Carb HIstory Question

Daegan

Member
I have seached a lot lately to find out more information on original carbs. I am not sure what a "533a" carb is specifically, what else there was out there originally by year/model, what exactly I have, or what is a good replacement carb/Jetting for my year and model. According to DrAtv:

THE STOCK SIZE JET FOR THE Z50 HARDTAILS IS 50 JET, MAIN (#50)

THE STOCK MAIN JET FOR THE 1970 CT70K0 AUTO IS (#62)

THE STOCK MAIN JET FOR THE CT70HKO IS #60

THE STOCK MAIN JET FOR THE CT70HK1 IS #58

THE STOCK MAIN JET FOR THE CT70K1 TO 1977 IS # 58

THE STOCK MAIN JET FOR THE CT90K0 AND CM91'S WERE #72

THE STOCK MAIN JET FOR A 1978/79 CT70 JET, MAIN (#55) 99101-116-0550

The way I see it, if I put an after market carb that is for "KO-78" on my bike then I have some jet size considerations to make in addition to the air boot into the body remaining as well. Can anyone out there elaborate on what should have come (Carb part Number) on an HK1 standard? What is the history of these carbs? Also, all other things considered equal; when replacing with an after market carb, should not those factors of jet size still be relevant?

This is what I have:

Red HK1 original carb with a #58 jet dialed in and runs great

Yellow HK1 with an (Identical looking) after market carb (very clean), #58 Main jet, idles fine, carb was cleaned by following a tutorial on this forum, smooth/consistant throddle, but very little top end like it just doesn' want to "open up". This bike has plenty of torque but I do not know the compression (I know:buy a tester). Very low miles on it and the head was rebuilt a few years ago as it was frozen in hibernation back then. Valves are set correctly, new points/timed. Have adjusted the throddle needle back to the middle notch. Cam timing was checked and perfect. Spark plug is great/new/gapped. The advance on the flywheel appears to be functional and no different than the other bike. Runs well, just no balls. Am I missing something like a cable adjustment or some other item? I have read about air flow being different and particular for some carbs.

I am in the high desert of Arizona, in the pines at about 5,500 ft. Temps are moderate with 70 degrees being the time of day I attempting adjustment/diagnosis.

I suppose I could swap carbs and see if that is the difference but that still leaves me with what I do with this after market carb to make it perform if that is the case. Thanks for any info or suggestions.
 

hornetgod

Well-Known Member
533A is the correct carb for an HK1. The 58 jet is correct too but, I doubt you would feel much of a difference in a 60 main jet.

I suggest swapping carbs between your HK1's first before going further.
 

69ST

Well-Known Member
Without more information, that is unlikely to ever see the light of day, comparing jet sizes between different model number carbs may not be valid. It's not only possible for two different stock carburetors to deliver the same air:fuel ratio with different size main jets, it's also possible that the one with the smaller main could actually deliver more fuel and run richer because of it.

The combination of jet needle and needle jet profiles can have a huge impact on how much fuel flows through the main jet. Airbleed orifice sizing also has a big impact. These topics always were arcane, even back in the day. Circa 2014, they're closer to "black arts". When in doubt, source the rebuild kit/parts listed for your specific model/year carburetor and you should be good-to-go.
 

Enginedoctor

Well-Known Member
daegan if the carb is clean, i think you should move the needle clip to the top position, then the bottom. see which one improves the top end the most. if it likes it lean, get a smaller jet, if it likes it richer, get a bigger one.

it just seems the motor (though we don't know compression) is either starving for fuel on the top end, or is getting too much to burn off and really get into the power.

The above posts are good advice, per usual, and bob brings up an excellent point about the 2 different carbs most likely flowing different rates with the same jet. there's no way to know with the aftermarket casting vs/ the stock
 

cjpayne

Well-Known Member
I had a AT7B carb(not sure what model CT70 it was from) that had a #35 pilot and #65 main. I tried the #65 in my 533A(#58main) and it went "pig rich" real quick. It took off quicker, but lost alot of topend.
 

Daegan

Member
Without more information, that is unlikely to ever see the light of day, comparing jet sizes between different model number carbs may not be valid. It's not only possible for two different stock carburetors to deliver the same air:fuel ratio with different size main jets, it's also possible that the one with the smaller main could actually deliver more fuel and run richer because of it.

The combination of jet needle and needle jet profiles can have a huge impact on how much fuel flows through the main jet. Airbleed orifice sizing also has a big impact. These topics always were arcane, even back in the day. Circa 2014, they're closer to "black arts". When in doubt, source the rebuild kit/parts listed for your specific model/year carburetor and you should be good-to-go.

That is exactly what I am getting at. It seems to me, that just because I have an after market carb that looks the same as the original on my other HK1 does not necessarily mean that it is jetted correctly. Enginedoctor, I did move the needle up and down and did find a sweet spot but it was still lacking the power. I then adjusted the cable at the cap thinking that perhaps I am not getting full throttle. Well I got full throttle it seems but the umph is still not there. I suppose I should mover the needle up and down with the cable adjusted out too, just to rule all that out. I am at 5,500 ft but that doesn't seem to make a difference with my other bike with a #58 main.

I think I remember Old CT having different performance with and without an air boot or something. It seemed to make a difference there. Any thoughts on that?

