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Air fuel ratio meters


jiggawhat2k
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Hi there, I've got the herald running better (spec is 1300 mk4 spitfire engine, hs4 carbs, 421 manifold and exhaust), done some driving now and noticing from 1k-4k there is a bit of a flat spot in power (and occasional hesitation on pop from the carbs) and then from 4k-6k it picks up nicely. 

Carbs are balanced and when idling the mix is good. I'm wondering if I play with the needle type (currently running AAR needles) I might be able to improve the mid range. 

A guide I read on tuning su carbs said that an air fuel ratio meter is the best way to get the mix right across the whole rev range. 

Is this something others have done? What meter might you have tried/installed before? 

Thanks for the help! Jim

 

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Google wideband oxygen sensor. Lots out there. If permanently fitted to the car I would not expect great life, as carbs are not usually accurate enough, but ideal for tuning.

I have an LD performance one in my yellow spitfire, sensor has just died (despite being EFI) but that is down to poor sensor position, and also leaving the sensor (ignition) on for long periods without running. (modern cars do not switch the sensor on until the car has been running a few minutes to increase sensor life). I have a plan for the sensor position, and the new sensor is clever and only comes on when the exhaust is hot enough to be safe.

In terms of tuning, it is like turning a light on. You can see exactly what the fuel mixture is at all times, no guessing involved. 

The other option, for a similar price to a wideband, is to get the car set up on a rolling road. I have done that with a couple of cars. In an hour they car do (better) what would take me days. And they alter the needles to suit. 

One other point, remember the distributor in all of this. If that is giving the wrong advance it will loose power/torque. But it should be fully advanced by 3000rpm. You can try "mapping" the distributor, needs an assistant, and record the advance at various RPM using a timing light. You can get various advance springs to alter, but the std ones to match the engine would be a good start as they do stretch over time. Likewise, is the distributor the correct one for the engine? The advance specs should be somewhere on the interwebby....

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What Clive said... but, you might be better looking at finding a rolling road and tuning shop where they know their way around SU carbs? HS4 on a 1300 is a slightly "Frankenstein" set-up so they would probably be able to grind carb needles to suit the engine/carb combo better than you could choose off-the-shelf needles based on AFR readings?

 

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Cheers for detailed response Clive and Spam (if that is your real name).

I didn't realise you can munch through sensors over time. Where did you position it? 

Very keen on a rolling road but the current situation makes that a little tricky at the moment plus I don't mind doing it badly myself, part of the fun! 

Cool I'll check out the LD one, thanks again, Jim 

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8 hours ago, jiggawhat2k said:

Cheers for detailed response Clive and Spam (if that is your real name).

I didn't realise you can munch through sensors over time. Where did you position it? 

Very keen on a rolling road but the current situation makes that a little tricky at the moment plus I don't mind doing it badly myself, part of the fun! 

Cool I'll check out the LD one, thanks again, Jim 

I have a very vertical manifold, 4>1. The sensor is on the outside of the bend at the bottom, almost horizontal. 

http://blog.innovatemotorsports.com/important-tips-to-get-the-maximum-life-out-of-your-o2-sensor/

With carbs I would set them up, then remove the sensor. 

If interested, I have an older JAW wideband sensor/controller somewhere... documents here under "legacy support" Had little use and I used it on a couple of cars using a homemade pipe that clamped into the back of the exhaust. https://www.14point7.com/pages/software-and-documentation

 

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On 05/05/2020 at 07:46, clive said:

I have a very vertical manifold, 4>1. The sensor is on the outside of the bend at the bottom, almost horizontal. 

http://blog.innovatemotorsports.com/important-tips-to-get-the-maximum-life-out-of-your-o2-sensor/

With carbs I would set them up, then remove the sensor. 

If interested, I have an older JAW wideband sensor/controller somewhere... documents here under "legacy support" Had little use and I used it on a couple of cars using a homemade pipe that clamped into the back of the exhaust. https://www.14point7.com/pages/software-and-documentation

 

Brilliant yes thanks, I'll direct message you. Cheers! 

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