New Exhaust Now Blah!

[mkksat13]

Update: Thanks for the picture and a lot of great info. I successfully sealed the reed block area, no more varying RPM's when I shoot the contact cleaner around it. The Zuma is still dying, however. It's hard to start, taking several kicks plus using the electric start. When I get it going it will run on the stand for maybe 5-10 minute, and I let it idle and also rev it up. The electric choke is functioning because after a couple minutes the rpms drop, plus I tested its function off the bike. It seems like as soon as the scooter really warms up it won't stay running at idle anymore. The last test drive when it died it wouldn't go over 30 mph, also concerning. This last time after it died I pulled the spark plug and shot the temp gauge on the cylinder head, it was 230, so not too hot. This bike has the Malossi 70bbk OR pulley/variator, and an aftermarket 14mm Chinese "stock sized" carburetor. The bike ran 50 mph+ with the original stock exhaust and this same setup. The only thing I did different was install a Technigas silent pro exhaust because the stock one was beat up and had been welded/repaired. Since this is a 70bbk do you think I need to mess with carb? I installed same exhaust on my stock pre bug and it worked out great, didn't have to change a thing. I don't know what happened to this bugeye, only thing different was that exhaust. I have been scouring these forums on here trying to figure it out. Any ideas? What do you think about the stator possibly? I'm going to do a compression check but the bike was great before new exhaust so I don't believe that's an issue. Hard starting and dying at idle when warm.... HELP!

[aeroxbud]

If it's dying at idle when warm it could be the pilot jet blocked. Perhaps the choke is letting enough extra fuel in to allow it to idle. But once this shuts off the idle circuit can't keep up.
You might also need to adjust the mixture screw. Not only for the new exhaust, but because of the new warmer summer temperature.

[bigmatthew86]

Are you running the stock air box? When I installed a 70 kit on my old bug eye with stock car & Leo Zx pipe I went up to an 84 jet. You can order one from the Yamaha dealer. Also drill out the air screw cap so you can adjust it if you haven’t. What year is the scoot. If it’s a later one it has a throttle stop that keeps you from being able to open fully that you need to mod.

[mkksat13]

This is a 2002 Bugeye with the stock air box. The pipe that I put on is supposed to be slightly better than stock, but nothing like the performance pipes so stock carb should be fine I was told. Since the bike has a bbk, do I need to do anything with the jets? I did not have to do anything to my prebug when I installed this same pipe on it, does the 70bbk that my 02 has make a difference??

The air mixture screw is there, and I've tried 1.5 to 2 turns out and still stalls.
Aeroxbud, which way should the air screw be turned? The plug looks good, color-wise.

[aeroxbud]

When the bike is warm, turn the mixture screw a bit at a time. Till you get the highest idle.
Clockwise will make it richer.
Did you adjust the jetting when you fitted the bigger bore?

[rtnifty]

I was going to ask if you upjetted after putting on the pipe. Sounds like a not enough fuel issue. While the enricher is on, there's enough gas to start up and run, but then when that shuts off, starved for fuel. If it's doing it on the road and on the stand then it may be both the pilot and main need adjustment.

I would follow what aeroxbud suggested at first, since that's the easiest adjustment. If that doesn't work, then I would up jet the pilot and let it run on the stand, - dont rev it. See if it shuts off after enricher kicks off. If it stays running, then adjust air screw for fastest idle as suggested. After that, while still running, snap the throttle wide open and see if it bogs or hesitates, if it does that, then the main is likely lean. Lastly - I've built a couple motors with that Malossi 70. In the instructions (right? one of the only kits to come with instructions. I like Malossi) they say to fully raise the needle, especially on a stock carb. Did you do that? If not, I would strongly suggest that you do.


- just a shot in the dark given what I've read so far. Hope you get her running.


"aftermarket 14mm Chinese "stock sized" carburetor." - This also gave me some concern. Not saying you need to ditch the carb, but I've had one go funny on me in a very similar way. It would make the bike run lean and then shut off. I think I determined that it was a matter of a leaky airscrew. I sorted that out and it ran for a bit, but then it started acting up again. In fact, it was this carb exactly - www.vmcchineseparts.com/collections/2-stroke-carburetors/products/chinese-2-stroke-carburetor-version-26-keeway-50cc-scooter.

