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Discussion Starter #1
Hi all,

One and a half years ago I bought a 2000 nord engine for my GT coupé, complete with the original carbs, and all the gubbins (starter, alternator etc), out of a 2L coupé one year from my car in production.
I rebuilt the whole engine, fully, somewhere between 6-12 months ago (it spent a while with my old neighbour in the UK before I was able to pick it up).

Ever since then, I haven't been able to get it to run properly and have only driven the car once, and it wasn't right.

Of course that could be down to me, though I am a by-the-book type and did the rebuild very patiently and paying attention to detail, especially regarding valves and timing etc. The V6 rebuild went well enough as did the one of my old 1300 nord engine. We (my friends that have been trying to help and myself) all suspect carburettor issues. I'll try to be concise and not drone on too much.

Weber 40DCOE76/77, original carbs for this engine. They'd clearly never been off the engine when I bought it. They have the stepped type mixture screws as opposed to the pointy ones, apparently this can make a difference in mixture setting.

Ignition and timing have been checked and re-checked, carb rubber mounts are more or less new. Plugs new and type recommended by Alfaholics for a stock engine. Spark and compression more or less the same on all four. Manifold tightened up nicely with loctite 518 as well as the new gasket. Air leaks not suspected. Both carbs stripped down, put in ultrasonic cleaner bath, rebuilt with new gaskets and O-rings.

I'm calling the cylinders 1,2,3,4 in order from the windscreen to the front of the car. Initially the car ran on 3 and 4 but not 1 and 2, after which we did the first tests but suspected the carb. It was checked and in sync before we tested anything else. We took the suspected carb off again, re-cleaned and rebuilt it, put it back on, to no change. Still on two cylinders. As in, pull the rear two plug leads, no change - pull either of the front two, it tries to stall.

I then took both carbs off, swapped the handed bits over, and put them back on the other way around. This confirmed the issue coming from the carbs as then, the car ran on 1 and 2 but not 3 and 4. Good to confirm the source of the problem.

So off it came again, for a third, more in-depth and longer cleaning session. Put back on the car, no change. (sigh) .

Then I realised something - don't ask me how it happened after two previous rebuilds - but the O-rings that are meant to be at the top of the idle jets weren't there. Ah! So, out of curiosity, I swap them with the idle jets on the working carb. It then worked back on the front two and not the back two.

Eureka!

So take the O-ring-less jets back out, fit O-rings on them, nice easy solution to a big problem!

Except no.

Now with O-rings on all four idle jets, the car is back to working on two cylinders... The back two again. Strange.

This evening I've been messing around swapping the jets around from one carb to the other in different combinations - there are 24 combinations in all, I've tried 8 so far. I've been doing the 'pull a plug lead and see what happens' test and the 'put your hand over the intake body and see what happens' test.

Previously with O-rings on the idle jets of only one carb, it was a case where putting your hand over either cylinder intake of the working carb tried to stall it, where putting your hand over either cylinder intake of the non-working carb would make the other one work harder and the engine rev more. The 'pull the plug' test was very obvious with two leads making it try to stall and two leads having no effect.

With O-rings on all four idle jets, as they should be (and the same O-rings not different diameters), between different combinations I'm getting all sorts of weird and wonderful, and contradicting results.
I've had it work on the front two, the back two, the middle two, all four (I think) very faintly but with a rough sound, sometimes the plug lead test confirming fairly clearly which cylinders were firing, sometimes - more often actually - it being pretty inconclusive with little noticeable effect on any of the four... Which is weird as obviously, the engine is running so it must be firing somewhere. I really can't make sense of it. All four idle jets are clean as a whistle by the way. 55F17 all four the same and as per spec.

