Author Topic: 77 CB750 - resurrection (Read 2324 times)

xhevi · « **on:** October 20, 2021, 07:28:34 PM »

hi all.

Its a 77 CB750 that was left to rot and now is in my garage, I am scratching my head trying to figure out what to do with it. It has 29K miles and had been put away in 1990. It was ridden about 1000 miles between 1990 and 2017, last known to run was in 2017. Its going to need of lot of work, patience and money to bring it back to life, a frame off restoration.

I intend to get it running first and then decide how far I wanna take this project. I would like to bring it back close to stock but that might be cost prohibitive so I might turn it in some form of a bare bones cafe racer.

Have not spent a hole lot of time with it but below is a list of what i have noted so far:

1) missing air box - I bought a used air box on eBay and will get rid of the pods.

2) carbs to engine rubber insulators are cracked and need replacement - the ones I find on line are crazy expensive for what they are IMHO. Any aftermarket cheaper replacement to these?

3) Missing exhaust, it has some short straight pipes but I rather have proper exhaust on it. I have seen on eBay 4into1 and 4into2 headers but don't know what size exhaust I need. Also reproduction ones can be had for around 400$ - any suggestion/recommendation on these?

(this is point where money spent on parts will equal the bike purchase)

4) plenty oil leak stains on the engine cases and some around the head gasket - it needs a top end rebuilt and replacement of all rubber seals

5) plenty surface rust on the frame and oxidation on the engine cases. - considering striping and painting or powder coating the frame and maybe bead blast the entire engine if scrubbing does not work (I know many in here will be against blasting the engine and I am aware what the risks are.)

6) master cylinder and caliper rebuilt, new brake lines.

7) all wiring needs a thorough check wire by wire and all electrical connections gone through.

#1 cylinder spark plug is stuck and is currently soaking in BP blaster. #4 spark plug looked oily. the #2 and #3 looked ok.

9) some corrosion made it past the points cover and they need a thorough cleaning and replacing points and condensers. also considering the Dyna ignition. minor rust on the advance unit.

10) will also need new coils as i cant get any reading through the plug wires, plug caps are also to be replaced.

11) cabs are in on piece and all the inner bits are OEM. previous owner said he did clean up the carbs and they do indeed look relatively clean and rubber bits look new. they look clean enough to get the bike running first and afterwards will rebuilt.

(this is point where money spent on parts will be double the bike purchase)

12) tank paint look horrible have not had a chance to look inside for rust yet.

13) the black painted rims, spokes and hubs are flaking and also need work.

14) steering stem bearings need to be replaced.

15) fork need to be rebuilt.

16) Missing seat.

here are some photos

I do not mind the work and not shy with wrenches, this will be my third project, so far I have restored a 76 CB550 and a 73 BMW r60/5.

.... I am afraid this 750 might be a goner indeed, unless I find a donor or a sugar mama.

Alan F. · « **Reply #1 on:** October 20, 2021, 08:10:12 PM »

That'll clean up, no need to go too far overboard on a 10 point restoration.

beemerbum · « **Reply #2 on:** October 21, 2021, 08:51:38 AM »

You are proving once again a fundamental principle, as if more proof was needed. There is nothing more expensive than a cheap or even free old Honda CB 750.

xhevi · « **Reply #3 on:** October 21, 2021, 09:34:30 AM »

An old airhead would be more expensive.

Could pick up a new bike, but that's no fun without hurting knuckles, swearing and kicking it.

BenelliSEI · « **Reply #4 on:** October 21, 2021, 09:46:44 AM »

The head gasket and base gasket look OK in the photos you posted. Get it fired up before you spend any money and then decide. I’ve walked away from a few recently. The economics made no sense….

Don R · « **Reply #5 on:** October 21, 2021, 09:49:37 AM »

I'm also a big fan of getting them running first, do a couple safe test rides.

xhevi · « **Reply #6 on:** October 21, 2021, 10:07:50 AM »

Quote from: BenelliSEI on October 21, 2021, 09:46:44 AM

The head gasket and base gasket look OK in the photos you posted. Get it fired up before you spend any money and then decide. I’ve walked away from a few recently. The economics made no sense….

the attached picture is of the cylinder bore as it meets the head. at the bottom right corner you can see a line at the head gasket. (bad quality photo with an endoscope). also on the outside i can see the head gasket disintegrating (did not take a photo).

