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MG TD TF 1500 - Miss fire when working hard

I have had this problem for some time now, I get a serious miss fire after a prolong run at a constant 55 mph stop at say a round about when taking off miss fire starts or if climbing a long hill will start to miss when nearing the top.will run all day if speed is say 45 mph water temp 80-90 oil pressure 55. have changed distributor cap, rotor, condenser and points all supplied by distributor doctor. if I immediately change plugs the fault disappears for 50 miles then the fault returns, replace with old used plugs then again ok for 50 miles or so then fault returns I have also replaced the coil twice now on one that runs ok on another car. engine updates years ago, 1/8" off cylinder head larger valves high ratio rear axle, carb. jet needles changed from se to ap as suggested by MG Tuning specialists ignition Timing 13 btdc at 3000 revs. Rebored +.020" 1000 miles ago. Recent visit to Jersey filled with their standard un leaded, ran 100 miles with out a miss. checked fuel supply carbs always full
Could this be ethanol related? your comment would be appreciated
Adrian Wells
Thomas WELLS

I'm thinking your spark plugs are not shedding heat and you're getting a misfire. When they get too hot ignition can occur.

Try a cooler plug...?

.....
MAndrus

Your timing advance of 13 degrees at 3,000 RPM sure seems off. I would think you would need more - I run 32 or 33 degrees advance at 3,000.

But that doesn't address your description of the problem.
DLD

Adrian or Thomas,
might want to change out your wires,,, , That seems to be the next most inexpensive and easy thing to try.

Steve
SPW Wincze

Thanks for your replies, I am using NGK BP6E plugs do you know the number of the cooler plugs? I have also changed the HT leads to no avail. If timing set at 32 degrees at 3000 rpm what would that be static?
Regards to all, Adrian Wells.
Thomas WELLS

Probably close to TDC. I'd have to look at one of the published dizzy curve charts but 13 degrees at 3000 rpm would be well retarded.
MG LaVerne

For a later cylinder head (3/4" reach) the standard plug would be the NGK BP6ES. The hotter plug would be the BP5ES. For an early head (1/2" reach) the plugs would be the BP6HS and BP5HS.
See http://www.ttalk.info/Tech/Advance_curves.htm for a look at most of the advance curves. Bud
Bud Krueger

From Bob Grunau's article: http://www.mg-tabc.org/library/distributor.htm


The MG Factory recommended static setting for all distributors as points just opening at TDC ( Top Dead Center ) . However with modern gasolines and higher compression the timing can be advanced to give much better performance. Static settings of 5 or 6 degrees BTDC ( Before Top Dead Center ) are recommended with a total advance of about 36 to 38 degrees BTDC at approx 3000 to 3500 crankshaft RPM.
Gene Gillam

The symptoms you describe could be the result of ;ow voltage/poor generator performance.
Monitor your voltage with a reliable meter. I installed a voltmeter to keep tabs on our electrical performance on the road. The most critical period is driving in the dark, in cold, rainy weather, with headlights, wipers and heater humming. I can study the voltage and tweak the regulator for a fraction of a volt to suit.
JRN JIM

Dont rule out the post I made regarding low tension voltage leakage that was created by the tacho drive gearbox rotating so there was minimal gap between the 12V connection on the distributor and the gear box case.

I had exactly the same symptoms you are describing car ran fine until the motor was under load. Electricity will find its shortest path to earth when other circuitry is under load.
G Evans

Your brakes aren't dragging?

......
MAndrus

Thanks again for your comments. I checked the clearance of the low tension wiring, I have a large gap. I am now chasing the cooler plugs, this does seem the favourite at the moment. I must say after 50 years of motoring and owner mechanic this must be one of the first times I have been beaten.
Regards Adrian.
Thomas WELLS

The timing (13 degrees verses mid 30's at 3000 rpm) seemed pretty convincing to me Adrian. Be very interested in the outcome of this vexing conundrum. Cheers
Peter TD 5801
P Hehir

Adrian, another issue to be considered is your very high compression ratio, approximately 9.5:1. Your petrol could be a problem, along with the retarded spark. Bud
Bud Krueger

Hi guys, latest on miss fire, stripped and rebuilt carbs. in doing so I found the jet needles were EU and not EW as I thought, these needles are 1/4" shorter than the std. ES and slimmer so were giving a very rich mixture when throttle was well open ie. fast cruising and when pulling up a fairly steep gradient, it seems this could carbon foul the plugs giving a miss fire I have now fitted the std. ES needles and now gives a reading of 4.5 to 5.5 on my carbon monoxide meter, now cruising at 60mph and uphill and no misfire.
it seems strange as the plugs look the same colour as with the richer needles.
I will keep the weaker needles and the cooler plugs up my sleeve for future reference.
By the way the cooler the plugs the higher the number so I now have a set of BP7HS the centre electrode is thicker so dissipates the heat more effectively
I will keep you informed.
Regards to all, Adrian.
Thomas WELLS

Adrian, earlier you mentioned using BP6ES plugs, i.e., 3/4" reach. You just mentioned using BP7HS, which are 1/2" reach. Which head do you have? Bud
Bud Krueger

This thread was discussed between 26/06/2015 and 01/07/2015

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