Questions - as I'm still learning about this (I had lots of 'fun' with the L-Jet injection on my GTV6 after that one's rebuild ... And now find myself wishing I had the same issues here).
1) Those air flow meters with the rubber bit you stick in the carb intake; one of my friends has one and has brought it, whenever we've fiddled we've gotten the same airflow, eventually, on all four, despite all four not working properly. Are these only an indication of airflow, or of mixture too?
2) Mixture settings... I have the Brooklands 105 series workshop manual which has been very good so far. However on the Weber carbs section, under the 'reassembly' paragraph it says to screw the mixture screws home and undo them one turn then turn them in as necessary to achieve smooth idle. On the following 'carb adjustment' paragraph it contradicts itself by saying to turn them out two turns. Not knowing better, I've gone in the middle at 1.5 turns. Although I've seen elsewhere on here that the stepped type mixture screws for the 76/77 variants are not uncommonly out 5 whole turns - that's quite far out from the manual. Opinions? Right now it runs not smooth at all and lots of misfiring - possibly due to two cylinders trying to compensate for the other two. Is there a way to know my mixture setting is good, besides looking at the plug colour?
3) any advice is appreciated, I can't make head or tail of this any more, despite being persistent and trying to understand it all. I'd like to avoid going down the very costly route of a pair of new carbs if it's not necessary.
4) one thing I don't get - back in the 1300, I once melted a plug and had to come home on 3 cylinders. The difference in performance was huge! It really lacked power. The one time I did take the 2000 out for a quick run to see one of the friends who had been helping me whilst wrongly thinking I'd finally got it working on four - not far thankfully - he said straight away when I pulled up 'that's running on two!' which was quickly confirmed. However... A 2000 on 2 is 1000 and a 1300 on 3 is nigh-on 1000 yet the 2000 on 2 ran better than the car ever did with the 1300 in it (on 4!). Could it be down to condition; the 1300 having only had a head upwards rebuild unlike the 2000 which had a full stripdown and everything renewed? Or could it be that the issue with it only happens at idle/low revs?

Thanks for reading, and thanks in advance for any help and advice!
Dan
 

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Instead of using air-flow meters, just visually synchronise the throttle butterflies. They should all close together and all open together. Then tighten (gently) all mixtures screws in until they have bottomed and turn all screws exactly the same number of half turns (I find it easier to count half turns than full turns). Unless you have bad or a varying mix of mixture screws, and/or air leaks, this method should synchronise your carbs easier. Air-flow meters work better in my humble opinion on SU type carbs, and yes they just measure air flow. If you have a varying mix of mixture screws, throw them in the rubbish bin and replace.

Also have you disconnected the brake booster hose and plugged the manifold port to eliminate a leak in the booster causing the cylinder closest to the windscreen having a massive air leak.

Oh and 2000s have a thing called torque, so I assume that is why your 1300 felt less than half a 2000.
Pete
 

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40DCOE76/77 are emissions carbs. You have the correct idle jets. Idle mixture screws are usually about 4 turns out from fully closed but it can vary between screws if someone has ever screwed them in too hard and then they could be impossible to adjust. The only way to tell is to remove the carbs and examine the idle mixture holes in the bores and see if any of them have been wallowed out.
Also, these carbs have idle air bypass screws which you should fully close.
When you put the carbs back on the engine and you have the linkage connected, remove the inspection covers from above the progression holes and using a flashlight verify that the edge of the throttle plate is in EXACTLY the same place on all four barrels. If 1 & 2 are different than 3 & 4 then adjust the synchronizing screw until they are the same. If you see a difference between 1 & 2 or between 3 & 4 then you need to have a Weber expert fix them.
Finally, convention is that the cylinder at the front is #1.
 

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Finally, convention is that the cylinder at the front is #1.
Yes, I have enough trouble trying to figure things out without having to reverse my thinking! I guess if you timed your cams thinking that cylinder #1 is in the back it might not run right. :surprise:
Have you checked your plug firing with an inductive timing light just to see if any are missing? I've seen plugs carbon up pretty quickly on a too rich engine just idling in the driveway. You've probably heard the old saying, "90% of all carburetor problems are with the ignition system." It does sound like you've checked that in paragraph 5 of your treatise but it's worth mentioning. Good luck!
 

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Discussion Starter #5
Thanks for the help, guys, in response -

Yes, I know my plug order is back to front... But when doing my tests, with the carbs in front of me, it's easier to count 1 to 4 left to right. It doesn't necessarily matter which cylinders are firing, just that all four do...

I checked the plugs again, generally dark in colour but most likely due to the numrous backfires and unburnt fuel more than mixture.

I removed the butterfly inspection screws, as far as my eye is able to discern they look to be spot-on synchronised. If there's any difference, then I can't distinguish it so it must be very small.

I thought the air bypass screws (with locknuts, correct?) were just to hold the choke tunes in place; when I removed them I simply un-nipped the lucknut as little as possible so as to put it back in the same position and removed the screw; in theory they're screwed in back as far as they were previously - however if I'm meant to release the locknut further and see how far I can screw the screws in, I'll do that. Is the end obvious or do you have to be very careful like mixture screws?

Mixture screws don't appear to have damaged their seats, they're all the same, and I've gone from 1.5 turns out to 4 turns out, although it doesn't seem to have changed anything, still lots of backfiring and no even running.

