Misfire After Prolonged Idling

[Gibber]

My Healey 3000 MK II BT7 is idling great and revving strong to over 4000 RPM. My problem is after idling for 15 to 20 minutes I have a bad misfire over 2000 RPM. If I let the engine cool off for 10 to 15 minutes it revs fine again. It is not over heating, after idling 15 to 20 minutes it is at 212F up from running temp of 195 F. I have a Crane XR 700 electronic ignition with a MDS Blaster 2 coil. I replicated the condition and tested the coil with a multimeter, the primary was .3 and secondary 6.0 both within spec. I have new plugs and leads. Not sure what to try next.

 

[JPSmit]

Seems like it is heat related. Do you by chance have another coil that you could swap in and try?

Coils are complete voodoo magic to me in terms of understanding how they work but, I had a misfire and a new coil (Bosch Blue) straightened it out completely.

 

[Editor_Reid]

My Healey 3000 MK II BT7 is idling great and revving strong to over 4000 RPM. My problem is after idling for 15 to 20 minutes I have a bad misfire over 2000 RPM. If I let the engine cool off for 10 to 15 minutes it revs fine again. It is not over heating, after idling 15 to 20 minutes it is at 212F up from running temp of 195 F. I have a Crane XR 700 electronic ignition with a MDS Blaster 2 coil. I replicated the condition and tested the coil with a multimeter, the primary was .3 and secondary 6.0 both within spec. I have new plugs and leads. Not sure what to try next.

Boiling fuel?

Curious: Under what conditions/cicumstances do you idle the car for 15-20 minutes?

 

[Bob_Spidell]

Coils are simple devices (essentially half of an AC transformer): DC current flowing through one coil of wire creates an electromagnetic field around the coil. When the current stops flowing the field collapses and induces current into a nearby, electrically isolated (insulated) coil. The voltage induced in the secondary field is proportional to the ratio of coils in the primary coil--the one points or electronics send current to--to the number of coils (wraps) in the secondary coil; e.g. if the primary coil has 10 wraps (coils) and the secondary has 1,000 wraps, the secondary field will have 1,200 volts (1 to 100). The number of wraps in an ignition coil is much higher, of course, and that's how an ignition coil can turn 12VDC into a 10-20KV spark across the plug gap.

To be efficient, the current to the primary coil should last long enough to saturate the primary field, which is a fixed value called the 'time constant' ('TC'). The TC in a points system varies with engine speed--too long at idle, which can cause the coil to heat up, and too short at high RPM, causing a marginal spark--but electronic systems produce a fixed TC which makes them preferable for efficient operation. The two coils in an ignition coil must be electrically isolated to each other, otherwise there'd be a short circuit between them. If/when that insulation breaks down, which can happen with heat, you will get inconsistent operation or outright failure. IIRC, older coils used a liquid, oil-like fluid for insulation but modern coils--I've never deconstructed one--use insulated wire or some sort of dielectric (insulating) film between them. Power line transformers, which 'step down' 44KV or so line voltage to 240V or 120V--in a series of steps--used to use PCBs for insulation, until it was learned the stuff is a hideous carcinogen.

Edit: I think Reid's suggestion is the most likely culprit; I get similar symptoms on hot starts, but they only last for a mile or two.

 

[Gibber]

Seems like it is heat related. Do you by chance have another coil that you could swap in and try?

Coils are complete voodoo magic to me in terms of understanding how they work but, I had a misfire and a new coil (Bosch Blue) straightened it out completely.

Thanks for the response, I have a new MSD Blaster 2 high performance coil that I just purchased. I can try that, I have ordered a new rotor, but not sure if that will help. I am just trying to cover all bases.

 

[Gibber]

Boiling fuel?

Curious: Under what conditions/cicumstances do you idle the car for 15-20 minutes?

Thanks for the response Reid. I live in West Vancouver Canada and we have a three lane alternating suspension bridge to Vancouver. At many times of the day with only one lane you can end up idling for up to 45 minutes to get across. If it was boiling fuel I believe the engine would stall. It continues to run but has a mis-fire even after I get moving again and the temperature comes down.

 

[Gibber]

Coils are simple devices (essentially half of an AC transformer): DC current flowing through one coil of wire creates an electromagnetic field around the coil. When the current stops flowing the field collapses and induces current into a nearby, electrically isolated (insulated) coil. The voltage induced in the secondary field is proportional to the ratio of coils in the primary coil--the one points or electronics send current to--to the number of coils (wraps) in the secondary coil; e.g. if the primary coil has 10 wraps (coils) and the secondary has 1,000 wraps, the secondary field will have 1,200 volts (1 to 100). The number of wraps in an ignition coil is much higher, of course, and that's how an ignition coil can turn 12VDC into a 10-20KV spark across the plug gap.

