Lotus Elan

Stromberg carbs

PostPost by: Tonyw » Sun Nov 15, 2009 2:18 pm

Hi all,

I posted a wanted request for the adapter plate that goes between the balance tube assembly and the carbs on the wanted part of the board, lots of lookers but as yet no offers so I thought I might make two.

The original adapter plates connect via an "O' ring to the balance tube assembly side and IIRC the manual says that tension on the "O" ring is required but a few thou gap should be measured around the circumference this as I understand it allows for some flexibility of the carbs, not really sure why his is needed so can anyone offer a reason??, lots of other twin carb cars do not have that type of fitment.

My other question is can I simply make an adapter to bolt the carbs directly onto the head or do I really need the balance tube assembly??

All academic if I find some adapters but i like to have a plan B.

Thanks in anticipation that the collective brain power of this great group will come through with some answers for me.

Tony W
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PostPost by: Gordon Sauer » Mon Nov 16, 2009 5:07 am

Apparently it's easy to snap off the adapter plate outer corners (3) --I'm pretty sure its cast aluminum and easily breaks--mine didn't originally have them, one I got had to be heliarced to make it whole again. I used thackery washers at those three bolts although maybe not necessary. Gordon Sauer
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PostPost by: oldelanman » Mon Nov 16, 2009 6:28 am

Hi Tony,
The flexible mounting of the carbs using "O" rings and thackery washers is to reduce frothing of the fuel in the float chambers due to engine vibration.

You do need the balance pipe between the two carbs, it's required to smooth out the pulsations in the manifold caused by the firing order of the engine particularly at idle and low speed. Without the balance pipe the CD carburettor will try to follow the inlet manifold vacuum fluctuations as the valves open and close and the air/fuel ratio supplied to the engine will vary accordingly. The air valve piston damper also helps with this smoothing.

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Roger
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PostPost by: Tonyw » Mon Nov 16, 2009 2:02 pm

Gordon & Roger,

Thanks for the explanation, it would seem then that finding these adapters may be problematic if they break off so it looks as if I will need to make a pair.

Is there anyone who can tell me what the thickness of this adapter plate is? some with a car in bits perhaps.

Almost have the engine completed, gear box done soon to be fitted into the chassis/frame.

Tony W
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PostPost by: Yum-yum » Mon Nov 16, 2009 5:34 pm

Hi Tony,
I'd read your ad and have been meaning to check if I had those parts. They'd be buried in a box when I put them away many years ago. I'll have a look for them tonight. As an aside, I replaced the manifold years ago with machined magnesium blocks getting rid of the o-rings and spring washers. I'd gotten tired of chasing air leaks. Magnesium does a very good job of dampening vibrations, unlike aluminum. At least 50k miles and I have not had any outward signs of fuel frothing. I've maintained the balance pipe. If you are getting anything machined up, I might suggest this arrangement. Here is a picture:
DSCN0222.JPG and
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PostPost by: oldelanman » Mon Nov 16, 2009 5:38 pm

Hi again Tony,
That plate will be tricky to make as it's not just a flat sandwich affair. It has a radiused lip around the centre hole to locate the inner diameter of the "O" ring which itself locates in a recess in the balance pipe block, the tolerance on that will be critical to achieve a good airtight seal while still allowing the flexible mounting mentioned before. If you have a workshop manual there's a section through the carb which shows the lip. Also I'm pretty sure that the plate has countersunk studs which the carb bolts to and I think they are pressed in.
You can see the plate in the attached pic still bolted to the carb body but it's reassembled to the engine now so can't give you accurate dimensions but the overall thickness of the plate (excluding the lip) looks to be about 10mm.

Hope this helps. Good luck.
Regards,
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Carburettor overhaul 011.jpg and
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PostPost by: collins_dan » Mon Nov 16, 2009 11:32 pm

Can someone post a picture of a proper installation of strombergs, specifically the

"The flexible mounting of the carbs using "O" rings and thackery washers is to reduce frothing of the fuel in the float chambers due to engine vibration."

I'm having trouble with misfires at 4500 rpms on the highway, and its been suggested that it could be due to fuel frothing.

When I look at how my strombergs are mounted, there's nothing flexible about it. The previous owner disconnected lots of things rather than repair them, or did a shortcut wherever possible.