If there are suggestions to further ignition tests, I'm all ears. All four plugs are new and spark well, timing is as good as we can get it, coil is the same one that worked fine on the 1300 - and we did swap with the one that came with the 2000 but no difference. We've tried both rotors and caps, no difference. Not sure what else to try really. But the fact that it works on different cylinders when mixing and matching carbs and carb bits doesn't really point to an ignition fault in my eyes.

In theory the brake booster doesn't have a vacuum leak - I suspected that was what caused the rearmost plug to melt in the 1300 so I put a new hose on it. Any suggestions for easily plugging it just to be sure?

In the meantime I've ordered a Gunson's Colortune and a Morgan Carbtune. I'm sure they'll come in handy for this and other things in the future. The colortune especially will be good not just to get the mixture right, but to visually see exactly which cylinders are firing, and how well.

Dan
 

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Discussion Starter #7
If you put four items on a table in front of you, do you count them left to right or right to left?
I know cylinder 1 is at the front, but for my tests counting left to right was less confusing than right to left and the order was irrelevant, only how many and which cylinders appeared to be working.

The problem isn’t my counting order, it’s the engine not running on all four and trying to diagnose and fix it. A non-constructive reply doesn’t help progress there.
 

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If one carb is the problem and it appears to be running rich, make sure that the cold start valves are fully closed.

You correctly identified the idle air bypass screws. Loosen the lock nuts and makes sure that the screws are all the way in then tighten the nuts.
 

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The problem isn’t my counting order, it’s the engine not running on all four and trying to diagnose and fix it. A non-constructive reply doesn’t help progress there.
Dan, with all due respect, these kind people are trying to help you and you have a very caviler attitude. I suspect your timing is 180 degrees out; good luck.

Perhaps this will help but I like to shine a bright light in the throat of the carbs to check the progression holes relative to the butterflies:
Daves home page
 

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I should have added that if the problem follows one carb then it could be flooding. Your fuel pressure should be between 2.5 and 3.5 psi. You can check the float levels by removing the emulsion tube stacks and measuring the distance from the top of the well to the surface of the fuel. There are posts about this by the usual suspects that you can find with a search.
 

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Discussion Starter #11
Thanks for the advice again - I will double check the chokes are 100% closed in the off position - and the same as each other on both carbs, tighten up the air bypass screws as much as they will, check the float levels (one thing I hadn't done previously). I think my workshop manual has all the information on what's correct there.

I can guarantee the timing isn't 180° out. I made the huge mistake of doing that on my GTV6 and it caught fire (briefly - I also had the foresight to keep an extinguisher nearby when trying to start it after the rebuild). So rest assured that when doing this engine, it was checked multiple times in a very OCD manner because I didn't want a repeat of that!

Interesting thought - so far I'd been thinking that one carb 'wasn't working' as in not chucking in the fuel (if you sprayed some down the bores it sure picked up the revs on all four) however it hadn't occurred to me that perhaps that one was running closer to how it should be and the 'working' one was flooding - it's all relative, and another route to go down to try and find the cause of the problem. I'm away 9-21st but when I get back my colourtune and carbtune will have arrived so I'll be able to tell the richness more accurately than looking at plug colour then.
 

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Discussion Starter #12 (Edited)
Chokes both fully closed in the 'off' position.

Air bypass screws nice and tight.

Float levels... Ha! Not quite the same. Measurements taken from all four corners of the float bowl and averaged out, one visibly higher than the other.
So where when rebuilding them the first time my friend said 'don't touch the floats!' turns out one was off-spec. I've done as the manual said and carefully bent one the tabs so that the two specified distances were correct. Both now have - as best as my eyes can compare to the calipers - the same, and the correct distances from the lid when just touching the valve and when at the stop. The float bowl heights are now much more similar - I'll accept slight inaccuracies in measurements here as it's hard to be spot-on.

Tried it again. It sounded better, but still not right. I'm aware that I've got my mixture screws at 4 turns out as a starting point only; I'll be patient and wait for my new gadgets to turn up to adjust them properly.

There's still lots of backfiring unfortunately - any ideas on the main causes of that?

Now however going by the spark plug lead test, we have the car running on 3 cylinders (halfway to sorting out the problem haha!). It's the rearmost one that's not making any change when you pull the lead and i'm aware that this is the one that can be affected by vacuum leaks from the brake booster.

After my previous issue with this plug melting in the old engine, I did fit a new vacuum hose as well as an in-line one-way vacuum valve which I thought wouldn't do any harm - and could only improve braking (a common mod to the GTV6 when the original valves that screw into the engine have the rubber balls go a bit hard - I didn't see any reason it would be any different here). So *in theory* no vacuum leaks, but I'm not sure how to confirm that.

