New BBK tuning, high rpm soft seize?

[grapevino]

Just want to start with an apology, it’s my first post and I’m really new to scooters and especially 2stoke engines. I used the search, and wasn’t able to find quite what I was looking for.

Just installed a 70cc onto my 2t vino. Oil pump intact, I put a cap in the tank (25:1ish) stock airbox, I already had the exhaust tuned on the stock cylinder ran great.
Got the BBK in ran fine without jetting kept an eye on the temperature obviously got a little hotter but not over 325F for a shorter ride not a lot of WOT. Tore down the carb found out the jets I ordered don’t fit the stock Yamaha carb but my bits came in so I bumped it up 2 sizes, tightened the mixture screw moved it out 1 1/2 for good idle, ran super rich on the needle so I moved it from the second richest position to the second leanest, still rich so I drilled a hole in my airbox, Fkn mint. Based off my temps not really going past 250F cruising, I assume it’s still a little rich.

Anyway my issue. When going for top speed at WOT engine shuts off. First time I thought I blew it up, rolled it down the street to check it out, couldn’t find anything started right up on the starter. second time it was like my starter button wasn’t working for a minute. Last time the lights were dimming when trying to start, so I rolled forward to “move the CVT” hopefully freeing something. Started right up. I had 3 of my friends try it out on a flat street and it did that 3/6 runs. I think but I’m not sure it only happens when you let off throttle. The last time it happened I could hear the engine moving just not firing. So I’m not sure it’s a soft size. My RPM pickup is a little finicky but when it’s working 10500 is max. It wants to go higher.

The exhaust flange leaks, I’m going to make a gasket today, i haven’t checked the cvt since BBK running stock 5g weights and stiff clutch springs and stock torque spring. Thinking of halving my rollers and putting 3 6.5g in. New torque spring set and 4g rollers on the way.

Any thoughts?

[bigmatthew86]

What are your temps getting up to when that is happening?

[grapevino]

Jul 27, 2024 16:18:56 GMT -5 bigmatthew86 said:

What are your temps getting up to when that is happening?

314F I believe, further diagnose it’s running great for everything besides long hauls at WOT and when it happens it seems to want to happen again shortly after starting and continuing. I’m now assuming I need to upjet some more because maybe it is leaning out up top I just haven’t had it there long enough to see temps rise?

My plug was pretty fouled It’s definitely running on the richer side low end.

[Zino]

You are soft seizing

You are using way too much oil it is fouling plug
The pump will keep up with 11k rpm’s on its own
I have never added oil to the gas at break in on any builds
When you add that much oil you lean out the mixture and it will run too hot
When you premix with modern oils you can run 50 to 1 ratio

If you have an exhaust leak that is the second reason you are seizing
You cannot upset enough to offset the exhaust leak

[grapevino]

Jul 28, 2024 8:59:31 GMT -5 Zino said:

You are soft seizing

You are using way too much oil it is fouling plug
The pump will keep up with 11k rpm’s on its own
I have never added oil to the gas at break in on any builds
When you add that much oil you lean out the mixture and it will run too hot
When you premix with modern oils you can run 50 to 1 ratio

If you have an exhaust leak that is the second reason you are seizing
You cannot upset enough to offset the exhaust leak

I was running the same mix when I put the exhaust on and the plug looked great. Should I get a different plug? I’ve heard of people going for a colder one. I haven’t seen my head temp go that high yet, I don’t know how accurate it is though.

I made the gasket for the exhaust the leak. I totally goofed my ratio in the post, it is 400:1 in the tank 6L tank with a cap of oil being about 15ml. The upjet helped a lot, but it still sized after a pretty long ride with tons of rpm’s. Currently running 22.5g worth of rollers. 3x7.5g (definitely a little light)

If I want a better time tuning should I be looking at getting a new carb or just a needle for the stock Yamaha. Last night it felt kinda all over the place for how it was running.

[bigmatthew86]

Like he said there is 0 reason to add any extra oil if your injection system is working & you’re using decent oil.

[Zino]

My current vino set up on both scooters
stock carbs 72 mikuni needle one notch richer than stock
no extra oil besides what stock oil pump provides
malossi sport 70cc ,wild lion exhaust ,malossi variator 6.5 gram sliders ,medium clutch springs ,stock contra
NGK Br8hs spark plug
this set up has run flawless for 1000's of trouble free miles and peaks in the low 50's
Temps on the temp gauge peak around 330 degrees
rpms peak low 9000's and it is very spunky on and off throttle It wheelies very easy

What 70 cc kit did you put on if it is a
sport level kit (single exhaust port cast iron) With stock pipe you should be running about stock weights in the variator or less Run the NGk BR8hs Plug

If it is a racier kit (aluminum single or double ring more than one exhaust port) with loud and proud
expansion exhaust you would run lighter weights than stock weights the more extreme kit the lighter the weights and very stiff clutch springs for example I ran the yasuni R with a malossi 70 cc and was running 5 grams in the variator and hard clutch springs and you would run the NGK BRhs 9 plug .

I think your weights are way too low for most set ups

[bacon]

Shot in the dark, but have you checked the carb for fuel flow into the bowl? I had an old snowmobile that would run great for a bit and then fall on it's face. A minute later it would start up and do the same thing again. Lower speeds seemed to be fine, but WOT would bring it to its knees. Turned out the filter or inlet seat was obstructed and it wasnt able to flow enough fuel to keep up with WOT. You could also check your float level - unlikely to change and cause issues but it's an easy check.

You could also pull the head off and look at the piston to see if it's scuffed. That would confirm soft seizing.

And for those that may be confused by "running lean with extra oil"...rich or lean refers to the air to fuel ratio, not the amount of oil used. The more oil in the fuel, the lower the amount of actual fuel in the resulting mixture. Adding more oil seems logical, but it displaces fuel and causes it to run lean for a given jet size.

[grapevino]

Thank you all for the insight. I learnt quite a bit from this ride, wish I started toying with it years ago. Poor thing sat in my backyard for a hot minute.
Definitely could be on something with the fuel flow. There may be a gas cap screw in the tank… I tried getting a colder plug but my parts stores didn’t have them so I’ll get one on order asap. Along with a needle.
I took off the boot on-top of the airbox, bored the pilot a bit, lifted the needle, and Stock 5.5g weights. Runs awesome, goes 55km/hr before the variator maxes. Just been keeping it out of higher rpm’s until more drill bits show up(after checking for that lost screw in the tank) Currently at .04” I believe is #101.5 main jet.
Cylinder is a eBay iron special single non baffled expansion exhaust.
Currently looking for the “clutch” bearing the one that the pinion rides on. Hoping the find dimensions, just about to dive into the search bar.
No oil in the tank anymore and Polini green springs. Might try the 6.5g weights again, but the take off feels almost perfect right now.

Cheers, thank you again.

[bacon]

101 main sounds huge for your build. What carb did you say you run? Id think you'd be in the 85 range

[bacon]

Also, just order a set of tuning jets for your carb. Drill bits or reamers will never give you the quality or consistency you need. Look at the build threads section and find some similar units and try to dial in the proper jets