Results 1 to 16 of 16
  1. #1
    CB750 Member theredbarn's Avatar
    Join Date
    May 2020
    Location
    Kingston, WA, USA
    Posts
    7

    My son brought home a toy for me to play with...

    I'm going to start riding again! Gave up my last bike after my son was born - 26 yrs later he brings this 79 CB750L home so I can have something to play with! (Mostly because he want's me to ride with him!) Been sitting a lot of it's life, only 4K miles on the odo. I have the Carbs off and am giving them a thorough clean and rebuild. Started and idled but RPMs would not settle if revved. I found 2 of the CV pistons stuck in the closed position. PO was starting to do ready work on it before his Mrs. told him it had to go,(oil change, new tires, forks rebuilt). Lots more to do, I'll be around to ask questions!


    Click image for larger version. 

Name:	IMG_0478m.jpg 
Views:	18 
Size:	312.8 KB 
ID:	9378

  2. #2
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,377
    SET THE VALVES and usually never done. What makes them scrap. Recognize that most people have to clean the carbs 3 times to get them right. The idle off/idle circuits have a weird seesaw action that makes them problematic. The air cuts do even more and get rid of them. High hanging idle is usually vacuum leak like the forward rubber manifolds. They crack to leak with time and heat.

    The chain tensioner bolts and valve cover bolts strip out easy as spit.

  3. #3
    CB750 Addict Medyo Bastos's Avatar
    Join Date
    Oct 2012
    Location
    Mid South
    Posts
    205
    Geez I was afraid to click on your post! What a great toy!


    Sent from my iPhone using Tapatalk

  4. #4
    CB750 Member theredbarn's Avatar
    Join Date
    May 2020
    Location
    Kingston, WA, USA
    Posts
    7
    The stuck pistons in the carbs were due to ham fisted-ness of the PO. The main jets in both look like a bent piece of macaroni and the needle jams when they seat. I came across Oldskoolcarbs on the facebook page. I think I may just send them to him and get about cleaning and detailing/ wiring checks etc. on the rest of the bike. I want to get a chain too - no telling how old the one is on there. Could be original equipment for all I know...

  5. #5
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,377
    The slide pistons match to the caps and if forced they may well be scrap. The slides being delicate as hell. The needles WILL jam if the slide and carb body are not level at install and they are supposed to flop around loose. If you have any contact wear at the bigger top ring-grooved OD of the slides they are probably again junk, they should NEVER touch, they are separated in use by differential air pressure. You absolutely cannot sand on the big OD either, only lower down on the main body that wears to snag sometimes in the lower carb bore.

  6. #6
    CB750 Member theredbarn's Avatar
    Join Date
    May 2020
    Location
    Kingston, WA, USA
    Posts
    7
    the slide pistons are fine it was the jet holders the needles insert into... check these out.
    Click image for larger version. 

Name:	IMG_0493.jpg 
Views:	9 
Size:	174.9 KB 
ID:	9379

  7. #7
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,377
    I see one possible wrong part (or three), other than that you are not showing the important parts there. If the needle jet (jet holder) is too tight to stick the needle the needle may be damaged too.

  8. #8
    CB750 Member theredbarn's Avatar
    Join Date
    May 2020
    Location
    Kingston, WA, USA
    Posts
    7
    The 2 in the middle were bent enough out of shape to grab the needle.

    I have a full kit with new needles coming. What a bitch to order parts. Everywhere I looked asked or a year and model but had the wrong kit! I finally found a reference that showed what years/models got what carbs and was able to order a correct kit by looking for the 81 KC model... Anyway, I'm a weber guy (I do VWs) and have jet measuring tools as well as a set of reams so I'll go through the existing jets and measure reuse as required.

  9. #9
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,377
    I see the slight bulge on one side now. There is no KC model, it's either a K or a C. For an L I would use K parts. Regardless, all the carbs are closer than the specs seem to show, you can even run 900 carbs, the CV feature allows it. Biggest thing is getting the screw in pilots instead of pressed in.

    One of the parts picced does not seem to have the same hole pattern, roll them around and compare all 4 as to the holes, they must match. If too small a hole for needle it will stick even if straight. That carb type was used in plenty of models with small parts changes everywhere.

  10. #10
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,377
    Hey, check out the steel piece that installs over the parts there, it can be easily installed upside down to damage a jet holder like that. One end is slightly smaller than the other and the convex end fits against the jet holder.

