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Engine Serious Missfire Under Load


Henry Boler
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Hi All,

 

Not sure if this is the right place for this question, but here goes anyway.

 

When my GT6 is under full load at high speed for more than about 5 seconds, it starts missfiring, spitting and popping from under the bonnet, and when is pops I can smell fuel so it's a front fire, or whatever.

 

I currently cannot work out what is causing it. I have taken the fuel system apart, blasted out the fuel lines, cleaned the filters, checked the fuel pump, cleaned the carb valves which were already clean anyway, checked the fuel flow, all appears really good to me, still failing though...

 

Do the Stromberg choke units ever cause issues like this? Mine is a single shared choke unit. Also the air bleed valves? Haven't had a chance to look at either of these bits.

 

I've checked the timing, currently 13 BTDC, but I'm sure that is what it has always been at and the delay part makes me think it isn't timing, but I could be wrong. Also changed the rotor arm and cap.

 

I'm currently up in Manchester 250 miles from home so I don't have any spares or tools with me but if there is anything obvious I can try whilst I'm here, I might.

 

The car will cruise at high speed all day, It's not until it starts climbing a hill or I need to overtake someone quickly that the issue starts, and I get the 5 seconds at full power and then have to back off once the issue starts again...

 

Do these engines suffer with sticky valves or anything like that? Or do the coils go funny after prolonged high revs, seems unlikely... It has been doing this for a few weeks now, so I don't think there's an issue getting home or anything, but it would be nice to solve this!

 

Any Ideas?

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The manual says 13 BTDC but that is for 5* fuel which is difficult to come by these days. Mine is around 7 BTDC. Easiest way to reset it is slacken the dizzy clamp bolt and turn the dizzy for maximum revs then back slightly. Take it for a run and see how it behaves.  I'd try that before stripping anything down. Also check the vacuum line to the dizzy is connected and working.

 

A friend of mine had badly worn teeth on the timing chain gear and the timing went slowly off over a week or so until the chain finally stripped the gear's teeth completely. Hope t's not that!

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Yeah, funny you mention that as I did wonder why it was exactly bang on what the manual said having had the discussion a few years ago about 95 octane timing. Maybe I'll try that before I drive back.

 

I'm not sure it would be valve timing as it runs so well. But having said that, would there be any obvious noises as a result? I do have an interesting noise that has been there since I changed the timing cover oil seal a while ago. I wasn't sure if the tensioner had gone back in properly, but it has run fine ever since...

 

Henry

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under full load at high speed for more than about 5 seconds, it starts missfiring, spitting and popping

 

 

spark brek,n doon under load is my bet

look at plugs, leads, rotor arm, condenser,coil, dizzy cap for traces of arcing,or burning ont electrodes.

 

look at yer dizzy rotor / moveable shaft, is it moving freely, so as the mech advance is working

note, closing up of points will alter timing figs

 

Lack of fuel will also doo this, as its running weak

Yer diaphragms ok,

non returns ok,

sure yer pump is OK, checked the lever, is it still there, they doo wear away!!!, which will give less fuel pumpage

 

Sure its not 10 degs either,  not that this will be much t,mek it flutter lower int rev range.

butt, will be at max revs .

 

 

M

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Some good questions.

 

The old dizzy cap center electrode was burned out, but that's no longer an issue. Haven't looked at the plugs or the coil, leads look fine, and are only 2 years old. I have an electronic discharge unit on the points to stop them wearing out, but it still has the issue with it switched off so I don't think it's that, unless the wiring is iffy, I could unplug it completely to test.

 

Fuel pump is fine, took it to bits to see and the levers are all fine, I've had to do emergency repairs on the diaphragm link before with a welder as I got stranded down in devon! Fuel delivery seems fine, on idle it fills a jam jar at the carbs in only a few seconds...

 

I might just tweak the timing tomorrow morning before I leave and see if that helps me at all.

 

Henry

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Henry.

 

I do not think this is a fuel or timing issue; it sounds more basic than that.

 

The fact that you can do about 5 seconds of heavy load driving before the issue occurs points to an electrical breakdown in its early stages.

 

Does your assisted points system run with a condenser ?? Are your points gapped correctly, bearing in mind they will be working ten to the dozen at that speed and load ?? Points tend to be problematic through the rev range; whereas a condenser starts giving up when under strain - such as you have described.

 

Even though the HT leads look fine and are only 2x years old means absolutely nothng these days with irregular quality.

 

Finally I always have my doubts about such items as electronic discharge units. You are either running with unassisted points or you are running with electronic ignition. Running discharge units just adds another potential problem to ignition systems; do yourself  favour and get a proper electronic ignition. It does not have to be a 123 unit or similar, I run my Vitesse with a straight forward SimonBBC electronic ignition.

 

I think if you are doing the distances and at a speed you mention this really should be a priority for you.

