Project: 1983 CB750SC

[bkh2]

I picked up a 1983 Honda CB750SC project bike a couple weeks ago; thought I'd share the progress. I've never dismantled a bike before and simple carb extraction from the Ninja 250 was the most experience I've had. Hopefully, you guys can offer some guidance when I inevitably get stuck.

The bike was not running when I purchased it. The carbs were already extracted and dismantled, so I had to install it to make sure everything was fine before going further.

The Overview:


The Engine:




Front:


Jumping the bike to make sure it works (after reinstalling the carbs):


The Dash (33k km's on a 31 year old bike?):


The Back:


The Headers:


Foreshadowing the inside of the engine:



Front Sprocket:


After making sure it works, the tear-down begins:

Carbs and airbox out:

[bkh2]

First bolt out:


Removing the rear shocks:




Removing the rear passenger footpeg bracket:


Removing one side of the frame for easier engine extraction:



General aftermath:





Ultrasonic cleaner!



Massive reorganization needs to be done so I don't lose any parts! Engine extraction to follow.

[250rr]

Way to take the plunge! You can do it! I'm in the middle of adjusting my valves, removing and replacing the timing belts and replacing a broken oil cooler. Also, removing rust out of my tank. And I had less experience spannering on motorcycles than you.

Then again, I haven't tried starting it yet. If it doesn't blow up I'll let you know.

[Klondike1020]

Looks good ! the 750s are a blast . Heavy but fun.

Whatever you lack in Experience you should make up for in Research!

looks like you have it under control! good luck

[bkh2]

Got home from work and reorganized:



Engine out and cylinder head cover removed:




So before I begin digging into this, I am supposed to find Top Dead Center, which I believe that I have done (not pictured). However, my question is what the purpose of this is. If anyone could let me know, that would be great. Thanks.

[alex.s]

Originally Posted by bkh2

So before I begin digging into this, I am supposed to find Top Dead Center, which I believe that I have done (not pictured). However, my question is what the purpose of this is. If anyone could let me know, that would be great. Thanks.

at TDC the valves are closed which means none of the cams are pushing down.

on a standard crank, at TDC1, piston 1 is up, valves closed, piston 2 is down, valves closed, piston 3 is down, valves closed, piston 4 is up, valves closed.

after that it's suck, squeeze, bang, blow, right? so just after tdc1, depending on which side stroke is next, the intake opens up on 1 as it goes down, cylinder 4 fires as it goes down, cylinder 2 is closed and coming up and starts compressing, and cylinder 3's exhaust opens and it exhausts as it comes up.


edit: thinking about it, i might be a bit wrong on the order

[bkh2]

Oil pan removed, and the gasket broke, so I ordered a new set.


New gaskets!


TDC obtained.


Removing the cams and stuff:




Gross...






Thanks for the TDC explanation, Alex. I've been thinking about getting pods to replace the stock air box, but I've heard that cross winds can really mess with it. What are your guys' opinions on this? Also, I ordered shorty shocks to drop the rear. The stock shocks are 13" and the new ones are 11-3/8"; this should drop the rear by about 4 inches. I think I'm going to bob it because I dislike the OEM look.

[fast1075]

Please put the cam caps back on the head in the exact order and orientation they were in. They are align bored at the factory and if reassembled in the wrong place/wrong orientation you are in a world of trouble.

As a side note, that is the very reason to never buy an E-bay head with the caps missing. I guess some morons think they have to sell ever part separately, when all they are doing when they sell a head without caps is sell someone a 10 pound piece of scrap metal.

Also, the piston tops indicate some oil burning. make sure the piston clearance and cylinder finish are right when you put it back together, and always check for valve guide wear and install new valve guide seals.

[bkh2]

Originally Posted by fast1075

Please put the cam caps back on the head in the exact order and orientation they were in. They are align bored at the factory and if reassembled in the wrong place/wrong orientation you are in a world of trouble.

As a side note, that is the very reason to never buy an E-bay head with the caps missing. I guess some morons think they have to sell ever part separately, when all they are doing when they sell a head without caps is sell someone a 10 pound piece of scrap metal.

Also, the piston tops indicate some oil burning. make sure the piston clearance and cylinder finish are right when you put it back together, and always check for valve guide wear and install new valve guide seals.

Hey Harry, thanks for the heads up regarding the cam caps. I will definitely be making sure that they go back exactly how I took them off. Also, thanks for the diagnosis of the piston tops. I'm working through step by step right now, but if I find that the clearances are good, can those symptoms (burned oil) be indicative of another problem?

