New carb fouling plugs, but dies with choke off (carb tuning)

Setup up front:
Yamaha Vino 2T (YJ50) minarelli horizontal engine
70cc BBK Athena sport aluminum
Yasuni Exhaust (derestricted)
Polini Air intake cone filter
brand new Dellorto 21mm carb

Initially I was running the stock carb (tiny), but even with a massive main jet it was running too lean so I bought a (stage 6) dellorto 21mm PHBG. Now I'm trying to dial in the carb. I've read through some PHBG tuning guides, but I'm still running into problems with idle.

The default idle jet (50) seemed to rich so I switched to 45 and at the end of yesterday I finally got the idle dialed in. constant sound, responsive to throttle, but quick to return to idle. This was with the engine fully warmed up. Today when I start it (choke on) it has a much slower inconsistent idle and sounds like it's about to die. leave the choke on for 30 sec or so and when I turn the choke off it dies. start it, runs a few seconds, dies. then wont start. I check the plug and it's dry fouled. (of note, that's how the beginning of yesterday started before the engine was warmed up. I fouled an additional plug yesterday when I let it sit in idle for a few mins. I'm running out of spark plugs here)

To me this says that I'm probably still too rich? But I'm confused why it worked so well yesterday and struggled so hard today. Also why, if it is too rich, does it die when I turn the choke off? Seems like that would indicate it's too lean if taking out that extra fuel kills it. And lastly, is it normal for a 2T scooter to die if left in idle too long or should it be able to idle forever? (I've heard some dirt bike guys say their bikes foul plugs and die if left in idle on other forums).

Thanks in advance. If I'm saying something dumb feel free to point it out. I'm very new to scooter tuning.

(additionally, if it matters, I don't premix. I run the stock oil pump (not adjustable) and add 1 cap of full synthetic oil to a full tank on fill up)

 

Personally l don't feel l have the knowledge to answer alot of your questions but just wanted to say it sounds like you got a good setup there.

 

It sounds to lean to me if it dies after you let off the choke.
Try letting more fuel in with the mixture screw.
You should be able to idle for ages without fouling the plug. Dirt bikes are a different ball game, they make all the power at full throttle and run like shit anywhere else.

 

I've never had much luck with 21mm carbs on any 70cc scooter; 17.5 and 19mm's yes, but 21's don't seem to want to play with me.
Try moving the carb slide needle up by changing the circlip to the bottom slot on the needle and make sure the air/fuel mix screw is set at about 2 1/2 turns out from fully home.
If it still won't idle properly then start to go up one main jet size at a time.
Thinking about it, you would need to re-jet anyway with a new carb, simply because 9 times out of 10 the jet that is fitted when you get it is the wrong size for the engine performance.

 

its quite possible to have the idle mixture set too lean (stalling with the choke off) and the main jet too rich (but still fouling plugs whilst riding)

screwing the mixture screw in on a Dellorto 21mm shuts off the fuel, and screwing the mixture screw out turns on the fuel.

get the engine up to temp and keep the engine running by blipping the throttle, screw the mixture screw in all the way, and then slowly start screwing it out until the engine will idle on its own and respond criply to the throttle.

when you get to the point it seems correct screw the mixture screw in abit and slowly out again, you will know when it feels right.

if your mixture screw ends up all the way out, then the pilot jet is too small, and you will have to fit the next size up.

then when you are happy with the pilot jet/idle mixture you can move on to riding and checking the main jet.

 

Not quite that simple, also depends on the position of the slide needle which would allow fuel to be sucked passed it even if the mixture screw was all the way in.
https://www.dellorto.it/wp-content/uploads/2015/10/dellorto_manual.pdf

 

normally the middle clip position on the slide will be right to start off with.

the needle position will affect the mid throttle position mostly.

sorry didnt look at your link, just going by experiance.

 

Other info from the carb: 92 main, 45 idle, 60 choke, W7 needle, 40 slide, clip 2nd from the top, mixture screw 2.5 turns out.
It seems like some stuff indicates too rich and some stuff indicates to lean.

My gameplan is to install a new spark plug, drop the idle jet to 40 and see if that's too lean. If I was already too lean then I think I wont get it to start and that should start to build the picture. Or it'll start but be noticeably too lean. (i don't think i'm going to seize a piston just running a bit in idle even if it is too lean). I'll take out the spark plug and check the color if it's able to be tuned in with that jet.

I have a hard time believing I'm rich when I fouled a plug from brand new just tuning around in idle/ 1/8 throttle revs. I'm starting to think that the reason the engine died without choke was just too slow of an idle speed? I set the idle speed screw before I installed the Carb by starting all the way out and then going in until the slide just barley started to move up, then went 1.5 turns in.
Any other techniques for setting idle? My ear isn't trained well enough to go by that. I don't have a rpm gauge or anything, but from what I've heard you can hook one up (rev counter or something?)to test it?

Really appreciate all the replies. I'll keep you all up to date with how it goes. If this doesn't start shaping up I'll start looking in to a list of other things to make sure it's really the carb tuning that's the issue. reeds, squish, compression, whatever

 

You checked for air leaks?

