High idle and no power when hot

[motorpsycho]

ummmm yeah, my stock 49 c.c. 2 stroke will push my 20" bike to 30 mph, with a stock (but re-jetted) nt carb, high flow air filter, expansion chamber exhaust, stock un-milled slant head, b6hs plug, accell superstock plug wire, stock cdi, stock magneto, 41 tooth sprocket. I weigh 150 pounds, and the bike is a 20" frame, with 20" wheels and a 22 inch tall cheater slick on the rear.
I have about $300.00 total in the entire thing.

 

[Perthbiker]

Sounds like you've got a hot one there. I reckon it it probably producing the good top end power by having the pipe tuned to the high revs, carb mixture right, and the standard head not causing detonation due to moderate compression. I opted for a more torquey power delivery and hence the 70cc with the high comp head, reed valve kit and tuned pipe that works at mid revs. Should give a better riding experience when sorted.

 

[Perthbiker]

Latest is that I cobbled together a CDI from the various posts on the subject on both forums. I customised it by adding quite a few options selectable via jumpers. After a long 70km test ride on Sunday, I have to declare it a success. The bike runs much cooler at high revs and seems to pull better at low speed too. Still battling to get it to rev much past 6000rpm so will be concentrating on the main jet settings on the Speed carb to hopefully cure that problem. Might also take a look at the header on the SBP pipe to see if it can be shortened to try get a pronounced boost in a certain rev range.

 

[Deleted member 12676]

Thing is that the ignition timing affects the pipe resonance so really you have to settle on a final setting for it before attempting to find the ideal header length for your bike.
But I will bet you need to lengthen it instead of shorten it. On my site are suggested header lengths for different engines that you could use as a starting point.

 

[Perthbiker]

Ah true, I hadn't thought of that angle.

The reason I was thinking that I may need to shorten the header pipe might be equally flawed. Basically I noticed when I've had the barrel off that there are 'burn marks' on the side of the piston opposite the exhaust port so was thinking that the exhaust is effectively bouncing back after the piston had already passed the port on the way up. If the header was shorter surely the gas will be pushed back into the chamber earlier and before the exhaust port closes?

The SBP pipe is mounted using the standard exhaust port fitment (filed out to try match it to the port as best as possible), the 45 degree copper elbow and just enough of the supplied copper pipe to link it to the full length of the 'J pipe'. About 12" from port to entry into the expansion box.

The bike certainly pulled much better low down and mid revs with the SBP pipe versus the standard pipe, but no power spurt in any rev range. Better performance can just as well be attributed to less restriction rather than any tuned pipe effects. I was wondering whether the power spurt is effectively hiding higher up in the revs that I'm not getting too because of the previous ignition and mixture issues? Maybe the benefits of SBP pipe has been oversold?

 

[Deleted member 12676]

The burn marks are just because of excessive heat in the cylinder heating up the piston too much, especially at the exhaust side. Get ready for a piston seizure if you dont lessen the compression or richen the mixture or put a decent CDI on it. The return wave of the pipe is sonic in nature, not a wave of fire.
These engines, without the ultimate modifications, will never give you a spurt of power at top rpms. There is just too little power to begin with.
With porting, reeds, 18mm Mikuni, performance CDI, and pipe perfectly positioned you might feel a little surge at top rpms. I will know for sure once I get my newly designed racing pipe on my 55cc with Rock Solid reeds and 18mm Mikuni.

 

[Perthbiker]

I have 72 and 74 main jets on order from RSE, and have richened up the mid range mixture on my Speed carb by lifting the needle 1x notch.

Recently fitted the RSE reeds and they went in with only slight rework needed on one cooling fin.

Am also toying with getting a Mikuni carb and have been recommended the 16mm unit, but I'm keen on at least trying to get the best out of the Speed card before ditching it.

Are you running the 18mm Mikuni now or will you be fitting it at the same time as the RSE reeds? Good luck with the results.

 

[Deleted member 12676]

I am going to see what the flow area of the reeds are before I make my final decision as to what size carb. No use putting on a 18mm carb if the flow area is equivalent to a 16mm hole.
If the reeds are more than adequate then you need to use the formula incorporating the engine size and max rpm to know the right size carb. no guesswork needed.

