Measuring Heads ?

[Vmax540]

Measuring Heads here is what I found with six different 540 heads. Give me your thoughts, I know this is not cc'ing but would this not give you an idea on the head ?

5 measured between
37.33mm - 37.69mm
1.46in - 1.48in
1and15/32 - 1and31/64 in fractions

1 Measured (and has detonation or debri marks) Is this safe to run any thoughts ?
36.22mm
1.42in
1 and 27/64 in fractions

*Also one 1981 SRX440 Measured
36.91mm
1.45in
1 and 29/64 in fractions
*The last picure shows the damaged and cut head on the right and a stock head on the left*

[Bob Vehring]

Chuck, I don't know what to say, I have never tried to measure that way. How consistant that method is would depend on how stable Yamahas casting was. I assume they cut in the chamber indexed off the head surface so how thick the casting is really wouldn't matter.
I know being a consultant for Briggs Motorsports Div. that each and every part has a low and a high tolarance limit on every part they make. All production engines have this. Its because of many things, variance in castings. Wear on the machines that cut the parts as they make thousands of them. The same with the tooling and how often they change it, and of course,especially back in the 80's there could be some human induced variance. ( commonly refered to as either the Monday mourning, or Friday afternoon part).
Again I don't know about Yami production technics but I would try this. Look for a flat machined area in the chamber, I don't have a head here, but maybe the spark plug hole has been back cut. Now use a flat bar. I use a 1/2 bar, long enough to span over the chamber. Now measure down with the stem of your caliper from the top on the flat bar to your flat surface. If you measure several, you should get a feel for how consistant Yamaha was, and/or be abel to tell if a head has been cut.

You also could try the old method of fishing in a lenght of solder threw the plug hole, then rolling the engine over to flatten the solder, this is used to check the distance between top of the piston and the edge of the squish band but besue you have the solder out to the end

[opsled]

In you pics the head on the right has been cut. You can tell but the fact that the black area by the T-stat is gone. All factory uncut heads will be like the head on the left in that area.

I wouldn't run the cut head on a stock engine without having it cleaned up and redomed to make sure it's right.

Most that you see cut like that were on a ported engine. When the exhaust ports are raised it needs to be done to bring compression back. How much depends on how much port work was done.

opsled

[YAMMIEGOD 3:16]

LOOKS TO ME THAT VMAX HEAD WITH SLIGHT DEBRIS WAS CUT. I WILL CALL YOU. THANKS CHUCK. 3:16

[Vmax540]

Thanks Guys, very interesting !

- "cleaned up and redomed "= then you'd have to shave the head to regain compression ? Is there a ballpark safe tolerance for shaving a stock head for performance or in the case of warpage and still be able to use pump gas ?
- Would you ever use a shaved head on a stock engine ?
-You guy's remind me how little I know.....

[YAMMIEGOD 3:16]

i will play with that head chuck. send it. i,ll get it CC ed and might just work mint with yamaha updated porting for 540 vmax. i,ll run atleast 100 octane. thanks man. 3:16

[opsled]

The head on the right is stock. The one on the left has been cut and redomed.

There are many reasons to do this including just repairing a head that has been damaged from a burn down. Raising intake/exhaust ports decreases the amount of compressable air space on the top of the piston so it can lower the compression (psi #). To bring it back up cutting the head which lowers the CC volume in the cylinder when compressed is common. There are limits to how far can be gone depending on many factors. The factory Yamaha specs are usually in a "safe" range and there is room to cut a head some without being in a danger range. Many variables on what to do and when to do it. I understand the pricipals and applications for such mods but don't do my own and don't have any specific numbers I could give you.

opsled

[Bob Vehring]

Sorry I got nothin for ya either pal The engine that came with my sled was done by a complete hack and I have no idea where the head has been cut to. On the new engine, which I haven't even touched this year, we cut it to acept Rotax chambers so now I can put in any compression I want for the day. The bigger reason was to get rid of that offset dome Yami used back in the day.







Many engines use gaskets instead of O rings to seal the head. On my Red SRV which at one time had race gas only compression, I now simply use thicker head gaskets to drop the compression back down but I can still use the head I had both the center and the deck cut on

[Vmax540]

Again very interesting ! Why did they use ? "offset dome Yami used back in the day"

[Bob Vehring]

I really don't know and other then SRV, VM, I don't know where else they were used.
Now remember this is only my little twisted mind thinking these thoughts but.....
I noticed several thing on my '83 that were quite different then most other sleds (brands) I had fron that era.
offset domes,
dual fuel pumps,
injecting oil downstream of carb bowl
Power jet carbs

I have nothing to base this on, haven't heard any stories, but maybe were they alittle gun-shy after the 81-82 SRX years?
I don't remember any of the things on that list as being used much on other performance sleds.
Again just a thought based on absolutly nothing

[Vmax540]

Relibability

offset domes,= ?
dual fuel pumps, = insure there is plenty of fuel flow
injecting oil downstream of carb bowl = Lube the carb slides
Power jet carbs= provide extra fuel in mid range

[Bob Vehring]

Maybe, but let me ask this, and I really don't know.
Did Yami use these things on their later model 2cy engines?
Did other manufactorers follow this lead?

My thoughts, dicussion welcome.

Offset domes, I would think it is fine for factory HP level, as you push the limits and raise HP, one side becomes more prone to deto. Haven't seen offset used by any race teams on any custom built engines

Dual pumps, plenty of fuel, no doubt. but I feed my 160 hp Rotax in the drag sled with one pump, no problems. Regans oval sled runs one pump, again no problem.

