Flat at top of throttle - SOLVED!
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Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Fri Apr 26, 2019 12:36 pm quote
Ok brain trust, help me understand whatís going on here.

I just put a GS race piston in my Polini 177. It winds out fine, and pulls hard, but when it gets to the top of the rev range it stutters and surges, like itís going rich.

Using a 20/20 carb, 160AC, BE4 mixer. I started with a 120 main and have gone progressively down to a 110, but it still wonít rev all the way out. Donít think I want to go any leaner.

Starts fine, accelerates fine right up to 3/4 throttle. Temps never get higher than 250į.

Tried swapping carbs, same thing.

Plug looks good, compression is good, timing is dead on 18į. Brand new BGM fast flow tap.

What say yea?

Last edited by SoCalGuy on Sat Apr 27, 2019 7:53 am; edited 1 time in total
Hooked
59" Allstate
Joined: 03 Aug 2018
Posts: 188
Location: TC Florida
Fri Apr 26, 2019 4:26 pm quote
I don't have the real answer, but the next thing i would try is going richer on the main. 2 sizes to start and see if it gets worse.
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Fri Apr 26, 2019 4:58 pm quote
Just tried a 125. Same.

Iím thinking maybe itís electricalÖ almost feels like a rev limiter.

I donít have a tach but my butt dyno tells me itís happening somewhere around 6000 RPM.
Hooked
1955 Allstate 1958 lambretta 1965 Allstate 1968 sprint
Joined: 20 Jul 2017
Posts: 379
Location: Central california
Fri Apr 26, 2019 6:09 pm quote
Flat engine
Well it sounds as if you have the carb thing covered. Sounds as if you may have a coil breaking down at the high revs. I have had that happen to some of my cars. Just an FYI I ohmed out the coils and they tested good. But they did not test good on an oscilloscope. Swap the coil....then cdi box. Also donít forget the pickup. Hope all is well scott
Ossessionato
1980 P200E, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 3764
Location: San Diego, CA
Fri Apr 26, 2019 6:21 pm quote
Why the new piston? What exactly are the differences between this and your old one?
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Fri Apr 26, 2019 9:01 pm quote
Scott: Coilís a possibility (no CDI on this bike). Points are new. Iíll try swapping out the coil and condenser.

John: The GS piston is essentially identical to the Polini ... itís made specifically for the 177 kit, same size, same compression height ... but lighter and stronger than the Asso piston Polini uses, and has thinner (1mm) steel rings.
Addicted
Joined: 26 Jan 2019
Posts: 633
Location: california
Fri Apr 26, 2019 9:57 pm quote
Given my recent inquiry for opinions on electronic ignition v points... feel free to ignore.

However - I have a favorite trick for electronic issues.

Start bike with cowell off motor - in the darkest place you can find - and look for spark arcing from electronics.

Sometimes high rev issues come from bad plug wire/ weird unseen coil crack/ etc.

Super easy to spot when done in dark room - feel like a genius when you pinpoit them.

Neighbors won't love you - but hey - fu*k the neighbors - not easy to sleep when your scoot is misfiring.

:-)
Molto Verboso
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 1457
Location: London UK
Fri Apr 26, 2019 10:28 pm quote
Re: Flat at top of throttle
SoCalGuy wrote:
Using a 20/20 carb, 160AC, BE4 mixer. I started with a 120 main and have gone progressively down to a 110, but it still wonít rev all the way out. Donít think I want to go any leaner.
On this AC160 is too big and for a BE4 it should be an AC120

Probably something to do with the electircs though.

You get the holes cut in the GS piston ok?
Molto Verboso
Kymco AK550
Joined: 09 Feb 2008
Posts: 1113
Location: UK
Fri Apr 26, 2019 11:56 pm quote
In addition to other suggestions check the fuel flow by running fuel from the carb end of the pipe to a container and make sure it is not being restricted by tank airlock or similar. If the tank breather is an issue it would usually be worse with a full tank.
Ossessionato
1980 P200E, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 3764
Location: San Diego, CA
Sat Apr 27, 2019 6:38 am quote
So is the piston a push for higher rpm/power then?
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Sat Apr 27, 2019 8:08 am quote
Ba boom!

