I have seen guys remove port bridges. Not a chance I would want to take. I want this thing to last. I'd be bummed if the ring snagged and tore the motor up.

Lube holes are definitely required for bridged ports. Most of the time they are pre drilled but not always. Sure will make you sweat when you're poking those holes and hoping you marked the bridge in the right spot.

I popped the head off the other day, out of curiosity about my actual exhaust port width. It measures 61mm currently, at the widest part of the oval. Could go a lot wider, although considering the position of the main transfers I think what I'll more likely do is square it off and tighten up the radii at the upper corners of the port. I don't want to raise it, and I don't really need to if I add some area above the transfers in the upper corners.

Next time I have the jug off I'll grind on it. I'm in no hurry, it's running strong as it is. I just didn't realize my port was only 64% of the bore diameter. That is very conservative, and if I don't widen it I can square it off considerably without affecting ring life.

Would end up something like the early RD400 port shape:


I'm still contemplating a bridged port. Yamaha didn't do that on the YZ490 but still managed about 45 horsepower at the wheel. I don't know anybody that ever rode one of those and felt like it didn't make enough power. Decisions, decisions.

Turbo I did talk to Joey he hadn’t been by his moms house to look I told him I would go with him it’s only about 2 miles from my house. I had a YZ490 and it would fly with my skinny ass I was 6.2’ and weighed 140 lbs

The only big bore air cooled two stroke I ever seriously modified and raced originally had a bridged exhaust.  I carefully measured the port and decided that if I removed the bridge, the port would be just at maximum width according to what I read in Jenning's "Two Stroke Tuner's Handbook".  Against conventional "HP" wisdom, I shaped the port into a gentle oval to help with ring life.  Transfers were left untouched. 

This was in the late 1980's and early 90's. It ran very strong.  Torquey and fast. Rings lasted forever.  I competed very successfully against very expensive "pro built" engines with my home brew mods in extremely competitive professional short course off road races.  So don't be afraid to follow your instincts and try something different!

Modern water cooled engines do very well with bridged exhaust ports...  the whole porting layout is different than it was back then and water cooling is a huge advantage...  heat is the enemy of exhaust bridges.   I have a small Kawasaki two stroke street bike with the KIPS system.  Talk about massive exhaust port area with the auxiliary ports flanking the already large bridged port!  No wonder they get well over 30HP from 150cc. 

 

The development continues.

I checked compression yesterday, got up into the mid 140s on the gauge. That's a little high in my opinion for this big-ass air cooled 95mm bore. Today I pulled the head and took some more material out of the dome, sanded it smooth and polished it with compound.

I revised the exhaust port shape somewhat, broader up in the shoulders of the port with more or less vertical sides. The width is still conservative so I could probably square it even more.



It is running really strong. I was out hot lapping it this morning on a stretch of dirt road. Pulls through the gears to 80-90 mph each time. It's happiest being run like that. The only place it seems temperamental is cruising 45-50 mph. The increase in chamber volume seems to have improved that considerably if not completely.

I tried shoehorning the cone pipe from the 360 on last week. It nearly fit, but it was designed to be installed with the flange in place and I can't really do that with the 500 parts. I did discover that a 2" inner diameter tube slips perfectly into the 500 spigot and directly over the 360 head pipe flange. It could be made to work but considering the stinger diameter on the 360 pipe it would almost certainly burn the 500 up in short order. I'll just have to wait for a proper pipe.

damn that is a fast bike!
I am riveted to this thread, love the updates!

It is definitely gearing limited. It gets up to the top of fourth gear pretty quick. Pulls hard especially considering the speedo is already reading 70+ and climbing quickly.

I did find an 18 tooth countershaft sprocket that would fit, as if the 17t wasn't tall enough. Then again, the brakes are already seriously overpowered and it would be questionably sane to make a point of going faster.

If possible I would like to submit this old article for the archives. Got another one coming today, I hope one of these magazines did a dyno test.

I'm quite sure that it's running faster in the 1/4 than the numbers cycle published. More than a little bit.

What speedo?
a real 65 on the worx 73 MY is best I got.

cliff

I am guessing 13.8 in the quarter. Hard to launch, limited top end.
 

Funny reading the scary magazine writeup from the day.
That writeup probably came out after we'd assembled ours or probably would have sent it back still in the crate haha.