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 the "How to tune PHBG Carbs" guide and it's very useful, 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)

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

Well. First I'd highly recommend you try a scooter website.lol

This is a vintage 2t pedal moped forum. But, it is a 2t. So someone might chime in. Search Puch maxi to see what kind of bike were into.

49ccscoot is a good one.

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

Lucky Duffy /

Interesting! looks pretty cool. I could get into that in the future. I'm just getting in to tinkering with the 50cc I have. I took your advice and posted in 49ccscoot. I was going to leave this post up just in case someone has some feedback unless that's bad form.

cheers

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

I'd keep checking here. I'm sure quite a few people have some experience with scooters and can point you in a good direction.

My gut is telling me you should try a hotter plug to cut the fouling. If fouling is the problem at all anyway.

What jets are you running, other than the idle I mean.

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

Chris Carleton /

Tank vented?

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

Lucky Duffy /

Jets currently are 45 pilot jet, 92 main, 60 choke. Additionally (pretty standard) it's a W7 needle, 40 slide, and clip is 2nd from the top.

As far as tank vented: i'm not positive. if its built into the gas cap then probably yes? (gas cap 'clips' on and is a little loose, have to insert the key and turn to remove it.) Otherwise the gas tank goes to a vacuum petcock (ran the vacuum line from the carb to the petcock so it would work) and then to the carb

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

Lucky Duffy /

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, just the stock oil pump)

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?

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

Yayyyyy, enjoy your tuning adventure!

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

Lucky Duffy /

Just to close the loop here:

After some more research on idle circuit operation, a better understanding of lean/rich symptoms, and some testing, I figured out the problem.

The issue was I had my idle set too slow (via the idle speed screw) 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. Either way, problem solved and I'm on to tuning the rest of this carb.

Cheers

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