2 base gaskets or no?

[william16339]

hi i have a 86 250r that i recently took apart quick to through on my PT ported cylinder, fresh bore, new topend, CR style headgasket, vforce reeds, CT pipe, and 38mm carb. i was wondering, what would the 2 base gaskets do? if i doubled them...more topend? more lowend? i always wanted to try it, but dont know if its worth the extra 9$ or not :lol: ... if i do 2 base gaskets, should i do the thin CR head gasket or the stock one? letme kno guys thanks!

[350xorbust]

Its all about comp ratio. 2 base gaskets will lower your comp ratio, but by how much???

[09RappySE]

Why would you want to do this? you will have a power loss. The only way I would ever do 2 base gaskets is if the piston to head clearance was too close.....all motors love compression.....of course I am a 4 stroke guy, I don't how it will affect the port timing.

[barnett468]

it won't do much other than reduce compression which you can regain by installing the thinner head gasket.

it will raise the ex port which will sort of raise the overall operating range but it will also raise the intake port which will shut off the intake sooner which will sort of lower the overall rpm range so it not only sort of balances things out so it should sort of run the same it might make it run worse. i have done this just as a test with no noticeable diff in perf so i run one base gasket.

[atc300r]

Back in the 80s when I put a cr head gasket (91 cr) in my 85 250r people were telling me Id have to put a second cylinder gasket in to bring the port timing back. It was hard to convince them the that when I changed the head gasket I didnt change the port timing. Some of the guys said they ran an o-ring head and I ask them if they added a second cylinder gasket and they said no. But they were telling me the opposite using the cr head gasket. Also if you putting the cr gasket in to add comppresion then putting in a second cylinder gasket your defeating the purpose of the thinner cr gasket.Just my to cents. My 85 ran great with the cr head gasket and 1 cylinder gasket.

[RIDE-RED 250r]

The thicker base gasket trick was a popular trick on the Ski-Doo Mach-Z 800 triple. Rich Daley at dynoport showed a 3-5 hp gain in peak HP on a track dyno doing that trick on the first Rotax 809 he got his hands on back in '98. They often refer to it as the "poor man's port job".

But, it wasn't simply installing 2 regular base gaskets. It was a special gasket with a thickness of .059" (if I recall) that you can still get from Crankshop. What this does was raise the port timing to put slightly more focus on the upper mid-top. BUT, you have to have the cylinder head milled to compensate and bring the compression back up due to the pistons not coming as high at TDC, thus lowering compression. If I could find the write up Rich did on the total step by step build up of the '99 Mach-Z with dyno testing at each step I would post a link...

I said all that to say this:

1st-Just slapping in extra base gaskets will do more harm than good unless you have the head shaved to compensate and keep compression where it should be.

2nd- This mod yielded 3-5hp on an 800cc TRIPLE cylinder sled engine that is already inherently more mid-top oriented in its porting due to the normal operating RPM's a sled engine sees. I asked pretty much this same question a few years ago if an old sled engine trick would work on the 250r engine. I can't remember who told me, but it made perfect sense.. If an 800cc triple, set up for a sled only sees about 5hp gains with this mod, gains on a little 250cc single would be barely noticeable at best. While in theory it should work, the meniscual gains would not be worth the time, effort, and expense to have the head shaved. Plus you now have 2 seams at your base that have to be air tight instead of 1.

Bottom line: Run a single, normal spec base gasket.

[C.J]

And Cj says "I told you so" :Bounce

[phil53]

Has anyone who replied actually done it? I did and noticed a difference with the butt dyno

I currently am running 2 base gaskets in my 86 250R, I warped my cylinder deck and it had to be shaved, this upped the compression too high as before this it was about 200psi, it went up to about 220 after the decking... So I added the extra gasket and just left it like that till I was going to change the rings out for a refresh and figure out what to do. It does knock the compression down a bit too much IMO.

It does change the engine port timing. It has more in the top, but, took away quite a bit of the bottom end grunt it had. It made it easier to ride, however, less fun as it is a bit harder to lift the front up with just throttle.

Mine isn't crazy modded either, 36 TMX, boyesen power reeds, fmf pipe, and a reed spacer. At sea level.

[RIDE-RED 250r]

Has anyone who replied actually done it? I did and noticed a difference with the butt dyno

I currently am running 2 base gaskets in my 86 250R, I warped my cylinder deck and it had to be shaved, this upped the compression too high as before this it was about 200psi, it went up to about 220 after the decking... So I added the extra gasket and just left it like that till I was going to change the rings out for a refresh and figure out what to do. It does knock the compression down a bit too much IMO.

It does change the engine port timing. It has more in the top, but, took away quite a bit of the bottom end grunt it had. It made it easier to ride, however, less fun as it is a bit harder to lift the front up with just throttle.

Mine isn't crazy modded either, 36 TMX, boyesen power reeds, fmf pipe, and a reed spacer. At sea level.

So, just because I already looked into it and found out it wasn't worth it, are you saying I don't know what I'm talking about?

[barnett468]

Has anyone who replied actually done it?From post 4.
i have done this just as a test with no noticeable diff in perf so i run one base gasket.





I did and noticed a difference with the butt dynoSince the thickness of the gasket is fairly small, if one happened to gain any performance change from adding just 1 gasket, the change would be so small as to typically only be noticed by experienced test riders.

The test would also have to be done back to back the same day.

Also as mentioned, the compression would need to be increased to the EXACT same level as it was with just one gasket to eliminate a difference in compression from affecting the test results.