[fast1075]

Unless it is added to the fuel, there are only two ways for oil to make it into the combustion chamber. It either gets past the rings, or it comes down the intake valve guides.

The deposits on the pistons indicate oil is being burned, causing the heavy, hard carbon deposits. In severe cases, in addition to the carbon deposits, there will be perfectly clean and shiny area on the piston. Dry sooty fluffy deposits are from excessively rich fuel metering.

Reasons for oil getting past the rings: (a) Excessive cylinder to piston clearance either from cylinder or piston wear. The clearance can be so large or the surfaces uneven enough to allow the pistons to rock in the bore. Close visual examination of the pistons skirts usually tells the tale in conjunction with evidence the upper piston land is running against the cylinder. (b) Worn rings due to mileage or wear. Rings seal by their radial tension and surface profile. This is often associated with poor maintenance or poor/absent air filtration. (c) Worn piston lands allowing the rings to flutter. This actually pumps oil up past the rings.

The valve guides, especially the intake can wear. The excessive clearance makes it difficult to seal. The intake guides as part of the intake tract are under vacuum any time the engine is running, with the highest vacuum occurring with closed throttle deceleration. This can suck a lot of oil into the chamber. Worn exhaust guides do not contaminate the piston domes or combustion chamber by and large. Loose exhaust guides cause buildup on the back sides of the exhaust valves and down the exhaust tract.

Loose valve guides cause excessively worn (wide) seat area an are a major cause of leaking exhaust valves which are manifest by burned seats and eroded valve faces. On another thread there is a picture of a 250 head with a damaged exhaust valve. Classic failure.

[bkh2]

Link to original page on YouTube.

Long time no update... I have (hopefully) put everything back together, and I have been messing with the carbs for the past few weeks. During cleaning, I didn't polish the needle seats good enough and the carbs kept overflowing, spewing gas all over the place. This was due to my incompetence so it was a big learning process for me (hence the month of no updates). After a few iterations of this, I finally stopped the leaks and got it all back together.

The good part is that I have gone from a two-wheeled paper weight to a functioning machine. The bad part is, from the video above, that it is not running well. Through some research, a hanging idle and exhaust popping (both apparent from the video) mean that the mixture is lean, right? I pulled the plugs and they seem pretty black and a little wet. This means that the mixture is rich right? I don't really know how to proceed now, apart from iteratively Googling and trial and error-ing, so if anyone could help me diagnose this problem, that would be great.

[alex.s]

sounds rich

[alex.s]

i would check the idle air control flow

[jkv45]

That didn't sound that bad to me. Was it fully warm? How does it ride?

Are the needles adjustable? Have you adjusted the idle mixture screws and synced the carbs?

[bkh2]

Originally Posted by jkv45

That didn't sound that bad to me. Was it fully warm? How does it ride?

Are the needles adjustable? Have you adjusted the idle mixture screws and synced the carbs?

It rides fine, but the idle is inconsistent, which does not inspire confidence when coming to a stop; it either maintains abnormally high revs or threatens to die. The video was indeed taken after I took the bike for a ride, so it should have been sufficiently warmed up.

I am uncertain about whether or not the needles are adjustable (for clarity, of which needles are you speaking?). I have experimented with the idle mixture screws, which eliminated the majority of the backfiring, but some pops are still evident every so often.

As for synchronization, I have not done this. I was watching videos where the person taps the vacuum through ports on the engine side of the carbs. I quickly checked just now, but did not find any similar port. Perhaps I missed it, or perhaps it requires yet another removal of the entire assembly to synchronize visually? In any case, I will have to take a closer look in the morning.

Thanks for the advice thus far, guys. I know this isn't a Nighthawk forum but I reckon the symptoms and associated solutions should be similar; I appreciate the help. I'll make sure to branch out to more specific forums in the future...

[fast1075]

Sounds like the carbs still have problems. Do all four cylinders have fouled plugs? Remember, even though the carbs are connected, each one controls it's own cylinder.

If it runs ok at mid throttle, but has problems only at idle, the air bleeds and pilots or the passages are plugged still. By the 80's I had moved on to Kawasaki, so direct help for the carbs.

The sync ports most likely have screws in them. You remove the screw and install a special tap that connects to a vacuum gauge. My sync tool which I have had since 1971 uses 4 gauges to monitor vacuum on all cylinders at once. You adjust the carbs first to get them even on vacuum, then adjust the idle mixture, and readjust for vacuum if the mixture adjustment changes the idle vacuum.

