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TR4/4A TR4 High RPM miss

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Tim Tucker

Tim Tucker

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All,
My TR4 starts popping out the exhaust and missing (no visible smoke) when I try to sustain RPMs >4k for ‘long’ periods. I am defining long as being a sustained drive >4k RPM in 4th gear or a lengthy acceleration period >4k RPM in 3rd. The quicker accelerations through 1st and 2nd go perfect. The car is free from any stumbles or other drivability issues. The plugs look good for both mixture and time (per the timing article). I am running a pertronix ignition, Lucas Sports Coil, the uprated rotor and cap as sold by the roadster factory, champion plugs, and a set of wires from TRF. I installed a new WIX filter last weekend.
I suspect (in order): misadjusted bowl floats and low fuel pump delivery. I checked the flow from the tank to the pump: seemed to be MORE than adequate. I pressured checked the pump pressure: ~2-3 psi (my gauge is a bit iffy at that low of a pressure.)

I would appreciate any feedback or ideas.

Thanks
Tim
 
Tim-

Don't rule out electrical yet. I know others will chime in and could easily correct me, but I think you'd see fuel delivery issues come up at other RPMS if that were the problem. I'm more thinking you might have a weak spark and its just not keeping up under load - could give you the occassional pop out the exhaust too if you weren't firing properly every time.

Randy
 
Weird problem.

I agree that it sounds like it's misadjusted fuel bowl floats. But I'm going to keep checking this because this is interesting!

You have not neglected the ignition system, sounds like it's bulletproof so I would rule that out.

Perhaps the car is running too cool, what thermostat are you running? Highly doubtful but still a thought I had. It happened on an MG I had.
 
Does it start running smoothly instantly when you let up on the throttle, or take a moment to "catch it's breath". The moment tends to indicate fuel delivery IMO, as it takes a bit for the bowls to fill back up.

Another test is to tee your pressure gauge in at the carbs using a tube long enough to run out and prop under a windshield wiper. Then take a drive and duplicate the problem. If you still see at least 1 psi or so, it's not fuel delivery (to the carbs at least).

Misadjusted floats would cause problems all the time, not just at high rpm.

Were the wires carbon-core? I've had problems with those that initially only showed up at high rpm.

Personally, the first thing I would do is convert back to points. In fact, when I did, my hard starting problems disappeared. I eventually found another problem (bad battery) that was probably root cause, but the Pertronix was complicating the issue.
 
I just went through something very similar... check your coil out to see that it is working properly.
Also, check to be sure you are getting a full +12v to the coil.
 
Geo Hahn said:
What plug gap did you use? -- I know some open the gap up when they use the sport coil and possibly over-do it.
Click to expand...

Jeff Schlemmer (from Advanced Distributors) has advised me .035 is about where you want to be with the Sport Coil. Any more than that is too much.
 
Guys,
Thanks for the great feedback, First: everything is suspect.

Geo Hahn: I had gone to a wider gap. Adjusted gap to .025 last Sunday and did a lot of driving.

Randall: I would have to say instant. I believe they are carbon core, and they are old. I think investing in a set of top quality wires would be a wise choice. <span style="font-weight: bold">What would you recommend?</span> I try that and then try the points+condenser.

Elliot: I borrowed a friend's spark tester and measured ~20Kv at the coil wire. I also measured ~15kV at the cap end of each wire and as low as ~6 at the spark plug end.

Given their age and price: top shelf wires first w/new plugs then if needed: back to points fr a test.
<span style="font-weight: bold">
Who has the best wires and what are they?</span>
Tim
 
Sorry, I don't have a good answer for best wires. The wires I've been running for over 20 years are no longer made (probably because they could never sell a second set for the same car :smile: )

But they were tinned copper-core wires with discrete suppression resistors at the plug caps; which is the configuration I plan to stick with. I found some nice 145 degree caps made by NGK (for motorcycles), now all I need is some good solid core wire.
 
FWI, I just had a Lucas Sports Coil completely fail on me after one year of driving, maybe 3-4000 miles. Completely went dead.

Local Brit repair shop also mentioned they had a few go bad. I notice Advanced Distributors doesn't sell them, so I am now pretty wary of the new Lucas coils. I am running my original and will buy a Bosch in the future.
 
Ken G. at British Frame and Engine once wrote of buying a case of Lucas coils, and finding that over 1/3 of them were either totally dead or not up to snuff on the tester, right out of the box.

I'm running one at the moment, but only because it was the cheapest thing I could find when my original crapped out at TRfest a couple years ago. If it gives me any grief, I'll probably install the cheap K-mart coil I had on the TR3A for some 20 years :smile:
 
Last week I installed a new Pertronix 40,000 volt coil and it was $32.95 delivered through Amazon.
 
Just a thought, but high speed miss could be the result of valve float. You might want to check for a weak or broken valve spring not allowing one of the valves to close.
 
Elliot,
I just ordered one from Amazon myself. I borrowed a friend's 'Peak KV tester and played with the Tr4 a bit today. The maximum voltage I saw from the coil was 12Kv (occasional 9Kv blinking). The cylinders all had 12kV-6kV at the plug. 9Kv was most common. 12kV was seen very little.

The meter's manual spells out an acceleration test. Each of the wires showed a slight increase (~30%) to the 15-12keV range when engine was quickly 'goosed'.

I also tried the coil that was on the car when I bought it in 1981: a Lucas coil with a label that reads: 'Power Plus'. It actually had a bit more output than the "Sport Coil". I actually saw 15-12Kv with the old one.

I had a list of items to order from TRF anyway, so I threw a set of their wires in there. That with coil and new plugs leaves only the Pertronix ignition on the electrics side. I checked for good voltage at the coil.

I'll be out of town this weekend, but I hope to drive the car as it's configured with the Peak KV tester hooked up. I'll update what I see when I start missing.

All the great feedback is appreciated.

Tim
 
Tim- Sounds like you are on the way to tracking it down.
Let us know how things go. Good luck!
 
Tim check for loose wires. A couple of years ago on the way to TRF summer party I was having a 3 to 4 k miss in the TR3. Only got out of town about 10 miles and had to pull over it was getting so bad. Checked everything I could think of. Did that a couple of times on the way and even chanced the points condenser (high rpm miss is a system of a bad condenser) and that straightened it out for a while. It was getting dark and it was happening again so I pulled over and while the car was idling I could see a small arc in the spade connector from the coil wire to the distributor. So I pinched the terminal and drove the next 2 hours with out a hitch. So pinch those spade terminals and maybe change the condenser.
 
I put an O-scope on the primary lead of the coil today. I found burn times of ~.9-1ms. Increased a bit when I hit the throttle fast. <span style="font-weight: bold">The amount of 'hash' (as they call it) on spark burn line was very excessive.</span> The thing looked like a dampening sine wave right up until the end of spark pulse. I then had 3 coil 'oscillations'. I think replacing my coil, wires, and plugs will be a good start. Looks like Randy (and others) were right: electric..:smile:
Tim
secondary.jpg
 
Thought I would let you guys know that it must have been ignition related. I replaced tie plugs(Champion), wires(TRF), and coil(pertronix) and was able to drive yesterday. No more miss. The car did start running lean (judging from the plugs) and started stumbling when accelerating from low speeds. I enriched each carb a bit and the hesitation went away and the plugs started looking good. Seems to have a bit more power...:smile:

I had wanted to test each piece: wires, coil, etc...I guess I'll just drive her now...:smile:
Tim
 
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