Advance timing on the timing light

[jtalk]

Hi Guys,

So I have a 49cc (39qmb) that we did a 49mm bbk to. Unlike any other bbk builds. This one had no power after the build. After the build I upgraded the cam to a A9, adjusted lifters at this time, upped the jets, and put on a pod filter. Exhaust is still on the way. After a lot of testing. I could not figure out why there was no power gains like any of the other motors I've done. So I put a timing light on it to see if it was off. With the engine off. The two lines on the cam line up with the head and the mark on the flywheel lines up on the mark with the "T". With the timing gun after warmup of 10 min. The timing light says that the timing is advanced 42 degrees. None of the other mopeds are running that advance. Maybe 1-3 degrees advanced only.

I don't believe these things can advance timing this far on their own.

Does anyone have any suggestions as to what"s going on here?

I've done two bbk's prior to this one with great success. Both running great even before upjetting the carbs. 

[captincvmn]

I have zero 4t experience
Could the timing marks be incorrect?

[scooter7878]

I would think probably the cdi is possible advancing the timing out of wack or something with the flywheel maby the fly wheel nut lossened up and flywheel slipped on the key way the ignition timing is triggered off the crank and flywheel and the cdi is involved. Cam timing is not going to cause a ignition timing issue not on a scooter anyways some newer cars may use a cam sensor input for some ignition timing calculation. Only things involved in ignition timing are crank, flywheel and keyway and cdi. The cam being off would be considered valve timing not ignition timing

[FrankenMech]

check the flywheel key

[jtalk]

So I just managed to take a look at all of that tonight. The flywheel key and all looks like it's all where it's supposed to be. one thing I did notice is that the magnet on the flywheel is slightly bigger and longer. Here is a pic of the two side by side that I had on hand. Both flywheels are being held with the keyways and timing marks lined up. The one with the bigger magnet is the one that was on the engine.

[scooter7878]

What one was on it originally have u tried both if the magnet it longer in one that would trigger the spark sooner than the shorter is this something that was running good than started having a issue or was it never running or running correctly. Unless u or the last guy had swapped flywheel I can't see that being it unless it came to u running like this and u have no idea how or if it ever ran

[jtalk]

This was a brand new engine. It didn't have a stator on it. So I just pulled one off of a different motor I had laying around. It seemed like the perfect candidate for a bbk build. It did run, but just felt like it was lacking power. The build was done right before fall. So my friend never fully broke it in. He just pulled it out of storage this summer while moving into a new house and brought it over to me. I've been trying to tune it every which way. Still feeling like it was lacking power. I decided to put the timing light on it to see if maybe we had the timing off on the cam. All the marks lined up, but my timing gun was saying that timing was advanced 42 degrees. The flywheel with the smaller magnet matches all my other 49cc engines. (139qmb's)

Does this small difference explain an additional 40 degrees of timing though?

I've seen a video where modifying the pickup to move it back physically can increase the timing, but I don't think it was by this much.

[scooter7878]

It's hard to say but yes it can advance it if the trigger on the flywheel that is longer hits the crank sensor before a flywheel with the shorter flywheel it's going to trigger sooner. Is try the other flywheel for shits and giggles can't hurt to try. And I have my sensor advanced by elongated the holes but it's not by a lot but I def noticed a difference

[scooter7878]

I'd def try the other flywheel and if ur really wanting to know I'd find something to slide through both flywheel in the key way slot to have them exactly lined up and put a straight edge on the leading edge of the longer trigger and see exactly how much difference there is but I believe that's ur issue. I'm pretty sure with some math calculations u could using the exact difference in the two triggers and the exact measurement of the outside of flywheel get a number of how many degrees advanced it is

[captincvmn]

Are the key ways in the flywheels aligned with each other in relation to the magnet placement around the outside?

[scooter7878]

Jul 21, 2021 5:34:04 GMT -5 captincvmn said:

Are the key ways in the flywheels aligned with each other in relation to the magnet placement around the outside?

I believe he did it by eye but he did say he had the key ways lined up I suggested finding something to fit both key ways to get them exact

[FrankenMech]

The difference in the magnets will give less than 10° difference, just like moving the timing pickup. Compare it to a degree wheel or protractor. The timing light may be defective, try it on another engine.

Any obstruction, mismatch, or misalignment in the intake or exhaust will also cause sluggish performance. People will also cause performance problems when trying to port match or gasket match an intake, -don't do it. Flow must always go from a smaller port to a larger port. Same size ports will ALWAYS cause a flow obstruction when assembled.

[jtalk]

I lined up the key ways as best I could eyeballing it, and the timing marks on both flywheels all lined up. That was when I noticed how much longer the bigger magnet was. At first I just figured maybe it was a different maker that's why the magnet could be wider. Then after lining up keyways and timing marks, I figured something was different.

Timing light is working fine. Checked the timing on my other two mopeds that are running great and I get the same results. 1-3 degrees advanced only. checked it on my car and it reads as normal.

triple checked the timing on the cam to the flywheel. Even had someone holding and making sure the flywheel was lined up while I redid the cam. Just to make sure it's spot on.

And there is no port matching done. I don't believe you really need to go that deep into it just for a fun ride. If the 139qmb is not enough for you. You might as well get the bigger engines. I don't think doing a swap on these things is all that hard anyways.

[GrumpyUnk]

I do not know what procedure you are using to check timing with a timing light. Do note that the metal tab on the outer of the flywheel is NOT at TDC. It is wherever convenient to put the pickup dictated its spot on the diameter.
That said, I think I would try a different CDI. The internal workings depend on charging/discharging a capacitor. That 'time' will depend on voltage, and if the capacitor is charged to full farad limit, by an aggressive/efficient CDI, or the other direction, a capacitor that is on its way out, timing would vary.
It's an easy thing to swap, and it may be the cause of your measured timing. I would expect 40+ degrees of advance at cranking rpm, or kick-starting rpm would be cause for kick back regularly. IOW, making it HARD to start, much like a lawnmower that has sheared its timing key. That would put the flywheel a bit ahead of where it should normally be when aligned, and I can attest to the mower REALLY grabbing the pull cord handle, to the extent that it hurt when it pulled unexpectedly. It was MUY difficult to start, and I may have been unsuccessful starting when the key was sheared.
I did not think the flat metal timing button on the outer of the flywheel was magnetic. I think it is just metal that comes close to a pickup and generates a 'pip' of current to trigger the CDI. The voltage is miniscule, I think.
tom

[FrankenMech]

Sorry for being late to post and not seeing the post above, fixing lunch and very slow typing.

Have you replaced the CDI? I still can't think of anything that would advance the timing that much. The timing and ignition on these things is stone fence simple. Timing that advanced would probably make the engine run backwards or be very hard to start. People have found that even a few degrees advance by moving the pickup or using a 'racing' CDI will make an engine kick back or hard to start.

BTW, very few people do engine swaps due to the difficulties involved in going from a bottom mount (50cc) engine to a top mount (125/150cc) engine, frame mods etc. I mentioned the port matching due to the original low power problem and the fact that people do try it...