Vitesse 2L Mk2. Misfire at higher Revs

[PatK]

Hi All, I hope that you can help with a problem that I have with my Vitesse. The engine pulls fine at lower revs but once the revs go beyond 3,500 there is a persistent misfire and failure to accelerate. This is particularly noticeable when hill climbing.

After a 40 minute run, I have removed the plugs and they all appear to be fine, a light straw colour. The ignition timing is set by strobe at 13 degrees BTDC at idle with the dizzy vacuum pipe disconnected. The distributor is as new, the distributor doctor reconditioned it ( expensive ) this winter and printed out the correct ignition curves against revs and vacuum advance. The fuel flow appears fine, I cleaned the filters recently.

The spark plugs are NGK BP6ES, gaps correct and the ignition is Luminition electronic with a Lucas Sports coil.  I am at a loss to figure out the problem, should I change the spark plugs, they have been on the engine for a few years ?

Any help would as usual be much appreciated.

Many Thanks

 

Pat

[Paul H]

8 minutes ago, PatK said:

Hi All, I hope that you can help with a problem that I have with my Vitesse. The engine pulls fine at lower revs but once the revs go beyond 3,500 there is a persistent misfire and failure to accelerate. This is particularly noticeable when hill climbing.

After a 40 minute run, I have removed the plugs and they all appear to be fine, a light straw colour. The ignition timing is set by strobe at 13 degrees BTDC at idle with the dizzy vacuum pipe disconnected. The distributor is as new, the distributor doctor reconditioned it ( expensive ) this winter and printed out the correct ignition curves against revs and vacuum advance. The fuel flow appears fine, I cleaned the filters recently.

The spark plugs are NGK BP6ES, gaps correct and the ignition is Luminition electronic with a Lucas Sports coil.  I am at a loss to figure out the problem, should I change the spark plugs, they have been on the engine for a few years ?

Any help would as usual be much appreciated.

Many Thanks

 

Pat

I had similar with my Mk2 Vitesse . Turned out that the rotor arm was catching the Accuspark electronic unit . Changed to the correct red rotor arm and problem fixed . Have you tried traditional points to see if this cures your problem ?

Paul 

[PatK]

2 minutes ago, Paul H said:

I had similar with my Mk2 Vitesse . Turned out that the rotor arm was catching the Accuspark electronic unit . Changed to the correct red rotor arm and problem fixed . Have you tried traditional points to see if this cures your problem ?

Paul 

Thanks Paul. When the Distributor Doctor overhauled my distributor, which was fitted with the Luminition magnetronic, ignition, he checked all clearances and replaced the rotor arm with a new one and checked all the clearances. He remarked that some rotor arms ( cheap e bay ? ) sit too high and force the carbon brush in the centre of the distributor cap hard up into the cap also causing misfires, but I have checked mine and there is good clearance. The clearance between the rotor arm and the hall effect sensor is fine, so I don't think that is the problem.

[Pete Lewis]

i would suggest a plug misfire  and maybe the ngk problem of insulator contamination

try a set or Bosch W78     dont get any plug with an R in it 

if the misfire is more violent rather than a rythmical plug dropping out  then have a search for rubber slivers in the carb float valve zones 

Pete

 

[Iain T]

It does seem as though it's the plugs. I found Bosch W78 bought from Greensparkplug work much better than NGK. 

Iain 

[PatK]

13 hours ago, Pete Lewis said:

i would suggest a plug misfire  and maybe the ngk problem of insulator contamination

try a set or Bosch W78     dont get any plug with an R in it 

if the misfire is more violent rather than a rythmical plug dropping out  then have a search for rubber slivers in the carb float valve zones 

Pete

 

Thanks Pete, good advice as usual, I have just ordered a set of W78 plugs. What I forgot to mention in my post was that there is no misfire when revved up from idle, it only happens when the engine is under load and the accelerator is floored, and the throttle butterflies are fully opened above 3500 revs. Do you think that it could be restricted airflow in the airbag, which is fitted with e bay pancake air filters?

