CHOP CULT HOME
Email Password
Search
  1. #1
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default The “slingshot”

    I built this 81 Honda CB750 from the ground up. I had an extensive build thread on another forum but it’s gone belly up.

    So here’s a basic run down.

    I got the stock bike in trade on a 5.0 swap into a Datsun pickup. Test fired and figured good enough to tear into it.

    First was to jig the frame cutting away everything but the neck. That way I retain the stock VIN # for DMV sake. The backbone is 1 1/2”x.120 wall 4130 cromolly. The rest is 1”x.120 wall DOM.

    I built the girder from scratch using brass/bronze bushings n washers I sourced from McMaster Carr. The fork is 1”x.120 DOM with thin wall trusses for rigidity. Took a bit of tuning to get the geometry and rake and trail where I wanted it. The spring was sourced from a 50r80cc quad. Wasn’t sure it would be enough but works great!

    Front wheel is a nordisk 21” off my 87 husqvarna 500xc. I also used its Bembro caliper and master. The rear wheel is the stocker off the CB, with its drum brake.

    The tins were take-offs from my sportster. The king and queen seat were fabbed with the idea of making a wide rear seat for the old lady to ride along. In hindsight I wish I had made it a bit more slim...

    I had it pretty well together n took it for a maiden voyage. It ended with me learning to fly and a seized engine. Long after the concussion wore off and my flesh grew back, I put another crank and pistons in it. I ran it in the garage a bit before a rod came apart n scored that crank! It sat for a year with me flat pissed off at it.

    I eventually stumbled into a cb900 shafty engine complete though without carbs. All the research I did pointed to the innards of the 900
    fitting in the chain drive 750 cases. They did! The 900 exhaust cam had some chunks out of the lobes(mabey pitting) and had been run that way. The 750 cam was very similar in spec but had more overlap.

    It wouldn’t run for crap with the 750 carbs so I built steel 1into2 or 2into4 manifolds and bought a set of mikuni vm36-4s. I got the 1to2 cable setup from cycleX. I wouldn’t recommend it as it’s been problematic and still causes it to hold at 2,000 often. Figuring the jetting was a nightmare eventually accomplished by a drill bit. It runs on the rich side at low rpm but great above 4,500. it will foil plugs if I ride it around much under 2,500. I literally just ordered a couple pair of 340 and 330 jets(I think I’m at 350ish currently).

    The exhaust is over 30 foot of 1 1/2” exhaust tube. I included some simple baffles and a small open chamber between the pairs. It’s only 1 1/2” long so probably don’t do a whole lot... the baffles neither, it loud!!!

    Well that’s roughly were it’s at.

    Click image for larger version. 

Name:	5C6F262C-5DC3-46B2-8BC7-730B130767D1.jpg 
Views:	29 
Size:	96.5 KB 
ID:	102722

    Click image for larger version. 

Name:	5E961039-3198-4B36-AF83-E1D999C867D1.jpg 
Views:	21 
Size:	128.4 KB 
ID:	102723

    Click image for larger version. 

Name:	5CC3114E-83DC-4BAC-B833-4488E2BEB231.jpg 
Views:	14 
Size:	122.2 KB 
ID:	102724
    Last edited by KG; 04-27-2020 at 4:26 PM.

  2. #2
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Mar 2012
    Posts
    272

    Default

    Didnt read anything but I like the pictures

  3. #3
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2017
    Posts
    2,537

    Default

    excellent, well done. Cool to know about the 900 guts fittin' in the 750 cases. what plans did you have to build that girder...still got 'em to share?

  4. #4
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Oct 2014
    Posts
    973

    Default

    No fun having to fight crappy engines, hope it behaves now!

  5. #5
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    Quote Originally Posted by TriNortchopz View Post
    excellent, well done. Cool to know about the 900 guts fittin' in the 750 cases. what plans did you have to build that girder...still got 'em to share?
    I didn’t have “plans” kinda made it up as I went. My neighbor offered to take my measurements and put them in his CAD to make templates for upper and lower trees. I still have those...
    I didn’t understand rake and trail so built it at 0. DOH!!! Ended up modifying it till I reached -4 1/2”. Was nearly unrideable at 0!!!

    Click image for larger version. 

Name:	B1306A90-FE63-4AAA-9B5E-BF2C68CAB89A.jpg 
Views:	3 
Size:	62.2 KB 
ID:	102737

  6. #6
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    Quote Originally Posted by datadavid View Post
    No fun having to fight crappy engines, hope it behaves now!
    So far so good. Have mabey 1,000 miles on it now. Only problem is the jetting n its no biggie if I keep it wound tight! Have more jets coming to.

  7. #7
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    Quote Originally Posted by markwade74 View Post
    Didnt read anything but I like the pictures
    Thank my buddy took them on a ride last weekend. The words are just jibber jabber anyhow!

  8. #8
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    Click image for larger version. 

Name:	D9F8BE61-CF3D-4445-A973-BE7A94B0E355.jpg 
Views:	5 
Size:	94.9 KB 
ID:	102738

    The girder off the bike.

  9. #9
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    A couple more pics I dug up.

    Click image for larger version. 

Name:	6243723C-62CF-4918-8DC2-5EBB60141EAD.jpg 
Views:	1 
Size:	127.7 KB 
ID:	102739

    Click image for larger version. 

