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Stumble on Acceleration

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Rob Glasgow

Rob Glasgow

Jedi Knight
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For the last few weeks, I've noticed my BT7 stumbles on acceleration after waiting at a stoplight. The car cuts out 2 or 3 times initially when starting out and then accelerates and runs smoothly. It only happens after being at idle for 15 seconds or more, like stopped at a light. If I stop at a stop sign for a few seconds, it does not stumble on acceleration.

Any ideas where to start to solve the problem?
 
Hi Rob,

It’s a little difficult to assist with the limited information provided as the problem could be the result of a number of issues from an intermittent ignition circuit ground to bad gas. Have you modified the ignition from the original points setup? Have you adjusted your carburetors or ignition lately? When was the last time you drove the car without a stumble? Have you checked the gas in your carburetor bolls or gas line filters (if installed)? Although I assume you have already checked most of these possible areas, I would rather have definitives to create a base-of-understanding.

Look forward to your answers,
Ray (64BJ8P1)
 
Just a SWAG, but it sounds a bit like vapor lock. My BJ8 will stumble a little for the the first mile or so after it's sat for a while after a long run, especially on a hot day. It's only done this for the last few years, roughly corresponding to the addition of ethanol to California gas (which may or may not mean anything).
 
I had this exact problem for a long time. It turned out to be a defective Moss Gold Lucas Sport coil. I replaced it with a Pertronix un-ballasted coil around 6 months ago and haven't had the problem since, including lately with several drives in 90+ degree weather.

Tend to concur on the ethanol-n-gas -- I bet it makes vapor locking more common, as well as hot-flooding.

Remember the old adage: "90% of fuel problems are electrical."
 
After rethinking this, I've decided the stumble is caused by vapor lock as Bob suggested. Several years ago I lived in the Central Valley of California where the summer temps often reach 100 plus. After driving the car for a while and then stoping for a few minutes, the car would stumble for a minute or so before it finally smoothed out. I knew the symptoms but never thought it would happen here on the coast where an 80 degree day is unusual. In thinking about when this happens now, it is always after I've been driving around town for a while. I drove the car today and had several stoplights but no stumble. I realized I had not been driving very long so no vapor lock. Just smooth acceleration.
So now that I think I've found the reason, vapor lock, what is the fix?
 
How's your carberator heat shield? Does it still have the asbestos insulation on the underside? If so, maybe your fuel line is too close to the exhaust. If not, maybe add some insulation to the fuel line in the engine compartment.
 
John, My original carb heat shield is in place, but after 52 years, it may not be as effective as when new. I think I will start with some insulation on the fuel lines to see if that helps. I don't really like adding non-original things that are visible, but I will give it a try. Thanks for the suggestions.
 
Got to admit, I don't understand vapor lock. Ok, from an engineering perspective I understand the relationship between pressure, temperature and boiling point of a liquid which can cause a mechanically driven fuel pump to stop supplying fuel but that doesn't apply in a Healey.

Normal 'vapor lock' occurs when the sudden depression in a mechanical fuel pump as the diaphragm moves causes the pressure inside it to drop below the fuels boiling point and instead of bringing in fresh fuel, a vapor bubble forms. Under the right conditions the engine heat transfers to the fuel remaining in the pump chamber and vaporises it so the engine starves. The Healey is different in that the fuel pump is at the back so the majority of the fuel system is positively pressurised. Only the section from the tank to the pump can get vapor locked and this seems unlikely without also hearing the pump sounding like it is running out.

Maybe we use the same term for different things?

otoh, if we are talking about fuel boiling in the float bowels then that is a different story. That is most noticeable as difficult to start hot and is effectively cured by ceramic coating the manifolds.

Andy.
 
It has always been my understanding that vapor lock in a mechanical pump is caused by a weak pump. Time for a change-out?
 
I had this exact problem with my BT7 when I first started driving it last summer after restoration. I concluded that the problem was the fuel in the carbs or the line near the carbs getting boiling while sitting at a light. This seems to be the consensus for your problem, and I agree. I fixed the problem completely by installing a 5-bladed plastic fan from Denis Welch. This moves a LOT more air than the original fan and keeps the engine compartment (and the radiator) cooler while sitting a light.

