- Fuel percolation Mk 1 Rabbit with DFEV Weber 32/36 carburetor
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    1. 09-22-2013 10:30 PM #1
      Fuel percolation. This subject has been mentioned before, but the correct name for the problem doesn't show up in a forum search, so I thought it would be nice to start a new thread with it.

      This problem has given me a fit this summer, and I am going to be working on some ideas to solve it, such as

      Thicker base gasket
      Plastic washers under anchor stud nuts
      Heat shield
      After-run fan auto or manual
      Wrap on downpipe

      Using an infrared thermometer, I found that on a hot day of running, the temp of the carb base climbed from 138F to 170F after 5 min. of shut-down.

      The intake manifold has a coolant passage for keeping fuel atomized on a cold start, but I have wondered if it helps or hurts the carb temp after the engine is shut off.

      So I am wondering if the heat into the carb is coming from the hot coolant in the intake manifold, or from the heat of the exhaust manifold coming up through the intake manifold?

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    3. Member ps2375's Avatar
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      Aug 13th, 2003
      Meridian, ID
      80 Rabbit(DEAD), 94 Golf, '69 Type1 Sedan
      09-22-2013 11:33 PM #2
      Quote Originally Posted by chickenfriend View Post
      So I am wondering if the heat into the carb is coming from the hot coolant in the intake manifold, or from the heat of the exhaust manifold coming up through the intake manifold?
      I would venture to guess both are contributing to your problem. I would suggest ceramic coating instead of heat wrap, cause the heat wrap will cause the dp to corrode and fatigue to failure at a much faster rate that if it weren't wrapped. The manifold is a big heat sink, coated or not, it will be hot, the do coated will be touchable 15-20 minutes after motor shutoff. (with a good coating) And if you don't drive in cold weather, you can bypass the intake manifold, or just bypass it in the warm months.

      I would try it with the manifold bypassed and see what the temps do.
      Tradition is the art of making the same mistake repeatedly, on purpose.

    4. Member
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      Mar 23rd, 2012
      Globe, Az.
      Rabbit pickup 79/80?
      10-28-2013 09:15 AM #3
      I used the Pierce Manifold Kit with a DFEV 32/36 Weber to do my changeover. I don't run any coolant through the manifold and it seems to be doing OK. There seems to be no perculation problem as it usually will fire off without turning a full revolution irregardless of the time since last shutdown. I have drilled a series of holes in the plastic grille where it is supposed to be blanked off on the right side between the grille bars so it is not noticeable and I think I am getting quite a bit of increased airflow to decrease the bay temperature.


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    6. 11-15-2013 08:21 AM #4
      Thanks for the replies and ideas.

      I am thinking that I'll need to try several things to fix this.

      The underhood temp fan system of the mk2 should be something good to add, carb or not. Perhaps a manual over-ride would be good to wire into that system as well.

      The basic problem is that we want heat in the intake manifold for starting and running, but we don't want it when the car shuts off. The heat that is in that area is hard to get rid off because the exhaust manifold, head, and block are heat sinks. All that heat is trapped under the hood unless the rain tray has been deleted.
      Last edited by chickenfriend; 02-11-2018 at 08:04 PM.

    7. Member DEWEED's Avatar
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      May 2nd, 2003
      84 Rabbit
      02-13-2015 09:15 PM #5
      Why not use aluminum? It has almost double the heat capacity of stainless steel. You could even tryout a really ghetto mock up of a couple sheets of household foil folded over with some high heat glue to see if it helps. It wouldn't be able to go under the gasket I suppose, but you could figure out a way to get it between your intake and exhaust manifolds. Then fabricate a new shield if that design test pans out.

      I like the fan idea. You could build a heat sink onto the exhaust manifold and use a fan mounted next to the radiator to push air through plumbing and over the manifold's heat sink. You could probably pull air too, but I'd be worried about the life span of the fan. Or, the more complex idea of adapting an AC system to pull heat from the manifold. That would be pretty funny actually, adding AC to your car to cool the exhaust and not the interior.

