Fuel system - Carburettor Air leaks

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Poor idling, poor running generally, underpowered running, backfires, misfires... worse when engine hot, choke off; worse in cold weather sometimes.


Air leaks... carburretor spindle bushes, carb/manifold flanges, inlet manifold/head flanges/gaskets, perforated manifold, vacuum hoses on carb or manifold. Air filter to carb leak, less severe symptons

Check for obvious problem areas visually; use WD40 or volatile spray around joints to detect change in idling speed or note (see case History)

Likes choke to run smooth above idle

Alternative diagnosis:

Heater plate under carb; diaphragm pull-down unit; idle stability mechanism; accelerator pump; All ignition problems, esp. coil.


Service carb fully (check and clean) jets/float chamber, butterfly and spindle bushes, float chamber.

Remove manifold and inspect, re-make manifold joints at head, with gasket. Carb/manifold joint, re-make if suspect.

Check all vacuum and breather hoses to manifold or carb. Fix or block-off as appropriate to function and condition

Check air-filter~carb connections.

Case history

asahartz: My bus (1.9DG with Solex carb) doesn't get used regularly - like a few others I guess. A couple of weeks ago I took it on a good run up to Manchester.

About halfway back it misfired a few times, with a backfire at one point, but after that it was no trouble.

Since then, it stalls at junctions as soon as it gets warmed up, and misfires quite a bit. I cleaned out the carb - scraped some crud out of the float chamber and blew it through with compressed air, and it seemed to run fine - but of course it wasn't fully warm. Today I took it for a little three-mile local trip and it was hell to drive - stalling at every junction, misfiring, no hill-climbing power.

I'm not sure where to look next - any suggestions?

maxstu: coil, leads, dizzy cap, rotor, plugs, fuel filter, air filter, fuel tank, filler tube, water in fuel, crud still in carb?

But not necessarily in that order Wink

My guess is the coil.

asahartz: I'm now pretty sure this is a carb problem now, though I can't work out how to solve it.

Changed coil - old one had a damaged insulator under the HT lead cover, started up fine, idled sweetly, no problems. Until I drove it. 200 yards down the road, as soon as the temperature gauge lifted off the bottom, it began to misfire.

Further investigation shows that if I close the choke flap and start it, it runs fine. As soon as I touch the throttle and the choke flap opens, it stalls.

As I have a Solex carb on a DG, it looks like it may be a good time to source the correct carb... Others have mentioned the manifold at the same time - is it completely different, or can I adapt the one I have?

CovKid: Weak mixture through an air leak somewhere? Check all hoses are connected and not perished
asahartz: Well I'm baffled. Had a good spray round, couldn't identify anything.

Went out to play hockey, came back to a cold engine. Started it up then sat & watched it. Perfect for about 5 minutes, then starts to misfire and won't idle without the choke flap being held onto to the second or third step of the cam. So it's clearly something that alters when it gets warm, but what? Help!

asahartz: By jove, I think I may have found it!

HarryMann wrote: If it really is heat and not that the choke has finally come off then re-do WD40 test with something more volatile, like carb cleaner.

I'd run out of carb cleaner, so bought a new can today. After stripping and cleaning the carb again, blowing it all out with compressed air again, reassembling it all very carefully, it started and ran very nicely - for a few minutes. So I started spraying the carb cleaner around the flanges again, and this time there was a distinct difference when I sprayed around the offside head/manifold joint!

So it looks like I need to lift the manifold after all. That will be fun. Is this just gaskets, or something more sinister, like a crack?

CovKid: Spraying with WD40 or Carb Cleaner around the carb, flanges and manifold is the way to find the air leaks.

These VWs HATE air leaks. It can't idle evenly if theres another source of air. Those who've owned bugs will know that worn throttle flaps did the same thing.

NB. I must thank Roger Ball (2 Totlands Drive, Great Clacton) for the many many carbs he rebushed for me using phosphor bronze bushes. He still does carb bushes if anyone is out that way.

Carb Renovation - Throttle Flap Wear and Idle Issues

CovKid: Roger Ball in Clacton specialises in refurbishing throttle flaps on almost any make of carb. Carb makers tend to fit nylon bushes to throttle flaps as standard. He dumps these and fits phosphor bronze bushes which have superior wear characteristics and lube nicely. Roger will refurb worn throttle flaps via mail order, or dropped to him if local.

You can reach him via: Rogerballnumber.gif

Providing ignition is in good form (and it may need investigating if the following doesn't help), in laymans terms for the engine to idle succesfully, it likes a very predictable and even 'suck' through the inlet manifold and carb body (as a unit) to ensure it recieves just the right air/fuel mix. Its finely balanced and relies on the free flow of air and no leaks along the way. At idle, its this vaccum (or suck) that pulls in a small but steady trickle of petrol through passages in the carb (carb is basically inactive at idle remember). Therefore if theres a leak below the carb (inlet port gaskets, bottom carb gasket, brake servo pipe etc), the suck through the carb is seriously reduced and the mixture goes weak - weak enough to kill the idle but possibly not when revving the engine as fuel is injected down the carb throat at that point and then the engine doesn't give a fig really. In fact this predictable vacuum is the whole basis of setting the idle speed/mixture correctly. Once set, a certain volume of air is assumed so if this is upset by a sudden change in that, for whatever reason, it can alter how much fuel is drawn in and destroy the idle speed and probably across the range at various points too with generally poor performance. A weak mixture also tends to make the engine run hotter and can cause damage.

Insufficient air (ie through a clogged air filter) won't supply enough oxygen and again the suck through the carb is reduced, mostly at idle although it'll probably pop and bang too on deceleration. A worn throttle flap also messes up the idle since air then gets sucked in through here (often randomly) and kills off the 'predictable' vaccum I mentioned earlier and often causes a 'hunting' idle or even sudden stalling as you approach junctions. A mixture screw set too lean can also do that but I don't advocate resetting mixture (or touching ANY carb screws) if its run ok up to now - find the cause first. If you don't you'll just be compensating and not very well at that.

The carb itself can become full of foreign bodies too which can clog the tiny passages/jets that supply the idle mixture. The above applies to all carb engines from Type1 onwards. It also applies to 2ltres with twin carbs which technically are one carb split into two and merely act as one carb when its running correctly.

Sudden power loss at high speeds (ie forcing it from 60mph to 40mph) is usually due to level in float chamber falling below demand, caused by blocked fuel lines, filters, a worn fuel pump pushrod or indeed a tired fuel pump. Carbs in a nutshell really....

In fact, even an apparently knackered carb can be made to run as good as new if the throttle flap is rebushed and all seals, diaphrams and gaskets replaced. Its often significantly cheaper than buying a new carb or chancing your luck on a second hand one.