Author Topic: The CB750F RAT BIKE project: Ratty old daily driver!  (Read 13295 times)

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ws6transam

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The CB750F RAT BIKE project: Ratty old daily driver!
« on: April 17, 2008, 08:04:13 PM »
April, 2007:

So I was between bikes, shall we say, when I realized that I couldn't stand it any longer and I needed something...ANYTHING... to appease the motorcycle craving that was eating me alive. My hands were shaking, and I found myself wearing my Arai helmet while driving my minivan with it's side doors open in an attempt to get a little bit of that roar in your ears feeling that says life's good! When I dropped the passenger-side wheel off the asphalt while cutting the apex too close, and my wife & kids screamed at me, I knew that I had to do something more drastic than simply live off my memories and fantasies. I was a mess: Wearing an undersized Hein Gericke jacket with an AMA 18-year pin on the lapel, something had to be done!

...Then it happened that someone posted an ad with a blurry cell-phone picture of a purple motorcycle: They said it was an old Honda 750 four-cylinder but that it hadn't run in about six or seven years. The price was right though, at only $300. So I bought it sight unseen, figuring at the very least I could sit on it in the garage, while riding the center stand, making spitting noises in my helmet while staring at some old Wayne Rainey poster on the wall...

It turned out that my hunk of old iron was a very dirty '77 Honda CB750 Supersport with flat, rotted tires. However, it appeared that there wasn't any rust anywhere, and it rolled over with the kick starter. The key was in the ignition, in the "on" position, and had been that way for years & years, guaranteeing that the battery was stone-cold-dead. The gas tank had the petcock broken off, but it wasn't dented in any way. I hem-hawed a little, and the owner tossed in a set of old Tiawanese performance tires, still mounted on junkyard Kawasaki rims. Yeah, they looked dirty but they also looked like there was no wear, and more importantly, no cracks. The $300 sealed the deal, and I rolled it onto the trailer with the help of the owner. Geez, this thing must weigh 600+ pounds!

Well, getting it home, the first thing I did dumpster-dive for a petcock at Slick's Auto Salvage in Mason, Michigan. Eighteen bucks later, I had me a petcock that seemed to do the job. Then, I filled the gas tank, hooked it up, turned on the petcock, and watch gasoline piss all over the floor from the carb overflow tubes. A few good raps later on the float bowls with a screwdriver handle stopped the petroleum hemmorage. Next, I put in the new battery. Most of the wiring appeared okay, except the positive terminal which had fatigued and broken. One electrical repair later ,and a shot of ether into the carb throats, and a key of the starter brought a whuff..whuff..putt! out of the exhaust. Hmmm... Ignition problems, perhaps?

I checked the points, and when closed, each one had between five and fifty ohms of resistance across the contacts. So, I grabbed a little emory paper, and filed those suckers until resistance went to zero ohms. Another shot of ether, and a quick key-over brought about half the cylinders to life, as the engine coughed and sputtered blue haze into the garage... Yikes, is that burning oil, or burning mouse nest? Oh well, guess it's time to pull the plugs!

I check, and cylinder # 1 is stone cold with a black plug. Cylinder # 2 is warm with a tan plug. Cylinder # 3 is also tan, and cylinder # 4 is whiteish. So I install the new set of spark plugs, but the engine still runs like crap: It sounds like we're missing a couple cylinders: Could it be a bad coil? I check, and cure enough, #1 has several hundred Kohms of resistance, and plug # 4 does as well. Parts store run # 3 nets me some new resistor-type plug boots. Plus, the bike was equipped with the wrong plugs: A resistor type spark plug on top of a resistor type plug boot. Ugh. go figure.

Anyway, with new plug boots in hand, the magic ohmmeter says all is good on the coil side. As I start the bike, it instantly comes to life this time! Woo hoo! Now we're cooking! Uh, oops! we might just be cooking soon, because those carbs sprung a leak again!

