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Intake manifold
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boomtown
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PostPosted: Tue Apr 11, 2017 2:51 pm    Post subject: Intake manifold Reply with quote

Next question.
What makes the intake manifold cold to the touch after driving around for an hour?

Thanks ,David
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PostPosted: Tue Apr 11, 2017 3:01 pm    Post subject: Re: Intake manifold Reply with quote

Clogged heat risers..
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PostPosted: Tue Apr 11, 2017 3:05 pm    Post subject: Re: Intake manifold Reply with quote

Describe what part of the manifold are you talking about.
If it is directly below the carb then yes it will be cool.
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PostPosted: Tue Apr 11, 2017 4:50 pm    Post subject: Re: Intake manifold Reply with quote

wcfvw69 wrote:
Clogged heat risers..


No heat risers
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PostPosted: Tue Apr 11, 2017 4:53 pm    Post subject: Re: Intake manifold Reply with quote

gt1953 wrote:
Describe what part of the manifold are you talking about.
If it is directly below the carb then yes it will be cool.


Yes directly below the carb. on the drivers side. Dual carbs and the manifold below the passenger side carb is not cool. does that mean that carb is not working properly?
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PostPosted: Tue Apr 11, 2017 5:02 pm    Post subject: Re: Intake manifold Reply with quote

Probably. Could be that cooler carb is spraying way too much fuel, so that is cooling that side down too much.
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PostPosted: Wed Apr 12, 2017 5:20 am    Post subject: Re: Intake manifold Reply with quote

Eric&Barb wrote:
Probably. Could be that cooler carb is spraying way too much fuel, so that is cooling that side down too much.


It runs rough too. So I'm thinking the drivers side is too rich and the passenger side is too lean. Now how to control since there is only an idle adjust screw nothing to adjust main jet. Maybe the throttle is not opening equal or the driver side is overflowing from the bowl. ??????

David
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PostPosted: Wed Apr 12, 2017 6:29 am    Post subject: Re: Intake manifold Reply with quote

I'm betting the reason your driver's side manifold is cold is the same reason it's running rough: either your no. 3 or no. 4 cylinder (or both) isn't firing. Check your exhaust manifolds on that side and see if they are also cold. That will confirm if the cylinder is firing properly or not. I was going to ask you if it was running rough, because the only other reason I can think of why one manifold would be cooler than the other, is if one side had a metal gasket under it and the other side had a thick fiber gasket. But I shouldn't think that would make it run rough, at least not noticeably so.

Adjusting the main jets is not like adjusting the idle mixture (but it sure would be nice if it were!). You have to physically remove the jet and replace it with a different size. Some carburetors make this job a lot easier than others. I'd make sure the problem isn't ignition or vacuum related before tearing apart the carburetors, though.

Good luck, man. Let us know what you find.
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PostPosted: Wed Apr 12, 2017 7:26 am    Post subject: Re: Intake manifold Reply with quote

boomtown wrote:
Eric&Barb wrote:
Probably. Could be that cooler carb is spraying way too much fuel, so that is cooling that side down too much.


It runs rough too. So I'm thinking the drivers side is too rich and the passenger side is too lean. Now how to control since there is only an idle adjust screw nothing to adjust main jet. Maybe the throttle is not opening equal or the driver side is overflowing from the bowl. ??????

David


If it was running fine before, doubt that a simple carb idle adjustment has gone that bad. Float could have cracked or too heavy, shut off valve loose, stuck open or clogged with debris, or even a push rod has snapped so an intake valve no longer opens up, or main jet is loose.
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PostPosted: Wed Apr 12, 2017 1:52 pm    Post subject: Re: Intake manifold Reply with quote

What make/model are you dual carbs?

Normally, the mixture of air+fuel passing thru the intake will wick heat from the intake manifolds. This is the same way gasoline or alcohol on your skin feels cool as it evaporates, more so when there is a flow of air to draw the heat from your skin.

With the short intakes found in dual carbs this is typically not an issue. It becomes a BIG issue for center mounted carbs suspended over the engine with no heating. The heat riser tubes from the exhaust are specifically there to warm up the intake runner to prevent the fuel from condensing out of the flowing mixture.

The heat from the head should be enough to warm the short dual carb intakes. The stock intakes used metal intake gaskets at the head to transfer heat more effectively. With dual carbs even this is not needed.

I'm leaning towards agreeing w/ Floating VW... your #3 and #4 cylinders are probably not firing or if they are they are not creating a lot of heat (low compression?).
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PostPosted: Thu Apr 13, 2017 10:19 am    Post subject: Re: Intake manifold Reply with quote

ashman40 wrote:
What make/model are you dual carbs?

Normally, the mixture of air+fuel passing thru the intake will wick heat from the intake manifolds. This is the same way gasoline or alcohol on your skin feels cool as it evaporates, more so when there is a flow of air to draw the heat from your skin.

With the short intakes found in dual carbs this is typically not an issue. It becomes a BIG issue for center mounted carbs suspended over the engine with no heating. The heat riser tubes from the exhaust are specifically there to warm up the intake runner to prevent the fuel from condensing out of the flowing mixture.

The heat from the head should be enough to warm the short dual carb intakes. The stock intakes used metal intake gaskets at the head to transfer heat more effectively. With dual carbs even this is not needed.

I'm leaning towards agreeing w/ Floating VW... your #3 and #4 cylinders are probably not firing or if they are they are not creating a lot of heat (low compression?).



