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MG MGA - Can anyone work this out

My Mark 1 MGA has had the same problem for a few years during which it has under gone a body off rebuild but the problem continues.
At 3000 to 3500 revs it has a tendency to lose power (very noticeable) after backing off on the accelerator the loss of power clears and acceleration continues. This is not all the time.
When the car was rebuilt 3 years ago the coil was replaced but the distributor was not rebuilt. The cylinder head was converted to unleaded but the half engine was not rebuilt. The carbs were replaced with new twin SUs. After this the problem continued.
Last year I had the distributer rebuilt by the Distributer Doctor and reset to factory settings, but the problem continued.
I have just completed building a new engine and half way through swapping them over......will the problem continue???
Any ideas as to what I am missing?
Thanks Peter
P B Chappell

Peter, what do the plugs look like? Picture maybe?
Lindsay.
Lindsay Sampford

My favourite: rear air filter on upside down, blocking the carburettor internal air balancing hole to the piston.

Steve
Steve Gyles

I was thinking along the lines of "chinese" air filters like the ones that stuffed my engine! "Holding back" when accelerating and then going when you back off sounds like weak spark, coil has been changed and the distributor has been overhauled, so the next thought is fouled plugs caused by incorrect or choked air filters.
Lindsay Sampford

Peter

Just in case you are wondering what I am on about, have a look at this link on my website: http://www.mgaroadster.co.uk/odds_and_sods.htm , second item down.

Apparently it is a very common 'error'. When I was speaking to Bob West a while back he said that a large number of cars come to him with running problems and he finds the rear filter box inverted.

Steve
Steve Gyles

Is the fuel pump delivering properly? Backing off the accelerator could be allowing the float chambers to refill.
Malcolm Asquith

Had similar symptoms on two different MGAs over the years, the first car used to begin to misfire one I reached 3500 rpm or over about 70mph in top. It turned out to be the new replacement SU fuel pump I had been sold worked perfectly except for the fact it was actually for a Morris 1000! A touch of fuel starvation.
The 2nd car with similar vague symptoms used to misfire after 4 or 5 miles normal running and then the misfire would worsen until the engine cut out completely. Usually the car would re start after about 15 minutes and the misfire sequence would begin again after a further 4/5 miles and so on.
I changed numerous plugs & points, condensers,2 coils, I even soldered the points connections to eliminate bad connections, all with no effect.
It turned out to be some clear silicon that had got into the glass covered fuel filter and was acting like a one way valve. It was invisible inside the filter and it wasnt until James at Bob Wests took it apart to check that we found it..
I would therefore check the fuel flow first, ideally at the instant the misfire begins, switch off the engine and take the top off the float chambers to have a look inside. We found that the front float chamber was not filling properly and then began to investigate further.
Barney's site on MGA Guru will no doubt have the data you need to check that your fuel pump is delivering enough fuel.

Hope this helps
Colyn
Colyn Firth

It's easy to check you fuel pump with a measuring jug and a stop watch and comparing it with Barney's flow rates, but think how hard it would be with a mechanical pump! I'd still like to know about the state of those plugs and what sort of air filters you've got, where are you Peter? Not stuck under your car I hope!
Lindsay Sampford


Had this same problem with an old '73 FXE Harley (shovel head). It turned out to be a reversion problem due to an incorrect carb spacer thickness in combination with an after market, off spec intake.

The way I figured out it was reversion was when I one day rode the bike with shorts on and felt my right knee go cold between 3000-3500 rpm (about 3250). It was fuel blowing back out the mouth of the carb and getting blown back on to my knee.

So Steve's reference to filter box problems rings a bell. Anything upsetting air flow may be causing fuel starvation at certain rpms.

--Jack "cold knees" Morris
JM Morris

You could well be right Jack.
Just a little concerned about the problem of finding a someone wearing shorts who is willing to ride astride the engine facing rearwards at approx 70mph so as to position his right knee next to the carburetters.
Any volunteers?
Colyn
Colyn Firth

Thanks for the various ideas. Attached are the photos of the plugs that I took out when I striped the head yesterday to go for flowing the ports(waste of time as it had been stiched welded at some previous date so head two is now going to be converted and flowed).
The air filters are Moss "original type". I have checked andthey don't apear to have a right way up - am I missing something.
The current plan is when the engine is back in to take it to Peter Baldwin's rolling road to set it up and see if he can see anything. My thoughts were coil and dirt in the fuel but....
Peter

P B Chappell

Peter

Post a picture of the attachment side of your 'original type' Moss rear filter box and I will tell you the right way up.

