Fuel system - Carburettor Air leaks

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Revision as of 22:33, 21 September 2008 by HarryMann (talk | contribs) (→‎Diagnosis)
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Symptons

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


Diagnosis

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.

Maintenance

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.