787 Detonation with my current setup

[fzrace]

I have a 1998 SPX that I've just finally put back together and detonated the pistons to failure in short order.
I have an MSD ignition set to the stock curve, Group K bored carbs, Group K porting, Group K cylinder head, Wossner 1mm over pistons, deleted stock fuel pump replaced with a Mikuni dual output high volume fuel pump and lastly, a Spec II pipe.

After the build, I had 180 psi compression in both cylinders, measured using a new compression gauge from Craftsman tools. Max EGT was 970 while riding.

Jetting on the carbs is as follows, Mag main jet-147.5, PTO main jet-150, high speed screws out 1/4 turn. Pilot jets are 72.5 with the low speed needle out 1 1/4 turn. Needle valve is 1.5 and pop off is 30 psi using the 80 gram spring.

My opinion on my current jetting is that the pilots are to big, and I could go bigger on the mains.

Mr Klemm of Group K fame, had warned me that this would be a detonation accident waiting to happen without race gas. At 180 psi compression, I didn't think it would be a problem, so I took the chance of riding without race gas. but it would appear I that I made a mistake.

My question is, considering my setup, does it seem reasonable to detonate itself to death?

If I run the stock head and leave everything else as is, would I still stand the chance of detonating another set of pistons?

 

[mikidymac]

The key here is it’s not just compression that requires race gas and no magic number that requires race gas. With the combination of your pipe, porting and compression you obviously need race gas and Harry would know as he has thousands of hours testing these things. Jetting alone isn’t going to solve your issue apparently.

 

[fzrace]

Miki, I might have mis-led you with my post.

I agree with you completely that jetting wont fix my issue. I only listed everything in hopes to avoid having people question what pilot jet I have installed, when in fact, ylu and I both know thats irrelevant to my detonation.

What I'm really trying to figure out istwo things.

1: if I put a stock head back on to lower my static compression, would I be able to run pump gas leaving the remainder of setup the same?

2: Could the dynamic compression of the Spec II pipe be too high even with a stock head, that I would need to run race gas?

 

[mikidymac]

Hopefully @Matt Braley will chime in as he has the most experience on this around here.

 

[Matt Braley]

Jetting should be around 75 lows and highs should be 150 mag, 152.5 pro at least.
2 things I’m not hearing..... initial timing with the Enhancer and squish measurement.

 

[fzrace]

I have the pilots at 72.5, and it blubbers at 1/4 throttle as if it's rich.

I agree with you on the top end with the mains and was going to try the exact combination you metioned.

The MSD is set at +6 deg and retardation occurs at 5000 rpm and reduces-6 degrees.

 

[fzrace]

I forgot to mention that the squish is .035.

 

[mandriva]

35 squish is too tight
you need stock head
you gave cylinder porting and carburetor true 40 mm... require more gas!
mikidymac told you everything 2#post reply
the spec 2 was no the best for rec rider

 

[POPPS]

.035 is way too tight, mid /upper 50's is much safer, At 180 you need around 100 octane...my retardation is 13 degrees, initial is 6 degrees, retard start at 5000 like yours, rev limit set at 8700. Start with dark fat plugs,,,and then work your way slowly up,,,changing top ends is a pain in the pocket and down time,

 

[fzrace]

I'm also going to purchase a new timing light. As I've been pondering my situation, I'm thinking that my light might need an upgrade. It's a lot dimmer than I remember and I sometimes struggle to see my marks. I've had it since it was brand new 30 years ago.

I'm just wondering if it's giving me false readings, making me skew my ignition setti gs the wrong way.

 

[Keno]

I have the pilots at 72.5, and it blubbers at 1/4 throttle as if it's rich.

I agree with you on the top end with the mains and was going to try the exact combination you metioned.

The MSD is set at +6 deg and retardation occurs at 5000 rpm and reduces-6 degrees.

What is your mechanical timing? -6 at rev limit seems like not enough, especially depending on what your max timing is. Its going to be basically impossible to run 180psi on a spec2 with 91 octane. You could try 23 degrees max, start retard at 4000-5000 rpm with 17 degrees total at 7000rpm, rev limit at 7250 or 7500. This is basically stock timing You'll want to keep WOT rpm to ~7000 on pump fuel. Increase mid pipe jet to bring rpms down.

Drop compression to 150-160, 0.055-0.06 squish.

OR Listen to Harry, because he knows about his motor.