Steve
Steve Gyles

Definitely lean. Check pump delivery, both from a separate feed and then from the tank. Blocked tank vents drive you nuts. Check float levels, mixture needles, and all filters. Plumb a fuel pressure gauge in so you can see it whilst underway.

FRM
Fletcher R Millmore

Peter, well it certainly doesn't look like fouling plugs, so maybe it's something to do with fuel or mixture. Your Moss "original type" air cleaners don't by any chance look like the attached picture do they?

Lindsay Sampford

Lindsay

I have probably commented before on those (old age and memory etc!), but my 'original' Moss canisters had the same filters. They are dreadful. Car ran rich, I could never lean the carbs out.

Steve
Steve Gyles

When I had them fitted to my car I had three clean plugs and one fouled one, the engine held back and misfired slightly at around 3,000rpm, and although it would get up to 70+MPH, its performance on the way was gutless. Fitted K&Ns and it now goes like a little rocked and sounds terrific!
Lindsay Sampford

Lindsay/Steve The picture that Lindsay loaded are the same as my Moss filters - do I gather from your comments that they may be contributing to the problem? What are the best alternatives - type and make?
Lindsay forgot to answer your earlier question - I am the other side of Bury St Edmunds Peter
P B Chappell

Peter

Dare I put my head above the parapet and say yes, that is the source of the problem?. Most of us fit K&N filters. Mine are the E9143: http://www.knshop.co.uk/air-filters/E-9143/prod_2977.html

I think there is a slightly better fit available as mine are a very tight fit in the canisters if you also intend to keep the mesh grill. Someone (probably Lindsay) will give you another number. The only modification you have to do with the front filter is carve a notch out of the filter casing to accommodate the air breather tube inside the canister.

All my tuning problems went away when I fitted the K&Ns.

Steve
Steve Gyles

Peter

have a look at Barney's page: http://www.mgaguru.com/mgtech/carbs/cb201.htm

He says to use the E3180: http://www.knshop.co.uk/k-and-n-filters/custom-air-filter-5-7/8inchod-4-1/2inchid-1-3/4inchh/prod_2631.html

Steve
Steve Gyles

While the filters can cause problems, would that not tend to lead to running rich or fouling! The plugs, or two at least show signs of running lean suggesting fuel starvation or too far advanced ignition?

Worth making sure the filters are ok and I can also recommend the K&N, but would suggest to continue to confirm fuel delivery is ok.
Neil McGurk

Peter, you said "after backing off on the accelerator the loss of power clears and acceleration continues. This is not all the time." Does this mean that your problem is temporary? After backing off, are you then able to rev through the 3000-3500 RPM range whiith little problem?

If so, I would think this may be related to a sticky vacuum chamber piston in the carbs.

If I am mistaken, then please ignore this post.

Chuck
Chuck Schaefer

Peter, get those filters out and fit some K&Ns (or anything else!). Those things played havoc with the way my engine ran and the engine breathing (Rocker box cover pipe breathes through the front carb filter). They led me right up the garden with plugs, timing, auto advance, mixture, I even got the distributor overhauled, all to no avail. Threw those crappy filters out and suddenly I had a proper MGA that went and growled like a sportscar! There ought to be a law against people selling those awful things!
Lindsay Sampford

Peter, see archive posting "Plug colour" 22 September 2009 and you will see some of the trouble those filters caused me. When I said "where are you" I meant where are you as regards this post as we hadn't heard from you for a while! But as it happens we are nearly neighbors, I live in Burwell, just the other side of Newmarket from you. If you are interested, we have a MGCC natter at The Bull, Burrough Green 2nd Saturday in the month at 12.30 lunchtime. The next meeting will be 17th April.
Lindsay.
Lindsay Sampford

Sorry Peter, in the post above it should read "3rd Saurday in the month", must have had a senior moment there! Lindsay.
Lindsay Sampford

Carb needles !!!!
Gordon Harrison

Thanks everyone - the filters are on their way out the door. When the new engine is in I will try with K&N and let you know how it goes. Peter
P B Chappell

Peter

Good idea. One fix at a time or you will never know what caused the problem.