On the photo you can see some rust pitting dots right below the ridge on the cylinder bore, this might affect compression. This was the worst that i have found so far on cylinders 2,3 and 4.

i still have to do the leak down test after i pull out #1 spark plug that will not budge.

i will get it ruing before i tear it apart, but i doubt it safe to ride.

xhevi · « **Reply #7 on:** October 21, 2021, 10:14:01 AM »

regarding exhaust.

i am looking at Used 4into2 Mac headers with the intent to use aftermarket mufflers. does anyone know that the OD of the collector is and what mufflers will slip on?

at this stage i am looking up parts just to get an idea what it will all add up at the end $ wise.

thank you.

desertrefugee · « **Reply #8 on:** October 21, 2021, 11:13:08 AM »

I have seen way worse made very nice with commensurate application of both elbow grease and money. Good bones there.

I will add my voice to the chorus on seeing how it runs before going too far - but confess that I've not always followed that mantra myself. I've been pretty lucky and can usually "smell" a runner. Besides, it's hard NOT to start tending cosmetic issues while waiting for parts to arrive. (Having a spare motor or two in the wings doesn't hurt).

xhevi · « **Reply #9 on:** October 23, 2021, 08:19:04 AM »

Today I started to check the ignition bits.

The advance unit is frozen solid and there is rust pitting on the cam. It's a 300 advance unit.

Has anyone has used the 4into1 replica? I don't mind trying to save mine but a new one might be in order.

Thanks

Alan F. · « **Reply #10 on:** October 23, 2021, 10:25:25 AM »

That advancer is a good candidate to soak in 50/50 acetone & ATF, even if only as an experiment.

flybackwards · « **Reply #11 on:** October 23, 2021, 07:25:56 PM »

Ahh, the stories I could tell - and yes it became ridiculously expensive, but mine was running (sort of) when I got it and there were no missing parts.

xhevi · « **Reply #12 on:** October 23, 2021, 07:38:43 PM »

Ridiculously addictive I would add

It's my third in 3 years.

Given it's condition when I got it, this one will have to be a modest restoration and in a tight budget.

After spraying with pb blaster and letting it do it's magic for a few hours, I was able to take apart the advance unit. I think it's reusable.

Having a hard time removing cylinder #1 spark plug. I hope I don't strip it the head.

Alan F. · « **Reply #13 on:** October 23, 2021, 07:44:34 PM »

Spray some of that PB Blaster down on those threads too, it's good stuff. You can also heat the relevant area of the head with a heat gun before your next attempt.

xhevi · « **Reply #14 on:** October 23, 2021, 07:59:57 PM »

Been spraying pb blaster few times now. It worked on the other stuck spark plug. Also used the heat gun. Will have to try the torch.

It's stuck so tight that I did strip the the spark plug tool. Had to get a 17mm deep socket hoping it would help.

Its not unlikely that those threads are already busted by previous owners.

Alan F. · « **Reply #15 on:** October 23, 2021, 08:13:09 PM »

That Honda spark plug tool is a little weaker than I'd have hoped, I've been through that part myself.

Good luck, perhaps a little: tighter- looser - tighter - looser like tapping threads?

xhevi · « **Reply #16 on:** October 28, 2021, 11:51:58 AM »

Update.

I managed to get the #1 plug out, took some effort so I hope I did not damage the threads.

Also managed to cleanup the advance unit (need to get new springs).

My plan to get it running first did not really work out.

I did a leak down test and found that cylinder 2 and 3 exhaust valves are leaking like they are stuck open, (pistons at TDC and valve tappets had some play). As I rotated the crank I could see all valves going up and down. Yet both #2 and #3 leak 100%.

On the 1 and 4 cylinders the are no leaks at valves and after squirting some oil I got about 10% leak at rings. I am happy with #1 and #4 considering this engine been sitting for years.

So, next I will remove engine from frame, take the head off and take it from there. Hopefully it's just carbon built-up keeping the valve from seating.

Any other thought what that might be?

If it turns out that that I need the head rebuilt any idea what it would cost and any recommendations for a reputable shop?

Jumping ahead of myself....

What are your thoughts on nikasil plating the cylinder bores? I don't see it discussed much in this forum, but it seemed to be a cheaper way to go with an airhead, considering the cost of boring + oversized pistons and rings.

