Other Group K wrote 760 has seperate cylinder timing curves can anyone answer this ?

[Freakrider]

How is the different ignition curves for both cylinders accomplished ? I believe the plug wires are numbered 1&2..but, there is one coil and one pulser coil (pickup coil), so does the 760 CDI control different ignition curves seperate ? Have wondered about this for some time now can you explain this ?

From Group K:

"We consider the ignition system of the B2 to be of primary importance because it's the first twin cylinder pwc to have different ignition curves for both cylinders. This is done to offer a more retarded timing for the rear cylinder (for more info on this, see our website document "Rear Piston Seizures"). This retarded timing, for the rear cylinder, helps to keep the operating temperatures of both cylinders more symmetrical when the drive live is heavily loaded. We found no advantages to changing these timing curves. Furthermore, the B2 driveline does not seem to respond well to being "revved" beyond the abilities of the stock limiter (about 7100 rpm). All of our modification sets retained the stock ignition curve and rev limiter."

 

[chriselmore_1]

hmm ive never seen a 64x stator(flywheel cover mounted) in person. but i do know group k is extremely conservative with their setups

 

[SUPERTUNE]

Triples have different curves, I don't believe that info is correct for the 760, maybe a nano second worth with the resistor wires on the 760 coil.
Although the 760 engine has 2 different chamber cc's in the stock head.

 

[OCD Solutions]

Although the 760 engine has 2 different chamber cc's in the stock head.

That's a good little tidbit to know for all the guys parting out couch engines for superjets. Was it enough of a difference to give different comp readings?

 

[Matt_E]

maybe a nano second worth with the resistor wires on the 760 coil.

Not possible to delay the signal with resistors.

 

[waxhead]

the 760 flywheel has a small lump on the flywheel as well as the normal two large lumps so the ignition knows what cylinder is what
This is to give the retard on back cylinder. You can take it off and it will run the same curve on both cylinders. I was doing this when I was using the 760 to run total loss ignitions using sxr elctronics on a superjet

The 760 is a single channel ignition so it still fires both plugs at the same time just not at 180 degrees apart
Im not sure of the actual difference in degrees

 

[Matt_E]

Good info, thanks.

 

[2XLR8]

Wax you are correct . That is why the GPR uses only one pickup. Much much better than the 1100.

 

[Freakrider]

the 760 flywheel has a small lump on the flywheel as well as the normal two large lumps so the ignition knows what cylinder is what
This is to give the retard on back cylinder. You can take it off and it will run the same curve on both cylinders. I was doing this when I was using the 760 to run total loss ignitions using sxr elctronics on a superjet

The 760 is a single channel ignition so it still fires both plugs at the same time just not at 180 degrees apart
Im not sure of the actual difference in degrees

Yes, Wax good info....thanks.

Group K further indicated: " The '96 Yamaha Blaster II ignition fires 2 degrees retarded at all times, and the rear combustion chamber has considerably less compression than the front. All the 1997 701 models, from Yamaha, also have this "staggered" compression arrangement. Because of crankshaft torsioning the rear cylinder is lagging slightly behind when it gets it's ignition spark. This means that the rear cylinder is firing 2 to 3 degrees more advanced than the front cylinder."

SJ's don't seem to have this rear cyl heat problem due to heavy pump loading...

OCD, I have always seen 5 psi higher compression on the 760 front cyl with the stock head...

 

[#ZERO]

I was told from a reliable source that the 760 engines have different timing curves for each cylinder that is control through the CDI's electronic circuitry. Even though the coil has the secondary leads tied together it can control the timing through the primary wires. If you notice on the outside area of the 760 flywheel there are two large pick-up points for the ignition timing points at 180 degrees apart. There's also smaller pick-up point that's used for the cylinder identification to control this feature through the CDI unit.

 

[yamanube]

That's a good little tidbit to know for all the guys parting out couch engines for superjets. Was it enough of a difference to give different comp readings?

I would like to know this also. I am getting about 10 psi difference on my 760 with a stock head, wanted to know if it was the head or if I lost some compression in the rear.

 

[#ZERO]

I would like to know this also. I am getting about 10 psi difference on my 760 with a stock head, wanted to know if it was the head or if I lost some compression in the rear.

That's normal because the squish depth on the cylinder head is quite a bit deeper at the rear cylinder.

 

[Freakrider]

I think all this info confirms that the 760 electronics used in a SJ only retards performance for the rear cylinder (2 degrees retarded) and that it was designed to accommodate heavy pump loading /crank torsioning of the couch....perhaps the 760 cyl in a SJ would benefit from the more advanced timing 701 ignition ?

 

[kevbo]

They were trying to get minimum octane requirement down enough to survive two year old 87 marina gas.

 

[Freakrider]

They were trying to get minimum octane requirement down enough to survive two year old 87 marina gas.

Kevin, where did you get that info from ?

 

[kevbo]

I pulled it out of my a$$. How would you design an engine that you knew would be operated by idiots?

 

[WFO Speedracer]

Old thread but I have a question, if I remove the small lump will both cylinders fire exactly 180 degrees apart ?

 

[waxhead]

yes

 

[WFO Speedracer]

yes

Thanks

 

[madscientist]

very nice info..