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Old 08-26-2012, 08:54 AM   #1
DisorganizedVince OP
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Noob carb tuning - bigger or smaller main jet?

About a month ago now I picked up a CM200T pretty cheap, I'd been without a bike for a while and it was the only thing I could afford. When I pulled the carb off of it it was a horrible sight, not only was the float bowl, venturi, and throttle slide full of gunk, somebody had been at the pilot jet with a pair of grips, and somehow snapped the end off of it, the main jet was seized (over tightened?) into the needle jet, which also had grip marks down the side of it, on top of all that pretty much everything rubber inside it had deteriorated. I ended up buying a "cheap" (60) chinese "plug and play" carb to replace it.
Immediate problems with the chinese carb were that despite being advertised as jetted and set up ready to work, the idle mixture screw was screwed all the way in, the needle in the throttle slide was all the way down, making it way WAY too lean, and the top of the carb pointed upwards instead of at a 90 degree angle. I've managed to sort it all out (adjusted idle mixture, put the needle in the halfway position, and swapped the cap from the old carb, putting in the seal from the new one)

Now here's my problem - she runs pretty damn well, (and the plugs confirm this) up until the throttles pretty wide open, where she starts to bog (there's noticeably less acceleration at full throttle compared to 3/4 throttle, and she slows down on hills if you try and open the throttle too wide)
At first I thought, especially with everything else set to run lean as hell that the main jets probably too small, (advertised as being a 110, stock for the bike, but it is chinese and there's no markings on the jets to confirm it ) but if you snap the throttle to fully open from idle it almost sounds like the engines flooding. That being said though, I snapped the throttle open, let the engine die and pulled one of the plugs out to see if it was wet, and it was bone dry (not sure if it'd be for long enough to get it wet though?). Also, put it on half choke, whack the throttle open, and she revs up nicely, both of which to me would indicate running lean.

Any carb gurus got any suggestions before I start ordering new jets? If it weren't for the "flooding" (if that's what it is) I'd assume lean straight away, but I wouldn't have thought it'd provide THAT little petrol.
So please, fire away, do I need a bigger main jet? Smaller? Just a cheap piece of chinese crap that doesn't belong on a bike? All suggestions welcome
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Old 08-26-2012, 09:12 AM   #2
troidus
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I think the first question is, can you even get replacement jets for that carb?

The only way to sort this out is with a wide-band O2 sensor.
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Old 08-26-2012, 09:24 AM   #3
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Quote:
Originally Posted by troidus View Post
I think the first question is, can you even get replacement jets for that carb?

The only way to sort this out is with a wide-band O2 sensor.
First part I totally agree with, valid question.

Second part, I'm not sure. People have been tuning carbs for nearly 100 years without a wide band sensor.

Trial and error can at least get you pointed in the right direction. Get main jets that are the next couple sizes larger and smaller. Do trial runs with all of them and compare WOT performance. When you get nice crisp WOT response you now have your starting point. Adjust other parameters (needle, air screw, pilot) from there.

The last couple bikes that I've tuned by trial and error using the butt dyno have had nearly perfect A/F ratios when hooked to a wide band on a real dyno.
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Old 08-26-2012, 09:26 AM   #4
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Quote:
Originally Posted by troidus View Post
I think the first question is, can you even get replacement jets for that carb?

The only way to sort this out is with a wide-band O2 sensor.
The carb itself is a (bad) copy of a Keihin PD22, and they've copied it well enough for Keihin jets to fit in it, so getting the jets shouldn't be a problem

Unfortunately I don't know anybody with an O2 sensor, or even the beginning of a clue how to use one, and I can't really afford to give it to a bike shop to sort out

Would you agree that the problem lies in the size of the jet or elsewhere? Beyond the basics my knowledge in carbs is lacking, to say the least

@slickwill

Trial & error's definitely going to have to be my tuning method for this one care to hazard a guess as to wether she's running rich or lean? If there's not enough informaton to "diagnose" the poor girl I'll just have to buy a few jets both bigger and smaller, but I'm flat out broke as it is, it'd be nice to save a bit of cash only having to buy in one direction

DisorganizedVince screwed with this post 08-26-2012 at 09:32 AM Reason: Reply to slickwill
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Old 08-26-2012, 10:54 AM   #5
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Open the choke on it and see if that helps or hinders some. Otherwise start moving the needle up and down of possible.
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Old 08-26-2012, 11:02 AM   #6
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Open the choke on it and see if that helps or hinders some. Otherwise start moving the needle up and down of possible.
With the choke halfway on it revs freely, should the position of the needle affect her getting to WOT? I thought it mostly affected "mid range," but I really only know the basics about carbs, I'll try moving the clip down a notch either later on tonight or tomorrow and see if it makes any difference,
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Old 08-26-2012, 11:38 AM   #7
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Needle is normally mid to upper rpm only. Depends on the carb as to where they truly effect on overlap. The choke is an enrichment circuit. It will effect all rpms if left on, more so on the bottom side. Get her moving and apply it or leave it open, see if you can it to the problem area in the revs and if it makes a difference. Note- you cant tune on free revs. Best is under a load or dyno it.

