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Rough running, low power in engine

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Rough running, low power in engine
#1
  This topic is about my 1967 MG MGB
Banana Skin Avatar
Banana Skin Stewart C
West Hills, CA, USA   USA
Hi all,
Going to describe my rough running engine in the hope someone can diagnose for me.
Engine starts fine and I have the idle set to around 800rpm. Once the engine warms up the idle seems to drop and there is a loss of power when I drive. I can feather the throttle to compensate but at a traffic light without throttle management it will stall. If I set the carb idle higher when I stop at the light it's screaming. Any thoughts would be appreciated.
Stewart

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Wachtmans Avatar
Wachtmans Wouter Strodijk
OVERVEEN, Noord Holland, Netherlands   NLD
1974 MG MGB "The Bee"
1974 MG MGB "The Bee"
1974 MG MGB MkIII "The Bee"
Might be a tired fuel pump......check, double check.

HiPowerShooter Avatar
HiPowerShooter Gold Member James Booker
Oshkosh, Wisconsin, USA   USA
1973 MG MGB
If it's been awhile...rebuild/refresh your carbs or at least start from the beginning and dial them in again. It's fuel delivery.



"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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RAY 67 TOURER Avatar
RAY 67 TOURER Ray Marloff
Fort Bragg, CA, USA   USA
1967 MG MGB "My Girl"
Have the throttle shafts ever been replaced in your carburetors? If not, you may be getting large vacuum leaks around the shafts. This causes the carburetors to function properly at one speed, and poorly at another. Impossible to balance then correctly. RAY

Rick Fawthrop Avatar
Rick Fawthrop Gold Member Richard Fawthrop
Seattle, WA, USA   USA
How many carbs do ou have?

Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
If your car "starts fine" and idles with no choke at first cold start of the day you are running too rich.

My cars need choke on first start and will need choke for at least 2-3 minutes or they will stall.

When warm they start with a click of the key and will idle at least 20 minutes (longest I have tried)

If it's a Weber please disregard...



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

Banana Skin Avatar
Banana Skin Stewart C
West Hills, CA, USA   USA
Thanks for the notes on this. I decided to get the carbs refurbished and sent them to Lawrie Alexander at British Sportscar Centre. He did a fantastic job and they're installed. Everything seemed good. The car idled perfectly around 850rpm with minimal adjustment. It wavered occasionally but nothing extreme so I took it for a drive. Temps rose quickly (unusual) and then the revs got stuck up at 1500 mark and would not drop. Climbing the hill to my house I lost power again. Is this a timing issue or fuel delivery? Distributer cap looks clean and the rotor arm too. I also noticed the timing marks are missing from the front of my engine, any ideas on how I can set timing without them ? Am I looking at rebuild of the Distributer ? I have new plugs already. Just wondering which direction to go next.
Thanks

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barry s Avatar
barry s Silver Member Barry Stoll
Alexandria, VA, USA   USA
1972 MG MGB GT
1974 MG MGB
1976 Triumph TR6
1980 MG MGB
Whether carbs have been rebuilt or not, one still must tune them properly. Smooth running at idle is a poor measure of proper carb mixture. There is a pretty large range of rich/lean that will still allow a decent idle. Once under load is a whole different story. A carb rebuilder may set a set of carbs to the initial setting (in the tuning process) to allow starting the engine. Once mounted the carb mixture must be fine tuned. References to 'road tuning' mean assessing the car performance and adjusting the carb accordingly until proper acceleration is achieved. The final step is rebalancing the carbs and setting idle rpm.

BTW I agree with Dan on a likely rich mix.



Edited 1 time(s). Last edit at 2017-09-13 11:14 AM by barry s.

HiPowerShooter Avatar
HiPowerShooter Gold Member James Booker
Oshkosh, Wisconsin, USA   USA
1973 MG MGB
Read your plugs...they'll tell you what you need to know.

Too rich/Too lean and ensure equality among the four cylinders individually and two "banks" as primarily fed by each carb.

No book/spec will substitute for driving and road tuning.

I usually dial my advance all the way back when I set base timing so that I can easily roll in more on the road.



"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...

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
Pretty sure the timing marks on your 67 are on the bottom of the crank pulley (like mine)

PITA when doing timing but you get used to it.

If you don't have a Hayne's manual get one soon - it will help in things like this.

For timing here is what I have settled on.

- Turn the engine crank until the crank mark is aligned with 10* on the block mark.
- Make a new mark at 0*
- This now gives you a reference mark for 30* and means you don't need a dial back timing light

Start the engine and adjust idle screws until you get 3,000 RPM.
Adjust timing to about 32* for the HC engine - your new mark just past the 20* mark.

In reply to # 3591195 by Banana Skin I also noticed the timing marks are missing from the front of my engine, any ideas on how I can set timing without them ? Am I looking at rebuild of the Distributer ? I have new plugs already. Just wondering which direction to go next.
Thanks



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
Red Betty - http://www.mgexp.com/registry/GHN5UH418165
Bart - http://www.mgexp.com/registry/GHD4U146898G

. Hide banner ads & support this website by becoming a > Gold Supporting Member <

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