Hello
I think my carbs are running a bit rich, I get a strong smell of fuel when I put my foot down and get some mild popping from the exhaust when going
down hills with no throttle, plus I seem to be using more fuel than normal. And my last friendly MOT tester said it was running rich too.
1.8 Zetec, Bike Carbs (ZZR I think supplied by Bogg Bros) and Megajolt, all still running fine over last 5 yrs and 10,000 miles of spirited road
use.
Can I try to adjust the 4 fuel screws under each of the carbs?
How sensitive are the screws? Do I just turn them all in by say 1/2 turn and see what happens?
Anything else I can check/adjust at home?
Its a pity you didn't ask the MOT tester to tune it on their gas tester. You could try using a Gunson Colortune their actually really good and
robust I know this cos I once forgot to remove it and drove into town with it still fitted 
. Comes with full colour picture instructions of what you
are meant to see through the plug window and dead easy to use. Probably pay for itself very quickly from fuel savings. If your running rich I'd
expect to see soot on your spark plugs or a lot of build up.
What colour are the spark plugs? Your looking for a straw brown.
White is weak and sooty is rich.
Ah yes, good idea, I will check the plugs tomorrow in the daylight and report back.
Thanks both...
The screws under the carbs are the pilot air screws and effect the idle and low throttle openings only. If it is running rich at high throttle openings then you may need to drop the needles. seems odd that they have been ok for 5 years. Have you made any changes to the fuel system?
Bike carbs don't like too high a fuel pressure from the pump.
I had to put a pressure regulator in and the Facet pump I use is only rated to 4psi.
Too high a pressure will push fuel past the float stops and straight through the jets to the engine
[Edited on 17/9/20 by feckn7]
quote:
Originally posted by feckn7
Bike carbs don't like too high a fuel pressure from the pump.
I had to put a pressure regulator in and the Facet pump I use is only rated too 4psi.
Too high a pressure will push fuel past the float stops and straight through the jets to the engine
+1 for the colortune. I did mine with one. Went to a RR they said it was as good as they could get with needle twiddling. Shimmed the needles on one bank to change the fueling profile but the other was spot on. They look jank but work well.
The mixture screws are just for idle mixture.
Things I would check over what has already been mentioned are the float heights - if fuel level is too high it will affect the mixture. Mine was
popping loads on overrun and dropping the float height reduced it a lot. Float height too high will also often result in a smell of petrol.
The main mechanism for controlling bike carb mixture under load is the interplay of the main jet and the needle. Is the needle height adjustable?
Raising it will richen the mixture, lowering has the opposite effect.
Using a wideband AFR meter is a good way to set up at home.
Stu
I checked the plugs, all 4 are dry and straw coloured, the gaps are all still at 0.039" or 1mm, it runs fine, the mild popping on overrun does
not bother me too much. So I think I'm going to leave the carbs alone. I will see what the MOT guy says next time when he does the emissions
test, at 3.5% CO2, 0.12 HC.
The fuel pump is a bike one, supplied by Bogg Bros along with the carbs back in 2008, it's all run fine with the megajolt for years I've not
touched the ingition/carbs at all since I built the car.
Think my Dad has a colourtune so I might borrow that and have a play.
Thanks for the advice.
From what you say its must be pretty close to being right. As for the popping I've had loads of cars and bikes that did that, even the old
landrover, it's part of the fun . I don't think you get that these days with boring injection cos it turns off the injectors on overun.
Main thing is your plugs sound healthy and that would be good enough for me 
When I take my older cars to the garage I'll say if the mixture or head light aim is out just sort it, that way they never fail