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MG TD TF 1500 - Backfire Please, Please Help Me!
|A few months back the TF developed a dreadful backfire - so bad it blew the core plugs out of the inlet manifold. After days and days of checking and resetting the timing, carb balance and mixtures I eventually stripped down the front of the engine to check the valve timing was OK -it was. Since the coolant had been drained I also lifted the head and there appeared to be some blowing between cylinders 2 and 3. I thus had the head skimmed and replaced everything using new gaskets. The car was then fine but since lots of components had been removed and cleaned I am not certain of what the culprit was. Today, however, I started the car and it was running well but after about 5 mins of tick over the backfiring has started again with flames from the exhaust pipe! The engine starts OK and ticks over well. open the throttle and it refuses to rev and backfires. I have taken out the electronic ignition and returned the distributor to points and tried a replacement coil. I have even bought a compression tester and all cylinders are the same. The static timing is set at a wisker before TDC and the jet nuts are eight flats down. The only parts that I have not replaced are the distributor cap, rotor arm and leads. The plugs are brand new but are very sooty. If the face of the block is distorted would this show up with the compression test?|
Any advice will be VERY much appreciated
|Why haven't you replaced the distributor cap, rotor and leads? I'd eliminate those as potential problems as well. Good luck.|
|A potential reason for such a backfire is that you have a valve that is not closing or seating properly. This can happen, for example, when there is a build up on the stem under the valve head which hits the bottom end of the valve guide.|
|From my own experience the rotor arm and distributor cap are more likely to cause misfire not a backfire. I have seen a failed cylinder head gasket cause spitting back through the carbs. |
To get flames out of the exhaust it needs fuel and a spark. This could be an exhaust valve not closing fully(could it have moved during early run if not tight enough) so a check of tappet clearances would be advisable. Otherwise an accumulation of unburnt fuel going down the exhaust pipe caused by a carb. float problem or a float needle valve sticking. A flooded carb would restrict revs if very bad.
I think it is very unusual to get a distorted block and yes I would have thought any further problem between cylinders would show on the compression test.
Hope this helps
|Thanks for the very prompt responses. I should have said I have not substituted the cap, rotor or leads. They were all new as were the valves when the car was rebuilt about a year ago. The puzzle is the car has actually run very well since its rebuild and has suddenly started to become a nightmare. I will buy a new cap, rotor and leads from the Octagon Club next Monday. It is possible that if one cylinder is not firing properly, unburnt mixture will be exhausted into the manifold and explode but why the explosions in the inlet manifold - so bad the core plugs in the balancer pipe blew out. One other point I have a Facit fuel pump fitted. I wonder if this is providing too much pressure? Another thing for me to return to original.|
|Jan. That being the case I think I would check the fuel level in the jets and make sure that it is not so high that it is drbbling into the inlet manifold. Also check that when the choke is in, the jets are fully up. If you remove the dashpot covers, pistons and needles, you will expose the jets. The fuel level should be around 3mm below the top of the jet - if not, bend the fork in the float chamber until correct.|
You may be correct. I have just removed the dashpot and piston from the numbers 3/4 carb (the one which was backfiring) and the throttle tube was wet with fuel. I dried this and again the fuel rose to the top of the outer, jet support tube with the meniscuseven higher. The Facit pump was clicking continuously but I think they do this anyway. I previously set the forks with a 3/8'" drill. I now propose to dig out the gravity feed petrol can I used before the tank was fitted to see if the Facit is providing too much pressure and if it is I will refit the SU. I will also set the bend in the fork by looking for the fuel level in the jets.
|Its the most accurate way to set them - the other method with a round bar is hopeless as the shape of the fork varies. If you want to do it very accurately, lower the jet until level with the meniscus, and measure how much you have lowered it with a micrometer or tyre tread depth gauge. That is the way that I did do it in the past, but not now as visual inspection seems to work well enough.|
If the car was running OK before then I would think float.
Sinking float, stuck Grosse needle or stop tab on float lever letting the lever go too low and stick ?.
I switch my fuel pump off before shutting down (anti theft) and once found the float dropped low enough to stick.
Ray TF 2884
|Ray Lee raybar2(at) tiscalidotcodotuk|
|So is/was the car backfiring or after firing? In one post you talk about core plugs being blown out of the intake manifold (backfiring) and in another flames out the exhaust (after firing). Exactly what are your current symptoms? |
As you know, All these engines need is compression, proper fuel mixture delivered to the cylinders and a good spark delivered at the proper time. The test equipment to measure and set those few parameters is relatively inexpensive.
a bit back you posted about a mysterious tapping in your head area.
I had the same soon after fitting new valves and guides. It was a valve not fully returning when hot, by the time I got the rocker cover off to check clearances it had cooled and re-seated, the excess tappet clearance when hot was making the noise.
Just a thought.
Ray TF 2884
|Ray Lee raybar2(at) tiscalidotcodotuk|
|That mysterious tapping can also be the rockers hitting the valve cover, especially if the head has been skimmed and / or its an aftermarket cover.|
This thread was discussed between 05/08/2016 and 06/08/2016
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