Steve
Steve Gyles


Interesting that the 2 center plugs are definitely lean but the fuel mixture appears more normal in the outer cylinders. Something momentarily blocking or altering air/fuel mixture to the central cylinders. After seeing all of the above posts, I'm betting on the air filters, too.
JM Morris

You could run without any air filters, and see if that cures the problem.
If not, another possibility is a blocked exhaust. I had a 2 stroke BSA Bantam that slowly died until I removed the silencer - it was plugged with oily carbon. On an MGA it could be a loose piece in the silencer that pops up at speed. (but a bit unlikely)
Another Peter.
P. Tilbury

Peter, my carbs are fitted with inlet ram pipes on which are fitted Piper "air socks" as filters. They are so easy to remove and to clean. No wrist breaking, knuckle skinning, or swearing required!
I have checked the performance with or without the filters and I havent noticed any difference so they must flow really well.
It may be worth comparing the prices of these with the K & N types before you buy.
If you look on the other thread I started titled "Something in the water" I have uploaded a picture of my engine bay in which you can just see the ram pipes and filters.
Colyn
Colyn Firth

Colyn,
Where did you get the ram pipes, and how much were they with the filters?
Peter.
P. Tilbury

Peter, the ram pipes and Piper Air socks are from Moss ( there are probably other suppliers ) and are listed separately. Altogether they would be just under 50 approx for the two ram pipes and a pair of Piper Air socks.
The K & N filters appear to be about twice this price, (the catalogue doesnt make it clear if the K & Ns are sold in singles or pairs. I would expect that they are sold as single filters )
I hope they will be less expensive in Canada.
I really like the look of them on my car and I would not go back to the original type or K & Ns.
Colyn
Colyn Firth

Amazing this problem, not for PB Chappell of course. Is that what you would call "typical charge robbing effect of siamesed inlet ports" ? If so, couldn't it come from one wrong engine component on cylinder 2 & 3 ?
Guy RENOU

"Interesting that the 2 center plugs are definitely lean but the fuel mixture appears more normal in the outer cylinders"

This is a function of our motors having siamesed intake ports. The the outer cylinder has more "port flow time" than its inner twin and gets the lion's share of fuel running down the port floor.

As others have said, high rpm loss of power that is "cured" by coasting is usually a fuel supply issue. Some tests:

Next time you feel the power loss, back off five seconds then floor it. If it's fuel flow, you should loose power again and the timing should be repeatable.

Try pullin the choke. If the car runs fine as long as the choke is pulled, you are either tuned too lean or you are having preignition due to chamber hot spots- the extra fuel cools combustion.

Personally, I'm betting on worn distributor bushings. Worn bushings allow dwell and timing to vary. This variation is often RPM- senstive and can sometimes be "powered through" by stomping the loud pedal. Electronic ignition is the fix or you can be like me and buy every used distributor you find under 20 buck and hope one of them works better. I suppose having your distributor rebuild could work but I am against such drastic measures as a matter of principles ;-)

Have fun,
Bill

BTW- I finally gave up and converted to pertronix. I plan to use my distributor collection as wall art- I envision something similar to the "Evolution of Man" although, to the untrained eye, all the dizzies look the same.
Bill Eastman

Bill
Distributor bushes was my initial thought and I had it rebuilt. It was needed and I thought that this would fix it but no such luck! Peter
P B Chappell

Dear Bill,

"This is a function of our motors having siamesed intake ports. The the outer cylinder has more "port flow time" than its inner twin and gets the lion's share of fuel running down the port floor."

Could you please explain for which reason this phenomema takes place, I imagine this is coming either from a camshaft miscalculation or both exhaust valves localized in the middle of head producing an internal hotter spot?
Guy RENOU

Peter, as I said in my previous post, the symtoms caused by my air filters prompted me to have my original distributor overhauled, and like you, I thought it would fix the trouble but it didn't. As Peter Tilbury says, try running without the filters and see if it cures problem. If it does, you know what to do, if it doesn't, you've proved it isn't the filters and it's cost you nothing except your time.
Lindsay Sampford

Read twice, answer once- I should know this by now.

I misread your post- thought the distributor was not rebuilt.

Firing order is 1-3-4-2. Rearranged this is 2-1-3-4. It is one and four that run lean because they fire right after their "twin."

Regardless, you are running lean. Since the obvious causes seem to be eliminated, diagnosis is in order. Try twisting one of the carbs two flats richer. The plugs shold change. You are looking for an adjustment whose effect is unexpected. If you go 2 flats fatter and nothing changes, that carb or associated parts probably has an issue. This isn't a problem, it's a treasure hunt- an opportunity to spend some "quality time" with your favorite MG.

Bill
Bill Eastman

This thread was discussed between 19/03/2010 and 01/04/2010

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