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two stroke bike four-strokes at midrange

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#1
hi, a friend asked me to built his engine of a yamaha dt 50 with a 55mm cylinder but everything was used from diferent bikes... i put a new piston and a new stroker crank but the cylinder was used and already ported by someone that loves to do holes...i haved checked everything(i think) but the bike doesn´t stop to four-stroke at midrange, on the low end it´s ok so it is when the bike enters in it´s power range... i thougt could be the exhaust port very close to the main tranfers(about 1 mm) so i epoxy the holes just to have sure but nothing...
someone could help me???
 
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#2
helio, just bumping this up to the top of the forum posts to make it more visible for you.
If you want someone in particular to answer it, perhaps you could reply to your own post and make that request.
Meanwhile here is a bump up. Hope it helps.
 
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#3
Try lowering the carburetor needle 1 notch. It sounds like it is running too rich. If that helps, lower it another to see if it's any better. Increasing the displacement frequently requires that the jetting be leaned. It may also be necessary to get a smaller main jet. By the way, the needle is lowered by putting the clip in a higher notch.
 
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#4
Heilo.

I agree with slodad, sounds like mid-range jetting. If adjusting the needle and needle jet does not help, try going to a bigger cutaway on the slide. (The opening on the back of the slide.) If you can't find a slide with a bigger cutaway, you can open up the cutaway on the existing slide by grinding it. Don't go more than 1/2 a millimeter at a time or you could cut too much out. Also, be sure to maintain the same radius as the original cut-away.

This lets more air into the engine at mid-range. I would try this before changing the ports.

Good luck. :cool:
 
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#5
was the first thing i checked... i spend hours jetting the bike, i just don´t change the cutway because i could need it more richer... but jetting doesn´t help much... and the engine just don´t stop "four stroke" either the jet is lean or rich,the throttle just cracked or wide open...
the only thing that help is raising the compression ratio...
and by the way the bike also runs a little hot without any special reason...
it seems the combustion is too slow by any reason...

thanks by the "bump up" olderndirtmom :cool:
 
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#6
Does it have the stock air filter? Is it clean? Is the sparkplug new? Is the sparkplug wire cracked?
 
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#7
have a aftermarket filter without a box... i haved already tryed new plug with diferent temperatures range... the only thing i dont replace was the cilinder, the stator, the flywheel and the ignition coil... i think i have another coil so i will try it....
i´m realy frustated because i even change the carburator for a smaller size, from a mikuni 24 to a dell horto 22 just in the case the carb have some problem...
it is not a usual problem i think...
 
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#8
In your first post, are you saying that it runs well at low throttle positions and also at high speeds, but misses in the middle? If that's the case, it has to be related to the needle. Is the original carburetor the one that came with the bike? You mentioned that the engine was assembled from a variety of parts, so I wonder if the carburetor is from some completely different machine and just happens to be the same size as standard.
 
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#9
i apologise, but what i want to mean is at low engine speeds the bike it´s ok also in higher engine speeds, either the throttle is just open, half open or full open... in the mid range rev (after the first 3000rpm and before the last 3000 rpm) the engine runs like it´s so rich that just cannot acelarate the bike in more that the second gear... no matter what are the throttle opening....
 
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#10
Your translation of an engine tuner as a person who likes holes is fantastic!If someone has modified the volume of the lower end you are never going to get that cylinder to perform correctly.Do a search on this site for Professor Blair and Eric Gorr.If you are dead set on making this engine perform I believe you would need the virtual 2 stroke software program.
 
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#11
helio lucas said:
have a aftermarket filter without a box...
it is not a usual problem i think...

Have you tried to run the engine without the filter? Is the filter too close to the carb intake? There has to be some dead air space between the carb intake and the back of the filter. If the back of the filter is too close to the intake it will disrupt the air flow into the carb and cause a similar problem as you are describing. Does the engine use reeds on the intake? If so, are the reeds in good condition?

