Popping Heads!!

Windswept · 08-07-2006, 07:28 AM

I recently acquired a '69 912 that a previous owner had started a restoration, then had to abandon. The owner a "engineer" had rebuilt the engine and that with the 5 speed gearbox and other parts I thought was well worth the $2,000 I paid for it. Being unsure how good the motor was I figured the only way to really find out would be to run it. I installed it in my '65 and it started easily and idles well but had a loud popping sound when you revved it up. A mechanic friend listened to it and thinks that the heads are loose. He suggested pulling the engine and retorting the head bolts and maybe I'll get lucky. I was wondering if anybody on this list has any other ideas about this.

Mike

Nige-912E · 08-07-2006, 11:32 PM

Depends where the popping is coming from, if its back out the carbs then suspect the carbs to be too lean/lack of fuel or air leak at the base of the manifolds.

If its popping from the exhaust suspect a leaky joint/holed silencer.

I'd check these areas first before contemplating the heads, a compression test would reveal if the heads aren't sealing anyway, so do that first.

When I installed my engine back in the 912e the original builder had mushed up one of the sealing rings between head an barrel, effectively that piston wasn't sealed at all. From initial start up I could tell something was wrong, it sounded like a diesel tractor with not exhaust, yes it was popping but the noise was horrendous!

Windswept · 08-08-2006, 06:20 AM

Thanks Nige, I've checked the carbs and it has a new bursch header and muffler. I'm not sure what a unmuffled diesel would sound like but this is awfully loud when you rev it up. I think you are right a compression test is in order before I do anything else.

Mike

912UK · 912UK's Garage

Carbs carbs carbs carbs carbs there I think I have said it all..

99.999999999% of the time when a car does this and ESP if rebuilt by a street mech. ( only ones that have not done Porsche home work ) Will rebuild the carbs with a NEW gasket kit !!!! Fresh..... NOT!!!

a gasket kit will NOT undo 32+ years of wear and tear of the carbs. These need to be taken off and sent away to a carb place that 912 and 356 boys recommend your side of the pond it's going to cost you but they will be

1 Chemicaly cleaned
2 Fracture tested
3 Pourous Tested
4 machined bases
5 Drill and rebush the carb rods ( MOST MOST!!!! important bit air leaking in here 100% )
6 Rebuilt
7 Jets corrected if a mixture of jets
8 sent back to you as brand new carbs
9 put them on set them up and balance them and they will be fine.

Nige I think if I could get $10bucks for every time I have said this and people have done it and worked I would be able to buy a 356 Pre Cab for cash!!

Mike don't get me wrong we get this question about once every 3 months.. Nige and I have been into Porsches for years!!

$2000 for a 912 is a bargain I picked up a 65 in Sept last year for $4500 and I thought that was a good price intodays Ebay days.. So spend some money on the car like easy $2000 again and you could be well on the way to a $8000 912

Any pictures I am happy to help with advice.

Mark UK

Windswept · 08-13-2006, 07:03 PM

Mark,
Thanks for the carb info, but I did a compression test and things don't look good, on the right side cylinders 1 & 2 I got 130 and 150 psi but on the left side both cylinders read 80 psi. At this point I'm going to pull the motor put the orginal back in for the fall driving season and put this one on the stand and probably pull the heads and see whats going on.

Thanks Mike

PS Here is a picture of my '65 number 554

Nige-912E · 08-13-2006, 11:13 PM

Double check your valve/rocker clearances before pulling the heads, if the valve isn't closing properly because of wrongly set clearances you will see a difference in compression per cylinder.

912UK · 912UK's Garage

Hey Mike that looks a sweet bargain for $2000 and if the engine costs you $4000 to rebuild properly the car is still going to get you back the $7000 mark

Nice one it looks a great car to start to build on and work towards getting a real mint car. because you got a great starting point of a 912...

Here's mine still working it all these years later new project is adding the air con I picked up from a 65 car I have here. And still rebuilding my 125BHP 912 motor!!!

I to started a great into price of $2000 £1200 back in 1999 so you are the same as me.. what you spend on the car will still pay you back later on... nice one.

Green 912 · 09-11-2006, 07:46 AM

do a leak down to see where the air is going.

Windswept · 09-11-2006, 07:49 AM

Pardon my ignorance but how do I do that.

Mike

Green 912 · 09-11-2006, 08:16 PM

Put the cyl in question at top dead center of compression stroke so both valves are also shut and put compressed air in through the spark plug hole. (same position as when you adjust the valves per the book) If valve leak you will hear it coming out the carb or tail pipe. If bad rings you will hear it in the case, listen at the oil filler. If a poor seal at the head to jug you will hear or even see it vis using smoke. Hearing it is more than enough. If the head is leaking and you have run the motor tear-down is needed. You can purchase leak-down testers that give you a measured value but for the above rough testing an old spark-plug with the electrode and ceramic removed and an air line attached will work fine. Check cylinder each one at a time. Be careful the air can make the motor turn part way around.

Windswept · 09-12-2006, 06:35 AM

Thanks Green 912,

If the head is leaking do you think it will require fly cutting the heads to get it to seal?

Green 912 · 09-12-2006, 07:43 AM

Depends on why they are leaking. Cutting a head if the cylinders are leaking from not being level with each other will do nothing. All that can be said at this point with info at hand is that if they are leaking you should go in and find out why. The jugs need to be dead level with each other on each side or the head will cock and if out of spec enough leak regardless of how hard you torque the heads down.

snowman · 09-15-2006, 11:45 AM

If the head was reassembled without the washers under the head nuts, this kind of symptom can be produced.

The carb base is always bent enough that a signigicant air leak can develop. Try blowing some propane around the joints. If the engine smooths out, its leaking. I have had to resort to rtv on both sides of all the carb base gaskets. It works, but use verry very sparingly.

Valves must be adjusted before any compression and leak down tests. With compression tests always make sure the throttle is wide open

tono · 10-08-2006, 03:11 PM

Diagnosis is classic...
Crack heads in a 40 years old car...at left side. Check piston rings.. If the crack is from spark plug hole to the valve..you are looking for a rebuilt heads...$1000-1500 a pair.
Be prepared to get other surprises..!!!! inside the engine.

snowman · 10-08-2006, 08:16 PM

If a head does actually have a crack, consider having it welded and reworked, or just replacing it with a rebuildable used one.

tono · 10-09-2006, 02:31 AM

If the crack is as I mentioned....from spark plug hole to valve seat...

throw them to the trash.

Put rebuit ones , not welded.

Consider upgrading your engine to a full flow oil / oil cooler system to avoid overheating..

in a future..

or else you probably will have to replace again crack heads.

snowman · 10-09-2006, 07:44 PM

Valve seats in 912s are replaceable. Cracks can be welded very cheaply. Why replace them? They will be better than new.