3.8 Starts Begrudgingly, Runs Rough, Runs Great, Dies at Idle

Greetings Folks,

I tried to find a previous post, but I couldn’t find an exact match. Hope its OK to start a new thread.

My 1964 3.8 OTS has been running a little weird lately. I’ve had the car for a couple months and I’ve put about 1500 miles on it without any problems. It’s getting to be a nice September weather here in VA, but nothing cold yet. But for the last few drives (usually I go out about 50 miles) it eventually starts after much prodding, i turn the choke off after about 5 minutes and it runs lumpy and rough (with the occasional “carb-backfire” and once warm it then runs absolutely great on the highway, but “lumpy” at slower speeds (engine lumpiness) and seemingly great on sweeping backroads. But after 40 minutes or so, I inevitably come to a stoplight and as soon as the RPM comes down it has died several times like this.

Today it quit once after 45 minutes the first instance where it returned to idle and I was able to restart, then 5 minutes later it quit again and it REALLY wouldn’t restart. I was still moving and I also tried popping the clutch in 2nd gear as I was rolling over to the shoulder with no luck.

I was stranded for 30 minutes with a very helpful MG owner offering advice and an even more-helpful police officer making sure we didn’t get hit. Plus a buddy of mine on the phone offering suggestions. There was plenty of fuel in the post-filter line, but we noticed all three dashpots were low on oil.

The way I “fixed it” was to just give it 30 minutes at which point it fired right up. I drove 10 minutes to the nearest NAPA with police escort and turned the car off whilst inside looking for a suitable dashpot oil. After 30 minutes, I restarted just fine and drove 30 minutes home. No problems whatsoever…

I haven’t looked at the plugs lately, the car has electronic ignition, a non-OEM fuel pump which I can hear working, there seems to be a small fuel leak (now for the first time) right at the glass fuel filter, slow dripping. There is also a strong fuel smell from back near the boot. The fuel filter element seems OK.

Sunday afternoon my big plan is to check the plugs for signs of rich/lean problems, perhaps regap and.or replace them, fill the dashpots, check the carb synch with my Unisyn tool, use my Joe Curto carb tools to check the idle piston movement and other maladies, see if anything is obviously out of whack on the carb adjustment, check the floats and the “needle and seat” (I think that’s what that’s called) in the float bowls, and figure out what the heck is going on with the fuel smell and the newly discovered dripping gas leak at the glass fuel filter.

Anything else I should look into for my initial investigation? I’m no expert, but I rebuilt my dual Strombergs in my TR250 using one of Joe’s kits and I think I have a general idea of how the SU’s work.

Many thanks,


If you’re getting a backfire through the carbs I would think the problem is ignition related. Points/condenser?.

Possibly failing ignition coil, breaking down when hot. When\if it fails again, check for spark, but be warned, it may have enough grunt to fire a plug that’s not under compression, but not when it’s installed. Had it happen to my 3.8. New coil turned it into a new car. Coil was only 12 years old at the time BTW.

Like all internet opinion posts, I have no idea what is actually causing your problem.

As with all classic cars, it’s either fuel, spark or compression.

Being reasonable, we can leave compression to one side.

So all you need to so is solve the spark vs fuel conundrum.

With an E Type, fuel is controlled by SU carbs. They are basically set and forget. Mostly.

So you’re money’s on spark.

No spark.

Symptom: car won’t start or run at all.

NOTE: assumes NEGATIVE earth, points in distributor.

To Confirm: Put a plug tester in series with a plug. It should flash when engine is cranked. If no flash check other plug leads as well. No flash = no spark.

If you do have a flash the problem is NOT spark per se, although it MAY be plugs. See 9.
Otherwise the problem may be timing, or fuel or compression. These steps will not help those things.

Take each step one at a time, in order. At the end of each step try to start the car.

  1. Battery flat.
    If the car cranks over it is almost certainly OK.

  2. Engine earth.
    Spark requires a good earth to the negative terminal of the battery. Check that the engine is earthed with an ohmmeter or voltmeter between the battery and the block.
    Visually inspect the engine earth lead (LHS behind the reaction tie plate. If in doubt run a thick cable (jumper lead) from the battery negative terminal to the engine.

  3. Check power to coil.
    Remove the positive connector to the coil. Put a 12v test light in series and turn on the ignition. The light should come on and be steady.
    Jiggle ignition key to eliminate switch fault.
    If no power, run a wire directly from the positive battery terminal to the positive coil terminal and try ignition. If it works problem is between battery and positive terminal wire. Check fuse 7 and chase wiring with multimeter. Recheck ignition switch. NB starter button will not affect spark.

