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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGA - Spacers for SU fuel bowl

On the Moss site there is an item called a spacer, which is said can be fitted between the SU fuel bowl and the cover lid.
It is claimed that it was used by the factory on the Twin Cam engine. I have never heard of it. To me it defies all logic. It is not to be confused with the anti-vibration mounting of SU's, in the same way as Webers.
Has anyone have any knowledge of this item? (Preferably with some documented evidence).
Or is this item in the "snake oil" category?

http://www.mossmotors.com

Insert 370-485 in the part number search, but leave the selection on "Any British Cars". Although the MGA is not listed, when you go to any of the other car models it describes the fitting "as used by the factory on the mga twin cam".

Mick
Mick Anderson

Mick - complete snake oil for sure.

Ironically, the only model of carb that experienced the problem, the H6, isn't serviced by this item - they only fit HS6 carbs - which never had the problem in the first place.

Moss - for shame....useless parts, and misinformation.

(This came, I believe, from something one of the race cras did to try and fix the lean situation - before they knew it was a vibration problem)
Bill Spohn

Bill:

The twin cam fix was related to a small group by Henry Stone when he was reviewing some cooling and fuel modification suggestions during the restoration of an MGA twin cam race car. He actually drew the details up with approximate dimensions.
This was back in the 80s but came instantly back to me when the HS series spacer kits were brought to my attention by a long time MG shop owner. The spacer kits are made by a small company here in California and have been used on a number of race cars that tended to run lean under full acceleration due to fuel bowl level drop.

They are stated to be racing parts that could be used on high performance street engines and no intention was made to misinform or promote useless parts.

Kelvin Dodd
British Marketing Manager
Moss Motors, Ltd.

KJ Dodd

and thats why i love this site.its the best.thanks,cc
c.callaway

It also says something about a company when one of their managers takes the time to review the forum and provide such answers. Well done.

Les
Les Bengtson

While I appreciate Kelvin's comments, the spacers serve no purpose, racing or otherwise.
You only have to apply your own logic to see that they are uneccesary.
If you place a spacer beteween the bowl and the cover lid it does raise the fuel level in the bowl and at the jet. However now you have to raise the jet to get the mixture correct (if you could get enough jet travel). You are back where you started!
There is more fuel in the bowl, but that will not prevent leaning of the mixture, even under racing conditions. The pump must alway be able to keep ahead of consumption.
I would like an explanation of their supposed operation.

Mick
Mick Anderson

Mick:

I have to state that I am not an expert on the dynamics of carburation, but here goes.

The problem as explained to me was that the fuel bowls were being emptied faster than the pump could replenish. They tried going with larger needle valves, but there was difficulty keeping the larger valves seated under light loads.

By using the spacers the bowl was enlarged enough to prevent fuel starvation on the main straights without the need to go to the larger needle valves.

It's interesting that we ran into a similar starvation problem trying to feed the MGB supercharger with a single carburetor. Under heavy load the float bowl would empty, which was fixed by going to a MG Metro Turbo .096" inlet valve.

and err. thanks for the other comments.

Kelvin.
KJ Dodd

Thanks Kelvin.
This is becoming interesting, but unfortunately not yet the answer.
A larger capacity bowl does not help a fuel starvation problem on a SU. Fuel starvation will happen when the fuel level in the bowl drops in relation to the top of the jet. This could happen, as you say, by the needle valve capacity being insufficient, however this cannot be cured by having more fuel in the bowl.

Mick
Mick Anderson

Kelvin,

Well after much struggling with this I think I can see where you are coming from.
The spacer raises the position of the bowl cover lid. This raises the level of the fuel in the bowl.
The mixture is not altered as the fuel used is determined by the space between the jet and the needle, not the fuel level.
Now, because of the higher level of fuel in the bowl, more fuel can be used than the needle valve supplies before the level drops enough to cause fuel starvation.
A temporary reserve of fuel until lower consumption, due less engine demand from lower engine loads, allows the needle valve to replenshish the bowl.
Thanks Kelvin in helping to pass a day of miserable weather here.

