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Post by katt on Sept 27, 2018 21:13:32 GMT -5
Why hunt "real 20mm" carb and pay like $50 when you can get legit 24mm carb for (125/150cc) for around $20 of amazon/ebay?
I want to install 24mm carb on 80cc QMB139 will this work?
From what i understand you cannot go wrong with bigger carb, air ratio will be proper even if theres bigger air flow trough carb engine only consumes as much air as it needs.
All have to adjust is the proper fuel flow (smaller jet for 80cc) to achieve correct ratio and then i dont even think there will be fuel overcompensation compared to my old carb.
Only think i know this 24mm dont have acceleration pump and 20mm carbs do have it, wonder if it makes much difference.
I already have jet kit for 150cc ordered by mistake lowest 3 main jet are 90-92-95. should be fine because i currently use about size 90 in my 17.5 carb for my 80cc.
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Post by eclark5483 on Sept 27, 2018 22:35:00 GMT -5
You'll lose vacuum in the venturi if you go too big. Bigger doesn't always mean better. Good rule of thumb on max jet size is mouth opening x 5... IE 17.5mmx5=87.5 or 88 main max. 24mm on an 80cc is too much IMHO.
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Post by katt on Sept 27, 2018 22:45:47 GMT -5
Meaning what exactly i heard installing zero resistance filter also loses vacuum and messes up carb operation however i tried without filter at all and it all works same.
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Post by gsx600racer on Sept 27, 2018 23:15:22 GMT -5
A larger carb will have a larger venturi. A larger venturi will cause less vacuum effect on the fuel circuits as well as the diaphragm circuit(especially @ 80cc's). You might be able to compensate some by playing with the jetting, but its not going to have the same response as a smaller bore carb. You're going to find with the larger carb will be sluggish in performance. The "bigger is better" mentality not going to pan out in this situation. I'd just save up your money and get a larger scooter.
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Post by katt on Sept 27, 2018 23:46:31 GMT -5
Vacuum supposed to only matter on idle speeds, higher RPM produce more suction and higher vacuum, currently i have low vacuum from having A9 cam shaft installed have problems starting.
My goal behind buying 24mm is to save money and also if i decide to swap later to 150cc engine i will need one anyway, since they come without carb.
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Post by eclark5483 on Sept 28, 2018 0:19:18 GMT -5
I'd guess valve lash is the problem, not vacuum. 24mm would be a huge mistake. Will explain more when I get home at work right now
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Post by gsx600racer on Sept 28, 2018 1:10:35 GMT -5
So your going to buy a bigger carb and making the problem you have worse ?
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Post by katt on Sept 28, 2018 1:26:10 GMT -5
Its not that bad its just takes few extra seconds to start when cold.
Like it starts as usual, but then engine dies in a second, just have to keep starter spinning past that moment when it dies and then it fires up okay.
Valve lash is within spec i checked with feller gauge.
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Post by eclark5483 on Sept 28, 2018 1:49:10 GMT -5
WOW!! You think things will get better with a bigger one? And if you have removed the filter and got the same results, you are not properly jetted. You say you have a 90 in your 17mm carb? That is TOO MUCH fuel. No, not too much for the piston, it is too much for the carb. Again, as I mentioned before, a good rule of thumb here on the jet, is 5xmouth opening. Reference: www.tuningmatters.com/how-tos/how-to-quickly-determine-the-carburetor-main-jet-size/As for the filter you are using, guess I need to copy and paste what I explained to another guy: I've always used a vacuum cleaner analogy to explain how the back pressure gives you more power (especially at launch). My explanation is think of different vacuum cleaner attachments and how much suction they have, the wide attachment has less suction then the corner attachment. Another way to observe this, is make a big "O" with your lips, get a piece of tissue and suck in, try to hold the tissue up with just suction. Now do it again with a tighter "O". Notice how a smaller "O" will produce more suction. If you try holding the tissue with a huge "O", you run out of lung power real quick, and it takes more effort, but a smaller "O", you can hold the sheet of tissue for quite some time. You can think of the air filter tweaking the same way. You look for the right amount of backpressure (resistance), which will in turn create a better suction. Remember, a carb does not push gas/air into the piston, suction/vacuum does. And when you twist back the throttle, you are not giving it more gas, you are in fact, giving it more air. When you can find a tighter vacuum without restricting flow too much, you'll be able to pull in more fuel from the bowl which will mix with the air in the venturi. Getting this just right will actually improve fuel efficiency as well. If the engine doesn't have to work as hard to pull fuel from the bowl, you won't need a huge main jet to compensate. I think this is probably the main mistake people make when trying to jet. They'll read the plug and think.. well, I'm running lean, better feed it a bigger jet, when really what they need to focus on, is how well the fuel is being delivered, not so much how much. Hope this helps. For an 80cc (72cc) displacement on a 139QMB, in order to use a #90 jet properly, you also need the proper carb. 20mmx5=#100 main max. My personal recommendation for a BBK'd 139QMB engine, would actually be an 18.5-19.5mm carb. I would lean more towards the smaller 18.5mm one for best performance. Or 18.5mm x 5 = 92.5 jet max, which would be sufficient for your bike and provide you with the best vacuum for the intake opening.
