My car had a lumpy idle and I traced it back to a broken vacuum hose. It was the hose that’s attached to the back of the intake manifold that runs to the air filter. You car wouldn’t have that hose but a leaky intake is worth checking. I think we used to spray starting fluid around the intake and see the revs went up to find the leak on our old MG’s and Triumphs.
1 Like
I performed a compression test today. Results:
#1 155 psi
#2 157 psi
#3 165 psi
#4 162 psi
#5 157 psi
#6 155 psi
Those numbers seem pretty healthy.
High to low readings within 5%? Very healthy, indeed.
I pulled the cam covers and could not open any of the valves by hand. So I’m feeling OK about the condition of the cylinder head especially given the good compression numbers.
I also swapped out the spark plugs but no better.
I applied the color tune to the other cylinders and see blue, black, then orange indicating a miss. All cylinders were the same. I cannot see a spark when the cylinder goes dark. So it seems like an electrical issue.
Once I increase the revs past 1000rpm the problem goes away.
Any other ideas?
Did you check for a vacuum leak?
Dave thanks I’m planning to check this in the next couple of days. I’d rather not spray starting fluid all over my engine so my plan is to wave an unlit propane torch around the intake. Does that sound reasonable?
2 Likes
Perhaps it’s just masked
Compression ok
Carb feeding adjacent cylinder ok
Plug ok
Other cylinders fire ok
Something between distributor and plug
Try observing ignition in dark , spray with water mist under load , bad cap ,wire, plug cap
Scope on ignition system if you have that ability could use a timing lite to verify miss with color tune
Do a leak down test and listen at. exhaust, intake, breather
And ditto vacuum gauge
Yes it does. In fact, it sounds a lot smarter than using starting fluid. Good luck. These kind of problems are so annoying.
The best way to check for intake leaks if you have compressed air is:
Drill two holes in a jar lid and fit two hoses in
Tape off the air intakes, attach one hose to the intake and the other to the air
Put a newspaper into the jar, light it, immediately close the lid and blow the smoke into the intake; it will perfectly show any leaks and you can see exactly how bad they are and where it comes from.
That way you can rule out leakage, I don’t think that’s the issue though.
Smokers would just blow in it lol 
2 Likes
I held an unlit propane torch all around the intake manifold and didn’t get any surfing so I don’t think there is a leak.
You’re right these types of problems are very annoying!
Do you have a ‘Unisyn’, or similar airflow meter to verify and adjust the airflow at idle and off idle in the carbs(?). I have a colortune, but have never found it easy to get the mixture right using it. It sounds like you have a mixture problem, which can be from a number of sources. float chamber source, or just mixture setting possibly. The backfire through the carb throat sounds like a lean condition to me.
Simpler. If it persists with a bit of choke applied (we know it’s not running rich and yellow) it’s not the mixture.
Thanks guys.
When I enrich the mixture with the choke the idle rises a bit which, I believe, masks the problem. I can still detect a bit of a misfire with the idle at 900-1000 rpm.
I do have an air flow meter and I checked and balanced the carbs a couple of days ago.
I opened up the float chamber for the rear carb about a month ago and didn’t find an issue but I should check the others as well.
Checked the floats and they are within spec. Also no gas inside the floats themselves.
I went for a drive on this somewhat humid night and on return I saw condensate on the rear carb body but none on the other two.
What does this mean?
Robert, my thoughts would be:
Liquid fuel does not burn, it must be vaporized. The carburetor atomizes the fuel into small droplets. These droplets then vaporize in the air stream. To vaporize a liquid, one must add heat- such as when water is boiled. If the fuel vaporizes, it will require heat, and if vaporization occurs without added heat, it will take the heat where it can, and if heat is taken, the lack of heat is now cold. I believe that is what is happening. The fuel is vaporizing, and enough heat is not being added, thus the cold which the humidity condenses on. Jaguar adds heat by running hot engine coolant through the intake manifold. I wonder if for some reason the flow of coolant through the intake manifold near the rear carb is restricted?
Tom
Thanks. I hadn’t thought of a coolant flow restriction. Seems easy enough to check with a temp gun. I’m not sure this is related to my underlying idle problem though.
If by chance, and I really doubt it, it would be cold enough to freeze in the carb, it would be an issue. But I never heard of that on these cars.
Tom
On a different car I did experience carburetor icing. Conditions had to be just right. 35° and raining. The carburetor heat tube had long since disintegrated so there was no heated air from around the exhaust manifold. Car would run worse and worse until it wouldn’t run. Then if you waited 10 minutes it worked perfectly. Until it did it again.
My Triumph Dolomite used to do this. One scary early symptom would be the throttle plates would freeze open. Had to coast to the side of the road and let engine heat melt the ice before proceeding.