      I've also been looking into using a stock aux. air regulator on a switch connected to a fitting on my exhaust manifold's unused Lamdba sensor port to help with cold starts. Or maybe a new solenoid valve. I like adapting stock parts to new uses though. Either way, a little recirculated air would improve cold starts and help keep emissions down

    8. 02-13-2015 09:47 PM #6

      There is a shield on the Opel site which is custom made for the DFEV which looks interesting. It acts like a carb base gasket. I noticed that it bends down immediately off the carb base mount so that it will clear nearly all the linkage around the throttle trunion. Good idea.

      The OPELs have the same problem of fuel percolation because the carb is directly over the exhaust manifold. Reading about their problems, it seems that even the heat shield is insufficient for preventing fuel boiling off. As an example, watch the youtube video

      Here is a picture of the OPEL shield being sold:

      From the picture, it looks like a width on three sides equal to the width of the carb base. However, there is a fair amount of extra area a heat shield could have around our carb, more than the OPEL.

      I consider a heat shield step 1. The shield would mount like a gasket on the four carb mount bolts, with thin gaskets above and below, then a spacer/insulator on top of that, then maybe another thin gasket depending on the type of spacer, then the carb.

      Step 2 would be a cooling system.

      Currently I am thinking the mk2 fan-after-run system makes the best first step. The temperature sensor is mounted on the cylinder head, which is good because that is close to the carb. The electric radiator fan would move a lot of air around in the bay and would *have* to be of some benefit, I figure.

      Like I said before, the basic problem here is that there is no place for heat to escape in the hood of the VW. The vents in the hood are for the fresh air intake in the rain tray; otherwise they would be great heat escapes if over the engine.
      Last edited by chickenfriend; 02-11-2018 at 08:08 PM.

    9. Member B4S's Avatar
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      Apr 16th, 2003
      Marionville, Ontario, Canada
      2002 Forester S, 1984 Volvo 244
      02-14-2015 11:13 AM #7
      I'd go with a heat shield between the intake and exhaust manifolds. It's tight, but it could be done. Also, there's an injector cooling fan from the Audi 5000s that could be used to cool down the intake manifold itself, or direct cool air downwards along the DP. Ducting that functions to direct air there while rolling could help too.
      1984 Volvo 244 DL...with carb.

    10. Member
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      Aug 5th, 2003
      87 GTI 1.8T
      02-15-2015 10:40 AM #8
      Quote Originally Posted by chickenfriend View Post
      Fuel percolation
      Never heard of that but it sounds like what many refer to as Vapor Lock
      Last edited by L33t A2; 02-15-2015 at 11:10 AM.
      '87 GTI 1.8t, '98 Dakota 4x4, '88 Shadow VT800C
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    11. 02-15-2015 12:22 PM #9
      Quote Originally Posted by L33t A2 View Post
      Never heard of that but it sounds like what many refer to as Vapor Lock
      The physics is the same, but the location of the problem makes them different.

      Vapor lock occurring in closed environments, like fuel lines and in pumps; percolation, in open environments, like a carb bowl.

      Both caused by the same thing, phase change of fuel, from liquid to a vapor.

      At the time I am editing this post, there are numerous videos on YouTube showing gas percolating in carburetors even at temperatures around 125 degrees F.

      I saw one video where a clear hose had been used to the carb as a test, to show gas boiling even before reaching the carburetor.
      Last edited by chickenfriend; 02-11-2018 at 08:13 PM.

    12. 07-17-2015 10:38 PM #10
      I have noticed my spark plugs are getting fouled by the percolating gasoline. Black and wet. I expect it soaks the back sides of the valves, too.

      To fight the problem with the plugs, I just switched from standard plugs to NGK platinums. The electrode is much smaller than the standard electrode. I figure this could help fire the engine because there is less area covered with gasoline on the plug bottom. #6 is the standard heat range, but I thought I would also try a hotter plug, #5, so I put two of each in and will compare all of them later.

