MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

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 - lack of power issue

I am looking for guidance please. I have recently acquired a 1961 1600 Coupe and am just getting used to it and checking it all out. It seems to be just fine around town and sits comfortably on dual carriageways/motorways at 50ish mph but if I try to get above this speed it struggles and indeed loses power, falling to around 40 mph. Any suggestions please as to what the issue might be? I'm hoping it will be something fairly simple but would value views of the experts on this BBS. Many thanks. Tim
TJ Prime

Hi Tim

Check timing, but I suspect fuel flow. Had a similar but more dramatic problem with mine turned out to be fuel flow which was fixed with a carb rebuild and a new pump. Now it flies.

Barry
B Bridgens

I had this years ago Tim when I fitted a new SU fuel pump to my 1600.

The car ran great to about 50 mph but then the power faded away.

It turned out that my spare fuel pump was actually a Morris 1000 pump and not the HP model that the MGA needs.
Obviously a Morris 1000 doesnt quite have the same fuel supply demands as the MGA!

So it is worth checking your fuel flow Tim, uncouple a fuel pipe from the carbs and put the end into a measuring jug.
When you switch on the ignition you should get more than 1 pint per minute (10 gallons per hour)

If you are getting this much fuel flow, then the problem lies elsewhere.

Best of luck

Colyn
Colyn Firth

I don't know off the top of my head which high pressure pump is used in the 1600, but according to the Burlen Fuel Systems, the standard high pressure pumps (AZX 1331) should produce 8.4 gallons per hour @ or 1.1 pint per minute (1 pint in 55 seconds). If you you are using the LCS pump (AUA 157), it will pump 15 gallons per hour (or 2 pints per minute). If you are using the later AZX 1307 pump, it will pump 18 gallons per hour (or 2.4 pints per minute).

When checking the fuel flow, look for bubbles in the fuel stream, which would indicate an air leak somewhere between the tank and the pump. Cheers - Dave
DW DuBois

Colyn
Good advice by you as always, but couldn't help thinking your penultimate paragrapgh sounded familiar - then I remembered what my eurologist had said !!

Graham
Graham V

90% of fuel problems turn out to be electrical.

The important thing here in my opinion is to go through one process at a time, then a test run. Start off with the simple things so that they can be written out of the analysis or indeed solve the problem.

For my money a check through the ignition circuit first in a mini service. Plugs, points, timing. If that does not solve anything, then proceed to the fuel side.

Steve
Steve Gyles

Thanks all.

After I bought the car a few weeks ago I did replace the plugs and the ignition leads and I have also today replaced the consdensor/points with electronic ignition. The loss of power issue has been there from the outset and none of the electrical changes I have made seem to have improved the situation. The fuel pump was installed by the PO in 2009 and is an SU AZX1331 which does seem to be the right pump for the car. It may of course be that the pump is defective in some way? According to the receipt that came with the car the pump is described as "AZX1331 Fuel Pump 62-4 B -ve". My car is positive earth but I believe these fuel pumps are dual polarity?

Tim

TJ Prime

Dave, the original SU pump for the pushrod MGA was called the HP pump (the Twin Cam pump was the LCS model)

According to the WSM the HP gave 10 gallons per hour and the LCS 12.5gallons.

Graham, sorry to hear about your restricted flow rate :-) Just watch out for bubbles in the flow like Dave said, it means that you must have an air leak! :-)

Steve, amazingly, I have never ever had a misfire due to ignition problems on all three of the MGAs I have owned.
Every time I have had a misfire, it has turned out to be caused by fuel starvation.
So I am always inclined to look at fuel flow first, its really easy to diagnose but not always so easy to fix! :-).
Then once I have eliminated fuel problems I would look at the ignition.

Colyn
Colyn Firth

Colyn - The HP pump in the manual is now the AZX 1331 (after having gone through several dozen other designations). The older HP pump may well have put out 10 gallons per hour and the AZX 1331 may still put out that much fuel (I have found that the output of the pumps are usually better than the specifications from Burlen Fuel Systems). Cheers - Dave
DW DuBois

Oops DaveI wasn't trying to correct you, just wanted to let you know where I got my info from.

Maybe if I had noticed your email address earlier I would have kept quiet😊.

Cheers
Colyn
Colyn Firth

Hi

I'm with Colyn, having tried for days trying to find my power loss problem buying plugs, points, distributor, coil and leads I eventually found a fuelling problem. I now won't neccessarily go for electrics first.

Barry
B Bridgens

Tim

Good, that is electrics out of the way.

