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 MGB Technical - Sticking valves?

I am also frustrated by the same symptoms experienced by our other MGB drivers whose car looses power and misfires after 2 - 3 miles. Mine is a 1980 LE roadster and after many months I am still struggling to find a solution. I have already replaced the plugs, leads, complete distributor (now 123 electronic type to eliminate points), ignition coil, ignition relay, petrol pump, fuel filter and checked the HIF carbs for free flow and that fuel cap is not causing tank depression but still the car runs for two or three miles before spluttering starts again.

However, someone mentioned sticking valves which made me wonder. When the engine cools down it makes an occasional 'click'. I had put this down to contraction somewhere but could it be a stuck valve returning to its rightful position? I am using unleaded fuel with Valve master additive. If stuck valves it is, an unleaded head conversion would probably be justified. Anyone had this happen?
J.F.G. Brown

J.F.G.

When this happens and the engine miss fires and looses power. Doing a compression test there an then will identifye the problem of sticking valves , I've never used an additive to my fuel other than a very old additive called Red X and then only occasionally , modern fuels should not produce residue that will contribute to valves sticking, I always use a 97 octain fuel to avoid ethonal additives which may damage your fuel system . You seem to have changed most electrical components which may be the cause of your problem so yes it could be a mechanical failure.


Regards Patrick
P.T. Tighe

J.F.G.

I did not realise you were running your engine with a leaded head naturally you will need some sort of valve additive to run the engine.

Regards Patrick
P.T. Tighe

Hello Patrick,

The trouble is that the misfiring and loss of power only occurs intermittently. Checking the compression in the garage may produce an OK result as the engine runs smoothly when started but when it has reached running temperature after 2 - 3 miles the fault returns while being driven. It would seem impossible to check the compression while on the move. I don't have a compression tester at present but may well get one to attempt a static check.

An unleaded head supplier believes the valves may have become corroded and consequently stick if the car is not in regular use (and mine isn't) but a local MG garage mechanic says he has not experienced an intermittently stuck valve in a MGB in 20 years. As the coil is getting hot, he reccommends a better quality coil - which will be the fourth I have fitted.

I know that I did have an LT ignition fault at the distributor points (which seemed impossible to cure with the replacement points currently on the market) so I invested in 123 electronic ignition. This seems to have stopped the backfiring but the loss of power remains.

Thanks for your reply - probly seen you on the Woldsways over the last 10 years.

Regards,
Joe
J.F.G. Brown

Joe,
I tend to associate back firing with a weak mixture and possibly retarded timing, I know that when I worked in the auto world and they started selling unleaded petrol we use to retard the ignition some what, my ignition is set around 12 degrees at 500 revs yours may be set around 6 degrees, I suppose you have tried running your car with the choke partially out to enrich the mixture.
I suppose you have checked that you have got your coil wired correctly, the ignition feed wire and contact breaker wire connected to the wrong terminals will make your coil heat up and cause misfiring.

Regards Patrick
P.T. Tighe

Valves will stick on cars that are not run regularly, especially if they have been in a poor state of tune.

A valve snapping closed behind you at 3AM is a real experience!

Carburettor cleaner in a squirt gun can be squirted on the valve stems above the guide. let it sit a while, crank the engine over. repeat a few times. Often works.

Sticky valves will show up on a vacuum gauge as sharp fluctuations, and with a long hose can be read in car on the fly.

And one more time - ethanol does NOT cause any problem at 10% or less. We've had it for more than 20 years. IF you have fuel system problems, it is something else.

Further, unleaded fuel has turned out to rarely give problems. Cars that run high speed for long distances, or run with late timing, may suffer seat recession eventually, but you will know, as the valve clearances will keep closing up. Neither I nor any of my customers use additives and have not had to fit hardened seats for anything but normal wear and excessive regrinds.

Well worth running a can of "fuel injection system cleaner" through once a year or anytime you suspect fuel system, or have poor running problems.

FRM
FR Millmore

One thing that's been overlooked is the pre-filter, inside the fuel tank. This can become clogged to the extent that the car will run fine, for a while, and then run out of gas. When I was experiencing similar problems with my '67, I did a fuel pump check and measured the output over 30 seconds. I blew the out the line, from the pump to the tank, and nearly doubled my fuel rate. Worth checking. RAY
rjm RAY

Joe,
Being a 1980 model your coil would be the ballast resistor type and therefore supposed to run on 6 volts if for some reason you had 12 volts perminantly at the coil it would over heat and cause missing.

Regards Patrick
P.T. Tighe

Hello Patrick,

Yes, the coil is connected the right way round. It's a 12V coil connected directly to the fusebox as the ballast resistor has been by-passed.

Ray: No I've not checked the tank filter and am not sure how to do that at present as there's half a tank of fuel in there. Its a new pump but I guess it would be doing its job if there is some sludge in the tank.

FRM: Thanks. I will think about checking the inlet vacuum but I have no vacuum gauge at present.

Thanks for all your comments,
Joe
J.F.G. Brown

Common test vac gauge is also a fuel pressure gauge, Can tee that into fuel line and read that on the fly as well. Cost like $10 at Cheep Autoparts - cheaper than typing on here!

FRM
FR Millmore

To clean the fuel tank pickup filter, you'll need access to a source of compressed air. A portable tank, that can be filled at a gas station, will work just fine. You don't have to drain the gas from the tank, but some fuel will be lost in the blow back process. RAY
rjm RAY

Thanks Ray,

I guess an inflated spare wheel would do but I'll have to think about a suitable connection.

Regards,
Joe
J.F.G. Brown

Be careful compressed air/gasoline is a bomb, can go off very easily. Remove the cap and keep air as low as possible - OUTSIDE!

FRM
FR Millmore

You don't need very much pressure to dislodge the crud that has collected on the tank pickup. 10 to 15 psi will do. I always wear nitrile gloves, when working on cars, and they make a great seal for connecting an air line to your fuel pump inlet line. FRM, is that a balm like in Monty Python's Life of Brian? RAY
rjm RAY

This thread was discussed between 16/07/2012 and 19/07/2012

MG MGB Technical index

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