  4. Check points are opening and distributor is turning.
    You can do this visually. Remove dizzy cap and get someone to crank the engine. You should see the points open and close. Use a torch; it’s dark down there.
    Put a 12v test light between the negative coil terminal and the black/white wire to the distributor. Crank the engine. The light should flash off and on as the points open and close. This should work with electronic ignition modules as well because what you are testing is the circuit through the points (mechanical or electronic) to earth.
    Note: the light may stay on or off when not cranking depending on whether the points stop closed (likely) or open (unlikely). This isn’t important.
    Check the points gap (14 to 16 thou) and inspect the electrode faces for pitting. If any doubt replace points and reset gap. Even when you’re sure it’s not the points, suspect them. It’s always the points.

  5. Condenser.
    A dead condenser looks just like a good condenser. Just replace it. They can be tested with an ohmmeter but if you put a new one in and it doesn’t fix the problem it probably isn’t the condenser.

  6. Check the coil.
    If the points are working and the condenser is OK. Get a spark plug and a plug lead. Connect the plug lead into the HT coil connector. Earth the plug by resting it next to a head nut. Turn on the ignition. Use a nonconductive (plastic) tool and open and close the points manually. (Alternatively you can connect a wire to the negative LV connector and tap this on an earth.) There should be a spark on the plug each time the points open. If you have spark the coil is OK. Move on to 6.
    If NO spark AND you are happy with 1-4 above, the coil may be faulty. Check the resistance of the low voltage (primary) circuit by connecting an ohmmeter to the two LV terminals. This should be between 0.5 (low resistance/sports coil) and 3.5 ohm (standard coil). Check the HT (secondary) circuit resistance by measuring between either LV terminal and the centre HT terminal. This should be in of the order of 5000 to 15000 ohm. Note that coil failure can be exacerbated by heat so even if it checks out cold it may be faulty hot.
    Replace the coil anyway with a known good one. (You can just sit one next to the old one and connect the 3 wires to it).

  7. Leads
    Remove the coil HT lead. Inspect for cracking or corrosion. Coolant can leak from the thermostat housing down onto the top of the cap and cause corrosion, especially with “screw in” contacts.
    Check resistance with ohmmeter; it should be virtually zero with copper core wires.
    Check the resistance of each of the plug leads by removing the plug cap and using a multimeter between the end of the wire and the corresponding contact inside the distributor cap. With copper core wire it should be virtually zero. If not check the cap socket for corrosion.
    Modern cable resistance is more complex and you would need to check the figures with the manufacturer. As a general guide though a lead should be between 2000 and 8000 ohm.

  8. Plug caps
    The original plug caps have a carbon resistor in them. They will have a resistance somewhere between 5000 and 15000 ohm. Modern or reproduction caps should be spot on 5000 ohm. If you suspect the caps, replace or eliminate them. You can solder a ring connector onto a fine 1” self-tapping screw. Screw this into the lead in place of the plug cap. Use the ring connector to connect directly to the threaded end on the spark plug.

  9. Spark plugs.
    Remove the plugs. Check for fouling and check gaps. If no success, replace with new plugs.

  10. Distributor cap
    Inspect for cracks or corrosion. The cap really should look brand new inside. Clean up the lead connector sockets if at all corroded. The central contact for the rotor button should have a resistance of the order of 30000 ohm. If the cap looks OK still try replacing it with another one, or a known good cap and set of leads.

  11. Rotor button
    Inspect and replace if it looks worn, pitted, burnt or otherwise faulty. Try another one anyway if it looks OK.

  12. Distributor
    Remove the distributor and carefully inspect it. Ensure that it wired correctly. Specifically check the insulators between the points and the coil and capacitor leads are in the correct place.
    Check that that the coil lead is connected and conducts to the capacitor lead.
    Check that the internal earth lead is connected to the distributor body and the centre plate.
    Check that the distributor turns freely and is mechanically intact.
    Check that there are no small screws or other foreign parts loose inside or causing a short.

  13. Other things
    If you have got here and not fixed the problem.
    The checklist above is fairly complete. Sometimes though electrical components can look OK but be faulty. Replacing each component, one at a time, with a known good (not necessarily new) component will sometimes smoke out a mystery.


If you’ve done all this and your car won’t run, try fuel next.

@abowie I m printed that list and throwing it in the glove box…great checklist.

This seems to summarize my personal daily routine! My diagnosis: middle age? :laughing:

Seriously though - good advice above. I’d start with spark!!


As I’ve said before, the first thing I do when getting a "new’ car is to pull the distributor and clean it out good, checking both the vacuum and centrifugal advances, because usually at least one is not working properly. That usually solves about 80% of the issues.
Make sure a spark is getting to every plug (use a timing light on each, one at a time). If not, a plug is fouled or a wire is bad or sometimes the distributor cap has failed.Then quickly check the timing.
At this point, if there still is a problem, you have narrowed it down to fuel.

Society has discovered discrimination as the great social weapon by which one may kill men without any bloodshed.
Hannah Arendt

1 Like

Centrifugal advance stuck “open”? That would mean too much advance at idle and off idle but good advance for cruising. Also make for hard starting.