... Aw heck, I think I'll just sit on the damn thing, have a beer, then put on my helmet and make spitting noises onto the inside of my face shield. Plllllbbbbbttttttt!!!!!!!
----


May, 2007: The carbs are rebuilt and she's running pretty well now, though it stumbles like mad and my spark plugs are whiter than carved elephant ivory... I've been putting around town, and haven't actually given it a good goose of the throttle yet... Maybe I should try! I hop on the bike for a short, short little ride....find a deserted side street, fan the clutch, and let it wail in first gear. As I shift to second, the bike suddenly QUITS, and I coast to the side of the road. I mean, NOTHING! Total electrical failure.... Dang, I even forgot to bring my cell phone with me and I'm supposed to be home in five minutes!!!

June, 2007: The old fuses have all been replaced with new ones, and the junk tires are tossed out in favor of the slightly less junky ones, which actually cleaned up nicely... I nearly busted my fingers though, as I had to re-learn how to wield a set of tire irons... I hadn't used them in eighteen years! So far, the cost of parts has been $180, including the EBAY set of front brake calipers and rotors that were necessary due to siezed-up pistons and busted-off bleeder screws... My rat bike is now a $480 rat bike. However the chameleon purple paint has really begun to grow on me, scratched and marred as it is...

July, 2007: A whole MONTH of riding is in without incident! Waa-hoo! 44 miles per gallon! Yeah, it still coughs and stumbles, and it doesn't run well when it's cold, but when it does get to chuffing it seems to do all right.... Plugs are still white though. It starts every time now.

August, 2007: I nearly got spat off the bike, in front of my WIFE, no less, due to the chain, which mysteriously got so loose that it crawled up the sprocket and came off. Now I need to re-check and tighten it each week. The chain is now pretty darned noisy and is obviously stretched beyond its useful limit. However I've managed to roll 2000 miles onto the odometer so far this summer...

September, 2007: I have to keep cleaning up the points with emory paper each week or else I lose two cylinders. These points are obviously shot and in need of replacement.

October: Ah, it's kind of nice to ride in October. Less bikes on the road, especially on mornings when it's 29 degrees out. However, the rat bike is getting hard to keep lit in the cold weather. It's flame-out city at anything over 3/4 throttle so it's running way, way too lean. I'll have to tackle that, along with the new points, and the chain next spring, IF I keep this thing.

November25th : Last check, nearly 3,000 miles ridden so far this season. This is just back & forth to work stuff, the kind that I would have done in my Grand Cherokee. The GC gets 14 MPG around town. The CB750F gets 44 MPG. Let's see: 3000 miles / 14 mpg = 214 gallons. 3000 miles / 44 mpg = 68 gallons. 214 gallons - 68 gallons = 146 gallons of fuel that I didn't burn. That's oh, about $350 worth of fuel that I didn't use! However, the bike is running rough, I dont trust the chain anymore, and it seriously needs some carb work. I've got snowmobiles to work on, anyway, and one of them needs the CB's battery.

March 15th, 2008: Well, snowmobile season is over, so let's get cracking on the rat bike. First things first: Install the new points and fill up the gas tank. Big surprise, it starts instantly! It's amazing what a little bit of fresh fuel will do when you properly winterize...  That, and even though it's only 35 degrees outside, the motorcycle seems to be running decent.

April 14th: My RK chain showed up in the mail, and I've just converted it to a 530-style X-ring chain. The 17/48 ratio is a bit taller than the original 15/45, but it feels strong enough, and the chain is dead quiet! Driveability is INCREDIBLE now that there's less slop in the system... Definitely worth the money, though now the rat bike cost stands at $680. Oh, I forgot: The seat vinyl cracked over the summer, so I also installed a new EBAY seat cover. Certainly not original looking, but it's comfortable and in keeping with the rat-bike philisophy: Reliable, low-buck repairs.