You may want to check your carb. It somethings gone wonky with it, it can throw everything off.

[mkksat13]

Thanks for the reply. No, I have never up jetted or messed with the aftermarket Chinese stock-size carb. I never changed the needle position either.  It ran great with the 70 kit and stock exhaust, never needed to change anything. Since the new exhaust it runs or barely runs I should say. The 70 kit was in there before. So now because of the exhaust I need to up jet? I believe probably both main and idle jets because it does bog and tries to catch up when I twist the throttle on the stand. My Chinese carb is similar to that one rtnifty but it was $25 off Amazon and has the dual throttle. I feel I need to invest in a quality carb that is super easy for me to tune. Any suggestions?

[rtnifty]

mkksat13 okay, this is gonna be a long one. Bare with me.

it's quite possible that the stock pipe was keeping the revs in a certain range where you weren't really Burning It Up, but you were perhaps on the very edge of lean. With the new pipe it sounds a lot like you've wandered into lean territory.

What you're describing sounds like the auto enricher being on when it's cold you're able to start the bike and run it. When the enricher shuts off and the bike warms up it shuts down, that sounds like an air fuel issue. If you're snapping it open on the throttle and it bogs then you're definitely lean.

I would say start with that pilot. Go up a size. Put the bike on the stand start it see if it lasts. Adjust your air screw so that you're going to be in the right range as far as your RPMs go. If it stays running and it's happy then you pretty much got it on the pilot. Once you got that set I'd say go ahead and raise that needle so that you're getting adequate fuel in the mid-range.

For your for your main you're going to want to up jet a little bit as well. You should be able to snap that throttle and that bike come alive without any bog or hesitation. The last bit of consideration, and what I use for my final test of the pilot is coming off of Full Throttle. If I let off the throttle and I feel it hiccup or skip, then I know that I need to go up on that Pilot One More Time because it's lean coming off of wide open. Just a little extra protection for the engine. Some folks may have different experiences but That strategy served me well many times.

Last bit as far as the tuning goes. You need a cylinder head temperature sensor. I can't stress this enough it will save your butt when it comes to tuning and it will stop you from burning up the motor while you're trying to get things dialed in. And when you're riding around that temperature sensor will save your butt if something goes wrong and you start burning it up. You'll be able to notice and shut that bike off before you do any real damage. I put one on all my bikes now. A lot of people like Trail Tech nothing wrong with it great great sensor. You can also shop around I've used run leader before. They're a little sketchy so buyer beware on that one. I've had good ones and I've had faulty ones. Trail Tech I haven't had any problems.

Let's look at it this way playing with the Jets is a fairly cheap and easy attempt to fix right now without dumping the carb just yet. If you can get her running right with just the jetting and that's a win and you'll have learned a shitload about tuning. It'll be less intimidating when you move on to buy a better carb.

As for your carbs, there are a lot of choices out there. Polini CPs are a good starting point. Easy to tune. Also OKO which is more like a keihin. Of course there's Dellorto which makes the 17.5 phva. Not a bad carb pretty easy to tune just to step up from what you're running now actually. Malossi also recommends 17.5 for that kit. You can go bigger like a 19mm if you want. That case the delorto phbg is a highly popular choice. Phbgs can be a little finicky a little difficult to tune, but they're good carbs too. I believe Malossi also makes a phva 17.5 so you could match up if you want.

So you can see there really a lot of great carbs out there with a range of prices. I recommend that you think about what you want the bike to do and what you're intended use might be. Use that to guide your research when you do move up to a different carb.

And of course don't be afraid to ask people here. Lots of great folks that are willing to help you out and give you good advice as well as the benefit of their experience.

[mkksat13]

Thank you Thank you Thank you!! I love this forum. I will update after I buy some jets, temp sensor and do the install and test!

