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Misfire at high revs

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4tecdog

4tecdog

Jedi Hopeful
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Can anyone put some though to an issue that has developed in this warmer weather (may be a coincidence and not related at all) My Bj8 has started missing above 3500 revs under hard acceleration also with an occasional backfire in the exhaust. The car will drive perfectly up to these revs even under heavy load in top climbing a steep gradient it will not miss at all. As soon as I slip out of overdrive and give it some beans it feels as if it is being held back and sometimes wakes the dead with that backfire.
The coil is fairly new with petrinox ignition, running on twin SU carbs with a tubular manifold and side exhaust. The valve clearances are ok and the plugs seem to be burning the correct colour if a little on the lean side.
 
I would look into the lean condition if You feel good about all the ignition aspects. Fuel flow, jet settings etc.
 
Had similar symptoms from my BJ8 years ago, caused by a 'leaky' distributor cap. Run the engine in a dark--but well-ventilated garage--and look for any arcing around the cap or on on the secondary wires. Leaky cap was Bosch, replaced with a genuine--this was before counterfeits became common--Lucas cap and problem was gone (and has been for many miles).

When I got my overhauled engine installed it had a nasty backfire due to the #6 wire making contact with the heater hose shutoff valve and causing that cylinder to misfire.
 
I thought this would be easy to answer, until you mentioned Pertronix; had it been a points/condenser ignition, the block wearing down, or a faulty condenser would've been a sure-fire diagnosis.

Back-firing under load is almost always indication of an overly lean condition, so I'd verify that your fuel pump is delivering sufficient quantities of fuel (partially blocked pipe or filter, as well as the pump itself). Unless someone has been playing with the carbs, they shouldn't have gone off just like that; can that be ruled out...?
 
Thanks for your suggestions I will be doing more diagnosis this evening, the distributor cap (Lucas) rotor and leads were changed the same time as the coil, the carbs have not been touched except for checking dash pot oil, fuel pump was replaced some eighteen months ago for an SU electronic type, not to say there is a fuel restriction somewhere which to my way of thinking could be the cause. Back later with any findings.
 
Fuel is fresh did 120 miles in her yesterday and refuelled shell optimax. Fuel tank is old though so who knows could have picked some crud up .
 
Should have said V power don't know where optimax came from some Marine engine I suspect, anyway looked at distributor cap rerouted HT leads as no six was a bit close to heater valve and rechecked plugs was susprised to note the engine has recently been running a little on the rich side except cylinder no 1 which seems a perfect colour. Not much in it though. Checked the choke cables and Vaccum pipe both ok. Went for a fairly fast run did not get any backfire however does still seem to hold back a bit at high revs not in overdrive. Once in overdrive would not know there is a problem as she is doing well over our speed limit at 2700 revs. Could it be the ignition is not advancing for some reason causing a flat spot.
 
It is possibly fuel related - my similar problem was on a BJ7, different carbs of course, but one of the jets was blocked at the connection into the float chamber great economy but lousy top end speed could not get above 60mph.

:cheers:

Bob
 
Also check the rotor arm. If there is any movement it can start 'floating'. I had the a similar problem at the top end of the rev range. I fitted a red rotor from the Distributor Doctor and that cured it. I would recommend getting one of these rotors anyway, even if it doesn't cure this particular problem. Excellent quality.
 
How old is your fuel pump? I was having similar issues on my E-Type and when the 50+ year-old pump was replaced with a new one last year the misfire went away.
 
My 3000 engine (in 100-6 body) is showing similar issues when reving over 3000 RPM. Have replaced coil, checked everything - now ordering Pertronix ignitor to see if that fixes it. The comments above about running lean are interesting - the carbs were recently overhauled, perhaps that is a clue.

4tecdog - have you found a smoking gun yet?
 
Petronix recommend you increase the plug gap by 5 thou. Also make sure the carbs are balanced and check the timing with a strobe light. I'm reliably informed that these cars like to run a little rich.
AJ
 
I don't see why Pertronix would need the plug gap widening. Pertronix uses a Hall effect magnet to replicate the opening and closing of the points, everything else remains the same.
 
DerekJ said:
I don't see why Pertronix would need the plug gap widening. Pertronix uses a Hall effect magnet to replicate the opening and closing of the points, everything else remains the same.
Click to expand...

I don't believe Pertronix needs the extra gap opening; it's just that you can get a stronger spark with it. Here's my theory:

- Pertronix uses a Hall Effect switching mechanism, which enables faster rise and decay times in the primary current in the coil than points
- The 'sharper open and close' of the (non-) points creates a potentially stronger voltage in the coil secondary circuit
- Increasing the gap increases the resistance to the spark jumping the gap*, increasing the voltage, and hence the strength of the spark

* As the secondary field collapses, the current (spark) is looking for a place to go. With a narrower gap, the spark might jump the gap at, say, 10,000 volts. Increasing the gap requires the secondary circuit to produce a higher voltage--say, 15,000 volts--before the spark can jump the gap. The secondary circuit in the coil will dissipate the current when the spark jumps the gap to ground; increasing the gap will create a stronger spark (hence why Pertronix recommends the gap be increased). This is the same reason that 'sports' coils use a larger gap, as they can potentially create a stronger spark.
 
I had installed a plastic in-line filter before the pump, and noticed it had some crud in it evev though I had a new tank that had been professionally cleaned. Symptoms were the same; ran fine until the engine was under load at high RPMs. Then it would miss and sometimes backfire. Replaced the filter and all is good.
 
Jack T said:
I had installed a plastic in-line filter before the pump, and noticed it had some crud in it evev though I had a new tank that had been professionally cleaned. Symptoms were the same; ran fine until the engine was under load at high RPMs. Then it would miss and sometimes backfire. Replaced the filter and all is good.
Click to expand...
I had a similar problem several years ago. Fuel line had a lot of dirt in it. Disconnect at the tank and fuel pumpand carbs and blow out with an air hose.
 
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