Exhaust System

[nitro5185]

I have an 1983 CR125. No one makes an exhaust system for my bike. I got a FMF Fatty Pipe off of ebay and custom fit it to my bike by cutting off the pipe inbetween the silencer and the pipe and doing some mounting mods. I think there is too much back pressure on my exhaust system cause In 6th gear and WOT it seems like it cant get the exhaust out. I saw that some companies make universal Spark Arrestors/Mufflers/Silencer. They also make 1"/1-1/4/ and 1-1/2 diameter pipes. I was wondering if it will hurt my engine to use a larger diameter silencer/spark arrestor and a large diameter pipe inbetween the pipe and the silencer? Also what is the difference between a Spark Arrestor/ Silencer like the Universal FMF Silencer. Thankx, Nick

 

[David Trustrum]

In 6th gear and WOT it seems like it cant get the exhaust out.

Ok you have some visualisation of a preserved problem here. :think:

Never the less we will run with it for a second:
Lets start with the supposition that your pipe is from another 125 (presumably a CR of a later year) so the stinger pipe will be near enough the right size & you used your standard muffler, but you just shortened the stinger on the pipe. This will decrease the backpressure slightly but not enough to lose any sleep over. A bigger stinger than std will lose hp. Quite a bit with that sort of change.

Your muffler is not collapsed & you have rebuilt it with some new packing I hope? OK, then no point spending more money.

Your next concern will be the jetting is very likely changed, most likely gone a little lean on the main. Try going up a couple of sizes on the mainjet & see if it is any better. Always go bigger before trying smaller until you get a feel.

 

[nitro5185]

I have put new packing in the silencer. The stinger as u called it used to be 7/8 ID pipe. When I made a new one I used 3/4. Now I want to try using 1" which is what I think normal exhuasts are on MX Bikes. Cause I have 1" Silencer and the opening from the Pipe is 1". So I dont know

 

[nitro5185]

Yes, I forgot my FMF Fatty pipe is for an 95-97. Also when WOT is will act like someone put there hand some what in front of the silencer to block the exhaust from coming out. Then if u hold WOT longer after a little while it will clear out and hit the powerband. I think the pipe is too small and too many bends in it that the air flow is not smoothly coming out. If u still think it might be too lean than tell me. Also, my bike normally had a 34mm Keihen carb and the people before me put on a 38mm Mikuni. So I dont know If that has anything to do with it?

 

[David Trustrum]

No the pipe is not too small nor have too many bends. You are thinking that gasflow is paramount so therefore bigger must be better, this is rarely the case. As long as the silencer is not collapsed or the stinger not obstructed smaller than it was on the FMF or original pipe then you will be fine in that area. Just to appease your line of thinking remember that there is not a constant stream of gas coming out of the exhaust. The port is closed half the time & the pipe acts like a surge container. 10/10ths tuning will see tuners reducing the stinger as small as possible, but stock or FMF will have got the figure as close to right as you will want.

OK so more info comes out. Yes I do think it is your jetting. Further fuel to the fire is the somewhat large carb on an older bike. It might be possible to tune the 38 ok but the std 34 or a 36 would have been a better bet I would have thought.

Who can tell where the jetting is wrong with a carbswap? it may well be lean on the mid position & rich full open -or any combination in between, which makes it struggle (esp with such a large size) to clear itself out. Really you need to readup about jetting, there are lots of articles on the web.

If you could find a 2nd hand std carb I think you would have a lot more fun trying to get it setup right.