Rough idle when hot

Hi All,

Looking for some guidance. My 1962 car has had a rough idle when hot basically since I bought it 3 years ago. Idle is set to about 800rpm but dips down to below 500rpm. On the road it runs great but I get an occasional misfire through the carbs when accelerating.

Work done:

Carbs professionally rebuilt
New wires
New distributor and cap (123ignition set to curve 2; I had this same problem even before changing)
New coil (I’ve tried multiple new Flamethrower 3ohm coils, currently have a 1.5ohm)
New spark plugs
UE needles
ITG filter
Fuel pressure is 3psi

The car seems to “miss” on the rear carb at idle when hot based on exhaust note. When I get back from a drive it’s so bad it seems like the engine will die.

My first thought was that I’m running rich at idle.

Today I put a Gunson colortune on cylinder #1. What I noticed is that when the engine misses I see no flame in the combustion chamber. So I’m losing spark from time to time.

The only electrical item I have not changed recently are the wires. I was not suspecting these because it runs well on the road. But I suppose my occasional misfire while accelerating could be from the same issue.

Do I change the wires or does anyone think I’m missing something else obvious?

Appreciate the help.

I would suspect you’re running way to Rich. And when it warms up it’s so rich it won’t fire. You could try leaning out the mixture or Try a leaner needle.
If you can drive right away without the choke, it’s too rich. On my car it needs a good 2 miles to warm up before you can fully push in the choke. If you push it in too soon the engine bogs and backfires through carburetors.


If it runs better when cold it must be the mixture. Lift the pistons by 1-2 mm and if it doesn’t die (too lean) or rev up a little (too fat) it’s not the mixture. It might be too rich and when it’s hot it starts to miss (but no flame at all?)

It could also be vacuum in the tank, lift the cap and see if there’s air rushing in. That’d explain why it takes a while.

If the ITG filter is an air filter, are your needles matched to this? A less restrictive filter causes a leaner mixture.

Also, do swap #1 and #2 wire or pop in a spare and also try the colortune in another cylinder, like #5 and see if that runs well all the time.

Here is a link to a video I took:

That helps; I don’t know what’s wrong with it. But try another wire. Good luck!

Bill I also have to warm the car up a bit before I push in the choke. I was thinking this was a mixture issue all the way but the colortune seems to indicate otherwise.

Do a compression test on the missing cylinder.
If one of your valve clearances is too tight, it’s possible when it gets warm the valve doesn’t fully close. I realize this is a longshot but it might be time to check your valve clearances.

Yeah good idea. I actually just adjusted all of them last month and they are all in spec.

I have had a slightly splashy idle on my 69 with triple carbs forever.

It is worse when the engine is at its warmest. I have rebuilt everything and tried all sorts of mixtures.

I did put a colourtune on it, and at idle, the flame colour was not consistent. It seemed to vary from blue to slightly yellow.


Robert, Dennis,
I had a similar problem about 10 years ago with my 1969 FHC. Sometimes the idle was very smooth and at other times it was quite lumpy like it was missing on one cylinder. The lumpy idle was only felt at the left tail pipe indicating one of the forward cylinders. The first time I did a compression check everything was fine so I swapped plugs, wires, adjusted valve clearances, but nothing seemed to help. Then I tried Gunston Colortune and observed an intermittent misfire on #5 cylinder going from blue, to yellow, to black in random order while the other cylinders had nice blue flames. It seemed that I was troubleshooting forever and nothing seemed to work. The problem finally went from intermittent to constant lumpy idle and a subsequent compression check showed constant low compression only on #5. I pulled the camshaft covers, cam shafts, and tappets and noticed on a cold engine that I could easily push down on the #5 exhaust valve and open it with my thumb. I could not do this on any of the other valves. I should not have been able to open a valve like that with just my thumb. So I pulled the cylinder head and discovered that some of the valve springs, but particularly #5 exhaust valves had failed (age, heat, ?) and were soft. I could easily compress the valve springs closed in my fingers. At that time I swapped in an excellent 4.2 L cylinder head that I had on hand from a 1985 XJ6 parts car and then drove it with that head in place until last year.
Bottom line, your problem could possibly due to failing valve springs that are not closing as they should and are preventing proper compression for combustion to occur. This is what happened to me. In addition the valve guides were a bit worn contributing to the compression issues as the valves wandered a bit in the guides.
I had my engine bottom end and original cylinder head rebuilt and put together again last year. The car has been running great ever since and it just went over 1,000 miles since the rebuild with a very smooth lump free idle.:grin:
I hope this helps.



I just reviewed your YouTube video and it looks very much like when I had the failed valve springs in my 1969 FHC 10 years ago or so. That was a while ago, but that it what I recall seeing as I tried to sort our my intermittent lumpy idle. It didn’t make sense to me at the time, until I removed the camshaft covers and camshaft and pushed down on the errant valves (my case it was #5) with my thumb and discovered that I could actually open and close the valve easily with my thumb (but shouldn’t have been able to do that). I suspect that you need some new valve springs if not a cylinder head rebuild.


A few years ago one of my '70’s Chevys would be fine when cold but start having idle and low speed problems especially on hot days after driving for awhile. It was fine at higher speeds. I fixed a couple of small vacuum leaks and was better but still not right. Did lots of troubleshooting with the carb and checking for vapor lock but could find any problems. Checked the coil wire and coil and found wire and connection in the coil to have corrosion so I changed them both. Problem solved. Just a thought if you think you might have any marginal wires, especially coil wire. I could not tell in the video if there was spark but no flame.

68 E-type FHC

Paul thanks for the insight. I’m going to do a compression test this weekend.

But I hope that’s not it!!

Thanks David that will be easy to check. I could not see a spark in person but maybe I just can’t see it.

For what it’s worth I also put the colortune on cylinder #2 and observed the same result.

Based on my experience with my 1969 FHC I suspect that you have a combination of valve spring and valve guide issues that create compression issues that result in random misfires. In addition to the compression check I recommend that you remove the camshafts and tappets to see if you are able to push your valves open with your thumb. You should not be able to do that. If you can do that, like I was, then you will need to remove the cylinder head for a closer inspection of the valves, valve guides, valve springs, and valve seats.


Hi Paul

I would have never thought of this as a cause

I put my head together with a spring compressor for the valve springs and it was a battle. But there could be a broken spring.

I will check the compression but I think that a weak or broken spring may not be a problem at cranking speeds but only have symptoms show up (ie valve float) when engine is revving higher

Thanks for taking the time to reply

This is what makes this forum great!


1 Like

You don’t have to remove the cams to check the springs. Just use something that won’t scratch the lifter but fits the space beside the lobe.

Well if you know how to remove the valve springs and have the spring compresser handy, it might be worth the effort for you to buy a new set of springs and swap out the old ones and replace them with new ones.
I happened to have a spare 4.2L cylinder head and a head gasket kit on hand so I swapped out my original head for one that I believed was in excellent shape (and it was) to prove that the problem was the springs. Luckily for me that was my problem.


Paul, this is a useful piece of diagnostic work for all of us, I think. Also worth considering is the use of a good old fashioned vacuum gauge. I have two and have used them to good effect over many years now. They’re not expensive to buy - my new one was A$45.00. Below is a link to a typical diagnostic table that covers: burnt valves, sticky valves, weak valve springs and a multiple other complaints. FWIW. Paul.
AutoSpeed - Using a Vacuum Gauge for Engine Diagnostics.