49mm Steel Sleeve --- She Rips!!

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ImpulseRocket89

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OK, tore it down again to get port timing.

Durations:
Exhaust: 159.5 ( we can call it 160 I guess)
Transfer: 128
Blowdown: 15.7

I am no porting pro....but what I have read it seems there is a discrepancy here, and these durations would give basically what its doing. Good low end, OK low-mid range, and mid-high falls flat fairly quickly.


Seems I want the BD to be significantly higher to get the power band up a bit more to the top, but I cannot do so without raising the exhaust port. I have already decked the jug for squish. Decking it more to raise the jug would get me into the top cooling fin and significantly reduce trapped volume. Would that be the way to go to spread the power band out a bit more toward the top?

Is my thinking correct here?
Wow that blowdown is horrendous, and is definitely the cause for the lack of higher RPM power. You could try to lower the jug to reduce the transfer duration and increase the timing ATDC to try and help improve the blowdown some. That would buy you a little room to raise the exhaust port. The downside is the port floor being a turbulence zone due to the piston blocking part of it would get worse. Plus doing so would only buy you a couple of degrees at best, and as you mentioned the jug is already decked so that will cause other problems.

You could raise the exhaust port a little (A couple degrees) without likely causing many issues. 160 degrees of exhaust duration isn't too terrible. Another thing you can attempt to do is buy yourself as much evacuation as possible in that limited blowdown by widening the exhaust port at the top. The risk here is if you shape it wrong you will snag a ring.

Another option still would be to ramp the piston on the exhaust port side to get it to open sooner relative to the transfers. You will sacrifice a little compression and part of the squish band as a trade off but it may be worth doing in this case.

Obviously any combination of the three could also be made to work.
 

TheWizzerd

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Wow that blowdown is horrendous, and is definitely the cause for the lack of higher RPM power. You could try to lower the jug to reduce the transfer duration and increase the timing ATDC to try and help improve the blowdown some. That would buy you a little room to raise the exhaust port. The downside is the port floor being a turbulence zone due to the piston blocking part of it would get worse. Plus doing so would only buy you a couple of degrees at best, and as you mentioned the jug is already decked so that will cause other problems.

You could raise the exhaust port a little (A couple degrees) without likely causing many issues. 160 degrees of exhaust duration isn't too terrible. Another thing you can attempt to do is buy yourself as much evacuation as possible in that limited blowdown by widening the exhaust port at the top. The risk here is if you shape it wrong you will snag a ring.

Another option still would be to ramp the piston on the exhaust port side to get it to open sooner relative to the transfers. You will sacrifice a little compression and part of the squish band as a trade off but it may be worth doing in this case.

Obviously any combination of the three could also be made to work.
Yeah, all that is basically what I assumed.

I know how to do the physical part of porting to where I wouldnt snag a ring etc.. Just not super hip on durations and whats ideal on these.

I do have another jug that isn't decked. It has practically identical durations to this jug. I think I can tweak it in better than the current jug as I have not decked it yet. At least where I'm thinking now tells me I should be able to.

I actually already have a piston that i ramped so I could play with the timing without scrapping a jug, then transfer those found timings to the jug so I can maintain squish and put the non ramped piston back in. Just havent swapped it in yet to play.

I actually love the low end. Its smooth. I can cruise slow, at very low throttle/idle and it has decent torque to keep it rolling nice and smooth, and its got some a$$ WOT right off of a low cruise...it just peters out the last half and revs with no additional power, and hence no speed gained......and just wasting fuel up there.

Its actually not horrible to ride, but having the top end do almost nothing bugs the snot out of me. I could sacrifice a bit of the nice low end to spread the power out a bit more, but honestly, I think maybe just dropping a good ole 47mm top end on may be the easiest way. If I dont get very far on it with the new jug, that's what I will do, and maybe put this engine on a mountain bike for trail riding. It is much better suited for that purpose as it is.

I like to tinker, but we only have so much to work with on these jugs and it looks like I happened to get some that are on the lower end of tunability due to port/sleeve locations. There are a few example of these jugs out there that are pretty close to stock doing very well on the top end.
 

Street Ryderz

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what stroke are you using? 160 degrees exh duration can be raised to get the bd better suited, i find 165-168 works well without much sacrifice of low end.
 

TheWizzerd

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Ok so raising the exh port roof by 1mm will give you 168 duration and bump the bd to 18.64 this should work out quite well.
Thanks for your input man!!!

I did ramp a piston on the exhaust to try that out before potentially ruining a jug. If it seems OK, I will do the jug and drop in a regular piston to maintain squish all around the dome.
 

TheWizzerd

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I was able to give the ramped piston a ride.

It does have much better top end, low end is the same, however the mid range has suffered quite a bit.

Didn't hurt the low end at all. Once I'm cruising close to the top, and hit it, is moves out much better and gained about 5mph for a top of 38 (its definitely got more in her I think) . But if cruising mid range and hit it, its significantly worse than before the ramp.

Mid range cruise is smoother now however..... Cruising at 25-30mph is quite pleasant for a china doll, and can still idle around nice and smooth at about 8-10mph. Which is quite nice to be able to do. My other builds' usable min speed is around 18mph.

This seems to be the pipe mostly at this point. I think the pipe is tuned for the very top end of the rpm range of the current port timings. Doesnt hit it until you are already pretty much at the top. Its quite abrupt, and you really feel it hit once it does. Just runs out of engine to make use of the pipe up there.

I could tweak this pipe some, but I have a jog pipe on the way for this build. I think that will be the ticket. Just figuring out how the hell I am going to get it on there.

I am going to go ahead and raise the exh. port and put the plain piston back in.
 

Street Ryderz

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Lower the needle in the carb to get improved midrange, it will be fat there below the pipe and putting more heat in it sooner will lower the point of the hit then you might get more out of it.
 

TheWizzerd

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Lower the needle in the carb to get improved midrange, it will be fat there below the pipe and putting more heat in it sooner will lower the point of the hit then you might get more out of it.
No needle. Its a tillotson/walbro style pumper. Both low and high needles are on the lean side at the moment to do just that.

Its fine for now until I get the pipe.

I do have several other carbs I could try, but I really prefer the pumpers. So easy to tune when everything else is right.
 

Street Ryderz

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No needle. Its a tillotson/walbro style pumper. Both low and high needles are on the lean side at the moment to do just that.

Its fine for now until I get the pipe.

I do have several other carbs I could try, but I really prefer the pumpers. So easy to tune when everything else is right.
yes true enough.
 
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