Author Topic: CB750 Project Chopper (First Build) (Read 8114 times)

captaincrash80 · « **on:** August 14, 2008, 09:15:28 AM »

So here we go with my first full-up build of a project bike. Bought this bike for $750. Some of the specs:

70-71 CB750 motor
72 CB750 framed, already raked
125 carb jets
836 Bore Kit
drag exhaust
Missing speedo, headlight, tail lights, signals, ignition. Seatpan was completely rusted out. Frame needed to be repainted and some of the welds ground down, etc. So I decided to take it down to the frame and put it all back together.

Here she is when i got her:

Not too bad, right? Everything seemed to be in order with the bike. Kicked it and it barked like a dog waiting to get out. Decided right then that the bike would be mine. Loaded it up and took it home. Knowing my fair share about engines I decided that the first order of events should be cleaning the carbs out. So I pulled them off to see what I had to work with:

Ugly, i know $:-\$ .

Not to be disgouraged, I grabbed a couple cans of carb cleaner and went to work

Floats came out pretty good.

Covers after the initial cleaning.

Float bowls after initial cleaning. Cleaned them second time, and that's where the problems started. When I went to install the mainv jet back into the 2nd body from the left the threads that hold it in place disentigrated

. So that also went onto the list of things I need to buy.

Since i was low on funds I decided that the next thing on the list would be to cleanup the frame. So out came the engine:

All naked

So pretty

Next I wanted to address the NASTY welds that the PO had left on the frame. A dremel and alot of patience and I got 2 of them down to where I wanted them:
From this:

to this:

While it was taking forever I was at least making progress. Then, while working on one of the welds, I noticed a powder being thrown everywhere. So I broke out my drill and wire wheel and found !BONDO!, and lots of it. The entire front 1/3 of my bike was covered in the stuff. And they laid it on THICK.

So now I know I have my work cut out for me. Will be making regular updates, hoping to have it finished by spring. All comments welcome and encouraged.

chrislib · « **Reply #1 on:** August 14, 2008, 02:30:29 PM »

Looks like a cool project. Don`t be too put off by all the bondo, cleaning it off will give you an opportunity to inspect all the welds where the neck was raked. IMO I would`nt trust any welds I cant see, esp something as critical as the neck.

olds-cool · « **Reply #2 on:** August 14, 2008, 06:26:46 PM »

You have a nice start there. I wouldn't worry about the bondo too much. It was common in the 70's to mold the frames for a smoother look. What I would worry about is if the previous owner accounted for proper rake & trail when they butchered up the frame. Go over to hondachopper.com and get a copy of the chopper book they have scanned. It's individually scanned pages and you can read it online or download each page & print it out like I did. It covers rake & trail really well in a form that even a novice can understand after reading it once or twice.

mrbreeze · « **Reply #3 on:** August 14, 2008, 06:35:45 PM »

Good info Olds-cool....I thinks a cool look but I would definely bust the bondo out and the welds inspected. Unless you have another scooter now,you are missing this ridin' season but you seem to have a cool project and giving yourself plenty of time (next spring). Keep us posted for sure!!!

mrbreeze · « **Reply #4 on:** August 14, 2008, 06:38:39 PM »

Oh.....and by the way.........those carbs looked like total sh!T!!!!!!!!! Hope everything turns out better when ya get it done!!!

mrbreeze · « **Reply #5 on:** August 14, 2008, 06:41:30 PM »

Wow!!!.....went back & looked at the carb pic again. Looks like someone dumped a gram of coke in your #1 carb.

captaincrash80 · « **Reply #6 on:** August 15, 2008, 07:35:13 AM »

So I was thinking about it last night and wanted the boyz opinion. I have no idea how long the motor has been sitting, but I do know that there's good compression. Since I've already pulled the motor would a top-end rebuild be recommended? Check valves, replace gaskets, etc?

