So I ride a 50, one cylinder, no exhaust valve, all port. I have altered it from stock with a higher compression piston, a pod filter, carbon fiber reeds and a rejet. Recently I noticed that the o ring on my head pipe was blown and crud was spitting out where it was bolted to the head. i replaced the ring and sealed it back up and the following condition now presents itself.

water temperature is increased on average as much as 5 degrees.

the bike lays down at the top like it's choking.

I have a new plug in there and I believe the main jet to be just a tiny bit too big but when the exhuast was leaking it pulled solid all the way to redline and would happily hum along sitting there throttle cracked wide open. only after the o ring was fixed did it start losing power up top.

I have a theory and I wanted to present it for critique by those here with chamber building knowledge.

I think the factory pipe is too small. The bike now flows more air through the engine than it did off the showroom floor. What I sort of understand about chamber design is the following. (stop me if I am wrong)

1. Sound waves leave the engine, hit the reversion cone and bounce back shoving the escaped incoming fresh charge back in at the right time as the piston closes off the exhuast port.
2. These waves pass faster through a denser medium so a chamber of the same dimensions reacts at a different rate depending on the pressure of the gas contained in it.

Taking this basic theory and applying it to what I am seeing I conclude that.

1. The repair of the o-ring has increased gas pressure thus changing the speed at which reversion occurs making the pipe "too fast".

2. This is packing the intake charge back in alright but also a lot of spent exhuast causing chamber temps to increase and the charge to be contaminated creating both the temperature rise in the water and the loss of power.

If this in fact the case I believe that means I need to either lengthen the belly of the pipe to retime the pulse or put a larger stinger on it to lower exhuast pressure. Am I on the right track?

nobody?

How long did you have the bike running 'well' with the new engine mods (compressions, reeds, filter etc) before you changed the exhaust o-ring / gasket?

I'm no expert on these matters, but if its choking at top end, with the exhaust leak now sealed, your pipe itself could be partially blocked. If the combusted gases are not exhausted out of the cylinder properly after detonation, it will run hotter.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30

half a year running well with the blown o ring..

you could be right by thinking the length of exhaust is the problam because we had a few c90s to play with and we made an exhaust for mine using diferent length bmx handlbars and a kr1s end can and we found the short one was doing the same as your describing but the longer pipes cured it and in the end we settled on an in between that gave briliant midrange and sounded like a crf
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ive seen more tarts than mr kipling

before I go cutting anything I am going to loosen the headpipe so it leaks again and see it the power comes back.

o o


Last edited by DifferentStrokes on Mon Sep 28, 2009 7:36 pm; edited 1 time in total

How would a leak equal more air? The leak was bleeding exhuast pressure out so that would be the opposite.
I do plan on rejetting soon regardless. My current jetting is too rich. I jetted it in the winter with cool dense air so it runs best at night and a bit fat during the peak afternoon heat.

o o


Last edited by DifferentStrokes on Mon Sep 28, 2009 7:36 pm; edited 1 time in total

Ah, I see what you mean. I am going to have to approach it very methodically methinks.

How old is the pipe? Has it choked up - might just need a good de-coking.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
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GTS1000 oddball

original pipe. I will definately trying to clean it out. Kerosene maybe?

From my experience I'd say that if the pipe was sealed and jetted correctly, but still made less power than the blowing pipe then there's a chance that the tail pipe may be too small a diameter (or too long) for your tune. A small diameter tail pipe will give good power, but only on very short bursts (drag racer for example), due to the heat build. A larger diameter tail pipe actually reduces power (usually) but allows this slightly lower peak to continue without over heating and the enevitable power loss. The difference may only be 1mm, but it will make a big difference in the way power continues to be made as things heat up.

You say you think the chamber is too small. What part do you think is too small? The shape of a pipe depends on many things, porting being just one of them. A big bellied pipe can over scavenge if the the porting is not designed for this type pipe, or a small bellied pipe can leave a gp style cylinder gasping in the midrange if it doesn't pull hard enough in the first stages.

Clean and seal up the pipe, jet it properly and see what happens. Can't make any judgements if things aren't running properly.

Well the engine is the same dimensions just a higher compression so I am confident the belly of the pipe is the right diameter. I think first that the stinger is to small in diameter which is increasing pressure in the chamber and speeding up the exhaust waves.
I see two ways to fix this.
1. decrease pressure through a very slightly larger stinger.
2. retime the waves by increasing the length of the belly.

As has been suggested I think cleaing it out as best I can should be the first step. This is the original pipe and is certain to have carboned up inside. Cleaning it can only do good things, it is not destructive and only requires figuring out a way to do it.

The stinger is the simplest change, lop it off weld on a section of larger pipe. I can then experiment with differing restriction to get the pressure right before settling on a final end can.

The belly is more involved but I think offers the greatest flexibility for future tuning. My plan would be to cut the belly in half across the diameter and changing the length to retime the return wave.

I'm no expert but I've heard caustic soda in mildly warm water (not hot water) is a good way of cleaning out a pipe.

Basically you plug it up, some people use potatoes, and then fill it up with the solution of caustic soda.

The more times you do it and the longer you leave it the better the result. For example leaving it for 24 hours then draining and refilling for another 24 hours.

You can probably get more info such as the correct solute/solvent ratio by doing a simple search. I'm not sure how well kerosene will work but I'm sure it would turn out to be more expensive.

You can buy caustic soda at the supermarket, or just use drain cleaner.