Heat Riser Too Hot

[txasylum]

I have a 66 Beetle with a 73 motor 1600 DP stock. I completely rebuilt this a year or two ago. Only have about 400 miles on it right now, but I have noticed that the left heat riser gets extremely hot, which I believe causes vapor lock when I let the car sit for about 15 minute.

I have double checked everything. The thermostat is set correctly and working, the vents work properly, all plugs look similar (none hotter than the other).

I have a thermometer and I checked some temps with the car running. THe head temps are similar. I checked the exhaust where it bolts to the heads. Both sides read around 330 degrees (so both are same temp). However, when I check the heat risers, the left side reads 276 degrees and the right side is 85 degrees.

What is causing the left heat riser to get 3x hotter than the right. I should also mention I have cleaned out the intake and risers when I had it off, so it is not plugged or clogged. It is clear.

Suggestions?

[Cusser]

[txasylum]

I put the lid kit on it so air flows into the motor, but shouldn't both heat risers be very similar in temperature? Why is the left 3x hotter?

[anthracitedub]

What kind of muffler do you have? I ask this because some mufflers don’t have the proper plumbing to help create the flow through the heat risers.

[diablosandwich]

The right side of my heat riser is hotter than left side on my rebuilt 1300. I am running an Ernst muffler that was NOS when I rebuilt engine. I thought one side was to be hotter than the other? On mine, the paint has burned off on the right side, but not left. I cannot touch either after it has run for a while. What kind of muffler are you using? Maybe this would help. https://www.thesamba.com/vw/forum/viewtopic.php?t=160919&highlight=ernst+heat+riser

Casey
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1966 Java Greens

[wcfvw69]

[hazetguy]

Which heat riser gaskets are you using? Some have a considerably smaller hole in them.
Without more info, my wild guess is that you have the copper colored gasket with the small hole on the right side, and the "normal" gasket on the left side.
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[KTPhil]

Since the purpose of the heat riser is to transfer heat to the intake manifold runners, there must be SOME side-to-side temperature differential if it is working correctly. How much is open to debate.

What muffler is it connected to? Do you have the right gaskets, with the right hole sizes? There is usually a difference in hole size to encourage flow through the heat riser. Some mufflers have the size restriction built in (or welded in), some rely on the use of gaskets with a restriction. Some flow left-to-right, some flow right-to-left.

And as posted, keep the fuel line away regardless.

[Slow 1200]

the heat riser is probably clogged, the other side should be hotter than that, and most likely unrelated to vapor lock

[Q-Dog]

Gee, it sounds perfectly normal to me.
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Brian

'69 Dune Buggy
'69 Beetle Convertible
'70 Beetle

[txasylum]

It is a stock muffler for the motor. I wondered if that could be the problem, but don't know.

[txasylum]

I have the lid offsets so air flows while driving. But after sitting a bit it still vapor locks. Could the muffler be the problem? I did not put a new one on when I rebuilt the motor. It is a stock muffler.

[txasylum]

Both sides have the same diameter. I didn't put the smaller holed gasket on the setup.

[txasylum]

Earlier in the summer I took the intake off and ran an old clutch cable thru it with a drill. It is clear and air moves freely through the risers.

[klroger]

Mine runs a lot hotter on one side. I think this is the way it's supposed to work as the manifold will absorb the heat from the exhaust gas as it passes under the cooler manifold. To me this is working the right way !!

Roger
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I Didn't know what to do, So I Didn't do anything!!!!

[bluebus86]

[KTPhil]

There was a thread and photos showing which side the small-hole gasket goes on. It is an important part of the design to ensure directional airflow through the heat riser. It will not be hard to loosen things up enough to slip the right gasket in there.

EDIT: These threads might clear it up:
https://www.thesamba.com/vw/forum/viewtopic.php?t=520603
https://www.thesamba.com/vw/forum/viewtopic.php?t=443868
https://bobhooversblog.blogspot.com/2006/11/vw-heat-risers-education.html

I'm not sure I found the pic/text to one part... some aftermarket exhaust systems omit the external pipe that is essential for the heat riser, and some go as far as to weld up one side with a small opening left, so two regular gaskets can be used-- in effect they built in the small-hole gasket into the muffler, ensuring it's done right. But that external pipe is critical and many leave it off.

I was interested in the Bob Hoover discussions liked above, regarding the "fweem" sound and how critical that heat riser flow is... good reading!

Some have the external tube on the right, some on the left. The consensus is to put the small hole gasket on the side with the tube.

So DON'T do this:

[bluebus86]

[Eric&Barb]

Could also be not vapor lock, but instead too much fuel being poured down the throat of the carb just after hot engine is shut down.

Had same hard to start problem in hot weather and found needed a second added gasket between the fuel shut off valve and the upper section of the carb. This pushed the valve slightly downward and same with the float in the carb, so the float is more submerged in the fuel causing a slight bit more pressure up against the valve.

Check that fuel is not flowing up out the float chamber vent tube in the 10 or so minutes after shut down.
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[KTPhil]