I did think to swap the plug with another one, but no change. At the moment they're looking a bit black, but possibly still due to lots of startups with choke and the time various cylinders have spent not burning fuel. I'll clean them up properly before playing with the mixture screws.

Whether it's due to mixture etc or not there is a bit of lag and revs dropping when you give it some throttle (as in, if you're not gentle it tries to stall). My friend said that was the accelerator pumps not working as good as they should do. Also the revs don't go down very quickly - and the linkages are set to be fully off with the pedal up. Thoughts? My thoughts at this point are get the mixture and synchronisation right first, and then try it again. The revs not lowering quick could be too much fuel? Or (possibly) an effect of the air box not being fitted to restrict airflow whilst I'm doing all these tests?

I realise for best mixture setting etc the airbox should be on whilst fiddling, though I'm also a bit apprehensive of putting the air filter on with all these backfires happening.
 

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The airbox should make no difference to the low rpm carb settings.
Excessive ignition advance can cause the revs to hang up,
You can clamp the vacuum hose closed to see if it is contributing to the problem.
 

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Discussion Starter #14
Also the accelerator pump having come off of its spring on one carb could cause the lag in picking up revs :)

Not sure how that happened - I removed the pump when rebuilding the cab the other night, but didn't dismantle it. Somehow between being taken out and put back in, the hook had come off of the spring without me noticing. I expect a slightly better throttle response on the next startup! Though after having pumped them a few times to make sure I could hear positive squirty noises from both carbs, I'll let the fuel dry up a bit before trying it again, for fear of a huge backfire.

Looking again just now, despite the floats being adjusted to the correct; and moreover the same; distances from the carb lids, I've still got one side filling up slightly more than the other; about 2mm by the looks of it. Not sure how that's possible.

When I start it again I'll try clamping the vacuum hose too.

Lots of little things, but I feel like slowly but surely things are headed in the right direction. I am grateful for the help I'm getting on here by the way.

In the daylight once I've set up mixture synchronisation etc properly I'll again check the timing to make sure it's at its optimal position.
 

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Leaky/worn float needle or seat could contribute to more fuel getting into one carb.
A leaking float that lets fuel inside could too - no evidence of fuel sloshing around inside a float when you checked float height/droop?
I am currently chasing a sooty plugs problem, and noticed revs hanging after screwing mixture screws in too far. Your hanging problem might go away now you’re 4 turns out?
 

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Discussion Starter #17
The floats aren't leaking, thankfully. The needles were changed with new ones from the rebuild kit so ought to be good. The hanging problem has been vastly improved by hooking the accelerator pump back onto it's spring base! Not sure how that came off.
Still a bit of rev lag but that's due to it running on 3 now.

Thanks for the idle screw tip! See what happens when you let a friend tinker with your car eh :) I'd never messed around with carbs before and was a bit apprehensive about it and this chap has done up many engines over the years. Perhaps he's getting a bit forgetful now, or we've done other alterations since and should have reset it. It was 2 turns out, now 1/2 turn out.

The backfiring has nearly all gone (yay!) except for the one cylinder that's not firing. It was nearly constant before, and now the car idles (with choke on, but I haven't yet set mixtures) and only pops and bangs when revving it.

Mystery:
Two starts ago, it fired on all but the rear-most cylinder.
One start ago, it fired on all but the front-most cylinder. I'm not imagining things, my dad was out there with me and pointed it out too.
Today it started on all but the rear again.
Clamping the vacuum hose made no difference.

... Hmm, weird. He suggested maybe my coil wasn't powerful enough, for what it's worth I'll put the one which came with this 2000 engine on it and try again, it is bigger if that makes a difference. I also have an electronic ignition kit I'm considering putting on too see if it makes a difference.
 

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Discussion Starter #18
Well... After more fiddling I feel ‘nearly there’! It runs, though it feels like some fine tuning is needed. No more backfires. Good idle.
It made it to the local beer fest and back today without incident 😄
 

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The fact that you need choke makes me think it's lean.

Unless it snows where you live, you don't need choke with these carbs, just pump the accelerator pedal a couple of times and she should fire up and be easily kept running by a tiny bit of accelerator pedal work. If you don't like being a wizard with the accelerator pedal, pull the fast idle lever out and walk around the car checking tyres, etc. for a minute and she will be ready to go.
Pete
 

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How does the car run with the throttle open at revs, Ie running on the main jets. do all four run then. try closing the idle screw completely at idle on the cylinder that isn't running. if no change then its probably not getting fuel.
 
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