Carb problem?

2.25 petrol (yes). Prior to a complete rebuild the engine had been starting and running very reliably, although with a lot of black smoke/rich. I'd fitted a Weber 34ICH overhaul kit last year while trying to find a fault that turned out to be the pump, which made no difference to how it ran. After the rebuild (rebore, crank polish, new bearings everywhere, valve guides replaced, seats cut etc) it started instantly and ran smoothly, although still rich. I checked the float level and found the level too low/measured gap too big by about 1mm, so corrected that and checked that the range was still OK. (But now wonder if someone had been in there before and set the level low in order to try and correct this problem). Followed the Weber instructions on stop and idle jet setting, no real difference. I'd given the bottom of the air filter a clean and oil change, so thought I'd check the top mesh. No problem found, but when I put it back on the engine was now running visibly clean and the smell of part-burned fuel gone. Off then for an MoT. It drove pretty well on the way over, but when the tester drove off to check the brakes it kept cutting out, and I thought maybe he was stalling it? When he came back he took it straight in to the test bay and didn't comment until after the test. He showed me that the HC had been over 5%, so had 'adjusted the carb' for me and got it down to 0.33%. Said 'it's the linkage' several times, and assumed he meant idle speed/cut out was due to some slack that he'd fixed. As for the HC levels, err, Ok, thought I'd got it sorted, but obviously the air cleaner wasn't the problem. Thing is that it was running terribly. Wouldn't idle, back firing on the overrun and would only idle with choke. So I'm thinking maybe the filter to carb hose had a blockage/collapse and so disconnected it at the carb. No difference. Then saw that the locking tab on the spindle was undone and the nut loose, what's he been doing because I hadn't touched it when I rebuilt the carb last year and it was locked then? Tightened that, no difference, but I don't yet know if it's been put together wrongly.

I'm now thinking maybe some dirt from the tank (which I'd cleaned out after the misfuel episode last summer) had dislodged when it was standing on end and being manhandled in and out during the rebuild. Somebody tipped me off about the neat little filters that solder onto the pick up pipe - bought one but haven't fitted it yet.

btw. Ig timing absolutely spot on 6 deg before tdc (high comp engine).

I reckon possibly two problems here. A. Running rich. B. Something happened as I drove it for the test and it's coincidence that the the tester drove it immediately afterwards.

Anyway, long post but trying to give as much info as possible.

I don't know anything about the Weber carbs, I am speaking purely from my knowledge of motorbikes but have you looked at the possibility that a coil might be on the way out? I have often come across that when a coil is on its way out that you can get a carb set up right but the coil will cause all sorts of problems that look like they are fueling problems.

That's a good thought. I'm going to give the carb a good staring at today, but that goes at the top of the other causes list (with HT leads, condenser and diz cap which I was too mean to replace...).

boguing said:

That's a good thought. I'm going to give the carb a good staring at today, but that goes at the top of the other causes list (with HT leads, condenser and diz cap which I was too mean to replace...).

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http://www.carburetion.com/Weber/adjust.htm

I'd replace the points and condenser with an electronic ignition module, they're only about £35 and reduce a lot of hassle and uncertainty.

Col

Dippypud said:

http://www.carburetion.com/Weber/adjust.htm

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Thanks Dippy, I have that and had followed the process up to the point where I may need to go away from the standard jet. I wanted to make sure that everything else was OK before going on to that.

+1 for checking the ignition system. I spent several weeks chasing a carb problem that turned out to be poor quality (brand new) points that were overheating.
However, it sounds like the MOT tester set the mix too lean to get the HC reading down. Dippypud's link has all the setting info, but a few suggestions:
- check for choke linkage binding - affects both mix and idle speed.
- worth looking at the little filter on the carb inlet, especially if you don't have a good inline filter fitted.
- are the jets right? I can't see them wearing much, but a previous owner may have fitted a wrong size, or damaged one while cleaning it with a bit of wire.
You should be able to get a reasonable idle + mix setting with the standard jets. If you need to go more than one or two sizes up or down to fine tune, it may indicate a problem elsewhere.

ExMil109 said:

+1 for checking the ignition system. I spent several weeks chasing a carb problem that turned out to be poor quality (brand new) points that were overheating.
However, it sounds like the MOT tester set the mix too lean to get the HC reading down. Dippypud's link has all the setting info, but a few suggestions:
- check for choke linkage binding - affects both mix and idle speed.
- worth looking at the little filter on the carb inlet, especially if you don't have a good inline filter fitted.
- are the jets right? I can't see them wearing much, but a previous owner may have fitted a wrong size, or damaged one while cleaning it with a bit of wire.
You should be able to get a reasonable idle + mix setting with the standard jets. If you need to go more than one or two sizes up or down to fine tune, it may indicate a problem elsewhere.

