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Flying, thank you very much for the highly detailed info and the time you put into it . . I’ll just post some general info and try to be brief . . I know you’re a sharp guy and likely know some, if not all of what I’m posting below, so it’s mainly for some others whom might at least find it interesting if not helpful . . I am also aware that you know this particular style of carb very well which is why I have referred others to you when I saw they had a carb issue with one, so none of this will be about me making suggestions to tune this particular carb, especially since I don’t know this particular carb well, plus you have already been there and done that.
Some of the info will also be irrelevant to most because people are simply not going to change some of the things I mention just to try and reduce the hesitation their engine has which is understandable, so again, some of it is mainly for general informational purposes . . I know that El Camexican is also familiar with some, if not all of the info below, so I’m a little surprised that he hasn’t made a post on the op’s thread at the very least, but I haven’t seen him post much lately so maybe he just isn’t around . . I am also not the most knowledgeable person in the world on these subjects, however, I at least know enough to get by and where to go if I need more info.
None of the mfg’s are dummies, but on occasion, but all of them, including Kawasaki, have built something that has had one type of quirk or problem in the past, therefore, none of this is to rag n Honda…too much, lol . . Problems like this just happen sometimes . . The mfg’s certainly learn from their mistake and eventually fix them on future models.
CAMSHAFT
A cam that has a lot of overlap will idle poorly due to low engine vacuum and a “dirty” intake charge that is diluted and contaminated by exhaust gases that haven’t been completely evacuated from the cylinder, therefore, one of my guesses is, that with this particular bikes combination of ex pipe and ignition timing and compression etc, the cam has too much “overlap” . . Camshaft lobe overlap is affected by a few factors, one of them being the distance between the cam lobes which is referred to as “lobe center angle” or “lsa” for short . . Most stock automotive engines have lsa’s in the 115 to 116 degree range . . This is to make the engine idle smoothly and be more “streetable” with good throttle response, especially in traffic etc . . Most performance automotive aftermarket cams use a lsa of around 110, which is a huge difference . . If all other things are equal on a cam, the one with a lsa of 116 degrees will idle far smoother, and have far more engine vacuum and better throttle response than the cam with a lsa of 110 degrees.
I would be interested to know what the amount of overlap the 250sx cam has compared to another 250 cc engine that didn’t exhibit this problem which has a non accelerator pump carb . . If the lsa was increased on the sx cam, the hesitation would be greatly reduced if not eliminated if it had a properly working and tuned carb, but that’s going to affect the power characteristics, plus changing a cam is not what most people would want to do to improve this problem.
ENGINE VACUUM
The more vacuum an engine has, the better throttle response it will have and the easier it will be for the engine to draw fuel from the carb . . I would be interested to know what the vacuum level is on these engines at idle and when the throttlke is stabbed . . the stabbing test is not a definitive test due to some variables but it still might yield some useful info . . One just doesn’t know unless they try it . . If the vacuum at idle or around 1/8 throttle is less than around 14 hg, an engine can start to exhibit carb related hesitation and or poor idle characteristics.
CARBURETOR SIZE VS ENGINE VACUUM VS THROTTLE RESPONSE
In general the smaller the carb, the higher the vacuum will be . . It is possible to run a 40 mm carb on a 250sx and have it not work much, if any worse, than the stock 27 mm carb providing the intake tract size is reduced sufficiently . . This gets a bit complicated and it will reduce power but it will still work . . this concept is exactly the same as the restrictor plates that Nascar occasionally uses . . To this end you could get a piece of pcv pipe that will slip snugly into your carb adapter or the exit side of the carb and see what affect that has on throttle response . . you can also check the engine vacuum before and after . . Of course you would need t rejet your carb and obviously this will reduce peak power but it might still give some useful info, and if it does significantly reduce hesitation, and someone uses the lower end of the operating range most of the time, it might be a viable solution to their hesitation problem.
IGNITION TIMING
If an engine has insufficient amount of timing it will have both a hesitation and will spit back out the carb . . This is easy to test if someone wants to . . Just reduce the timing on your engine by 10 degrees and check it out . . With this in mind, I would look at the ignition pick up and see if it can be easily modified to advance the timing by 4 - 6 degrees . . This amount is definitely enough to alter the performance enough to feel, however, it’s not what could be considered excessive . . If this reduces the hesitation and the engine doesn’t ping, it seems like a simple, partial cure.
