Is this detonation occurring on 2.0 engine at idle (500RPM or less)?

Garrett · 10:07 AM

I was tuning my carbs today on my 2.0 w/ 9.8:1CR I recently rebuilt. The rebuilt engine (top end) has about 100 miles on the rebuild, and this is the first time to tune the carbs as I rebuilt them several weeks ago.

While tuning the carbs (Webers), I got them all set, and when the engine speed drops really low, around 500RPM or so, I hear an occasional "spit", but I also hear what sounds like detonation!!! I have never heard this before - it didn't occur in my engine before.

The engine runs great, timing is correct at idle speed and 5000RPM full advance. I never hear this at any other time - is it detonation?? It kind of sounds like a backfire from within the engine... it's not a spit from the carbs or a backfire (I've got a custom sport muffler so I can hear backfires pretty clearly!). It sounds like "pinging" to me! You'd think that $2.50 per gallon would give me gas that didn't detonate!!! (at the wrong time I mean...)

What do you guys think?? Is everything OK???? I think it's OK, since it "goes like stink", as Superman would say!

Steve@Rennsport · 09-05-2003, 10:54 PM

Hi Garrett:

As you can well imagine, its more than just challenging to troubleshoot noises from afar, but metallic sounds at ultra low RPM can be many things. Does this happen when cold or hot?

Here are a few things:

1) Piston slap

2) Wrist pin clicking

3) Timing chain and tensioner noises

4) Throttle plates rattling, depending on cams and how badly your carbs are worn.

ein 6er · 09-06-2003, 02:08 AM

garrett,

beauuuuuuutiful car and engine!!

sorry, i can't help with your question, but ........ can you pleeeeeeeeeeease tell me, is this your oil pressure sender, (arrow)? can you please give me a part # and your gauge #. do you still have an idiot light? what year is your engine?

i have a '67 2.0 that i want to put a pressure gauge on.

thanks!!!

doug waters

dotorg · 09-06-2003, 02:35 AM

Is it? I thought that was the oil temp sensor. No wonder my efforts futzing around with that to get my temp gauge working have been a failure

Where's the damn temp sensor on 2.0L engines, then?

ein 6er · 09-06-2003, 02:52 AM

"Is it? I thought that was the oil temp sensor."

unless i'm mistaked, the temp sender is down by the fan. i only have a press switch where his press sender is.

dotorg · 09-06-2003, 04:10 AM

Quote:

Originally posted by ein 6er
"Is it? I thought that was the oil temp sensor."

unless i'm mistaked, the temp sender is down by the fan. i only have a press switch where his press sender is.

Yeah, a couple people have told me that. I don't have anything at that spot. Come to think of it, maybe thats why the gauge doesn't work.

Any idea if there are differences? Could it be somewhere else on the engine?

Superman · 09-06-2003, 06:47 AM

Hi Steve, hi Garrett. Beautiful car, beautiful engine. Remove the sound pad. Ignore all but the most obvious noises, especially those at engine speeds outside the operating range. Use only the best gas. For peace of mind you could play with the ignition timing and see where it begins to knock. Then back it off three or four degrees and you're safe. Two degrees, really.

Early_S_Man · 09-06-2003, 07:12 AM

Garrett,

How much advance at 6000 rpm are you running? 'S' engines used less advance than 'normal' or 'L' engines and there was an adjustment range, even back when 98 octane gas was avilable for $0.27 per gallon in 1967! Try backing off your advance 3° or so and see if the noise disappears ...

cstreit · cstreit's Garage

Garret,

You said you are hearing detonation at 5 HUNDRED RPM? Did you mean 5 Thousand?

Garrett · 09-06-2003, 09:54 AM

Hi Guys! Thanks for all the input and off-topic discussion!!

The noise was happening at 500 RPM, only at idle, and when the engine was warm. It sounds like the carburetor spit - this type of fast, explosive type of sound but from within the engine. I had the air elements off the engine, and I could tell it was NOT the carbs spitting. And with the noisy sport muffler, it was not a backfire either.

I've hung around my engine a lot over the years, and never had this noise. The P/C's are "reconditioned" - they looked brand new and came from a reputable source so I'd hope it was not piston slap, wrist pin noise or any of that. I don't think it is since the sound is infrequent and inconsistent. Occurs only every once in a while, and not really in the same spot (left sometimes, right sometimes...). It does NOT happen at speeds higher than 1000 RPM.

Warren, my advance is set at 35 degrees at 5000RPM (I didn't do it at 6000 RPM since it is so fast and my fingers might get sucked in, and the advance doesn't appear to change over 5000RPM!).

Garrett · 12:17 PM

The temp sender is at the back of the engine - below the fan on the right side where you guys have drawn the arrow (very cool by the way!). The pressure sender is the one I just sealed yesterday with lots of goop because it was still leaking down by the tranny mating surface; it's the top arrow in those images.

