Feels like hitting the brakes... but I'm not. By thecableguy (Read 773 times)

[thecableguy]

Here's a good one for ya. My scoot is an Icebear PMZ150-3C scooter (2 wheel) with the standard 150 GY6 clone engine.

Here's what happens. Sometimes, once it is fully warmed up, I'll be traveling at high speed (40-45 or higher) and it feels like an invisible hand starts applying the brakes, enough to slow me down 5-10 mph pretty quickly. If I slow down, it will return to normal running, but once it starts happening, any time I speed back up it'll do it again. The motor doesn't drop out, but the RPMs do decrease as I'm slowed. There is also no sound associated with this happening, no scraping, squealing, no change in the engine note.

The scoot runs great, pulls hard to 50~55, top speed about 60 on the flat @7500 RPM when this is not happening.

Things I've done to the scoot since it started happening, both to try to fix it and general upgrades:

Emissions delete
Hoca variator and 10G sliders
Gates belt
Yellow contra spring
Inspected clutch
58.5 BBK including new head and cam
New carb and intake/air filter with 38 pilot and 120 main
Manual petcock and larger fuel filter
Replaced all fuel and vac lines
Performance exhaust
Set, checked and rechecked valve gap .004 both
New tires
Inspected brakes front and rear
Several different CDIs I'm now running the Casoli from tvnacman (Thanks John!)
New coil and plug wire
NGK Iridium plug
New tires
Engine oil and gear lube checked and changed as needed

I thought I had it fixed this spring with the carb and BBK/head/cam install but it happened again last night after a long ride, as I was on a 45 mph road nearing home.

Could this be valve float? Hard to believe that problem would follow even after changing the head and cam.

I'm at my wits end here...

Jim

[trashtruck12172]

try a different valve gap. sounds like it's losing compression as the motor gets warm and the valve is not seating completely.

[crwford]

my wife's brake slide caliber stuck. I took it off pull it apart apply silicon past to slides back in order. Just lift wheels off ground and turn wheel see if it's holding back a little.

[thecableguy]

Yeah Trash, I guess that's what I'm leaning towards now. Maybe I'll start by moving the exhaust out a thousandth and try to make it happen again. I'll try that tonight.

Crwford, I've looked the brakes over front and rear and they are working properly.

[cyborg]

Low vacuum condition caused by sustained high throttle position and vacuum fuel Valve closing?

[thecableguy]

Sorry cyborg, that was one of the first things I did, put in a manual petcock. No vacuum petcock for me!

[rks]

Sure sounds like fuel starvation. Have you checked the tank vent?......how about the tank itself, could there be something floating around in there, partially blocking the outlet at times? What does the spark plug look like?

[richardv]

If it was fuel starvation the engine would slow down and he has said it doesn't slow down. Just him.

[rks]

I read it differently, I guess: "The motor doesn't drop out, but the RPMs do decrease as I'm slowed."

I took 'doesn't drop out' to mean, doesn't quit..... "but the RPMs do decrease"....as they indeed would, if fuel supply was reduced by a restriction.

[thecableguy]

It's possible that it could be a fuel restriction, or at least fuel starvation of some kind. I figured out that yesterday when it did it, I was very low on fuel. Perhaps with not much left in the tank, the "fuel pressure" available in the gravity fed system wasn't enough to keep up with demand.

So I went ahead and reset the exhaust valve to .005, filled the tank and gave her a test run this evening. Not really a true test as I didn't have enough time to ride around long enough. But no issues and she seemed to actually pull a bit harder up hills. I gained 3MPH going up my test hill by my house!

So perhaps I had more than one issue going on...

I'll keep riding and let you all know if I can get it to recur, while keeping the valves set to .004 in .005 ex and making sure I have gas in the tank!!!

[cyborg]

Tank venting can also be an issue,,,I had that problem on 2 separate bikes

[thecableguy]

Just an update to all, I have not had the problem recur, but I also haven't let the tank get below 1/4. So it was either related to needing the exhaust valve a bit looser or just not enough gas in the tank...

Thanks for all your suggestions!!

Jim