Author Topic: 78 CB750F - Craiglist find, Rebuild (Read 21943 times)

ZanVooden · « **on:** March 03, 2011, 08:39:43 PM »

Hey all,

So this is my second rebuild. First one was a 78 CB750K (http://forums.sohc4.net/index.php?topic=57379.msg620853#msg620853)

I pick up this 78F down in Davenport for $150.

Heres the good: almost entirely complete. except for the seat, handle bars, front headlight, and one side cover emblem. The engine turns with the kick start, No battery yet so havent tried the starter. 22,001 miles. Oh and it was only $150.

The Bad: Tank looks rusty and the carbs seems to be completely stuck. The electrics are entirely questionable. No title (I've started the process of retitling it WI)

The Plan: first thing is to check compression and see what the engine may need. I dont intend to do anything extreme with this bike. Going to go stock, except for superbike bars and probably one of those new corbin seats. Otherwise stock rebuild.

-Dave

ZanVooden · « **Reply #1 on:** March 06, 2011, 04:22:40 PM »

So started tearing in to the bike today. First up was to take the windjammer off (anyone want one).

Then I pulled the air box and carbs off. Turns out the carbs werent stuck after all just need new cables.

Then did a compression test. Here are my results.

#1 #2 #3 #4
110 100 85 85 (psi. typical)
Then added some oil to the cylinders
#1 #2 #3 #4
150 145 150 145 (psi. typical)

So it would appear that I need new rings. Is this correct?

MoMo · « **Reply #2 on:** March 06, 2011, 04:45:55 PM »

Not necessarily so. Buy some Seafoam and put at most an ounce in each cylinder. Let it sit for a couple days and then kick over(make sure you have the plug holes covered or you might stain your ceiling-I never did that

) to get the Seafoam out. Then check compression again. I had a similar situation to yours and did that cycle a couple times, eventually compression came up, bike started and to the best of my knowledge is still running today. It was customer's bike. I've done that several times and never needed to tear down. The Seafoam might be dissolving the gummed up rings/cylinders. Good luck...Larry

ZanVooden · « **Reply #3 on:** March 06, 2011, 05:29:09 PM »

Larry,
Thanks for the advice. I'll have to give that a try.

Also when taking off the carbs I found a couple surprises from the PO. The bowls had some old gas in them that must have been mixed with a fuel additive because all of the bowls had the green solutions in them and smelled of really old gas.

when pulling the bowls off I found that the PO must have striped the original bowl screws because he felt the need to replaces them with metal sheet metal screws.
Any one have any ideas on how I should fix this. I would like to put the right screws back in. Would tapping the holes simply work?

BeSeeingYou · « **Reply #4 on:** March 06, 2011, 05:43:24 PM »

You could try and retap them or could you retap them the next size up if the threads are really tore up.

I see the seat has a place of honor on the workbench. Planning on reusing it?

MoMo · « **Reply #5 on:** March 06, 2011, 05:57:44 PM »

If you tap to the next large size, you may have to drill out the float bowls so the screws will pass through. Make sure there is enough metal left so the float bowl does not crack when you tighten it. Think it out before tapping. If a nut will fit on the carb side, you could use it in conjunction with a longer screw/bolt. Be thorough with the carb cleaning, you'll probably have to completely disassemble.

The green slime smells wonderful, doesn't it

...Larry

ZanVooden · « **Reply #6 on:** March 07, 2011, 07:09:02 AM »

I should also mention that before doing anything to the bike, it was about a quart low on oil. I topped it off before doing the compression test.

UK Pete · « **Reply #7 on:** March 07, 2011, 09:34:18 AM »

Great project you have there , the carbs look a bit messy, my experience with carbs when they have been butchered like that by a PO is to find another set , they often go quite cheap for these f's
Good luck with the bike if you need any advise just ask
Pete

noahspop · « **Reply #8 on:** March 07, 2011, 03:41:32 PM »

Can't wait to see how everything turns out.
Great find.

captaincrash80 · « **Reply #9 on:** March 08, 2011, 05:24:37 AM »

Looks like a fun project at a great price. Waitin to see how this one turns out!

ZanVooden · « **Reply #10 on:** March 08, 2011, 08:36:31 PM »

Last night I put some seafoam in the cylinders. Now I'll wait a couple days and try a compression test again and we'll see if this method works for this engine. (fingers crossed)

Tonight I ordered: handlebars and a kick stand springs (the bike was missing one and it was getting very annoying)

ZanVooden · « **Reply #11 on:** March 10, 2011, 08:31:04 PM »

So far I've ordered Carb kits, Headlight bucket, a Full lock set w/ key (ignition, seat lock, & tank lock), Accelerator pump kit, Throttle tube, grips, and a new gas tank lid.

Time to take a break on buying parts before I break the bank here.

ZanVooden · « **Reply #12 on:** March 10, 2011, 08:39:47 PM »

Alright, so I can honestly say I am in slight amazement of Seafoam.

I did as photolar suggested and added some to each cylinder and let it sit for a couple day and redid the compression test.

Here are my results:

#1 #2 #3 #4
120 120 118 115 psi (typical)

So what do you guys think? they are all pretty close, but is the compression high enough for this engine?

Also started looking at the tank. Below are a some pictures. What do you think? is that the dreaded Kreem liner? Is there a good way to salvage this tank?

somesuch · « **Reply #13 on:** March 10, 2011, 08:41:00 PM »

I have Helicoiled the stripped carbs before, but small Helicoils need care when installing. You can snip the coils that stick out, but you'll need very nice dikes for that.