The engine is the original early 65 engine with the center-fed cams. I have the oil gauge that has the pressure side on the right of the gauge, with the red light at the bottom of the gauge that comes on when the engine is not running, and when it's idling very very low.

The part number sold by our host Pelican is:
Oil Pressure Sender (requires additional sealing ring below), 911 (1965-73)___
A-741-551-01
$46.

http://www.pelicanparts.com/catalog/images/John1/early_oil_pressure.jpg

Garrett · 12:18 PM

You know, I am not sure now that I think about it....

The "idiot light" must be on newer cars. Pelican has both for sale - a pressure warning sender, a temp sensor, AND a pressure sender. My engine has just two senders - the pressure and temp.

And what about that detonation? I will try a little retarded ignition timing to see if that helps...

dotorg · 09-06-2003, 12:00 PM

Whats this "idiot" light? The light on the bottom of my pressure/temp gauge is the alternator light.

Garrett · 09-06-2003, 12:15 PM

You know, that's what I thought. Thanks dotorg for the reminder. I should just read my manual and eat breakfast before replying to this Board!!!

ein 6er · 09-06-2003, 12:26 PM

garrett,

wish i could help you with your problem. i've never heard that on mine, but i try not to let my rpms fall to 500!! (-:

thanks for the info. i only have a pressure switch where your press sender is, like the one in pp's cataloge. the switch has one wire that goes to the low oil pressure light (idiot light) on the gauge. this is in a 914-6, the factory 914-6 gauge doesn't have a pressure gauge only a temp gauge and the idiot light.

"Is the red idiot light on the gauge wired to the pressure sensor in addition to the gauge??? Or is there a separate sender for the idiot light."

this is what i have been trying to figure out. do you have 1 or 2 wires on your press sender? later engines have the press sender mounted just above the temp sender, mounted into the cam oil line in addition to a press switch, located where your sender is. i'm trying to add a gauge and keep the idiot light.

i'm wondering if the early sender is a dual press sender/switch (idiot light).

the 911 gauge i want to add is from a 74. so with your sender (68-73) i need to get a 68-73 oil pressure gauge.

dotorg, do the early 911s not have a low oil press light, (idiot light)??

doug

dotorg · 09-06-2003, 12:36 PM

I have no idea... Is that maybe what the top light is? I have a pressure gauge, too. I don't think I've ever seen the top light on that gauge light up.

Garrett · 09-06-2003, 12:38 PM

The top light is the hand-brake indicator light.

Ok, I got my "antique" manual. It looks and smells like parchment - it should as it was made in Jan. '65!!

It says, "The alternator indicator lamp indicates alternator operation, fan belt condition, and cooling blower operation. The lamp will light up as soon as the ignition is switched on, and will go off as engine speed increases. If the lamp should begin to flicker, or go fully on while driving, it may be an indication of a loose or broken fan belt necessitating adjustment or installation of a new belt. However, the cause may also lie in a defective voltage regulator in which case it will be necessary to have the defect corrected in a reputable shop equipped for this task. The alternator indicator lamp is located in the lower part of the large combination instrument."

Hope that clears that up; it does for me!!

Early_S_Man · 09-06-2003, 04:56 PM

Garrett,

I misread your initial posting about where the noise was occuring in the rpm range! It very well could be chain rattle due to tensioner inconsistencies at such a low rpm level ... why not set your idle to 850 - 900 rpm?

Your 35° advance setting seems too much for 9.8:1 compression on today's gasoline! I believe 31° was the maximum advance recommended on early high-compression 'S' engines ... on 98 RON octane gas!

Your oil combination gauge pic answers a question I posted a couple of years ago and never got an answer to ... which early car(s) had the 280° F oil temp gauge??? Thanks! Great paint job, too!

350HP930 · 09-06-2003, 05:04 PM

At very low rpms on a carburated engine you can create a combo of high manifold pressure, a lean mix and spark timing that will result in complete combustion before the piston can reach TDC.

Thats detonation.

Its much like the pings you can hear when starting some cars.

Garrett · 09-06-2003, 08:41 PM

Thanks Warren - I will lower the timing to be on the safe side. I think it is detonating at a low RPM - it doesn't sound like chain rattle. The tensioners are rebuilt (that was a fun task!) so hopefully doing their job; but the sound is more in the forward part of the engine, in the cylinder area. It's been said that the low RPM range is the unsafe range due to various factors I don't know of.

You've convinced me to lower the timing, and raise the idle. I usually like the 500 or so idle speed because if it's a bit higher, it seems to always sit at about 1200 - either 500 or 1200!!! I'll see if I can get it better now that I've just re-done the carbs.

Thanks for everyone's input. I really appreciate it.