TheSamba.com :: View topic - CHT seems way off; Tuning / Troubleshooting help request

Mon Oct 27, 2025 2:36 pm

Newbie (to the VW world) here, looking for some help on a fresh engine build.
This is newly built engine (2110), all new components. Unfortunately sat for a couple of years (covered, indoors) as the Bus restoration it went in lasted much longer than expected.

Symptoms: Starts and run "OK", but idle is a little rough. As I'm working through the tuning, I'm seeing the right cyl head temp at 300 degrees and the left at 160 degrees (measured with an infrared gun right below the valve cover). This is pretty consistent over a couple of tuning sessions. Before the engine went in, I had the blower tin off, and am certain there are no obvious restrictions. This engine build does not have temp control flaps, so if both CHT were too cool I'd get it, but this seems odd.

Specs:
Engine: 2110 with about 50 light miles on it. High lift cam.
Ignition: 009 distributor, pertronix, 8 BTDC, verified all cylinders are firing.
Fuel: Dual Kadrons 40mm; idle at 950, both mixture screws set about 5/8 of a turn out (idles best there). Mechanical fuel pump.
Valve lash was set during the build, but I have not verified it since.

I have a DIY O2 sensor and gauge I can stuff up the tailpipe to get a rough indication of AFR. At idle it's pegged at 19 (as lean as it can go). Raising RPM past idle and AFR drops to 11-12, about as expected (maybe jetted a little rich)

The differential in cyl head temp has me concerned. I could see some varition due to mixture, but this seems extreme.

Any suggestions as to next steps in the diagnosis?

Thanks much in advance!

Tue Oct 28, 2025 12:48 am

What happens when you drive it?

Tue Oct 28, 2025 5:24 am

First off is that 50 light miles is not doing the engine any favors. You need to work it hard to get the rings to seat. Hopefully it is not too late!

I am assuming that you are taking these temperature and AFR readings in the driveway idling. My comments are based on that.

The next is that being new to VWs I am doubting that it is firing on all 4 cylinders. A hot cam with dual single barrel carbs is going to idle very badly at best so you may not be picking up that you have cylinders dropping out at idle. Kadrons in my limited experience are notorious for a 2 cylinder idle. The fact that you have no temperature control is going to exaggerate the problem because a cold engine does not like to idle.

160*F is an air compressor not an engine. Also a reading of 19 AFR indicates to me that the oxygen in the mixture is not being consumed. Remember that your AFR meter is measuring unused oxygen not fuel and normal air that we breathe is 20.8% oxygen.

To pick up which cylinders are not firing point your heat gun not at the heads but at the exhaust pipes close to the heads. My Kadron equipped engine would read 160 to 180*F on the cylinders that were not firing and 300 to 450*F on the cylinders that were. The front two cylinders were the ones that dropped out at idle.

If on the other hand you find that the cylinders that are not firing are both on the same head you have a carb linkage imbalance or an idle circuit that is plugged.

I might also add that initial timing of 8* may not be helping any. Depending on your compression ratio and grade of fuel 12* might be what you need but be careful that you do not have too much on the top end as 28 to 30* total may be all the engine will handle in a Bus.

I hope this helps.

Tue Oct 28, 2025 6:02 am

Is this at idle? Because Kadrons are notorious for poor side to side idling. Do you have the giant balance tube running between the intake manifolds on the banks?

Tue Oct 28, 2025 7:37 am

Thank you (especially oprn) for your replies.

The problem is most obvious at idle, and yes, I'm taking the temp measurements at idle in the driveway. Off idle (ie driving) it runs reasonably well. I have run the engine under load to seat the rings.

The cool temps are both on the left side (cyl 3 & 4), and it seems obvious now I'm idling on two cylinders. I presume I'm getting 4 off idle, but in any event it would appear I have a carb issue. (Yes, there is a balance hose (not tube)) between the intakes).

oprn, thanks for the suggestion on timing. I thought 8* would be reasonable (if not optimal), but I don't think that's the principle issue right now.

Tue Oct 28, 2025 7:56 am

You need to go around the balance/mixtures a few times, I expect you know that.

Tue Oct 28, 2025 8:37 am

My 4 channel CHT shows 3 and 4 rather cool at idle compared to 1 and 2. Up to 50-75 degrees cooler depending on it's mood. Once driving 3 and 4 are warmer than 1 and 2. You really need a driving tune with a CHT that maybe you swap between sides if you suspect one side is abnormally hot. Then solve from there. Your idle problems could be any number of things. Figure out why it's idling poorly and only worry about CHTs at speed.

Tue Oct 28, 2025 8:40 am

I found that with a detailed carb balance the balance tube made no difference in how it ran. When the carb balance was not perfect it did make a little difference.

Your problem is most likely in the linkage then if it's side to side. The stock Kadron linkage is not ideal but can be made to work reasonably well. It's weakness is that it changes as the engine temperature varies. Do your linkage adjustments with the engine up to full temperature if you can.

Another reason to get the thermostat back on and working.