--Nick

ZanVooden · « **Reply #14 on:** March 13, 2011, 07:58:56 PM »

Did some more work on the cycle today.

Installed handlebars and reattached all the controls and such.
Also rebuilt and cleaned the carbs using carb dip, carb cleaner, and compressed air. Still need to check the float heights though.

As I mentioned earlier the PO must have stripped out a couple of the bowl screws and just used sheet metal screws in place. I'm going to attempt to tap to the next size.

Before:

After:

Free Booter · « **Reply #15 on:** March 14, 2011, 08:34:18 AM »

HOLY CRAP $150!?!?

MoMo · « **Reply #16 on:** March 14, 2011, 09:30:35 AM »

ZV, are you checking with the throttle wide open? The readings you have should start the engine. Once it starts you should see the readings go higher. Good luck...Larry

ZanVooden · « **Reply #17 on:** March 14, 2011, 11:21:14 AM »

Quote from: photolar on March 14, 2011, 09:30:35 AM

ZV, are you checking with the throttle wide open? The readings you have should start the engine. Once it starts you should see the readings go higher. Good luck...Larry

Yes, kind of. Im doing the compression check without the carbs on the bike. So essentially wide open.

Quote from: Free Booter on March 14, 2011, 08:34:18 AM

HOLY CRAP $150!?!?

Yep. The guy was asking $250. I offered $150 and he took it.

MoMo · « **Reply #18 on:** March 14, 2011, 12:24:14 PM »

Quote from: ZanVooden on March 14, 2011, 11:21:14 AM

Quote from: photolar on March 14, 2011, 09:30:35 AM
ZV, are you checking with the throttle wide open? The readings you have should start the engine. Once it starts you should see the readings go higher. Good luck...Larry

Yes, kind of. Im doing the compression check without the carbs on the bike. So essentially wide open.

Quote from: Free Booter on March 14, 2011, 08:34:18 AM
HOLY CRAP $150!?!?

Yep. The guy was asking $250. I offered $150 and he took it.

Give it another dose of Seafoam and check it again..Larry

ZanVooden · « **Reply #19 on:** March 14, 2011, 12:39:06 PM »

Quote from: photolar on March 14, 2011, 12:24:14 PM

Quote from: ZanVooden on March 14, 2011, 11:21:14 AM
Quote from: photolar on March 14, 2011, 09:30:35 AM
ZV, are you checking with the throttle wide open? The readings you have should start the engine. Once it starts you should see the readings go higher. Good luck...Larry

Yes, kind of. Im doing the compression check without the carbs on the bike. So essentially wide open.

Quote from: Free Booter on March 14, 2011, 08:34:18 AM
HOLY CRAP $150!?!?

Yep. The guy was asking $250. I offered $150 and he took it.

Give it another dose of Seafoam and check it again..Larry

I did forget to mention in my eariler post that I added more seafoam on Thrusday night, and check my compression on Sunday.
I am now at 120 PSI across all 4 cylinders.
Photolar, are you thinking these numbers still seem low?

MoMo · « **Reply #20 on:** March 14, 2011, 02:54:14 PM »

No, you should be able to get it started with 120.. Most of the bikes I bought back to life came up to 120/130 with Seafoam and eventually 140 to 150 after it started. I did not know you gave a second dose so that is why I suggested it. I have even freed seized motors with it that started and are running to this day. I am sure it will run. Good compression, spark at the right time and fuel in the correct mixture and any motor should start...Larry

ZanVooden · « **Reply #21 on:** March 14, 2011, 09:43:22 PM »

A little progress tonight.

Got some new grips, so had to put them on to check them out. Im pretty happy.

Started cleaning the tank out. Drained whatever old gas was in it and noticed that there was nothing but a piece of plastic left of the in tank filter. So pulled the petcock off and pulled the filter out. Got plastic end cap for the tank outlet. Then put in 2 quarts of aircraft remover. (wow, they are kidding when say use gloves and be in a well ventilated area... That stuff burns...) put in a handful of short screws, and shook the tank around till I was tired, then started making an apparatus to put in the tank for electrolysis cleaning. I shook the tank around about ever 30mins or so. Then after 3 hours I flushed it out with water and shook all the screws out. Below are the result.

Tomorrow night I will do the electrolysis on the tank to try to remove the rust.

ZanVooden · « **Reply #22 on:** March 16, 2011, 08:59:48 PM »

So the tank electrolysis didnt work as well as I would have liked, but thats because I had to resort to a crappy charger than was pretty unreliable. I'll probably just fill the tank with vinegar and let it sit until the bike is ready for it.

So decided now that the carb are clean and the engine has compression it would be time to try to start the engine and see if it would run. So I rigged up a remote tank, put the carbs and the air filter on and gave it a couple cranks. No go. Decide to pull a plug and see if I had spark. No spark at the plug.
Checked the points for spark. No spark at the points.
Checked a bunch of connections and they all seem good.

So at this point Im looking for advice as to where to start looking and testing. I have a digital multimeter, just curious where to start checking.

Any advice or recommendations is appreciated.

MoMo · « **Reply #23 on:** March 17, 2011, 04:33:29 AM »

The points need to make contact, did you clean the points? Most every old bike that has been sitting needs to have that done...Larry

MasterChief750 · « **Reply #24 on:** March 17, 2011, 07:44:08 AM »

Is the battery charged?

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Author Topic: 78 CB750F - Craiglist find, Rebuild (Read 21943 times)