Cylinder 4 Running Hot

[Gr0unded]

It was a nice afternoon, I decided to take the beetle for a little ride around the block (the long way). It was roughly 88F, partly cloudy and humming down the road around 65mph. The save my bug sensor starts flashing on the oil light of the speedo. I didn’t mind the flashing too much until it started glowing more solid to which I looked for a place to pull over.

I got out the infrared gun and started taking measurements all around while it was idling. The usual spot I measure is just under the oil pressure sensor, which happened to be around 230F. Shot at the oil sump below and it measured 210F, then shot at the oil pump area which was around 246F.

While I had the infrared gun out, took some more measurements around the heads. I typically shoot right at the exhaust side just before it meets the exhaust outlet, the following values were taken:
Cyl 1: 272F
Cyl 2: 256F
Cyl 3: 210F
Cyl 4: 406F (Yikes)

Curious what could be causing this spike in heat for cylinder 4? I’ve had the engine leak down/compression tested within the last month, each cylinder holds good compression, from memory the results were the following:
Cyl 1: 142
Cyl 2: 142
Cyl 3: 144
Cyl 4: 160

Could the cylinder with higher compression be the link to a hot head?

The engine is an aircooled.net special that ran great from the get go. But had to be rebuilt because of dumb me operating the engine too lean, burned cyl 3 and 4 in the early part of 2024. 1800cc (74x88), cheater cam, L3 heads. The engine has had at least three different carb arrangements since being rebuilt, first a weber progressive 32/36, then a 39mm PICT 3, and it now has freshly rebuilt dual 40mm Kadrons. Seems of late, no matter what carb combination this engine has, the oil gets hot when outside temps are 85F plus.
From what I’ve been able to gather, all of the tin is in place except the small square piece in the heads. It does not have a thermostat nor the air flaps, this engine never had them to begin with. The hoover bit is missing based on what the last mechanic said when he had it apart last.

At one point this engine has run cool after it was rebuilt, but seems like a fluke. It had an SS Rally 4 tip exhaust, 32/36 weber progressive, but the heat risers were not hooked up…however that combo ran great and cool. The only downfall of that combo was during winter, it was balky until the engine warmed up and ran silky smooth. Then I had to change the exhaust to a 1-3/8” single QP, replaced alternator, drilled out the heat risers for pre-heat to base of carb then ever since it will run hot when temps are greater than 85F. If I keep the traveling speed below 55mph, oil stays within tolerance at any temperature outside.

The biggest question is, what could be the issue with cyl 4 running so hot compared others?

As much as I lurk and read these forums, constantly reminded how much I still have a lot to learn.
_________________
B r i a n
1976 VW Type 1 Beetle - Standard

Stop Dead Photo Links! Post your VW photos to the Samba Gallery!

[raygreenwood]

Get or make a thermocouple and plut it under the sparkplug on each cylinder. Run long wires up front and alternately plug them into your volt meter to read very close and much more accurate temps.

The infrared guns are notocirously inaccurate and can be fooled by a lot of things.

But...#4 has a little higher compression and usually its probably a little closer to the exhaust. Ray

[Gr0unded]

[raygreenwood]

[Gr0unded]