To be efficient, the current to the primary coil should last long enough to saturate the primary field, which is a fixed value called the 'time constant' ('TC'). The TC in a points system varies with engine speed--too long at idle, which can cause the coil to heat up, and too short at high RPM, causing a marginal spark--but electronic systems produce a fixed TC which makes them preferable for efficient operation. The two coils in an ignition coil must be electrically isolated to each other, otherwise there'd be a short circuit between them. If/when that insulation breaks down, which can happen with heat, you will get inconsistent operation or outright failure. IIRC, older coils used a liquid, oil-like fluid for insulation but modern coils--I've never deconstructed one--use insulated wire or some sort of dielectric (insulating) film between them. Power line transformers, which 'step down' 44KV or so line voltage to 240V or 120V--in a series of steps--used to use PCBs for insulation, until it was learned the stuff is a hideous carcinogen.

Edit: I think Reid's suggestion is the most likely culprit; I get similar symptoms on hot starts, but they only last for a mile or two.

Thanks, I will try the new coil.

 

[Editor_Reid]

Thanks for the response Reid. I live in West Vancouver Canada and we have a three lane alternating suspension bridge to Vancouver. At many times of the day with only one lane you can end up idling for up to 45 minutes to get across. If it was boiling fuel I believe the engine would stall. It continues to run but has a mis-fire even after I get moving again and the temperature comes down.

I've been there myself, in a 1968 Lotus Elan several years ago, watching the temp needle headed for melt-down territory. The car didn't explode, as halfway expected, or even burst a hose, but it certainly sucked all the joy out of the drive and sent the Stress-O-Meter way past the redline.

The experience did motivate the installation of a thermostatically controlled auxiliary radiator fan, complete with a manual override.

 

[Gibber]

Thanks, I will try the new coil.

Yes, it can be frustrating. An external electric fan is definitely on my radar.

 

[BoyRacer]

To me, 212 degrees is overheating a Healey engine. Unlike modern engines they are not designed to run happily at 212. And if sitting in traffic is a regular occurrence then you definitely need an auxillary fan.

Do you remember the scene in Ford vs. Ferrari where Ken Miles drives a customer's car (an MGA I think) ) at high speed around the block to blow out the carbon so the car will run right again? I think the same principle applies here.

 

[Michael Oritt]

Mixture, timing, heat-wrap on pipes, upgraded fan and/or auxiliary fans, premium (?) water pump: there are many fixes we chase--some that actually work--to keep engine temps down.

Sometimes simply putting a piece of duct tape over the temperature gauge to get it out of sight and out of mind is the best solution. I've done that while climbing a pass and/or on long desert runs in high ambient temperatures when nothing can be done but to carry on.

 

[Gibber]

To me, 212 degrees is overheating a Healey engine. Unlike modern engines they are not designed to run happily at 212. And if sitting in traffic is a regular occurrence then you definitely need an auxillary fan.

Do you remember the scene in Ford vs. Ferrari where Ken Miles drives a customer's car (an MGA I think) ) at high speed around the block to blow out the carbon so the car will run right again? I think the same principle applies here.

I know that 212F is hot but if I am on the highway at 212 F the engine still runs smooth without mis-fire. In the case when idling at 212 F it mis-fires over 2000 RPM and when the temp comes back to normal 190 F temp it still mis-fires, this is the mystery.
Thanks for the input.

 

[Gibber]

Mixture, timing, heat-wrap on pipes, upgraded fan and/or auxiliary fans, premium (?) water pump: there are many fixes we chase--some that actually work--to keep engine temps down.

Sometimes simply putting a piece of duct tape over the temperature gauge to get it out of sight and out of mind is the best solution. I've done that while climbing a pass and/or on long desert runs in high ambient temperatures when nothing can be done but to carry on.

I know what you are saying and I do the same on the highway n hot summer temperatures. It may be hot but still runs well. This is a case in 60 degree weather misfiring over 2000 rpm after idling for long periods.

 

[JPSmit]

Thanks for the response, I have a new MSD Blaster 2 high performance coil that I just purchased. I can try that, I have ordered a new rotor, but not sure if that will help. I am just trying to cover all bases.