April 17th: Today I took a stab at the tuning, along with some miscellaneous welding on the muffler. The MAC 4-into-1 pipe had the wrong model muffler on it, and I really didn't like the muffler swinging around on the end of the collector like that. So now it's properly attached to the frame mount with proper steel.. Nice and solid. I also raised the needles one position in the clip, and it help reduce stumbling considerably. However, the plugs still show white after a 20 mile ride at 60 MPH, so I've ordered some #110 jets, and will go up one size to see how things go. I might even raise the needle jets one more notch while waiting for those jets to arrive in the mail.  My idle mixture screws are 3 1/2 turns out in order to keep it from dying when I crack the throttle. Now it looks as if the ratbike has passed the $700 mark. However, I hope to save that much in gasoline again this summer, especially with $3.50 a gallon gasoline! 3000 miles = $500 savings over top the Grand Cherokee.... Shoot, it almost feels like printing money! :D  Best part is that I no longer have to wear my helmet in the minivan! My wife appreciates that part a whole lot.




Offline rachet

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #1 on: April 17, 2008, 09:28:23 PM »
This is a great write up!  Very enjoyable reading and I'm glad to see that you can build a reliable bike on the cheap!  Keep us posted!

Rachet~
But I need Tacos!  I need them or I will explode!

Offline mystic_1

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #2 on: April 17, 2008, 09:37:20 PM »
This is a great write up!  Very enjoyable reading and I'm glad to see that you can build a reliable bike on the cheap!  Keep us posted!

Rachet~

I agree.  I laughed, I cried, I'm looking forward to the movie adaptation :D


ws6transam, I presume you've been through all the regular maintenance stuff like valve adjustment, carb sync, etc?  Good job resurrecting the bike!

mystic_1
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Offline seaweb11

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #3 on: April 17, 2008, 09:56:52 PM »
Nice read.
Welcome to the Forum ;D

ws6transam

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #4 on: April 18, 2008, 03:15:00 PM »
Glad you all liked it.

NEWS FOR TODAY is: I moved the clips on the needles to the #2 position (second from bottom) and the mid-range stumble is now gone. Cold idle with choke on is still not very good, but it's liveable. You can feel the power drop off under WOT now, so I've ordered the next size up jet, the 110, hoping that will cure the WOT blues. However, around-town driving is now much, much better, with no stumbles. The plugs still look whitish, but at least they dont look like they are made from bleached chicken bones anymore, as there's now a few beige discoloration patches here & there. Thankfully there wasn't any sputtered aluminum anywhere!


Offline 750goes

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #5 on: April 18, 2008, 03:22:10 PM »
a good read indeed.... keep up the good work   you are an inspiration to all us other tight wads...... ;D

Offline seaweb11

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #6 on: April 18, 2008, 03:42:14 PM »
What is the stock main jet size for that model?

I know the 78K starts at 110.......and I have 136's ..........pods......... ;D

Offline scondon

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #7 on: April 18, 2008, 03:52:00 PM »
What is the stock main jet size for that model?

  105, Seaweb. A bit less compression than the 78K as well. :)
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Offline chrislib

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #8 on: April 18, 2008, 04:38:12 PM »
awesome read, made my night!
Chris...closet Idlefiddler
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1973 CB750K...CANDY BACCHUS OLIVE

Offline mystic_1

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #9 on: April 18, 2008, 06:50:22 PM »
BTW, is it just me, or does $300 seem to be some sort of magic number when it comes to CB750's?  I paid $300 for my bike, and it seems I'm always hearing people say "I got this CB750 for $300."

mystic_1
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Offline Joel

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #10 on: April 18, 2008, 06:58:59 PM »
Where can I pick up one of these magic $300 bikes?