On the fuel side, others have given you things to look for. However, I have not mentioned this one for a while, but having the rear air filter box on upside down can have the effect of just running on one carb. I did it once and power loss with misfiring were some of the symptoms. When I spoke to Bob West about it some years ago he said that 50% of his customers who came to him with these types of problems had the rear filter box on upside down. Having it that way blocks the air balancing ports to the carb piston. See here on my site (first photos): http://www.mgaroadster.co.uk/odds_and_sods.htm

Steve
Steve Gyles

Colyn - "Oops DaveI wasn't trying to correct you, just wanted to let you know where I got my info from."

Not a problem - I'm as prone to make a mistake as the next person - perhaps even more :-). I do have some literature on the various pumps from the old days that I intend to check to see if they will shed some light on the discrepancy.

I just got a book on the SU company from Burlen Fuel Systems, but have not had time to read it. Interestingly, the SU company was part of the MG company for a period of time - it apparently bounced around through a lot of different companies over the years - just in my time with MGs they bounced around through several different companies before they came under BFS. Cheers - Dave
DW DuBois

Thank all. Steve - I'll check the air filter. I made that exact mistake myself on my AH Sprite air filter just last year- it is indeed very easily done and it definitely affects performance. Tim
TJ Prime

My brother once took his Marina TC to the SU service department in Birmingham, for a tune up. When he got it back the air filters were upside down!
Dave O'Neill 2

Tim
The one electrical item you did not mention having changed is the coil - a notoriously dodgy bit of kit, particularly modern look alikes. It's essential you get that item changed as well and then you can proceed to other elimination in a logical manner. Imho.
Best
Bruce.
B Mayo

Thanks again to all. New coil on order and I'll try this out later in the week at the same time as checking the pump output flow and the air filter bolts. The car has what I believe are velocity stack air filters which I haven't come across before. Cheers. Tim
TJ Prime

Dear All. An update. Checked the fuel flow per Colyn's suggestion and the flow was only about an eighth of a pint per minute. A lot less than the expected pint per minute and therefore I guess this must be the culprit. I am a bit surprised that such a reduced flow would permit the car to perform acceptably at low speeds but that does seem to be the case. Should I be?

My next step is to buy a new pump, install it and hopefully the car will then fly! I've replaced a mechanical fuel pump before but not an electric one - any particular tips? I'm hoping to be able to do it from the hatch behind the seats or might it best be tackled from below?

New coil also on order but I'll see how a new pump affects the performance before doing anything with it.

I did also follow Steve's advice and check the air filters. The car has stacks (see photo) and all are the right way up but its hard to see how air can get to the air balancing ports through the miniscule apertures there are. The gaskets are quite old so I probably ought to replace these at some point.

Tim




TJ Prime

Before you fit buy a new pump have a look at the old one, assuming it is the SU type pump it has a built in filter in the inlet valve which may simply be clogged up.
Also,you may have an after market in-line filter hidden down by the tank like mine does.

My after-market filter was partially blocked by some clear silicone which was invisible inside the glass filter body but which restricted the fuel flow so much that the engine cut out after a few minutes of driving hard, although I did find that if I drove at under 20 mph, it would run all day!

When you take the pump off (which is much easier done by jacking up and taking off the RR wheel) try blowing through the fuel pipes (forwards to the carbs and rearwards to the tank) to make sure they are not partially blocked.

best of luck

Colyn
Colyn Firth

Not sure if I'm looking at the photo correctly, but it doesn't look to me like the velocity stack has a balance hole, just an indentation in the casting.

I'm also not sure the stack has been fitted the correct way round, as the radiused edge has a gasket on.
John Bray

Would have expected something similar to this


John Bray

I agree with John. I can't see all the way through to the tartan rug. That is going to contribute to the problem. Is that stack meant to fit inside the canister? If so it's unlikely to be very efficient. Not enough curvature. We designed and manufactured a version on this BBS. See here: http://www.mgaroadster.co.uk/Stub_Stacks.htm

Steve
Steve Gyles

John - I think you are probably right that it is just an indentation in the casting. Would you seek to adapt by boring holes that line up with the balance holes? Tim
TJ Prime

Steve

No, see attached photo.


TJ Prime

Drilling through is essential in my opinion. With your configuration this will also affect the front carb, both need to be drilled out. You have both carbs running sluggish and fuel mixtures all over the place at any rpm.

What is your rocker cover vent pipe connected to?

Steve
Steve Gyles

Tim

Back home now. My money is with with Colyn on partial fuel blockage. I think your low flow will cause your symptoms and is unlikely to be the pump as SU pumps usually either work or don't. An explanation as to why it runs fine and then doesn't could well be caused by the 'gunge' sucking forward when you are requiring more fuel and then dropping back when you stop.

