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VW 2.0 Carburator Engine - Bad cold idle and now vacuum reading
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laxen
New User
Apr 15, 2016, 1:16 PM
Post #1 of 5
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VW 2.0 Carburator Engine - Bad cold idle and now vacuum reading
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1979
VW LT31
2.0L Petrol(CH)
--------------------
The problem:
When I start the engine when the engine is cold, idle is very bad, If I release the accelerator dies engine. When I try to drive with the engine cold, the car like jumps and have low power and sometime coughs.
When the engine is hot, it goes well, and keeps idling good, though somewhat high(1100 rpm).
At first I thought the problem was the choke. Previous owner had installed a manuel choke and adjusted it to close the entire gap. So I have set up with 3-4mm gap, and it didnt help me with the cold idle problem. So Choke does not seem to be the problem.
I have changed vaccum hose between the carburettor and the vacuum unit.
I sprayed start gas around the carburetor and it seems that there is no vacuum leak. But Im not totally sure.
When I plug in a vaccum gauge to the carburetor when the engine is hot, I get 11 inch.hg. However, when I screw on idle or air / mixture screw will not change anything on vaccum meter, unless I pull into the air / mixture in the bottom, then the value goes down to 5 and the engine almost dies. So the vacuum meter doesnt seem to change or get higher or a bit lower even if I screw the mixture screw very far out(Only thing that happens is that the rpm rises).
The needle is stable between 10 and 11 inch.hg.
If I step on the throttle and release it quick the vaccum goes down towards 0 and doesnt hit higher than 15 inch, and then settles down to about 10-11 again.
Is the cold idle problem and the low vacuum caused by the same problem?
Briefly, Dies at idle cold, idling good when hot. Only 10-11 vacuum.
Thanks
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Apr 15, 2016, 2:07 PM
Post #2 of 5
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Re: VW 2.0 Carburator Engine - Bad cold idle and now vacuum reading
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First off I don't know this carb or car for sure at all but there are universal carburetor basics of operation.
Choke has to go back to an automatic - way too many folks do that not realizing there's a fast idle step cam it can't climb up and unless you know exactly how to hold gas pedal while applying choke it will mess it all up.
Please tell if this is NOT a choke plate but some other method to deliver richer mixture for cold starts as there could be something I've never seen which ends it for me.
Idle mixture screw are not to be played with once right. Now the setting is lost so almost anything you bottom them out and count out about 2.5 turns per screw or equal. Remember where you left it for later fine adjust then never tough those again.
Your Hg reading from carb isn't valid. Take vacuum readings and they are telling from "manifold actual vacuum" with a "T" into a hose such that item hose operated still has vacuum. Real vacuum is the lowest port(s) on a carb or the intake manifold. If you chose one higher it's for another purpose like venturi air speed vacuum which will confuse anyone - get real readings they do tell what's going on.
You should see about 18Hg at idle if smooth when right. this 10Hg if that was real it wouldn't be running at all.
So - hope you have the parts to fix for the original choke or may get stuck for a whole new carb not rebuilt or no value to exchange what you have - bummer.
What a carb is doing and they can well to see diagrams and understand it is about an inch thick book nobody remembers now what area is doing what and what sets the assorted mixtures unique to some way of doing it a VW will differ from others no doubt.
You measure choke plate pull off either with known measured rods (drill bits work) in the right spots of air horn or angle of the place with a bubble level thing shown in specs for that exact carb.
It's supposed to open by itself from fully shut instantly when it first fires in a nano second. Not by messing with a choke knob or leaving it in one spot. You do set how far it instantly opens from fully shut.
What to do now if this was in front of me: Find the parts for the automatic choke. Most failed because they didn't shut off because they didn't heat up or spring that reacts to heat broke and think you can put on a manual which you could if you found the whole OE carb made that way or fix the automatic parts that are broken, clogged or failed in some way.
There's not quick easy explanation of how to adjust this and set it properly without info on the exact carb with specs and should show where you make adjustments for a choke and the whole list but begin with getting that back to the way it should be and go from there. I'd even rather see you find a used junk than mess with this mix breed that's messed up now if that's what it takes.