Thanks.

xhevi · « **Reply #17 on:** November 01, 2021, 06:22:52 AM »

Does this oil filter bolt belongs to the 77 cb750 (26mm head)?

The parts diagrams and pictures online show a different bolt.

bryanj · « **Reply #18 on:** November 01, 2021, 06:29:15 AM »

Thats aftermarket, there was a time when acorn bolts/nuts were all the rage

xhevi · « **Reply #19 on:** November 06, 2021, 06:32:30 PM »

Good evening.

Would an earlier K (up to 76) oil tank fit on a 77K? Are Mounting tabs location and oil lines the same?

I know the side covers are different to allow for the oil tank cap that was visible up to 76.

Thank you.

KB02 · « **Reply #20 on:** November 11, 2021, 03:57:29 AM »

Quote from: xhevi on October 20, 2021, 07:28:34 PM

.... I am afraid this 750 might be a goner indeed, unless I find a donor or a sugar mama.

There's no reason that machine can't be brought back to life! Looks better than mine did when I got it (see my avatar). My advice: Build the engine back to stock. I went crazy with mine and it has been giving me all kinds of problems. I've got a box full of stock '78 parts. if you're looking for something specific, drop me a PM and I'll see if I have it.

xhevi · « **Reply #21 on:** November 11, 2021, 04:17:33 AM »

Thanks.

Have made some progress breaking it down. Engine is off the frame and on a stand. The PO had spray painted the engine whole with rattle can, I am in the process of cleaning the crud and stripping the crappy paint.

The OEM paint of the bottom end is also coming off. Depending if need to spilt the cases I will decide if I will paint them or just leave it bare aluminum.

Hoping to get the bottom end off today.

Will post photos.

xhevi · « **Reply #22 on:** November 14, 2021, 01:51:30 PM »

I had some time to take the top end apart.
Found that the camshaft and holder in the cylinder 1 side are shot.

eBay seem to be the only source of reasonably priced used parts from 50$ 150$. I know it's a gamble but need to keep this built in tight budget. I don't think any of the eBay sellers go through the trouble of checking if these parts are within spec, so I have to go by the photos and find one that looks in decent shape. I know that I should check for any wear on the bearing and journals. Any pointers as what else to check? Any one has a spare camshaft and holder for sale?

So far only checked the primary chain slack and I got 67.5mm, seem to still be some life left on that chain. The cam chain looks tight.

There is some wear on the primary chain tensioner, the wheel is not smooth and I can only describe it as slight sawtooth indentations, otherwise it's round and moves freely. I am debating whether I should split the cases and change all seals (no idea if any seal leaks).

I have yet to check the head, pistons and cylinder bores. But there is plenty oily built-up.

bryanj · « **Reply #23 on:** November 14, 2021, 02:24:12 PM »

If the bearing surfaces in the cam tower and on the cam are visually OK the chances are that they are fit to use.
You need to pull the cylinders and clean out/check all the oil ways including the restrictor in the head or it WILL happen again

xhevi · « **Reply #24 on:** November 15, 2021, 09:50:24 AM »

Finally I was able to attach some photos.

I am shopping for a used cam chain and holders and need some help with what parts fit a 1977 CB750K.

Camshaft part number is 14101-392-000. Partzilla and CMS indicate that this part fits any CB750 from 1975 to 1978. The bad camshaft i have is marked K4.
From what I gather in the forum is that all camshafts are interchangeable from 1969 to 1978, BUT the earlier cams have higher lobes and put out a bit more HP at high revs. Is that correct? I also found here that markings don't matter as old stock camshafts (lower R) might have been milled afterwards for the later year engines and the only way to find out is to measure the lobes on the cam.

Camshaft holder part numbers are 12010-300-030 superseded by 12010-300-040 and lastly superseded by 12010-410-000 (i might be wrong on the order). Partzilla and CMSNL indicate that this part fits any CB750 from 1973 to 1977. Parts diagram indicates that both the left and right sides are the same part number. I noticed that mine have different markings. do these marking matter any?

So bottom line... what camshaft fits what holder and what is recommended to get for a K7.

Any suggestions or links to threads I might have missed on this topic, much appreciated.
Thank you.

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Author Topic: 77 CB750 - resurrection (Read 2324 times)