5 g rollers causing a short bog down By scootvirginia (Read 2,350 times)

[scootvirginia]

Ok so I have a stock scooter 50 cc typical qmb139 engine. I changed the variator rollers from 8g to 5g and nothing else. RPM was roughly 5500 and is now 7000 which is what I expected to happen. However on the RPM climb at about 6000 the bike dies for a second and then picks back up. As a test I swapped out three of the 5g for 8g and the bog comes and goes but is still there. Switched back to the 8 g rollers and all is good again.

So any suggestions? I was thinking that I need to pair a change to the clutch as well? If so what?

Thanks

[Alleyoop]

Where do you want to run at 5500 or above? If it is above the 5500 then you need more fuel, you are running LEAN at the higher RPMS. Got squat to do with the weights except at higher RPMS you need more fuel.
Alleyoop

[Alleyoop]

You have to tune your Carb, you should be able to run at whatever the engine is capable of.
Of course you do not want to over rev it. But the specs on the scoot should tell you at what RPMS the engine makes it MAX HORSEPOWER. And then you tune the CVT and Carb to run at those RPMS or very close to it. Running above those rpms will do nothing and will actually loose power on you as well as the life of the motor.
If the Carb is tuned right you can change weights all day long and it will run good according to the grams. Heavy weights will lower your rpms and lighter weights will raise your rpms.
for every gram your rpms will change by 500-600 up or down. So you can decide at what RPMS you get the best top end with what weights.

SO THE FIRST, thing you want to do is tune your carb so that you can run at high rpms with enough gas and not running Lean.
So put on the lighter weights and work on the carb, go up on the Main Jet OR you can try and Raise the Needle in the Diaphgram.
If the Needle has slots and a "C" Clip you can raise needle to get more fuel at mid-range and Wide Open Throttle. If it does not have a "C" clip then you need to get tiny washers to put in to raise the needle.
Once you get your carb adjusted right Then you can play with the weights and decide which weights gives you the best TOP END and good start offs and mid-range and also decide at what rpms.

Also Variators make a big difference as well, some Variators have different angled ramps. Some are angled to give you great starts and mid-range and top end at lower rpms than you would expect. The one I found that does that is Scrappys Performance Variator. You will not get a lot of rpms but it gives enough low and good top end.
Alleyoop

[Bashan]

Dang Alley! That's good stuff, let me write that down . He's right on, your scoot never got up into those RPMs so now you'll have to adapt to the new situation. More gas and air are needed so you'll have to follow Alley's advice and see what shakes out. Rich

[scootvirginia]

Great info, read it many times around different forums and trying follow that path already. The first thing was to unchoke this carb a bit as I felt it was restricted more than the standard known practices. Checked them all, drill out the screws and cap, no clip to adjust (will need washers) have washer and o-ring on the fuel/air needle. Slide moves all the way up. The backround is this we have two scoots of the same model bought 3 weeks apart from the same place. Her's runs at 7000 good take off and climbs the hills great and can get to 40 to 43 mph if needed. Mine runs at 5500 and goes up hill at about 25 while she is way ahead by now. If I get to 38 39 mph I am wearing lighter clothing that day...

Somewhere there is a differance between the two, I just cannot fiqure it out. Her throttle is way more snappier. So the intent is really not so much to purposly increase the RPM but to get the same running bike as hers.

Ok so here is the current state: stock 18 mm carb with a 88 main and what we think is a 30 pilot which was in it from factory. It reads H30 on the side. Due the main being increased I adjusted the air/fuel to compensate for the change and then made the idle adjustment. On the stand it does seem to have a slight hesitation when increasing the throttle steadily so I took the cap off the air box where the air first comes in and that seemed to lessen the hesitation. After the re-adjustment it seems to be better, idle is at 1800 Running down the road you cannot feel it to any real noticable degree. I think the sluggishness is in the stock parts of the variator and clutch assembly. The chinese carb is coming off and a 20 mm from Scrappy is going without any unkown restriction that none of us may know about. Peace of mind for me I guess. Of course I will have to go through this again but it is worth the learning curve to get better at this.

The book says max torque at 5500 is 3.0 N.m.

Ok so I am going to put the wieghts back in since I have the carb in a good state at this point with what I have right now and see what happens. Will go to the auto parts store and pick up some small washers and see what that does. Raising the needle will richen the throttle at 3/4 to WOT. I do not have a jet kit, I picked the 88 as it seemed the one people use on stock carbs accross the forums. Not sure if the pilot jet should be changed or run a uni filter. I was trying to stay with stock stuff until it became necessary to change. Also changing one thing at time make life easier for trial and error adjusting.

One more thing...Alley you are well appreciated, just reading the straightbanging posts to help her was amazing. I hope she gets going good soon.

Thanks will post results later

[scootvirginia]

Update on carb: right now it is this:

88 main, 30 pilot, 1 3/4 out on a/f screw, idle 1800 and two #4 washer on slide needle. The washers work great on the top end you can feel the pull from 3/4 to WOT. THAT WORKED GREAT....

After review of the jets from my notes I did change the 38 pilot back to stock and then installed the 5g for the above changes as Alleyoop suggested. So a round of beer for Alleyoop for the knowledge and one for Bashan for the Library section, it is so nice too have all that stuff in one place to go to. Awesome Work

The one thing I need to do though is a plug check just to see where I am at but I think it should spot on based the performance at this point.

I wanted to post some pictures but it seems I cannot press the choose file button it is grayed out for some reason...maybe it is my iPad.

Anyway going a good test ride with the wife be back soon ....

[scootvirginia]

Another update: while cleaning up the tools I found the two air box pieces I took out; one was inside at the the tube before going into the intake tube and the other was the cap with holes in it on the end of the tube going to the air box.

So far so good... I think now it is time to swap 1/2 the weights and get the RPM down closure to 6k... even at full on throttle it hovers at 7000-7100. The wife and I are on even bikes now. Although she weighs in at 160 to my 185 she most likely will win out anyway.

Will test and report results... I have a yellow contra spring with yellow clutch springs but I am not sure what that would yield other than take off at a higher rpm due to the strength of the springs. I also looked up the variator Alleyoop suggested and may order that too but I want to get the new carb on first and get that dialed in.

[scootvirginia]

Yet another update: the mix of rollers worked great and moved the rpm to 6100 to 6500 and top end got a tick better on the long runs. Still need to do the plug check just verify the set up. All in all some good education and seat time.