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Post by Clank on Aug 30, 2015 16:44:31 GMT -5
I read this in an article about CVK carbs and determining where to start with a main jet. This is an approximate setting +or- 1to2 sizes. All it is is the Ventura size in mm x 5.
So my stock carb is 18.5mm x 5 = 92.5 or 93. But I being at high altitude need to reduce that between 5 and 10℅ which I x by .9 giving me 83.25.
I've just dropped it from a 89 to 85 and its a tremendous difference.
So again carb ventura size x 5 = main jet +/- 1.
Try it out and let us know your results.
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Post by tiny on Aug 30, 2015 16:46:59 GMT -5
Cool Beans might giver her a try.
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Post by katastroff on Aug 30, 2015 18:15:27 GMT -5
Thx!!!  |
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Post by spaz12 on Aug 30, 2015 20:19:17 GMT -5
I read this in an article about CVK carbs and determining where to start with a main jet. This is an approximate setting +or- 1to2 sizes. All it is is the Ventura size in mm x 5. So my stock carb is 18.5mm x 5 = 92.5 or 93. But I being at high altitude need to reduce that between 5 and 10℅ which I x by .9 giving me 83.25. I've just dropped it from a 89 to 85 and its a tremendous difference. So again carb ventura size x 5 = main jet +/- 1. Try it out and let us know your results. He wrote quite a few good articles. |
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Post by 90GTVert on Aug 30, 2015 21:35:22 GMT -5
Other than the constant (5) that formula is at least 30-40 years old, because I know it was in carb tuning manuals from the 80s. Prob much older. It's a good idea though and the 5 works pretty well as a starting point in many cases compared to the ~4 constants most stated originally. Some of my past setups would be seized since it predicts an 80 and I used a 100 (as I understood it in the one I read it was stated for scooter carbs, not specifying CV and I believe a comment even said CV carbs didn't work out with it or something like that). Quite a few would be close or even right on. My TaoTao used 79 to 95, same size carb. Pretty sure I could make more changes and extend that range of sizes even more, so you can't expect a one size fits all. I don't mean to knock it, I'm just saying I wouldn't put a jet in and expect it to be right. There's no substitute for tuning.
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Post by paulmohr on Aug 30, 2015 22:08:12 GMT -5
You simply can't use a formula with a constant multiplier that is only based on venturi size. The venturi size of the carb is no where near the only factor in how much fuel an engine will need. And that is assuming you have the correct sized carb to begin with. With cars that isn't that tricky, there are formulas and calculators to figure out what size you need and carbs are sized by CFM. I haven't been able to find any such thing for small engines. I can't even find how many cfm these carbs flow, there doesn't seem to be a rating for them.
For starters I think 92-93 is going to be a bit rich for a bone stock 49cc scooter. I don't think so rich that it would cause problems, but my guess is you would be using more fuel than you need to.
What if you were one of these people that decides to put a larger carb on their scooter because think it will go faster that way. If you put a 23mm carb on it you would need a 115 main jet according to that formula. That is just going to be much fuel for a stock 50cc. Heck that might be too much for a heavily modded 50cc scooter.
Proper jetting relies on so many more factors than just the size of the carb. Like engine size, rpm range, cam design, how restrictive or un restrictive your intake and exhaust are and the weight of the vehicle and rider and other things.
Just look at how finicky these things are with simple mods. Something as simple as removing the stock airbox can require up jetting by 15 sizes. To think that two scooters, one stock and the other heavily modded would require the same jetting simply because they have the same size carb is absurd. A scooter with a 100cc bbk, cam, exhaust and pod filter is not going to use the jetting as a stock 49cc scooter. The one scooter has almost twice the volume, a longer intake and exhaust duration and most likely will spin faster. This will most certainly require more fuel than a stock engine.
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Post by spaz12 on Aug 31, 2015 0:04:45 GMT -5
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Post by birdman on Aug 31, 2015 4:28:12 GMT -5
Im going to use this formula to hopefully figure out my 1990 Jog once I get back to in a month or so.
