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Hi,

This is a question from a friend who races in Mini 7 class here in NZ (1000cc)

The question :

I have 1000cc motor with a late model 12940 cylinder head (std small valve head ) on it. I have pocketed the block and redone the holes in block. It is used in how mini 7 series here in New Zealand. We are having problems with the inlet valves on number 2 and 3 leaking after a few race meetings. We do a cylinder leakage test on it and those valves and it is about 20 to 30 %. We have run both 98 and 100+ fuels. This happens to a few racers and others are fine, any ideas, maybe dry necking?

Can anyone offer some suggestions that I can pass on?

Thanks
Steve

Valves leaking what?

Oil or water?

dry decking would be okay to do but TurboMini's dry decking loads of tech talk on there

If you don't mind waiting until tomorrow for an answer, or at least a pointer in the right direction, I'll be visiting an associate who races Migs and (used to) Se7ens, in the morning.
He should be able to figure the problem out.

As an aside, i think all the Se7ens are running dry decked engines now.

Thanks for any help

If I read this correctly, you are doing what we call a leakdown test, where the cylinder is set to valve-closed TDC and then pressurized with air through the spark plug hole. The leakdown gauge tells you how much is air leaking as a percent of what is being supplied. You then use a very sophisticated tool called your ear to identify the source of the leak, if you hear air escaping from the exhaust pipe its the exhaust valve, if you hear it at the carb inlet its the intake valve, if you hear it at the valve cover its the rings. So he hears air at the carb inlet and his gauge reads 20% to 30% loss. That is a lot, the sort of number you might see with a broken piston ring. A good motor is going to read maybe 2% to 5% leakage. I would repeat the test after checking a few things. First, make sure the crank is definitely at closed-valve TDC and then make sure the crank is restrained; put the car in gear and/or put a wrench on the crank before connecting the air pressure. If the air pressure forces the piston down, the crank turns, and the gauge will read funny numbers. Then if you do that and you still get those same 20% to 30% readings, pull the valve cover and make sure there is definitely some valve clearance at TDC, that the rocker setting has not closed up.

I have copy / pasted exactly what he e-mailed me.

I'm sure you are right about the test he is doing though. I'll pass on what you say to him.

Cheers

Steve

When you do the leak test see to the engine is warmed up! Solution of the problem might be the fuel. Race heads of 12G940 needs additive like Miller Oild VPS to handle led free fuels type Shell V-Power .

Tommy

My associate couldn't come up with a definitive answer to the question asked Steve.

Dry decking would certainly help matters though.
He also said that, on his car/s, after every 2 race meetings the head would be taken off and all the valves relapped regardless of the state of tune etc.
Maybe the latter is the answer to the problem if no other cause can be found.

Sorry, I'm just getting my head around this head

And the 1000 cc block is a small bore?

Now the 12G940 is actually a big bore head, so does your friend mean it's a 12G940 with small valves (as found in ordinary 1275's)?

Has he taken the head off and assessed the valves and seats? With numbers like that, I'm guessing that the valves are bent or not closing. I'm not sure that dry decking would help this. If it were Exhaust Valves on Cyls 2 and 3, then maybe as this is the hottest part of the head.

Also, when doing leak down tests, I usually back the rocker screws right off, just to be sure nothing silly is going on there and that maximum spring pressure is applied to the valve. Also, might be worth repeating the test with a squirt of gear oil down each cylinder to give a temporary seal if the leakage is due to rings. It will start off OK, but quickly show leakage as the oil is forced in to the sump.

Just some ideas, might well be off track.

SteveG wrote:Hi,

This is a question from a friend who races in Mini 7 class here in NZ (1000cc)

The question :

I have 1000cc motor with a late model 12940 cylinder head (std small valve head ) on it. I have pocketed the block and redone the holes in block. It is used in how mini 7 series here in New Zealand. We are having problems with the inlet valves on number 2 and 3 leaking after a few race meetings. We do a cylinder leakage test on it and those valves and it is about 20 to 30 %. We have run both 98 and 100+ fuels. This happens to a few racers and others are fine, any ideas, maybe dry necking?

Can anyone offer some suggestions that I can pass on?

Thanks
Steve

Are you running open inlets without a filter? if so the amount of dirt which is sucked in and impacted upon the valve seats may well be causing poor seating, this combined with corrosion between races could explain your problem. We have the same issue on our 1293S. I understand that it is not uncommon to lift the head and lap the valves in every two meetings if you want to have minimum leak down pressure loss?

Regards

Eddie

I suspect this could be a problem caused by too much measuring.

As Steady Eddie suggests above minor damage that manifests itself between races could cause this type of symptom, where as if the engine is run up to temperature before the leak down test is carried out the symptoms will probably have gone or at least been dramatically reduced.

Is the leak down being done hot or cold?