Super Jet Tired of going through top ends - HELP

[escore]

This is in a Blaster but I figured it might get more views here.

So I blew the top end in my Blaster for the 4th time since i built it a while back. Here is a picture of the pistons as they where when the cylinder was taken off. You can see some exhaust residue on the bed plate which was also on the rear cylinder and manifold. When I first put the engine together it went 3 times in 4 hours and then I put double base gaskets on it and it ran a year and a half until this. It has port matching, factory mod pipe, reeds and angled spacer, reed stuffers, MSD CDI, lightened flywheel, stock carbs, dual cooling, and most importantly an ADA head with 35cc domes. any insight based on this picture would be awesome. I'm really tired of this.

 

[kevbo]

This is in a Blaster but I figured it might get more views here.

So I blew the top end in my Blaster for the 4th time since i built it a while back. Here is a picture of the pistons as they where when the cylinder was taken off. You can see some exhaust residue on the bed plate which was also on the rear cylinder and manifold. When I first put the engine together it went 3 times in 4 hours and then I put double base gaskets on it and it ran a year and a half until this. It has port matching, factory mod pipe, reeds and angled spacer, reed stuffers, MSD CDI, lightened flywheel, stock carbs, dual cooling, and most importantly an ADA head with 35cc domes. any insight based on this picture would be awesome. I'm really tired of this.

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Your engine is set for higher octane than you are using. The two biggest factors are compression and timing, back off both of those and this problem will disappear. Carb tune also contributes greatly, the top screws should be set a little rich of peak power for maximum reliability. Running the tank low will negate this without a vapor separator so no speed runs with less than 1/2 tank. Get 37cc domes and check the squish with one base gasket, then retard the timing a couple degrees. Tune the carbs well rich of peak and run 10 gallons before you start leaning it in.

 

[escore]

What makes you say the octane needs to be higher? The picture? Maybe an air separator would be a good idea I guess.

 

[txsuperjetfreak96]

what was your compression? Should always check your compression so youll know what octane fuel to run.

 

[kevbo]

What makes you say the octane needs to be higher? The picture? Maybe an air separator would be a good idea I guess.

I can see detonation erosion on the aft piston. It is possible that air in the line pushed an otherwise sound setup over the edge. Lowering the compression and timing will increase your margin of error. Also open the top screw more on the aft carb since it feeds the hot cylinder.

 

[shepster]

I've been here before with my blaster and it sucks to eat pistons!!
As Kevbo said the set up needs to be right, Blasters place a lot of load on the motor and it needs to be set up differently than say a superjet. Lower compression, correct jetting and the right timing. Looking at your pic the jetting looks to be very close to being dangerous.
What is the motor size, carbs, jets. head pipe size etc, cooling? All these will contribute to a nice reliable or an expensive set up

 

[Dutchie]

Was'nt the rear cylinder made with lower compression by the factory couse of this problem ?

And who just runs an race engine without knowing what compression they are on? That is asking for this. It is not fun if your ski breaks down all the time. indeed the compression and timing should be checked.

 

[escore]

I had checked the compression last year when it kept blowing up but it has run good for the last year or more so I cant remember now what it was. I am thinking right around 175. It has dual cooing and the head pipe is opened up to the right size which I know forget what that is. This next bore will put it over 83mm. Stock carbs and I forget the jetting in them again its been a while since I was into them. I think I am going to try going back to a stock head for a while and see how it runs then.

 

[JetManiac]

Looks lean especially rear cylinder. As others have said, the exact setup matters for reliability. You need to know the variables and measure squish when building your motor. If you need extra thick base gasket to adjust squish, then use 3 layer thicker gasket , but dont use doubled up regular base gasket. Check compression after it is built.

If you switch to stock head, then your performance will suffer alot, but the lower compression and jumbo squish will help reliability.

 

[Mark44]

Have you checked your squish and have you tuned the carbs properly? Look in the tech section for the thread "carb tuning for rookies". It explains the proper procedures to tune the carbs it can not be 100% accurate by ear or feel.

 

[Myself]

That is definately lean on the rear cylinder and getting too hot. I suspect the exhaust scavenging is more efficient on the rear. Are the exhaust manifold runners the same length? Does one have a straighter path to the expansion chamber (rear)? The design of certain exhaust manifolds will inherently cause a hotter rear cylinder by design. The only way around the issue would be to stagger the jetting and run fatter in the rear. You might also check with the pipe manufacturer to see if newer versions of the same pipe use a larger stinger outlet. This was a known problem on early Factory 717 pipes and the later ones had the opening enlarged.