Tim in Ohio · « **Reply #7 on:** August 15, 2008, 08:09:27 AM »

I am watching your build with interest. Keep those pics and updates coming.
Olds: Great link there... hondachopper.com cool stuff.

captaincrash80 · « **Reply #8 on:** August 15, 2008, 08:44:32 AM »

mrbreeze,
Sadly this is my only 'real' bike i have right now. I've got a 125cc dirtbike that I take up and down the street every once in awhile but it's not street legal. And the only other scoots are my 78 CB400A that's completely torn down and my friend's 83 VT500C that we're rebuilding the carbs on

captaincrash80 · « **Reply #9 on:** August 18, 2008, 07:29:08 AM »

Haven't gotten a whole lot done on the bike as of late, but did manage to get the rest of the frame stripped down:

and tried to organize the parts a bit:

So here's a question for those who have done this many times before. Once I picked up the motor to drop ot on my rolling cart

a nice oily water mixture started to come out of th bottom of the engine, making me think that the poor thing spent some nights in the rain. Compression on the bike is awsome, but due to what looks like it could be some watery oil would a top-end rebuild be in order? Discuss!

captaincrash80 · « **Reply #10 on:** October 31, 2008, 01:41:41 PM »

Ok, so I know that it's been forever since my last update. Here's what I've been doing in the meantime:

That's the 78 CB400T mentioned in a previous post. Rebuilt it for my uncle using a 1981 CM400C donor bike

.

Anyway, back to the chopper.....

Finally stripped all the paint/bondo off the frame. The welds all look good. Also figured up the rake/trail and while not perfect, it is still very rideable. Tommorow I'm going to pick up 3 project bikes (one of which has my new carbs on it). Still aiming for spring on the chop, need to Kong the frame and repaint it, finish cleaning the engine up, etc. Will update more often now that I have some time to start working on it again.

mystic_1 · « **Reply #11 on:** October 31, 2008, 02:41:30 PM »

Thanks for the update. Out of curiousity, exactly what trail figure did you arrive at?

thanks
mystic_1

captaincrash80 · « **Reply #12 on:** November 01, 2008, 12:33:16 AM »

mystic_1,
Best i could figure it was between 10 and 12 inches (tape measure has always been a little off). I do have some concerns, seeing as how stock trail is around 5 inches. I think it could stil be rideable, but I leery about sinking more time/money into it if I can't even ride it when it's all said and done. $:-\$ I'm planning on moving ahead with it as of right now, but may change my mind depending on what you guys, the experts, think.

Hush · « **Reply #13 on:** November 01, 2008, 12:58:42 AM »

Love the rake of that frame, hope its all straight.
Nice job on the 400 too.

mystic_1 · « **Reply #14 on:** November 01, 2008, 03:20:30 AM »

Quote from: captaincrash80 on November 01, 2008, 12:33:16 AM

mystic_1,
Best i could figure it was between 10 and 12 inches (tape measure has always been a little off). I do have some concerns, seeing as how stock trail is around 5 inches. I think it could stil be rideable, but I leery about sinking more time/money into it if I can't even ride it when it's all said and done. $:-\$ I'm planning on moving ahead with it as of right now, but may change my mind depending on what you guys, the experts, think.

Well, I'm certainly no expert, but from what I've read trail should be between 4 and 8 inches. Ten to twelve sounds like a lot, would be a chore to ride at lower speeds and won't corner especially well, but will be very stable in straight-line highway cruising.

The bike pictured in my avatar has an calculated trail of 5.3 inches, it has +8 forks on a stock frame. I'm going to rake the frame to level it out and my calculations show I'll hit a trail of around 7 inches by extending my downtubes (+4 inches) to arrive at a steering head angle of 38 degrees (+5 degrees over stock), using shoter (11.5 inch) shocks in the rear.

mystic_1

Tim in Ohio · « **Reply #15 on:** November 01, 2008, 08:22:17 AM »

I'm liking ths thread. Glad to know my old buddy mystic is a chopper builder. Will come in handy on my next build.