Mick
Mick Anderson

Mick (and Kelvin) - some of the Lotus engines ran on a common float bowl, but didn't experience starvation - they simply ran a suitably sized fuel pump.

Kelvin, this mod may have originated with the MGA Twincam, but it was certainly not effective in solving any real life problem because draining the float wasn't what ailed the Twincam (it was a sticking float - a totally different problem that also resulted in lean running).

While I suppose you could theoretically arrange a situation where a race engine would run lean by draining the float at high RPM on a long straight, most racers would solve that problem very simply and without resorting to anything as Byzantine as float bowl spacers. I doubt that anyone running a race engine would still be running a stock fuel pump, and I also suspect that even the double ended SU pump used on Twincams and Jags would prevent any such problem.

I still think that the Moss description is misleading and intends to lead people to believe that by using this part they will experience some improvement in performance - that is patently false in all but 0.000001% of your buying public, and that guy (yes, he is probably a single person) could solve his problem without this bodge.
Bill Spohn

I thought long and hard about this when I saw it in the catalog, and I fully agree with Mick & Bill.
The fuel level, if properly set, is very near the top of the jet in its correctly adjusted position. Raising the level via this spacer would create a too-high fuel condition, leading to flooding at engine stop or low end/idle, unless the level was set lower, giving zero net effect under normal conditions. It is possible that some slight effect could be obtained by having the fuel level SLIGHTLY too high and utilizing the "extra" fuel to compensate for excess demand for a very short time. There might be some gain in needle valve capacity due to the float dropping a bit further (and thereby opening the needle a bit more) before starvation sets in, but all of this is a pisspoor way to compensate for low fuel delivery, and an excellent means of screwing up the correct needle calibrations. It is conceivable that using the spacer AND lowering the float chamber mounting on the carb, so that the initial fuel level setting is the same, would give a bit of extra capacity for high end use, albeit at a too-low but still working fuel level. This would let the needle valve be wide open at full acceleration. Still a bodge, and not hard to do on an H series, but v difficult on an HS. A marginally useful version would involve a larger diameter float bowl, and I have read of floatbowls from larger carbs being used on smaller ones. I suspect this mod originated with the Amal carbs, which are sometimes fitted with spacers. However, these actually increase the fuel VOLUME without changing the LEVEL.
FRM
FR Millmore

So since I sparked this debate on the twin cam site and Mick was kind enough to open up to the wider audience here, I guess I should add my 0.02c worth.

Always keen to learn and this is what I have summarise from the above.

A small gain can be made at the top end using these spacers, but the spacers should be used in conjunction with lowering of the float chamber mounting on the carb. To prevent over richening at lower speeds

Perhaps the kit should come with an offset bush or bolt to lower the fuel bowel?

But perhaps this is all just a smoke screen for a poor fuel pump.

Cheers <MARK>
Webber user !!!
Mark Hester

I'm confused- If these spacers are used to contain more fuel because of low fuel pump output per Kelvin's statement " fuel bowls empied faster than the pump could replenish",how much more fuel could a spacer add? Or are the main straights shorter oversea's- Gary
gary starr

Gary-
YOU are not confused! By rough estimate, the likely amount of fuel is about 7 feet in terms of distance travelled. Or, about a tablespoon or two at a consumption rate of a quart a minute or mebbe twice that.
FRM
FR Millmore

"But perhaps this is all just a smoke screen for a poor fuel pump."

It would be if any cars actually suffered' from this, but I doubt anyone would unless they run at tp RPM with a stock fuel pump that is on the way out.

What this whole thing appears to be is someone at Moss saw a spacer made to try and correct a situation caused when someone fitted non-stock carbs to a Twincam with float bowls mounted other than level. It has no real life application to any street cars and probably none to any race cars either, bar a racer too thick to fit a proper high volume fuel pump to his car. The spacers offered don't even fit the carbs that gave rise to the product!

I think this may have been a bad call by Moss, poorly understood, attempting to offer something they thought might be attractive to owners, not realising that it wasn't a useful product. If they have any integrity (which I know they do) they should reassess this product and pull it from their catalogue - if not, they are deceiving their customers.
Bill Spohn

This thread was discussed between 19/06/2006 and 22/06/2006

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