Funny carburettor issues

[Gadge]

I took the 126 out for a blast yesterday and noticed something very peculiar When I returned home. The engine was hunting very badly on tick over. Intermittent shakes and sputters which at first I put down to an ignition fault so I re gapped the points and set the timing up so it was spot on the 10 BTDC and it still did it so I hooked up my gas analyser and is was saying it was rich. Perfect I thought, I'll just adjust the mixture down so I did and it got worse. I fiddled with the fast idle which didn't really help and in the end the only way I could stop the hunting was to turn the mixture screw right out so the gas analyser read a whooping 10.5%!! So somehow it's seems it's running lean yet gives off a reading of 5% and only cures when it's up at 10%

Does anyone have any ideas?  the car runs fine when driving and doesn't miss a beat (although it does seem a little down on power). I do have an electric pump fitted and at first I thought maybe it was over fuelling the carb but I have a switch in the car to turn it off and it still ran poorly even with the pump off

[ChrisRLewis]

Perhaps an air leak somewhere? The carburettor base plate perhaps may have a problem? Adjusting for a richer mixture just compensates for the air leak. Just a thought 

[ronb1967]

try spraying some wd40 around the carb base if it improves idleling then you know its drawing in air somewhere,or distributor vacuum advance /retard diaphragm split, remove vacuum pipe from carb end and try sucking on the pipe you shouldn't be able to suck/blow throu it   

[Gadge]

Great stuff guys it's looking more likely to be some sort of air leak. I'm going to have a good look today or tomorrow. A quick look last night didn't throw up anything obvious - I did grab the top of the carb and give it a shake and it was solid so I'm wondering if something has vibrated loose. The vacuum pipe is a good shout so I'll give that a good look too

[Bambino126]

Could it be the gas analyser was reading wrongly in the first place? I have the Gunson one and you have to leave it to settle down for a good while in fresh air before taking a reading.

With my carburettor I've often found tiny bits of contamination in the float chamber which can block the jets leading to a hunting effect. They're from the float chamber rusting on the inside from ethanol in the petrol I believe, otherwise the fuel filter would have caught them if they were from anywhere else. Anyway compressed air to blow out the jets might be a good idea then return the mixture screw to its original position??

[Gadge]

Ok had a quick look today after work and can't see anything obvious - vacuum hoses are all ok and a sucked through the ignition advance and that's all ok too. So tomorrow I'm going to remove the carb and fit the spare and see where we are with that.

[ChrisRLewis]

Could even be something as simple as a blocked jet, don't forget that the mixture adjustment is just for the idle range.

[Bambino126]

Could even be something as simple as a blocked jet, don't forget that the mixture adjustment is just for the idle range.

That's what I'd put my money on.

[Gadge]

Well, had a mammoth carburettor day today. Firstly I removed the carb and fitted my "spare". This spare carb was bought a good few years ago and I've never actually used it before. I opened it up and scraped out years and years worth of crud and scale from the float camber, blew out the jets and lubed up the linkages...... So with great enthusiasm I fitted it in which, for anyone who's ever changed a carb on a BIS "in situ" isn't easy. Primed the fuel pump and fired it up. And it sounded hirendous! It wouldn't idle at all and on medium revs it sputtered and coughed like an old motorbike  the kettle was boiled and I sat back and sulked for 15 minutes.

15 minutes after that I had the carb off and dumped it on the bench - it was obviously a duffer as there was nothing wrong with all the vacuum hoses and the base plate and manifold were all good. I guess years being covered in crud took its toll. So back to the original carb. I removed the top cover and gave it a bloody good blow out with the air line down all the jets and chambers. I also noticed that the float height was way out so I readjusted that and also repaired the linkage using parts scavenged off the duffer. At least it was good for something.

Once done I refitted it again, fitted a new fuel filter and it started - still a bit rough on tick over but once the gas anyliser was on and the mixture was reset it ran great and (relatively) smooth running. So I'm putting it down to blocked jets and silted up fuel lines. What a palaver and a waste of a day but I got there in the end :

[Bambino126]

Well done at least you got it sorted.
I get the same problem with mine from time to time.
I put it down to ethanol in the petrol rotting the insides of the carb. How else can dirt particles get inside there if all the petrol is being filtered?
When I rebuilt my carb the bottom of the float chamber was clearly damaged by it and rusting, and tiny rust particles were present in the fuel in there. This then blocks the jets.
I think cleaning out the carb is going to be an annual job for me unless we can find a source of ethanol free petrol.
I used to run super unleaded because they didn't put ethanol in it, but now even that contains 5%. I emailed  Shell, BP, and Esso and they all said the same thing.

[Gadge]

Yep, not sure exactly where the blockage was but it seems ok now. What I did notice when I removed a large screw from the side was a build up of what looked like clear jelly. I believe this is old petrol  that goes off. I Took it out for a 200 mile blast yesterday all the way over to Monmouth and back and the car didn't miss a beat