Runs great for 30-min then loses power

They also have the best points and condensers.

When it happens, pull over to the side of the road, wrap a cotton towel around the coil, and moisten it with water. See if that doesn’t take care of the problem.

exact same thing on my Spitfire, except…it would be fine if the tank was full. If the tank was less than 1/2 then it would do that. Shut it off wait 5 min and it would be ok again, until next time.

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Bill,
That problem with your Spitfire sounds a lot like the problem that I had with my E-Type when the fuel cap vent hole got blocked. If this happens again, remove the fuel cap and see if you have a loud whoosh sound from the vacuum being released. My 1969 E-Type had a fuel cap with a small hole drilled it in to allow equalization of the tank pressure with outside air as the tank drained. That vent hole got clogged with metal polish when the prior owner polished the fuel cap before I purchased it. My car would stop running more quickly when the fuel tank was less full and run for longer with an almost full tank. If I were you I would check to see if your Spitfore was supposed to have a venting fuel cap, if it still has one, and if the vent is clogged. Perhaps the original venting fuel cap was replaced with one without a vent hole, or the vent hole is clogged. Cleaning out the fuel cap vent hole fixed my problem, and now when I polish the fuel cap I make sure that vent hole is clear afterwards.

Paul

Another time I had an issue with the 69 S2… also time -related. It turned out that the rotor had a pinhole in it and would, after awhile, cause the car to stumble and die. A cursory exam of the rotor did not show anything, it was under magnification that it showed up. In any case, a good replacement rotor might help eliminate that issue. In the 67 corvette, a similar issue was a bad condenser, and another time, a bad ground strap. These cars get really picky in their old age!

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Ben,

You are getting a lot of good suggestions here but I wonder if some/many of them could be eliminated if we had a little more diagnostic information from you. You state that after 30 minutes of good driving the engine “starts to lose power”. You press the gas pedal to the floor and the engine won’t rev above 2K.

Some questions that might help narrow this down:

  1. Does the engine run rough when it loses power or just doesn’t pull as it did earlier? I would expect ignition problems (many of which are suggested so far in the thread) would result in a misfire which would cause rough running.

  2. When the problem occurs what do you do? Do you just drive with the reduced power, or pull over and leave the car for a while?

  3. If you turn off the engine when this happens, what happens if you immediately try to restart it? Is it back to normal, or still in “low power mode”?

  4. If you have to leave the engine off for a time for the problem to go away, how long a period does it take? If it is a relatively short time (say less than 30 minutes, so the engine is still warm), does the problem recur after more driving and does it take 30 minutes more driving or significangtly less given that the engine was not cold to start with?

I know how frustrating it can be to have a problem that takes so long to reproduce (30 minutes of driving in your case), and having 20 things to try becomes impractical. If we can narrow it down somewhat, many of the theories could hopefully be discounted as inappropriate in your case.

-David

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Great idea David. Hopefully this explains it in a bit more detail:

  1. Does the engine run rough when it loses power or just doesn’t pull as it did earlier? I would expect ignition problems (many of which are suggested so far in the thread) would result in a misfire which would cause rough running.

A: Does not run rough when power loss occurs, just doesn’t pull.

  1. When the problem occurs what do you do? Do you just drive with the reduced power, or pull over and leave the car for a while?

A: I’ll drive it with reduced power until I can come to a complete stop, which seems to reset everything and power is restored. I don’t turn the ignition off.

  1. If you turn off the engine when this happens, what happens if you immediately try to restart it? Is it back to normal, or still in “low power mode”?

A: when I turn the ignition off then restart, it fires up with no hesitation and power is restored.

  1. If you have to leave the engine off for a time for the problem to go away, how long a period does it take? If it is a relatively short time (say less than 30 minutes, so the engine is still warm), does the problem recur after more driving and does it take 30 minutes more driving or significangtly less given that the engine was not cold to start with?

A: Leaving the engine off for an extended period does make the issue go away. However, once I start driving again the cycle repeats (after 30-min drive the power loss comes back).

Again, appreciate all the help! What a great community.