Thanks, Dan
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PostPost by: Yum-yum » Tue Nov 17, 2009 4:53 am

Tony,
Are these the plates you want?
Stromberg plates.JPG and

They are off a Federal S4 elan, I would guess they are universal but I don't know for sure. The plate part is about 3/8" thick, but as oldelanman mentioned the o-ring lip rises above this plane. The studs are about 7/8" long. If you want these send me a PM and we can work something out.

Dan,
There is a pretty good exploded view of the parts in question on the rdent website under parts manuals/elan/engine/induction/2ndthrottle page.
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PostPost by: oldelanman » Tue Nov 17, 2009 9:14 am

Dan,
The "O" rings must be the correct material diameter, too small and they will be crushed completely and you will end up with metal/metal contact between the adapter faces and consequently no vibration damping, too large and it will be impossible to assemble. It is also critically important to tighten the hardware slowly and evenly. Correctly assembled you should have a 70thou gap between the faces of the two adapters which you can see in the attached pic.

"Flexible mounting" implies that it allows movement but this is a misleading description as even when it's all correctly assembled you won't feel any movement by pulling up and down on the carbs.

Regards,
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Carburettor mounting 002.jpg and
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PostPost by: Tonyw » Tue Nov 17, 2009 12:28 pm

Thanks all for the replies very helpful.

Alex i would like to talk to you about those plates of yours, tried to PM you but I am having trouble getting the mail out of my "out" box can you email me on lancelot2"at"westnet.com.au happy to pay a reasonable price or swap if I have what you need.

The wealth of knowledge on this site is astounding.

Thanks all

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PostPost by: collins_dan » Tue Nov 17, 2009 1:41 pm

Thanks for the pictures. I suspect my o-rings are bad or have been overtightened. I have no gap. Dan
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PostPost by: Tonyw » Tue Nov 24, 2009 12:36 pm

To all

Thanks all for the replies and advice Alex has found a couple for me and they are winging there way from the USA to Australia as we speak, thanks Alex you are on my Christmas card list.

Tony W
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PostPost by: MintSprint » Tue Nov 24, 2009 12:46 pm

oldelanman wrote:It is also critically important to tighten the hardware slowly and evenly.


Worth checking the adapter plates with a straight edge, too - I've seen instances where the previous owner has over-tightened and actually distorted the aluminium of the plates as a result (in which case, there's no way they will ever seal correctly without being over-tight and causing fuel frothing).
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PostPost by: collins_dan » Tue Nov 24, 2009 1:41 pm

To replace the o-rings and ensure even pressure, can I just unbolt the whole carb assembly, replace the o-rings and rebolt the whole assembly back on, tightening the last bit one bolt at a time? Thanks, Dan
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PostPost by: oldelanman » Tue Nov 24, 2009 3:40 pm

To replace the o-rings and ensure even pressure, can I just unbolt the whole carb assembly, replace the o-rings and rebolt the whole assembly back on, tightening the last bit one bolt at a time? Thanks, Dan


Hi Dan,
Yes you can but it's a bit fiddly and you will need a lot of patience ! You don't need to remove the carbs completely but you will have to disconnect a couple of things to allow access and enough movement to get at the "O" rings. Disconnect the throttle cable from the nipple at the "W" clamp which connects the two throttle spindles and remove the airbox to give you access to the lower nuts - if you have an Elan access is still very tight but I think there is probably more room on +2. The carbs are joined together by the fuel pipe and the throttle linkage but you don't need to disconnect these. Remove the 6 nuts and thackery washers that hold the adapter plates to the balance pipe blocks and pull the carbs away from the engine off the mounting studs - you can now get at the "O" rings. As MintSprint says it will be worth checking at this point that the adapter plates are not distorted by previous overtightening. To rebuild place the "O" rings in the recesses in the balance pipe blocks and carefully slide the carbs back onto the mounting studs keeping the mounting faces parallel and you will feel the lip on the adapter plate enter the "O" ring, you will probably need an assistant to hold the carbs in place while you replace the hardware. Fit the top 4 nuts first and finger tighten them until they just contact the thackery washers, now fit the bottom 2 nuts and finger tighten until they just contact the washers as before. Now gradually tighten the nuts one flat at a time (do not fully tighten one and move on to the next) Continue until you have a parallel gap between the two mounting faces of 70 thou. If any of the thackery washers becomes fully compressed with no gaps between the coils something is wrong and you will need to back off and start again.

Hope this helps. Good luck.
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