RPMs top out at 8K, restricted cdi?

[islandscrub]

Aloha!

I've ridden 3 139QMB based bikes so far that happen to have rpm meters on them, and they all seem to top out at 8k rpm. I read about electronically restricted cdis that do this, but the ones I've seen (on 2 strokes) actually have an external sensor that pings off of the clutch bell. None of these bikes have the bell sensor, so I guess its something else doing it, possibly fully in the unit itself, is this possible? I also read threads about clipping a wire and such, but not sure about that method.

Anyone here know how to derestrict a stock cdi or know of a reliable, verified aftermarket derestricted one? Not looking for a 'racing' one, the stock timing is preferred.

Mahalo!

[90GTVert]

The CDIs can be made to limit RPM on their own. Over the years, I've heard 6,500, 8,000 and 10,000RPM limiters being mentioned. A 10,000RPM rev limiter would do essentially nothing on most 139QMBs because they don't want to rev there anyway. What you mention with the clutch bell is a speed limiter. That system "knows" what RPM the clutch bell would need to turn to reach X speed and if it spins that fast the limiter kicks in. It shouldn't care what RPM the engine is at, as long as the clutch bell doesn't rotate past whatever RPM.

Most of these stock don't want to go much over 8,000RPM anyway. For me, peak power was 7800-8200RPM and with stock rollers and everything it didn't get there that much. Could be that you're seeing the natural limit of the setup, rather than an electronic restriction.

An easy way to check is to secure the scooter on the center stand with the tire well clear of the ground and rev the engine. Don't hold it because you could damage the engine if it's indeed no restricted. Just hold it long enough to see if the tach goes above 8,000RPM. If you see say 10,000RPM, the CDI isn't holding you back. If it still stops at 8,000RPM, then most likely the CDI should be replaced. Stock replacements are super cheap on eBay and amazon and don't tend to be restricted. Good to have around anyway as a spare.

Some have wires that can be cut, but in many cases it's programmed into the CDI and there is nothing you can do about it. Well, I'm sure a really determined person that is into electronics could... but it wouldn't be worth it when a replacement is $10.

[ratdog]

The engine puts out 3 hp at 7500 rpm. Above 7500 rpm, your power will start to drop off. Most of the stock bikes I have seen are hard pressed to pull much above 30 mph which is real close to the 7500 rpm level. As mentioned, easy to tell by Erving under no load, or simply ride down a hill at WOT. If the bike gets up to 8000 going down the hill and starts to misfire, then it’s restricted, if it revs past 8000 going down the hill, then it’s fine, you just don’t have enough power to pull past 8000 on flat ground.

[islandscrub]

Thanks for the responses.

I forgot to mention, two of them are decked out with a 52 and a 52.4 cylinder respectively, plus some other goodies, they both still top at 8k, but I will ask them to try the downhill and stand tests.

[ratdog]

Well if they have BBKs, then they should be able to pull way past 8000 rpm. In that case, just buy a replacement stock CDI. I would stay away from the so called racing CDIs because they are worthless. Of course I have no idea what it is like where you are, but here in the US, there are many, many places to buy CDIs. Both on the internet and in person, and they would not be restricted in anyway. Around here they are cheap, in the $10 to $15 range