Name:	8FEB68C7-3F7A-4677-B224-34618926F45F.jpg 
Views:	2 
Size:	179.0 KB 
ID:	102740

    Click image for larger version. 

Name:	9F1638B7-A1D2-4D23-A140-24BDC46EC292.jpg 
Views:	9 
Size:	164.3 KB 
ID:	102741

    Click image for larger version. 

Name:	FCE3A787-2CFC-4A62-BAE0-133F2445F340.jpg 
Views:	4 
Size:	171.5 KB 
ID:	102742

    How it was when I got it.

    Click image for larger version. 

Name:	BF366EF5-FDB3-4602-8F9C-FDCA265099CE.jpg 
Views:	4 
Size:	161.2 KB 
ID:	102743

  10. #10
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    Working on getting the mikuni vm36s dialed. Got a sync gauge, also ponied up for an afr gauge!
    Though I had I close but apparently not! Pulled carbs for a deep cleaning. Verified 35 pilot jets and matching needles are 6f6 and match. Got the idle circuit dialed in to roughly 13 on the afr. Revs great with a roll on. Falls on it face WOT. Pulled the 310 mains n swapped the 340s in. Very hard to look down to see the afr at WOT, a lot going on!!! So I moved the afr to the crossbar in handlebars. Need to go for a test ride to see what it reads now. Doing it right is time consuming and very frustrating!!! But it will all be worth it when I’m done!

    Click image for larger version. 

Name:	840EEA35-4F53-41DF-AA07-AED9CEB84189.jpg 
Views:	1 
Size:	112.7 KB 
ID:	103166

  11. #11
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Sep 2012
    Posts
    712

    Default

    Of course you've got a Scout. LOL

  12. #12
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    Quote Originally Posted by Hatchet54 View Post
    Of course you've got a Scout. LOL
    It was a body off resto project I was doing for a friend. It cruzing the Mendocino coast now.

  13. #13
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    Think I’m as close as I’m going to get with the jets. Current 330 mains with the needle clips maxed rich. Had an issue with the charging system. I’m betting it has to do with the extra draw of the afr gauge and the preheat for the o2sensor. 2 of the 3 yellow charge wires had overheated and shorted between each other. Burned another tank of fuel and it’s ready for more!!!

    Click image for larger version. 

Name:	AE545C5B-A915-4EC0-B6F6-DA12A788AF1A.jpg 
Views:	1 
Size:	63.7 KB 
ID:	103806

    Click image for larger version. 

Name:	31638A57-5464-4CD7-A2E2-BBF7765B9E3F.jpg 
Views:	1 
Size:	73.4 KB 
ID:	103807

  14. #14
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2015
    Posts
    209

    Default

    Bad ass mo fo !

  15. #15
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Apr 2013
    Posts
    9,145

    Default

    Draw plus high resistance at one or both of the splices melted the way between them. Inspect the full run of both wires for overheat and cut back some of the insulation as overheated wire is distinctive even when the insulation jacket survives.

    I'd replace both full lengths of wire to the O2 sensor with heavier gauge, and use bare crimp splices and dielectric grease to keep out corrosion (solder melts, crimps melt at much higher temps than solder, and bare crimp splices permit accurate inspection vs those auto store pieces of shit (and the worse cheap crimpers typically sold with them).

  16. #16
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    I’m ditching the afr meter and 02 ASAP. Mabey 1 more ride to verify readings.
    I soldered n heat shrinked them. I cut enough to offset the splice so they won’t be stacked. Found some other questionable connections and cleaned them up also. Cleaned and fresh dialectic grease in most/all the plastic plug connections.
    Definitely charging now!

  17. #17
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Apr 2013
    Posts
    9,145

    Default

    If it happens again look to the regulator/rectifier as those on second glance look like two phases from the alternator shorting together. A load downstream of that can do a variety of damage.

    Later high amp Shindengen (avoid fakes, get from a legit Jap bike supplier) are well regarded and often used to retrofit other machines. A bit of searching should get you some part numbers.

  18. #18
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    Quote Originally Posted by farmall View Post
    If it happens again look to the regulator/rectifier as those on second glance look like two phases from the alternator shorting together. A load downstream of that can do a variety of damage.

    Later high amp Shindengen (avoid fakes, get from a legit Jap bike supplier) are well regarded and often used to retrofit other machines. A bit of searching should get you some part numbers.
    I’ll look into it.
    Yes it’s 2 phases shorting together.

  19. #19
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Oct 2014
    Posts
    379

    Default

    Yeah, Farmall is spot on about the R/R- I don't touch anything electrical without swapping out for a modern Mosfet R/R.

    Good thinking doing the AFR. If the sensors are in a hidden spot, you might just want to leave them, but just disconnect them. It'll make any future TS quick and easy.

  20. #20
    Senior Member

    Content on this page requires a newer version of Adobe Flash Player.

    Get Adobe Flash player


    Join Date
    Dec 2012
    Posts
    222

    Default

    I have the sensor setup as a tailpipe probe. Constantly tuning for people so I ponied up to take the guess work out of it! I got a couple bungs but it won’t be on long enough to make it worth welding one on. Takes about 3 min to put it on or take it off.

Share This



Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Log in

Log in