The “slingshot”

Collapse

Desktop Ad Forum Top

Collapse

Mobile ad top forum

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • KG
    Senior Member
    • Dec 2012
    • 221

    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:	1
Size:	96.5 KB
ID:	1346641

    Click image for larger version

Name:	5E961039-3198-4B36-AF83-E1D999C867D1.jpg
Views:	1
Size:	128.4 KB
ID:	1346642

    Click image for larger version

Name:	5CC3114E-83DC-4BAC-B833-4488E2BEB231.jpg
Views:	1
Size:	122.2 KB
ID:	1346643
    Last edited by KG; 04-27-2020, 4:26 PM.
  • markwade74
    Senior Member
    • Mar 2012
    • 293

    #2
    Didnt read anything but I like the pictures

    Comment

    • TriNortchopz
      Senior Member
      • Dec 2017
      • 3233

      #3
      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?
      If buildin' old school choppers was easy, anyone could do it... ain't nobody said it's gonna be easy...

      Comment

      • datadavid
        Senior Member
        • Oct 2014
        • 1022

        #4
        No fun having to fight crappy engines, hope it behaves now!

        Comment

        • KG
          Senior Member
          • Dec 2012
          • 221

          #5
          Originally posted by TriNortchopz
          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:	1
Size:	62.2 KB
ID:	1317873

          Comment

          • KG
            Senior Member
            • Dec 2012
            • 221

            #6
            Originally posted by datadavid
            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.

            Comment

            • KG
              Senior Member
              • Dec 2012
              • 221

              #7
              Originally posted by markwade74
              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!

              Comment

              • KG
                Senior Member
                • Dec 2012
                • 221

                #8
                Click image for larger version

Name:	D9F8BE61-CF3D-4445-A973-BE7A94B0E355.jpg
Views:	1
Size:	94.9 KB
ID:	1317874

                The girder off the bike.

                Comment

                • KG
                  Senior Member
                  • Dec 2012
                  • 221

                  #9
                  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:	1317875

                  Click image for larger version

Name:	8FEB68C7-3F7A-4677-B224-34618926F45F.jpg
Views:	1
Size:	179.0 KB
ID:	1317876

                  Click image for larger version

Name:	9F1638B7-A1D2-4D23-A140-24BDC46EC292.jpg
Views:	1
Size:	164.3 KB
ID:	1317877

                  Click image for larger version

Name:	FCE3A787-2CFC-4A62-BAE0-133F2445F340.jpg
Views:	1
Size:	171.5 KB
ID:	1317878

                  How it was when I got it.

                  Click image for larger version

Name:	BF366EF5-FDB3-4602-8F9C-FDCA265099CE.jpg
Views:	1
Size:	161.2 KB
ID:	1317879

                  Comment

                  • KG
                    Senior Member
                    • Dec 2012
                    • 221

                    #10
                    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:	1318218

                    Comment

                    • Hatchet54
                      Senior Member
                      • Sep 2012
                      • 721

                      #11
                      Of course you've got a Scout. LOL

                      Comment

                      • KG
                        Senior Member
                        • Dec 2012
                        • 221

                        #12
                        Originally posted by Hatchet54
                        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.

                        Comment

                        • KG
                          Senior Member
                          • Dec 2012
                          • 221

                          #13
                          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:	1318727

                          Click image for larger version

Name:	31638A57-5464-4CD7-A2E2-BBF7765B9E3F.jpg
Views:	1
Size:	73.4 KB
ID:	1318728

                          Comment

                          • shank1kole
                            Senior Member
                            • Dec 2015
                            • 227

                            #14
                            Bad ass mo fo !

                            Comment

                            • farmall
                              Senior Member
                              • Apr 2013
                              • 9970

                              #15
                              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).

                              Comment

                              300 mobile ad bottom forum

                              Collapse
                              Working...