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MG TD TF 1500 - Missfire
|I am chasing an intermittent missfire on my XPAG TF.|
It can occur hot,cold or anywhere in between.
It sounds as though it has gone onto 2 cyls but by the time I stop and get the bonnet open it is merrily ticking over on 4!!!!!.
So far new- points,condenser,dizzycap,leads,plug caps,coil and plugs.
I thought it may be sticking valves but there is no ticking you get when a valve does not seat. I have a later dizzy so it is not the reduction box shorting it out.
Any ideas as I am stumped.
Ray TF 2884
|Ray, define your 'later dizzy'. If there's no outside terminal, it's not a standard ignition. Bud|
It is a later Lucas dizzy I bought from Brown and Gammons in 1996 (27H 5138) which they still sell. It is modified to the correct advance curve and was(is) a popular mod over here.I am more interested in my car running well rather than being period correct.
I have had it off the car and it checks out OK.
|Worn valve guides? |
Hot or cold, they can slip or stick on the valve.
When was the last time they were changed out? And what kind were installed?
|Have had sticking valves before and they usually make a slack tappet sound when it happens. I renewed the seals recently and checked the valves were free and not worn. I have a valve spring compressor I use without removing the head.|
|Does it occur at a certain rpm? It might be worn needles in the carbs causing a mixture change. Is it an acceleration issue. Maybe check oil levels in the dashpots as that may influence the mixtures as well.
The moment the misfire occurs shut the engine off immediately and pull the plugs and look at them are they consistent with one another?
Try a fresh set of plugs. You might have a bad one. It happens
Also check all your fuel filters for debris. Carbs, pump(s) and tank check to see that the bowls have the same level of fuel. Which may indicate one of the carb filters is fouled. Look for debris in the bottom of the bowls as well.
I would be looking more toward fuel delivery issues once you ruled out a faulty plug. Given the checks youíve already made to the ignition side
Having been fighting my own gremlins I know how frustrating it becomes when the obvious doesnít seem so obvious. Good luck keep us posted on your progress. Also make only one change at a time and document your results. If what you did doesnít change the problem put it back the way it was and try something else. That way youíll know exactly what the issue was and can refer to your notes in the future should the problem pop up again. At my age I find I donít remember as much as I use to.
|W A Chasser|
|beware of brand new faulty capacitors !|
better try an old one.
|Carbs and fuel lines OK ,bowls full. Tried external capacitor and it still happened. No rhyme or reason to it, it will sometimes run for days with no problem.|
I have been through the change one thing at a time regime as I was taught during my apprenticeship in the mid 50's when this car was new.
It will not beat me.
|Ray, do you have solid core ignition wires? They cause misfires in electronic ignition systems.|
Sorry to sidetrack the thread but where did you get the tool which allows you to change the valve seals without removal of the head ??
|J A Davies|
Jumping in here -I have an old screw up style valve spring compressor that I've been using for years and it works great
I had to extend the two hook ends with a touch of weld by about 1/8" so they would pick up the inner springs as well but it's been my main valve spring tool of choice for several years
Best thing about it is that it holds the springs and cap compressed without having to hold anything, giving you easy access and unlimited time to fit the 'O' ring seal and keepers, then simply unwind it
You could possibly pick one up on fleebay BUT it will need the little extension job done to it to reach the inner valve springs
I have a later dizzy so it is not the reduction box shorting it out.
What is the fact especially?
|Tim, solid wire and old fashioned points.|
John, it is one I made myself,the valves are held up with air pressure. pic posted
W. Mueller, it is a newer type distributer and the low tension lead enters through a seal
|Drawing posted if you want to make one, the handle needs to be bent up at 45degs to clear the carbs, I never thought of that when bench testing.|
|Sounds like an ignition issue to me, though it seems you checked and replaced almost everything. The only thing that I can think of that is not mentioned above is the rotor arm. Needs to be clean, seated correctly, good contacts, no tracking, etc, as you know.|
|I think Dave might have the answer. The rotor seems to be the only link in the chain that you haven't replaced. Good hunting. Cheers|
Peter TD 5801
|sorry, should have said new rotor arm checked in another engine.|
I am going to have another look at the valve spindles and guides.
|It sounds like a fuel supply problem to me, like a partly blocked foat bowl filter or low fuel pressure/supply that has time to refill the bowl by the time you get to pull over to check|
|I thought of that and switched off the moment it happened and found both float chambers full, no crud anywhere in system. The pump shifts the right amount of fuel at 3foot head and all filters clear.|
The only thing I have not done is fit my complete spare dizzy (original type).
Can't take it out on the road yet as the salt/gritters have been out.
1" of snow and the UK shuts down!