[Zino]

Rtnifty is giving great advice.
temp gauges will save your hard work .
and also give you big clues where your tuning is at .

here are some carb tuning checks
to test the pilot jet
after the bike is well warmed up
with it it idling
blip the throttle
3 things will happen
1)the idle speed will hang out a higher speed then normal
This is lean and you need to go richer by turning the air screw richer
2)The idle speed will drop back to normal almost immediately This is perfect for a daily rider
3)Idle speed will drop below normal speed and then clean up Racers like this because after they come off of wide open throttle They get a blast of extra rich fuel for cooling

For a daily rider go for 2 first 3 second
Racers take number 3

You want a pilot jet that you can control between 1 to 3 turns out with the mixture screws

Do the wide open throttle test .
On race day we used this to make sure we had right jetting for the day
after your scoot is well warmed up .
Try running wide open on a flat surface a few times
Back off the throttle by 1/4 turn.
This will make you a notch richer temporarily .
3 things will happen
1)Rpms will rise or speed increases .This means you are a notch lean on the main jet and need to go up a jet.
2)Rpms/speed will drop slowly this is close to spot on
3)Rpms speed drops fast this is a notch rich try a leaner main.

For the needle
I just go for the most responsive on and off throttle position if 2 feel close to the same I go with the richer needle setting.

[mkksat13]

Great advise! I will do this when I get the new jets and temp gauge. As the bike sits now, when I give it a quick shot of throttle on the stand, the rpm's stay high, then drop and it dies, so this is another indication I am definitely lean. I feel like I'm working on a puzzle...difficult but enjoyable!

[rtnifty]

Yep totally agree! It truly is a puzzle. Lolol. Gonna be great though. Some things will come easy, others are gonna make you wanna chuck the part/piece in question out into a lake somewhere. Take your time and do one thing at a time. oh an keep notes. They don't have to be crazy, just enough to jog your memory. That really helps too. Always good to have something to refer back to.

Good luck!

[repherence2]

May 13, 2023 20:17:05 GMT -5 rtnifty said:

Good luck!

my Hawaiian plant medicine kahuna told us, "there is no such thing as Luck, so I will only Wish you Success."

this is the machine world.  it is all logic.  "if this, then this.  if this, then not this.  if not this, then this. etc."

Success comes when people install Cylinder Head Temp gauge and a Tachometer.  until then, it is completely guess and test.  with Temp and Tach, you have numbers.  with a Temp sensor, it is easy to see when you are about to soft seize.  it is easy to see what main jet give the best performance.  it is easy to see what needle setting is best throughout the throttle range.  it is easy to see what is the best pilot jet, sometimes you need to compromise on the pilot jet because most people do not realize that there can be massive off-throttle heat spikes if your pilot jet is small. 

with a Tach, roller weight tuning (shift RPM) and clutch tuning (engagement RPM) is so much easier.

this is machinery, there is no Luck.  there is only Success.

---i think it is a jetting issue because from what is described in the first post, it sounds like it is soft seizing because not enough fuel.  ---

[mkksat13]

Here's an update: My Malossi sport bbk which was a 2004 serial # had approximately 15,000 miles (bike is 2002). The compression was really low so I pulled her apart to take a look. Cylinder & piston in terrible shape, scorched almost, probably due to the soft seizing. Rtnifty was right, it was running on the verge of too lean for at least the last year with the new sport exhaust. It was a good run, can't believe it made it that long. So I have since replaced with identical 70cc kit. I still have the Technigas exhaust, Malossi reeds, complete Malossi OR kit, used the old 14mm carb that I up jetted to 85 main.

So this is my current issue --- it's bogging on take off AND when I come off the throttle, let the revs come down and give it gas again = bogging. It catches up, but would like to clean this low rpm issue up. Do I have to move the needle clip up? Do I have to change the pilot jet too? I'm not good with rich/lean conditions, but seems rich to me, like it's "spitting" at lower RPM's. Still in the break-in cycle, but seems to run great and accelerate strong after the bog. Thanks for the help!

[Zino]

It might not be the carb

if your coming off then throttle and at take off it might be the cvt
in your variator how heavy are your weights what color contra spring are you running and whatcolor clutch springs ?