BBK Stumble and misfire By scootcanic (Read 1,885 times)

[scootcanic]

Here's the details. We just installed "100cc" BBK. Performance cam, big valve head, new blue style CDI, fresh ignition coil (orange if it matters), 95 jet. Stock intake, carb and exhaust. It idles wonderfully can not complain about that. I have followed Alleyoop's carb tune procedure to a tee. All jets are clean ports are clean, idle speed changes with mixture screw when adjusted. All is good until we drive it. 

When we take it for a spin, it stumbles on acceleration. I have tried richer and leaner, does it less when lean on acceleration but then the second portion kicks in, when on the throttle constant it feels like a misfire here and there. If it is richer it doesn't do that as much but still noticeable. When lean it is very bad. I cant find the happy medium unless there is something I am missing all together.

[Alleyoop]

Ok try this when it stumbles does or acts up BACK off the throttle 1/8-1/4 of an INCH and hold it there.
Now pay attention to what the motor does.
a. IF IT SPEEDS UP it is LEAN at that point
b. IF IT JUST SETTLES down it is RICH at that point.

Now depending on where in the Throttle position this happens Different settings have to be done:
1. If it stumbles as your starting off - Adjust your Air/Fuel Mixture
2. If it stumbles in the mid range - Adjust your Needle
3. If it stumbles at WOT - It could be your Air/Fuel Mixture or you may have to Re-Jet.

Alleyoop

[Bashan]

Alley? Does the stock carb, air filter, and exhaust disturb you at all with the big arse BBK, cam, and valves? Rich

[scootcanic]

Jun 12, 2014 22:16:21 GMT -6 Bashan said:

Alley? Does the stock carb, air filter, and exhaust disturb you at all with the big arse BBK, cam, and valves? Rich

Stupid thing about all the performance parts but stock intake and exhaust is I have a second one I did basically same thing to. That one gets up and goes no sputtering on the way to WOT and full rpm. Once there its great for a solid mile or two. Then its like someone shuts off the key and turns it back on. So far only when hot and WOT. May meed to get some jets in between 90 and 100 and play with it more. That one has a carb slide where the needle goes of 17mm with accel pump. The current one in this thread is a 20mm slide no accel pump.

[scootcanic]

@ Alleyoop

I tried the backing off at wot when it acted up and it didn't accelerate right away, just smoothed out then slowly will accelerate or maintain speed. So that tells me smaller main jet. That will happen this weekend. Again like last post, may need to break down and get a full set of jets between 90 and 100 to iron it out perfectly. I only have a 90, 95, and 100 currently readily available.

[Bashan]

Jun 13, 2014 4:22:04 GMT -6 scootcanic said:

Stupid thing about all the performance parts but stock intake and exhaust is I have a second one I did basically same thing to. That one gets up and goes no sputtering on the way to WOT and full rpm. Once there its great for a solid mile or two. Then its like someone shuts off the key and turns it back on. So far only when hot and WOT.

Could the bowl be running dry? Or maybe a vent problem? How about valves, what's your gap?

[Alleyoop]

Jun 12, 2014 22:16:21 GMT -6 Bashan said:

Alley? Does the stock carb, air filter, and exhaust disturb you at all with the big arse BBK, cam, and valves? Rich

BASHAN,
No the stock carb, filter and exhaust does not disturb me. Freeing up the exhaust will just free up the motor little more power, less backpressure. You can get any motor to run good and the only thing you may be loosing is power but a motor will run good if the valves, fuel and air mixtures are good throughout the throttle range.

SCOOTCANIC,
You may want to Try raising the NEEDLE 1 notch(move the clip DOWN) that will give it more fuel in the midrange and also a little more on the top end(IF it acted as if it was lean) One notch up or down on the needle jet is like going up 1-2 on the main jet size.
Alleyoop

[cyborg]

gas tank venting ok???just went thru this same problem with my vespa and didn't notice until i put clear fuel and vac lines on it,,,i drilled a small vent hole ,,problem went away

[scootcanic]

BASHAN,
Both valve lash is at .004 and was double checked again when I was still having issues.

ALLEYOOP,
I have tried putting it all the way down and it was not good just stumbled in midrange and would not get past the sputtering no matter how i did the throttle position other then backed all the way off. I could go to the 100 and move the clip all the way to the top and see if it makes a difference if it makes a 1-2 change in effective size.

CYBORG,
I could do the same by leaving gas cap off. We can try that tomorrow .

[cyborg]

When I found the problem I just cracked it a bit till I could work on it

[Alleyoop]

Be more clear by what you mean put it all the way down and it stumbles more( NEEDLE DOWN OR CLIP DOWN ON THE NEEDLE) BIG DIFFERENCE.

If you mean you put the CLIP down all the way(RAISING THE NEEDLE) and it stumbled more then it is RICH not LEAN(In this case move the CLIP up(LOWERS THE NEEDLE) to LEAN IT OUT.

If you mean you put the NEEDLE down all the way(LOWERING THE NEEDLE) and it stumbled more then it is LEAN not RICH(in this case move the CLIP DOWN(RAISES THE NEEDLE) to RICH it OUT.
Alleyoop

[scootcanic]

Hope to eliminate confusion this post.

I just moved CLIP from center to top of needle, makes it leaner on acceleration. Test drive results were good at idle and midrange on acceleration. At WOT it sputters. Backed off 1/4 and it just settled down. Did not have a noticeable gain in engine speed or power. This says I am RICH at WOT. Going to put in smaller jet (90), rinse and repeat.

[scootcanic]

So I have installed the 90. The sputtering at WOT is cured. Just now have the issue where it intermittently will skip out while driving now. At WOT going down the road it seems like it loses spark or fuel then regains power again. Just like if the key was shut off. I have the same issue with the other scooter in the garage. Both run, idle and accelerate fine, just the miss at WOT while driving. Most of the time it is fine. Just sporatically while driving. Have not been able to pinpoint a good way to duplicate it.

[Alleyoop]

Does it happen at a certain RPM? If it does and it drops then recovers I think your hitting a REV LIMITER. Check if your scooter has wires going into the CVT some have a REV limiter reading the crank. If so Unplug it.
Either that or try a different CDI. If you have a BLUE with some diagram on the face of them so called performance CDI those are CRAP many have complained about those and taken them off. Also check the Coil wires make sure the two wires on the coil connector are nice and tight and not loose and or touching. If a little loose put a plier to them and squeeze crimp on tight.
Alleyoop

[scootcanic]

Alley, it is funny you mention that. We got it running good. Likely the same time you were typing was the same time I had that same thought rev limiter. Got my pal over here and test drove both at the same time. Found out happens at same road speed and more so on flat and downhills. Exactly the rev limiter as mentioned.

Is there some tell tale sign of a crap cdi that you have mentioned. We have that blue one installed and seems to be working fine. I would like to know what to look for and a recommendation on a good one if that is the case.

@all that replied: Thanks very much for your help! I can not stress that enough! A good forum with awesome advice is very hard to find.