Offline mystic_1

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #11 on: April 18, 2008, 07:06:41 PM »
It'll be hiding under all the rust and aluminum oxide :D

mystic_1
"A ship in harbor is safe, but that is not what ships are built for."
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My build thread:  http://forums.sohc4.net/index.php?topic=68952.0

Offline 750goes

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #12 on: April 18, 2008, 07:23:47 PM »
Here is my $300 starter project from some time ago


Offline mj_honda

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #13 on: April 18, 2008, 08:56:32 PM »
BTW, is it just me, or does $300 seem to be some sort of magic number when it comes to CB750's?  I paid $300 for my bike, and it seems I'm always hearing people say "I got this CB750 for $300."

mystic_1

Well, I picked up my 78K 750 for the cost of fuel. At the time it was about $2.50 so round trip was around $100.

Carb rebuild kits: $130.

New cables and filters. Choke, clutch, throttle, air and oil: $75

$305 so that sounds about right.

I figure that is a solid number. My bike was a a gift so the extra cash was easy to spend :)



Offline bwaller

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #14 on: April 19, 2008, 04:12:09 AM »
Thanks for the humorous read, nothing like a good chuckle before work!

I just wonder though if rat-bike status will continue or if in the future the need for a little weekend fun leads to engine rebuild, Dyna ignition goodies, oil cooler....... you know how contageous these things are.

If it happens and it usually does, don't keep track of the cost, it gets depressing!

ws6transam

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #15 on: April 24, 2008, 12:10:41 PM »
So, how does this look? Running good, or perhaps still a little lean??



Here's one of the spark plugs after going up two positions on the needle. It no longer stumbles, and fuel mileage is climbing, even with the added fuel! I still haven't gotten my #110 main jets in yet, so I've been staying away from WFO, as much as I'm tempted.

I filled up after putting 130 miles on the bike, and to my surprise, it only took 2.6 gallons. That's err, um... 50 miles per gallon, right on the nose!  4.7 liters per 100 KM, I reckon, for you progressive metriccentric Europeans...


Offline 750goes

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #16 on: April 24, 2008, 02:59:58 PM »
from that photo - I think it is definitely too lean.... should be a light tan on the plugs not white

my 2cents  :)

Offline tom8Toe

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #17 on: April 24, 2008, 03:10:41 PM »
It'll be hiding under all the rust and aluminum oxide :D

mystic_1

I have yet to find one. Any help would be greatly appreciated.  ;D
-Chris-

"A model built for comfort, really built with style Specialist tradition, mama, let me feast my eyes" - Led Zeppelin

Current:
2014 Triumph Scrambler

Past Rides:
'89 Kawasaki EX500 (Crashed)
'72 BSA A75 Rocket3 (Sold)
'65 Honda CB450 Black Bomber (Stolen, never recovered)
'89 Honda GB500 (Sold)
'75 Honda CB400F (Sold)
'08 Triumph Bonneville T-100 (Sold)
'14 Triumph Scrambler

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MrHellpop

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #18 on: May 09, 2009, 01:04:50 PM »
I think the key to the mystical $300 bike is to not live in California! You must live in an area that has long periods of 'unridable' weather, as there is a higher chance of people shuttering their bikes for the winter, only to find them 10 years later and realize they don't want it any more! Kids/wives are also a significant factor in someone getting rid of their project. I just got one here in Dallas, 4/29/09... 1977 CB750k for $300, no joke! The bike is in great shape overall, with the only rusting I've seen on the exhaust pipes and the chain. I think it's mostly vigilance though. I have checked craigslist obsessively off and on for the last 3 years, and finally found one that fit the bill! I think the guy really wanted to sell it to someone who really wanted to buy it and ride it over someone who wanted it just for parts. Lucky for me!

Offline MJL

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Re: The CB750F RAT BIKE project: Ratty old daily driver!
« Reply #19 on: May 10, 2009, 05:56:52 PM »
Where can I pick up one of these magic $300 bikes?
I found one on CG, in Muncie IIRC. I was going to go get it that weekend, but the guy said he had another feller coming to look at it, and I didn't want to drive down if it was going to be already sold.


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