Millenium YZ295 compression test

 I bought an 03'YZ295 with an Eric Gore 295 top end awhile back. It didn't last long before the 

plating between the main and boost ports flaked off. (very narrow). It was warranted, and Millenium made it right with a new cylinder. The thing runs, starts, and idles well, (after much jetting). But has never made a lot of horsepower. I,m not needing a lot of power as the thing was ordered to be a pump gas trailbike.  I recently took a compression test and only got 175 PSI, is this correct? It has the double headgasket, and takes a good boot to start, (more than stock 250 that had 210 PSI. The power valve was observed working through the L.S. vent cover hole. It has an FMF SST pipe, per Gores original setup. It also has a 12 oz. flywheel weight. It pulls down low, but will not "bang to the bottom" without stalling, like a well tuned KTM 200 that I also own. Experimented retarding the timing, incrementally, but it justs smoothed the mid range a hair, but it never really had a hit in the first place. What gives? A stock 250 will run away from it.

What if I reduce the thing to:

How much

PSI should I see with Millenium mobetta 295, pump gas, double gasket.?

On 11/8/2017 at 0:11 AM, ventabular said:

 I bought an 03'YZ295 with an Eric Gore 295 top end awhile back. It didn't last long before the 

plating between the main and boost ports flaked off. (very narrow)...

This is quite alarming. 
Do you know if this is common for Eric Gore's 295 big bore cylinders?


Been pretty common to get F'ed up work out of him...if you could reliably get a good cylinder fit and finish, and actually get the damned things to run right, I wouldn't be looking at KTM 300s. I'm actually willing to deal with the rest of the KTM bullshit for a 300 that runs correctly and doesn't eat itself.

I disagree, Eric Gore/Millenium has done my YZ twice in the last 6 years.  First time was just the 295 top end and second time was a complete rebuild of the motor including installation of full WR gears.  I have had ZERO problems

The Eric Gorr cylinder probably had 50+ hours on it. BUT the exhaust bridge between the main, and PV ex. ports was extremely narrow, guess 3-4mm, The millenium cylinder, it is more like  8-10,.The rest of the porting is much more conservative. The power is also less. That divot worn in the bridge, allowed the rings to contact the side PV. Millenium has clearanced all of them a bit more.

His track record with my builds and that of my riding buddies is around 50%. That might be generous too.

I don't know much about his builds or yz300's but if you wanted a big bore trail bike 175psi sounds about right. 

I think the ktm300's are around that range stock.  

Other big bore engines typically run lower PSI and trail bikes also run lower PSI than you typically see in MX built bikes.   I bet that bike would run fairly well the whole way down to like 130 PSI (not that you should ;) ) 


Subzero-thanks for the answer. I just took the head off, and everything looks good. Been running Maxima 927 at 40-1, with some spooging, and there is no carbon deposits on any thing. No signs of flame front or detonation, looks like a babys ass in there. I had some det. when I cracked the throttle off idle. This was after the temperatures dropped. I have since rejetted. I don't think Millenium did anything with the squish. I am checking it now. the bike has been hard to jet.


Anybody's thoughts on that SST pipe?

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