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car stumbles on acceleration.

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rusty1c Avatar
rusty1c Peter D
Antioch, CA, USA   USA
1961 Austin-Healey 3000 BT7 "RUBY"
1963 Chevrolet C10 "Rusty"
Hope someone can help me with trouble shooting what appears to be a carb problem. The car has just had a total restoration including carbs, distributor and fuel system. The trouble starts when the throttle is opened all the way. The car starts to stumble, hesitate and has flat spots. The distributor has all new internals including points, condenser, cap and wires. The fuel supply system is completely new. The carbs have had a complete rebuild including new shaft bushings and the jets have been centered and the the carbs synchronized. The car idles fine and has had the idle mixture checked with a color-tune unit, nice blue flame present. The timing is set at spec for a BJ8 cam.
The trouble occurs when the throttle is opened all the way under hard acceleration. The car starts to stumble and has flat spots and lacks power. Outwardly everything seems fine until I step hard on the throttle. I'm at a loss as to where to go from here as everything seems to be set to specs. The car cruises fine at freeway speeds and below until I bury the throttle then the trouble shows up.
Rusty1c
Pete

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63mechanic Avatar
63mechanic Ted S
Langley, BC, Canada   CAN
When you hammer on the throttle and you experience the "Flat spot" does it sound like there's any popping noise from the engine >???
Have you ever plumbed a fuel gauge inline of the carbs just before them with a long enough hose to reach your driving position... And take it out for a ride and simulate the conditions which you experience the "Flat-Spot" and see what the fuel pressure is >????
Waiting on your reply
spinning smiley sticking its tongue out



Edited 1 time(s). Last edit at 2017-03-29 08:27 PM by 63mechanic.

ch47insp George B
Birmingham, AL, USA   USA
1957 MG MGA 1500
1959 Austin-Healey Bugeye Sprite ~ For Sale ! ~
1961 MG MGA 1600 ~ For Sale ! ~
1962 Austin-Healey 3000 BT7
Pete
Have you checked the dashpot oil level on the carbs? I had the same thing happen to me on an MGA and thought I had a more serious problem until a friend suggested checking the dashpot oil. Topped them up and problem solved.
Good luck
George

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rusty1c Avatar
rusty1c Peter D
Antioch, CA, USA   USA
1961 Austin-Healey 3000 BT7 "RUBY"
1963 Chevrolet C10 "Rusty"
George, Ted,
Thanks for the suggestions. I'll look into what you have told me to check for this weekend.
George Its going to be a bit tough plumbing into the fuel line as the fittings appear to be British Whitworth threads and there isn't anywhere around here that I can get those type of fittings. I sure don't want to start cutting into brand new fuel lines to install a "T". I'll give it some thought and let you know what I come up with.
Thanks,
Pete
rusty1c

Robert Alan Avatar
Robert Alan Robert Scalla
Simsbury, Simsbury, USA   USA
1964 Austin-Healey 3000 BJ8 "The Big Healey"
1994 BMW 325i
1995 Volkswagen Eurovan
I had the same problem. It took me 10 years and 12 British Specialists to finally find the correct guy. He adjusted it and it ran smooth for the first time and has run great ever since. Unfortunately he is in MA, you in CA. Keep looking for the 'good" specialist who knows what to do. Lots of guys claim to be good but aren't.

CPBOL Avatar
CPBOL Chris Bollinger
Mentor, OH, USA   USA
What did the guy adjust? Carbs?

Robert Alan Avatar
Robert Alan Robert Scalla
Simsbury, Simsbury, USA   USA
1964 Austin-Healey 3000 BJ8 "The Big Healey"
1994 BMW 325i
1995 Volkswagen Eurovan
Yes the carbs. He adjusted everything but in particular the carbs.

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rusty1c Avatar
rusty1c Peter D
Antioch, CA, USA   USA
1961 Austin-Healey 3000 BT7 "RUBY"
1963 Chevrolet C10 "Rusty"
Found the cause of the hesitation. Apparently I had to much advance in the distributor as a result the car ran a bit hot and when I shut it down it went into heat soak mode, (heat from radiator fed back into the engine and carbs). As a result the gas in the incoming fuel line got hot and started to cause vapor lock. After the car was moving for awhile and everything started to cool down the problem went away. I reset the timing and have not had a problem since. The timing was around 16 deg. BTDC, Now set to 12 deg. BTDC. Also found out why my adjustable advance timing light wouldn't work on the car. Apparently since it is an inductive light it has to have resistive type wires. I am running solid core "Bumble Bee" wires and the electronics in the light don't like it.
Pete
Rusty1c

ch47insp George B
Birmingham, AL, USA   USA
1957 MG MGA 1500
1959 Austin-Healey Bugeye Sprite ~ For Sale ! ~
1961 MG MGA 1600 ~ For Sale ! ~
1962 Austin-Healey 3000 BT7
Pete
Thanks for the update. It is always great to follow through with the cure for the original problem.
George

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San Jose, CA, USA   USA
FWIW, my Craftsman advance meter works fine on stock, stranded copper wire secondaries. However, my car is pos. grnd. so the case is hot and i have to be careful to not let it contact the body in any way.

MichaelPistone Michael Pistone
Fitchburg, USA   USA
Hi Robert. I'm in MA. Just curious who "your guy" was that fixed your carbs?

Robert Alan Avatar
Robert Alan Robert Scalla
Simsbury, Simsbury, USA   USA
1964 Austin-Healey 3000 BJ8 "The Big Healey"
1994 BMW 325i
1995 Volkswagen Eurovan
Brittanic Motors 110 Memorial Dr, Avon, MA 02322,
Bob Nicholson made my car run better than ever.

(508) 588-9241

mgtf328 Alan Jones
Frampton, Gloucestershire, UK   GBR
I've just been thru an extensive learning curve on tuning BJ8 carbs. See "Tuning carbs" post on British Car Forum.
If you've balanced the carbs correctly and you're sure the timing is right, I would suggest that you're running a weak mixture. Have you taken the plugs out to check the colour. They should be a chocolate colour and my experience indicates that if you're close to getting it right they are likely to have a thin trace of soot on them if you check after letting the car idle rather than shutting the engine off at more than 1500 rpm. It's generally acknowledged that BJ8's like to run a little rich.
AJ

petnatcar Avatar
petnatcar Silver Member Peter Carbone
Watertown, NY, USA   USA
Pete,
I know you've had the carbs rebuilt but check the gap of the float bowl level just for the heck of it.
Your owners manual should have the correct info for that gap.

Good luck,
Pete


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