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  1. #1
    CB750 Member theredbarn's Avatar
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    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!


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  2. #2
    CB750 Guru amc49's Avatar
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    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
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    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
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    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
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    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
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    the slide pistons are fine it was the jet holders the needles insert into... check these out.
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  7. #7
    CB750 Guru amc49's Avatar
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    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
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    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
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    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
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    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
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    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
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    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.

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