hydrocarbons excessive fail

[xpress]

hi there. mot failed on excessive hydrocarbons. I found a vaccuum hose between the vaccuum advance trumpet to the carb undone at the carb end. plugged it back in and the idle went smooth once more and restored power. took it back to mot man but still got a reading of 1600 raising to 2500. I need 1200 to pass.

I can see some black scoring on the cap and rotor arm but it doesn't look excessive. I am going to change the rotor arm, cap, plugs and leads anyway since the van hadn't done many miles but they haven't all been changed in a good while and it brought the hc emissions down when I changed them when it happened a few years ago.

can changing the oil filter and the engine oil bring the hydrocarbon exhaust emission reading down?

would the unburnt fuel caused by incorrect timing advance and the air leak in the carb take a few more miles and a good long drive to blow it out and get a healthier fuel emissions reading?

[hammy44]

apparently changing the oil and filter can make a difference but check the vacumn hoses arnt defective only a few pounds to buy a mtr off it.

[itchyfeet]

if it still won't pass you could try disconnecting the oil breather pipe to carb accidently

[weimarbus]

also check the vacume pipe to the brake servo from the inlet manifold this sometimes becomes porous behind the braiding

[xpress]

would basically more air give a less rich reading? if the air filter wasn't in its box? I would have thought at idle the mix would have been less rich with the vaccuum hose to the carb from the vaccuum advance not connected. just connecting it up I thibk the van would need a longer run to clear out any unburnt hydrocarbon.

undoing the oil breather how would that change the idle reading? I guess making a mark at the top of a screwdriver and adjusting the mixture screw a bit leaner would pass it? is it anticlockwise to lean the mixture? any advice is appreciated. I don't much like wallet busting garages without much sympathy to older classics. it's not smoky and it's running perfect. it's frustrating.

there's no catalytic on this van it's pre cat. the mot man didn't seen interested in testing the van at higher idle speed and had to be asked and only did it for a few seconds. please advise and experience appreciated.

[kevtherev]

High Hydrocarbons is unburnt fuel and burnt oil.
Removing the breather stops oil mist being burnt
Mixture too rich/weak will give high readings
Ring wear will give high readings
Worn valve guides the same

You need to eliminate the easiest ones first

[itchyfeet]

have you checked the ignition timing?

[Aidan]

to test for inlet air leak spray some carb cleaner or similar stratgically onto the various inlet pipes, pay especial attention to the braided rubber hose that goes to the brake servo as this is often porous and allows air in one side on a dg , you adjust the carb but then one side is running lean one rich high hc, any cleaner that is sucked into the engine will increase the revs slightly, if inlet system is airtight then it will eveporate on the hot engine parts, have a fire extinguisher to hand and keep away from moving engine belts when checking with engine running

[xpress]

thanks all. about three years ago had high hysrocarbons and new plugs distributor cap and rotor fixed it. I have only done 3 thou max since then so should these and the oil filter even need changing?

I don't suspect any timing or excess air intake issues. she idles and pulls well.

[xpress]

van is doing 55 and could do 60 full out maybe 65 but I don't go over 55. will do a compression test when I pull the plugs. am I right in thinking the plugs aren't worn after 3k miles in a few years but might not be at their best if mixture was running too rich. everyone says it smells a bit rich but it's a pre cat van. they do smell rich don't they? especially on a cold start as the choke snaps back.

is it best to tweak the mixture after changing the plugs cap rotor arm? does anyone here tweak the mixture in the middle of a plugs cap rotor arm wear cycle?

reading up about the oil breather exhaust air intake setup.

btw anyone handy from the brighton area reading this? message me! thanks.

[paul66beetle]

Mine was air leak.

[xpress]

hi paul. I found the vac hose from the advance to the carb undone. I don't think by plugging it back in and driving 2nmiles for a retest would make enough difference to pass can someone confirm that it might have needed a longer run first?

[itchyfeet]

vac hose to dizzy would cause the timing to be out and give an air leak, the distance you drive should make no difference.

[kevtherev]

...and it would send the HC reading into orbit

Air leak.... As we said

[xpress]

all i can do is change the plugs cap and rotor leads and change oil and ride the mix. kev. would plugging the hose back on the carb instantly take the hc reading back down?