Mot Fail On Emmissions Twin 45 Weber Setup

[swervygerdy 0]

Hi Folks

 

Apologies in advance for the long winded explanation.... I've been 'restoring' a 205gti for about 5 years now and last Fri was its first mot in quite a while, more a sensibility check. It failed on emissions - specifically hydro carbons at 6500 ppm (limit is 1200ppm). I was quite happy, thinking that the carbs could be 'fettled' to get me through...... not so easy it would seem.

 

My local mechanic (great guy and carb guru) explained that no amount of tuning would get that number down to a pass, (though he did try!) He reckons one of two things could be the issue.

 

1. The timing may be out.

 

OR

 

2. The head has some sort of performance cam fitted (see other post) and to get the emissions right I would need a vernier cam pulley to allow some adjustment.

 

I can check the timing myself using the 10mm dowel method explained in the Haynes manual, but the vernier pulley is completely new to me

 

Is their a particular pulley to go for?

Can it be swapped for the existing pulley in situ?

 

Have any of you guys experienced similar problems when running carbs?

 

I should add that the engine is running really well and revving cleanly.

 

 

[Biggles 65]

Not sure I agree with your man on the basis of what you've posted but obviously he's got all the info - I've got a pass from a 1400 on twin 40's from a similar number and I'm far from a carb guru.

Hope you get it sorted.

[Simes 248 2 Cars]

I used to get 3500 hydrocarbons on a standard mi16 inlet and cams on idle.

As soon as it revved at 1100rpm it was fine.

It used to pass.

[swervygerdy 0]

@ Biggles - what did you adjust to get them down?

 

the h/c did drop to about 2100ppm at best with the air filters removed, bonnet up and engine revved to about 1500rpm but we have dedicated MOT test centers in NI and it needs to be right when it goes in. The MOT tester will not adjust anything - he's simply there to test the car.

 

A £20 note cellotaped to the exhaust may be the solution!

[Biggles 65]

Set up by the book I would get about 5000ppm, lean out the idle mixture and it would be borderline at idle but HC can be passed at fast idle so it was fine. It was a case that when it was MOT time I'd give it half a turn in on the idle mixture screws before going to the test then half a turn out when I got home. Happy days.

[jimmi 0 1 Cars]

Swervygerdy: You sure your cam timing is correct ? - I had same prob with Hydrocarbon levels but I found the cause. If its a standard cam pulley it may have been re-dowelled... hence using the timing pin holes will leave your cam timing way out. Most likely too advanced. Make sure you are getting full lift on ( Inlet ) no.1 as per the camshaft spec sheet ie: at the correct ATDC figure