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MG MGA - SUguru - H4 carburettors

Maybe I have been living under a rock or my parents are too restrictive with my internet access as I have just found this blog from Tom Bryant with some interesting articles about SUs.
What really pricked my attention was the bit about the hole sizes in the mixture levers supplied by Moss being wrong, the comment not to use a fuel filter between the pump and the carburettors and teflon O rings on the jets. I thought this guy knows what he is talking about.
What do you all think about his articles (Volvos aside)?


https://thosbryant.wordpress.com

https://thosbryant.wordpress.com/2014/03/01/su-carburetor-rebuild-h-4/?fbclid=IwAR2GamBGg-e_6Ln8ZEa5S3dXgb0eWuc47z4tOPjGRYB3Av_F1SJ4ny8hc_c

Cheers
Mike
Mike Ellsmore

I read some of it but the articles were long, so if you don't mind Mike, what was the reason he said for not having a fuel filter between pump and carbs please?

I did note (with great glee) that he recommends Mobil 15w-50 oil for the dashpots, a (real) synthetic oil!! :)

Unfortunately we've not had the car version available in the UK for a good few years now, I know as I used to use it in my MGs much to the horror of some.


Nigel Atkins

With some confusion over the placement of a fuel filter when using an SU electric pump it is once again time to clarify it.
If using a filter it must be placed between the pump and the carburettors, never between the tank and the pump.
The suction of the pump is due to the operating of the solenoid.
The pressure of the pump is due to the pump spring alone.
With the filter after after the pump only the spring is trying to force fuel through a blocked, or partially blocked, filter. No electric current is flowing because the points are open at that point in the cycle.
If a filter becomes blocked and is between the pump and the tank the solenoid is stalled with the points closed the current flowing through the solenoid.
This current flow continues non stop.
The solenoid will overheat and fail.

Mick
M F Anderson

I'm not sure if I am in trouble or not with my setup then Mick, my MGA came with an in-line filter between the tank and the fuel pump, then later on, I fitted another in-line combined fuel pressure regulator between the pump and the carb. So it has two in-line filters.
I run with an uprated Facet pump and a Weber 45 DCOE.

Going back to your post Mick, how long would it take to burn out the solenoid?
I would like to think that most owners would switch off the ignition once the engine stopped running.

Cheers
Colyn

Colyn Firth

Colyn,

I am not familiar with the workings of a Facet pump. I will have to look up what happens if a Facet pump is stalled.
With your filter between the tank and the pump I believe you are OK because you are a maintenance fanatic and would carry a spare pump anyway in your massive tool kit.
With the SU pump between the pump and the tank you may have a partially blocked filter, enough for the engine to run but the solenoid working overtime.

Mick
M F Anderson

Nigel, his comment on fuel filters is
"BTW, see that fuel filter, just beneath my right hand? Get rid of it. Do not use any fuel filter on your SUs. Totally unnecessary. There is nothing that will go through your fuel pump that won’t also go right through your SUs without doing any harm. Fuel filters on SUs are, quite simply, a breakdown waiting to happen. You’re better off without them. And putting a fuel filter upstream of the fuel pump is particularly detrimental, as it can cause vapor lock, starving your engine for fuel."

I am in agreement with this statement, remembering that H4 carburettors were originally fitted with a brass inlet screen in the float bowl cover and an inlet screen in the SU fuel pump (not sure what Mick thinks of these). Another point is that fuel quality, storage facilities and transport tankers are far improved from back in the '60s so particulate contamination in fuel is a lot less these days.

Clogging of an inline paper fuel filter (or the inlet screens mention above) is an indication that you have a corroded fuel tank that needs to be addressed rather than disguising it with more fuel filters.

My 2 cents worth.
Cheers
Mike
Mike Ellsmore

You are correct Mick, I do keep a spare Facet fuel pump in my spare battery box, I isn't likely that I will ever need it but I feel better knowing that I can fit it in about 20 mins and be back on the road if I do.

I did suffer with fuel starvation with this set up when I first got the car many years ago. The DPO had used clear silicone sealant somewhere in the fuel tank and a piece of it was floating around in the in-line glass fuel filter between the the tank and the pump.
It would run ok for about 5 miles and then the engine would develop a misfire which worsened until the engine stopped.
So I would switch off the ignition, check the ignition power supply connections, play about with the plugs, points, ignition leads, condensor etc etc. Then after about 15 minutes of fiddling around, it always started again and so I thought that one of the things I had adjusted or changed, had fixed the problem.
Then after another 5 miles, the problem would return.

After a few months of disastrous breakdowns, Bob West found that there was a large piece of clear silicone floating around in the fuel filter which was virtually invisible when immersed in fuel.

After 5 miles or so, this would be gradually drawn over the filter outlet and block the fuel flow.

Then when you switched off the ignition, the fuel pump switched off and so the silicone would then float back down to the bottom of the fuel filter and the car would run again.

We only found the silicone when we took the filter off the car to clean it.

It was a nightmare, it almost made me give up on the car and sell it to someone else to sort out the problem.

I'm glad I didn't though, this MGA has been a brilliant hobby for the two of us, it has since taken us all over Europe and I hope to drive it for many more years.

Cheers
Colyn
Colyn Firth

Mike,
thanks for that.

I didn't notice until your post about the H4 inlet screen filters.
Nigel Atkins

Colin

Having used Facet pumps in the past, I’m pretty sure they stipulate that a filter should be fitted before the pump.
Dave O'Neill 2

I have always used Facet pumps on my race and rally cars for best part of 50 years! I have always used a filter before the pump, to protect an expensive pump!
I usually use the CAV period type filter with replacement paper element.

Never had any issues!

Colin
Colin Parkinson

Hardi says to have a filter before its pump but I've never had one on my Q&H which I think is a Hardi and it's been on for 13 years without issue.

Now I've run out of stock of the smaller in-line plastic bodied paper filters that I changed annually, and because they're no longer so cheap to buy, I might just go back to no filter at carbs, Bs had filters at carbs but not Spridgets anyway.



Nigel Atkins

I don't entirely agree with the statement that a fuel filter is a bad idea. It will trap MUCH finer particles than the SU screens. The statement that the particles won't damage the carbs is correct, but what about the engine itself? Small, hard particles passing through the valves and potentially being scraped by the rings is a concern. That said, I don't think it's a critical issue. Another problem is that small particles can find their way into the jets and eventually clog them. I've seen this happen.

Having a clear filter helps you keep an eye on the condition of rust and debris in the tank. The note about it causing vapor lock is an interesting one, but I would want to see evidence that this is any more of a contributing factor than other things, like jet tubes directly over the exhaust manifold or uninsulated fuel hoses running through high heat areas.

I do agree with not placing fuel filters upstream of SU pumps. Facet pumps don't seem to have this problem, but an SU can be damaged. The filter doesn't have to be completely clogged to burn it out. A partial clog can do the same, slowly killing the pump as it struggles to suck fuel. I had this happen once several years ago (not an MGA) when the inlet screen clogged up with small particles. The car was running just fine at high speed, and then the pump suddenly quit. Disassembly showed the problem - lots of sediment clogging the screen.
Steve Simmons

This thread was discussed between 11/02/2021 and 15/02/2021

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