Intermittent sputtering

[RAC68]

Hi Nevets,

Although I appreciate the reasoning others have extended with regard to an ignition fault, I would suggest you look back into your fuel system. I am assuming your fuel pump is of the original Lucas type and connected, without interruption, to the fuel tank. It is not uncommon to find dirt or foreign substances have collected in your fuel pump, and is diminishing carb refill flow. This condition usually becomes most obvious after periods of stagnation when the bits pack and harden. If flow is insufficient to meet the higher demand for fuel when under load, a sputtering will occur.

All the best,
Ray 64BJ8P1 (original owner)

 

[nevets]

Ray

That's interesting. The car just came out of winter hibernation when the problem first occurred. The pump is the original SU type, although it is fairly new. Is there a way to check for debris in the pump? And a way to clean it? I'll consult the manual, NN's tech tips. At some point, I should install a filter to protect the pump. As I mentioned earlier in this thread, I did not discover particles in the float bowls.

As a first step towards sorting this, I intend to replace the aforementioned ignition components (coil, cndenser, etc). If that fails to fix the problem, I'll again look into the fuel system, including the fuel pump, which I had not considered previously. Though, if it were the pump, wouldn’t the symptoms be evident upon start up?

Thanks for the help.

 

[RAC68]

Hi Nevets,

Depending upon the pump, there are access plates that can be removed to clean the filters of debris, however, I find it much easier to take the pump out of the car to clean the chambers and filters within the body.

Lucas indicates DON’T PUT A FILTER BETWEEN THE TANK AND PUMP. Should the filter clog, stopping the supply of fuel, the pump will burn out. The pump is capable of handling quite a bit of crap before becoming blocked. Even when the pump is clogged, sufficient fuel should be in the chambers to prevent the pump from self-destructing.

Depending upon the type of debris restricting pump output, sufficient flow may be present for start, idle, and light run. In some cases, as with bits of paper or silicon sealer used inappropriately to seal the gauge unit, partial blockage and fluttering (as a result of pump vibration) may allow varying amounts of fuel to pass and only indicate a problem intermittently.

All the best,
Ray

 

[nevets]

Ray

Thanks for the follow-up and for the caution about placing a filter upstream of the fuel pump. Removing the pump before attempting to dismantle/clean certainly sounds easier than doing the job in situ.

 

[Galen]

nevets

I had the same problem recently. Please refer to thread "Tri carb vapor lock" After many, many hours going through the fuel system I checked the vacuum and that was my problem. Get a vacuum gauge and check the pressure. I adjusted the timing based on the vacuum pressure and that solved my problem.

 

[nevets]

Galen

Interesting...excuse my ignorance but what is the connection between ingition timing and intensity of vacuum pressure? I realize that vacuum is used to advance the timing as engine RPMs increase, and that the higher the RPMs the greater the vacuum, but how would changing the static timing increase the vacuum pressure? Isn't that rather like the tail wagging the dog?

Thanks

 

[Keoke]

.


Interesting...but how would changing the static timing increase the vacuum pressure?

Just consider the engine as being a vacuum pump "OK"

Isn't that rather like the tail wagging the dog?
Not really Nevets




Many racers and engine tuners (I get caught myself sometimes) have forgotten one of the simplest, as well as most accurate tuning tools ... the vacuum gauge. However, it is most accurate and useful on a healthy engine.----Keoke-

 

[Galen]

you should have about 20 in mercury at 3000 rpm. I was only getting 10. This explained the back pressure in the carbs. When driving it felt like i was running out of gas, sputtering @ 3000 rpm. It was because of the low vacuum pressure. I set the timing based on getting the maximum vaccum. I got about 18 in. mercury. And this solved the problem.


The mystery remains though as to why the harmonic balancer is off by 60 degrees from the timing mark? I have just completed a complete re-build.

 

[tahoe healey]

Where are you measuring the vacuum?