I’d also check the float needles. Those suckers can stick on one of the three bowls and your engine will be fuel starved on two cylinders, which is the symptom you describe. They can come unstuck at any time, and will get restuck again, until you disassemble the float bowls and either clean or replace the needles. Note that the original needles are rubber and may deteriorate in today’s fuel. The newer and better replacements are called Gros Jets, which are steel, and can be cleaned of varnish.

The way to test this without having to disassemble is, with the car idling, tap the top bolt securing the overflow tube on the float with a hammer to dislodge the stuck needle. If the car idles properly again, you’ve identified the problem.

Do you know if/when the carbs were rebuilt? 15-20 years ago some poorly machined jets and needles made it into the market that resulted in difficult to control carbs. Often too lean, but sometimes too rich. Mine were rebuilt again 5 years back and they’re way better.

The fuel pump can make for some interesting hesitations and stumbles when they start to fail, and it can take a while to finally fail.

I’d certainly start with the usual suspects like wires, plugs, caps, rotor, points. But don’t say replace the dizzy or coil until you’ve ruled out fuel.

I had a problem with my 3.8 car that doesn’t exactly match your symptoms, but it might be worth a check. The car would be hard to start (sometimes) and would always run terribly, sometimes backfire, until it warmed up then would run perfectly. This happened consistently. One time I started the car when cold, opened the bonnet, and luckily noticed a coolant drip from the thermostat housing onto the distributor. As it turned out, when the car was cold, coolant dripped on the distributor. The drip stopped when the engine heated up and the distributor would dry out. Car ran perfect. Couldn’t find the problem when the car was hot because it didn’t drip when hot…

Sage advice! My '64 FHC had the same exact issues, we started looking at ignition voltages and the like, and I am convinced now it was a couple of things (I know, silly excuse…) that included a poorly wired coil/Pertronix Ignitor (low voltage), very tired starter that would dim the lights next door when cranking, and a tired dizzy. Once started, she would run fine - could not shut her off and start her back up 5 minutes later though. Had her die a couple times at idle in awkward places - and got used to putting a click of choke in as I came rolling to a stop sign. She also would fire (partially) on start up with the very first cylinder and then nothing for awhile. Seems with the fuel pump going we would eventually get a spark. So - I went to drain the fuel with the fuel pump - and at 1/4 tanks started getting bubbles. Lots of bubbles - so the suction side of the archaic fuel pump had a leak - new fuel pump. I have some odd looking “flocs” in the fuel - white to translucent 1/16-1/8" size flakes - Running a lot of fuel through now on backroad jaunts. I also replaced the coil with a sports coil and put in a 123 dizzy. Big improvement. Still have some stumbles and surges when running at 2500 in 3rd or 2nd - seems smoother in top, but the bumps are still there, more like pushes than stumbles. I have yet to dig into the carb bowls - replaced the fuel filter and will do so again - had improvement there and there was a bit of gelatinous stuff on it - probably the flocs - thing I got a bad tank of gas locally - working it through now with premium and some Seafoam. She starts right away now, but still feeling there is a fuel starvation issue as noted in the start of this thread - the floats are the last item in the equation. Will sort through this next. Starvation seems to flatten her power curve at about 3000 rpm - should pull smartly through that Rev number I am thinking - OR? Also seems to run better as she heats through.

Thanks Erica for the insight and everyone else too - This forum has been a huge help with my LED signals/taillights and other issues…pete

Had one go bad on a TD a while back. Cold, good to go. Hot, no go.

Norm, that is not an uncommon problem. The water pipes are placed very carefully over the distributor, ain’t they!
(PS Gertrude says hi)

Society has discovered discrimination as the great social weapon by which one may kill men without any bloodshed.
Hannah Arendt

Not much to add to all the good advice, but you mentioned the low oil in the carb dashpots. I can’t see how that would account for hard starts and bad idle. So your problem is somewhere else. (Also, many of us use automatic transmission oil for the dashpots.)

1 Like

Bob, I’ve run out of dash oil in the middle of the night along a long, lonely stretch of Interstate 5. Yes, it causes a variety of nasty symptoms, and can result in you sitting along the shoulder for a long time if you aren’t aware of it. The manual says to check it at every fill-up, or some such overkill. But when the pistons can’t move up and down freely, you can experience both too rich and too lean in a quarter of a mile.

Society has discovered discrimination as the great social weapon by which one may kill men without any bloodshed.
Hannah Arendt

Interesting… I wonder where the oil can go? It can’t leak out as it’s contained within the piston, doesn’t get “used” other than to slow / damper the vertical movements of the piston, won’t really evaporate, or at least not quickly.


Some folks fit the adjustable needles to their Strombergs. Unlike the fixed needles you and I have, the adjustable needles fit to a piston that has a hole in the bottom out of which the dashpot oil can leak. There is an O-ring to seal around the base of the needle in this configuration, but in time it will leak…