Making good progress on my dt125 build. Put a mx piston in for higher compression, and a 28mm carb. planning on converting to mono shock here soon. only thing now is porting and new exhaust. Will a fmf fatty pipe provide me any gains? stock exhaust right now is clogged and is really restrictive. I do not have the silencer either so Im blowing exhaust straight onto my tire and bottom of my seat.

Umm... again not that it matters too much since you're talking wild modifications that probably nobody here has actually tried... You either have a 1974 DT125a or a 1976 DT125c.

There is no such thing as a "1974 DT125c".

Can be very important for parts to correctly identify your model and year... registration and title paperwork can be notoriously inaccurate. Need to look up VIN in Tech Library.

Not that FMF ever mad a pipe for your bike anyway... so you'd need to hack something together from an existing pipe that may or may not work well. There is a lot of free info pipes and pipe design on the Internet if you want to try to learn.

are you sure there was no "c" model? I have the service manual for it and it has instructions for the "c" model

There was no "c" model in 1974. 

1974 = DT125a
1975 = DT125b
1976 = DT125c

 

MX piston won't give higher compression but might have a modified skirt for "go faster" stuff. But could try an MZ-YZ head for more squeeze. I "think" any 125 head held by the 4 long deep bolts--call them nuts-- would fit, & center plug is always good. Think yours might hve a center plug anyway.

the mx piston is around 60 percent as tall as the stock one is, and has one piston ring instead of two, but same top design. didn't notice a change in compression once I got it installed. I do not have a center plug head, but I will look into a yz head once I get a new pipe on there. Would like to get my 28mm carb tuned; currently I cant get it to rev high enough to get into the power band... #35 pilot #140 main unknown needle put im 4 down on the clip.

Why don't you just sell it and get a monoshock YZ that has more power, has a factory monoshock, and isn't going to fail after 2 hours of hard riding. I'm not saying your conversion to monoshock is going to be shitty, but even teams like maico were failing over and over and over. I can be completely wrong and you may be right around the corner from reinventing the wheel and i should shutup, but i wasted a lot of time on pipe dreams when i was in my 20s that i wish i would have just listened to people who know what they are talking about. I'm not one of those people but markt and RT325 are probably the most knowledgeable people on this forum.

YZ's in my area go for a TON of money, which I cant afford. I picked up 4 dt125's for $80. selling them in running condition still wouldnt be enough for me to be able to buy a non trashed YZ. Why not do a little custom build on one of the bikes and experiment a bit? MarkT and RT325 are the reason I have a running bike right now as they've answered the questions to the problems I had when I started.