Well I do not want to add more confusion to this problem but --- using a HF infra red sensor gun the exhaust for #3 & #4 is about 100 degrees hotter than #1 & #2. Plugs for 3, 4 have carbon build up. Plugs for #1 & #2 appear cleaner.

Going to Saturdays show we had no power. Monday morning I checked all carb settings and turned idle mix screw to just 1/2 turn on both carbs and backed off accelerator pumps slightly.

Some other VW guys running same carbs suggested air leak but spraying starter fluid at manifolds and carbs does not produce a surge.

We are running dual kadrons/solex 40/44 I think. I'm sure of the make but not the size because I wrote the info down but don't have access to it at present. I used the Kaddie Shack YouTube videos to make my adjustments.

Sorry for such a complicated problem.

David
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PostPosted: Thu Apr 13, 2017 12:29 pm    Post subject: Re: Intake manifold Reply with quote

boomtown wrote:
wcfvw69 wrote:
Clogged heat risers..


No heat risers


That would be your answer.

Intake gets cold because the evaporation of the gasoline takes heat.

In cool humid weather this can actually cause ice to form in the manifold as the loss of heat freezes the moisture in the intake air. This is called carb icing and causes all sorts of driveability issues.

Which is why VW equipped the car with heat risers and intake air preheating.
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Last edited by sjbartnik on Thu Apr 13, 2017 12:31 pm; edited 1 time in total
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PostPosted: Thu Apr 13, 2017 12:30 pm    Post subject: Re: Intake manifold Reply with quote

boomtown wrote:

Well I do not want to add more confusion to this problem but --- using a HF infra red sensor gun the exhaust for #3 & #4 is about 100 degrees hotter than #1 & #2. Plugs for 3, 4 have carbon build up. Plugs for #1 & #2 appear cleaner.


If 3&4 were affected with a vacuum leak those plugs would be free of carbon build up from running too lean.

Ideally the gas/fuel mixture will burn cleanly and as completely as possible in the cylinder so little to no carbon on the plugs.

So somehow you are getting too much fuel from that carb or for some reason like a burnt valve, broken push rod, etc.. Might try switching carbs or parts of them and see if the problem switches sides, or do a compression test. The latter will be the quickest to do.
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PostPosted: Fri Apr 14, 2017 12:27 pm    Post subject: Re: Intake manifold Reply with quote

Try adjusting the valves. Maybe they're off whack on 3&4 and you're getting no/poor combustion.
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PostPosted: Sun Apr 16, 2017 5:33 am    Post subject: Re: Intake manifold Reply with quote

Thanks for the great Info and advise. We are on vacation now but on return first item on agenda will be compression check on all cylinders. I have a feeling it has something to do with valves so before compress check I'll check valves.

Thanks again and I will keep you informed if I interested.


Thanks to all
David
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PostPosted: Sun Apr 30, 2017 11:34 am    Post subject: Re: Intake manifold Reply with quote

I'm back.

More recent information to add to the previous . I removed the plugs for a compression check and found what appears to be fouled plugs in #3 & #4. This could be why the intake manifold on the drivers side is cold to the touch. Too much gas from that carb and it is evaporating inside the manifold. Remember the intake manifold on the passenger side was neither cold nor warm to the touch.

The compression test showed #1 &#2 to be 90 lbs. and #3 & #4 at 110 lbs.

Also when it was running and I pulled the plug wire from 3 or 4 it did not change the idle but either 1 or 2 and the engine stalled.


Image may have been reduced in size. Click image to view fullscreen.

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I'll need to check my repair manual but I think the compression should be within 5 lbs. of each cylinder

David
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PostPosted: Sun Apr 30, 2017 11:57 am    Post subject: Re: Intake manifold Reply with quote

90 seems a bit low. For that matter so does 110. At least on a fairly fresh engine.
Did you run the test with the throttle Wide Open? It will make a difference. Also, make sure to crank the engine at least 6x. Some will say until the gauge stops climbing. It's up to you.
Once you have your baseline readings. Squirt a little oil in the cylinder through the spark plug hole and run the test again. If you have bad rings the readings should climb. If they stay the same, you could have a valve letting compression by.

Good Luck

EDIT: some say to run the test on a warm engine. That's kinda hard with dual carbs. If the carbs are off then you already have it wide open.
10-15% difference between cylinders is ok.
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PostPosted: Sun Apr 30, 2017 12:35 pm    Post subject: Re: Intake manifold Reply with quote

I will try the oil down the spark plug hole thing. Never thought of that.😀

The engine is out as I respond so it will be easier now. If it were one stuck valve how would it affect two cylinders?

David
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PostPosted: Sat May 13, 2017 1:10 pm    Post subject: Re: Intake manifold Reply with quote

To all interested, the engine is running like a tiger. While engine was out I switched the carbs from side to side. I then had the same problem BUT now on the opposite side of the engine. It had to be the carburetor. For the fourth time I removed the carb for cleaning. This time after removing the main jet I snaked a thin wire down the main jet starting at the emulsifier opening and a tiny brass piece dropped into the bowl. What a find. Have no idea what it is or how it got there. Reassembled and everything runs top shelf.

Thanks for all the comments and suggestions!

David
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PostPosted: Sat May 13, 2017 4:32 pm    Post subject: Re: Intake manifold Reply with quote

boomtown wrote:
I switched the carbs from side to side. I then had the same problem BUT now on the opposite side of the engine. It had to be the carburetor.


Good troubleshooting.


boomtown wrote:
a tiny brass piece dropped into the bowl. What a find. Have no idea what it is or how it got there. Reassembled and everything runs top shelf.


Great.
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