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Newbie here with just purchased 81' CB750

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Hello all, new member after just picking up my 81' CB750 Custom yesterday. Met a guy that was just tired and giving up riding so he sold her to me for 300 bucks... In pretty good shape, ride-able but rough from neglect, chrome is all perfect as is frame... Little to no rust, seat perfect... Frame is very nice... Tank has a couple of dents I will remove them with a new paint job... Nothing missing, everything works... Wanna restore original all chrome 4-4 exhaust or new 4-2. Look to make her look like she did at the showroom floor...

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Wow you got a steal. My 80 cost 100 more and is not near as nice. But I'm planning sort of a rat rod/Enduro version with mine. Nice pick though!
 
Wow you got a steal. My 80 cost 100 more and is not near as nice. But I'm planning sort of a rat rod/Enduro version with mine. Nice pick though!

Pic of yours? Get much from this storm system that passed?


The good thus far: Cams chains are both in excellent shape; both sprockets are good; drive chain looks great; great compression tests all across...

Confirmed needed: carb job; valve work; front fork seals; brake pads and shoes, will replace fluid...

Ordered Mac 4 to 2 all chrome pipes to do with carbs... Ordered new tires- Pirelli Route w/ white lettering...
 
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To save some very possible grief do the valve adjustment first, many perceived carb issues are actually the valves. And hold to a minimum of .004" and preferably .005" on valves, the book spec had lots of people burning valves or engines didn't run as well.

Compression? The spec is 170 psi.
 
all we got was a bunch of rain as you can see in the pic of my garage floor.

the PO of my bike had tinkered with the carbs himself. the bowl overflow tubes all had cracks in them and his answer was to put a piece of tygon fuel line over them to seal them up. that probably worked for a year or two but that tygon stuff sucks. i went to ace and got some 1/8" copper tubing and replaced all four of them. he also seated the pilot jets a little tighter than lightly seated and two of them broke off inside of the carbs, but i got those out. bought carb kits for all four, accelerator pump kit, going to do a air cut off valve bypass and then when i was trying to clean up some varnish off the floats i dunked them in hot ultrasonic water and found all four of them had hairline cracks and were leaking. next is valves and coil with plug wires. odd thing was with even with two pilot jets broke off in the carbs the bike ran so i know the brakes, clutch and trans all work. plus two bran new tires and a new (cheap, going to be replaced) battery.
 

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PO of my bike had tinkered with the carbs himself. the bowl overflow tubes all had cracks in them and his answer was to put a piece of tygon fuel line over them to seal them up. that probably worked for a year or two but that tygon stuff sucks.

Got your work cut out for you, I am paying a pro to do my carb work...

To save some very possible grief do the valve adjustment first...

Thanks for all the info and advice...
 
Tank and panels off to be painted, pipes are in as well as carb stuff, brake items, etc... work begins...

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Just got the tank and side panels back from the paint shop... The bike is at another garage getting the carb work and valve work done plus putting on new 4 to 2 all chrome pipes. Can't wait to get these put on! Older paint is in previous post pic... They even repainted the cursive word "Custom" back on top like Honda had it!!! Computers are great!!!

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Got her all back together and the carbs were leaking like crazy... Had them professionally cleaned before when I had the valves done so ordering a rebuild kit...
 
Saw a pic of a conversion making or changing the carbs to two with each serving two cylinders... Any benefits or drawbacks? Would this work with the stock airbox to keep the original look?
 
The stock carbs where specifically designed by engineers to work with that engine in that bike with that airbox.

Drawback: any other carbs were not.

Benefits: Regardless of the configuration, you can increase total horsepower by increasing the total CFM of the carbs and installing an exhaust pipe to match but then your getting into customization, not restoration.
 
What do you do to keep them in good working order?

Me? The biggest thing I do is drain the carb bowls when I know the bike will sit for more then a month. I also have tiny inline fuel filters in the middle of each of the 2 fuel lines.

When I got this bike in the pic, I rebuilt the carbs myself replacing all the jets with new ones and using brass floats then let the shop sync them for me.
 
Picked her up again today. Had the valve work and carb work done (cleaned and rebuilt)... New pipes, new tires, new brakes and shoes and pads and fluid... Replaced all cables inc choke... Fork seals redone... New levers (both), new handles for both sides...Drivable again, 85% finished... painting finished... mostly detailing remaining to clean her up... had her out for about 50 miles today riding, testing, and playing..., my god, what fun, what power... She's fast...
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One thing for the experts. When I first got her (before any work was started), after she warmed up, the rpm would remain 3-4,000 when the clutch was pulled in...Throttle cables replaced, carbs rebuilt and cleaned... sync'd... valves done... NOW ON PICKING HER UP TO RIDE HOME, after she warmed up, the rpms dropped to about 3,000 and not to idle rpm... It was like this when I bought her before any work was done...

WHAT DID I/WE MISS? How do I get her to drop to idle rpm when clutch is engaged\pulled in and gas is let off\not engaged?
 
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Some finished pix showing the paint detail, etc... AMC, can you tell me what you think of my question in the previous post? Thx...

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Idle hanging high if the idle has been reset to correct when engine HOT is almost always an air (vacuum) leak of some sort. You MUST have too much air getting into motor above the idle requirement, then pulling engine up higher is normal use and letting go of throttle makes the idle hang. Can be one carb off in sync, very common to leak at the head to carb manifolds even if you think they are in great shape. More rare but throttle cable with not enough slack in it, you must have a small amount of slack left with throttle 100% off before rolling it open pulls the butterflies open.

Very common on lesser quality older engines to have the idle set high enough to hang due to old engine needing more air just to keep turning over but not likely maybe in your case.

To tell if carb bank is the problem, with it off and in front of you at the same settings that hang idle high, look at the 4 butterflies from front of carbs and ALL must be virtually 100% closed and at the dead same spot. There is a small hole in bottom center way forward that feeds idle fuel (curb idle port) then a few holes further back ( off-idle transfer holes)that act like accelerator pumps as the throttle just begins to crack open, proper idle position will have the very front hole of those in the group to be either barely covered or EVER SO SLIGHTLY exposed, over half of hole showing is too much air crack there. All 4 need to be the same too, one off can hang the others and if vacuum synced to end up like that it's evidence of one cylinder not pulling to match the others. There is one other small hole forward of the butterfly and up on the right side a bit, the aircut vacuum port. The #2 carb will have another hole all to itself for supplying vacuum to the fuel demand valve on later models.

I personally bench sync to make all 4 butterflies barely cover the front hole there and all dead even. Often if rest of motor in good shape you are synced well enough to drive the bike pretty good then. If you have the first hole showing solid that is probably enough air to hang the idle.

In post #5 of this thread higher up 91octane in his third pic clearly shows a butterfly open way too far for idle, it shows the entire first hole and part of the second open there and would hang like no tomorrow. Would also be running rich as the hole amount exposed is feeding fuel once the butterfly opens past it. They are only to feed fuel instantly as soon as the butterfly cracks open, they keep mixture going to engine while the transition from pure idle to primary system is taking place, the accelerator pump hasn't begun to work yet due to shaft not moving far enough to activate its' check valve.

Aircuts leaking at diaphragm are a vacuum leak source too.

Most of the time it's the carb manifolds to head leaking.

Hope I haven't choked you there......................
 
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