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Idle Problem

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DSVW Dennis S
Ellicott city, MD, USA   USA
Hi, 1972 MGB here.

I have a problem with my idle that I just can't figure out.

After the car gets to operating temperature it will sometimes drop idle speed from 1000 to 500. This happens at a traffic light or stop sign. I have replaced the entire ignition system (except the coil which measure 3 OHMS and around 10K Ohms from center to either + / - contact).

I can't find any vacuum leaks. The new plugs look good after several hundred miles of driving. New needles and seats in the carb. Carb idle and mixture set. I like the idle at 1K so that's where I set it. New fuel tank, electric pump, hoses and filter. Whole new distributor, wires, cap, rotor, plugs, vacuum advance. Timing set.

It has a new 195 degree thermostat, water pump, hoses and fluid. The car runs slightly above N temperature and stays there.

It starts right up and runs beautifully at highway speeds, for a couple hours straight driving.

This only happens when I am sitting idling or at a stop sign or traffic light. It never stalls, just drops 500 rpms

Thanks!

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HiPowerShooter Avatar
HiPowerShooter Gold Member James Booker
Lake Winneconne, WI, USA   USA
1973 MG MGB
Are all of your plugs consistent in color tone? Should look like Hershey's cocoa. If darker, you're still too rich which (in my experience with HIF's) will really create erratic idle issues. A local owner(he's also my UPS driver) has a 74 which he had me take a look at for a similar problem just a week ago. Although he thought his mixture was good...his plugs were too dark, his idle was all over the place, he didn't need choke to start in the morning and his MPG was only around 22MPG. I leaned it out and the idle issues disappeared. He also gained roughly 5mpg. Cleaned the plugs and told him to pull them in a 100 miles to verify which he did. Looked very good.



"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

Basil Adams Avatar
About 12 miles from Sears Point, CA, USA   USA
When idle speed drops, it may not be because of ignition but because of air delivery. It could be a slack throttle cable, a PCV, any of the smog-related items hanging on the intake manifold, or just an idle screw that is out of adjustment. The next time it happens, pull the choke out about a quarter of the way - it may be time t go through the carbs. Let me know if I can help. Thanks. Basil 707.762.0974 basiladams@yahoo.com



Basil C. Adams
1956 MGA Coupe (Show Car)
1957 MGA Roadster (Driver)
1958 MGA Coupe (Racecar)
1959 MGA Coupe (unrestored)
1960 MGA Coupe (unrestored)
1960 MGA Roadster (Driver)
MKIII Elva Courier (E1056)
1967 427 Cobra
1972 Alfa Romeo Montreal
A coupla late MGBs
1960 Austin Healy BN7
More Cars than Brains

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spikemichael Avatar
spikemichael Platinum Member Michael Caputo
Ocean Shores, WA, USA   USA
1973 MG MGB "Freebie"
1973 MG MGB
1974 MG MGB "Spike"
1979 MG MGB "MegaBeanie"    & more
You have a '72 and say "carb" not "Carbs" will you please identify your intake system?



Michael J. Caputo
'79 RBB and '73 CBB owner with extensive experience in 12v Audio System design and installation.
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B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Do you see a variation in your base timing setting when the idle drops?
First things first: determine if its fuel or ignition. If ignition stays stable, then its fuel. If your timing drops with rpm, then your problem is in the new distributor. ,

If you have HIF carbs and the problem is not ignition, I'd look at the temperature compensators. Like any spring, they fail with time and use.



jeff@advanceddistributors.com

DSVW Dennis S
Ellicott city, MD, USA   USA
Thank you, All. I have twin SU HIF carbs. I think they are original to the car. I will try all that is suggested. Do you think these aftermarket air cleaners could be the issue. The filters are clean.


Attachments:
SU HIF carbs.jpg    37.9 KB
SU HIF carbs.jpg

HiPowerShooter Avatar
HiPowerShooter Gold Member James Booker
Lake Winneconne, WI, USA   USA
1973 MG MGB
Remove them, go for a drive and find out. A short drive(assuming you don't live on a dirt road or out in the "sticks"(does Maryland HAVE "sticks"?? ) going for a drive without a filter won't damage anything.