BTW this is the coil I use. I still have the ballast resistor on the car - disconnected- in case I need a regular coil but I love this thing.

12V Bosch Blue Coil with Mounting Bracket, US Version, 00-012US - Aircooled.Net VW Parts

This is a factory replacement Bosch Blue Coil with the 3 ohm impedance, compatible with points, and points replacement devices, in 12V electrical systems. There are 5 DIFFERENT 'Bosch Blue Coils' last we checked, only ONE is correct for you (This one)!

vwparts.aircooled.net

here is a fascinating read:

Bosch Blue Coils

Even if you have one in the parts bin or from another friend or club member - just to try

 

[DerekJ]

I agree with Reid re boiling fuel. Happened to me on a long trip back from a Healey event when I was in a long slow moving queue on the autoroute.

 

[Gibber]

BTW this is the coil I use. I still have the ballast resistor on the car - disconnected- in case I need a regular coil but I love this thing.

12V Bosch Blue Coil with Mounting Bracket, US Version, 00-012US - Aircooled.Net VW Parts

This is a factory replacement Bosch Blue Coil with the 3 ohm impedance, compatible with points, and points replacement devices, in 12V electrical systems. There are 5 DIFFERENT 'Bosch Blue Coils' last we checked, only ONE is correct for you (This one)!

vwparts.aircooled.net

here is a fascinating read:

Bosch Blue Coils

Even if you have one in the parts bin or from another friend or club member - just to try

Thanks, hopefully it will help.

 

[red57]

I know what you are saying and I do the same on the highway n hot summer temperatures. It may be hot but still runs well. This is a case in 60 degree weather misfiring over 2000 rpm after idling for long periods.

I think its fuel percolation - But not so much the engine temp, rather the fuel temp is the problem. I have been fighting the same problem and I believe it's down to air flow - when you are on the highway there is lots of cooling air flowing past the carburetors and fuel lines but when you are sitting idling there is little air flowing past the carbs and fuel lines. Heat soak from the head and manifolds slowly heats the fuel and it vaporizes.

This is the same phenomenon we know as "hot restart" problems when the car runs like crap for a few miles after a short stop to get gas or take a break. After a couple of miles fresh, cool fuel to the carbs settles things down and it runs fine again.

I have 2 5wire UEGO air/fuel monitors in my car (one in each header collector) and when the stumbling happens I can clearly see the A/F needles swing rapidly all over the place from rich to lean corresponding to the sputtering of the car. I think this fluctuation reflects the liquid vs vapor in the system - carbureted engine MUST have liquid fuel to meter correctly. This is why, in my case anyway, I'm confident it's not some electrical issue.

I've been researching the issue a lot lately and several things I have learned - One reason we are suffering this more these days than the past is the formulation of gas these days is a lot different than it was in the 50s, 60, and 70s. Some of the additives have a lot lower boiling point and vaporize easier, most modern fuels burn much slower therefore getting the exhaust hotter than it used to be (some are finding advancing the spark at lower rpms helps because the slower burn rate needs more time to fully combust inside the combustion chamber rather than still burning in the exhaust tract). On top of that, there are very few carbureted engines so they formulate for fuel injection which is at much higher pressures and may not be as subject to fuel vaporization as our 2-3psi systems.

I think one of the main problems is the fact that our fuel systems are dead heading - when the float chambers are full, the flow cuts off and the fuel then sits in the lines and has time to get hot. One of the solutions is to have a bypass system with a return line to the tank so when the float chambers get full, the fuel continues to flow so it doesn't sit and get hot...thereby getting a continuous supply of cool fuel from the tank. Datsun did this with the 240Z when they copied the Healey and most all modern fuel injected cars do this.

One solution I'm playing with right now is blowing a cool air stream onto the carb bodies and float chambers to help keep them cool. I have made a 'manifold' out of pvc pipe that is mounted just below the carbs with holes blowing air up to the base of the carbs and float chambers. I have a thermostat snap switch set to on/off at 140F as an initial test (I have read that a lot of modern fuels evaporate around 120-140F). I found a small 12v computer cooling fan I mounted in the bottom inside of the inner fender well for a source of cool air. I'Il have a chance to drive it in a couple of days and know more then.....

If I don't get some success with that approach, I plan on getting a bypass pressure regulator and plumbing a return line - but since that is a much bigger project, I decide to try the 'blowing cool air test' first.

Not a very good picture but shows the general idea (and obviously temporary with cable tie mounting) - if it works, I'll make something much nicer