Question for two stroke experts regarding port height and jetting

[Mr. Demeanor]

Last year I raised the ex ports quite a bit and have been shortening my tuner.
Both of these changes have required going down (leaner) jets in my carbs.
Not just a little either. I have gone down 5 index drill bit sizes to get the same piston wash as I had prior to the changes.
Im just curious as to why raising the exhaust ports and also shortening the tuner would make the mixture richer.

[Mr. Demeanor]

Just wanted to add I also have had to lean it out at idle so its throughout the entire RPM range.

The only theory I can come up with is increased signal to the carbs.

My fuel consumption has decreased a lot. At the Suwnanee I can usaully just make the long Saturday run on one tank of fuel. I have made that run maybe 8 times and usually am running dry at the end.
This year I had a quarter tank left.

[JPEROG]

I am waiting for an answer because its the opposite of what I am used to doing. When you flow more air, it usually takes more fuel...

Joe

[dwilfong]

Did you change the head cc when you raised the ports?

[Mark75H]

Increased signal or decreased airflow, even though that's not what you intended. A or B

[FMP]

I am going to suggest that you have improved on the timing of the return pulse in relation to the ex port closing . YOu have put the pulse in the degree timing window that gives the most advantage. This is resulting in more of the charge being pushed back into the cyl at the desired rpm .

[Magcat 62]

I DONT consider myself expert but have done up some omc v4 crossflows by raising the exhaust and short tuner your not trapping mixture charge and is escaping instead of building cyl pressure. i raised intakes and shaved heads with good results short tuners never worked for me kinda like too big a cam i a 4 stroke better mileage is baffling! no pun intended lol art

[Magcat 62]

I am going to suggest that you have improved on the timing of the return pulse in relation to the ex port closing . YOu have put the pulse in the degree timing window that gives the most advantage. This is resulting in more of the charge being pushed back into the cyl at the desired rpm .

great theory dynamic compression should be higher = more power

[FMP]

But raising the ex and short tuner would put the pulse in earlier, against what should work at the 5500rpm . But at the 7500+ it's right on time again or closer to it. Crank rotation speed gets ahead of the return pulse more as rpm goes up.

[Mr. Demeanor]

Im reading responses and thinking.....

[Mr. Demeanor]

My carbs and extensive intake work may have been too big for the exhaust timing/tuner. This caused a weak signal that required big jets. Gordon, Ray Nuedecker, etc. were all surprised at how big I had to go on jet size to get it dialed in.
The signal improved with the mods.
This motor does seem very sensitive as I run two size smaller jets on the port side to get a consistent wash. The only explanation is that its the difference in exhaust length due to the staggering of the cylinders and/or crank rotation.
Even the stock motor has leaner idle screw settings for the port side carbs.

[Mr. Demeanor]

Im also getting less fuel/oil under the cowl so I think the answer is signal to the carbs Im just not sure why the ex/tuner changed it dramatically.

[dwilfong]

A shorter tuner will scavenge the chest quicker drawing down the case pressure.
It will draw all the way to the carb with the piston at BDC.

[baja200merk]

What if the ports are too high for their stock roof and wall angles.

[Mr. Demeanor]

I would say the ex port height reduced main jet size and the tuner really affected idle mixture.
When I cut the tuner I started getting oily goo from the exhaust when it was idling on the hose and dripping out where its parked.