2-stroke exhaust pipe

Hi,

My 1986 Maico 500 is slowly coming together after months
of waiting for parts and many hours of work. And, if I may say so, it
comes together quite nicely. Maybe too nicely, because I have doubts
whether to soil it in the dirt again or not. Still, if my will
prevails, into the dirt it goes - when ready.

On to the topic: The exhaustpipe is dented, cracked and rusted, but
not beyond repair. And I`d like to do it myself, through MIG-welding.
If for no other reason than price, a new pipe is EUR 250,- + tax.

What I`d like to know is how the finish of the inside of the pipe
affects engine performance. Need it be super smooth to allow the
pressure pulses to work their magic, or is that of less importance
than removal of dents and closed leaks? As it is, top end power can
only be reached through a transition phase of coughs and stuttering.
It still pulls furiously, that must be said, but I feel there`s more
power imprisoned in there. And I`d like to liberate it.

No amount of carb-twiddling, short of changing jets (it has original
jets, and so should run OK, yet perhaps a tad rich, in high RPM`s) and
ignition tweaks has helped and the top end has had a fairly recent
hone and a new ring, so my guess is that the pipe is the culprit.

Any thoughts on the subject are most welcome, as is - to those so
inclined - interesting thread warps.


TH

 

According to the math, dents will make very little difference unless
they are a major percentage of the cross sectional area at that point.
There was a good article many years ago in Cycle World magazine that
compared straight theoretical pipes with real world somewhat non-linear
pipes sold by the aftermarket suppliers. The "tuning" aspects of the
pipe relate mostly to the distance down the centerline from the exhaust
port to the transistion. In reality, the interior does have a real world
effect but I would suspect more natural problems like carbon/oil crud
buildup would have more effect than the dents. We used to boil out the
two stroke pipes, especially with older oils run at higher mix ratios.
If you do weld, MIG should be the best though even then stress relieving
the whole pipe after might be necessary to reduce cracking adjacent to
the welds. There's a lot of vibration going on in the pipe wall. Lastly
depending upon the age of the pipe, improving the stinger might produce
more usefull power increase than anything else.

 

Under what conditions does it exhibit "coughs and stuttering"
Acceleration or steady state? Will it idle? Are you sure there are no
air leaks in the cases, past seals, around gaskets or anywhere in the
intake tract? I always pressure check a two stroke engine when I
assemble it. (plug the exhaust port and intake tract, pressurize to 9
to 12 psi, if it leaks down faster than 1 pound a minute, you have a
problem. Use a soap and water solution to find the leak) Is it too
lean in some places and too rich in others? A worn slide, needle jet
and jet needle can cause all sorts of weird problems. All the parts
should be measured, wear that can do harm is not always visible to the
naked eye. Are you sure all the jets and air passages are clean? Is
the float set at the right height. Are all the gaskets in the carb
soft and sound? This bike is no spring chicken so it could be anything
including a bad ignition, though unlikely if it runs okay on top.

The only way the pipe could cause the symptoms you describe is if it
fell off, or squashed flat, or the stinger/silencer/muffler was
completely plugged.

Pull the dents and gas weld wherever required. It doesn't have to be
pretty to run right, just so long as it has the same general shape the
factory shipped it with. Sound/pressure waves in the pipe will work
harden it over time which is why pipes that have been run a while
begin to crack. Do all your repairs then have a heat treater anneal it
and it won't crack for a while. The process of heat treating it will
cause any carbon build-up inside the pipe to crack and fall off and
you can just shake it out.

Bill Smith

 

i've had better luck on thin, old, dirty, work-hardened exhaust pipes with
oxy-acyet (over MIG or TIG)

the bead on the inside should look as good as the bead on the outside;
perhaps 99.9% penetration would be prefered

 

Well, thanks to everyone for their input. This place sure is a
fountain of knowledge if ever there was one.

Here`s more data for those interested:

As to the carb, it is a 40 mm Bing, the piston and the slide are
within wear tolerances, float height is correct, it is clean, the
gaskets are good. It has an idle jet, a main jet - both standard - an
air screw, an idle screw and a needle that clips in three positions.
At summer temperatures I use the leanest notch, as this seems to work
best, i.e. less pronounced stuttering/cleanest power delivery.
I use Castrol TTS at 2.5% in premium unleaded. The cylinder base
gasket is 0.2 mm thicker than standard to compensate for the lack of
leaded fuel and increased squish area due to slightly larger cylinder
bore. There is no knocks or pinging.

The air filter is clean and oiled, and the fiberglass reeds are in
good nick. I use NKG plugs, heatrange 9, which don`t foul and last a
long time. The spark is strong and blue. The silencer is well
maintained.

The engine`s stuttering seems to be RPM-relative, as the load doesn`t
matter too much - unless it`s on frozen lakes in winter on tyres
without studs (I`m currently making my own studded tyres from
semi-worn MX-tyres and sheet metal screws). With no load, the bike
just spins out. And yes, it idles fine, but can be a bugger to start.
Which is not unheard of in large 2-strokes. I see it as an inbuilt
rider-warm-up-feature...

I haven`t done a leak down test, so perhaps I should do that. It
doesn`t burn gear oil, but air might sneak it`s way through at the
ignition side for all I know. Otherwise the engine is sound. BTW, they
still make them - the ATK Intimidator`s bottom end is similar to my
Maico`s motor, the difference being 200 more cc`s in the top end. But
I digress.

Counter to what K. says, it has good low down throttle response. In
fact I think the bike is a nice woodland terrain and single track
tool. Throttle control is paramount though. One needs to be a good
throttler...

I`ve gathered from what`s been said here that pipes are relatively
tolerant to dents. But what about leaks? Could leaks in the pipe, at
the cylinder/pipe and pipe/silencer connections cause RPM-related
hicups? Of course I will find out when I`ve fixed those things, but
thoughs on that is, again, very welcome.


I`ll post a follow up when the bike`s barking again.


TH

 

you might consider a quick
pull-in-the-clutch-whilst-hitting-the-kill-switch-and-coasting-to-a-stop
routine for a plug check next time it happens