If it was mine, I would pull the carbs, remove all the jets and give then a nice long bath in 50/50 pinesol/water. Three or four days at least. When you clean them out. Get several cans of brake clean and check to make sure it goes thru the transition ports (the tiny holes in the carb body near the throttle blade when the throttle is closed) if these tiny ports are plugged you have problems.

The idle circuit is the hardest to get in order because the orifice sizes are so tiny. The air bleeds which get the air from the carb inlet must be clear. The idle circuit needs the air emulsify the fuel at idle.

Way back when, a company sold a kit that used a log manifold to mount a single carb, and was popular with some that only toured, and didn't care about performance because of the hassle of keeping four carbs in sync.

I did the exact same thing on a KZ-440 that had set up and the carbs were ruined. Ran it with a 34mm Mikuni. I'm not suggesting this, just a view "outside of the box".

Get the cabs clean, and if it is not an oiling issue (ring seal, etc) you will be fine. Those old Hondas are tougher than a concrete block.

[CC Cowboy]

That bike is begging to be a cafe.

[jkv45]

Originally Posted by bkh2

It rides fine, but the idle is inconsistent, which does not inspire confidence when coming to a stop; it either maintains abnormally high revs or threatens to die. The video was indeed taken after I took the bike for a ride, so it should have been sufficiently warmed up.

I am uncertain about whether or not the needles are adjustable (for clarity, of which needles are you speaking?). I have experimented with the idle mixture screws, which eliminated the majority of the backfiring, but some pops are still evident every so often.

As for synchronization, I have not done this. I was watching videos where the person taps the vacuum through ports on the engine side of the carbs. I quickly checked just now, but did not find any similar port. Perhaps I missed it, or perhaps it requires yet another removal of the entire assembly to synchronize visually? In any case, I will have to take a closer look in the morning.

Thanks for the advice thus far, guys. I know this isn't a Nighthawk forum but I reckon the symptoms and associated solutions should be similar; I appreciate the help. I'll make sure to branch out to more specific forums in the future...

I would start by setting the idle mixture screws at 1 1/2 turns out. They may be an air bleed or they may be a fuel bleed. I would find out which. If they are an air bleed, turning them out would make it leaner - and the opposite if they are a fuel bleed. From there I would change them all in 1/2 turn increments until you have the highest idle speed - then go 1/4 turn richer (that's why you need to know which type they are). Then use the idle speed adjustment screw to bring it back down to the right RPM.

As far as needles go, they are inside the slides and stick down into the main jet. If you raise them (move the clip on the needle down) you are lifting the needle out of the jet and making it richer at the same slide opening. Usually you start with the clip in the middle position. Push the slides up with your finger (from the back of the carb) and check that they all drop down correctly. Torn or damaged slide diaphragms can also give you problems.

That's for a basic carb tune. To really get it tuned you first need them all in sync. If they are not in sync you may never get it to idle properly.

One other thing to check would be a vacuum leak. Spray carb cleaner around the boots from the carb to the engine and listen for a change in RPM. Older cycles frequently have cracks in the rubber boots or sealing problems where they bolt to the engine and will leak.

[Klondike1020]

And that is why dealers will not touch an old 4 carb w a ten foot pole

I tried to take my cb to the Honda dealer... They said no way

[Motofool]

Originally Posted by bkh2

...........During cleaning, I didn't polish the needle seats good enough and the carbs kept overflowing, spewing gas all over the place. This was due to my incompetence so it was a big learning process for me (hence the month of no updates). After a few iterations of this, I finally stopped the leaks and got it all back together.

The leaks may remain if the float valves are old or worn, only that they happen only when the carbs vibrate.

Please, read this:
http://www.dansmc.com/carbs.htm

The level of fuel inside the carbs may be reaching too high of a level, making the mix rich and inconsistent.

[bkh2]

After an extremely busy month of June at work, I've been able to get back to the bike. I went through another iteration of full carburetor disassembly and cleaning. After reassembly, I touched the headers and noticed that cylinder one was cold. I switched the plugs #1 and #4 which caused four to be cold and one to be hot; this confirmed that the spark plug was at fault, which is strange because it was a brand new plug. I put the old one back, synched the carbs, and this was the result:

Link to original page on YouTube.

There is still some tweaking to be done, but it is definitely leaps and bounds ahead of where I was a month ago. I took it for a ride around the block and it seems to ride quite well. It's definitely a different feeling than riding the GSXR, but I think it will just take some getting used to. Now I have to shift gears and think about the cosmetics; this is where the real struggle begins...

Thank you to everyone who contributed some knowledge. While not all of it was required in this project, substantial learnings have been gained that will hopefully translate to other projects in the future.