 

Alright, gave it a go. same results.
Swapped to a 40 idle jet, new spark plug. She fired right up with the choke on. When I let the choke out the first time it died so I started back up, twisted the idle speed screw in 1 twist and then it was totally fine when i took the choke off (and sounded like a normal idle).

Side note: That's what i think happened the morning after I was tuning the first time. The idle was just too slow which made it die when I took the choke off.

Sweet! so I get to work fiddling with the mixture screw. It sounds kind of lean in that when I rev it takes a little too long to get back down to idle. So I make some small changes back and forth to get it just right, but then the engine dies again and wont start after. sure enough. fouled plug. like flat black carbon buildup. (side note, I ran with a new tank of gas, no oil added)

I would feel strange going smaller than a 40 idle jet given my setup and the engine sounds lean when I rev it. (the stock 14mm carb I was running before had a 46 idle jet...) Is there anywhere else I should be looking for issues?
Could the squish play a role? or the reeds? I'm using NGK BR9HS spark plug. Additionally I live down at sea level.

I made sure all the connections from the reed valve to the intake was tight, but I don't really know how to check for air leaks if there's a specific technique? I also haven't tested the compression, but I have a compression tester on the way.

 

Spray some wd40 on the intake rubber. While it's running, if your idle picks up or drops or whatever, you have an air leak.... Simple test...
Really satisfying when you find an air leak that way

 

"Quote - It sounds kind of lean in that when I rev it takes a little too long to get back down to idle. "

That's actually an indication of it being too rich.
Like I said, move the needle circlip.....

 

After quite a bit more research, I'm starting to understand the idle and progression circuits a little better. I think the idle jet is still too big since I'm fouling plugs being primarily in idle. I think there's too many factors to determine if the idle jet is too lean or rich based on inconsistent revs into the progression alone.

@scubabiker thanks I'll try that out to test for air leaks

@Stevep thanks very much for posting the dellorto manual, that helped cage my brain quite a bit. I think your last comment was a misspeak though. from the manual: "If, on the contrary, the jet is too small, the engine responds better to the accelerator (except when it stalls when the jet is much too small), but when the throttle is closed, the speed (rpm) doesn't decrease immediately, and the speed remains high for few seconds before settling down to idle" - granted that's talking about the idle jet and not the needle, but in general my understanding is holding on to revs too long is a lean problem.

in my mind step one in this process is to figure out why I'm fouling plugs in idle. I'd agree that the needle plays a part in idle, but a very minuscule one with the slide (nearly) completely down compared to the progression where it really matters. Thus I think I should be able to fix my idle problem with the idle jet and mixture screw before messing too much with the needle.

My plan is to drop the idle jet a few more points and play with the idle speed screw and mixture screw until I get a good sounding idle that's moving the back tire a little. (I do have a tach/rpm gauge on ordern now, but it's going to take forever to get here based on COVID). Then I'll start messing with small revs and see how it responds keeping in mind it's both about the idle jet and the needle.

The only thing that doesn't make sense out of what I've read is: (not from the manual, but another website) "What comes out of the idle passage is metered by the mixture adjustment screw in addition to the idle jet. During progression, flow through the progression hole changes direction; instead of air going in, fuel comes out. This is not affected by the adjustment screw, rather it's solely metered by the idle jet." That seems odd in that I could create what sounded like rich or lean conditions when revving solely by adjusting the mixture screw? Am I just crazy?

 

40 idle is big for your setup, its somewhere around 32-38 buy sets of pilot jets and try, when you get your bike on operating temperature thats when u adjust the idle screw and see from there if its rich or lean depending on how much it wants air or fuel. The needle is the last thing u need to touch it always comes on the end after finding mainjet , idle screw and on the end u adjust and check needle and re adjust a bit mixture screw if needed. Also check what atomizer you are running, if its not AU 262 then change it to that, because the other one is for 4t so u will have even more trouble fine tuning the bike. Also if it takes too long for revs to drop down you are runnig lean or have air leak there is no other option, if u dont have air leak and your idle jet is right as soon as u get off the throttle rpms should drop down to idle without holding more than 1sec.

 

Just to close the loop here:

I'm confident now that the issue I initially had was that I had my idle set too slow so when I took the choke off the bike would die. And at the same time I was testing idle jets that were too rich given my carb size. I dropped the idle jet to 34, set the idle a little higher and was able to tune in the idle without fouling a plug or anything else weird. 34 is a bit lean (when tuned correctly the mixture screw is 4.5 turns out) so I plan to bump up to 36 and give that a go. I also realized that what I previously thought was "too lean and hanging on to RPMs" was not a correct assessment. With the 34 jet I got a chance to hear what too lean really sounds like and it hangs on to RPMs wayy longer than what I was thinking and if I went even leaner the scooter wouldn't even rev, it would just kind of gasp and struggle.

So that problem is solved and I'm on to tuning the rest of this carb!
Main jet should be simpler just by doing full throttle runs and checking the top speed and spark plug color.
Then on to the needle after that if necessary.

I did check the atomizer and luckily it is the correct 2T type. thanks!

Can't thank you all enough for all the help and information!
Cheers