Welcome to our resource for MG Car Information.
MG Midget and Sprite Technical - Mechanical fuel pump?
|Starting to think about what fuel pump to fit to my Mk1. I want to keep it looking as original as possible but as its going to have a 1275 in it Im not sure whether I should change to an electric pump. Luckily the block Im using has the hole for a mechanical pump so it would be easy to keep it original. Are the mechanical pumps reliable? Id have to buy a new one so Im wondering if they are ok.|
|A mate has just change a B engine from mechanical to an electronic electric and said it's so much better now. He fitted the Hardi which I think was probably the same as the Q&H pump I fitted over 10 years ago.|
|John, I have a mechanical pump I got for a fiver (but very clean condition) which I'm going to try in the 1275 for the Frog. I too have the pump cutout in the block and the cam on the camshaft. I don't see why it shouldn't work except I believe there's a delay in fuel delivery compared to an electric pump.|
|Mechanical pump obviously works and if you have a good one already fitted then why not stick with it but it can be bettered and if you have a choice, then well, er, you have a choice.|
|I'd say not really critical either way, all it has to do is top up the float bowl(s) at a rate commensurate with your driving style. Electrical is nice if you've emptied the system and want to hear it refill before you startup, but they can suffer from bad points, duff solid state electrics, poor connections etc. especially when positioned in pusher style under the rear of the car.
Mechanical I'd say is probably more likely to be reliable; as long as diaphram seal and valves are in good condition it will deliver fuel.
Thinking about old pumps - is there any problem with ethanol tolerance to be aware of?
|Having a mechanical pump causes a delay before the engine fires if the float bowls have emptied. Electric pump works as soon as you switch on so float bowls are full by the time you start. Therefore less wear in the engine.
Also mechanical pumps get less efficient over time leading to longer and longer cranking times.
I'm surprised the 1500 went back to mechanical. The last thing a 1500 engine wants is a long period of cranking before it fires.
Why not have both? An electric pusher at the back and a mechanical at the front. The electric will blow through the mechanical no problem, the mechanical will suck through the electric no problem. If either pack up you will still be ok. Twice the reliability?
|Rob, my last Mk1 had exactly that setup, the PO had done it as a touring mod. I never used the electric pump though as the mechanical one worked ok.
I know what you are saying about a delay while the floats fill but that should only really happen on the rare occasion they are empty. Also surely it would be better for the engine to get oil pressure at a low cranking speed before firing than instant start with little pressure?! I'll leave that for others to discuss!
The car I have now must have had a 1275 in it before I bought it and the fuel pump was just a Facet cube under the bonnet. It also came with a couple of SU pumps so I have one if need be. It didn't however come with a mechanical one so I'd have to buy one and I'm always a bit dubious about the quality of new stuff, probably a bit unfounded.
I like the simplicity of the mechanical setup, just a pipe to the front and two small rubber pipes to/from the pump. No extra wires or brackets in inaccessible places (there's no mounting point on my car either). I think I've made up my mind - I'm going mechanical!
|I believe the manufacturer went back to the mechanical as a quick fix when the government required that fuel not continue to flow in an accident. With a mechanical pump when the engine stops so does the fuel flow, not so with an electric pump which runs until the key is switched off.|
|Just a small point on the side:- I've put the original AC mechanical pump back in my 948, but the new Piper cam is only contacting approx. 3/8 of the shoe width.|
|Nick and Cherry Scoop|
|Rob's point is good. I know of someone who had an electric pump for priming on a 1500 powered by a manual momentary switch.|
Stan hit the nail on the head with his point too, about the fuel pump stopping when the engine stops. As I understand it that's the exact reason.
Don't worry about new AC Delco pumps. They never went out of production and they've just as good as they ever were. Lots of small engines still use them and they are super-easy to rebuild if you need to.
|I'd fit and electric pump. I have a facet silver top mounted on the offside rear in the same place as the post '64 spridget ones. Keeping it out of the engine bay reduces vapour lock problems as well, as cooler fuel is pumped to the carbs.|
|The fuel pump is the no1 reason for a failure that stops you down the road. The original mechanical pump fuel lines will give vacuum lock with modern fuel, unless you live in a cold climate without traffic lights or traffic jam. |
|So how do all the people get on with mechanical pumps if they suffer vacuum lock? There must be loads of people still using mech pumps surely?|
I don’t want to fit something that isn’t going to work at all!
|Flip, what is meant by 'vacuum lock' - I've never heard of it...|
|My Sprite has done 175,000 miles on mechanical fuel pumps (don't know how many). Since I've had it, we've never started badly, presumably because the float chambers have fuel in them, the fuel pipe is full to tank fuel level, and the pump only takes about 10 revs to get pressure up again.|
|Nick and Cherry Scoop|
|Mechanical pumps have been good for 100's of thousands of A series driven cars, not to mention the Triumph engines too. There is less to go wrong than on an electric pump, 'though they are generally pretty reliable too.
The principle down side to the mechanical ones is that if your float chambers are low, then to operate the pump the whole lump of the engine must be churned over on the starter motor, rather than a direct operation as with an electric one. Heavy on the battery, but then it does give a chance to prime up the oil pressure before starting.
Pros and Cons, as with many things!
|Thanks Nick, good to hear from someone with a mechanical pump that works! I still think I'll go that way even with the odd negative, as you say there are pros and cons either way.|
|sorry vapor lock, the routing of the fuel lines of the mechanical pump is close to the outlet manifold and in de warm wind of the radiator.|
|Although some Minis had electric pumps, the vast majority used mechanical pumps AFAIK. I don't think they have too many problems, that I am aware of.|
|Dave O'Neill 2|
|if the fuel pipe is routed near a heat source, then simply use some insulation sleeving.|
|My AA man (well - it felt that way at one point last year) fixed my pump on the way to the 40th (he said it was fun) by refitting the non return valve - in the dark! Then before I fitted the new pump he did it for me. Changed the long arm to short etc. (New pump was in boot ready to fit). Quote ‘simple stuff that’s fixable’|
As for filling up the dash pots - mine are generally full.
|I wonder if modern fuels are more volatile? Sucking fuel reduces the pressure on the liquid which lowers the vapourisation point. Heat makes it worse and in some cases that can lead to poor running on hot days stuck in traffic (because the fuel is stuck in a warm pipe for longer).|
|have got the original mechanical pump on my elan which has never had a problem ever--1970|
Yeah, you have to spin it over a tiddle if it's been sitting for more than a couple of weeks but that's ok
I don't know if you can still get them ,but there used to be mechanical pumps available with a priming lever to pump the fuel up if it's dry ,but why would you bother on a car with a starter motor to do it for you
|I know A35 pumps often had the priming lever, but they also came with a starting handle, despite having a starter motor.|
|Dave O'Neill 2|
|Yes, mine has the priming lever. I agree with Willy - but for the reason that getting the frogeye bonnet up is such a fag.|
|Nick and Cherry Scoop|
|I had a car once that used a mechanical pump and it had a priming lever. But that car didn't have a starter motor.|
This thread was discussed between 19/06/2018 and 23/06/2018
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.