EXHAUST SYSTEM FUNCTION AND DESIGN
In general, the exhaust pipes are not just a way to route the exhaust gasses to the rear of a vehicle, they are also intended to perform a function called “scavenging” which basically creates a sort of vacuum effect that actually helps pull the intake charge into the engine . . This affect is more pronounced on vehicles that have multiple cylinders with individual pipes that converge/merge together at the end where they exit into one single large tube . . In fact here’s a couple short, super low budget, simple tests, that visually shows the vacuum affect created by exhaust scavenging.
https://www.youtube.com/watch?v=pDQXsfeZGwk
https://www.youtube.com/watch?v=yMyKv_P80eI
https://www.youtube.com/watch?v=W7lbTcdaYpE
CARBURETOR SIZE
Based on all the info you have provided along with other info gathered, it is my guess that to reduce the hesitation, it is best to stick with a carb that is between 25 and 33 mm for this particular bike in box stock form anyway . . There is an oval bore Chinese pumper carb with a 27 mm outlet which would make it around a 25 mm carb . . The one I posted earlier was claimed to be a 30 mm but that might have been the outlet size . . If it is, then that carb is around a 27 mm . . It’s a bit hard to tell the true size of some of these because the sellers don’t always post the exit size and instead often just state a mm which in many cases I have seen is incorrect . . Either way, these seem to be in the correct range for this purpose.
THROTTLE RESPONSE TEST
Posted a few posts above this one.
LEAN CONDITION TEST
Posted a few posts above this one.
CARBURETOR HEAD/TOP CLEARANCE
That’s a very good description . . I will look for a top that might have an elbow on it, there are some . . Another option is to have the end of the cable modified by having an elbow installed . . If you look at the bike below, it looks to me like it might have a Mikuni on it . . I have no idea if they used an elbox on top of the carb though . . The carb is also mounted square to the ground . . I don’t remember if the 250sx carb is mounted at the same angle or if it is angled upward . . Either way, if the carb used, is angled upward, it would have a little less bend in the cable which might make it a little easier to use a straight cable on it . . You would certainly be one of the best judges of that since you are very familiar with how much head clearance they have for the carb.
CHINESE CARB QUALITY
I for one am not a fan of Chinese made parts in general, however, of the posts I have read regarding the Chinese made carbs, most were positive . . There were a few that were initially negative, however, it was later determined the either the float level was off, or the needle had debris that came from the persons fuel tank stuck in it etc . . I’m sure some of them simply don’t work well for some reason or another, but judging from what I have read, in the majority of the cases they seem to work just fine . . If I remember correctly, they may not use the exact same jet numbering system the Keihins use but regular Keihin jets work in them.
Also if I recall correctly, Damon told me that some, if not all of these carbs, are the exact same model that are used on some production Chinese motorcycles, therefore, in light of that info alone, I would guess that they should be fairly decent considering there are around 100 gazillion motorcycles in China but that’s just my guess.
ACCELERATOR PUMPS
Just buying a carb with an accelerator pump does not mean that is going to work better without being properly tuned . . One of the complications with carbs that have accelerator pumps is that one size does not fit all . . in other words, the pump may squirt too little fuel or too much . . it may also not squirt fuel long enough or it may squirt it for too long, and unfortunately, unlike the accelerator pump on most if not all aftermarket automotive carburetors, the pumps on a motorcycle carb are rarely, if ever, adjustable . . This means that if the pump on the carb needs some adjustment, you will likely have to modify or make some parts to do that with.
One example I heard of regarding problems with accelerator pump carbs, was related to the Keihin FCR model . . The problem is apparently so common, that Boysen makes a $120.00 kit with an adjustable jet to cure this with . . I read on the net that one guy was smart enough to simply buy a $5.00 jet the size he needed to fix this prob on his carb.
The point being, don’t expect any carb with an accelerator pump to have the pump setting be ideal for your bike right out of the box . . You might get lucky…you might not . . This being said, I’m guessing [hoping] that the ones on the small Chinese carbs will be pretty close for this particular app at the very least.
Please feel free to mention any errors . . I have no edit button so once it's posted I can't correct it.
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Originally Posted by barnett468