BTW - VW quit carbs on about everything by model year 1979 I though? Export or for assorted countries could do what they wanted. Site is US based so will lack specifics or specs easily for most of us,
T
(typo fixed- 'Can't' climb step cam was spelled wrong)
(This post was edited by Tom Greenleaf on Apr 15, 2016, 10:55 PM)
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laxen
New User
Apr 15, 2016, 3:04 PM
Post #3 of 5
(1514 views)
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Re: VW 2.0 Carburator Engine - Bad cold idle and now vacuum reading
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First of all, thanks for your answer.
The carburator is a Solex 35PDSIT ( http://www.ruddies-berlin.de/grafik/35pdsitVW.jpg ).
The carb don't have a fast idle step cam as far as I can see.
Original there was an electronic choke(part 59a on pic above). Do you think this is the problem? I adjusted the choke with a drill according to the workshop manual.
Okey, the vacuum reading was from the pipe on the middle part on the picture and is above the throttle body(that hose go to the distributor), so I think you right that it isnt manifold vacuum.
The only vacuum house I can find on the intake manifold is the one that goes to the break servo. Is it OK to have this unplugged when taking vacuum reading from the hole?
I don't know that :/.
(This post was edited by laxen on Apr 15, 2016, 3:05 PM)
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Discretesignals
Ultimate Carjunky
/ Moderator
Apr 15, 2016, 5:29 PM
Post #4 of 5
(1503 views)
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Re: VW 2.0 Carburator Engine - Bad cold idle and now vacuum reading
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At idle there shouldn't be any vacuum on that hose that goes to the distributor advance servo. Looks like the idle is set too high to compensate for something and exposing the ported vacuum port. Could be set to idle higher because the idle circuit isn't working in the carb, which might explain why you can turn the idle mixture screws in and nothing happens.
Maybe a compression check should be done first to confirm the cylinders are sealing. You can make every adjustment known to man on the carb and not have it run right if your engine is tired. Go from there.
Since we volunteer our time and knowledge, we ask for you to please follow up when a problem is resolved.
(This post was edited by Discretesignals on Apr 15, 2016, 5:32 PM)
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Apr 15, 2016, 11:57 PM
Post #5 of 5
(1488 views)
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Re: VW 2.0 Carburator Engine - Bad cold idle and now vacuum reading
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Laxen: A correction about "step cam" had that spelled incorrectly so it meant backwards of intention - sorry.
You seem to know what I was talking about. Carbs will have a way to hold idle UP for a time when cold and a step cam is a way or could use some eccentric it might slide up with pulling at knob. Not so sure about way too much with this specific carb so running on concepts it has to have to work.
Thanks DS (a nickname given for Discretesignals) for following this as you seem to know it - admit these were NOT my thing, brand or some styles not familiar to me off the top of my head.
Diagram helps Laxen. It should also have a page telling what each # is by name and probably needs to be in a few different languages?
Still stands that converting a choke to manual is almost always a mistake in an attempt to save on work or money that doesn't work out so well.
Again: Carb mixtures really get set once then not touched except for cleaning and put back. If off from when known good they can cover another problem nobody found is the problem with the idea that you can touch them at all but they can need cleaning.
Vacuum. Carbs are a top, middle and base I think I said. What port gets what for vacuum totally depends on how it's routed for actual vacuum or air speed vacuum as mentioned. Lots of terms like the "Venturi Effect" you need to know.
In the mean time as DS said take a check on engine compression! It can't have proper vacuum anywhere if compression is all low then the problem is not carb for now but that first.
They ALL ARE THERE TO MIX FUEL TO PROPER MIXTURE - That's it. Lot's of fussy parts just for that and it's that important.
Engine need all thing in good shape or adjusting anything is changing something else so basic compression on up thru tune-up items, timing of things, then would end with fuel issues and proper idle speed - LAST.
Notes on late carbs: It would be car makers live VW, Toyota, Datsun/Nissan and others made cars for world sale up to but not exceeding that county's regulations so they lasted lots longer in the world in cars then those sold new to the US and Canada. Some companies couldn't keep up and quit selling cars totally or entirely different cars. Fuel available is and was not the same world wide either by octane rating or additives no longer sold to US/Canada first then some others. Hard to know which design went where but it's known info somewhere you should find and have.
Again - Being a mostly US based site I/we are limited to exacting info quickly anyway.
Parts could be difficult if needed for this.
Last: Pay attention to fuel filtering anything especially fuel filter that may even have a water drain! Good luck, Tom
(This post was edited by Tom Greenleaf on Apr 15, 2016, 11:58 PM)
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