From: gunnj@ddn-wms.nmi.com (Janet Gunn)
Newsgroups: rec.motorcycles
Subject: Re: 2-Stroke Question: Low-End Pickup
Summary: 125cc 2-strokes aren't SUPPOSED to have low end power
Date: 23 Dec 92 04:50:01 GMT
Organization: Network Mgmt Inc., Fairfax, VA

In article <1992Dec14.101148.25158@galadriel.bt.co.uk>, ant@galadriel.bt.co.uk (Antony Espindola) writes:
> 
> Hi All.
> I've recently been having trouble with my little 125 2-stroke.
> The problem seems to be at the low-end revs.
> It won't pull very well until about 9,000 rpm!!
> It bounces off the scale at 13,000 rpm and makes a hell of a noise.
>
What kind is it? My TA isn't supposed to make power below 9k.
It says so in the manual
(For those too young to remember, the TA125 Yamaha, 2-cylinder, air-cooled
2-stroke, twin shock GP racer, based on the AS3 street bike, dominated
125cc racing in this country for 10 years in the 70s)

An engine needs 3 things to run well -
air
gas
spark
all at the right time.

This is just as true for a 2-stroke as a 4-stroke.
In 2-strokes the port timing (particularly the exhaust port) deterimines
whether you have a broad but low powerband (street bike) or a high but narrow
powerband (race bike).  Later Yamahas have variable exhaust port timing
(blocks more or less of the port, depending on rpm) to give you the best
of both worlds.  If you have one of these it may be out of order.
Others (notably Kawasaki) use rotary valves to determine when the exhaust port 
opens.  If you have one of these it might be out of order.
You might be sucking air through a dead crankshaft seal on the ignition side.
If so, that cylinder would be running noticeably leaner.  You might be sucking
oil through a dead seal on the clutch side -then it would run richer,
and probably blow smoke.  Your ignition timing might be off.  You might 
need new plugs (I was amazed what a set of brand new plugs did to my powerband
at Daytona this spring).

When you say low rpm, do you also mean small throtle opening?  If so, check
the pilot and needle jets. Also check for any other air leaks.  Do you get good
power at 13k, or just a lot of noise? Have you checked your rings?  That's 
what 2-stroke eat - oil and rings.  What is redline supposed to be?
 
> The low-end pickup is not good at all.
> Firstly, the carb seemed to be sticking at about 4,000 rpm, but this
> seemed to cure itself after the carb inlet rubber was tightened
> properly. I had to buy a new one due to starting problems ie. it
> wouldn't. This helped it start but brought problems of the sticking.
What do you mean by "sticking"?
> We also replaced throttle linkages and spark plug.
> 
> Now the sticking has been cured, the bike doesn't want to pull below
> about 6,000 rpm. (get a bigger bike I hear someone cry).
> The bike has a non-standard exhaust and air filter, but has been
> jetted accordingly. The bike was running rich with the needle adjuster
> on full so we brought that down a touch and it seems to be running ok.
> 
Have you tried smaller mains?  What do you mean "running ok" - I thought
you said it had no power.  Have you done a plug chop?  Have you checked
the tops of the pistons?
> I think it may be the pilot jet is blocked and will strip the carb
> (again) tonight.
> But if it isn't this what could it be ??
> And also, as a general 2-stroke tuning question, how COULD I improve
> the low-end pickup ??
>
1	change you ignition timing
2	change your carbueration
3	change your porting

GOOD LUCK

Janet gunnj@nmi.com