17 replies to this topic

[Ianoffulford]

I am new to SU carbs, so I am a little at a loss with this one....

 

My van does not want to run past 50 mph, as i get to 45-50 its acceleration (if you can class it as that) curbs massively and the van feels like it is gasping for breath at this point.  I know that she will probable never see 70, but a nice 60 cruising speed would be nice.

 

If I top the oil up in the carb, she runs much better for 10 miles or so, but the the problem returns.  The oil at this point is back to the original level, and never drops any further (12-14mm from the top of the tube).  As it doesn't appear to continue to consume this oil, I do not understand what the issue is. 

 

Sorry again if this is easy, I am learning my way rapidly with this van of mine :)

 

Cheers

 

Dirk

[Dusky]

I am new to SU carbs, so I am a little at a loss with this one....

 

My van does not want to run past 50 mph, as i get to 45-50 its acceleration (if you can class it as that) curbs massively and the van feels like it is gasping for breath at this point.  I know that she will probable never see 70, but a nice 60 cruising speed would be nice.

 

If I top the oil up in the carb, she runs much better for 10 miles or so, but the the problem returns.  The oil at this point is back to the original level, and never drops any further (12-14mm from the top of the tube).  As it doesn't appear to continue to consume this oil, I do not understand what the issue is. 

 

Sorry again if this is easy, I am learning my way rapidly with this van of mine :)

 

Cheers

 

Dirk

Enrichen the mixture a bit, or fit a stronger spring.

 

The oil adds weight, lowering the carb piston, and thus leaning the mixture ( so am I told).

[KernowCooper]

The oil in the damper causes the slide to raise slower and thus the air is restricted and it draws harder on the main jet and so runs richer. Have you done a basic tune and checked the mixture by using the lifting pin, engine dies out and stalls = weak engine revs up and stays there = to rich engine revs and settles back down = correct

[carbon]

Dirk - does sound like your van may be running weak mixture.

 

If you can give a bit of background info (year, carb type, any mods to inlet, engine, exhaust, etc?) then may be able to make more informed suggestions.

[Ianoffulford]

Hi, I have just been looking into removing the carb and setting it up correctly.  Having removed the spark plugs I wonder if it is running rich as the plugs are all coated in a carbon layer.  My Mini is running a single HS4 with a stage one kit including the seperate inlet manifold and lcb exhaust system.

[carbon]

Dirk,

 

Would suggest you take the dashpot off the HS4 (just 3 screws) and do a couple of checks:

- does piston move freely in dashpot

- does needle have any sign of wear 'groove' near the shoulder

- take needle out and see what markings are stamped on the end (three letters)

 

If needle is worn or not correct then it is quite possible to be rich on tickover but lean at speed

[Ianoffulford]

Hi, so I have had a look at the dashpot/piston.  The dashpot was scored, so i changed the dashpot across with a good spare I have.  Sadly that has not changed the situation.  The needle shows ADC with no visable wear....  I will check what the needle in the the spare carb is shortly.

[Alex_B]

Don't worry too much about the damper oil dropping, it isnt designed to be filled to the top, it fills a tube in the piston but above that you are just pouring oil down the side of the dash pot top around the piston that then gets sucked into the engine and burnt. The top of the tube that holds the oil is about 12mm from the top of the dash pot so that would explain that. 

[carbon]

If you're running a Stage 1 kit the standard needle is likely to give you a weak mixture at med-high rpm

 

You can have this with sooty plugs if the idle mixture has been set rich to try and overcome this.

[Ianoffulford]

Hi carbon.  Just checking, is ADC a standard needle?  I have a second one which is AAC...  How would I find out what i need exactly?

 

Thanks :)

[carbon]

Dirk,

 

Here's a very useful site which allows you to compare SU needles : http://www.mintylamb.co.uk/suneedle/

 

The 998 used quite a range of different needles as standard, depending on year : ABX, ADE, AAG, ABJ, ADD. Your AAC and ADX needles look very similar to these standard 998 profile, but weaker than some of them.

 

As a first step I would recommend fitting a new red dashpot spring - I had a similar problem with weak running on 998 auto. Would hardly get over 40 mph let alone 50! Putting a new red spring in made a massive improvement - the one I got came from Minispares and was definitely a lot beefier than the old red spring that came out.

 

If the new red spring does not fix it then would suggest ADD needle as next step.

[Ianoffulford]

Hi again.  I had ordered an AAA needle and a red spring after reading another forum thread.  I have installed those and the engine runs really strongly until about 4000 rpm (guess as I do not have a rev counter).  At that point it just runs out of power - like it is struggling to breathe.

 

Someone has mentioned timing the car by ear...  does anyone have any experience of this?  is it likely to improve the running?

[Ianoffulford]

Well, an update on this.  After changing the needle and spring.  I found there was wear in the distributor - so i fitted a new electronic one.  I have changed everything on the ignition side as a process of elimination.  The pump seems good, so I was beginning to feel at a loss until today.

 

Today she dropped a cylinder whilst driving round locally.  I checked to find that no.3 cylinder was not firing (correctly)  checking the lead and plug showed that the plug was indeed sparking.  Upon driving the car back to the garage, I could hear and smell the engine firing back through the carb, occasional fuel vapour clouds said something was desperately wrong.  I think the problem may have been head related the whole time.  Tomorrow morning I will check compression and remove the head to inspect....

 

Once again thanks guys for your help so far :)

[carbon]

Keep us updated with what you find.

 

Before you lift the head, are all of the tappet clearances still at 12 thou?

[Dusky]

Did you set the ignition timing after the swap with a timing light?