 

[HeadSmess]

the marks on the piston on the SAME side as the exhaust port (never seen them elsewhere) are yes, from the exhaust gasses impinging upon it. the engine say, covers from 1000 rpm, til 6000rpm.

somewhere in this range, its out of tune. at some point, the exhaust is going to get smashed against the piston while the ports closed. even when it IS in tune, it still smashes against a fair portion of the piston as the piston rises to cover the port. the piston doesnt just suddenly close the entire port instantaneously, nor does the pipe just suddenly decide to stop pushing. in fact, the piston on a ht never completely exposes the port, and then, it only opens it fully for an instant as it goes through BDC! the rest of the time, that pistons in the way, impeding flow, creating turbulence.

i rarely see these "burn marks" on a NON piped 2 stroke. burning looks completely different. its a lot...drier looking. blistered. and looking at the underside of the piston crown is a dead giveaway. tan is good. blistered black is not so good.

they ARE NOT burn marks. it is simply oil from the exhaust getting smacked against a a stationary surface.

ignition timing has nothing to do with pipe resonance. port timing and length is all. (yeah yeah, and temperature and atmospheric pressure)

it will affect total pressure placed upon the piston crown, and exact timing of where that pressures "peak" is. when the peak is too close too tdc, or even on the wrong side of it(! way advanced !) its like hitting a brick wall. the force line from con pin to crank pin to crank shaft is too "straight" to develop rotational power.

now, if the peak is too far behind tdc, the piston is descending faster than the gas is expanding, heat is lost, power is lost, la de da de da.

find the perfect spot, which does change as rpm changes, and you will get maximum torque at a given rpm. which is why theres advance curves and companies like honda can invest millions of dollars into just ignition timing... timing is critical, but does nothing to pipe resonance. all other variables the same, the pipe stays as a fixed system. a harmonica tuned to C will always play C. while that exhaust port is closed, the pipe may as well not be there. (impractical, maybe)

when everything is running as planned, combustion has completed and finished before 10 degrees ATDC, if not earlier. so there is, depending on engine, another 50 to 100 degrees of rotation before the ex port opens.
all that is happening is hot gasses expanding and forcing the piston down by doing so. the "flames" you see occasionally escaping exhaust ports, are merely incandescent particles of carbon. (fourstrokers with straight pipes!) the closer to blue, the more complete the combustion, the less free carbon present.

a flame is a byproduct of fast, heat liberating combustion. exothermic.

rust is a slow, cold combustion. you dont see any flames on rust. endothermic. an oxy torch is fast heat liberating combustion. exothermic.

hydrogen burns with an almost clear flame as it contains no carbon at all.


and then, i dont know about sbp pipes, ive never tried one. seen them, never tried one. why should i?

a real pipe, made to the exact, calculated dimensions required, will knock your socks off. without having TOUCHED anything else. (except main jet)

meh. my only major port job was ruined by a ring locating pin letting go. i stuck to pipes and cleanup only after that. too depressing. if it snapped a conrod or something it would be different, but a locater pin letting go? that makes you wanna sit in a cupboard and cry...

though....im getting over sulking a bit...this to do list just gets longer.... longer... neverendingly longer....


arrrrrgh! i hate commitments! why cant i just be a billionaire and do things instead of having to live day by day and never get ahead?

 

[Deleted member 12676]

The heat that burns the oil is a combination of the peak heat near TDC and the heat of the exhaust gas as it leaves the cylinder into the exhaust port and is exposed to the exhaust side of the piston. It has nothing to do with the returning sonic wave of the pipe.
The more advanced the ignition is, the cooler the gases are as they exit the cylinder, which does affect pipe resonance because the sonic wave is directly dependent on the amount of heat inside the pipe. Higher heat, faster wave speed. But advanced ignition increases the the peak pressure in the cylinder which raises the peak temperature.
Cylinder combustion can exist as far as 70 degrees ATDC. The peak pressure happens between 10 and 15 degrees ATDC.
If a good pipe increases horsepower at peak rpm by 50%, and an engine only produces 2 hp, then do you think the increase to 3 hp would "knock your socks off"? Of course not.