keihin clone cvk carb jetting questions pd19j

[Sir Scoots-A-Lot]

I finally got my scoot carb 100% dialed in with the a/f mix, idle settings...err maybe 90% as I still need to flood my engine a little by revving the throttle before and after starting, but then it idles just fine after warm up.

Now I want to go to a 47mm Big Bore Kit ... so naturally I want to order new jets to try out and so I don't overheat my engine by running too lean. I believe I have a #85 jet in my aftermarket pd19j carb, but I have heard many people with 72cc kits run between a 90 and 95 main. I think I need the 4mmx 0.7mm thread jets but can someone confirm that please?

Also I am wondering not only about the main jet and possibly the pilot...but also...what about the needle jet and jet needle?? Are there different sizes available for that? is the needle jet replaced using the same jet as a pilot or main?

I know very little about carbs, haven't gotten inside one just yet and it's my understanding there are 3 jets: pilot for idle, needle jet/jet needle for idle-3/4 throttle and main jet for top 1/4 throttle. So if the needle jet affects such a large range of throttle conditions, wouldn't it be a good idea to go larger with a bbk? or is the ONLY way to go larger on the needle jet to raise and lower the Needle? or can I replace the needle jet with a larger jet?

As always your wise answers are greatly appreciated as I am a newb in many ways and would prefer not to learn the hard/expensive way. Thanks!

[Sir Scoots-A-Lot]

anyone know what size/thread replacement jets to get for the pd19j kehin clones? anybody? or do the sizes vary oftentimes?

[kagetenshi]

It's the smaller 4mm. I have the same carb.

[ratdog]

You are only concerned with two jets. Your polit jet which controls you just off idle mixture and your main jets. The main jet is the important one because your engine will spend very little time below about 5000 rpm.

If you have too rich a jet, most of the time the engine will run just fine until it gets up to higher RPMs. Too rich will normally show its self as simply limiting your max RPMs. I personally feel I would rather be too rich, so if you have a 90 or 95 jet, go with it, after the engine is broken in, we can run some plug chops to check the mixture.

If you were building a drag race engine, we would get best preformance running slightly lean. But lean is HOTb and you are raising the compression so much when you go to a BBK, that you are already making that engine run hotter then it was designed to run.

My personal suggestion on a BBK is to NOT go with the cheapest you can find. Look at the pictures of the piston top. If it’s rough and looks like a bunch of small pebbles painted silver, look elsewhere. Try to find a kit that has a smooth top piston. I have heard a bunch of junk about how those rough piston tops are somehow good, but I have never seen any high preformance engine EVER that didn’t have almost polished piston tops

[Sir Scoots-A-Lot]

Thanks for the replies Ratdog and kagetenshi.

I suspected it was that m4 thread like you said kagetenshi after I did much research, thank you for confirming as I don't have mm calipers to measure.

Thanks for the info Ratdog. 95 does seem like a common size for people to go with and it is what I will be looking for. I found it on Scrappy Dog scooters in what appears to be the correct thread....BUT then I went to google their shipping times and I have heard that Scrappy has gone to the dogs lately, almost every review I saw from this year is negative and says that the owner has several sites and has recently been neglecting this one yet still taking orders and money. Im not sure if the reviews are true or just a few angry customers but it was enough to scare me off. 
I will be looking on PartsForScooters.com now as the owner is on this forum and seems to be reputable as well as very approachable and knowledgeable with answering any questions.
I found multiple sources on Amazon in the correct thread and jet sizes but most I have found are from China and I don't want to wait more than about a week. I suppose I will keep looking thru the plethora of vendors to hopefully find one close and fast that is a reasonable price and not part of a large expensive kit as I only want maybe 2-5 jets at most and will probably be happy with just the 95. I have #85 main and #32 idle now with metal cone filter and I need to flood the engine before I start it and sometimes also turn down the idle also before starting if the engine is very hot, then turn the idle back up once it starts. I have tried playing with the a/f mix screw and half the settings feel rich, half feel lean. oncw the scoot is running and I adjust idle it runs just fine. plug chops show fairly normal with a little dark buildup on electrode although I have only checked on this one very old plug and the buildup looks normal not excessive just a lil dark if anything, never a wet pluig. Once I install the kit I will try to get a new plug and post chop pics if I have any trouble for sure

[ratdog]

I’m sure you know that we don’t “read” the electrode. The insulator is the important part.

[Sir Scoots-A-Lot]

Sept 30, 2018 19:21:37 GMT -5 ratdog said:

I’m sure you know that we don’t “read” the electrode. The insulator is the important part.