Don't know if you have the attached material...but here it is just in case.



"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...


Attachments:
SU Carb Diagnostic Chart.JPG    51.7 KB
SU Carb Diagnostic Chart.JPG

Bentley HIF.pdf    2.16 MB
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tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Those pancakes are about the worst possible filter, but that is not your problem. I like Jeff's suggestion of the compensators. Is your carb heat shield in tact? It could be the coil. Cheap to swap.



Cogito ergo sum periculoso

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dawvid Avatar
dawvid Silver Member David B
Sharon, MA, USA   USA
I've read about a fair amount of idle issues with HIF carbs, especially when hot.

They modify the mixture based on the temperature using what is called the "temperature compensator".

You might want to use Google or just search this forum for more details.
Maybe something like HIF idle" or "HIF temperature compensator" as a search topic.
There are quite a bit of write ups to read.

David



74 Damask Red BGT
Davesmg@outlook.com

HiPowerShooter Avatar
HiPowerShooter Gold Member James Booker
Lake Winneconne, WI, USA   USA
1973 MG MGB
If it's intermittent, look at the carbs. If it's consistent look more at ignition.

IRT speaking of springs. Did you ensure both of your throttle return springs are tight?

Also, check your pistons. I've seen worn sets which will not always return to their exact "resting" location depending on just minor movement or even temperature variations. Although the piston drop test is good...take a very close look at them to see any witness marks where there may be some interference, especially at the bottom of the bell. I see the needles and seats were replaced. How about the jets themselves? Once they wear and take on a more oblong shape they can also have an effect on your idle.



"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

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nickj Avatar
nickj Gold Member Nick Jenkins
Novato, Marin Co, CA, USA   USA
1970 MG MGB GT
1972 MG MGB
I always think this is a wear issue. Carb pistons, cables, linkages, all the places Jeff and Basil suggest. Each wear point might not be noticed on its own, but there's a lot of them and they add up, and seem to get worse when the engine is hot. It's true this problem has been reported a lot, I think it's because these cars are old. Unless you replace everything with new, you may just have to set the idle a little higher than you like.

HiPowerShooter Avatar
HiPowerShooter Gold Member James Booker
Lake Winneconne, WI, USA   USA
1973 MG MGB
Pull the filters, start the car, allow it to idle(hot) then place your hand over one of the intakes until it almost chokes out. Note where the idle RPM RETURNS without blipping either/both of the throttle arms. It could be that your true idle is set at 500 RPM but is actually rising to 1000RPM most of the time. In that case, it's almost certainly an air intake issue either a leak or if none, with the pistons/return springs/linkage/seals.



"One test is worth a thousand expert opinions"--Alvin "Tex" Johnston...Boeing test pilot.

73 MGB. Tires: Round, black, hold air. Oil: Sometimes old, sometimes new...always slippery. Oil filter: Yellow, usually full of oil. Carbs: 2 SU HIF. Distributor: Yes. Headlights: Not that bright but bright enough. A bunch of other stuff most cars have but not really important enough to itemize. Oh, wait...it has a cool sounding exhaust with stickers on the chrome tips. Really slays the ladies...

DSVW Dennis S
Ellicott city, MD, USA   USA
I think I may have found the problem. After swapping out the coil and checking the resistance of the wires I decided to check again for vacuum leak. I found that when I spray carb cleaner into the area I show in the picture with the metal pointer, the car almost stalls. I am not sure what this is but I do see a rubber gasket of some type in there. Is this something I need to pull the carbs to replace? I see there is one on each end. Or maybe try permatex in the opening to block any vacuum leak? I will research more but it actually looks like linkage so I am not sure why there would be a vacuum leak in that area......


Attachments:
carb.jpg    26 KB
carb.jpg

B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, MN, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Looks like the seal is missing. Huge problem, but not likely the sole source of your issue.



jeff@advanceddistributors.com

tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Yes, it means your carb bushings are worn out. If they are HIF's you can do them yourself but be sure to get genuine SU parts, not aftermarket. Get the no-sink floats while you are at it. If older HS types, then it is send them to Joe Curto in NY for a rebuild.



Cogito ergo sum periculoso

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