Stuttering throttle response, mixture problem?

[The Cone of Silence]

1967 Healey 3000 Mk III BJ8

I had an issue with throttle/ fuel/ carbs which I could use some help and experience on.

Symptoms noticed:

Hot day on the NSW Central Coast. I drove to the hardware store for 30 minutes, after a warming up and the fuel mix was, as usual, a little rich....with a fuel odour coming from the exhaust. This is pretty normal.....I struggle with carb balancing because I'm a mechanical novice.,

Got to the hardware store then parked in the hot sunshine, 38 degrees C for 30 minutes.

Got back in the car to drive home and noticed the throttle response was very odd. Even though I kept my foot steady on the pedal, the car would stutter repeatedly. This wasn’t so noticeable at low speed but it was happening throughout the rev range and felt uncomfortable.

Here's the kicker though, when I accelerated hard to investigate the throttle issue more, the car repeatedly backfired, which I thought was an issue with an overly lean mixture? Going from rich to lean that quickly seems odd given I didn't tinker with anything under the hood.

Any ideas, chaps?

Bobby

 

[red57]

Sound like fuel percolation, sometimes also known as vapor lock. It was the sitting for 30 min after running at normal temps, as it sits the heat from the engine soaks into the carbs and starts to boil fuel. My brain works in F not C, but I'd guess 38C is downright hot, so that aggravated the situation. Two quick solutions - let it sit until the engine gets cool again, or drive until you have fresh, cool fuel from the tank in the carbs. Hopefully others will have more permanent solutions to offer, but this is not a new issue with a Healey. I haven't, but you might try searching this forum for help.

 

[Bob_Spidell]

Sound like fuel percolation, sometimes also known as vapor lock. It was the sitting for 30 min after running at normal temps, as it sits the heat from the engine soaks into the carbs and starts to boil fuel. My brain works in F not C, but I'd guess 38C is downright hot, so that aggravated the situation. Two quick solutions - let it sit until the engine gets cool again, or drive until you have fresh, cool fuel from the tank in the carbs. Hopefully others will have more permanent solutions to offer, but this is not a new issue with a Healey. I haven't, but you might try searching this forum for help.

That's my theory as well. My BJ8 started to do this about the time E10 gas became prevalent. I'm sure it's a coincidence.

 

[steveg]

Swap out coil, condenser, rotor - these can cause heat-related problems.
I had a similar problem when my coil got hot - replaced first with Pertronix, then Bosch - problem gone. Modern condensers have a lot of problems.

Is your fuel line too near the exhaust manifold? If so, heat shield or insulation.

Is there a way you could squirt the carbs and fuel line with water to see if that cured the problem, then proceed along those lines?

 

[The Cone of Silence]

Red, Bob, Steve, thank you very much, you've put my mind at ease.

I had a Pertronix electronic ignition installed only a few months ago but the coil isn't new. Fuel line is definitely close to the exhaust manifold but no closer than normal. Perhaps a little heat shield to help prevent this occurring again might help. We're set for a scorching summer here with a monster El Nino inbound.

I do keep a bottle of water in the trunk so next time this happens I might try that too!

Well done chaps....and thank you.

Bobby

 

[busybrit]

Vapor lock is what I get on a hot day after leaving the car, but it clears up real quick once the electric fuel pump fills the float chambers. So if you are driving the car home for 30 minutes and you still have the issue, it's not in my experience vapor lock. I suggest swapping your coil out.

 

[bdcvg]

Pressurized fuel does not evaporate as easily as static fuel, a pump not quite working up to par will aid percolation.

 

[The Cone of Silence]

Ok some interesting additions here. I did wonder how long it would have taken to get cooler fuel through and while the stutter response was worse when I first drove off and became less pronounced towards home, it was still occurring.

So, suggestions seem to be a new coil and perhaps fuel pump too.

The fuel pump on Ruby is very loud indeed and I've been threatening to change it for a long time but the classic car mechanic I have always used says it works perfectly well.

Questions, then.....
I'm happy to buy a new coil, but what will this achieve and how will it help?
What are the recommended brands/ products for optimum performance and reliability?
Re the fuel pump, is the suggestion that it's not pumping at a high enough rate?
What are the recommended replacement pumps, again for performance and reliability?

Finally.....is the replacement of the fuel pump and coil a difficult job requiring mechanical know-how? I'm probably most concerned about the fuel pump here in case of a spillage.

Your thoughts are, as ever, greatly and humbly appreciated.

Bobby

 

[steveg]

...So, suggestions seem to be a new coil and perhaps fuel pump too. ...

Suggest one thing at a time. Coil is easiest. Use a non-ballasted coil. Make sure you note which wires go to the + and - terminals.

Are you familiar with the Aussie suppliers, such as the Healey Factory? They will have the best ones.

If you're not in a club, suggest joining one as there's a lot of expertise available.

 

[DerekJ]

You can easily test the fuel pump flow. Just disconnect the fuel line near to the carbs and pump fuel into a litre container while you are timing it. I've forgotten the flow rate for a standard fuel pump but someone will jump into to tell you. Do this before you splash out on a new pump.

You can check the distributor advance. Advance it until you get pinking then back off a degree or so until you don't.

Have you taken the car out again for another test drive? If so is it still stuttering? The original symptoms definitely seemed like fuel vaporisation.

 

[Healey_Z]

I agree with the coil, points, condenser, wires. Like the old saying goes, 80% of carburetor problems are electrical.

 

[The Cone of Silence]

I enquired with the Healey Factory down in Victoria and they advised the following:


Make sure the rest of the tuning is correct
The coil should be matched to the Pertronix ignition as it may not be an off the shelf unit -
check with your local Pertronix supplier
Please check float levels and fuel mixtures as well.

So, as I don't know how to check the tuning or how to match the coil to the ignition, I shall see if I can find someone locally who can.

Thanks for your advice guys, please keep it coming!

Bobby

 

[dcarlg]

Hi Bobby,
Good decision to find a competent mechanic.
I suggest procuring a service manual for your car so you can get a good understanding of your car.
Also suggest that you observe the mechanic as he works his magic.
Good luck.
Douglas

 

[The Cone of Silence]

UPDATE:

Had a butchers at the spark plugs - sooty as a Victorian chimney sweep. Gave them a good clean with the Dremel soft wire brush and put them back in.
My former mechanic down in Sydney suggested that before getting new parts and making assumptions about this and that, to first lean the mixture a little. So, I backed off the slow run screws until the engine was sitting at a nice idle when warm...around 800 rpm, then backed off the mixture adjusting screws 180 degrees. The exhaust was still stinky so I backed them off a further 90 degrees and it seemed to help....I took her out for a 20 minute drive and all was well.

The job now is to warm up the car using throttle only (no choke) and then drive for an hour in varying conditions to test how it all feels and performs. So, I'm going to do that before I think about new coils and timing.

When I did this, the ambient temperature was 28 degrees C and there was no stuttering from the throttle and no backfiring either, so it could just be a case of a rich mixture and a vapour lock issue with the 38C heat of the day when the incident occurred (100.4F)

I'll keep you all posted. As ever, thanks for the tips and advice....which are humbly appreciated.

Bobby