Thanks for the photos and the drawings
I now know what I want to make
|J A Davies|
|Check this out first, I spotted it on eBay uk. Overhead valve spring compressor - removes springs without removing the cylinder head. Inexpensive as well. Not tried it, but am tempted to buy one myself.|
|Thanks Dave, that makes my one look Heath Robinson!|
I have ordered the one with lever/scissor action should make checking the valves a doddle.
|I've ordered one and will post a photo when it arrives.|
|Hi Dave |
Look forward to seeing your pictures
|J A Davies|
|I'm hoping the owner of the photo won't mind as it's promoting the product.|
|"found both float chambers full, no crud anywhere in system."|
Given the above this probably isn't the issue but what's happening could be the result of a loose connection to or bad ground for the fuel pump. A momentary shut-off would probably be masked by the reserve in the float bowls so I can't defend my suggestion but it may still be worth consideration.
|J. K. Chapin|
That looks exactly the same as mine, but as I mentioned earlier I had to extend (weld)the ends of the tangs so they would locate the inner springs, BUT, looking at the pic yours appear a bit longer than mine was when I got it so it might just do the trick as is
If you give the spring cap a little tap down on the edge with a small hammer before squashing the spring up it will release the keepers from being jammed in their taper and it all comes apart great
|Try these 2 tests:|
Make a direct connection from the earth battery post direct to the block,
Bypass the ignition switch feed to the coil by making a connection direct to the supply battery post.
Strange things happen to electrical connections when they are placed under load. An ancient mechanic advised me many years ago to always earth direct to the block.
From experience I have also found incorrect viscosity oil in the carby dash pots can create this symptom.
Tried that already. It can only be a dizzy fault or sticky valves.
I will leave it now until the snow and crud is off the roads.
When I find it I will come back to the BBS.
Presently having fun fitting a modern seal to the rear of a friends XPAG
|Are your valve guides the original material, or the bronze variety. Also, do you have valve stem seals, not the O rings, on the exhaust valve guides.|
|R A WILSON|
|I just had a miss at road speed. Off then on.|
After a tough going over, found under the dash, the main wire grub screw on the key switch was loose.
This would be like turning the key off removing power to ignition. When connected again, a back fire would occur.
After securing the wire, I put a dab of liquid electrical tape on the screw.
Went for a 20 mile run without a miss.
silicon/bronze been in for years no problem, "o" ring seals to allow some oil to the guides
still happens with a direct feed from the battery.
It can happen when pootling about or thrashing it.
|Is it possible to get it on a rolling road "dyno" with an ignition analyzer attached?|
Might not be cheap but may take you right to the source of the issue.
|P G Gilvarry|
|I can't see the image of M Grogan.|
Something with my browser or something with the image?
|Just a pic of the liquid tape bottle|
removed the valve springs with the head still on the car. The valves slide up and down freely and spin at all openings. Valve stems in ports inspected with an endoscope, no hard deposits or spalling.
I have rigged up a spare coil in the battery box with all the leads attached and the next time it happens quickly switch over.
While I originally said 'missfire', it is more like it drops to 3 or 2 cyls.
Ray TF 2884
Could there be some foreign rubbish floating around in the bottom of one of the float bowls that gets sucked into a jet-tube
Could be something in the passageway down there--just a thought
I've found pieces of 'O' rings trapped in the bottom of MGB carbs blocking the flow
Might pay to pull the dashpots off and lift the floats out and blow back from the jet towards the floatbowl with an air supply and see if anything comes out
|Wild guess here, but check the bolt at the bottom center of the intake manifold. These have been known to loosen or even fall out causing the divider baffle to move about. The result is an imbalance in the feed to the cylinders. It could be that at idle, there is not enough of an imbalance to cause a problem, but at speed, it could cause one.|
Easy to fix, but hard to spot and diagnose.
Carbs clear, I rebuilt the carbs when this started but the fault still appeared.
also checked the baffle.
Now I know there is nothing nasty wrong with the engine the worry has gone. It does not go on for long and I will find it.
I have put my spare dizzy in but can't go for a run because the roads are salted again.
I'm in summer mode here but your mention of salt reminded me of something
During winter, if I drive off in our Elan and head off onto the main road, I get about 2klm out the road and it starts to pull back and missfire, pull up on the side of the road and turn it off and within a minuite or two it will start up normal and drive away happily all day afterwards--Turns out it was freezing the carbs up and the telltale was that if I pulled up and looked under the bonnet straight away the carbs were dripping wet
Might be worth a look---------
It has been happening all through the summer so not that
You post reminds me how far south you are in Tasmania. We get a mindset of Australia as being all "Home and Away" and "Neighbours"
|Interesting about carb freeze ups. I bought a new Ford Ranger pickup in 1986, 4 cylinder engine. I still have it! Ever since day one when first started in the morning and after driving about a mile, it would start to miss, cough and spit back through the carb. Found out it was the design of the carb which would allow ice to form on the jet, pullover shut the engine off for a few seconds and restart, it ran perfect all day. They finally got it to run better by increasing the heat flow on the intake, but to this day that problem persists to a point. We only use this truck around the farm anymore, hasn't been on the road for 10 years, but still running. PJ |
|I had a 1954 TF 1250. On a road trip to Georgia on the interstate it would drop two cylinders but the moment a large truck would start to pass it would cut back to four cylinders and run fine until the large vehicle passed completely and then drop two once again.|
It was a small burr on the dash pot that prohibited the pistons' movements. Once cleaned and all was well.
