SU carbs and CO2

[scudderfish]

I have a friend with a 3.5 V8 TR7 on SU carbs that just had an MOT fail on CO2 levels. The car has been off the road for years, and has just been re-commissioned. I have no knowledge of SU carbs, so what would be a sensible plan of attack for pulling down the CO2 level?

[DaveEFI]

I think you likely mean CO?
The limit for this depends on the age of the vehicle. If between 1/8/75 and 1/8/86, it is 4.5% maximum, which rather obviously SU carbs can meet, given how many vehicles are fitted with them.

First check the ignition system is good. No misfiring, etc. And that the timing is to the factory spec for your engine version.

Make sure the air filter(s) is clean. If you don't know when it was last changed, fit new. Also make sure the crank case breather system (flame traps and hoses etc are clean and functioning correctly.

Now check the idle mixture. Run the engine until it is a normal operating temperature. Most SU carbs have a spring loaded piston lifting pin, situated on the main carb body under the dashpot. Lifting the piston slightly (a few MM) weakens the mixture slightly. If when you do this (one carb at a time) the engine speeds up and stays speeded up while the piston is lifted, the mixture is too rich. If it slows down, too weak. Ideally, it will stay the same speed, or speed up momentarily then go back to as before. High CO generally means a too rich idle mixture. Just for info, an engine will idle reliably with a too rich mixture to pass an MOT, but generally not if it is too weak.

If there is no lifting pin, remove the air intake trunking and lift the piston with a small screwdriver, etc.

I'm assuming the carbs are in reasonable condition - not flooding or whatever, and the engine idles reliably at the correct speed. But even worn SUs can generally be adjusted to meet the pre 86 standard. Post 86 (but before catalytic converters were required) the limit is 3.5%, and again SU's can meet this easily. IIRC, the factory spec for a pre 84 SD1 with carbs was 1.5-2.5%

[ChrisJC]

SU's have a habit of wearing where the throttle spindle passes through the carburettor body.

This allows air to get in which can make setting up the idle mixture very hard.

They need to be well balanced too.

Chris.

[stevieturbo]

MOT do not check CO2, although most 4/5 gas analysers will give that info.

MOT do check CO.

But....surely a TR7 is nearing the age of no longer needing an MOT ?

Exactly what were their full readings/fail ?

And is the exhaust divided per bank, or is it just a measurement of all 8 cylinders ?

[harvey]

But....surely a TR7 is nearing the age of no longer needing an MOT ?

As it's had the engine conversion to V8 it still needs an MOT regardless of age. IIRC in that case the MOT emissions are checked on whichever is older, the engine or the car.

[stevieturbo]

But....surely a TR7 is nearing the age of no longer needing an MOT ?

As it's had the engine conversion to V8 it still needs an MOT regardless of age. IIRC in that case the MOT emissions are checked on whichever is older, the engine or the car.

Pre 83 would be same regardless.

Again, need specifics on test results though.

I guess then as it is not a TR8 then...MOT is a bit more grey ? Although as the engine is one that was fitted to that chassis....it may not be considered extensively modified ?
It does state engine swaps are ok for the most part...although it does also mention number of cylinders. Although in the case of a TR7 if it is registered with the V8, you could argue it's just a TR8 so not really modified at all.

[scudderfish]

Yes, I did mean CO. Unfortunately I don't have a detailed ending to the story, but it did end up passing it's emissions test and now has a valid MOT. Hopefully some other poor soul will one day find this thread and appreciate the info within.

[GDCobra]

Although a car of this age could possibly claim MoT free status it is not an automated processs, it depends on the owner applying for it, I know of a few in the kitcar world who have done this successfully even though there is no way they could legitimately claim that they are not extensively modified!
Others, myself included, prefer to have a "second pair of eyes" to look over the car once in a while and many also consider that, although they may have successfully achieved MoT free status from a legal standpoint, if it were put to the test in the event of an accident this may give an insurance company sufficient wiggle room to refuse a claim.

Strange though that any TR7 should have emmisions complications, didn't think emmisions had been invented in their lifetime!

[stevieturbo]

Although a car of this age could possibly claim MoT free status it is not an automated processs, it depends on the owner applying for it, I know of a few in the kitcar world who have done this successfully even though there is no way they could legitimately claim that they are not extensively modified!
Others, myself included, prefer to have a "second pair of eyes" to look over the car once in a while and many also consider that, although they may have successfully achieved MoT free status from a legal standpoint, if it were put to the test in the event of an accident this may give an insurance company sufficient wiggle room to refuse a claim.

Strange though that any TR7 should have emmisions complications, didn't think emmisions had been invented in their lifetime!

Whilst most testers are generally clueless, as you say, from a legal standpoint having an actual MOT cert, despite how worthless it actually is....is a sensible thing to have.
That said, a lot of people working at their own cars etc...are equally as bad, so as you say in many cases another set of eyes cannot be a bad thing.

But no MOT is not and never has been a valid reason for insurance to refuse a claim despite some false claims online about it. A vehicle being roadworthy and having an MOT cert, are not the same thing. The MOT is a meaningless piece of paper ( or online ) at best only valid at the exact moment in time it is written. And even then it means little.