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Eric&Barb
Samba Member
Joined: September 19, 2004
Posts: 24757
Location: Olympia Wash Rinse & Repeat
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 Posted: Wed Oct 24, 2018 10:11 am Post subject: Re: Heat Riser Too Hot |
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| KTPhil wrote: |
Yes, and the generally poor quality of the float valves in carb rebuild kits makes such overfilling more likely. It was a problem 25 years ago and it still is. Keep your original valve as long as possible. |
Good point! Personally have never had to replace one of of those valves. Did have to replace the float that I am pretty sure was the one that came from the factory with the original 40 HP engine. Was causing same fuel drip out of the throttle shaft after hot engine shut down. Never had a problem with start up after that with the 1963 single cab.
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In Stereo, Where Available! |
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txasylum
Samba Member
Joined: December 15, 2004
Posts: 389
Location: Boyers, PA
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| Posted: Wed Oct 24, 2018 1:49 pm Post subject: Re: Heat Riser Too Hot |
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I messed around a bit more. The muffler I have is a German muffler. The heat riser tube winds down the right side of the muffler. So that side should have the gasket with the small hole on it, correct? I don't have a small hole gasket on that side, or on the muffler at all. Should I put that small gasket on that side? I don't see how that will make the left side riser any cooler.
| KTPhil wrote: |
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. |
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txasylum
Samba Member
Joined: December 15, 2004
Posts: 389
Location: Boyers, PA
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| Posted: Wed Oct 24, 2018 1:55 pm Post subject: Re: Heat Riser Too Hot |
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A thought - would running dual carbs be worth the money? It will eliminate the intake and heater risers, but is it worth it on a stock 1600?
| bluebus86 wrote: |
keep in mind different muffler configurations are around, left side or right side location for the small hole gasket are meaninglessness unless muffler configuration is known.
Also although the riser is hot, I doubt it is the sole contributor to vapour lock heat, the whole engine can be hot, the heatriser is a minor source of heat, it has a very small mass compared to the entire engine. It probably contributes little to the overall engine bay temperature after shut down.
You got a couple hundred poinds of hot motor, yet only a few pounds of heatriser, despite the higher temp of the r
riser, there is not a lot of heat in it do to its small mass.
A fuel supply restriction can be a source of problems, creating vapour lock, so make sure that is good, along with assuring the pump pressure and volume are correct, ensure the fuel cut off solenoid is working if equiped
Good Luck, Bug On |
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bluebus86
Banned
Joined: September 02, 2010
Posts: 11075
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| Posted: Wed Oct 24, 2018 8:02 pm Post subject: Re: Heat Riser Too Hot |
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| txasylum wrote: |
A thought - would running dual carbs be worth the money? It will eliminate the intake and heater risers, but is it worth it on a stock 1600?
| bluebus86 wrote: |
keep in mind different muffler configurations are around, left side or right side location for the small hole gasket are meaninglessness unless muffler configuration is known.
Also although the riser is hot, I doubt it is the sole contributor to vapour lock heat, the whole engine can be hot, the heatriser is a minor source of heat, it has a very small mass compared to the entire engine. It probably contributes little to the overall engine bay temperature after shut down.
You got a couple hundred poinds of hot motor, yet only a few pounds of heatriser, despite the higher temp of the r
riser, there is not a lot of heat in it do to its small mass.
A fuel supply restriction can be a source of problems, creating vapour lock, so make sure that is good, along with assuring the pump pressure and volume are correct, ensure the fuel cut off solenoid is working if equiped
Good Luck, Bug On |
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Millions of Bugs sold with heatrisers and no vapour lock problems. dual carbs are not easy, linkage must be perfect for syncronization, webers require holes cut in inner wheel wells to access adjustment, and spark plugs!
I suspect your vapour lock issue is not caused by the heat riser. It is supposed to get hot.
Bug On!
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Slow 1200
Samba Member
Joined: July 02, 2004
Posts: 2105
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| Posted: Thu Oct 25, 2018 12:27 am Post subject: Re: Heat Riser Too Hot |
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For the heat risers to work properly the tailpipes have to be inserted at the correct depth or the system doesn't have the expected pressure differential, at least that's how VW designed it.
in any case this is unrelated to your vapor lock problem, I'd look into making sure that all the tune-up basics are there (valves, points, timing, idle) and then look for things like leaking needle/seat, sinking float, float level too high |
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viiking
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Joined: May 10, 2013
Posts: 2668
Location: Sydney Australia
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| Posted: Thu Oct 25, 2018 1:49 am Post subject: Re: Heat Riser Too Hot |
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I've worked with large heat exchangers in chemical plants and the principle is the same regarding direction of flow and heat exchange.
The outlet pipe of the cooling or heating medium is usually restricted to ensure that the cooling or heating medium (steam, water, air) completely fills the heat exchanger. This maximises the contact area between the cooling/heating medium and the product being cooled or heated.
Taking this to the VW heater riser example, I would assume for maximum efficiency that there should be a restriction on the opposite side of the exhaust gas inlet to the heat riser (left or right depending on what side the inlet is). The exhaust gas is the heating medium, whereas the air/fuel in the intake manifold is the medium being heated. This restriction can be either done by differing pipe diameters or as someone has indicated, a smaller orifice sized gasket. I must admit I have never realised that the gaskets are different.
Over the entire heat riser pipe, there HAS to be a reasonable temperature difference between one side and the other. If there is NOT, then no heat has been exchanged and the heat riser is not working efficiently. The heat in this case could simply be taking the route of least resistance and just flowing through without giving up its heat. Or it could be blocked of course.
Take the example of a water filled radiator. When the radiator is working, the water returning to the motor is much cooler than the water entering the radiator after it has been cooled by the air rushing through the radiator. The air gets hotter, the water gets cooler. In a VW, the air/fuel gets hotter and the exhaust gas (going through the heat riser) gets cooler.
So in short, one side should be cooler. If a smaller gasket has to be installed (not sure if it does however) then it should be on the opposite side to the exhaust inlet.
This is of course very oversimplified and does not take into account heat transferred by conduction, convection along the muffler etc. But I'm not sure it is as significant as the heat exchange effect I've discussed.
Sorry for the physics lecture! |
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