What carb is this? And any tuning advice?

Discussion in 'Tech Area' started by Halfast, Nov 8, 2018.

  1. Halfast

    Halfast New Member

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    Hey guys, I recently picked up a 81 c70 that had this carb on it, no markings, jets have no markings and I'm hoping to get some baseline settings, ie A/F turns, needle/idle turns and float height settings and if anyone knows what the jet sizes that come with this carb, that'd be great.

    The little cub has been neglected for many years, apparently, but I got it running for about a minute, but it took many kicks, was choppy and rough and the plug seemed fuel fouled after it's brief run.

    Thanks in advance.
     

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  3. racerx

    racerx Administrator
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    Looks like an OEM Keihin of the correct vintage, very likely original to the bike.

    Break out the magnification. You should see a number stamped into the end of the main jet. It's screwed into the emulsion tube, the two-piece brass assembly closest to the carb in your photo. The other brass piece is the pilot (idle jet). They appear clean and may be but, appearances can be very deceiving. A lot can happen in 37 years time. The orifices in the wall of the emulsion tube along with the pilot jet orifices are minuscule and very easily clogged. They're not always easily cleaned, so be forewarned...chemical cleaning alone, oftentimes, won't do the job. Run a thin wire through every orifice in the emulsion tube. A twist tie, such as used with bread loaves, is a reliable & free source of such wire. Unscrew the main jet from the emulsion tube to allow cleaning access end-to-end. The pilot jet is going to be tougher to clean; it's orifice diameter is even smaller...which also makes it more susceptible to blockage.

    Set the the pilot airbleed screw ~1.5 turns out from seated. Clockwise, toward seated, richens the pilot (idle) mixture, counterclockwise makes it leaner. The usable range, from seated, is ~1/2 - 2-1/2 turns. For the initial curb idle setting, best you can do is go by eye. With the carb on your bench, look into the venturi and note where the slide just starts to move, then give it ~1 turn further open. Those settings should get you going. The engine needs to be warmed up-to-temp and running to dial-in the idle settings.

    Keep an eye on the petcock assembly. The packing (internal sealing washer) is probably kinda crunchy, thus there's a good chance that you'll end up with a fuel leak.

    This carb is probably rebuildable. Rebuild parts and an exploded diagram can be found here.
     
  4. wanrep

    wanrep Active Member

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    Most of the Chinese aftermarket carbs I've seen don't have any markings on the body or the jets.
    Didn't the C70 use a choke cable? That one doesn't look like it's made for a cable.
     
    #3 wanrep, Nov 8, 2018
    Last edited: Nov 8, 2018
  5. Halfast

    Halfast New Member

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    Thanks racerx, I got it running, it's still going to need some tuning, as it's still rich at 2 and 1/4 turns with the choke off. I may need to get a smaller main.

    And yes, the C70 does use a chock cable, this carb has a choke lever on the carb. And there are definitely no sizes on those jets, plus this carb looks practically brand new compared to the rest of the bike, so I'm thinking a Chinese aftermarket as well. Wish I knew the jet sizes.

    Thanks again.
     
  6. racerx

    racerx Administrator
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    That casts things in a different light. IMHO, kind of a toss-up between new OEM, new reproduction and taking a chance on some new Keihin jets. Some Chinese knockoffs are close to originals, close enough that they use OEM parts...like gaskets, seals and jets. With no numbers on either jet, you'll have to make some educated guesses and buy multiple jets to try. They're not expensive but, when you have to buy multiples, in different sizes, it can add-up pretty quickly. Three of each, for example, might exceed the price of a cheapie PZ19 carb...or the half the cost of a new, gen-yoooo-wine Keihin that's plug & play, i.e. direct, service, replacement.

    Starting without the choke and idling too rich, no matter how the pilot airbleed is adjusted definitely screams "oversized pilot jet". But, does this overrich condition go away as throttle opening exceeds ~1/8? Will the engine rev-out cleanly, under WOT? Jetting & tuning the carb involves answering these questions. For now, just for shits-n-grins, try setting the pilot airbleed 3-1/2 turns out. It probably won't solve the problem but, you never know. If you at least see some improvement, you'll have a little bit more light shining on the situation.
     
