Idle falls until car stalls

[SteveHall64Healey]

Gents,

The idle on my BJ8 is stable and correct when the car is first fully warm, but after the car has been running for more than about 20 minutes, the idle drops slowly until the car stalls. Raising the idle doesn't prevent the slow drop to the point of stall. Any advice?

thanks
steve

 

[Keith_M]

Steve,
I had this exact problem a few years ago, but I'm having trouble remembering what cured it. I THINK that it was cured by the addition of a 5-bladed fan, and I assumed that the problem was caused by inadequate air movement in the engine compartment causing the fuel to boil in the float bowls. Surprisingly, it happened even on relatively cool days.

Sorry I can't be me more sure about this; the old memory cells don't seem to work as well as they once did. But, hey, at least I remember that I had the problem. ...at least I think I had the problem. :dejection:

 

[Tech396]

It sounds like a classic case of the carbs being set too rich, or a sticking needle and seat assy, causing a rich air/fuel mixture..

 

[andrewss]

Fuel quality and vapour lock.. engine bay too hot... todays fuel doesn't suit our classics and as a result we have vapour lock issues.. I guess the issue only arises in town and not on the open road ?

 

[SteveHall64Healey]

yes - open road or even street driving is fine. The problem occurs only at idle, and only after sitting at idle for at least 30 seconds.

 

[Keith_M]

yes - open road or even street driving is fine. The problem occurs only at idle, and only after sitting at idle for at least 30 seconds.

It really does sound like you're boiling the fuel in the bowls. The stock fan doesn't move much air and it gets hot under there pretty quick, particularly if you've been driving hard and then come to a dead stop.

 

[andrewss]

You could do a number of things.. Install a Kenlow fan on the front of the radiator. Wrap the exhaust manifold with heat retarding tape. Add rubber/carbon spacers between the carbs and the inlet manifold to reduce heat transfer. All of the above will help but no one thing will eliminate the problem completely...

 

[Brinkerhoff]

If its heat buildup under the hood then the engine should idle fine with the hood open correct? If not look for another reason.

 

[andrewss]

No because the engine bay needs cold air flow to reduce the temperature in and around the carbs (usually achieved when driving)..

 

[RAC68]

Hi Steve,

From my experience, this condition is the result of a number of issues including a miss-adjusted carburetor set on the rich side in combination with heat build-up in the engine compartment. Even though the heat will drop when the bonnet is open, by that time the build-up of carbon and un-burnt fuel builds in the combustion chambers to cause the reduction of engine RPMs, it will continue. At driving speed, yes engine bay temperature will drop but an extended increase in RPMs will usually burn off the fuel build-up and blow out the carbon to overcomes this RPM-bogging condition until at idle again.

I don't believe that vapor lock is the issue unless the car builds temperature to cause substantial overheating at idle and the car is difficult to restarted (air-block develops in the fuel line). However, making sure the engine compartment is maintained at a cooler state through the creation of a more efficient directed air stream created by the addition of even the simplest of a fan shrouds placed around an aggressive fan and more efficient radiator core.

Again, we are addressing the build-up of an operational environment that will produce increasing amounts of carbon and unburned fuel that will eventually overwhelm ignition. Eliminating or slowing the development of this operational environment will require a hotter and more precise ignition along with the more efficient management of engine operating temperature through the installation of fan, shroud, and improving radiator heat-exchange efficiency. Any one of these may be already present in your car and has probably improved an even worst condition, and any one of the above may help and improve your present situation, however, I believe all are needed to increase the margin to make this condition disappear or become a rare condition.

Good luck,
Ray (64BJ8P1)

 

[SteveHall64Healey]

Thanks for the input. I'm away at the moment (actually in the Golden Lion Pub aboard Queen Mary 2 in the Irish Sea) but upon return I'm going to reset the carbs from scratch and see how bad the problem is after that. I had a problem with float bowls improperly set (too low) which resulted in fuel starvation at higher RPMs, and I have not set the carbs following that repair. The fuel mixture may be too rich with more fuel available to the carbs.

If the problem persists I may be inclined to simply tolerate it. I had the exhaust manifolds ceramic coated over the winter so temperatures under the hood should be much lower than normal. Even so, if it is a head buildup issue, I would not want to deviate too much from stock appearance under th hood by adding an electric fan- although a yellow fan with more blades might be a reasonable compromise.
Steve