I have to admit I'm not good at reading them, I think I have only tried twice, so while I probably read online that you are supposed to look at the insulator, I did not know the insulator is the most important part. Heck the way my insulator on the scoot spark plug isn't very prominent --it doesn't really stick out---so  I most likely didn't pay it enough if any real attention. That's good information to know for next time. Thanks

[Sir Scoots-A-Lot]

Now I must have played with it too much while struggling a lttle during startup and my air/fuel screw seems to be stripped on the threads if I back it out too close to all the way open it falls off while riding. Im lucky I didnt lose the entire screw, rubber gasket and washer, although I did lose the tiny metal washer. Im bummed out about this problem as my BBK is coming along with larger jets I was oing to put in this carb. It seems to fall off if I have the screw halfway out or maybe even less. A temp fix was to leave it screwed almost all the way in and put tape over it to catch it if it did still fall out during my 40 mile round trip. I made it ok but the scoot stuttered when quickly revving the throttle hard and I also had to set the idle too high so it wouldn't stall, which means the bike was trying to takeoff at stops which seemed to happen even if I set the idle ok before riding, it seemed like the idle speeded up after riding, maybe from the screw being loose or from the washer being gone I don't know or maybe it's just lean surging and not getting enough gas at times since I tried to leave the screw in far. I don't know...Does the a/f mix screw affect how fast the carb bowl refills?
Now I am stuck thinking maybe I will kinda glue the air/fuel screw in place once I set it to a medium setting that will hopefully work in cold and warm weather.
Or mayber I will drill off the security cap on my old carb, take the a/f screw from it and see if it will fir my new carb and solve the problem.
Or if the carb A/f screw female threads are stripped after trying a different screw perhaps I will attempt to switch out that part with my old carb if compatible.

I am just bummed about this and venting. It feels so messed up and it's not like I can properly set it anymore if it's falling out.

[tocoo]

for 10 USD, you can buy a repair kit on alieexpress, be super careful with the size of the needle compared to the size of the spring and where the rubber ring sits on this new needle

[ratdog]

The mixture screw only effects idle. In fact, if you have your throttle stop screw screwed in to far ( too high an idle) the mixture screw doses nothing. The setting procedure for the tile mixture screw is to first back out the throttle stop screw as far as you can and still keep the bike running. Then you adjust the mixture screw. When the bike is idling too fast, you can turn the mixture screw back and forth and see no change in the idle speed. I would put some blue thread locker on just the threads, screw it in all the way, then back it out one turn. Let it set for a bit, and you can most likely drive the scoot for a very long time with no problem

[Sir Scoots-A-Lot]

Oct 2, 2018 12:25:34 GMT -5 ratdog said:

The mixture screw only effects idle. In fact, if you have your throttle stop screw screwed in to far ( too high an idle) the mixture screw doses nothing. The setting procedure for the tile mixture screw is to first back out the throttle stop screw as far as you can and still keep the bike running. Then you adjust the mixture screw. When the bike is idling too fast, you can turn the mixture screw back and forth and see no change in the idle speed. I would put some blue thread locker on just the threads, screw it in all the way, then back it out one turn. Let it set for a bit, and you can most likely drive the scoot for a very long time with no problem

The first two sentences you wrote I have heard before from everyone who knows anything....but when I adjust that a/f mix screw, other than affecting the idle like it's supposed to, I notice a difference not only when Im standing next to the scoot and giving it a quick rev to WOT then immediately letting it back to idle---if I rev it like that while the screw is most of the way in (lean i think) it makes a little noise like a "backfire" or similar to when I have trouble starting it and i see it spitting a/f back thru the carb momentarily--and when I get off the throttle quickly and when I have it turned mostly out (rich) it seems to bog a bit when revving up quickly to WOT--I notice a difference then and also when I adjust the air-fuel mix screw and screw it in almost all the way and ride it like i have it now almost all the way in with tape over it so it wont fall out and get lost--the other thing that happens is when i go down a steep hill with the screw almost all the way in and try to go to WOT or pull the throttle hard and fast it's likt the bike isn't getting enough gas. I know it doesn't make sense because that screw everyone says is only supposed to affect idle but I noticed the same results when I adjusted this a/f screw from almost closed to almost opened and rode it the scoot acted different not just at idle. 
Thats why I asked: Does the a/f screw affect how fast the carb bowl fills with air or anything like that? or is it just plugging a little a/f circuit or is it opening the butterfly more? I would like to read a diagram of this to maybe understand it better as I do not have a good full understanding of every function of the carb. 

