Twin SU Spacers & Getting A Filter On

[artrider50]

I just saw a Marlin on e-Bay that had Twin SUs AND some pancake air filters!

I then saw some 'Mini' 1.5" SU spacers that were a lot thinner than the standard ones (mine are about 25mm), and can only presume that getting the filters on was by using the thinner spacers or none at all?

So the big question is what would be the implications of fitting the thinner spacers? A bit more heat to the carbs obviously, but would it be enough to significantly affect performance?

[robert]

Re: Twin SU Spacers & Getting A Filter On

I have the same set up as in the picture with thinner spacers and pancake air filters (and a small notch filed out of the aluminium angle at the top of the engine side panel). The heat shield looks like it's been 'adjusted' too.
I have had the problem of fuel vapourisation on one occasion when the temperature was about 30 degrees but other than that no problems.
If you have the older HS carbs, watch out for the throttle linkage which will hit the exhaust manifold if the spacers are too thin. I think the linkage is round the other way on the HIF carbs.

[amalgamated users from prior forum]

Re: Twin SU Spacers & Getting A Filter On

Message originally posted by: Stefan Carlton
Sigh. I've had a nightmare with this for nigh on 18months trying to find something that would fit. I've tried:

* Mini spacers & MGA spacers but it appears that the manifold nuts are in a different location & you'd need change the accelerator connecting arm as you can't push the heat shield back any.

* Creating an air-box, but this proved troublesome as you couldn't do it correctly - there wasn't enough space on the front carb to allow for smooth air-flow & when I looked at the cost it was going to be nigh on ?300 just for the box.

* Pipercross air-rams & socks, don't fit over the front carb.

* Buying an off the shelf scoop but due to the curvature of the bonnet they wouldn't fit.

* not worrying about it (didn't help).

Then I suffered a complete breakdown due to overheating & vapour lock and decided something had to be done. The car is currently (29th June 05) with a man in Darlo getting a custom aluminum scoop fitted (?80 all in) to allow the filter/ram combo to fit. He's also putting some slots in the back of the bonnet to help extract the hot air. Hopefully that'll work, but I'm also going to pick up an electric fuel pump and get rid of the old fuel pipe & pump in the engine bay. I've also got a new radiator fan on it's way which should work better than the ageing Kenlowe I currently have.

On the scoop front, I should have the car back this weekend, and he suggested that making/fitting another scoop wouldn't be too difficult/expensive (he touted ?50 to 60 for the scoop), let me know if you want pictures etc of mine.

[artrider50]

Re: Twin SU Spacers & Getting A Filter On

Somehow I knew it wouldn't be that simple!

I'm going to try Stefan's 'Option 5' I think

[amalgamated users from prior forum]

Re: Twin SU Spacers & Getting A Filter On

Message originally posted by: Stefan Carlton
Hehe, It doesn't work. Too many people would look at the engine and remark it had no air filters, every time I had to drive through a dust cloud I was panicking... I'd prefer not to have a scoop, but it'll be easier on the nerves with air-filters on!

I know Jez was trying to sort the same problem out but without the scoop, I don't know where he got to in the end mind.

[artrider50]

Re: Twin SU Spacers & Getting A Filter On

I used to have an old J4 van that utilised some ladies hosiery and an elastic band! Need replacing quite often though I look forward to seeing the scoop...

[AndrewKB]

Re: Twin SU Spacers & Getting A Filter On

Oh my gawd! This one will run-and-run. My car had a setup very similar to the one in the pic for years and it ran OK 98% of the time, only getting vapour lock when VERY hot. Since I put in a new engine, but with identical carb setup (i.e. thin spacers and those rather c****y air filters), I've had nothing but BIG trouble, with the result that I haven't been competing in trials this year (although the car is OK on long road runs). I'm pretty convinced that the only way forward is to restore the full-depth spacers, use decent K&N filters, and cut the bodywork apart to accomodate them. In my humble experience the swap from mechanical to electrical pump made not one jot of difference to the vapourisation problem. I've also got the exhaust wrapped in layers of insulation and THAT made b****r-all difference as well. AND, the lack of throttle opening is yet another problem. Sorry to be such a voice of doom-and-gloom.

[dogoncrazy]

Re: Twin SU Spacers & Getting A Filter On

Why not fit just one single su of a suitable size on a decent manifold .The old 'log' type manifold is a complete waste of time in terms of effiency and twin carbs will only give you a little more power at the top of rev band[providing that they are set up properly,which is unlikely!!]

[AndrewKB]

Re: Twin SU Spacers & Getting A Filter On

Alan - You are right, of course. I know of at least two trialling Marlins with single 1.75" SUs and the owners swear that the set-up is preferable to twin 1.5" SUs. One has had his on a rolling road and confirmed your assertion that it gives almost as much power at the top end.

This swop has, however, tedious complications on my car which are too long-winded to explain here which is why I am considering an engine swap - see separate posting.

[artrider50]

Re: Twin SU Spacers & Getting A Filter On

It's not all about performance for me though, it's about enjoying something that looks and sounds nice too. Twin SUs look and sound better - never swap for a single SU, it would be embarassing!

Probably the same reason why I wouldn't swap my Roadster in a million years for a friends much faster Caterham 7(yawn)!!

[amalgamated users from prior forum]

Re: Twin SU Spacers & Getting A Filter On

Message originally posted by: Don Sayers (Twin open carbs!)
The main problem is the upwards bend in the B-series twin carb manifold. several solutions suggest themselves.

A cut and shut job on the manifold, tricky as its cast iron.

Thinner spacers as suggested in the build manual.

Custom made inlet stubs, a doddle with a big lathe and some big ally stock. Question; is the link pipe that links the two carb really vital? what's it for?

A Fabricated manifold in iron tube, perhaps copying the type found on maestros (A series) where the inlet manifold ran in a big curve round the back of the carb, with the single carb mounted on the inside of the curve facing the engine. Supposedly good for torque.

Any ideas?

[AndrewKB]

Re: Twin SU Spacers & Getting A Filter On

Although I'm not an engineer I think some of the above comments miss the inherent problem, which is that the standard twin SU setup requires the full-depth (insulating)spacers to reduce the heat transfer from the manifold to the carbs. Sure, there are a dozen different ways to squeeze the carbs under the Marlin bonnet, but they all increase the likelihood of the carbs overheating, and the fuel vapourising, the problem being exaggerated, of course, by the tight confines of the Marlin bodywork preventing heat dissipating as quickly as it should.

The only real debate is why some apparently identical twin SU installations work perfectly all the time, some work most of the time, and some are diabolical all of the time.

[amalgamated users from prior forum]

Re: Twin SU Spacers & Getting A Filter On

Message originally posted by: Stefan Carlton
I've just got my car back My Scoop:
http://www.stefancarlton.net/pictures.php?menu=picture&albumid=100

At the moment I'd tentively say it's helping!

[artrider50]

Re: Twin SU Spacers & Getting A Filter On

Stefan, I have looked at the photos on your website one thing that strikes me that could be a contibuting factor is the disturbed air flow into the SU's when the air gets warmer, try some short, flared ram tubes which will aid induction of fresh air straight through the vents. Have a look at David Vizards section on Carbsin Tuning the A series.

[amalgamated users from prior forum]

Re: Twin SU Spacers & Getting A Filter On

Message originally posted by: Stefan Carlton
originally I didn't have space to fit anything like that on, with the scoop it's all sorted - I have sort ram pipes & air-socks as filters.

I'm just re-writing all the background code at the moment before re-doing all the major sections hence the lack of details on there.