Here's a simpler diagram.


It pretty much applies to all carbs. Aside from those basics theres cv(vac operated slide) vs slide (mechanical operated slide) carbs. About the only real user difference is a bog on fast opening a slide carb unless theres an accelerator pump on it. The cv uses vac off the intake to raise the slide slower on demand. You just regulate how much it can open itself via the butterfly. Needle and jet tuning is the same though. Some carbs have less systems,some more. Most have 3 like shown above. Idle,slow,main. If you want your head to explode I'll break out the dual siamesed carbs.

Easiest carb to learn..single cyl cv type. Its pretty straight forward and no worries of balance or rider error (usually).

Danjal screwed with this post 08-26-2012 at 11:56 AM
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Old 08-26-2012, 12:13 PM   #8
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Always start with the main jet and work backwards to the needle jet/position, the slide cutaway, and the idle screw.
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Old 08-26-2012, 01:00 PM   #9
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Just took her out for a ride - and here's the results

Moving the needle both up and down makes the bike run considerably worse in the mid range, and doesn't seem to make any noticeable difference to 3/4 - WOT

With the choke half on the bike runs worse at 3/4 - WOT under load, although when not in gear it will rev happily without flooding

The carbs a mechanical slide carb, and thankfully even though it's a twin, there's only one carb, so no balancing required (not quite you're conventional inline twin, the pistons move up and down at the same time, making it feel like you're riding a single)

I suppose she must be running rich then, I think I may just have to order a bunch of different jets and play around, maybe get a Keihin 110 as well, in case the chinese idea of a 110 jet is vastly different



Here's a thought - when I first got the bike it had the wrong heat range spark plugs in it, I replaced them with new ones, but the shop didn't have any resistorless plug, so I ended up getting plugs with resistors in them, (CR7HSA as opposed to C7HSA.) In all the bikes I've owned before they've never really cared wether they've had resistors or not, but given the bikes age, could the extra resistance cause the spark to be too weak, making it harder to ignite at full throttle?
I'm not sure of the likelihood of it causing a problem, but I'm sure somebody out there will.
Can't wait till I've got this thing running properly, it kinda ruins the fun of being able to ride again worrying about doing it more damage all the time, it's stopping me from going on any long distance rides due to fears of killing the thing
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Old 08-26-2012, 03:44 PM   #10
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Some plug connectors have resistors built into them. If the recommended plug is a non-resistor it's for this reason.
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Old 08-26-2012, 04:19 PM   #11
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Some plug connectors have resistors built into them. If the recommended plug is a non-resistor it's for this reason.
I'm pretty sure the plug caps do have resistors. I did ask the shop at the time if they had any resistorless plug caps to even out the resistance, but they didn't have any..
Guess that's a job for tommorow - I'll put the old plugs back in, and if she runs well I'll order some new correct plugs online. They're colder plugs than is recommended for the bike, but I suppose it shouldn't do any harm. I've always read that colder plugs are good for long distance riding anyway, but I've erred on the side of caution and stuck with what the manual recommends.
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Old 08-26-2012, 07:25 PM   #12
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in the past when befuddle by similar issues, Ive ran with the air box lid off, or even tape over half of the air filter, to see if the condition improves, or gets worse by the addition or subtraction of additional air to the mixture.
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Old 08-26-2012, 07:56 PM   #13
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Slickwill was taking you down the right path. Do a road test with a 100. Then do a road test with a 110. Does one make it run better and one make it run worse? If the 100 is better, try a 95. Better or worse? You'll get it dialed in. Revving it without a load doesn't tell you anything.
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Old 08-26-2012, 09:05 PM   #14
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And before trying to tune it, make sure your manifold and airbox boots are in good condition and properly secured to the engine, carb, and airbox. Also make sure the airbox isn't cracked and that you have the proper air filter and that it's in good condition and properly oiled if needed. Replace the fuel line and pull the petcock out to check the inlet screen for debris. Finally, run it in the dark and see if you have spark escaping anywhere. Once all that's squared away, then you can decide if the carb really needs work.
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Old 08-27-2012, 01:54 AM   #15
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In most cases the needle/needle jet fitted to Chinese carbs will be a long way from whats required on specific applications. It is possible to sort this out in some cases, but its quite common that parts will not be available, which unless you can make the items required, or find parts from a Jap carb which will fit, tuning a Chinese carb to work 100% is often difficult/impossible.

Best option here is to try and find a used OE carb for your particular bike, which will work ok and be a lot less hassle than trying to get Chinese one working.
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