Maybe my observations are some kind of other malfunction of this cheap carb other than the a/f screw and the malfunction from riding over bumps or whatever caused it happened at such a time that it fooled me into thinking it was because of the screw adjustment I made. Or maybe adjusting the a/f screw from one extreme to the other fouled the plug a bit causing effects over not just the idle range

I appreciate your advice on how to fix it Ratdog. I feel like your suggestion is the best way to go. It worked best before the thread stripped starting at a little over 1.5 full 360 degree turns out from closed so that's about where I will set it or maybe 2 full turns out. Right now I thi nk its  one turn out and seems to not be getting gas at WOT but only going downhill or if I open throttle too much too fast and again more so down hills

I am a huge supporter and fan of these Chinese engines--I am a firm believer that once you tune them and if you service them as much as you should then you can get some serious reliability and longevity out of these Chinese scoots just like you can out of a Japanese scooter at over 3x the cost...BUT this carb problem is one good reason I wish I had EFI. I can still get my scoot to start, run without stalling and not strand me every time, but it is a bit annoying and embarrasing having to fiddle with it for a minute or 5 every time I want to start it up now. Hopefully your suggestion will get it back to starting a lot easier like when I had it dialed in before the thread stripped. Other than that Im aslo thinking the carb might also need a bigger idle jet -- current is said by carb seller to be #32-- because I have a metal cone filter and even when I had it dialed in i needed to rev it several times to flood it a little before trying to start it

[ratdog]

The mixture screw only effects the air/ fuel going thru the carb at low speed. It has nothing to do with the float bowl

A pause when suddenly opening the throttle is often an indicator of a too lean engine. This can be improved a little by raising the needle, sometimes by increasing the size of the polit jet, and often going to a larger main jet.

I alway work backwards. That is, using plug reading, I get the main jet correct first. Then adjust the needle if necessary, then change the polit jet if necessary, and finally adjust the air fuel screw to fine tune the idle

For some reason, this has always worked for me, and I have never had problems.

Most of these little scoots have an little booster pump the squirt gas into the carb when you first open the throttle. I know of no way the adjust this little guy. In my experince, it always squirts the same amount of gas, regardless of how close you adjust the point of when it first squirts. Frankly, I keep this adjustment fairly loose. I have no way of proving this, but I think it’s best to delay it a tiny bit to give the slide a chance to start moving. The main reason they now include this little guy is because they jet the bikes new so damn lean. They claim this is the sstify the EPA ( same reason the seal the mixture screw on a lot of the new bikes). At any rate, unless your up a couple thousand feet above sea level, most of these guys will be jetted way lean. And most will have that lean surge when you open the throttle fast.

[Sir Scoots-A-Lot]

Oct 4, 2018 9:29:10 GMT -5 ratdog said:

The mixture screw only effects the air/ fuel going thru the carb at low speed. It has nothing to do with the float bowl

A pause when suddenly opening the throttle is often an indicator of a too lean engine. This can be improved a little by raising the needle, sometimes by increasing the size of the polit jet, and often going to a larger main jet.

I alway work backwards. That is, using plug reading, I get the main jet correct first. Then adjust the needle if necessary, then change the polit jet if necessary, and finally adjust the air fuel screw to fine tune the idle

For some reason, this has always worked for me, and I have never had problems.

Most of these little scoots have an little booster pump the squirt gas into the carb when you first open the throttle. I know of no way the adjust this little guy. In my experince, it always squirts the same amount of gas, regardless of how close you adjust the point of when it first squirts. Frankly, I keep this adjustment fairly loose. I have no way of proving this, but I think it’s best to delay it a tiny bit to give the slide a chance to start moving. The main reason they now include this little guy is because they jet the bikes new so damn lean. They claim this is the sstify the EPA ( same reason the seal the mixture screw on a lot of the new bikes). At any rate, unless your up a couple thousand feet above sea level, most of these guys will be jetted way lean. And most will have that lean surge when you open the throttle fast.

First off I want to say : Thank you Ratdog for taking the time to read my woes, questions and respond, especially thank you because this situation is a bit tricky since I am not providing the data of what's going on as well as I could plus my loose a/f screw kinda screws things up. I know sometimes it was getting looser during a ride from bumps and partially stripped threads and other times the screw straight up fell out and luckily when it fell out my engine would stall usually as soon as I hit the next stop so I always found the screw on top og my CVT case by the starter motor.

I drilled out my old spare carb security tab to harvest the a/f screw from it complete with a tiny rubber gasket washer, a tiny metal washer and this one had a spring over the majority of the length of the a/f screw. I guess that spring works in a similar fashion to the idle screw/spring to hold it in place from vibrations and bumps while riding. I don't remember a spring on the newer carb's a/f mix screw, but it's possible I lost it before I would have seen it. 

So I put the a/f screw and hardware from old carb into new carb. it looks almost identical except the old stock carb screw was a lot shorter above the threads where the screw head sticks out and has a slot for the screw driver. I guess it is shorter so they could have plaenty of room to put the security tab over it. I put silicone gasket maker on the threads which I know isn't ideal to hold t tight like thread locker but im hoping it will be enough to hold it while still being able to remove it when/if I need to adjust it.

