Results 1 to 9 of 9
  1. #1
    CB750 Member ReedNTX's Avatar
    Join Date
    Oct 2017
    Location
    Texas
    Posts
    5

    What Have I gotten Myself Into?

    Found a good deal on a Copart auction. Never built a bike before, but have built several aircooled and watercooled VW's.

    1978 CB750F it appears. I looked at some parts diagrams and it looks like I am missing the generator and cover (and several other pieces!).
    Attached Thumbnails Attached Thumbnails c502d635-71a2-469b-8acb-9b2c08973701.JPG   95d81834-446b-4716-88f5-4c65f08dacf2.JPG   1fbcc717-2eee-4e00-ac71-198f14fae7e6.JPG  

  2. #2
    Moderator dirtdigger's Avatar
    Join Date
    Feb 2012
    Location
    Black Hawk SD
    Posts
    2,526
    I would be afraid of what got in the motor while the cover has been off for who knows how long.

  3. #3
    CB750 Member ReedNTX's Avatar
    Join Date
    Oct 2017
    Location
    Texas
    Posts
    5
    Well, it starts and somewhat runs.

    I ended up putting on a battery, stator, exciter coil, stator cover, mechanical advance, points plate, and a set of points on it.

    I ran some temporary wiring in preparation for building a new harness, tapped the oil galley plug for a pressure gauge to verify oil pressure, and got it to start, rev, and poorly idle.

    Had gas leaking from 2 of the overflow tubes so I pulled the carbs today. I'm amazed the engine ran with the shape the carbs were in. Main jets were 2-3 turns out from being tight, slows were loose on 2 carbs, and one carb was missing the bowl gasket. One of the main jets looks to have been drilled out- they are all labeled as 110's, but this one is visibly bigger when held to the light.

  4. #4
    Moderator dirtdigger's Avatar
    Join Date
    Feb 2012
    Location
    Black Hawk SD
    Posts
    2,526
    Go to Jets R Us and get some new jets, they are inexpensive and the only way you will get accurate jet sizes. If they have been tampered with throw them away. Clean the carbs good and when you get it running again you have to do a running carb sync, the way it sounds this thing has been butchered and cobbled together so I guarantee the sync is screwed up.

  5. #5
    CB750 Member ReedNTX's Avatar
    Join Date
    Oct 2017
    Location
    Texas
    Posts
    5
    Quote Originally Posted by dirtdigger View Post
    Go to Jets R Us and get some new jets, they are inexpensive and the only way you will get accurate jet sizes. If they have been tampered with throw them away. Clean the carbs good and when you get it running again you have to do a running carb sync, the way it sounds this thing has been butchered and cobbled together so I guarantee the sync is screwed up.
    I'm assuming synch is similar to what I've done on cars? Start with a mechanical bench synch making sure butterfly openings match, then syncing vacuum once they are running?

    My carbs are 657A castings, rather than the PD43A that should have come on the bike originally. Engine case #'s identify it as a 78F.

    Should I go with the 110 mains and 40 slows that the carbs call for, or the 108/38 setup that 78 calls for? My initial thought is that the 78's ran a touch smaller because of the additional fuel provided by the accelerator pump.

    I'll be running a stock airbox.

  6. #6
    Moderator dirtdigger's Avatar
    Join Date
    Feb 2012
    Location
    Black Hawk SD
    Posts
    2,526
    Yes you will need 4 individual vacuum gauges. If you are the 4 into 1 I would start with one size bigger on the main jets and stock on the idle jet size. You still may have to change jetting depending on how it runs. Dont know if there are any internal mods...cam or port work, etc. Yes the PD carbs did run leaner, emissions became stricter, the accel pump was used to help cover up the lean condition but they were still too lean and run better one size bigger.

  7. #7
    CB750 Member ReedNTX's Avatar
    Join Date
    Oct 2017
    Location
    Texas
    Posts
    5
    Been a while since I've been on here.

    Rebuilt the carbs with all new jets- bike now starts after barely tapping the starter, and idles and revs beautifully with no leaks!

    Started with 110s and 40s, should hopefully get me by for a while.

    The factory seatpan was wasted. It was missing a bunch of metal due to rust, corrosion, and some previous damage. I sourced a fiberglass seat and tried my hand at upholstering it. It turned out so-so. It only cost me about $15 in materials, so if I get tired of it I am not out too much.

    Rear shocks have been switched to 11.25" units to accommodate my short legs.

    Missing speedo was replaced with an earlier, green-faced model, because I like the color.

    Tank is repaired and repainted.

    Upper tree has been stripped, sanded, and polished. Bar has been replaced with chrome clubmans sitting on 30mm risers to spare my back. Levers have been switched to adjustables.

    Headlight bucket is now a bronze and black combo from 4into1.

    Clutch was stuck from sitting. Pulled it out and friction thickness measured good, so I cleaned it up and put it back in.

    Still need to finish painting the seat frame and side covers (got those from Frankenbike's store).

    Have a new DID chain that needs to be put on.

    Waiting for some turn signals to arrive in the mail this weekend, then it's on to finally building the harness.

  8. #8
    Moderator dirtdigger's Avatar
    Join Date
    Feb 2012
    Location
    Black Hawk SD
    Posts
    2,526
    Give the jetting a try, think you will have to bump the mains up too at least 115's once you get to ride it and get it under a good load. With your elevation and pods and exhaust I think its going to be lean.

  9. #9
    CB750 Member ReedNTX's Avatar
    Join Date
    Oct 2017
    Location
    Texas
    Posts
    5
    Quote Originally Posted by dirtdigger View Post
    Give the jetting a try, think you will have to bump the mains up too at least 115's once you get to ride it and get it under a good load. With your elevation and pods and exhaust I think its going to be lean.
    I ditched the pods in favor of the factory airbox.

Posting Permissions

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