12 replies to this topic

[Richard e]

Hello. I am going to give it a go to set my carb up from the air to fuel ratio. I understand I need a wide band AFR gauge. I am not going to fit a sensor, so assume I need to stick it up the exhaust pipe - I’ve read that I’ll need a sniffer tube? I appreciate that it won’t read well on idle, but it should be good at higher revs? What gauge should I use please?

[bpirie1000]

This is a crazy large topic.

Need to be trimmed up first and carb will need to be looked at for wear and tolerances. Also helps if you have a range of needles. Also an ability to listen, look, feel and hear... years of experience are best left to specialists...

Ac dodd is the magical man to work the magic...

[Earwax]

A few years ago I went INNOVATE     - they have a portable gauge type kit  They are seriously expensive now.  No knowledge of the cheap versions going around - I would stick to ones that use a bosch sensor.

[Ethel]

You'd be better off with the sensor properly plumbed in. The engine needs to be under load to see what the AFR is really doing. With no load the revs will max out before the carb has barely opened.

[timmy850]

I use an Innovate MTX-L gauge and I installed it in an electronics box with a 12v accessory plug

I then used an Innovate 37280 tailpipe adapter (although you can make something similar with a length of exhaust pipe, weld in the bung and a method to hold it in the car exhaust)
https://www.innovate...o1pOYd6q15v0Mpn



I’d still recommend getting a bung welded into the exhaust if possible, an exhaust shop can do this very cheaply (cheaper than the innovate clamp) and it reduces the chances of exhaust leaks giving a bad reading & also lets you see the mixture at idle. If you do have an idle emissions test then it’s important to get that right

[68+86auto]

a probe say 20cm into the pipe will read fine. My most basic machine (wheatstone bridge type) uses about a 3m hose with about 20cm shoved into the pipe and it doesn't even have a vacuum pump to suck the gasses. It works surprisingly well. No issues at idle. The readings are very close to my much fancier machines which have a vacuum pump and can read 4/5 gasses.

[Rubbershorts]

I'll second the under load advice. Mine was tuned by AC and was found to be pretty lean when measured under load at full gas, when I fitted an AFR gauge afterwards. AC himself will admit that having a gauge fitted when he is tuning your car is a big advantage.

[babsbrown]

Obviously it's better to plumb it in. But equally I've found that the AFR when the car is static but revved is almost equal to that when the car is under load.... almost. If your getting an AFR and not plumbing it in, you might be better with a colourtune.

[Ethel]

While you could manage with a probe up yer pipe, it would be a poorer option if you're splashing out on a gauge for just this car.

 

You can get a boss for under a tenner, or in a length of pipe to bolt in for not much more.

 

 

 

 

....stationary will never be comparable as there's nothing to resist the revs. As soon as you come remotely close to filling the cylinders it will accelerate. A colourtune does have one advantage in showing you what's going on with a single cylinder. That's not insignificant for the siamese A Series.

[babsbrown]

Aye and if your not comfortable with welding you can get a ready made section of pipe with on already welded in. Sorry Ethel just mentioned that. Didn't read it properly 😂

Edited by babsbrown, Yesterday, 01:43 PM.

[imack]

If you're going to the expense of buying an afr gauge and sensor, get a boss welded into the exhaust so you can see what's happening on the road. Static tuning is a good starting point, but if like me you have neighbours, they might get pretty annoyed with the noise and fumes. Once you're happy with the tune, you can remove the sensor and fit a blanking plug.

[Earwax]

An interesting find in a twin SU carbed motor I have AFR tuned - after extended what ( think Long loong straight 30sec +) there was some leaning out  - so if i recall we were aiming for 12- 12.5  and were pretty happy give or take but on an extended straight in top gear it appeared to drift leaner.  We didn't do an immediate plug check - No idea if this is the norm but worth noting.

 

The set up on the innovate and probably all - allowed RPM input and saving readings - in the end I found the driver trying to hold differing steady revs and the passenger scribbling as we went along was reasonably effective 

[timmy850]

The Innovate 37280 tailpipe adapter can be used while the car is driving, so you can check it under load too. I use some electrical tape to hold the cable onto the fuel filler neck, then the cable goes in the rear window.

 

If you make up a sampling pipe yourself, you can add a similar clamp to hold it in while driving.