I am still haaving more trouble starting it than when this new carb was newer and I had it "dialed in" just by adjusting idle and a/f screw so that all I would do is  rev the throttle several times to flood the engine before starting. Now its like I rev it a few times before starting but it wont start unles I do that plus often I pinch the overflow shut before cranking it and rev it while cranking and sometimes I put my hand over the carb opening (with the cone air ilter taken off) while cranking and holding at WOT then while still cranking I stop revving and I have the idle screw turned all the way loose and eventually I hear the engine catching but I need to keep revving it so it wont stall while i turn the idle back up with my other hand. I noticed that if you pinch the overflow shut while revving the carb puts out more gas...

To me...it's neccesary I get it started to get where I need to go without paying an arm and a leg for cabs or walking at least 2.2 miles and waiting a long time for busses...but it feels ridiculous I have to do all this just to start it and have it running...I guess im lucky I even figured out how to get it started like I do and im lucky I can usually start it first or second try withing a minute to 5 tops if its cold or sometimes as much as 15 minutes to start it when I have been riding more than 5 or 10 miles and the engine is still hot.

Im thinking that the characteristics of what it takes for my scoot to run with this carb  has changed from when I first got it to now with a couple thousand miles on this carb. My new thoughts are it could be a multi part problem and multi part solution possibly. I did adjust the valves around when I got the new carb to about .003" intake and .004" exhaust...I have since read many manuals call for half of that, although it seemed ok after adjusting it to that, plus maybe I didn't tighten it in place enough and it has since come loose and adjusted itself tighter or looser (im thinking tighter since I don't think im hearing any excessive loud clicking) Also maybe the inernals on the carb like the float/float needle need fixed.

Definitley Ratdog I agree part, or maybe all of the solution lies in the car jetting. This  carb I have been run ning a couple thoussand miles ago claims an #85 main jet and #32 idle and Im running a non-restrictive metal mesh reverse cone filter on what seems to be stock 39mm bore and stock stroke. I have a 47mm big bore kit sitting in front of me as I type this, rings gapped and clocked, one circlip in and the wrist pin halfway in, all oiled up, gaskets and hardware ready to go.....I am just waiting to see if maybe I can make my carb situation better on the stock bore first and also waiting for a #88, #91 and #94 main jets to come in the mail today so I won't run it too lean although I have heard of a couple people running stock jet or #80-#88 apparently without issues that I know of--I dont know how that would go but I would rather run a lil rich as I run WOT whenever it's safe. I have been also thinking for a few days maybe I should get at least a #35 idle --not only for the BBK but maybe that would also solve my current starting issue as it seems I need to flood it and also turn the idle screw almost all the way down for it to start combustion. I dunno...maybe it's the funky PAIR system I have contributing a vacuum problem...But all I can do is keep making adjustments and recording results better than I have and going from there along with getting hte best advice possible onb here from guys like Ratdog who know their tuning way better than I may ever know. I  will try to get a new spark plug or two this weekend and try chops. Guess I will be ordering probably just a #35 idle jet also.

I hope I can run this new BBK at least as well as I have been able to start the stock bore until my new idle jet comes in  a week because I need/prefer this scooter for transportation over Uber cabs/walking and if I install the kit and it doesn't want to run it would be a huge pain and admitting defeat if I had to put the original bore back on. I feel like Im too proud and once the BBK is on it won't come off until it runs or gets sold/junked.

[Sir Scoots-A-Lot]

PS today the scoot was on and off but mostly on having the probblem I described that was only happeneing downhill before but this time it happened on all terrain but was pronuounced a lot heavier on downhills. To describe it again: WhenI hit the throttle to much too fast the tngine hesitates at least and if I were to for instance hit it from 1/4 throttle to WOT especially downhill and hold it at WOT even after it hesitates it would often jerk forward then hesitate and just lurch back and forth as far as getting less and more power even though I was holding throttle at same position. It's almost like a misfire r like when some dirtbikes are being revved for fun and hit the rev limiter. I can't decide if that's rich or lean. Last time I might not have described it as well. Last time I thought it was lean. now I think maybe rich? but not sure. also during the same ride this problem was happening most of the time but not 100% of the time and the intensity of how it happened varied also

[GrumpyUnk]

The description leads me to think the enrichment system is not working at all. Everything done to get it to start will cause a richer mix. Hand over intake, twisting the throttle to work the accelerator pump & squirt in some fuel, etc.
I would check that the enrichment passages are clean and the 'needle' retracted when the electric gizmo is cold.
tom