      Also suggested out there on the web is simply cracking the hood after parking on a hot day. That lets a considerable amount of trapped heat leave the engine bay. I have tried it, but I can still hear the fuel gurgling. Perhaps with the action of an after-run radiator fan, this would work better. Another problem is forgetting to close the hood.

      You would have to add a hood scoop to vent the bay, and I would hate to mess the hood up.

      I think adding a flat shield under the carb is going to help, but my hunch is the problem will continue. The heat will still come up of the manifold, the head, the downpipe, and the block, and be trapped under the hood. Not to be too pessimistic, but even though the idea of a fan directly trained on the carburetor, a computer fan, for example, sound great, the heat still has no where to go and it seems like it would just be blowing hot air around, doing nothing.

      Makes more sense to me to insulate the carburetor itself as much as possible, meaning, a shield and insulator on the base, plus sides and insulating material affixed.

      If they made a snorkel adapter for this carb, you could use that and even insulate the top, but they don't, so the top with the filter has to stay open to the air.

      Another crazy idea I had, which might work for other carbs as well, would be to make a thick spacer base from aluminum and drill a coolant passage through it, or wrap it with external tubing, and run the coolant to a small dedicated radiator, like a fuel cooler located someplace like the raintray so the heat gets out. Put a small electric coolant pump in line and control it like the VW after-run fan, or let it operate simply as a heat siphon. That would continually remove heat from the carb base until the engine cools down.

      One small improvement I made was sliding the ignition coil in its bracket to be farther away from the carburetor. During running, the coil gets quite hot and retains heat after the engine is turned off. Don't need heat from it radiating to the carburetor.
      Last edited by chickenfriend; 02-11-2018 at 07:55 PM.

    13. 12-21-2015 09:15 AM #11
      I ran across an interesting paper online published by the California Air Resources Board Executive order D-145 regarding the Weber DGV and the Cannon manifold.

      Here are a couple of interesting quotes I'll put here for archival purposes:

      "To prevent fuel in the carburetor bowl from percolating during the soak period, an insulator gasket is installed between the carburetor and intake manifold inlet."

      "In place of the fixed orifice (PCV) a conventional PCV valve (CV-584-C) is used [Volvo application]"

      "Two modes of heat transfer, radiation from the exhaust manifold and conducting from the engine coolant, are used to enhance vaporization of the fuel and to prevent condensation from occurring within its internal passages".
      Last edited by chickenfriend; 02-11-2018 at 07:56 PM.

    14. 02-11-2018 07:29 PM #12
      After putting this off for a couple of years, I finally got around to making a heat shield. It is still winter here, so I'll have to wait until warmer times before seeing how well it works. I made it out of sheet of 0.025" aluminum I bought at the hardware store. It is cut to be 10-1/2" from front-to-back and 11" from side-to-side:

      The difficult part was figuring-out how to cut the oval hole in the center. I came up with the idea of using a hole saw to make two overlapping holes, then I trimmed out the rest with a hand sheer, and took some sandpaper to smooth out the edges.

      You will notice from the picture that I have put a slight bend on the edges, this is to add rigidity to the sheet.

      Because of the throttle cable linkage dropping down below the horizontal plane of the carburetor base, it is also necessary to carve-out an hole there for clearance. I did not do a very neat job of that, but if I made another one, it would be better. Looking at my picture, about half of what I carved-out for the linkage, the rear area where the return spring is, was unnecessary.

      The way this thing stacks up is thin gasket, heat shield, another thin gasket, insulator gasket, then carburetor. I used High-Tac gasket sealant on the two gaskets which sandwich the heat shield. I made these two gaskets by hand from stock gasket material I bought at the auto store. To finalize the gaskets, I installed them on the manifold and trimmed any overlap over the plenum opening--I didn't want any gaskets sticking out of the edge of the aluminum.

