Topic: '78 CB750 Project

[sethmcalister]

Hey guys,

New to the forum here.  Sold an '01 Triumph Bonnie last year and regretted it.  Picked up a '78 a couple weeks ago for $600.  It's not running right now, but the PO indicated that it was before he put it in storage (for 2 years).  He was attempting to cafe it out, but never made it very far. 

I started to strip the bike before I had back surgery, so it's going slow right now.  I don't have an indoor shop so I am not pulling the motor until I can get it to a friends house to pull the carbs and see how they look.  I don't want to pull the motor apart if I can avoid it, but we'll see how the top end looks anyway and go from there. 

The PO had attempted to seal the fuel tank and when I checked it, it was all flaky.  Started by throwing some acetone in and that pulled a lot of the loose stuff and garbage out.  Sitting on my deck working today and threw in a thing of The Works to help it along.  From what I can tell, the inside of the tank is pretty dang clean so far.  I'll let it sit overnight and see how it looks, maybe try some paint thinner if I can't get it all out. 

I've ordered front/rear wheel bearings, upper/lower shock bushings, steering bearing and I'll change the oil and oil filter.  The old seat was a tad ratty, but it came with clip ons and a new cafe seat.  I plan on using the old seat pan if it's good.  I stripped the vinyl off; but the seat pan has some rust on it (hoping just surface and not too bad).  If it is then I'll probably go with a homemade seat pan and kind of a tuck and roll bratstyle seat so I can keep it 2up for my wife.

I'll post some pics up, but I have to resize them first since they're too big.  Lookin forward to riding this year for sure though !

[flybox1]

welcome to the forum.
can you try and get it running?
this will at least tell you if you need to dig into the trans.
3000 mi maintenance items are always a good starting point for a newly acquired bike.
shop manual can be downloaded here and contains all the info needed.

[sethmcalister]

I've definitely already downloaded the shop manuals and glanced through all of them.  The front caliper was stuck so I'll need to see if I can rebuild that or pick up a new one.  I ordered some new bronze swingargm bushings to replace the old stock bushings too. 

The front forks are leaking.  The PO had already picked up new fork seals, but hadn't replaced them, so that's on the list as well. 

I looked at the plugs and they all look pretty brand new, so I think the shop the PO took it to had replaced them and gone through it. 

I'll have the front/rear wheels trued, throw in my new battery (agm), need to do something about the headlight, taillight and blinkers, front/rear brakes, and air filter.  The other stuff will get checked too.

Side-note, the clutch and throttle seem pretty heavy (compared to my old bonnie).  Not sure if it's because it's an older bike or I need to loosen them up a bit - I don't care for such a hard pull.

[Airborne 82nd]

Welcome
 Is this a K or F I would not even think about pulling the motor. Like Fly said do the 3K tune after two years with todays gas I would go ahead and pull the carbs. As far as the manual the 77-78 stuff is in the back section and it says don't pull the slow jets but that is wrong as wrong can be. If it was running 2 years ago it should start right up after the 3K and carb cleaning. When it comes to these carbs there is a lot of little detail stuff that must be done right or you will be taking them off more than you need to. Fly is pretty sharp on the PDs so listen to him or me

[flybox1]

looks all K to me 

[sethmcalister]

It's a K.  I've already discovered (at least this is what the dealer told me) that there were two different types of carbs on the '78s - is that true?

I like the advice on not puling the motor, I may at least pull it out to clean up the frame.

All I've been doing is reading up on these bikes and the four carbs definitely have me a bit hesitant.  It was much easier with only two, haha.  There wasn't any fuel in the tank, and I'm hoping that they winterized it before he put it in storage and attempted to seal the tank, but I won't know until I pull them off. 

I know I'll be looking for advice, so thank you ahead of time.

[flybox1]

Look on the right end of your carb bank, on the seam, right above the bowl....there should be a stamp.    PDXXXXX
that'll be the carb model #
PD42B and PD41A's are common for the 77/78 CB's

[sethmcalister]

Here's a (crappy) picture of the carb #.  42B?

