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Carb Guru (s) wanted


Big Brother

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I have a problem that after a (very lengthy and painful) process of elimination, looks to be carb related.

 

My engine pulls well at higher revs but at idle it runs like a dog!

It's missing on number 2 cylinder and removing the plug leads does nothing (no change to engine note)

 

So far the following has been eliminated:

 

Spark plugs (swapped and replaced)

Plug leads (swapped and replaced)

Coil (wasted spark set up so leads swapped over)

Valves, cams, followers, head gasket (complete head was swapped - long story)

Piston, rings and bore (piston removed and inspected - all OK)

 

This only leaves the carbs or maybe some form of demonic possession!

 

All the jets look fine, nothing appears to be blocked. The idle mixture adjustment does nothing as the cylinder is not firing.

I'm now about to delve into the unknown and reduce my carbs to lots of little bits.

 

What can cause a single (individual) choke of a DCOE weber to fail at idle but to be fine at higher revs??? (the other choke of the carb is fine)

 

Steve

 

 

SE7EN-UP!

-----If everything seems under control, you're just not going fast enough.

-----

 

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There is a myriad of possibilities, but for starters:-

 

A small air leak on the carb mount will affect idle but not full throttle - spray WD40 or similar around the 'O' rings and if the idle speed changes you have this problem.

 

Remove the idle mixture screw and check that it and it's orifice are clean.

 

It is possible for carb spindles to twist so that one butterfly is shut at idle (wrong) while the other is slightly open (correct). The barrel with the shut butterfly then doesn't function at idle. This is often caused by having a return spring fitted to the opposite end of the carb spindle to the throttle cable. In this case removing the return spring will often cure the problem. The spindles sometimes get twisted anyway even if you don't have such a return spring and providing it's not too serious, you can correct for it with the bypass screws (under the white plastic caps). You need an airflow meter to measure the flow of each cylinder at idle to determine all of this.

 

Before you do any of the above carry out a compression test (engine warm and throttle fully open). If the cylinder with the faulty idle mixture is down on the others (by more than around 20psi) you need to fix that first. If it is higher than the others you might have a worn cam lobe, although this will nearly always show up as an extra large valve clearance on the affected cylinder.

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What size chokes do you have. I have 40DCOE's with 36mm chokes on my Zetec, can't remember off the top of my head the rest of the details for tubes, etc.

Anyway, at idle and low revs the car is very lumpy, it was explained to me by a Weber Guru that 36mm chokes create no venturi effect at low revs. If I want a more tractable motor he suggested changing to 34mm chokes.

 

NE Area Rep

Though cowards flinch and traitors sneer,

We'll keep the red flag flying here

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Before you do any of the above carry out a compression test (engine warm and throttle fully open). If the cylinder with the faulty idle mixture is down on the others (by more than around 20psi) you need to fix that first. If it is higher than the others you might have a worn cam lobe, although this will nearly always show up as an extra large valve clearance on the affected cylinder.

 

The problem has remained despite changing the entire head (inc tappets and cams). This was done as I had rattling tappet problems with the old head. Changing the head cured the rattling tappets but the rough idle remained.

While the head was off, the piston from No 2 cylinder was removed for inspection. It's only been in the engine for around 6000 miles and everything looks completely normal.

 

The old head shows that not too much had been happening on cylinder number 2 (its valves are a lot darker than the other cylinders)

 

At the moment, the carbs are on the bench so I'm going to overhaul them while they are off the engine.

 

As a compression test is done with a wide open throttle, is it possible to do with the carbs/inlet manifold removed?

 

Steve

 

 

SE7EN-UP!

-----If everything seems under control, you're just not going fast enough.

-----

 

 

Edited by - Se7en-Up! on 19 May 2008 12:34:59

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Martyn,

 

I have a manual and a Weber carb book but thanks for the offer.

I'm hoping it's just a blockage and something that can easily be found.

 

Tyrone,

I've got 34mm chokes but the problem I have is that there is no go from No 2 cylinder.

All the others are fine.

 

Hopefully stripping the carbs will reveal something simple!

 

Steve

 

 

SE7EN-UP!

-----If everything seems under control, you're just not going fast enough.

-----

 

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I'm always really impressed by someone who really knows his stuff
I knew I was in the wrong league when I posted my response and Roger had been there first. *redface*

 

NE Area Rep

Though cowards flinch and traitors sneer,

We'll keep the red flag flying here

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  • 2 weeks later...

Well, all now back together. Sump back on, oil in, coolant in, carbs rebuilt.

 

Turn the key and............

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Luvvly smooth idle, no smoke just the purr of a happy 16v Zetec 😬

 

Downside is I have no idea what caused the problem but rebuilding the carbs seems to have fixed it.

Now covered a few miles and everything that's meant to be inside the engine - stayed inside the engine.

 

I also managed to strip both front hubs, fit longer studa and get my new shiny wheels on the front.

It does feel strange driving with 185/70 on the front and 185/60 on the back *tongue*

 

Steve

 

 

SE7EN-UP!


If everything seems under control, you're just not going fast enough.


 

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