popping the bust plate and breaking belts?

Fuel Cut-off vibrating closed

I had the same thing happen several times before I figured it out, too. I added an overcenter spring to hold the fuel cut-off open during the run but could still manually close the valve with the lever. No more problems with vibrating the valve closed or leaning out and blowing the burst panel, at least not for THAT reason.

 

Possible Lean Out Conditions

True enough, unless there is a piece of grit floating/rolling around inside ONE nozzle that might cause ONE cylinder to instantly go lean. I could imagine a backfire under those conditions that would not show up on a plug reading. Very remote chance, I know, but still something worth checking out. Pull the nozzle lines and LOOK in the nozzle with a light and maybe even a magnifying glass if you are old and gray like me.

 

Fuel managment: i only run one high speed -6 poppet with a pill in it. (I went 5 numbers down on the pill and put 2 shims in the poppet. after the first run. We also changed the fuel pump to a fresh one (same size of course, enderle 1200). No change, it still broke the belt and popped the panel).

Fuel shutoff: i have the overcenter bracket and spring

Fuel Tank: It is baffled, mounted in front of the engine. I have never run the tank dry it is quite big. It has a -8 vent on it (one 90 degees hose end and 20 inches of -8 hose). The main feed line going to the pump is a -12 about 5" long, it exits the tank at the bottom and has a small sump. All the returns enters the tank at the top. Fuel tank is holds about 45 liters

I got the engine apart now everything looks good. Going to check all the lines and poppets today.

I ordered a new set of wires and some tighter valve springs. next time out i'll put more fuel in it and see what happens. It is odd tho according to the tuneup it should be very rich as it is now. I can't really think of anything else to try. As i have not found any errors.

 

vent too small

i would do the glug test as Will explained. Your vent should be able to replace the same volume as your pump supply but a -8 vent is too small for a -12 supply

 

I just now tried the glug test.

With all returns capped and the tank lid on and closed it drained without a single brup or glug.

 

Bearings

What do your bearings look like after this happens?

 

bearings

@ rb0804

All bearings are perfect, just done checking them.

 

First of all I'm no expert at all on this but I believe a #8 vent is fine for a # 12 inlet. I use a #10 on a 1 1/4" inlet and it passes the "glug" test no problem. Isn't a #12 inlet a little small for the engine he is running? Is this maybe the issue? Don't most us a 1" or larger inlet?

Corey

 

@ Will

The hose is only 5 inches long. I don't know the brand or make of it, But I can't squeeze it enough to flatten it, even when using both hands (don't know if that is worth anything).

I'll check the sd card on the grid.

I believe it was wide open.

No head temp gauge, it takes quite awhile to warm up at idle.

 

Just checked the sd card in the powergrid.

It didn't save the runs as i didn't launch using the transbreak which is setup to start the recording. It did save the data from the warmup in the pits where we tested the transbreak.

 

burst plate popping

Britt said back in the IHRA days running straight rotors the blower would do this if set too tight on end clearance. It is probably not your problem if it happens at the same point every time but worth checking. If end clearance is too tight the rotors will touch the end plates when they bow up from boost. Look at endplates for grooves or wear usually at the top near the tip. Touching will spark or heat up ignite the fuel in the blower. Breaks belt and pops panel. Motor bearings and plugs look like new. Had it happen a couple times before he figured it out. Also motor can be lean at some point and not show on plugs or bearings. Only have to be off one cycle to pop the burst panel, plug does not show everything, it is more of an accumulation over time.

 

Any chance it could be valve springs ?

 

How tight are the valve guides ??

 

Have you made a run without the grid? If it was me, I'd make a run on straight mag and see what it did. If it worked then a run on the crank trigger. Why do you need the grid if you're not moving the timing

 

Valve springs: they are between 310 and 340. But i ordered a new set, just to be sure

Valve guides: I don't have the numbers on them but they are i descent shape. (half a season old)

Powergrid: At he moment i am not using any features from it, but the rev limiters. But i plan to in the future. But yes next time out i'll have to do a run without it just to test if that is messing with anything.

 

This is way out left field but have seen this before, have any of the spark plug holes in the head been re-threaded with a steel heli-coil, motor kept putting the panel out after half track, it was the end of the heli-coil glowing.

 

How much boost? The springs are on the weak side for a bunch of boost for that combo.

 

There are no heli coils in the spark plug threads

Boost is about 30 psi