piston holing caused by exhaust design....explain please?

[Andy Pickering]

Ive just changed from a Rscoot to a PM, not changed the jetting but been checking the plug but no change..although I think im slightly rich anyway and as im not run in yet am content with for now.

[Train Driver]

nick read these it helped me
http://www.motorcycle.com/how-to/how-tw ... -3423.html

http://www.iwt.com.au/MOTA.HTM

[cezeta]

Perfect post by 6lda 28b which has provided the answer.

"The stinger acts as a pressure bleed, allowing pressure to escape from the pipe. Back pressure in the pipe, caused by a smaller-diameter or longer stinger section, helps the wave action of the pipe, and can increase the engine's performance. This, presumably, happens since the greater pressure creates a more dense, uniform medium for the waves to act on--waves travel better through dense, consistent mediums. For instance, you can hear a train from a long way away by putting you ear to the steel railroad track, which is much denser and more uniform than air. But it also causes the engine to run hotter, usually a very bad characteristic in two-strokes."

[drunkmunkey6969]

i have heard that some pipes cause piston holing. how is this caused? is it a suck it out but not back in - timing kind of glitch. anyone understand this and can shed some light on it?

i have not got a specific problem......just want to look good when i explain it to someone else some day

Broadly speaking, as far as i understand it, 3 main things in pipes cause problems....

1) Cracked pipes, leaky pipes, leaky/loose manifolds etc....cause excess air to be sucked in, lean running and therefore seizures etc.

2) Pipe stingers....narrow stingers cause larger amounts of back pressure and result in excess heat build up at the piston.....don't take it as gosspel, but i think some of the old PM pipes had this issue?

3) Expansion chambers (usually fat pipes - but not always) that give a hard and fast power band can be difficult to dial in accurately at mid-range, and differnt brand/size of carb add to this. The switch between low revs using pilot/top of needle and high revs using main jet etc can be a fast and narrow transition, the variables between engine revs, powerband, slide position and combo of pilot/main/needle/clip/atomiser etc can be a f@@ker to balance mid range.

Helpful?

[cezeta]

helpfull? yes very.

i wonder how many dealers/kits got the blame for scooters going bang when its just joe public like us that do not know the finer intracasies of setting things up.

i have always wanted engines delivered in a box ready to ride and with no effort on my part but have never found that level of service.

20 years of wishing and im realising i need to ride a bit more with my ears open to avoid failure.

any idea why the mugello had a reputation for holing pistons? was it poor compression ratio or merang pistons? they are not heavily tuned so cant be under too much stress?

[soulsurfer]

helpfull? yes very.

i wonder how many dealers/kits got the blame for scooters going bang when its just joe public like us that do not know the finer intracasies of setting things up.

i have always wanted engines delivered in a box ready to ride and with no effort on my part but have never found that level of service.

20 years of wishing and im realising i need to ride a bit more with my ears open to avoid failure.

any idea why the mugello had a reputation for holing pistons? was it poor compression ratio or merang pistons? they are not heavily tuned so cant be under too much stress?

I think a lot of dealers sell kits with little or no advice offered Good for business??

I believe the Mugello had a pooly designed head, but whilst on that subject, there are many kits that don't come with a cylinder head. I find this strange as surely certain kits require correctly machined heads to suit not only compression but also piston crown. When a head is supplied, they invariably aren't suitable. I've now found this on two totally different kits I think these are time bombs that one day and for what seems like no reason it will hole the piston. That's why I use EGT guages.

[Mick Abbey]

Hi Expantion chambers are very complex things from the header to the difusercone/cones to the baffel cone/cones and the stinger outlet, if your stinger outlet diameter is to tight it will cause the engine to over heat and cause a meltdown also if the pipe has an internal stinger (where the stinger sticks into the baffel cone to help keep the noise down this can also cause over heating if the diam is to tight ) what pipe builders are doing is where the stinger joins the baffel cone, the diameter at that point will be a couple of mill tighter eg 22mm then open out to say24mm on the stinger pipe to prevent over heating when on full throttle.
Dan is correct by saying that a loose fitting pipe with air leaks can cause the engine to run lean as when the ex port opens up and the ex charge rushes out down the pipe it causes a vacume which helps scavenge the cylinder and pulls the fresh un burnt charge into the pipe then when the pressure wave reaches the the baffel cone it starts to do a u turn and pushes the un burnt charge back into the cylinder just before the piston closes the ex port ready for the compression stroke to start the cycle again if the pipe has an air leak it can cause the return charge to be to lean and can cause the engine to run lean tis is not as detromental as having an air leak on the inlet side.
Changing a pipe or using an after market air filter or no air filter are the main factors where jetting the carb is important more than porting the cylinder .

Mick

[coaster]

any idea why the mugello had a reputation for holing pistons? was it poor compression ratio or merang pistons? they are not heavily tuned so cant be under too much stress?

I managed to hole 2 pistons on my mk1 186 muggy but that was almost certainly down to having a too small main jet. That said, the crowns seem a bit thin The Mk1 muggy did not come with a head, you had to buy them seperately. I bought a Reaper tuned head off ebay which was marked IMOLA/MUGELLO, I've since heard that Reaper heads tend to have the CR too high