[PatK]

13 hours ago, Iain T said:

It does seem as though it's the plugs. I found Bosch W78 bought from Greensparkplug work much better than NGK. 

Iain 

Thanks Ian, I have just done that and also ordered them from greensparkplug, as I am now unsure about ordering cheaper priced ones from e bay.

[Iain T]

9 minutes ago, PatK said:

Do you think that it could be restricted airflow in the airbag, which is fitted with e bay pancake air filters?

Probably not but take the pancakes off and take it for a spin. It'll run a bit lean but should see if they are the problem. 

Iain 

[Pete Lewis]

as a avid hater of aftermarket must haves i would do as Iain suggests and take them off 

most aftermarket  must have stuff might sound good but on a stop watch is normally a lot worse than standard 

but no one will ever admit to that   Ha !!

Pete

[Clive]

1 hour ago, PatK said:

Thanks Pete, good advice as usual, I have just ordered a set of W78 plugs. What I forgot to mention in my post was that there is no misfire when revved up from idle, it only happens when the engine is under load and the accelerator is floored, and the throttle butterflies are fully opened above 3500 revs. Do you think that it could be restricted airflow in the airbag, which is fitted with e bay pancake air filters?

What oil is in the dashpots?

And yes, cheap pancake filters are not helpful. Either go the whole way, cold air/free flow filter like K+N etc and a proper setup on the rolling road or using an AFR gauge so you can sort the fuelling. Or rather easier, keep it standard...

But usually misfire is down to plugs/leads/distributor innards.

[poppyman]

Not a partially blocked fuel filter by any chance Pat?

Tony.

[GrahamB]

My Spitfire came with cheap pancake  filters and  suffered from similar symptoms.  The cause was that the filters were too free flowing so there was not enough depression to suck enough petrol through the carbs resulting in the mixture becoming weaker as the revs increased.  I solved the problem by just fitting the standard paper filters direct to the carbs. Once the cause was confirmed, I purchased a second hand filter box so I could feed cold air to the carbs via the ducts from the front of the radiator.  If this is the cause of your problems, removing the filters will not help unless you also fit richer needles. You cold try pulling out the choke when the engine starts to loose power above 3500 rpm to see if there is an improvement.  If it gets worse then it would indicate that the mixture is already too rich.

The proviso is that the Spitfire is fitted with SUs and not Strombergs. 

[Dave the tram]

Hi

Just a long shot but I had a misfire with my GT6 that I could never resolve until recently. At full throttle, higher end revs and especially uphill - it would start to loose power as if starved of fuel, then immediately cough repeatedly as if back through the carbs. I think it was, as one air filter casings got a bit sooty! I sort of learnt to live with it having tried so many things, but couldn’t confidently overtake.

Recently I serviced the car and the only things I did that could have affected it were:

- check and clean plugs, all looked ok and gap correct

- check dizzy cap (it has electronic ignition)

- checked tappits, all correct.

In refitting the rocker cover, I noticed it was deforming a bit around the bolts and the washers weren’t seating well. Refitted with better combination of steel, nylon and rubber washers to get good seal. It has run well ever since and I think a vacuum leak was causing it via the breather pipes. I intend to try and recreate the fault to confirm this, but am currently in the middle of other jobs on the car.

cheers, Dave

[Pete Lewis]

8 hours ago, PatK said:

when the engine is under load

this is when plugs are most vulnerable under  max pressure the spark can jump off all over the place or not jump at all 

Pete

[PatK]

10 hours ago, Iain T said:

Probably not but take the pancakes off and take it for a spin. It'll run a bit lean but should see if they are the problem. 

Iain 

 

2 hours ago, Pete Lewis said:

this is when plugs are most vulnerable under  max pressure the spark can jump off all over the place or not jump at all 

Pete

Thanks Ian and Pete, I will take the filters off tomorrow and give it a try, then if it is still like it I will go down the rubber slivers route, although I did replace my fuel hoses with Gates Barricade rubber ones, but they too may have been counterfeit. I guess the Club shop is one of the few places that you can trust

[PatK]

4 hours ago, Dave the tram said:

Hi

Just a long shot but I had a misfire with my GT6 that I could never resolve until recently. At full throttle, higher end revs and especially uphill - it would start to loose power as if starved of fuel, then immediately cough repeatedly as if back through the carbs. I think it was, as one air filter casings got a bit sooty! I sort of learnt to live with it having tried so many things, but couldn’t confidently overtake.