[flybox1]

Yep, PD42B's   PD carbs have two nice features.  An accelerator pump, and a fast idle cam.
With these systems working properly, cold starts are less of an issue, and you'll have better acceleration with the first 1/4 throttle turn.  Certainly not as good as FI bikes, but an improvement.
There are heaps of threads here on how to get them functioning properly.  A couple going on right now.

[sethmcalister]

Can you shed some light on how the accelerator pump and the fast idle cam work?

[AintNoEasyWay]

Drain the float bowls & have a look at what's going on beforehand. At least undoing the bowls will give an idea of whether he left fuel in the carbs or not while you wait for safe cover to operate.

You can also check the accelerator pump without removing the carbs (you will have to remove the airbox though). There is a nozzle that sticks up in the venturi of each carb & shoots fuel horizontally into the chamber after a quick blip of the throttle. Here's a video: https://www.youtube.com/watch?v=4lJf2NGUNdQ

Guys helped me a bunch within the past couple weeks on a thread for my K7, worth the read to learn a lot: http://forums.sohc4.net/index.php?topic=135552.0

[flybox1]

Assuming these systems are operational:
Fast idle cam - when you pull the choke knob, the fast idle cam raises the carb slides to maintain a set (by you) idle rpm, like 2500, so you dont have to fool with the idle knob or stand there and throttle the engine till its warm

Accel pump -  the first 1/4 turn of the throttle actuates a diaphragm, located at the bottom of carb bowl #2.  the diaphragm squirts gas into the carb throats.

if you are in any gear, a moderately quick twist of the throttle, from the 0 position, will give you a shot of gas to aid in your acceleration, and less of that vacuum carb 'gulp' when throttled hard  (gulp is a technical term  )

With cold starts, a pull of the choke knob raises the slides and closes the choke butterflys.  Add in a quick twist of the throttle, (actuating the accel pump) primes the carbs.  easy peasy cold starts 

[sethmcalister]

Ahh!  I love learning new things.  This is some great info, thanks FlyBox!

Hopin to check the tank today, it's been sitting overnight with the Works TB cleaner.  Some of the sealer the PO attempted seems to have stuck.  Thoughts on trying to really get it all out?  My thinking is that if it appears to be sufficiently adhered to the tank, there shouldn't be any need to fully remove it, right?

I considered picking up some paint thinner for another little round, but is it needed?

[flybox1]

Although i have not needed to try it, acetone should work well to loosen any old liner.
Fill the tank with golf balls to take up some of the volume.  maybe a length of chain to help 'scrub' the tank as you shake it.
After all the liner is removed, Kleen Strip Prep-n-etch leaves a nice gray phosphoric coating and will last for years.

[sethmcalister]

I started with the acetone and some finish nails, that took a lot of the crap old liner out that was peeling.  Then I went with The Works.  I haven't dumped that out yet.  The inside of the tank, from what I can tell thus far, looks to be rather clean.  It seems that, in my opinion, the tank didn't even need to be sealed in the first place. 

Denatured alcohol will work just about the same, right?  Or at least prevent the flash rusting?  I had planned on attempting another sealer though simply because I want to make sure that the old sealer doesn't cause any problem, assuming there is some left over.

[flybox1]

DA or acetone will work just fine to dry the inside.
hair dryer in the fill hole will speed things along.

[sethmcalister]

Do you recommend re-sealing to be safe?  One guy had recommended Caswell.  I'm leaning toward re-sealing to be safe...

[flybox1]

reseal if you want, but i never saw the point if it wasn't needed.  good, unrusted metal tanks have been used a long time.
meticulously complete all the prep steps listed on whatever product you use.

[Geeto67]

I would reseal it. Stock tanks have a thin plating on them (it looks like bare metal but is is not) and once it rusts and you use chemicals to clean it the plating is pretty much gone and it will rust easier going forward. I have never seen an american gas pump that didn't have a little water in it so it is piece of mind. Don't forget to neurtalize the works with baking soda and water.

The good kits are Por 15, Caswell, and RedKote. I find that Por 15 is the easiest to work with for newbies. Prep is the most important part. Also buy the kit and not the liner seperately  - the metal converter is as essential to the process as the liner itself.