Still sounds like mid-range jetting. Have you done a plug chop? Try riding the bike in the range where it is missing and then hit the kill button. Don't shut the throttle off until the engine has completely stopped turning. Remove the spark plug and look at it. Is it wet, black, sooty or oily? If so, it is too rich and needs to have the needle, and or, needle jet changed. If this is the problem, increasing the size of the cut-away in the slide will help. Look on the slide, there should be a number stamped on it like 3.0 or 3.5. That is your cut-away size. The larger the number the bigger the cut-away or the leaner the slide.

Try moving the needle all of the way up and then all of the way down and see if it makes a difference either way. Too rich of a main jet will also cause a blubbering or four-stroking at midrange.

If the plug is dry, white or blistered it is too lean and the jetting or cut-away has to be decreased. I have a feeling you already know about jetting but I threw that in anyway. ;)

Since the engine is a collection of different parts there is no base line to go from. You are just going to have to try different things until you hit on the problem. You may want to try to find a stock cylinder to match the ports, (holes) and if they are too far off from stock, epoxy them back to the stock size. Also check the ports to see if they were lowered or raised to change the port timing. If the port timing is too far off, the engine may never run correctly.

I agree with foxforks. email Eric Gorr for questions about the strange porting in the cylinder. He will probably have some insight for you. :cool:
 
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#12
helio lucas said:
the bike doesn´t stop to four-stroke at midrange
I am afraid to admit that I have no idea what this means....

Are you saying that the power band doesn't kick in where you think it should?

If I am understanding it properly, you have bored and stroked this engine. I would think that the expansion pipe would need to change accordingly. What have you done in that regard?

Rod
 
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#13
in relation to the "tuner who loves to do holes", i discribed him like that because he simply open the boyesen ports, open the exhaust port and cut two 6mm holes btween the main transfers and the exhaust, linked to exhaust... but the holes have a mismatch in height of 1.5mm and one of each is simply touching the tranfer...
and the boost ports are open, one with 135 the other with 142 duration...
this guy tells my friend this cylinder was ported to a maximum!!!!

ok now what matters...
the engine has an intake port reed cage with 2 boost ports and its made by polini to dt 80. the reeds are in good condition but i have allready close the reed stops and change to a smaller reed cage just to have sure...

the exhaust is really not working properly with the engine but i think there is no reason to do such a four-stroking. but once again i swap the expasion chamber to a giannelli of an yz 80 (i think)
ok the bike has more low end and is lots more powerfull, but i´m not chasing power and the four-stroking continue and i think doesn´t change...

i change also the cdi to one with a diferent characteristics and change the timing back and foward-once again nothing...

i raise and lower the cylinder playing with the gaskets... nothing!!!!!!
the only thing that helps a lot is raising the compression ratio but now is too high...

one thing i note to is thebike is running too hot, and i don´t know why... could be the pump or the radiator or something....

when i was trying to jet the bike i also take off the filter. because the bike is a supermoto not mx. one time the four-stroking was better but the bike was so lean that almost cannot rev it, but helps a little bit... i thougt it was thw jetting too so i change the carb to a smaller one and jet it... just less power nothing else...

also as i said , the bike runs too hot and i also think the engine makes a lot and hard noise, the four-stroking sound is more like cracking sound. all of this with no aparent reason...

i´m sorry for the long text, and thanks!!!!!
 
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#14
one more thing, i´ll try to discribe the "four stoking":
the first 2000-3000 rpm are ok,the bike pulls well and clean, then the bike just make a horrible, hard, loud crackling sound... either the gas is open or almost closed, once the engine is on the pipe its ok again...

i´ll try to change the ignition coil, put new glasswool on the silencer and lower the primary (low end) compression ratio with a spacer between the reed cage and the face of the cylinder...
if this don´t work i will throw the bike to under a truck(a really big one) on the highway!!!!!!!!!!!!!!!!! :bang:
 
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