captaincrash80 · « **Reply #16 on:** November 01, 2008, 09:09:39 AM »

Ok, so here's an update. I measured my shocks and they're 13 inches from hole to hole. I also have lowering blocks that are 3 inches from hole to hole. I'm thinking that removing the lowering blocks may help to reduce some of the trail. Thoughts?

junkyardjon · « **Reply #17 on:** November 01, 2008, 09:34:53 AM »

raked triple tree's should solve your trail problem

captaincrash80 · « **Reply #18 on:** November 01, 2008, 10:00:25 AM »

Went ahead and took off the lowering blocks and reinstalled the shocks. Recalculated the trail and ended up at approx 8 inch trial. That should be a little easier to drive

captaincrash80 · « **Reply #19 on:** February 02, 2009, 09:32:25 PM »

So after a lengthy exile due to life being the general (insert appropriate word here) that it is, I'm finally back to getting the bike going. Have the engine up in a donor frame right now so that I can get it back to running order. All was going well till I discovered that the starter motor was spinning but not engaging. I want to hear it run $:-\$ ! So I tried to kick start it. Still has the same nice compression as before, but it wouldn't fire. I know that without the help of the starter it's going to be a pain to troubleshoot, so what would you guys recommend?

Tim in Ohio · « **Reply #20 on:** February 03, 2009, 06:03:12 AM »

I am back from Exile as well. Keep the updates coming. I am tuned in!

captaincrash80 · « **Reply #21 on:** February 03, 2009, 06:33:59 PM »

So I've been trying for the past couple of days to get my engine to fire with no luck. The starter isn't engaging so I decided to just kick it over. That hasn't worked either.

Things I've done so far:
Changed oil
Ran new wires (just the minimum needed to run, will be making a new wiring harness for the final build)
Rebuilt carbs
Cleaned spark plugs

After everything was said and done I've kicked it probably over 50 times. I sprayed a shot of starter fluid into the #1 chamber via the carbs and was rewarded with a small fireball coming out of the exhaust port (exhaust is currently being stripped). I also have the points cover off and can see it sparking. I'm pulling the clutch in while I'm kicking, and the compression is still good. The bike was sitting for an unknown amount of time before I got it, but reading about you guys getting bikes that have sat for 10 years running in a day or 2 has me worried that I'm missing something. Is there a step anyone can think of that I may have missed? I have an extra engine that I could start rebuilding but really want to try and get this once going before I take that route. Thoughts, opinions, advice, etc?

Alan F. · « **Reply #22 on:** February 03, 2009, 07:48:38 PM »

Minnimum wiring, ok so no kill switch to wory about right? That one gets everyone sooner or later.

Have you verified spark on all 4?

New plug caps may be in order, their resistance will go up with age and corrosion. Measure the resistance of the plug caps, they should be about 5000 ohms. before you screw the caps back onto the plug wires, cut the wires back a half inch then screw the caps back on, there are a few spots that will eventually corrode and weaken the spark. Also be sure to measure the resistance of your coils to make sure they're not garbage. measure across plug leads 1&2, then 3&4 as well as the thinner wires that feed each coil. you should find some resistance at each spot and both coils should read similarly.

captaincrash80 · « **Reply #23 on:** February 03, 2009, 08:53:00 PM »

Alan:

Tested the caps w/ my multimeter on the 20k setting and got between 9.xx and 13.xx on them (4 different readings). Also used TwoTired's method for testing coils (http://forums.sohc4.net/index.php?topic=17711.msg184209#msg184209), which tested good. I'm contemplating upgrading/replacing the wires and possibly buying new plugs. Will work on it more tommorow, if anyone has any thoughts between now and then I'd love to hear it.

IT WILL LIVE AGAIN!!!!!!

mystic_1 · « **Reply #24 on:** February 04, 2009, 04:41:06 AM »

Forgive me is this is a redundant question, but have you checked your point gap and timing?

mystic_1

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Author Topic: CB750 Project Chopper (First Build) (Read 8114 times)