Also as Colyn I have had problems with getting sealant in the pipes. The reason for this is that a problem with MGA tanks is that it can be difficult to seal the side mounted sender unit. If when fitting you use too much sealant or wrong type excess gets in the tank and hence the pipes. Basically make sure all pipes and filters are clear. The issue is most likely to be pipe between tank and pump or filter in pump. As Colyn says the blockage in this case will be transparent hence nearly invisible.

Although I as Steve normally 'think electrics' this doesn't sound like electrics to me but ...

The discussion on stub stacks is interesting but doesn't seem to fit your symptoms. This can easily be proved one way or other by just removing them and driving without stacks or any type of filters and see if problem is still there. I have driven many miles with 'bare carbs' with no issues so wouldn't worry for a test. You have great advantage here on some others problems in that the symptoms seem rather back or white.

Paul
Paul Dean

Tim
just to pick up on something that Steve said earlier.
The vent pipe from the valve cover would usually connect to the air fiter casings.
Fresh air is normally drawn from the filters down through the valve cover, crankcase and out through the tappet cover downpipe. This seems to be the best system

Make sure that it has not been connected into the inlet manifold as this would really mess up the mixture of the carbs.

(Early MGBs had a positive crankcase ventilation system that did use pressurised air to ventilate the crankcase but they need a special PCV valve and a connection to the inlet manifold to work)

Like Paul suggested, try running the car without any stubstacks or ram pipes to see if things work ok.

It looks like your stubstacks have mesh filters across them, I was persuaded to remove the "tea-strainer" mesh filters I had across my inlet ram pipes as the engine tuner (Peter Burgess) told me that they were creating a 3 bhp power reduction.

The SUs on my last 1850cc engine ran with long ram pipes and no air filters, it had a non vented alloy valve cover so there was no vent pipe from it.

Colyn
Colyn Firth

Paul

I don't think this is an 'either or' situation. It is most likely a combination of both. The air balancing pipe ways to the tops of the vacuum pistons have to be able to breathe but they appear to be totally blocked on the evidence so far presented.

Have I read on this forum before that those style stub stacks may look the business but without air filtration there will soon be substantial scoring on the vacuum pistons?

Steve
Steve Gyles

If you would like some air filters Tim I have a couple of spare Piper sock-type filters that will fit your ram pipes and you are welcome to have them FOC.
Email me if you want them.

Colyn

Colyn Firth

Dear All

Thanks for recent responses. I attach a photo showing the vent pipe from the rocker cover. Not that good a photo but you will hopefully see that the pipe doesn't go anywhere but just finishes down against the heat shield.

Colyn - thanks for your kind offer. I will email.

I'm pretty overwhelmed by the offers of advice/help on this BBS - thanks very much.

I'll try and get round to some of these tasks over the next couple of weeks - just need time in an already busy schedule!


Tim




TJ Prime

The reason I am voting for fuel pipe blockage rather than stubs/rams is finding a way to explain when the problem hits the speed actually drops rather than just refuses to go any higher which is rather odd. I can though see how a fuel blockage could cause this. There are also electric problems that come and go, e.g. condenser, but these either work or don't. Clearly I may be wrong.

Clearly it does look as if there may be a problem with the stub stacks. I have never had these or put the rear filter on upside so have no experience here. Surely if both stacks have no opening the engine wouldn't run at all?

Tim - If you just want to go back to standard filters I have a couple of spare ones if you want them. By the way I run slightly modified K&N elements in standard bodies.

Paul
Paul Dean

Paul - I'm new to these stub stacks too. Photo attached for info. Cheers. Tim


TJ Prime

Paul

I repeat. The ram pipe lack of drilling through the casting to the air holes is wrong and WILL have a major effect on performance regardless of other issues. i.e even when fuel flow is sorted the carb vacuum pistons will not operate correctly until the air passage to the top of both the pistons is drilled through the inside of the stubs.

In fact, looking at Tim's latest photo of the stubs I am not convinced they are suitable for our carbs.

Steve
Steve Gyles

After further study of that latest photo and comparing to John Bray's photo you can see that the two bulges in the casting adjacent to the mounting bolts are not drilled through (external drilling of stack required, not internal as I said before). Carb pistons will definitely not be working correctly. Worth a test drive with stacks removed. Drilling through will solve that problem but will allow unfiltered air into the piston chamber and in the long term will likely cause scarring to to the piston and chamber. In my opinion remove them and fit something else.

Steve
Steve Gyles

Steve, the H4 carbs that I ran on my 1850cc motor had ram pipes fitted and they were fairly similar to those on Tims engine (albeit a little longer). But unlike Tims, they had trumpet style bellmouths that opened fully outwards.

They are in my garage, I will have a look at them tonight, take some pics and see how the balance pipe holes compare with those on Tims ram pipes.