  11. #11
    CB750 Member theredbarn's Avatar
    Join Date
    May 2020
    Location
    Kingston, WA, USA
    Posts
    7
    The last of the carb parts I ordered came today... I'll post some pics of the freshly cleaned and assembled set soon! Oh, and valve clearances were spot on!

  12. #12
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,377
    Maybe not, if you used the service manual numbers. The manual allows for .002" as one end of the range and I can personally attest that that close can and will burn valves. I look at .002" as zero.

    Whatever number you get there subtract .002" from it as it is NOT real. Far better to use .004"-.006" while looking for .005" as an ideal. The engines using the lower service number of .003" can end up with nothing to burn valve before the 4000 miles to recheck them comes up. The .005" number can have the engine go 3X-5X longer before that happens.

    Why? The cam caps are looser than the valve clearance and the strong springs then push the cams around to make part of your perceived clearance as not real. Then you lose a little more when engine oil ramps up to center the cams somewhat in the wide clearance, that removes some of your valve clearance too. The valves are low quality steel with a thin heat treat, they were intended for low lead fuel, no lead is whoppingly different in wear and the valves then begin to recede to close up the clearances. Dead opposite of most engines.

    The OEM service manual spec on valve clearance is one big reason why so many of the bikes do not run well when you get one used. Or they run fine until the valves burn quick, mine did it in less than 3000 miles. Brand new bike, The next engine I loosened them up and it lasted forever.

  13. #13
    CB750 Member theredbarn's Avatar
    Join Date
    May 2020
    Location
    Kingston, WA, USA
    Posts
    7
    Running! Everything is back together! The bench balance I did was pretty good (used the guide on the Facebook page) - It idles OK and a blip of the throttle does essentially what it should. I have a carb sync on the way, should be here by middle of the next week. To clarify, valve clearance was fine. I couldn't pass a .003 or.006 feeler through so that puts me in fine area. Will prolly spend the weekend cleaning and detailing some more. Cheers!

  14. #14
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,377
    The 6 is fine but not passing a 3 says 2 and you WILL burn there. A 3 if solid (feeler MUST slip in) will go 4K miles likely but 4 would triple that. The routine adjusts are every 4k miles, my looser 4-6 setup will go 20K miles plus. Saves you yanking the cover every 5 minutes. Burnt valves and carb issues are the biggest killers of these bikes. Almost every one I ever took apart had at least one burning valve in it, most several.

    Yours and do as you will................ They never let you know until the damage is already done, it sneaks up on you in such a gradual loss of power you don't pick up on it until it's bad. Most of the time it begins as a funny little off note at cruise and often hard to pick it up. When you gas it up it will still run hard. But it's not. What makes so many people begin to chase the carbs. They never check compression which shows it instantly.
    Last edited by amc49; 07-18-2020 at 09:25 AM.

  15. #15
    CB750 Member theredbarn's Avatar
    Join Date
    May 2020
    Location
    Kingston, WA, USA
    Posts
    7
    I was confusing - the way I wrote that... a .003" slips easily so did a .004" and the .006" would not. So, no I'm nowhere near the thin side on clearance. I appreciate your warnings and concern.

  16. #16
    CB750 Guru amc49's Avatar
    Join Date
    Sep 2015
    Location
    Fort Worth TX.
    Posts
    3,377
    I understand.

    You of course do as well that I hate to see the bikes not run as well as they can as long as they can.

Similar Threads

  1. Piston to Cylinder Bore Play
    By Wez_ in forum Engine / Exhaust
    Replies: 5
    Last Post: 08-17-2017, 08:28 PM
  2. My new toy
    By snake89 in forum The DOHC Garage (1979-1983)
    Replies: 9
    Last Post: 07-04-2016, 09:28 PM
  3. Front sprocket play? Please help.
    By nsxguy7 in forum Engine / Exhaust
    Replies: 3
    Last Post: 07-02-2016, 11:28 PM
  4. Plug and play mono shock
    By Ridevana in forum The SOHC Garage (1969-1978)
    Replies: 1
    Last Post: 10-23-2014, 08:31 AM
  5. Hello from the home of the groundhog in PA
    By Carman24 in forum The CB750 Garage
    Replies: 4
    Last Post: 12-15-2012, 01:41 PM

Tags for this Thread

Posting Permissions

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