Ben

Ben,

Those answers are very illuminating and quite surprising, at least to me. The fact that the problem “resets” just by pulling over and not even shutting down the engine, says to me that this is unlikely to be a heat-related ignition problem. I’d be focussing on fuel delivery, but it will be interesting to hear what other folks reactions to your answers are…

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I agree. I think the fuel output is low. When you run fast enough, the fuel bowls gradually run down and the engine struggles. Then when you stop, the pump catches up, the bowls refill, and on your way. That doesn’t pinpoint the cause, but it suggests that you want to devote your attention to the filter, pump, lines and tank pickup. And maybe the screens in the carb inlet valves.

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Yep agree,

That makes a ton of sense. I changed the fuel filter and cleaned the fuel tank pickup tube screen (have a new one coming in a few days). Will continue down that path and visit the carb inlet valves and fuel pump.

Guessing I can test the pressure from the fuel pump to eliminate that as an issue.

it was a 76. its long gone. I was supposed to have a vaccum cap. However the first thing I did was put a vent hole in the cap.

I agree with David.

I’d siphon as much fuel as I could out of the tank and then drain and remove the sump. I wouldn’t be surprised if you find a lot of crud in there and a clogged screen on the fuel pickup. (Edit: I see you’ve done this.)

Next, IIRC, there is an inlet screen on the fuel pump itself. It could be clogging with crud.

Then there is the fuel filter under the bonnet. Make sure it’s properly installed. (Edit: I see you’ve also done this.)

The service manual has a procedure for checking the output volume of the fuel pump.

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Ben, as you know, the pickup tube fits down into the small sump on the bottom of the tank. It sounds like you may have removed it out the top, did you check for sediment in the sump? Also, you say it was collapsed? Was it jammed down against the bottom? if collapsed, that may be preventing enough area for fuel to get through.
Tom

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Ben
, just remember that this is half the fun of these cars. Thankfully they are built so that you can do all the repairs and maintenance, if you choose to. As per other cars from the 60’s and older, things age and lots of things may not work because of oxidized electric Al terminals, or bad grounds, dirty clogged gas filters and on n on
Just systematically go through everything.
Try to determine and test every system before you assume something is in need of replacement.
Great feeling when you correct a issue and all it was was a simple fix
Good luck

Thanks everyone for the suggestions and advice. I’m out of town but will report back after I get a chance to turn a wrench.

Unfortunately the issue has not resolved after doing the following:

  • Drained gas tank and pulled sump (it was disgusting the amount of crud in it). Cleaned thoroughly

  • Blew out the pickup tube and replaced screen filter

  • Pulled fuel pump and tested. Was converted to SU solid-state pump. All seemed to work fine

  • Replaced fuel filter with correct one.

  • Measured fuel pressure at filter - indicated between 3 - 4 psi

  • Replaced rotor and condenser

  • Replaced ignition coil with petronix unit

Next step is to replace wiring harness as Rocknjag69 suggested. I put this off for last as I’m assuming it’s a bit more of a pain than the other trials.

Interestingly it’s still very drivable, and the power loss periods have no regular cadence (sometimes it doesn’t happen).

Thanks again for all the help here! I feel like owning these cars is like raising a child - it takes a village!

Start the car. Wiggle the key switch ever so slightly.

After 50 years, the contacts can be burnt.

Clearly a fuel delivery issue IMHO. I might suggest beginning at the fuel tank, inspect the insides for rust then work your way forward. From your description it sounds like a form of fuel starvation.
Cost effective measures: Replace the fuel filter (along with air cleaner filter). Pull something like a .22 cal rifle cleaning brush thru each fuel delivery line. May have debris lodged in there obstructing/reducing fuel flow.
Check carb bowls for debris, a sure sign of fuel supply contamination AFTER the filter.
Then there is always the possibility of VAPOR LOCK. What fuel are you using? E10 is prone to this in Classic Cars! Non-Ethoonal (albiet more expensive) gas is the “now” preferred juice.
Final question. Does the problem go away after the car sits for an hour or so, engine area cooled down? If so I’d be more prone to VAPOR LOCK.

Let the Forum know your findings/results.

Happy Trails,

Dick

One of the best thing i added to mine is a fuel pressure gauge. This way you know right away if you have a fuel or ignition issue. I mounted the gauge under the dash, to the left of the steering column.

Couple of years ago i had a loose ground on the fuel pump which caused intermittent issues. Found the culprit right away.