Short of that I don't know. Don't even know if I should go up or down if I was to try a different main jet. I am getting the original carb back in about 3 weeks and I plan on doing some serious cleaning/ultrasonic/rebuild on that. Till then I always have the ever growing urge to swap the carbs just to make sure I have the issues of lacking power isolated. They have different length cables and different caps though and those could be potential issues in the first place at this point.

Thank you guys for all the information and suggestions. Please let me know if you can elaborate anymore with my additional comments. Much appreciated.
 

Enginedoctor

Well-Known Member
additional elaboration:

Okay Daegan glad you put some more tidbits in there. the 5500 feet really helps my diag. and i can shed some light on the air boot question, as well.

If your air filter is clogged, or causing a measurable restriction, it will richen your mixture, and of course, prevent that 'full throttle' feeling. plain and simple, if the cylinder can't get full air flow at lets say, 6000 rpm, the cylinder pressure won't ever be at it's max, and it might not get into those higher RPM's under any type of load whatsoever. If you remove the air filter, and whip the bike in a dust free environment, you can tell immediately if this is your problem. if the problem goes away, the air cleaner represents a serious restriction. if not, you can probably count it out.

Okay now onto your altitude. recently, i myself have gone from sea level (melrose ma, north of boston by about 10 min), to 7000 feet here in park city, then back to sea level (literally the beach in baja mexico), AND then back to 7k feet in PC. my prized possession (sorry ducati) ct70h came with me the whole way. the difference of a mile in altitude is astounding, in more ways than one. this is why i love EFI so much, and loathe the old fuel leak, unless of course it has an adjustable main jet, which motorcycle carbs don't (aside from the needle, but i'm not counting that… trying to make a point here)..

oh and that point is, the higher up you go, the less O2 there is around you. sure, same goes for N2 and other trace gasses, but they're inert. O2 is the one we care about. You can tell when you hike up a hill or climb 4 flights of stairs and find yourself feeling like you scaled everest without the luxure of a sherpa. I've been told for every 1000 ft elevation, you lose 3% power on your internal combustion, naturally aspirated engine. Owning a ducati that will pick up the front end in 2nd down in SLC, but not here in PC (2k feet higher), i believe it. what's more, is it changes your A/F ratio noticeably. With an 80 main jet in my little bike, i cruised the beaches of mexico pulling 60+ mph without an issue. even went 2 up a bunch of times, and one time pulling a surfer up onto a board. surprised even me. Now back to PC. the bike won't hit the upper RPM range, where the meat is, on a small motor like this, because it's delivering WAYY too much fuel. the needle dropped to lowest position, the bike runs, and hits high rpms, but still needs a smaller main jet. in my experience, probably a 76. And even with the perfect air fuel ratio, the bike will struggle to hit 55 up here. it is what it is.

But, more importantly, back to AIR FUEL RATIO. i would guess you need to go leaner. your old carb might have a slight restriction, due to it's arthritic age, and the 58 might be more like a 57 or 56… these numbers are arbitrary, though, not really 'Quantitative', siting that different carbs flow different rates.

My advice to you, put in a leaner jet (5500 is no molehill), see how it runs then play with the needle. I STILL think you should do the compression test, speaking of Quantitative numbers, you can compare pounds of pressure per square inch from one 72cc cylinder to the other. if the numbers don't jive, you're searching for a ghost.


Hope that sheds some light. high means lean, low means rich, in terms of how you should jet anyways. keep in mind, if you jet lean then go to death valley, you NEED to go richer on your jet. rich motors won't hit the high RPMS as efficiently, but rich runs cool. Lean kills engines, and in extreme conditions, overheats and causes a thing called 'detonation' or 'pre ignition'… not exactly the same thing, but we're not gonna get into that today, kiddies… just remember lean is bad.


OH yeah, almost forgot. take the carb off (keep the cable on) and twist like you're late for a date… see if the throttle piston disappears completely out of the carb's bore. if it does, the cable length is good. if it doesn't it needs an adjustment, or worst case scenario another cable or carb cap. When you do get either of these parts, make sure the carb piston drops all the way to the throttle speed adjustment screw, or you'll get a 2000rpm idle.
 
Last edited:

Daegan

Member
Enginedoctor: Thank you. That is very specific and detailed information given for my situation. It is going to take me a while to perform all those tests but I am sure that the solution is in there somewhere. First thing I am going to do is test the cable like you suggested and then try it without the air cleaner. There is another "boot" though that goes into the body right? Could the other boot being present or absent limit the performance? Getting the original carb and cap back will answer some questions I hope and with a good cleaning and rebuild, maybe eliminate all issues with performance. Your insight and personal experience in fluctuating elevations is much appreciated and really helps bring it home as far as understanding what is really going on. When and if I have to go there I will look into smaller jets and adjustment.

Compression test? Yeah, I'm ashamed to say that I went to the auto parts store today to get cotter pins and was so close to pulling the trigger on the compression tester but hesitated for a second, and then the feeling was gone. They had one for $49 bucks with the adapter and one for $25 bucks but I would have to buy an adapter. I will go tomorrow and get it for sure this time.

Thanks again-
 

Enginedoctor

Well-Known Member
daegan the 10mm spark plug adapter is a rare one… most cars don't use a 10mm(X1.0mm) thread. i had to buy mine from 'cornwell'.. a major tool retailer. the tester will likely be for a 14mm thread.
 
Last edited:
Top