Another carb question

[kevinandkate]

Fifi has the 281f carb with a mixture screw near the base. When she first arrived I had a bit of a pop pop pop on overrun so adjusted the carb as per Hayned, IE screwed it completely home then out 3 turns and voila no more pop pops. Strangely this has started again and I have had to screw the jet out another one and a half turns to stop it. I checked the timing and points gap and all was spot on. My question is, does screwing the jet out richen or weaken the mixture? The controle technique is due next month and they tend to be keen on emissions, my intension is to weaken it on the day as much as I can (everybody does that here if they can) and still let it run but do not know which way to turn it to weaken it.

 

[kevinandkate]

Wow, no replies, surely someone knows the answer even if Mr Haynes doesn't see fit to tell.

 

[pepper]

I was sort of going to answer it yesterday evening, but thought there were more eminent people around to answer so kept quiet!

IIRC screwing the mixture screw outwards allows more air into the carb and weakens the mixture.

 

[kevinandkate]

Thanks Pepper, I was begining to think I was typing in a white font. There is quite a range of screwing the jet out which doesnt seem to have much effect on the engine or performance until about 6 turns in total when the engine sputters. Will try screwing it out to that limit for the controle technique.

 

[malcolm]

Screwing the mixture screw out will richen the mixture. Normally 1.5 or 2 turns from fully in will be about right.

The pop on over-run is caused by a hot exhaust manifold. Ignition timing will have a large effect. Try not to compensate for bad ignition timing with bad mixture.

Having said that my GTL would pop a bit on overrun - not a bang just a regular sneezing noise. I quite liked it.

 

[Steve Hennessey]

Fifi has the 281f carb with a mixture screw near the base. When she first arrived I had a bit of a pop pop pop on overrun so adjusted the carb as per Hayned, IE screwed it completely home then out 3 turns and voila no more pop pops. Strangely this has started again and I have had to screw the jet out another one and a half turns to stop it. I checked the timing and points gap and all was spot on. My question is, does screwing the jet out richen or weaken the mixture? The controle technique is due next month and they tend to be keen on emissions, my intension is to weaken it on the day as much as I can (everybody does that here if they can) and still let it run but do not know which way to turn it to weaken it.

By screwing the air screw out you are enrichening the mixture and therefore compensating for something that's not quite right elsewhere. Something is causing your engine to run weaker such as an air leak in the inlet manifold or carb base gasket. Poorly seated inlet valves can do the same.

Popping noises can also occur if the exhaust flange gaskets or seals are leaking and air is drawn in on the overrun.

That little air screw can also tell you if the engine is running rich. If you find that you need to screw it in to cure lumpy running this can point to a dirty air filter, worn float needle valve or dirt in the carb.

 

[DaveP]

I got an absolutely wrecked engine (tickover 1500 rpm, high crankcase pressure, using/leaking as much oil as fuel) through a CT recently, they're only concerned with CO2 emissions, I did exactly what you're suggesting and ran it a little lean, but if it's causing a misfire I'd probably just get it rich enough to tick over effectively, lots of unburnt fuel coming through won't help I'm sure. Ask around about CT centres as there's those who take a sensible view on old cars and those who are a bit more "by the book" I've push started a mate's Ami8 into my CT centre which had no door handles and was hotwired and it passed first time.

Other places can fail you for having 145 tyres on a saloon.

Dave

 

[kevinandkate]

Thanks everyone for the information, will have a delve around the carb base. There was a slight leak on the exhaust where the manifold pipe joins the rest before passing through the wing, I thought I had cured that but its started again so that will need to be the first job. Will let you know how it goes. Dave I take your point about the different centres, have just discovered a more lenient one not to far away so Fifi will go there.

 

[billythefish]

According to what I have been told you tighten it and then loosen it one and a half turns.