Put a resistor plug in. Stutter disappeared instantly. Engine now revs cleanly all the way out. Goes like a madman.

Best $2.50 Iíve spent.

Thanks chaps!
Ossessionato
1980 P200E, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 3764
Location: San Diego, CA
Sat Apr 27, 2019 8:14 am quote
Nice. So itís a better performer than the old piston?
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Sat Apr 27, 2019 8:33 am quote
Iíll let you know after I get a few miles on it now that the rev issue is sorted. Havenít given it a real workout yet, but it feels like it spins up faster and produces a few hundred more rpm at least. Much more lively on the pipe.
Ossessionato
1980 P200E, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 3764
Location: San Diego, CA
Sat Apr 27, 2019 8:47 am quote
Cool. Grab a trail tech tach they are cheap and pretty easy to move bike to bike if you have a mind to. Itís useful for sure.
Addicted
Joined: 26 Jan 2019
Posts: 633
Location: california
Sat Apr 27, 2019 11:28 am quote
Awesome.

Funny how scoo'ts operating performance on any given day can affect overall mood!
Hooked
1955 Allstate 1958 lambretta 1965 Allstate 1968 sprint
Joined: 20 Jul 2017
Posts: 379
Location: Central california
Sat Apr 27, 2019 1:43 pm quote
Solved
What made you think or decide to put in a resistor spark plug? Scott[/b]
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Sat Apr 27, 2019 2:30 pm quote
I noticed that with the engine running the LED display on my temp gauge was cycling randomly without any buttons being pushed. I figured whatever was causing that was probably also affecting the spark (since the sensor is under the plug). I donít use a resistor cap, so I thought maybe a resistor plug might filter out whatever erratic signals were making the gauge act up. Lucky guess.
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Mon Apr 29, 2019 7:45 am quote
UPDATE: The resistor plug was a temporary fix. The problem was a bad HT coil (you called it Scott).

New Ducati coil installed and all is well.
Addicted
Joined: 26 Jan 2019
Posts: 633
Location: california
Mon Apr 29, 2019 9:05 am quote
SoCal - Glad to hear you tracked it down.
Highly satisfying!

Can you clarify what your specific electrical set up is on this one?
Understood you are points and condenser.
Something like this?
Assuming:
- Stock stator
- Stock points & condenser
- Upgraded coil?
- Stock plug wire
- Stock plug

Thanks!
- CM
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Mon Apr 29, 2019 10:16 am quote
Yup. Pretty simple:

-Stock double yellow stator
-Stock points & condenser
-Stock flywheel
-Ducati HT coil (avoid the CEAB units, theyíre junk IMO)
-Standard 7mm plug wire with crimp on brass fork connector w/old skool spongy plug cap (I like how it seals the plug hole & keeps precious cooling air from escaping the shroud)
-B7HS or B8HS plug

No battery, lights run thru a 12v TrailTech adjustable regulator
Addicted
Joined: 26 Jan 2019
Posts: 633
Location: california
Mon Apr 29, 2019 10:22 am quote
Tks!
Know you are a proponent of keep it simple.
Like.
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Mon Apr 29, 2019 11:17 am quote
Yeah, simple is good. Iím also a big fan of using factory parts wherever possible. There are exceptions - I like go fast stuff as much as the next guy - but the quality and tolerances of the pieces they put on these bike in the first instance were pretty damn good. Iím still impressed by how well made some of the 40+ year-old parts have held up on my scoots, and I donít baby them.
Addicted
Joined: 26 Jan 2019
Posts: 633
Location: california
Mon Apr 29, 2019 5:07 pm quote
Quote:
lights run thru a 12v TrailTech adjustable regulator
Clear on ignition - tks.
Regulator comment caught my eye tho.
Still new to lots of this.
Blind spots abound.
Didn't know they were a thing...

Just now reading up on use of regulator.
My system is hell on all components that have to put up with boom/bust of output.
Regulator sounds like a smart idea.
Can you give basics of where you have it wired in/mounted?
Is all electrical output - other than stator to coil - going through this?

Tks
-CM
Ossessionato
1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 4868
Location: So Cal
Tue Apr 30, 2019 7:58 am quote
Iíll pm you.
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