Before you have the liner put in, you might want to pressue test the tank to make sure you don't have any pinholes that need addressing. A minor pinhole the liner will take care of but bigger than the head of a pin and it needs brazing.

[sethmcalister]

Does the thin plating look grayish-blue in color?  There were some spots that appeared to have rusted over slightly...or where the plating was gone. 

I was planning on neutralizing it with denatured alcohol, I was under the impression that did the same thing as the baking soda and water, no?

[Geeto67]

I'm not a chemist, so yeah, sure. baking soda is cheap and we have plenty in the house so it is just what I use.

The plating looks like raw steel with a slight sheen to it. I don't know that it was a blueish color, the NOS tanks I used to have just looked like steel. any kind of blue color might be a reaction to the cleaning chemicals, some residual gunk, or any number of things including part of the oxidation process. I've seen the insides of tanks start to "rainbow" when they begin to oxidize. Old fuel varinish residue can sometimes have that effect if it is thing enough.

FWIW, if you used the electrolysis method of rust removal the byproduct us replating the tank with whatever coating was on the sacrifical annode in the first place. That is why that process is less prone to flash rust post cleanup. 

[sethmcalister]

Got a little work done on the bike today...only about an hours worth though. Tank still needs some love. Going to have to pick up something heavier,either pain t stripper or marine cleaner I think. And then I got figure out how to get the damn nails out I threw in there.

Here are some progress pictures though. Airbox says 750F, is that normal. I also included some welds from the front of the frame. I'm not sure if these are normal or if they were from the huge friggen fairing it had on it.

Another question, I would like to cut off the rear seat mount portion. How okay is it to cut into the gusset? I want to be able to cut off just in front of the part that sticks up. If not, I'll cut to the shock gusset and then cut the part that sticks up off and grind the frame.

Sent from my Nexus 5 using Tapatalk

[Geeto67]

The welds are not normal. Don't know why they are there.

The cb750f on the airbox is normal. They all say that.

Why do you want to cut the frame? Do you have a need to do it? And by need I mean did you get a seat and figure out that the frame rails are in the way? Or are you just cutting it because you saw it on the Internet and think that is the thing to do. You shouldn't be cutting anything without all the parts in hand and a measured twice. Once it's cut you can't go back, so make sure you have everything figured out.

[calj737]

Denatured Alcohol doesn't neutralize the chemicals you've used. BUT, it will dehydrate the moisture after you neutralize. It's an excellent step before you do seal the tank.

I would also encourage you to line the tank after the chemical baths re: thin metal plating becoming affected by the chemicals. You don't need to pressure test the tank, fill it with water (as part of your neutralizing step) and wipe the outside of the tank down with DA, and watch for leaks or seepage. Field is not under pressure during normal operation in these tanks so that test is superfluous.
The places most common the leakage are the badge mount areas, so inspect them well. But if the interior is nearly as clean as you say, leaks are unlikely unless there is physical damage to that tank.

[calj737]

Regarding your frame: the welds on the down tubes of the cage are not factory, and likely as you suspect from a fairing being mounted. Obviously the rust needs to be dealt with. If you undertake grinding the weld-globs off, do take the next step and re-weld that sleeve. Those sleeves were fine if left unmolested, but there is a good deal of rust as a result of the attachments there, and I'd go the measure of repairing that weld for surety's sake.

Once around the bottom, then up the sleeve on the exhaust side. This will restore all the integrity unless you discover some actual tube rot as you prep for the welds.

The good old seat gusset: if I understood your post correctly, you want to "bob" the seat hoop as far forward as the shock mount? If you do this, replace the cross brace (seat gusset) with some flat stock or square tubing, welded back into the frame at the shock mounts. This serves 2 purposes: lateral rigidity of keeping the frame (under good deal of stress at the shock mounts) in tandem, and will prevent the frame from being "splayed" from seat/rider weight.

There have been many who cut and do not replace this gusset, but it is not prudent. Obviously, check for tire/fender clearance with rider weight and suspension travel to determine where to weld the gusset back.

Apologies for the lengthy post- by the way, nice looking project to start. You live in some damn pretty country up there and were it not for the frickin' winters and black flies, I'd be jealous