Hello all,
While I continue to save money to have a set of heads rebuilt, I'm still trying to pick up more information on whats involved with a top-end rebuild, but also learning how these engines work conceptually.

Something I've been curious about - a friend and I drove 8.5 hours to and from Braselton two weekends ago for Petit Le Mans in the 914 and we dealt with engine temps the whole way. Granted, the VDO CHT is largely inaccurate, but I've at least come to understand that its usually reading around 10-15 degrees below what the actual temps are. Essentially, we could not keep the thing below 350-375 degrees at any point along the way, either during the day in Florida when the ambient was 85-90 or at night in GA when the ambient was in the low 50s. At around 65-70 MPH the car is turning 3,200-3,400 RPMs in 5th, but it seems like it just will not stay cool, at least at that throttle position. Otherwise, the car is tuned well and the carbs are synched. I have suspected a burned ex. valve on #3 for awhile, as this is the only mixture screw that doesn't seem to have much effect on idle. The paper towel over the exhaust trick also confirmed that it is "sucking."
My question is, could a burned valve, which is no longer seating fully, also be the reason why CHTs are higher for that cylinder? Since the contact between the valve and seat is responsible for soaking up a lot of heat (somewhere around 20-30%) in the head and dissipating it, wouldn't a burned valve negatively influence this?


Thanks for humoring my nerdy questions!
-George

How about you're not making any power on that cylinder (burned valve) and it's causing the other 3 to work harder to maintain the same load, hence the others run hotter?

I'm done!

While those head temps are high for the speed you were driving, they are still within a safe range.
Normally I don't expect to see head temps above 350 unless I'm pushing pretty hard.
Do a leakdown test on the engine.

Harbor Fright sells a point and click non contact thermometer for 15$. You maybe able to see if you have higher temps on one cylinder by taking exhaust pipe readings. HF
You have ruled out a lean condition. Are you getting full advance on the distributor?

Do you know what the actual temps are? Say no so we can move on. First that VDO guage is in 50F increments so it's not easy to read in the first place. Therefor, that little bit that is in question might be a lot. Verify or calibrate but no guess work here. OK, so you say "I've done that and it still runs hot" then I'd check timing or for a lean running condition. How are temps in town? Did the oil temp come up with the CHT temp? And there is that whole air cooled thing starting at the fan and........well you know. Know your Head Temps another bumper sticker.

Thanks for taking time out to reply guys.

SLITS/Ron, I would agree with that logic, but since the sender is on the #3 cylinder, if it was not firing at all, I'm not sure that you would see much, if any temp coming from that cylinder, right? Or would the heat from the other cylinder make its way back to the sender at #3?

Chris, I am aware that those temps aren't necessarily catastrophic, but obviously its not good for the engine. Being that is probably the original engine, I'm not too terribly concerned - its due for a rebuild at some point anyway. I will do a leakdown this weekend though. I've been meaning to do one for some time, but have been spending most of my time on that which I know much more about - body work. I'll stop being lazy and get it done

Bulitt, I'm not sure I've completely ruled out a lean condition, but I'm not sure how I would even get to the point where I could tell. Since I can bottom out the mixture screw for cylinder #3 without really making much of a difference in RPMs, I really can't even set a lean idle for that cylinder. So far I have basically set the others using the lean idle approach and got #3 as close as I could. That being said, if anything, I think the car runs rich most of the time - or at least it certainly smells like it. I have been thinking about getting a digital thermometer from HF, so again, maybe this weekend is the time to do it. I'll report back. As far as the advance, I set it to somewhere around 32-34 degrees with full vacuum advance, as suggested in the Pelican Parts article. I have played with the timing as well before, retarding it to see if it would change the temps, but all I succeeded in doing was making the car run more rough.

76-914, you are right, I don't know the actual temps. And I have not calibrated the gauge either, but I've used the same logic spoken about here on the forums previously. For ever 10 degrees over 72 degrees ambient, the gauge should be off by about 10 degrees, correct? Again, would a digital thermometer help here to verify the gauge readings? Temps in town are great. As long as I'm not driving a sustained 65-70 on the highway, it stays well below 350, usually around 300. Oil temps are great, almost never above 180 and I've never seen it hit 220.

The only other evidence I have is that when cruising, especially before the car has warmed up, I get a "lean pop" from the top of the #3, passenger side carb. Again, I'm assuming once the car warms up the valve starts to seat better, but its still not seating well enough to draw that heat out. I think this is just one of those things which will be easier to diagnose at the Tropical Ramble - someone else will just need to be there to see it.

So you get backfire out the carb sometimes but not out the exhaust?
Doesn't sound like a burned exhaust valve to me.

At idle, what happens if you pull the spark plug wires (from the distr. cap, using insulated pliers) one at a time?
They should all cause the rpm to decrease equally.
Since you can't get one idle mixture screw to have any effect I suspect you will get no rpm change on that cylinder.
There is probably a blockage in the idle circuit for that cylinder, and that condition has nothing to do with your temp under load.

Try moving the CHT sender to a different cylinder to see if the temps are lower under similar driving conditions.

This sounds exactly like a problem I just went through. I had 1 bad cylinder I could not adjust either, engine ran hot, popping through intake on bad cylinder, all spark plugs looked a little lean except the bad cylinder which was way rich. Turned out mine was a dropped valve seat that fortunately did not jamb the intake valve open.

I would not drive car until at least compression is verified.

You were on a long trip. 5th gear? I have probably had a CHT gauge longer than most and I have noticed a trend on the three different engines that I had it on. These engines like fan speed. In 5th gear the temperature will climb. Especially, 5th gear with some load like climbing a slight hill. Downshift and get the RPM up and your temperatures come down.

Remember, when they told you the engine was air-cooled, they forgot to mention that it is also oil-cooled. What were your oil temperatures doing?

Quit looking at the gauge.