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Post by katt on Sept 28, 2018 2:25:08 GMT -5
Right now i have 17.5carb x 5 = 87.5 i really doubt 2.5 sizes up from that (#90) makes much difference, i have used #98 main jet and notice no difference at all.
Well fuel economy not my main goal at moment vacuum sucking more fuel makes sense though, but i might swap engines in near future anyhow so i keep that carb handy.
I have vacuum gauge i can test the vacuum pressure on idle and on high RPM do you know whats the proper vacuum range is?
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Post by eclark5483 on Sept 28, 2018 2:30:22 GMT -5
Yep, that 87.5 would be the limits of the carb. I'd go no higher than an 88 on that carb, and that's with using a more open air filter. With a regular/properly tuned filter, you are looking at say an 85-87 jet for proper tuning.. BTW, the math involved also works on your pilot as well, but instead, you multiply by 2 to get the pilot. So in your case 17.5mmx2=#35 pilot, which is probably where you wanna be at.
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Post by katt on Sept 28, 2018 2:33:58 GMT -5
Yep, that 87.5 would be the limits of the carb. I'd go no higher than an 88 on that carb, and that's with using a more open air filter. With a regular/properly tuned filter, you are looking at say an 85-87 jet for proper tuning.. BTW, the math involved also works on your pilot as well, but instead, you multiply by 2 to get the pilot. So in your case 17.5mmx2=#35 pilot, which is probably where you wanna be at.
Right now i have size 42 pilot and its not enough will need to swap to 45 could be due to low vacuum at idle.
This is how i adjusted idle only using mixture screw and having butterfly in same position to cover progression ports.
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Post by eclark5483 on Sept 28, 2018 2:44:43 GMT -5
You have some serious issues going on if a 42 isn't enough. I use a #35 on my kid's 80cc 139QMB, and her's will pop right off in under a second. There is no way I'd go anywhere near a 38 or higher. Seriously think you need to retrain your thinking that throwing more fuel at it is going to fix the problem.
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Post by katt on Sept 28, 2018 2:49:16 GMT -5
You have some serious issues going on if a 42 isn't enough. I use a #35 on my kid's 80cc 139QMB, and her's will pop right off in under a second. There is no way I'd go anywhere near a 38 or higher. Seriously think you need to retrain your thinking that throwing more fuel at it is going to fix the problem. Did you read my link about progression, do you adjust idle in same way or you also regulate amount of air?
Do you know normal numbers for vacuum pressure i might have a leak, even though i used starter fluid to test and it did not show any difference.
Another thing i live in LA elevation 285′ and Spencer Iowa elevation is 1,312′ from what i know higher the elevation less oxygen in the air so you will need less fuel (lower jet) to get the right ratio.
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Post by eclark5483 on Sept 28, 2018 3:02:20 GMT -5
I control air first and foremost. I do this with both my kid's bike, my Spree's and my Indian. I honestly DO NOT worry about the idle speed on initial tune.
Start with recommended pilot and main jet sizes for your carb. This will typically end up being too rich, which is OK. Next I determine idle jet settings by a very easy test. I start the bike, warm it up, then test how it does from idle to a rapid blip on the throttle to 100% open. There should be no hesitation. If there is, I block off part of the air filter some more, or even tape it till throttle response improves. You are thinking too much into it on the progression circuit. While yes it does play a factor during transition, it still requires the same back pressure as the pilot jet. Again, you are looking for vacuum here. Once you get the backpressure locked in.. THEN rod it a while and read the plugs. If you have done it correctly, you should have a slightly dark plug indicating a slight rich condition. From there you step back a bit on the jetting, and then finally adjust idle mix and speed. You should end up with a bike with a proper A/F burn all through the RPM range, and your plug should be a nice milk chocolate brown.
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