 

[Patrick67BJ8]

you should have about 20 in mercury at 3000 rpm. I was only getting 10. This explained the back pressure in the carbs. When driving it felt like i was running out of gas, sputtering @ 3000 rpm. It was because of the low vacuum pressure. I set the timing based on getting the maximum vaccum. I got about 18 in. mercury. And this solved the problem.


The mystery remains though as to why the harmonic balancer is off by 60 degrees from the timing mark? I have just completed a complete re-build.

I absolutely have got to see this! Are you going to bring your car to the next "tech session"?? I want to know why your timing mark is off 60 degrees and your engine still ran and good until 3k rpm. "Fish anyone"?
Patrick

 

[Keoke]

want to know why your timing mark is off 60 degrees and your engine still ran and good until 3k rpm. "Fish anyone"?

Here if the require maximum advance is not being attained[ Timing improperly set at idle] "timing mark off" or "Wrong advance set at idle--Less than specification" it is possible that the engine will stumble in the higher RPM region.

Well Patrick, there are at least two reasons I am aware of that can result in the timing mark being off:

1]I have the item in my garage and I had to use a vacuum gauge to time the engine. Very simply, the notch was improperly located at the factory.

2] It is also possible that the vibration damper has spun in these cases where complete separation does not occur similar results can occur--------Keoke.

 

[Patrick67BJ8]

want to know why your timing mark is off 60 degrees and your engine still ran and good until 3k rpm. "Fish anyone"?

Here if the require maximum advance is not being attained[ Timing improperly set at idle] "timing mark off" or "Wrong advance set at idle--Less than specification" it is possible that the engine will stumble in the higher RPM region.

Well Patrick, there are at least two reasons I am aware of that can result in the timing mark being off:

1]I have the item in my garage and I had to use a vacuum gauge to time the engine. Very simply, the notch was improperly located at the factory.

2] It is also possible that the vibration damper has spun in these cases where complete separation does not occur similar results can occur--------Keoke.

I'm intersted to know if the vibration damper was rebuilt? If not, I would be suspect of it and take appropriate action. If it was rebuilt maybe it was reassembled incorrectly.
Patrick

 

[tahoe healey]

Where are you installing the vacuum gauge? At the T?

 

[Keoke]

I'm intersted to know if the vibration damper was rebuilt? If not, I would be suspect of it and take appropriate action.

Well Patrick,in my case I was suspect and replaced the damper with a known good one.However, this damper had not been rebuilt.



If it was rebuilt maybe it was reassembled incorrectly.
That is a good point that I had not considered.--Keoke- :thumbsup:

 

[Keoke]

Where are you installing the vacuum gauge? At the T?

You want Constant Vacuum TH. Use The Vacuum Port At The Front Base Of The Carb, The distributor vacuum line will be disconnected.--Fwiw--Keoke--

 

[Galen]

To connect the vacuum guage you can simply disconnect the vacuum line at the back carb (tri-carb)and plug in there. Or there is a port at the front carb. Simply unscrew the bolt at that point and check the vacuum.

Patrick I will certainly bring my BN7 to the next Tech session along with my vacuum guage for some fun. Bret Blades figured all this out and he can do the show and tell while i fry up some of those speckled sea trout to go with your coffee and donuts.

We can also debate the issue of the harmonic balancer being off 60 degrees and why the car still ran.

Galen

 

[tahoe healey]

Thanks, I was wondering if the advance line was okay to use or if it would change things by not having the dizzy advance connected. What pressure should I expaect at 6500 feet? Do I understand that it is a matter of max vacuum pressure I am looking for?

 

[nevets]

Keoke

So bsaed on what I'm hearing, vacuum pressure is a function of ignition timing? So assuming all the plumbing is good, I just need to make sure the timing is correct to asure propper vacuum pressure?

Getting back to my problem, if the issue is in fact timing/vac pressure, why would the sputtering come on once the engine temp got up to around 180-190?

 

[tahoe healey]

In reading up on this vacuum thing, there are differences of opinion as to disconnecting the vacuum advance. I did learn that I need to compensate the reading by one psi for every 1000 feet of elevation and since I am at 6500 feet that is significant. I had to do this for compression testing in the past.
Thanks for all the help.
TH