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Andrew's Mk IV Spitfire


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I suspected I was in a good place when looking around there was a Z4 race car on the ramp. A man tinkering with a BMW motorcycle and sidecar racer, and a "Micra" that was a tune frame race car. Oh and they wheeled out an isetta and MK2 gold GTi to get to the Dyno. An eclectic mix of interesting machinery!

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I went to Revivals, in Thriplow.  They were really good, and interested in my car and what I was doing to it.  I'm still deep in thought about the next steps for refining the fuelling.  Maybe modifiying some new AAM needles to make them richer where I need at higher revs. I'm still undecided upon the spring removal.  My understanding is that you want the pistons to reach maximum height at max power, so reducing the spring rate will help with this at the cost of reduced depression through the venturi (which is what drives the fuel out of the jet).  This makes sense if airflow is your limiting factor (particularly if the carbs are a meaningful % of resistance in the system), but if they aren't a restriction, then all you are doing is reducing the depression which is driving fuel flow.  Given I have 1 1/2" carbs, not 1 1/4" and a standard cam, I suspect that airflow is not my issue, and that fuelling is.  So more depression would be beneficial.  Of course then I need to alter the needle profile to be sufficiently rich at this lower station.

I've also got an issue with fuel height in the float chambers being different, which likely isn't helping, the rear (left) carb appears to have a lower fuel level than the right.

Finally there seems to be a good bit of oscillation in the carburettor pistons, which isn't giving consistent fuelling and is not optimum, but this might just be an SU carb thing?

If anyone can understand the ramblings of a man who'se spend a lot of time reading books on SU carbs and pass comment then I'd greatly appreciate any and all opinion.

Here's a video of one of the dyno runs looking down the carbs.

https://youtu.be/Tb8OG95RZ14

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Great result, considering the 75 hp DIN rating of a Mk.3 engine when new, on very different fuel...

1 hour ago, egret said:

Finally there seems to be a good bit of oscillation in the carburettor pistons, which isn't giving consistent fuelling and is not optimum, but this might just be an SU carb thing?

I'm pretty sure oscillation is normal for twin carbs on a 4-cylinder, if you think about how the cylinders are paired and the firing order, ...1-3-4-2-1-3-4-2..., the periods of flow through each carburettor are not evenly spaced so there will naturally be some oscillation. I've also experienced the situation where 2 and 3 run richer than 1 and 4 due to this.

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Well the su carburettor high performance manual has a few interesting sections in it.  It mentions that rover fitted very large carbs to one of their 4 cylinder engines as a sales gimmik, and restricted the piston travel to prevent full travel, and the poor performance that would have given. Maybe my carbs are oversized and putting the springs back in might help.

It has an entire chapter on the method of how to reprofile needles, so that feels like a good option in the future. But the most relevant part talks about how float bowl volume is critical to power output.  Given the visible difference in fuel height, this needs to be rectified before I fiddle with anything else.

Having said all that I'm not feeling negative, the car is driving much better than before. I just believe there are still some cheap/easy wins to be had :banana:

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Interesting stuff, and good to see the video. Is that with the dashpot springs removed? I'd agree with David Jumpingfrog about oscillation - doesn't look unexpected to me. Different damper oil and spring strength would change behaviour to some extent but not eliminate the wobble. Air flow down the carb throat is a train of pressure waves rather than smooth, so pistons will always react to that, I would think.

On fuel height, have you tried adjusting the float height in one or other carb? It's a bit trial-and-error, and difficult to get them bang on equal. I set mine so that (with dashpot & piston removed) lightly blowing on the top of either jet makes the fuel slosh enough to just be visible. Very unscientific... maybe a cocktail stick dipstick down the jet would be more rigorous. 

Carb size and piston opening is an interesting question; I know the upgrade from HS2 to HS4 is fairly common but, from what I've read, you need to take cam / head / exhaust mods a bit further before HS2's are a limiting factor. 

What was the rolling road bloke's explanation for removing the springs? Was it running too rich / less power beforehand? 

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I'm not certain if the springs were in or not with the video.  I'll have to check the timestamp, but I think this was before we took them out.

I've got it on my list to look at the fuel floats and try and adjust these better.  The rolling road man said to ensure you set them with the smallest weight on the needle valves, and don't just turn them upside down using the entire weight of the float.

Also I think that if I had the option I'd be running HS2s, it's only that I have the HS4 setup that I'm using it.  I feel like a reliable set of HS2s would be comparable to a fuel injection setup.

The reasoning behind removing them was that we'd run out of richer needle options and were still overly lean at the top end, so getting the needle up faster would put more fuel through.  It did seem to do this, but I still suspect that a richer needle with the springs back in might be the optimal solution.

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I agree that HS2s are big enough on a 1300 unless it’s fairly seriously tuned up in airflow terms. The HS4s are a bit big.

As I understand it, the dashpot springs  are used to set the working range of the carb so that the dashpot is fully up only at maximum airflow. 

Taking the springs out makes things richer by exploiting needle taper, but the flip side is it reduces vacuum in the throat so less fuel is drawn out…. 

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Your thoughts do make a lot of sense, as someone who went on this journey and had a lot of fun fettling the carbs on my Spitfire. The book you have is brilliant and it all works, I ended up making the little sticks with numbers on to put in the dash pots and custom needles on the drill press. You can get it into a very good state of tune without a rolling road, and you could even fit a wideband sensor to your exhaust if you want to confirm the results you think you are achieving. Ultimately I went down the fuel injection route as I liked optimising the car so much, in that way I guess the book and tuning carbs is a bit of a gateway drug towards EFI.

As Nick says I think the logic you've been given may not bear out in reality, but at least you've now got one good setup to fall back on, and can use that as a benchmark to experiment further.

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