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Post by iwiketuddlz on Aug 31, 2015 7:40:18 GMT -5
Great formula!! It will certainly give you an idea of where to start, you know, if you are clueless to the tunning process and all!!! Thanks for sharing!!!!
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Post by paulmohr on Aug 31, 2015 10:18:01 GMT -5
I notice the article does assume you have chosen the correct carb for your engine size and mods. Which isn't going to really apply to our scooters because we use pretty much the same size carb regardless. 18-19mm is the standard size and we just tune it by jetting up or down. It is a nifty formula but honestly you could come just as close by making an educated guess based on the stock jetting and engine mods. To fine tune it you are still going to need to buy a range of jet sizes to experiment with.
That article also leads me to another question I have been pondering. At the end it mentions plug reading. One of the ways to determine jet size is by reading the plugs using the "chop method" Where you run the bike at WOT under a load and then kill the engine. The problem is this assumes you have a clutch. You would run the bike at speed, hit the kill switch and pull in the clutch.
The problem I see is on these scooters we do not have a clutch. So if you ran at WOT then killed the engine the engine is still spinning while you are slowing down. Thus feeding fuel to the engine while the ignition is not running. It would seem to me that this would give a false "rich" indication on the plugs.
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Post by iwiketuddlz on Aug 31, 2015 11:17:58 GMT -5
Good point Palmer!!!!or it would give a false LEAN reading because of the access fuel would then WASH the plug!!!
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Post by 90GTVert on Aug 31, 2015 14:30:18 GMT -5
Brake quickly if you want the engine to cut out with a CVT. Don't hurt yourself though.
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Post by Clank on Sept 3, 2015 8:00:00 GMT -5
Other than the constant (5) that formula is at least 30-40 years old, because I know it was in carb tuning manuals from the 80s. Prob much older. It's a good idea though and the 5 works pretty well as a starting point in many cases compared to the ~4 constants most stated originally. Some of my past setups would be seized since it predicts an 80 and I used a 100 (as I understood it in the one I read it was stated for scooter carbs, not specifying CV and I believe a comment even said CV carbs didn't work out with it or something like that). Quite a few would be close or even right on. My TaoTao used 79 to 95, same size carb. Pretty sure I could make more changes and extend that range of sizes even more, so you can't expect a one size fits all. I don't mean to knock it, I'm just saying I wouldn't put a jet in and expect it to be right. There's no substitute for tuning. Your absolutely right 90GTVert, you need to tune it to get the optimum out of it, hence the plus or minus 1. I didn't share this as a solid for sure instant tune remedy, but as a good starting point. There are so many variables in a CVK style carb, and being both small and temperature sensitive along with the effects of altitude coming into play, tuning is a must. Think of this as a positive starting point. I will post links to all the articles I've found later today. (hopefully). But in all the articles they state that you HAVE to set the main jet before tuning your needle and A/F screw. Happy tuning, more to come..... |
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Post by Clank on Sept 4, 2015 2:41:23 GMT -5
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Post by Clank on Sept 10, 2015 23:07:03 GMT -5
OK so my formula came out to be 83/84 jet size. So I went from 89 to 85 and noticed it was a little more zip in the low end... Everyday life gets in way so dinking around on the scoot was nil. After playing with needle and A/F it felt like no mid at all.
Well I decided tonight to throw out the high altitude part of the equation which gives a 92.5. I only have a drill bit size .92mm and next size up is 1.04mm. So up to 92 and only got to run it up the street and back. Felt more mid and more low. Not enough of a run to get it in full high but started to pull harder just before breaking hard to avoid the fence. So Idk quite yet what to think.
Oh, and I guess I completely spaced mentioning it's a 47mm piston not a stock 39mm. And quite a bit cvt tuning and modding. Haven't had time to do my extensive/extreme tuning/modding idea yet.
Will continue to post, most likely not the same scoot time but always the same scoot channel. 49ccscoot.com
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