Reducing intake volume on 61x manifold....

[Cannibal]

So according to something SuperTune posted in a recent jetting thread, Group K does something to reduce the volume of the intake on 61x manifolds to improve the response for freestyle.

For owners that are absolutely dead set on running a single carb setup anyway, we would strongly recommend they consider using 61X cases instead of the 62T cases. It bears noting that on our single carb 61X packages, we modify the stock 61X manifolds to further reduce inlet manifold volume and increase signal …. It makes a difference in throttle response that can be easily felt on any single carb engine setup.

I have a spacer I was thinking of running on my 61x to increase intake volume (thinking about a 4 stroke and how longer runners increase torque), but that is apparently counter productive.

Does anyone know how Group K reduced intake volume? I could easily mill a little off the mount. Any other suggestions?

I wonder how the intake volume of the famed Wet Jet manifold compares to a 61x version. Maybe there is more to the performance gains from this manifold besides the reed stuffers.

 

[CARBONX2]

4mm off the side that bolts onto the carb side of the reeds

 

[Cannibal]

4mm off the side that bolts onto the carb side of the reeds

Sorry, that confused the hell out of me.

I don't know the terms, but there are 2 manifolds for the intake setup. What I call a carb manifold and a reed manifold.

4mm from the underside of the carb manifold? Or are you saying 4mm from the top of the cases where everything sits?

I can see that it would be easy to take some material from under the carb manifold.

 

[tbaer]

would this same theory work with dual carbs on a 61x. my 61x hits hard and is very responsive, but i am running a dual r+d manifold with 44's.

 

[Cannibal]

I would almost think the dual manifold would reduce the intake volume even more since it's a straight shot to the reeds?

 

[Big Kahuna]

the wetjet manifold has built in reed stuffers.

 

[kevbo]

Reducing the inlet tract will make your pilot/popoff a little richer. Difficult to measure any real performance gain.

 

[Cannibal]

Group K says that reducing intake volume will increase throttle response. SuperTune copy and pasted that and didn't disagree. They say it increases signal, so yes, it might enrichen it slightly. But if you set your carb up for that, the better throttle response seems like a nice benefit for me (I'm a fatty and need all the help I can get).

While looking at my manifold, I think I'm going to take 2mm from the carb mount on the milling machine tomorrow. Unfortunately, there will be no way for me to compare before and after because I'm still waiting on rings.

Steve

 

[Waternut]

One thing I've learned is you only do one questionable performance mod at a time...and only after everything is working properly. If not, all of a sudden you have people saying that the best performance mod they did was something like reeds when in fact they ripped the engine apart and fixed an air leak or freshened up a worn out top end at the same time.

 

[Cannibal]

I suppose you might be right. Although in my case, I just don't have the time for one at a time. My 650 (61x case, 6m6 cyl) that came to me with a 44mm Blackjack and rest stock is going up to a 701 with the same carb, B pipe, Riva 35cc head, MSD total loss all at the same time.

That said, I don't have to mill my manifold at this time, I could always do that at another time. In that situation I could always be able to compare back to back.

 

[Carbon Blaster]

One thing I've learned is you only do one questionable performance mod at a time...and only after everything is working properly. If not, all of a sudden you have people saying that the best performance mod they did was something like reeds when in fact they ripped the engine apart and fixed an air leak or freshened up a worn out top end at the same time.

This is a good theory & most of the time I would agree with you, eg: if you fit an aftermarket rideplate, scoop grate & steering nozzle all at the same time & your ski handled so much better, which one really made the big difference, or was it all three?
But with engine mods often once you've modified one part, you need to modify another to balance/support the effect of the first mod.
If you get fuel/air in quicker, you need to get it out quicker.
Any serious high output 2-stroke has 1 carb per cylinder. Sharing a carb like on many jet ski motors is a compromise to potential performance.
Sharing an inlet manifold makes the inlet tract very big (too big/compromised) so using smaller volume 61x cases helps balance things out.
If you want to run twin carbs, then 62T cases are better as they flow well, have bigger volume & with the shorter tract twin manifold, there is a better balance. You can now get it in quicker & with a pipe, get it out quicker

 

[WAB]

interesting. I might just have to buy and extra 61x intake manifold and try decking the carb manifold piece. If you can remove the carb mounting studs you could mill the top down also. Hmmm
On the one I run now, I actually ground some extra alum from inside the intake when I opened it up to accept the 46mm wetjet carb.

 

[Philip Clemmons]

On mine they simply milled the reed side of the manifold, such that I had to run shorter bolts or washers.................likely 3-4mm

 

[Cannibal]

The only thing I saw about milling the underside of the carb manifold is that the carb manifold and the reed manifold form an up side down V. By taking off material, the edges of the V no longer line up. You'd want to take off the reeds blocks, bolt the manifolds together and blend them.

My carb studs came right out. That's where I was going to take a little material from.

 

[tipafew]

I had mine milled on my old motor. A racer told me about that years ago. I just took a few mm off the the motor side of the manifold. It was a LONG time ago so I can't say whether or not I noticed anything.

 

[CARBONX2]

4mm off the underside of the carb manifold

 

[Mouthfulloflake]

waxhead always told me that an old trick on the 6m6 cases was to mill like an angle on the bottom of the manifold, to lean the carb out from the engine more.

he said it worked really well back in the day, a search of old posts might turn up the info he shared.

 

[Cannibal]

Angling the manifold is an interesting idea. Though it brings up a few more issues such as the bolts are no longer seating squarely. Might not be a big deal though.

I calculated the volume of taking 4mm from under the carb manifold and taking 4mm from the carb mounting flange.

Taking 4mm from under the manifold will result of a loss in volume equaling 2.26in^3, but taking 4mm from the top of the manifold will only give a loss of .41in^3.

If someone were really serious about doing something like this, they could take 4mm from under the carb manifold, 4mm from the top of the carb manifold, and 2mm from the bottom of the carb. Plus if you could figure out somewhere else to mount your throttle cable, you'd loose the volume of the cable plate and an extra gasket which is around 3mm total.

Under=2.26
Top=.41
carb=.20
plate=.30

total=3.17in^3 reduction in volume.

Not that I'd take on something like this, but it might be possible for someone trying to get everything they can from a 61x

 

[kevbo]

I used an 6m6 case with a ported 61x cyl for a season. I exchanged the 6m6 case with a 61x. The only difference was in the required popoff, the 61x case needed the next lighter spring and thats all. Im sure that milling a manifold will make a change, Im not sure if it would be a meaningfull one.

 

[sflsurfrider]

Maybe there is more to the performance gains from this manifold besides the reed stuffers.

the wetjet manifold has built in reed stuffers.

harrison, if you werent so busy effing around with peoples avatars, you mightve paid just enough attention as to not redundantly state the obvious.