Now, as to exactly what is occuring when the fuel gets too hot. I agree that the phenomenon is not true "vapor lock", which can only occur in a negatively pressured fuel line. Most of the Healey line is positively pressured. Boiling in the line near the carbs could be a problem though in that the line would deliver only vapor to the carbs for a while until the bubbles were expelled. Boiling in the carbs is also a possibility. Either way, the fix is to reduce the heat load on the lines and carbs either by shielding or ventilation.

I wonder if the amount of ethanol in the fuel makes this problem worse?

Keith
 
Rob,

Although I appreciate your conclusion, I do not feel your issue is caused by vapor lock. This feeling is based upon your comments that the ambient temperature was not very high, the problem would clear within a few minutes/miles of starting, and the issue has only recently appeared.

From my experiences, vapor lock in a Healey has become a catchall and, where the issue is tracked further, another cause has always been identified. Don’t get me wrong, I do believe vapor lock could be a valid difficulty, however, not as often as it has been pointed out.

Keith made some very good points and cooling the engine compartment is always a good idea. I would continue to check the ignition and fuel systems for anomalies. Also, winter gas changeover happened in most of the US during mid September and, depending upon the timing of your issue, could be the main or contributing factor.

Good luck,
Ray (64BJ8P1)
 
Wikipedia has a pretty good summation:

https://en.wikipedia.org/wiki/Vapor_lock

Looks to me like vapor lock is possible anywhere there is localized heating of a fuel line, carburetor or pump. Also looks like we can't blame ethanol in the gas (dang) as it has a lower vapor pressure than gasoline. As I stated, I have a similar problem in my BJ8 when the whole car is thoroughly heat-soaked, which stops as soon as the car is cooled by airflow.

Some piston aircraft engines are notoriously hard to hot start because of vapor lock. Hot startup procedure is typically to run the boost pump for a minute or more to purge the fuel lines prior to attempting a hot start.
 
This is a interesting discussion, and my guess is (given how common the phenomenon seems to be) that it's been had before in this forum. Being new here, though, I feel the need to continue for a while. :smile:

I stick by my guess that Rob's problem is caused by boiling of the fuel in the line or carbs, because the problem only occurs when sitting a light for while and it goes away after a few minutes. On the other hand, I suppose it could be caused by some component of the ignition system getting hot, too.

I confess that I'm completely confused as to the actual definition of "vapor lock". Maybe we can agree that for the purposes of this discussion, it means a bubble of vapor somewhere in the fuel line that disrupts fuel flow. I'm not convinced that ethanol isn't the culprit though. The boiling point of ethanol is around 170 F, but the boiling point of gasoline seems to vary tremendously depending on season and supplier. One site listed the range as 100-400 F! So, it would seem to me that the odds of having a "vapor lock" problem might depend on where you buy your gas, whether it's winter or summer mix, and the ethanol content.
 
Am going to restate I had exactly the problem described.

For instance, I'd be sitting at a stoplight on an 80 degree day for, say, 45 seconds. Drive away from the light and the car would run rough for a block or two then run smoothly. Driving the car normally, it seemed to require more throttle than in the past. This went on for a long time; I was testing for the percentage of ethananol and different percents didn't seem to matter.

Then I swapped my Moss gold coil out for a Pertronix coil I already had and the problem disappeared completely.

That was before the beginning of the summer and I've driven the car several times in 90 degree weather since and it has always run fine. No other changes.

In addition, speaking to a Moss employee at a car club function, he said there have been issues with some of these coils.
 
Like SteveG, I have cured vapor lock on two vehicles- one by changing the coil, the other by changing the dist cap as the carbon brush had disintegrated. I recently cured a bizarre problem that felt exactly like fuel freezing- a stumble and poor idle shortly after cold start and which turned out to be weak exhaust valve springs being pushed open by high cold oil pressure in the hydraulic lifters. Sometimes the obvious cause isn't the actual one.

If somebody could spend $25 on the Harbor Freight multimeter that comes with a thermocouple and tape it onto a carb float bowl/gas line to get the actual fuel temp then I might be slightly more convinced that the problem is due to fuel boiling.

Andy.
 
Steve, I installed Pertronix ignition including their coil several years ago. I also have an original Texas Cooler fan that works great. I'm still inclined to believe the problem is related to over-heated fuel. I am going to try some insulation on the gas lines and perhaps the fuel bowls if I can figure out how to do that.
Darn it, just when I said the California coast almost never gets above 80 degrees, today we hit 92. I didn't drive the Healey today so I can't comment on the stumble issue. I, on the other hand, was way overheated and stumbled several times......
 
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