• Hi Guest! Being a Paying Supporter is NOT a requirement to participate at BCF. However, subscribing will help ensure we can continue to serve the LBC community for years to come. If you enjoy BCF and find our forum a useful resource...Please consider supporting us by **Upgrading**
    (PS: Subscribers don't see this banner)

Rough Idle - What to check?

Chet Zerlin

Jedi Trainee
Country flag
I am really trying to diagnose and fix this issue without having to bring the car back to my local (and excellent) mechanic.

Yesterday I drove my 1959 100-6 about 30 miles and everything was working beautiful. Engine pulled through 4,000 RPM's smoothly, gearbox and overdrive shifted cleanly.
When I got within about one mile from my shop I stopped at a red light and shifted into neutral and let the clutch out. Suddenly the idle RPM began to drop and the engine sounded like it was going to stall. I kept the revs up to prevent that and when the light changed I was able get moving again but now the engine didn't want to pull any RPMs and started "spitting" and backfiring. I was able to keep the car moving and drive to my shop but the engine was clearly running rough. Pulled it in and turned it off. Then waited about 5 minutes and cranked it up again. It started right up but would not hold an idle. A few minutes later I started it again and it maintained a 700 RPM idle but was running very rough.

So I'm understanding that the usual things to check would be fuel and ignition. My gas tank was almost full and considering how well it ran prior to this happening I'm thinking it's not a fuel or carburetor issue. So I'm thinking ignition?

What would cause an engine that was running so well to suddenly exhibit these problems? What would all of you suggest I check?


Healey Nut

Luke Skywalker
Country flag
Re: what to check?

Fuel filter ?
Points issue ?
Cracked distributor cap .
bad rotor arm
loose HT lead
Dead spark plug .....the list goes on

John Turney

Country flag
Most likely electrical (they usually are).
A bad rotor usually means it won't run at all.
A loose HT lead or dead spark plug can be diagnosed by pulling the HT leads to the plugs one at a time (Make sure your hands are insulated or pull it with the engine off - it can be quite a bite otherwise). It there is no change when you do that, that is the cylinder that has the problem.
Country flag
Check your carb pistons: sometimes, a jet needle will come loose and drop into the jet and car will run on 3 cyls, then 'fix itself,' then break again.


Senior Member
Country flag
I experienced very similar symptoms recently. It turned out to be the cable from the dizzy to one of the plugs that had broken. I found out with the help of an ohmmeter.


Jedi Hopeful
Country flag
If the wiring is sound and the ground connections are good I would put in a new condenser and check out the coil.

Bob Hughes

Luke Skywalker
Country flag

Open up the float chambers and look in, you might have - Low petrol levels - maybe a pump problem or dirt in the bottom of the chambers - blocking the entry to the jets.

If fuel level is low get a bottle of know quantity and disconnect the fuel feed to the carbs and shove it into the bottle, turn the ignition on and see how long it takes to fill up.

I experienced similar problems once, and after spending a small fortune with a specialist to almost no avail I found that I needed new needles in the carbs - I had run the car for nigh on 13 years with those needles but in the end I was over fuelling - new needles solved this problem. (the old needles were totally different to the new ones despite having the same markings) Just in case have a look at your spark plugs if they are sooty black you could be over fuelling.




Senior Member
I had a very similar issue and the condenser was the culprit. After replacing, the car performed beautifully again.


Senior Member
Country flag
Intake manifold or exhaust manifold air leak will cause this. Mine went up and down 800 rpm until I tightened bolts. If you did any work in that area take a look and tighten bolts. Marty