  7. Halfast

    Halfast New Member

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    You know, that's something that wouldn't have occurred to me until after I had spent the money on the jets. I could see it being one of those, "I should've just bought a carb" moments.

    I did put it out to 3-1/2 turns and that is where it is now, it's mildly better and revs a bit more more smoothly, up to wot, but still way rich according to the plug chop. Since I'm compiling a want and need list for this little bike, I'll figure out whether I want o abuse myself with multiple jets or get a plug and play when I've completed my list. And thanks for the help and taking the time to reply!
     
  8. racerx

    racerx Administrator
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    That's what I thought...wanna know why?;)

    Plug chops aren't as useful as they once were. Pump gas has changed too much, most race fuels aren't much better in this regard...some aren't even gasoline(!). IMO, the best method for sizing the main jet is by finding the rich limit. IOW, the biggest main that still allows the engine to rev-out cleanly at WOT. That will get you to within one jet size, split between choosing the larger jet paired with a leaner jet needle setting or the smaller jet with a richer jet needle setting. This method typically delivers air:fuel ratios, at WOT, in the mid-12s...and nearly as accurately as using a wideband O2 sensor & meter. That's virtually ideal, providing strong acceleration/throttle response, controlling peak combustion chamber temps and giving you a bit of safety factor for that tankful of bad fuel.

    FWIW, sounds like you're within 1-2 jet sizes, with both. If only you knew what is in there, at present...:cautious:
     
  9. ctbale

    ctbale Member

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    Its very similar to this one.

    https://www.ebay.com/itm/Honda-pz-1...mm-LH-choke-carburetor-DAX-Lifan/163354827301

    I have installed a dozen of these carbs if not more. I have had great luck with them. The main is a 75 and the pilot is a 35

    for a 70cc I go down to a 72, but I am cold and at sea level. 90cc the 75 is pretty good. had to go up to a 78 on some.

    if your anything above sea level a 70 main would be a good place to start. I typically have to file the black insulator to make a smooth intake port.

    jets r us sells jets for them.
     
  10. Halfast

    Halfast New Member

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    Great, thanks for that, that gives me a solid baseline. Do you leave the pilot at 35? I'm at sea level as well.
     
  11. Halfast

    Halfast New Member

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    Well hey, thanks for sharing the wisdom you've gained from experience! And interesting tuning write up vs. chops. Something I'll have to consider.
     
  12. ctbale

    ctbale Member

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    I have never needed to change the 35 slow jet, can be fine tuned with the slow mixture screw. I always go as fat as possible on the main. I go up until the engine "burbles" at WOT. Then go back down to the next smaller main, then play with the needle clip, then dial in the mixture screw. Think you would be pretty safe with a 70

    PS we dont have any corn in our gas in alaska so thats nice.

    pure-gas.org for corn free gas
     
    #11 ctbale, Nov 9, 2018
    Last edited: Nov 9, 2018
  13. racerx

    racerx Administrator
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    That pretty well describes the main jetting process, in fewer words than I can...talk about setting the bar really low:ROFLMAO:

    FYI...don't get too complacent about gasoline. About 3 weeks ago E15 was green-lighted via royal fiat...errrrrr..."executive order":devilish:
     
  14. kirrbby

    kirrbby Well-Known Member

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    Maybe he'll use some to light himself on fire :)
     
  15. ctbale

    ctbale Member

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    Hate ethanol! People who think its a good thing .... clueless!
     
  16. racerx

    racerx Administrator
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    Ethanol can be a good fuel. It's high-octane and clean burning. E85 can be used as cheap race fuel.

    But a good thing...only if one has compensated for the tradeoffs... short life, increased rate of consumption. The worst of it is the way it attacks neoprene rubber parts, that have not been remade using different polymers, along with rust formation, due to alcohol's affinity for water. That's where most of us are varying degrees of screwed. The rules keep changing, after the fact. Owners of other bikes, with more complicated fuel systems, chain saws, boats and cars have it a lot worse.

    FWIW, I've been dealing with E10 for better than a decade now. Parts-wise, it's mainly been more frequent fuel line replacements, petcock packings and an O-ring, or two. Performance-wise, it's mainly been fewer mpg...until this year. Now, it's intermittent power reduction coupled with fewer mpg and increased peak oil temps. I wouldn't be a bit surprised to find out that E15 is already out there. And E15 is a bridge too far...
     

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