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How to get more low end out of my bike?

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I have a 1985 rm 125 and I was wondering how to get more low end out of it? I have a pipe off an 84 rm and a dg silencer and that's about it. How could I get more low end out of it?

 

 

Send it off for professional porting.  The port timing is the only way to really change the power delivery of a 2 stroke, especially an early power valve 2 stroke.

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Send it off for professional porting. The port timing is the only way to really change the power delivery of a 2 stroke, especially an early power valve 2 stroke.

what about one of those porting and polishing kits you can buy to do it yourself?

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what about one of those porting and polishing kits you can buy to do it yourself?

 

Do you know how much to raise/lower the exhaust port?  How big should the transfer ports be?  Should you even change the transfer ports?  How about cutting the piston skirt?  How much?  Unless you have a book or manual that gives you these specifications I would not do it yourself.  A professional that has the experience should be consulted for this.

 

However you can do a little work on just cleaning up the transfer ports, removing casting flash, smoothing the transition from cylinder to port, and even base gasket matching.  However you should NOT change the shape or size of the port itself unless you know how big to make it or what to change about it's size and shape.

 

I have done some 2 stroke porting on air cooled engines with great success, but only because I had a manual and chart that told me what and how much I should be changing.  Here is an example.

 

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I have a 1985 rm 125 and I was wondering how to get more low end out of it? I have a pipe off an 84 rm and a dg silencer and that's about it. How could I get more low end out of it?

 

A 125 motocrosser is a 125 motocrosser.  Not much low end but you can improve some things with what you have.    Keep the piston rings in tolarance and compression up.      Use good fuel and make sure there are no air leaks.    The 1985 RM125 is a good reliable bike  and leave it stock.

 

Agree with other replies......consider a bigger bike in the future and there will be no shortage of low end torque.

Edited by greg-15
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That dog won't hunt.  Low end grunt is not what 125cc two stroke MXers are made for.  Don't waste your money on trying to change that fact.

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Send it to eric gorr and have him port and bore it.that will make the most noticeable improvement. Or you could buy another 85 rm 125 for what you'd spend doing that.Or better yet an 85 rm 250

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You are already starting off with a bike that was a dog ass weak on the low end. The 85 RM125 had no low end or midrange for that matter and all of its power was in the top end of the range, and yes a big bore will def give it more bottom and mid power set up right. Probably the easiest way to help it out is to first make sure the top end is fresh and it has good cylinder pressure. Do a solder test and see what the squish clearance is. Lets assume its.063",  I would  base mill cylinder .030", This will do 2 things, lower all the port heights and tighten up the squish band to .033" and raise compression. If your original squish is less then .060' then you will have to cut the head  deeper to set the squish at what you want to run it at. I would shoot for 033"-035" squish clearance. This will probably cause the bike to run on at least 50/50 pump and race fuel but if you want to run it on straight pump you will have to remove material from the head bowl, about 1.5-2cc's, you will have to do the math. 125's without power valves don't really have any low end but this mod will bring the power down where you spend more time riding. Most all 2 stokes  respond to lowered ports to improve bottom and mid power.  Boyesen dual stage fiber reeds and swapping out the carb for a 35mm PWK air striker will also help with low end response, going to a bigger rear sprocket will get it into the meat of the power sooner, a little bit longer pipe will give it more bottom end. You can talk to Eric Gorr or any other reputable shop and they can prob base mill the cylinder and work the head for around 150 bucks. There are a lot of things you can do but you normally trade power somewhere, if you try to gain power in the lower ranges you will loose it on the top and vice versa. The early-mid 80's RM125's had huge transfer ports and that hurts response and low/mid power. As 125's gotten faster over the years I noticed the transfer tunnels have gotten smaller, go figure. If you like that particular bike your riding then go for it. Gorr should know this year bike and should be able to help you out

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