High hydrocarbons

[max biaggi]

I've got a 1984 petrol automatic 1.9 T25.

It is running fine apart from the MOT test, which is very strict here in Northern Ireland.

This year the hydrocarbon reading has been getting very hard to pass. With a lot of creative carb tweaking, (including temporarily disconnecting the vacuum advance!) we got it through.

The carb has been stripped and cleaned, a compression test indicates that the exhaust valves are fine, and the timing and ignition components have also been checked/renewed.

Somebody has said that water in the fuel could cause this though. 3 questions.

1. Can water really do this?
2. How?
3. Can some sort of in line water separator be fitted?

Many thanks.

[Aidan]

air leak on the vacuum pipe to the servo can also cause this as no3 and 4 will run lean, wrap the braided hose from the manifold in gaffer and retest, if it's ok then you have your leak source, the wire braiding reinforcement in the pipe rusts and the whole pipe can become porous

[eatcustard]

Is it burning oil?

[max biaggi]

Is it burning oil?

No. Compression test is fine so worn rings are eliminated.

[max biaggi]

air leak on the vacuum pipe to the servo can also cause this as no3 and 4 will run lean, wrap the braided hose from the manifold in gaffer and retest, if it's ok then you have your leak source, the wire braiding reinforcement in the pipe rusts and the whole pipe can become porous

Why would an air leak cause high HC though? Surely running lean would lower the HC in the exhaust?

I always thought it was due to fuel air mixture leaking past the exhaust valve on the compression stroke or incomplete combustion (eg.too rich a mixture / poor ignition etc)?

What about this water in the fuel theory? Does that make sense?

[Fritz]

High hydrocarbon emissions from a vehicle may be caused by improper ignition timing, faulty ignition, misfiring problems, vacuum leaks, worn piston rings or valves .
One fouled spark plug can result in extremely high hydrocarbon emissions. Your truck is years old and probably has high mileage. Start by replacing the spark plugs and plug wires. Replace every single vacuum line on the engine . Replace them one at a time, so as to not get confused. Have someone check the timing on this vehicle.


Regards

Fritz,,,,,,,,,,,,

PS, stolen from the Wiki....

[max biaggi]

High hydrocarbon emissions from a vehicle may be caused by improper ignition timing, faulty ignition, misfiring problems, vacuum leaks, worn piston rings or valves .
One fouled spark plug can result in extremely high hydrocarbon emissions. Your truck is years old and probably has high mileage. Start by replacing the spark plugs and plug wires. Replace every single vacuum line on the engine . Replace them one at a time, so as to not get confused. Have someone check the timing on this vehicle.


Regards

Fritz,,,,,,,,,,,,

PS, stolen from the Wiki....

Plugs, plug leads, plug caps, coil, distributor cap, rotor arm all replaced. Ignition timing set. Compression tests have ruled out worn rings & faulty exhaust valve/seat. No audible misfire in engine.

A couple of people have mentioned vacuum leaks, particularly the brake servo one, but it seems counter intuitive to me, as to me , extra air = weaker mixture = leaner combustion = less HCs

I'm working my way through the vacuum lines right now, though.

[kevtherev]

is the choke fully retracting?
float height ok

HC is un burnt fuel as you know, so that includes oil, don't discount the ring/bore wear.
did you do a leak down test to discount ring/bore wear?

[max biaggi]

is the choke fully retracting?
float height ok

Yes

HC is un burnt fuel as you know, so that includes oil, don't discount the ring/bore wear.
did you do a leak down test to discount ring/bore wear?

Yes. All good hence rings and valve sealing eliminated.

[Aidan]

carb doesn't know one side has an air leak so despite all the adjustment in the world you end up one side rich one side lean, but the rich side gives the high hc down the exhaust which is picked up on the sniffer, there wasn't much difference in the plug colour tbh, in normal running ok, but at idle.....
the leaky brake servo vac pipe is I know a weird one that I have personally come across and if I hadn't seen it with my own eyes I would not have believed it, just by wrapping my had round the pipe the hc fell away to fine, but before that resolutely 1500+, on a run in rebuilt engine that pulled great had ace compression and new everything inc a fully serviced carb, since then it's been ace, eh Dazzer

[max biaggi]

Many thanks, Aidan.

[max biaggi]

As you put you have cleaned the carb out but is everything spot on in it ie the main jet needle ???

Looked OK, but new one on the way to me.