Recently I serviced the car and the only things I did that could have affected it were:

- check and clean plugs, all looked ok and gap correct

- check dizzy cap (it has electronic ignition)

- checked tappits, all correct.

In refitting the rocker cover, I noticed it was deforming a bit around the bolts and the washers weren’t seating well. Refitted with better combination of steel, nylon and rubber washers to get good seal. It has run well ever since and I think a vacuum leak was causing it via the breather pipes. I intend to try and recreate the fault to confirm this, but am currently in the middle of other jobs on the car.

cheers, Dave

Thanks Dave, that is a most unusual fault, although I'm sure Pete has seen many more strange ones.  Thanks Dave I will bear that in mind, if changing the spark plugs and removing the air filters does not cure it.

[Iain T]

On 30/05/2021 at 19:01, PatK said:

The engine pulls fine at lower revs but once the revs go beyond 3,500 there is a persistent misfire and failure to accelerate. This is particularly noticeable when hill climbing.

Is this something new that's just developed? If so what was the last thing you changed prior to it running ok? 

[PatK]

8 hours ago, GrahamB said:

My Spitfire came with cheap pancake  filters and  suffered from similar symptoms.  The cause was that the filters were too free flowing so there was not enough depression to suck enough petrol through the carbs resulting in the mixture becoming weaker as the revs increased.  I solved the problem by just fitting the standard paper filters direct to the carbs. Once the cause was confirmed, I purchased a second hand filter box so I could feed cold air to the carbs via the ducts from the front of the radiator.  If this is the cause of your problems, removing the filters will not help unless you also fit richer needles. You cold try pulling out the choke when the engine starts to loose power above 3500 rpm to see if there is an improvement.  If it gets worse then it would indicate that the mixture is already too rich.

The proviso is that the Spitfire is fitted with SUs and not Strombergs. 

Thanks Graham. I have a standard Vitesse air filter box fitted and inside it are the 2 filters, it is fed via two cold air ducts from beside the radiator. I will see if it still misfires with the filters removed and if it is worse then I will know that there is too much airflow, if pulling out the choke cures it then we are on the right track. i realise that with the filters removed the mixture will be a little weaker.

[PatK]

8 minutes ago, Iain T said:

Is this something new that's just developed? If so what was the last thing you changed prior to it running ok? 

Thanks Ian. It has been there for a while, which was one of the reasons that I sent the Lucas distributor to the Distributor Doctor to rebuild, and that was the last thing that was changed. 

[PatK]

9 hours ago, poppyman said:

Not a partially blocked fuel filter by any chance Pat?

Tony.

Thanks Tony, I have one of the Club's glass filters which can be removed and cleaned. I did this recently as there was some small black sediment in it, but not much and it was not blocked, but I cleaned it just in case.

[Iain T]

OK then try all the above one step at a time so you can identify what (hopefully) works! 

We've all been there, annoying but satisfying when you find the fault! 

Best of luck. 

[PatK]

9 hours ago, clive said:

What oil is in the dashpots?

And yes, cheap pancake filters are not helpful. Either go the whole way, cold air/free flow filter like K+N etc and a proper setup on the rolling road or using an AFR gauge so you can sort the fuelling. Or rather easier, keep it standard...

But usually misfire is down to plugs/leads/distributor innards.

Thanks Clive, engine oil is in the dashpots ( 20/50 ) and I regularly check these. Once I have tried running it without the air filters then hopefully I will  be able to see if that is where the problem lies

[PatK]

2 minutes ago, Iain T said:

OK then try all the above one step at a time so you can identify what (hopefully) works! 

We've all been there, annoying but satisfying when you find the fault! 

Best of luck. 

Thanks Ian, very helpful, as you say try one thing at a time. Air filters tomorrow and plugs next when they arrive. I will keep you posted.