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MG MGA - Misfire above 3000 rpm

I'm running a 1800 with HS4 carburettors everything is standard. The car struggles at rev's above 3,000 misfiring and backfiring.

I have checked fuel flow and its producing a litre of fuel in 70 seconds, this sounds enough to me but can anyone clarify the correct delivery rate?

Any other suggestions on what to look for would be appreciated.

Regards...John
J Bray

John

Just possible it could be my upside down filter trick. See here on my website, second set of photos down the page:

http://www.mgaroadster.co.uk/hot_press.htm

Has this suddenly started happening, or has something been recently replaced or overhauled?

Steve
Steve Gyles

could also be as simple as a bad condensor.
scott (aka skip)

Steve,

I had already considered your favourite upside down filters, but they were on the correct way. The car also still does it with the air filters (orginal MGA) removed.

I have recently replaced the entire engine in case it was camshaft or piston related and I've also changed the carburettors neither of which made any significant difference. Hency my question on fuel flow rates as I had a similar problem a few years ago and it turned out to be a blocked pick up in the fuel tank.

My next step is to change the distributor and I will try 'skips' idea about the comdenser. I've already changed the coil and the distrbutor cap and leads.

Regards...JB
J Bray

Steve,

Do you know what needles you are using in your HS4 carbs, I'm currently using #5

...JB
J Bray

The workshop manual says the SU pump should deliver 10 gallons (45.4 litres) per hour. Your litre in 70 seconds is roughly 51.5 litres an hour so should be fine.

Malcolm
Malcolm Asquith

John

My other problem that gave flatness and misfires above 3000 ish RPM was caused by the wrong mechanical advance being fitted in the distributor by the restorer. i.e. The cam arm he fitted was only 7 degrees, whereas it should, I believe, have been about 11-12 degrees. This meant that I could only get about 24 degrees of dynamic advance (7 x 2 + 10 degrees static) instead of 33-34 degrees (12 x 2 +10). This site shows what I am talking about: http://www.jcna.com/library/tech/tech0015.html

I think my needles are #6 - have not looked recently.

In my experience most of these types of faults are electrical, not fuel. My 10 pennyworth.

Steve
Steve Gyles

Malcolm,

Thank you for the confirmation that the fuel flow is about right.

Skip

I had high hopes, but changing the condenser made no difference.

Steve,

I suspect you are right and it's ignition related, I will try and find an alternative distributor as the next step.
J Bray

IS the mixture set correctly. Is there oil in the dashpots?
dominic clancy

Also, check that the cars are balanced and that the link nuts are tight - could be that one carb is doing all the work.
dominic clancy

I took the distributor apart and it has a 10 degree cam arm, both springs are very weak and the whole assembly is very floppy.

Having sorted out another 25D distributor with 15 degree weights and much stronger springs I timed the car by ear and it now pulls cleanly all through the rev range and has a far crisper sound.

All I now need to do is a little fine tuning. Needless to say I now have a big grin on my face!

Thanks Steve and everyone elso on the thread for your input

John

J Bray

J, Check to see that the carb bodies are tightly connected to the manifold. Any air leak here can cause backfires. I had this problem and only found it when spraying carb cleaner arount the carb manifold area. I found the nuts to be loose and the gasket torn. Oh, The carb cleaner caused the engine to idle way down and almost cut off when sprayed in this area. Good luck, Neal Turner
Neal Turner

John

I am getting most worried. That is the 2nd diagnosis I have got right in 2 weeks. It's normally one a year!

Glad you got it sorted. As a matter of interest have you had the distributor overhauled recently, or has this problem gradually crept up on you as the springs weakened?

The reason I ask is that I believe my distributor was incorrectly set up by the reconditioner last year. After a chat with Bob West, I gather that he had a number of problems with the same company and now goes elsewhere for his work.

Steve
Steve Gyles

Steve
Could you kindly contact me off-line on this subject. Probably better that way - 01273 454022.
Most grateful.
Bruce
B Mayo

open the points up a hair.
wc wolcott

Steve,

I suspect that I just picked the wrong distributor out of my box of bits, I selected the one that looked in best condition and made the assumption it was correct. Untill comparing it with another one it looked OK.

In an attempt to solve the problem I even took the car to a well known MG garage near me, they fitted a new vaccum diaphragm and tuned the car which made no noticable difference.

I now need to identify which is the correct distributor advance cam for my MGB 18V engine. I have a similar set upto your car accept I'm using MGA air filters and I think you use K&N's.

Out of interest how did you arrive at the conclusion that 12 degree cam's are required? I've looked at the site you suggested but was unable to come up with a definitive amount of advance.

I need to re fit the air filters, so will try and remember to put them on upside down so you can be right for a third time!

Cheers...John
J Bray

John

I have asked others on this site what the correct cam should be but got no response. I have just worked on the figures given to me for dynamic timing: 33 degrees at 3000RPM (vacuum disconnected). Depending on which 1800 engine you are using, the static timing varies between 7-10 degrees BTDC. Therefore, the cam needs to be 33 - (static advance) divided by 2. This will give a cam requirement in the range of 11-13 degrees. Being a non engineer, but a professional strategist, I went for 12!

Please tell me if I have got it all wrong. In the meantime my 45D Metro distributor is doing a stirling job and, as I discussed with Bruce (see above), I am not touching anything while the engine is purring! My rebuilt original distributor with its modified (by me) cam and electronic ignition sits temptingly on the bench waiting for a rainy day - no I did not mean that - I meant dry day.

As an aside, when I tried the rebuilt distributor late last year with its 7 degree cam, I just could not get it going. If I set the timing statically I peaked on performance between 2400-3000RPM. If I set it dynamically I found I could not subsequently start it - too far advanced. The noises from the exhaust (booming) convinced me that I had blown the baffles and that was the cause of my problems. Wrong. The replacement stainless steel box made exactly the same noises. I now have a perfectly servicable second hand stainless steel box sitting in the garage. Any offers?

Steve
Steve Gyles

Steve,

Thanks for the info. We all had high hopes of your new distributor and it was strange but understandable when you reverted to the metro one (I trust it's an MG matro)

When I had the timing set by a local MG specalist they set it at 3000 rpm although the problem persisted because of the faulty distributor.

Can you tell me how/what you are using to set the timing at 3,000rpm?

I also changed the stainless rear box recently, it was a Falcon one and they supplied a new one under warranty. As I've disposed of the old one I'll never know if the noise levels were due to the faulty box.

John
J Bray

John

I use this one: http://cgi.ebay.co.uk/Timing-Light-gunsonS-HEAVY-DUTY_W0QQitemZ280136786193QQcmdZViewItem

They retail normally at about 60-70. You simply dial in the advance you want to set (say 33 degrees), set the engine at the desired RPM (say 3000), point the timing light at the timing marks and twist/click adjust the distributor until the pulley mark lines up TDC. Dead easy. Even easier if you have a modified front cover with the timing marks on top.

Steve
Steve Gyles

John

As a follow-on, although I have not yet done it, you can plot your own ignition curve. Set the RPM at a range of settings, noting the advance at each setting. This is easily done by simply rotating the advance wheel on the timing light until the pulley mark shows TDC and then take a reading from the wheel.

Steve
Steve Gyles

Steve,

Thanks for the info, I'll aquire a modern timimg light. I used to modify the distributor cams, springs and fix the base plate at the moment I will settle for it running cleanly throughout the rev range and a smooth tickover.

John
J Bray

This thread was discussed between 24/07/2007 and 27/07/2007

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