the carbs. If any of the carbs leak gas out the overflows I will readjust the float levels until they stop. I will also take the carbs and shake them and turn them side to side as if the bike is leaning or doing a wheelie. I even like to let them sit for several hours just to make sure that they do not decide to start leaking later.
I do not know how small the O-rings are so I cannot tell if they go on your carbs. They might be for the float bowl drain screws.
"negative throttle"? Why would you be twisting the throttle past idle? You should be able to adjust this out with the throttle cabels. The return spring should bring the rail back to rest on the stop screw. The rail will twist with force. It is only supposed to be strong enough to open the slides on the carbs. The adjusting nuts on the tops of the carbs are there so you can sync/fine adjust the carbs.
TOOLS

[efair]

I too am puzzled about the so-called overcross.  The shop manual addendum mentions this on page 15.  There are two springs on my setup: 1) the extension spring serves to return the pistons and throttle to the idle position when you release the throttle, and 2) a torsion spring that is part of the rack that allows "overcross".  This second spring is quite heavy, but it does allow the suggested motion.

[efair]

Here's a picture of my setup - you can see the extension spring at the top, and the torsion spring at the bottom.

[TOOLS1]

I have never taken a rack apart to have to deal with that spring. The "overcross" they are referring to is throttle free play. You can adjust that with the throttle cables.
TOOLS