My ram pipes didnt have the heavy mesh over the bell mouths though and I know that this mesh does reduce power a little.

The mesh I believe would have the effect of enriching the mixture very slightly.

Balance tube connections aside, ram pipes shouldnt have any detrimental effect on power output unless they are smaller in diameter than the carb venturi.

They are supposed to enhance power at certain rev bands.

Will post some pics of mine tonight when Im home from work.

Colyn
Colyn Firth

These were some Les Leston ram pipes that I had some years ago. You can clearly see the air holes for the piston chambers.


Dave O'Neill 2

Colyn

The performance of ram pipes is not the issue here nor the filters. We all know that any filter system will have an effect on performance. The issue here is the operation of the working parts of the carbs. We could do with an internal picture of Tim's stacks to see if there is an internal airway tapping to the hole on the carb manifold. If so, all is well. If not, "we have a problem Abingdon".

Steve
Steve Gyles

Personally I wouldn't run ram pipes on a road car, they don't offer any filtration. It would be interesting to know if the carb needles have been changed to match the ram pipes to prevent the car running to lean.

Better of with a pair of air filters ideally fitted with Steve Gyles style stub stacks and K&N filters or the stub stacks that were fitted inside the MGB filter canister but they do require a little fettling to fit.
John Bray

I have had another look at Toms earlier picture of the flange of his ram pipes.
They do appear to have little cut outs in them which are designed to allow air to flow through into the vent holes in the carbs flanges.

However, Tims picture appears to show that the gasket is blocking these cut outs and like Steve says, this will really mess up the running of the engine.

So Tim, make sure the flanges are fitted the right way round and not covered by the gasket.

I managed to take some pics of my ram pipes and they are quite a lot longer than Tim.
They do though have similar cut outs to suit the carb flange vent holes.
I have tried to take some pics but I am struggling to get thme small enough to upload.


Colyn Firth

A pic of the flange of my ram pipes showing the cut outs in them.

These pipes ran really well on my car and I used Piper sock filters on them to prevent dust ingress

Colyn


Colyn Firth

Colyn

They look the same as Tim's square cutouts from what I can see in his earlier photo. The question though is there a drilling from that square cutout to the inside bore? If yes, then fine. If not then there is no breathing to the top of the piston.

Steve
Steve Gyles

On my pipes the cut out sections extend sideways into the carb venturi.
On Tims there doesn't seem to be much evidence of this.

Can you remove the gasket Tim so that we can if the cut outs are actually there or not?

Cheers
Colyn
Colyn Firth

Colyn. The cut out section on mine is identical to yours. At least it was as there are now newly drilled holes matching up with the holes on the carbs. The piper socks will hopefully cover these and prevent dust from entering. The fuel pump filter had some grit in it but only a very small amount and I wouldn't have thought enough to cause blockage but I'll need to give it a test drive to see if any change. It is now cleaned up and back in place. Very wet morning in Edinburgh!
TJ Prime

Steve

You seem to be misunderstanding me. I am not saying there not being holes in the stacks isn't a problem it certainly is. What I was saying is I think a fuel blockage is more likely to match the quoted symptoms.

In fact if there are no holes connecting atmospheric pressure to the holes in the carbs through the stacks,, and hence to the void under the piston, I would have thought it wouldn't be possible to get the engine to do little more than tick over. This is because the partial vacuum above the piston wouldn't be able to lift the piston significantly as the void below can't fill with air. Clearly in the case of the rear filter being upside down the engine will run on one carb.

Paul
Paul Dean

A wee update. Air blown through inlet and outlet pipes to clear any gunge but still no improvement in the fuel flow. Listening to a couple of SU fuel pump videos on youtube it is clear that my fuel pump sounds nothing like it should so I've come to the conclusion that the pump is the issue. I'll order a new one and hopefully that will do the trick. I'll update once installed. Cheers. Tim
TJ Prime

I changed the fuel pump this morning and all is looking positive. I haven't had the opportunity yet to take for a drive but the new pump is producing the flow of fuel that it ought. Thanks to all for your advice. The filters at the carbs were also a bit clogged up with dirt and so these have also been cleaned up. Cheers. Tim
TJ Prime

Good luck...thanks for the update!
Gene Gillam

Sounds promising Tim,
I will keep fingers crossed that your new pump sorts out the problem.

I havent forgotten the Piper filter socks,I have been away from home a fair bit over the last couple of weeks.
I should be able to sort them out for you over the next few days.
Cheers
Colyn
Colyn Firth

Thanks Colyn. Regards. Tim
TJ Prime

This thread was discussed between 08/10/2016 and 03/11/2016

MG MGA index

This thread is from the archive. The Live MG MGA BBS is active now.