puch magnum exhaust

Hey,

I have a sluggish 79 magnum xk. It has low compression, around 90... but i think that's due to having only half of a base gasket, which i'm in the process of replacing.

However, i'm also concerned about not having enough back compression from the exhaust. The bike has a stock exhaust, but it never had anything holding the 2 major pieces of the pipe together. I mean the skinny part that connects to the engine doesnt seal well with the larger section of the pipe. I looked at another magnum and it had a very serious bolting device installed, and mine has none.

Any ideas what this piece is called and where i could locate it? any useful alternative would be appreciated as well.

thanks,

manu

Re: puch magnum exhaust

There's no need for exhaust backpressure on a stock engine with a muffler.. Too much backpressure can melt a piston but too little will do nothing.

The objective of a muffler is to kill sound. Unfortunately this requires the exhaust gases be bounced around and blocked so their energy is absorbed... and this causes resistance to flow (backpressure).

:

On a performance exhaust some amount of back pressure is needed.. pressure in the pipe encourages powerful sound waves and helps the pipe operate efficiently.. but even here too much backpressure is dangerous... more dangerous than on a low power, low rpm stock engine.

:

I think this myth about a lack of backpressure being bad came from something else.. Under certain circumstances a leak at the exhaut port can cause an engine to run lean as fresh air may be sucked into the engine and lean the air:fuel mixture. A lean mix can overheat the engine. But lots of 2-stroke engines run with no exhast system at all and run fine.

:

If you can't find the proper part to clamp the header pipe to the muffler, there are all sorts of mechanical clamps (plumbing and automotive) that might be substituted.. if possible (the muffler can still be installed and removed) have it welded. Or build a clamp that fits perfectly.

Re: puch magnum exhaust

msawkar /
manu OP

wow thanks for the thorough response. Truth be told, when the back part of the pipe is screwed to the frame properly, and the port is tighted to the engine, the pipe seems pretty sturdy. It's just that there is a little gap between the inner and outer pipes...

the exhaust has been that way since i've had it, and the bike used to run much better years ago (i'm just getting it back up and running after it sat in a basement for a couple years).

I think my problem is more related to low compression, like i said before i am getting only a 90 (using a friends compression tester). He said 120+ is more normal. hopefully replacing the bad base gasket will help as the rest of the engine looked ok and not to scuffed, and the piston and rings seemed fine.

Thanks,

manu

Re: puch magnum exhaust

well.. compresion is not related to the cylinder base gasket .. compression is determined by the piston rings and everything above the rings.

certainly do fix the gasket and also check the head and cylinder surfaces for flatness.. if it has a decompression valve in the head check it's sealing .. check it's cable adjustment.

Check head bolt torque with a torque wrench.. about 10 pount feet or so.

The leak is somewhere up there or it's the rings..They could be carbon-glued and stuck into their grooves.. pull the cylinder off and see. Or the cylinder walls could be glazed .. or something else prevents the rings from sealing. Might just be worn..Try squirting some motor oil around the rings and see of compression improves.

the bike probably just needs a thorough tuneup. the exhaust leak at that joint would bother the hell out of me.. i would do whatever t took to fix that.

Re: puch magnum exhaust

msawkar /
manu OP

hrm, i almost feel as if i should change the subject of this topic but i have to respond to your post because you are giving me such good advice...

my friend and I took took off the head and cleaned the cylinder surface... he said the rings looked pretty good (they expanded on their own and where a bitch to get back on properly) and the cylinder also looked ok (what do you mean check for flatness, i can take it apart again and look for something specific... what should the cylinder look like).

I dont think the walls were glazed, but what exactly does that mean. We put some motor oil on the cylinder and put the thing back together. Compression did increase a little bit... but i think it tops out at about 90... which is still low.

I dont believe i saw anything like a decomression valve. what does that look like? this is a puch magnum 1.5hp 1sp (XK from 78 i think)

I'm pretty sure the head bolts are on tight, i'm assuming i shouldnt tighten them an excessive amount... just till they appear to be pretty tight.

thanks,

manu

Re: puch magnum exhaust

i was referring to a warped head.. compression would leak from the gap between the head and cylinder...

if you checked rings and things look good, it's something else. I guess the ring grooves and the lands are ok so the rings lay flat and can seal.

To measure ring wear, gap the rings.. install a ring at the top and/or bottom of the cylinder and measure it's gap .. if its over 0.03" or so, its getting worn..

Cylinder glazing is accumulated, burned oil residue .. a good scrubbing of the cylinder wall will remove it.

a decompression valve is a little cable-operated valve in some heads.. opening the valve allows the user to pedal-start the engine very easily .. when some cranking momentum is built up the valve is closed, compresion builds and the engine fires.

Lots of people strip head-bolt threads by not using a torque wrench.. i recommend you find a small pound-inch wrench for moped work.

Re: puch magnum exhaust

msawkar /
manu OP

oh, i see about the warped head. I'll have to take it off and take a look at it. I've noticed that the threading where the plug sits is a little funky, i can get the plug in properly, but it always wants to go in at an inproper angle... i dont think that's a problem though...

as for gapping the rings... do you mean just place the ring inside the cylinder without putting the rings on the piston first? maybe i'll do a forum search.

i haent been able to find the correct compression figures for the engine, just a lot of hearsay... do you know what i should be work towards? number wise...

-M

Re: puch magnum exhaust

generally, 120 psi is the accepted norm .. some engines have more.. i've heard as high as 160. and i've read that about 80 is the minimum required to get a bike to start.

You're engine is shy about 40 psi and thats a whole lot, assuming the gauge and method is correct.

Yes.. gap is checked by slipping the ring alone into the cylinder. Since the bottom of the cylinder has the least wear and is likely to be the correct bore size, a manual would tell you to put the ring into the bottom of the cylinder to measure ring gap.

New rings are gapped before installation on the piston. If there is insufficient gap, the rings could bind when the engine is brought up to running temperature. If gap is too small, a bit is taken off with a file.

As rings wear thinner, this gap increases.

Re: puch magnum exhaust

the xk magnums are, from the two I have worked on in the past month, strange beasts. all magnum exhausts seem to want to slip off until you get a good system worked out to hold them on - the one it comes with hasn't cut it on any of the 7 magnums we've got here in sf. We hacked at our 1.5hp xk- drilling a few more holes in the inside of the exhaust (where the MKII has them and the XK doesn't), switching the 12 intake & 12 carb to 14. Tonight we switched a new xk's engine for a maxi 2 speed. Most xks seem hobbled, and capable of much more with a little tweaking.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account