Where I do see a problem is where its injected, The oil pump is throtle controled and adds oil accordingly. Lets say your driving down the trail for 2 miles at 1/4 throtle, come to a long lake and now WFO it. The float bowl is a resavoiur and is now filled with a very lean oil mix. You are running on that very lean oil mix until the full load can travel up to the carbs and fill the bowls with the correct ratio for full throttle. By that time, I've scared the shit outta myself and backed off

PJ carbs. Yes, it does offer one more jetting circut, but really the slide carb design allready offers 5 circuts to control jetting in every range. I think that about covers it. What I don't like is, on a non PJ carb, the main sits down inside the brass plug at the bottom. This acts as a resivour to trap fuel around the main jet even during time where the fuel is really sloshing around down there. Basicly it draws fuel from the lowest point. On a PJ carb the main sit not only flush on the floor of the bowl, it mounts upwards. This puts the point it sucks in fuel a good 1/4-3/8's above the bowl floor. Could it suck air under hard cornering loads when the fuel washes to the side? I didn't want to find out.

I won't say any of these things are related to any problems yamaha may have had, but I can't say any of these ideas became a new standard that other manufactorers followed either.

[Vmax540]

[quote="Bob Vehring"]Maybe, but let me ask this, and I really don't know.
1 Did Yami use these things on their later model 2cy engines?
2 Did other manufactorers follow this lead?
3 Dual pumps, plenty of fuel, no doubt. but I feed my 160 hp Rotax in the drag sled with one pump, no problems. Regans oval sled runs one pump, again no problem.

4 Where I do see a problem is where its injected, The oil pump is throtle controled and adds oil accordingly. Lets say your driving down the trail for 2 miles at 1/4 throtle, come to a long lake and now WFO it. The float bowl is a resavoiur and is now filled with a very lean oil mix. You are running on that very lean oil mix until the full load can travel up to the carbs and fill the bowls with the correct ratio for full throttle. By that time, I've scared the shit outta myself and backed off

5 PJ carbs. Yes, it does offer one more jetting circut, but really the slide carb design allready offers 5 circuts to control jetting in every range. I think that about covers it. What I don't like is, on a non PJ carb, the main sits down inside the brass plug at the bottom. This acts as a resivour to trap fuel around the main jet even during time where the fuel is really sloshing around down there. Basicly it draws fuel from the lowest point. On a PJ carb the main sit not only flush on the floor of the bowl, it mounts upwards. This puts the point it sucks in fuel a good 1/4-3/8's above the bowl floor. Could it suck air under hard cornering loads when the fuel washes to the side? I didn't want to find out.
quote]

1 Yamaha did use the the PJ Carbs on the 88-90 Exciter altough many tuners and aftermarket'ers removed the PJ's and rejetted on both of these sleds.
2 I do not believe others followed suit mostly because these were either unnecessary or did not add relibiality ?
3 same as #2 and definately not necessary.
4 Hence, the direct injection kits in which I have used for many years without issues.
5 I can't ride that hard or fast anymore.... You point out a good fact the main's are up out of the crap.

[Bob Vehring]

5, True they be out of some of the crap that settles to the bottom, but, my point is they may also be out of the gas under high cornering where the fuel moves to the side of the bowls.
Shifter karts useing either Mikuni or Keihen float boal carbs, removed the float system and went to a pump around/return line system just because of this.

[opsled]

My thoughts.

Offset domes?

Dono, Yammie has done it on some and not on others. Fan and liquid cooled. Could have something to do with directing combustion to even heat dispersal. Turbulance in a combustion chamber happens and no two designs are the same. Air intake and exhaust causes flow and I can see where in some cases offsets can help a piston live longer. This is a very common practice on any kind of combustion engine. Offset's don't bother me unless I saw evidence they were an issue. They were done for a reason.

Dual pumps? Could be many factors as to why. Again Yamaha did it both ways. I'm sure they had a reason. Would be interesting to know.

Oil injection UPSTREAM (not downstream). I like. My pet peave.

Carbs with oil in them are always cleaner and less prone to crud buildup. Oil is mixed with the fuel and not just dumped in after the carb. The oil will be better dispersed throughout the engine. The "lean oil" theory isn't true. Never been evidence to prove it shown anywhere by anyone. The oil injection system on any two stroke does not change oil flow rate to change mix ratio. It changes rate to match fuel flow to keep the mix ratio the same. Idle, midrange, WOT, makes no difference.

Fuel flow varies by RPM and load (throttle position) Oil injection systems are geared to the crank (RPM) and the flow control cable is attached to and moves with throttle position. It must be done this way or a lean oil condition will occure. Oil flow must match fuel flow!!

PJ carbs,

Work fine and allow for more efficient fuel management over all conditions. The problem with them is they are prone to failure when they aren't serviced/setup properly. Yamaha's PJ's also become almost inoperable because of the outlet location when air boxes are removed. Many engines went down when owners removed airboxes and ran their sleds. Yamaha dumped the idea with the Exciter 570's. To many people messing with stuff and no real control by the manufacturer or dealer. Burn your engine down, put the airboxes back on and take it in for warentee. Happened a lot. Yamaha eliminated the headache by eliminating PJ's.

All things are done for reasons. I won't condem without proof. Yamaha doesn't build junk. When they have had issues with things they fixed or changed them. Things they didn't change were left alone for good reasons. Not bad.

opsled