Big dog Dave ! The 75 - 76 cylinders are not that good since the boost port they added is stupid as it is fed behind the cylinder liner from the rear transfer ports , the transfers have no direction , To make it work good I epoxy between the boost port and transfers and designate it seperatly then the transfers can be hook at the rear. These really need alot of work . The earlier cylinders are a better one to work with and i can add a boost port correctly .

Rich

That is very good to know. Thanks Rich!

I have a 34mm Mikuni from an IT200 on my 74 DT250. I did do some minor porting. Lowered the exhaust port and cleaned it up. There was a very noticeable difference from the 28mm. I ran them back to back. I was running a pod filter.

Lowered the exhaust port? Not sure I understand what you mean.

Would love to see some pics of your DT250... I have a 75 that's one of my next projects.


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MarkT, I made the hole in the head going into the exhaust pipe larger at the bottom. Actually main reason I did this was because it caught a ring and chipped.

This post talks a little about my project. It is a vintage motocross bike. I am working on a custom plastic airbox using an rm85 filter to better mount to the 34mm carb. Too early to get any good pictures. yamaha-enduros.com/index.php/forums/gene...clutch-upgrade#69311

Thanks for the clarification! In my experience with porting, lowering the exhaust port below the top edge of the piston at BDC won't gain you anything at all... but understandable to fix damage. Just have to be careful that at TDC skirt doesn't uncover port and open crankcase to exhaust port.

Rambling... anyway... it sounds like the carb was probably the biggest reason for your gains... hope you post up about your airbox mod... sounds like a great upgrade.