 

Replace your condenser and points, and I think your problem will be rectified and also worth bearing in mind checking the ohms on the HT leads and that also includes the king lead.

 

Whilst doing this, check your low tension connections and also the heat of your coil after a run - it should be warm but not hot.

 

Hopefully the above will solve this problem for you.

 

Good luck.

 

Richard.

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That's interesting to know as I didn't know you could have a delayed electrical breakdown like that.

 

My assisted points system is a Sparkrite SX1000, inductive discharge system, takes the load off the points so they don't burn out every journey which got tiresome! It has a condenser but I believe that's only there so that when you press the off switch on the unit the system can run as normal which requires the condenser. I could take the unit off completely and drive it home to see if that helps, the car runs fine on points as normal, just get fed up of constantly having to file the arcing off!

 

Two reasons I never went for electronic ignition are one that I had the sparkrite lying around from an old car, also if it fails, I still have a working conventional ignition system waiting at a flick of a switch which appeals, and for the last two years it has been just fine.

 

I'll have a good play with the system when I get down south and try different leads and coil etc. until I find the fault!

 

Henry

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Misfire under load has all the hallmarks of a duff sparkplug

 

Henry you mention points burning out on every journey thats not good

are you running a 1.5ohm coil on 12v this burns points as current is doubled and also gives very high HT which will blow caps rotors and spark plug insulations.

 

but ot me a short hard mis under load is straight forward spark plug failure

 

make sure they dont have a R in the suffix

 

pete

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Hi All,

 
And the prize goes to Pete! One duff sparkplug!
 
I drove back from Manchester with the sparkrite in the boot and it didn't help. I took the carbs apart and found both diaphragms needed replacing, so I replaced them and cleaned it all up in the process, ran better but still had the misfire.
 
When I got back, I took the spark plugs out to reveal this... (see image)
 
I dug out my old set of spark plugs, which had only been used for a few hundred miles and still looked new, swapped them all over and she runs like a dream now, and my god she goes fast when you stand on the right pedal now!
 
Pete, I don't know what the resistance is but when I bought the coil I went for a 12V one specifically.
 
Henry

post-870-0-94805800-1480426662_thumb.jpg

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Well done Henry & Pete. Simple is often the answer.

 

How long have you had those plugs in situ and what make are they ?? The plug has ceramic damage and that is caused by: poor handling, plug use past its shelf life or more concerning too high heat range. NGK plugs are the preferred choice these days as their heat range is more generous than say Champion which really is a plug of the past.

 

Both diaphragms gone, mmmmmm.

 

When did you last service the car from top to toe ?? With your mileage (great for the car) the level of checking is even more critical in order to pick these faults up and increase reliability. Both of the faults you have flagged will have occurred over a period of time. 

 

12v coils are either ballast or non-ballast; with the former running at about 1.5 ohms and the latter approx. 3 ohms. Pete flagged up a test to check and I cannot recall it 100% so will let him come back on that. What make and model of coil are you using - Lucas Sports Coil ??

 

Finally, I think you should seriously consider going electronic. The improvement is very noticeable with better idle, mph response and fuel economy.

 

Good luck and well done, once again.

 

Richard.

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The plugs have probably been in the car for around 4 years, they are NGK BP6ES. It's weird how all the other plugs look absolutely fine though...

 

Yeah both, one was worse than the other, one had a big split whereas the other only had very small ones that probably wouldn't have affected the engine at all as I was stretching the rubber to see the splits.

 

Well, I only built it 2 years ago (all but carbs and plugs which I did 4 years ago, didn't bother re-doing the carbs and I used the same plugs). Yeah, only recently started having issues with the engines running, it is still very reliable fortunately! After today however the engine has now very recently had everything checked. Oil change, new filter, ignition service, carb service, fuel line, filter and pump service, cooling system service! Surely there is nothing left to fail now!!

 

The coil is a Lucas DLB 105 from Rimmers.

 

The thing is, that Sparkrite SX1000, completely stabilises the ignition timing as there is no arc across the points, it is all done in solid state electronics. My fuel economy is around 30 mpg. Also I took it to santa pod raceway, and the results I got there surpassed a MK3 GT6's expectations by about 2 seconds or something crazy! Running on the Sparkrite! Having said that, I may well look into electronic ignition at some point.

 

Henry

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I reckon you are back on track now Henry and I think the 4yrs recycle environmental bit was your undoing :rolleyes:  !!

 

That's the correct coil (Lucas DLB105) for non-ballast ignition systems.

 

Regards.

 

Richard.

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Ah yes, credits to GT6M!

 

I've never seen a plug like that before!

 

It's odd I know, but she ran beautifully, it was only when properly gunning it that I noticed anything at all... Idle was sweet as anything too...

 

Oh well, I'll keep a close eye on her for a bit and see how she goes I think.

 

Thanks for all your input guys!

 

Henry

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