Yamaha TRX850 Enthusiasts Forum

I'd like to get some inputs re my "poor" mid-ompphhh...

I don't have any recent nrs/charts, but i'll try to describe the issue.

I have a fairly tuned engine, with 878-kit, MAB full-system, Mikuni flatslides, stacks with pod-filters, CDI-mod. Cylinder head has been ported/plasted quite extensively including modified valves, welded exhaustports etc. Squish has been measured/adjusted, slightly skimmed head/cylinder. I also have Kent cams, that due to a mishap at Kent have been slightly modified in comparison to standard Kent cams. It's been on dynos a number of times for checking/adjusting A/F.

All has been done with the aim to add mid-range oomphh, not caring about peak figures. The porting has been done by a very skilled porter in Sweden, with the same aim, peak-figures/high RPMs not interesting. (Looks very different with for example KW's porting, my inlets are a lot narrower for example)

I won't go into numbers, as all knows, dynocharts can't be compared (but i've been happy enough). What has been puzzling me is the look of my curve, in comparison too other charts, both stock, mildly tuned and ones with similar specs as mine. My bike easily revs 9500, with very slight drop in power above max, it revs on and on... On the other hand, it has quite a loss in comparison in mid-range. All contrary too what was expected...

Over the winter, I've had the bike at a workshop, and asked the mechanic to check my cam-timing too see if there would be any gains changing the timing. Cams have been mounted with standard sprockets, so no room for errors frommy side. The cams should be 105/109 according to Kent specs, also with my slightly modified profiles. Turns out the timing is 110/100 instead. Bingo! That's the cause then! The mechanic slots the sprockets, and sets the timing correct. The gain in mid-range is approx. 2 hp, and losing 1.5 at top... (It also turns out the cams seem to be not completely round, he got different measures on valve gap each time he checked?)

I feel like i am missing more in the region of 4-5 hps in midrange (comparing curve shapes with similar), and would happily sacrifice 2 or more at top.

1, Any good theories on my poor mid-range?
2,What with the cam timing, I would have expected a much greater difference once it was correct?

So IIUC you have good low oohmpf and then it dips or flattens out to shoot at higher revs?

...and as i know you actually ported the head for more mid and i have no plastic and GET more mid.....and that with tornado cams wich are top end cams. Yes this is a mystery...really!

Going to do some job on mine when spring rush is over at the mechanics.
New rings and a hone, skim the head a bit and let him grade the cams perfectly....we´ll see what the results will be.
....and as i have the TCIP4 now we can work on the ignition in a PC

What is the A/F ratio at the "dip"? does it dip at the same place?

It has a dip in one of the A/F charts from last year where i clearly had a drop in hp as well, but we've fiddled around with jets/needles/accpump, tried different settings. I don't know what the A/F looked like this last time. I presume the mech managed to smoothen the A/F.

But still, like KW said, comparing his and mine, where he has aimed for peak and I have aimed for mid, is strange. The whole shape of my curve is more like it would have been the other way around...

I must say you´re right on installing the standard cams again on standard sprockets of cource to see what it does...

Did you measure A/F on one pipe?
Perhaps you could find something strange if doing holes in the downpipes and measure each cylinders A/F individually?

Surely the problem is clear - you've got Mikuni flatslides Everyone knows they are rubbish

Seriously though, I have no idea why you would have this drop. Does the AFR drop at any point?