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misfiring under load

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Jedi Hopeful
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My TR3 is as smooth as a sewing machine until....
I get a misfire above 3000 RPM under load. It goes away when I back off on the gas. If I try to drive through it, it seems worse.
Should the coil have a resistor? I thought the resistor only played a part at low RPM, what does it do at high RPM?
Might it be fuel? Am I too rich or too lean?
Might it be timing? Advance or retard?
I have new points, condenser, rotor, cap, plugs and wires.
 
I don't think the coil is involved. They usually work or don't work. Heat can cause malfunction, but I'm not getting that as a factor here.
Because I have been following a thread on another Forum that was similar, I suggest you examine the carbs diaphram.
Damper oil, too.
Of course, timing could be off as well, so that deserves a look if the diaphram, etc.checks out.
Keep us posted, please.
 
Did the misfire start after or before? - "I have new points, condenser, rotor, cap, plugs and wires."

If not go back and check those new components for defects.

Check to make sure you didn't transpose an ignition wire to the wrong spark plug.

Then check: wire lead from distributor CB terminal to coil - it fails due to internal cracks you may not see, loose ignition wires, carb damper oil, dirty fuel filter, fuel pump, coil, timing, lean mixture, valve timing, valve clearance, valves sticking or leaking.

Always perform the easy fix before the difficult.
 
I agree with Harry above.
Sounds electrical to me particularly since you've just changed some pieces, check that darned condenser (by substitution) those things can fail right out of the box.
I'm not sure whether your TR3 has a resistor on the ignition to the coil but I don't think so.
 
Definitely not the resistor, the resistor. the resistor limits the current flow at low RPM. At High RPM, the current is self limiting.
Now, did the misfire begin before or after changing the new electrical bits?
Given that it is at high load/ high speed, if it is electrical under this kind of condition, it will be most sensitive to either mis adjusted points (to much dwell) which won't give enough time to "charge" up the coil, also, at higher load, the pressure in the cylinders goes up so the voltage for the spark needs to be higher, which makes it more sensitive to all these problems.
I would double check all connections, low voltage and high, also dwell, and plug gap.
It would also be helpful to know if the missing is occurring on one specific cylinder (problem on HV side) or low voltage (low voltage).
 
I agree. I think your dwell is off.



You people have some issues with the way resistors work, but that'll just get me into trouble again.
 
how about vacuum advance and timimg? When does a dist reach full vacuum advance and maybe the timing is too advanced, did you adjust it?
 
Just an experiment to try : run it up a little above 3000 rpm with light throttle (which if I understand you correctly should still let it run smoothly), then open the throttle enough to cause the misfire. If the misfire takes a beat or two before it starts, then I would suspect problems with fuel delivery. Especially if someone has added a modern type fuel filter.

Whether a resistor is needed or not depends on the coil that is installed : the original coil for a TR3 did not require a resistor but it's possible someone has replaced it with a coil that does. But leaving out a required resistor won't cause rpm-specific problems. Instead it overheats the coil & points, leading to rapid failure. Last time I did it, it literally melted the rubbing block on the points.
 
By resistor... are you guys talking about a ballast resistor? Certainly this would not have been used (stock) on a TR3. As Randall said, the ballast resistor would only be required if you are running a ballast ignition coil.

You state the car begins to misfire when you exceed 3000 RPM under load. Is this true in all gears? (i.e. If you make it misfire in 4th gear this way, can you reach higher RPM without the misfire in 3rd when driving under otherwise the same conditions?) If you can reach higher RPM in a lower gear this would imply that it's not ignition related but fuel related. If the misfire continues at 3000 RPM regardless of the gear selected, this would imply something in the ignition system.
 
is there oil in the dashpots? It sounds like you are lean under hard acceleration and the dash pot oil slows down the lifting of the dashpot so that more fuel can get sucket into the engine. This is only an "accelerator pump" to use North American terms and would not account for the condition continuing under steady state.
 
I thought it was the opposite as far as the dashpot oil goes, no oil in the dashpots the needles come up faster which means a richer mixture until the air flow picks up
 
Nope, other way round. Holding the pistons down increases the velocity in the venturi, which increases the amount of fuel sucked through the jet. The taper of the needle has a much smaller effect than the air velocity.

But the description doesn't sound like dry dashpots to me. No oil produces a severe lean bog, not really rough but very weak; and is more or less independant of load. If you open the throttle gradually enough, the engine will keep pulling even under full load.
 
Related to what Randall just wrote, I read that racy types use a relativly heavy oil in their dampers. It's said that slowing the rise of the air valve piston gives that richer mixture needed for better acceleration.
That's contrary to what I was told long ago, that racers used a light machine oil like 3 in 1 for faster throttle response.
I like the way 5w 30 acts and I'm not sure that I could tell the difference that a straight 50w would make, but my curiosity may get the better of me....someday.
 
To get away with very light (or no) oil in the dashpots, you also need to run the mixture quite rich (which ISTR Kas recommended in the TR3 Comp Prep manual). Might help on the race track, but sure trashes fuel consumption on the highway !

I did do some experiments with different damper oils on my own 59 TR3A some years back. Even made a "vacuum cleaner" to suck out the existing oil into a Mason jar. What I found was that the difference with various viscosities was quite noticeable, and that I had been running oil that was too light for optimum response. Straight 20-weight seemed to work out the best for me on the street; although 40 or 50 would probably be more appropriate on the autox course (without changing the overall mixture). Anything lighter than that would produce a lean bog (which was sometimes only noticeable in comparison to the heavier oils).

However, I strongly suspect that what works best for you depends a lot on individual factors (including taste) so YMMV. Come to think of it, I might have also had 'rich' needles installed during my experiments (tho I later went back to standard).
 
To get away with very light (or no) oil in the dashpots, you also need to run the mixture quite rich (which ISTR Kas recommended in the TR3 Comp Prep manual). Might help on the race track, but sure trashes fuel consumption on the highway !

I did do some experiments with different damper oils on my own 59 TR3A some years back. Even made a "vacuum cleaner" to suck out the existing oil into a Mason jar. What I found was that the difference with various viscosities was quite noticeable, and that I had been running oil that was too light for optimum response. Straight 20-weight seemed to work out the best for me on the street; although 40 or 50 would probably be more appropriate on the autox course (without changing the overall mixture). Anything lighter than that would produce a lean bog (which was sometimes only noticeable in comparison to the heavier oils).

However, I strongly suspect that what works best for you depends a lot on individual factors (including taste) so YMMV. Come to think of it, I might have also had 'rich' needles installed during my experiments (tho I later went back to standard).
 
I also experimented with various oil viscocities, with equally obvious and dramatic results.

Heavy oils (50 weight engine oil?) masked everything and gave a terribly slow throttle response. The piston movement was far slower than the throttle blade, making the piston limit everything until it moved up.

No oil gave terrible drivability issues, as the piston clearly was crashing up and down (audable clacks as it hit the top and bottom) both from the vacuum signal and from bumps in the road. Lots of bog issues at street rpms. This all I found fascinating, as none of my motorcycles ever used oil in the dashpot and never had this problem. But both the ZS's and the SU's very clearly had problems with no oil damping.

Working around in between the two extremes let me select the balance I wanted. For myself, I found Dextron ATF to give me the quick response I wanted with a minimum of bog.
 
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