      With gasket scraps, I made 4 washers for the hold down nuts to help block any conductive heat coming up through the carburetor mounting studs.

      The insulator gasket/spacer was a Fel-Pro 60144. It is about 1/2" thick.

      Notice that it has a sharp corner which goes to the driver's front (closest edge to throttle cable). It also has writing indicating "top". Note it IS IMPORTANT to get this gasket oriented because it won't match the smaller primary and the larger secondary of the carburetor if you don't, and you will have an obstruction to the air flow between the plenum and the carburetor throats. This gasket shows as for Ford Pintos, 1973, and this one, which I think is newer production (costs more), has a varnish applied to it, giving it a black shiny appearance.

      If I still have percolation problems, I am going to work on using a manual fan over-ride or the mk2 fan deal which uses a temp sensor on the cylinder head.

      The first idea I tried was an aluminum foil turkey baking pan. I think it would have worked fine, at least for a while, but I decided against it due to appearance:

      I did like the way that shield had sides, especially on the cylinder head side, but really the edge of the other shield comes up almost to the height of the valve cover, so that radiant heat is going under the shield and not at the carburetor.
      Last edited by chickenfriend; 02-17-2018 at 11:12 PM.

    15. 02-15-2018 11:06 PM #13
      Back, before it was dangerous, they supplied an asbestos mat to be wired on your exhaust to help keep the heat down.

    16. 02-22-2018 09:18 PM #14
      That is interesting.

      The best thing about a heat shield under the carburetor, but over the intake manifold, is that the heat from the exhaust manifold is still able to heat the intake manifold. A heat is good for the intake manifold, but bad for the carburetor.

      I also run coolant through my intake manifold, which helps keep fuel from condensing in the manifold during the warm-up phase.

      I'll need some warmer weather to really see how the heat shield affects the fuel percolation problem, but there no question I have had an immediate, unexpected benefit, which is that I have had no run-on incidents (dieseling) at shut-off since I put the shield on.

      Now, I always thought my run-on was because the throttle plate was open too much, exposing the transition ports (and I have to very low idle because of that, which has helped), but now I think I was getting vapor lock and percolation, during engine operation.
      Last edited by chickenfriend; 07-16-2018 at 07:24 AM.

    17. 07-15-2018 12:14 AM #15
      send your exhaust manifold to Swain for thermal barrier coating.

    18. 07-16-2018 07:27 AM #16
      Thanks for the response.

      However, this summer, the heat shield seems to be doing the trick preventing run-on.

      I have not visually checked for fuel percolation, however. I need to do that.

    19. 09-20-2018 09:26 PM #17
      Small end-of-summer 2018 update on the carb:

      I did not have lock nuts on the carb studs or did the nuts have lock washers. I found myself on a trip with a coughing engine and soon discovered all the base nuts had become loose, resulting in a massive vacuum leak.

      I installed lock washers, but think I will use locking exhaust nuts down the road/ I have some of the copper plated ones. M8 x1.25 is the size of the studs.

      One interesting discovery on the carburetor was that I realized the stock Weber linkage spring, the one between the side plate and the throttle plate trunnion, was not applying sufficient tension to return the throttle plates to rest. I put the spring in the second hole in the trunnion and that worked.

      The problem is that the accelerator pedal has no return spring, so the spring on the carb has the responsibility for returning it. When the pedal is released all the way, there is often enough counter-pull from the weight of the accelerator pedal to keep the spring from returning the throttle plates to rest.

      Here were the symptoms of this problem. I would come to stop but the engine was at an idle higher than what I set it at. If I turned the key off at this point, the engine was likely to run-on.

      However, if I blipped the accelerator pedal, that would let the throttle plates come to rest and the idle would be at where I set it. The "blip" let the throttle close all the way before the back slack in the cable could act against it.

      In other words, the blip created a slack cable for a just enough time for the plates to close. It has taken me about 25 years to figure this out.
      Last edited by chickenfriend; 09-20-2018 at 09:47 PM.

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