Could it be possible that the trucks created a slightly heavier air pressure and that changed the fuel-air ratio?
|Must admit I never though of a sticking piston. As I said earlier I had rebuilt the carbs but another check wont harm.|
|Hi Ray, I had the same fault as you describe struggled for 18 months, eventually tracked to faulty plug caps, the originals were faulty giving me the original misfire I then replaced with champion caps and fault still there I eventually, after replacing everything as you do I checked the resistance of the caps, should have been 10000 ohms two had virtually no resistance the other two had massive resistance. I removed the caps fastening the HT leads directly to the plugs. The car was then unbelievable it ran absolutely perfect as it should. now fitted with Bosch caps without resisters bought from fleabay. Have also ensured the rec counter cable is clear of the dizzy electrical connection. Also try running in the dark with the bonnet open looking for tracking.
Let me know how you get on.
tried that, ran without caps but it still happened.
If it is not the carbs I will open up the plug gap as I have an electronic points assist which I know will handle it.
New plugs, points,rotor arm, condenser, leads,dizzy cap,plug caps and coil all check out on test, so now thinking it must be fuel.
Got 6 weeks or so before the salt is off the roads to find it.
Although you have rebuilt the carbs, it still might pay to check the jet supply from the float bowls by blowing them out from the jets backwards It would only take one troublesome floating piece of rubbish in there to cause an intermittent blockage
|Electronic points assist???|
|Had a friend with a TF some years who had misfiring issues. He had the carbies rebuilt twice resulting in no change with the problem. He finally worked out that the overflow pipes were not clear after they were soldered|
into the brass fitting above the fuel bowl lid. Just a long shot here.
it is an electronic system by Boyer Bransden.
The points only trigger the system and have no load. The only down side is that because there is no load you have to make sure the points are clean.
Easier starting and very smooth low tickover.
As usual, no connection to the company.
new glasses or spellchecker needed ;-)
|Have you tried running without that? If not, give that a shot. George|
that was the first thing I tried as it is only a couple of connections to change.
I am now convinced it is in the fuelling.
|Ray L .,you may be right ! However what actually happened in answering the |
Thread was I noticed in the initial, first comment I made was the word 'babies' instead of the wanted carbies. Corrected it somehow but it would not let me delete the second comment made-too late. Ugh !
|A few decades ago my TD with stock distributor would misfire at certain RPM around 3000 as I recall. Turned out to be a worn distributor shaft which would not hold the dwell at certain RPMs. Fixed with a Crain electronic ignition which was not affected by the slight wobble of the distributor cam and shaft. Been running fine with solid core wires ever since.|
|John Quilter (TD8986)|
|Ray, my 18 month search for misfiring and sometimes completely shutting down eventually led me to the fact that one of the needle valves in the float chamber was sticking shut. A small tap on it, or jerk to the car would free it so every time I looked, the float chamber was full. Except for once. Cleaned the seat of the valve with acetone and itís been fine since. I blame ethanol ... 😱|
|That is another thing to look at. As I said no hurry now as the gritters were out again yesterday. Everybody seems to have forgotten how to drive on a bit of ice.|
|May be solved,
It started to do it while warming up in the garage. I quickly switched off and as I opened the bonnet I heard a slight clunk. Took off the front air cleaner and pushed up the piston to the top and it stayed there. Took off both bells and slides, a quick blast with brake cleaner and both now drop with a good clunk. It was not the needle/jet out of alignment just a very sticky chamber
I think I may have been chasing two unconnected problems and had cured one only for the other to confuse me. At least I hope so.
Ray TF 2884
|Also have a good look at the needles - I noticed on one of mine a hard build up of black material at the base of the needle next to the piston and it took a scratchy pad to remove it. If left there it just might stop the needle and piston from closing properly.|
there was some crud around the needle but it was a stickiness in the bell causing the problem.
The roads are bone dry now so I am going to go for a run and hope this is finally it.
|Well that is 50 miles of motorway, lanes and town roads, no blips. About 15 miles of motorway at 70mph (3500 revs) rad blanked to keep up the engine temp.|
Maybe I can bring this to a close now and just wait.
Ray TF 2884
|Congrats...and thanks for keeping us posted on the results!|
This thread was discussed between 11/12/2017 and 16/01/2018
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