Jedi Knight
Country flag
One possibly telling thing you mentioned is that you preferred to not bring t he car BACK to your mechanic. While I empathize with the feeling, it begs the question of how recently your mechanic had the car and more importantly, exactly what was done to it. You might be able to focus in on what, if any, ignition or fuel system parts were removed/replaced/ disturbed. Not that it would necessarily be his/her fault if something they fiddled with failed but sometimes fiddling can accelerate the appearance of problems that were not yet appearing. The cars need fuel, air and spark delivered properly to run and there are probably more electrical possibilities than fuel potentialities but if the carb linkages were adjusted etc. something might have slipped. If the inside of the distributor was visited there are several things that could cause your issue. Please let us know what fixes it. I vote for a faulty condenser/rotor or an intermittant ground associated with the points or breaker plate in the distributor. Spitting and especially backfiring make me think ignition. An unburned fuel charge due to intermittant lack of spark can both sputter and cause backfire.
Similar threads
Thread starter Title Forum Replies Date
skystryd remind me it's fun. good idle. rough when accelerating Spridgets 6
RAC68 Loose Metal Knocking/Banging Sound during Rough Idle Austin Healey 32
M Rough Idle MG 15
Robert560 Weird "rough" idle problem after ignition changes. Austin Healey 18
MGB922 Rough Idle MG 13
A rough running after long idle or long low rpm driv Triumph 1
R 1989 toyota camry, rough idle when in gear, help Other Cars 6
64rocksprite rough idle, then races to 4000 rpm?? help! Spridgets 11
H Rough Idle and Sputtering Austin Healey 22
L Tach jumping, rough idle. Triumph 9
jcatnite Persistent Rough Idle Spridgets 22
J Rough Idle-Colortune-weak spark? MG 9
T TR2/3/3A rough running after 20 min shutdown Triumph 9
H TR4/4A 67 tr4a runs rough and losses power when hot but not over normal temp Triumph 14
J Spitfire 1979 Spitfire Running rough [sputtering, exhaust poping] at start up Triumph 2
N Stuck Needle Valve - Rough Running Austin Healey 15
glemon TR5/TR250 Stumped TR250 Running Rough--Follow Up Triumph 3
glemon TR5/TR250 Stumped TR250 Running Rough Triumph 8
D Rough running BT7 3 carb HELP Austin Healey 13
55modified Wanted Wanted rough BN1/2 hood for cutting up Austin Healey Classifieds 0
B Thoughts...rough running at cruise. Austin Healey 3
M TR2/3/3A Engine rough on acceleration Triumph 19
GTP1960 TR2/3/3A Rough running / a lot of carbon on the plugs Triumph 11
K TR2/3/3A Dead Petronix and rough running..... Triumph 10
I BJ8 is running a bit rough. . . Austin Healey 7
B Pics of my ’63 BJ7 and Questions on Hard Starting and Rough Running Austin Healey 11
bigbadbluetr6 TR6 TR6 carb question and running rough Triumph 13
N Runs Rough Under Load Spridgets 15
Gliderman8 General TR Rough looking '67 Vitesse Triumph 7
JPSmit Rough Bugeye hood Spridgets 0
K Rough patch inside bore of SU? Triumph 2
W Mighty rough..... Spridgets 1
justin_mercier Post-War Other 63 Herald in rough shape, MA craigslist Triumph 2
M Rough Running 99 XJ8 after Overheating Jaguar 42
T TR2/3/3A TR3 Running Rough - Cause[s]? Triumph 8
B diamond in the rough? Triumph 6
T Rough clearance checks Spridgets 2
K TR2/3/3A TR3 Running Rough Again? Triumph 7
bighealeysource BN6 runs rough on restart after running great Austin Healey 7
M 87 Series III XJ6 Rough Running Jaguar 16
G Rough running and backfiring MG 30
M TR2/3/3A Another rough running TR3 Triumph 1
T TR2/3/3A TR3 running rough Triumph 6
vping Running rough MG 9
bighealeysource Up-date on engine running rough on my BN6 Austin Healey 1
D XJ8 rides rough Jaguar 5
D GT6 Rough GT6--your thoughts? Triumph 9
T Rough budget +/- for 1275 cylinder head overhaul? Spridgets 10
R TR2/3/3A TR3 Missing [running rough not stolen!] Triumph 4
RickB MGB 70 MGB runs rough - sputters & backfires esp. cold MG 44

Similar threads