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Thanks for that, as I said, I've set the idle speed and bleed screw as per the manual, but there's something else going on, and your comment on the choke linkage could well be pointer that I needed to sort out this poor running problem. The original running rich problem could well be tied into it too. The main jet was new last year in the kit I fitted and although I can't remember the size off the top of my head, it was the standard fit. If I do need to open the carb up I'll certainly check the filter - it was almost solid when I did it last! And I will be fitting the filter I mentioned...

Your MOT tester has taken the idle mixture from one extreme to the other.
5% CO is too rich
It's better to set the CO between 2.5% and 3%
Your mixture is currently much too weak - which is why you need some choke to richen it up a bit.
A friendly MOT station will let you check your mixture using the exhaust gas tester for a small consideration; then you can tweak the idle mixture screw to get a good CO reading.
HC is usually measured in parts per million - so, I think you mean CO

boguing said:

Thanks Dippy, I have that and had followed the process up to the point where I may need to go away from the standard jet. I wanted to make sure that everything else was OK before going on to that.

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I had a similar prob on a mk3 escort after having a weber carb fitted,( company car),it turned out it was jetted wrong for said car,tis worth taking jets out and making sure they are correct,you might need to change idle jet/main jet.

I'd replace the points and condenser with an electronic ignition module, they're only about £35 and reduce a lot of hassle and uncertainty.

Col

I'd agree with those saying it's an ignition problem, often it feels like a fuel problem. Check the points gap and timing, or go electronic as already mentioned.

But also sort out the weak running!

Linkage problem. Double check the set up.

I haven't had much time to investigate, but the linkage 'looks' like it's working smoothly.

But I did drive it a few miles today and this is the odd bit.

It won't idle without full choke.

It needs choke to start however warm or cold it is.

You'd think this meant it's running weak? But.

It doesn't smell strongly rich (just a hint) when driving with full choke.

It drives quite nicely like this.

If I back the choke off just a little once moving it doesn't smell rich at all, and still drives OK. But when I lift off higher revs it backfires. If I back the choke right off it backfires more or less whenever I lift.

That's a sign of running rich? When the choke's off having been weak with the choke on...

A lot to check. Dirt in the filter. Blocked emulsifier from being allowed to dry out on a bench for six months, possible coil/plug leads, although they were fine right up until I got to the Mot station.

Have you checked the timing and points gap? A closed points gap can feel like fuel starvation and will also cause backfiring.

Hi Bern, when I rebuilt the engine I had the luxury of gapping the points on the bench (and was easily able to check at all four lobes for wear). The timing was exactly 6 degrees btdc. That's not to say either haven't slipped. Something else to check.

I had been wondering about the vac advance/retard, the new pipe's still where it should be and it tested OK a few weeks ago.

A faulty condenser can also cause all sorts of problems, even new ones are no longer reliable. For the sake of a few pennies it would be worthwhile changing it. I had similar problems when I first became a series owner and it was down to a badly adjusted air mixture screw which was also a little damaged on the tip. Mine is a Zenith carb though.

Col

As others have said, ignition problems can give symptoms exactly like carb issues. But if it won't idle at all without the choke, I'd check the throttle stop screw (i.e. the throttle isn't completely closed when the acc pedal is released) and the idle jet. If the idle jet is clogged, the engine will still run with the choke closed as the high vacuum will pull fuel through the main jet, but it won't be happy... Just a thought: you haven't got the weber with the idle solenoid? Failed idle solenoid will also give these symptoms.

boguing said:

Hi Bern, when I rebuilt the engine I had the luxury of gapping the points on the bench (and was easily able to check at all four lobes for wear). The timing was exactly 6 degrees btdc. That's not to say either haven't slipped. Something else to check.

I had been wondering about the vac advance/retard, the new pipe's still where it should be and it tested OK a few weeks ago.

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I would only gap the points with the distributor fitted to the engine, as there could be a bit of play in the distributor shaft which gets taken up when it's all fitted. Agreed it's much easier on the bench though!

You should be able to disconnect the vacuum pipe at the carb end and suck it, the points should move. If the points don't move and you can keep sucking then something is wrong! This would weaken the mixture.