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Discussion Starter · #1 ·
Ok last time I posted a Dyno run I did with my nephews 400ex. It came to my shop with just a modded stock pipe, K&N air filter, minimal head porting and a stage 2 hot cam. When I started the first run it was only making 29HP NOT SHOWN. I jetted it in with the stock pipe and gained 1HP taking it to 31HP. Then I put on a full Yosh System on it and jetted it in perfect and as you can see it didnt have a very big gain bout 1HP now up to 32HP. I then ordered a Dyna Tek ignition for him and installed it. I spoke with a guy from Dyna Tek and he said it advances the timing about 4 degrees. I found out also that Curtis Sparks makes a flywheel key kit that is only bout 20 bucks that is supposed to advance the timing 6 degrees. For some reason 400ex's have quite the retarded spark? You cannot do both, the key and the Dyna, it would be way too far advanced. I did not do the key adjustment because the Dyna does more than advance the timing and it also moves the timing curve and raises the rev limiter to what I found to be about 800rpm. Look at the dyno sheet it shows with the Dyna it only gained bout .25HP but it did move the HP curve longer on the top end.

Now the interesting part. During all the runs with the Yosh Pipe I had the spark arrester removed to improve exhaust flow of course and to improve the power right? Wrong with this Yosh pipe I found that putting the sparky back in improved torque and hp. This is the first time I ever seen this happen when the sparky was in. The gain was bout half a horse overall but not as much torque or power on the bottom end. Also look at how wavy the HP line is on the top end with the sparky in compared to being out.
 

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Alot of time open flow exhaust is not good for power results as you have seen. I think IMO these engines need some back pressure in opder to produce a better number. you may see more gains by upgrading the valves to a +1 size did you polish the intake side of the port runners?
 

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Discussion Starter · #3 ·
He bought the head used with the porting already done and the head was decked bout .010 so how good the port job is dont know but it did look to be alot better flow than the stocker. And no it does not have +1 valves it was sorta a budget build when we originally did the head cause my nephew was still in school. Yes some engines need back pressure and this is one of them.
 

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The only reason I asked about polishing the intake side is that you don't need to do that, you want some resistance of airflow coming in to creat a vortex polishing it will actually drop the CFMs but polishing the exhaust side benefits you by reducing carbon build-up. so if you ever remove the head for any reason you should be able to tell.
 

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Discussion Starter · #5 ·
I c. Well this bike isnt mine so i am not putting a lot of time into it but i do like to learn about things like this. How did you discover that about the porting? This 400 has had some head porting done to it but was not a high finish polish.
 

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I read alot... I actually ported my head I have a thread on here somewhere with the progress. I haven't installed it yet i'm waiting on my topend gasket set to come in. I wish I had a Dyno or access to one like you do, I want to make some pulls before and after to see if I have any gains. You can have all the H.P. in the world but if you lack flow to match it you wasting your time..IMO. I see all the bigbore kits and things they will make your quad scream but if you add in a good port and polish job man it's like night and day. I stuck with the stock valve size for now, not knowing how the head is going to work out, I didn't want to sink a bunch of green into the head to find I only wasted my time..lol but i don't think I did.
 

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Hey moose is it the same with a two stroke?I'm getting ready to port and polish my bikes head.
 

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Any port job is basicaly the same. The guidlines I followed were from the auto side I only had one chamber to port with a car you got 8 (4 on each head) I read alot and looked at alot of videos, plus I have done V-8 heads in the past. The key thing to remember is bigger is not always better when it comes to porting. you only want to remove wnoug material to maximize flow and just rough enough to create a swirl effect in the runners. you achieve this by grinding parallel to the port openings. You want to remove any cast imperfections wen the air hits these imperfections it actually causes the air to stumble through the port but removing these and cross grinding you create a smoot even flow. On the Exhaust side you want to do the same as the intake side but the final touch you actually want to grind it smooth enough that you can actually polish it. Also you want to clean and polish the underside where the piston will meet the head, doing this will not allow carbon to build up robbing your engine of power. I hope this helps. Oh yeah I also used a butt load of Spiral sanding cones and carbide deburring bits for the inital grinding.
 

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Thanks man.So I should make them both smooth but polish the exhaust side.And the head.OK.I actually meant cylinder when I said port.You know a two stroke.LOL
 

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Discussion Starter · #10 ·
Ya trx250 a dyno is nice but very addictive and expensive when it comes to R&D LOL. Your so right on all the bolt on stuff, its the internal equiptment that counts. Its not only about max HP either cause where the HP is throughout the revs is what counts more IMO, just cause it makes a gazillion HP doesnt mean its the best overall. Thats cool you do alot of research on your 250s i just wish there was more of a demand for those bikes, i really like the250s. This is why i am putting a bit of R&D in this 400 cause they made them now for 11 or so yrs and there is a but load of them out there, unfortunately the 450 class pretty much smashed the 400 class.
 

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hey guys off topic but where do i get a really good port at? You guys know a builder or a shop thats bad ass? i want to do a ported 416 and have it fast but reliable. thanks
 
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