Crackle & Pop

[cliff.b]

Well, after 6 weeks of work I have finally cleared all the mechanical issues on my list, despite the list growing significantly since I started l, especially after each test drive.

But today I went for a longer drive with a forlorn hope that nothing new would crop up and was very pleased to find that everything worked ok. The car felt good and infinitely changed since I bought it last month 👍.

The only issue, if it is an issue, is that when hot I was getting a fair amount of crackling & occasional popping on the overrun. Sounds quite good to be honest but I would like to understand why it's happening.

I assumed that it is due to unburnt fuel in the exhaust but looking at the plugs, I would say if anything I am running slightly lean. 

Someone has suggested that running lean is causing this but can't explain why. 

I will probably try making it s little richer to see if that makes a difference but it would be nice to understand the physics of what is happening if anyone can explain.

 

 

[PeteH]

This explanation came off an American Website:-

First, when you close the throttle, you are also closing the throttle plate. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. The A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.

1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.

So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire.

Pete

[Pete Lewis]

i would richen a tad and advance a tad , re adjust  the idle if it rises 

Pete

 

[cliff.b]

19 minutes ago, PeteH said:

This explanation came off an American Website:-

First, when you close the throttle, you are also closing the throttle plate. This reduces the air and fuel flowing into the motor, and increases the vacuum (lowers the pressure). This results in less air and fuel in the cylinder during the power stroke, which in turn results in a lower pressure in the combustion chamber. The A/F mix burns faster in proportion to the pressure applied? Well, when we reduce pressure this way, the mix burns slower. This results in two things happening.

1. The lower burning fuel generates less heat, and the cooling effect of the non-burning fuel tends to “quench” the flame front, or slow it down even further. Because the mix is burning much slower, the exhaust valve can open before all the fuel is consumed, and the unburnt fuel is ejected into the exhaust.
2. The engine designers, in order to promote smoother idling and better combustion, retard the spark when the throttle is shut, and this results in the mix being lit later.

So, now we end up with unburnt fuel in the exhaust, and burning fuel being ejected into the exhaust, and bang! Backfire.

Pete

Excellent, and in words I can understand lol. So I will try it a little richer and see what happens. I should probably check the vac thingy on the distributor is working as well but will all have to wait until next week now.

Many thanks for looking at this 👍

[Pete Lewis]

dont get confused the vac unit does rely on a spring inside the diaphragm to return/control the base plate in usa many had a twin vac to pull the spark(retard) on decel 

with things like OSC  valves in the pipe line (orifice spark control) a sort of one wat vacuum delay /hold on valve 

yours is simple a way to advance the spark when cruising to gain MPG   it has no effect on performance

the vacuum is developed from a small orifice close to the throttle plate so a vac is generated with high velocity low throttle openings  and not related to manifold vacuum 

as soon as you open or close  the throttles this vacuum is lost and the vac unit spring returns the base plate to static . 

if the internl spring fails the base plate can move to where ever it fancies 

a quick suck (yuk) on the pipe will show its moves and returns ok 

have a scotch handy 

Pete

[cliff.b]

12 minutes ago, Pete Lewis said:

dont get confused the vac unit does rely on a spring inside the diaphragm to return/control the base plate in usa many had a twin vac to pull the spark(retard) on decel 

with things like OSC  valves in the pipe line (orifice spark control) a sort of one wat vacuum delay /hold on valve 

yours is simple a way to advance the spark when cruising to gain MPG   it has no effect on performance

the vacuum is developed from a small orifice close to the throttle plate so a vac is generated with high velocity low throttle openings  and not related to manifold vacuum 

as soon as you open or close  the throttles this vacuum is lost and the vac unit spring returns the base plate to static . 

if the internl spring fails the base plate can move to where ever it fancies 

a quick suck (yuk) on the pipe will show its moves and returns ok 

have a scotch handy 

Pete

Many thanks for this explanation Pete. I knew it was a vac advance but had no idea regarding the intracacies of how it works & why.

Sucking the pipe had the desired effect. I seem to have got used to the taste of petrol over the last few weeks 🤢

[PeteH]

On 28/05/2021 at 10:38, cliff.b said:

I seem to have got used to the taste of petrol over the last few weeks 🤢

A new career as a Fire Eater Beckons?.😆

Pete

[Iain T]

27 minutes ago, PeteH said:

A new career as a Fire Eater Beckons?.😆

Pete

'No naked flames' does that mean it's OK fully clothed? 

[cliff.b]

2 hours ago, PeteH said:

A new career as a Fire Eater Beckons?.😆

Pete

I haven't got to the point of actually enjoying it yet 🥴

[68vitesse]

1 hour ago, cliff.b said:

I haven't got to the point of actually enjoying it yet 🥴

Perhaps not enough ethanol in the petrol yet, could end up driving a drunken car rather driving under the influence.

Try it as an excuse for failing breathalyser.

Regards

Paul

[cliff.b]

8 minutes ago, 68vitesse said:

Perhaps not enough ethanol in the petrol yet, could end up driving a drunken car rather driving under the influence.

Try it as an excuse for failing breathalyser.

Regards

Paul

Tell the Police it's because I had syphoned a lot of petrol, hmm 🤔

[Nick Jones]

Popping and banging on the over-run can also be a sign of an exhaust leak somewhere near the front.

 

Nick

[cliff.b]

16 hours ago, Nick Jones said:

Popping and banging on the over-run can also be a sign of an exhaust leak somewhere near the front.

 

Nick

Can't see any evidence of that but will look more closely if the enrichenment doesn't help

 

[cliff.b]

Adjusted slightly richer and then out for another run.

Crackling/popping now greatly subdued apart for a brief period when it started again but then ok until home. 

Colourtune now suggests slightly rich at tickover but looks good at anything faster & plug colour also looks good to me.

[Nick Jones]

If Colourtune says slightly rich at idle then it’s about spot on

Nick

 

[cliff.b]

Following up on this issue, the crackling on the overun is still only occasional since I made the mixture richer and seems to be only (but not always) when the car is a bit hot. Also, the power seems down when this happens but it all soon recovers.

The car has Waxstats fitted and I'm wondering if the issues these cause might be consistent with what I am experiencing?

Any thoughts much appreciated.

 

[Pete Lewis]

do the 1p (well on each you you need two 1P  )  and get rid of the wax capsule  

its an easy swap just pry the crimped cap of carefully  remove the wax capsule  add two 1p coins make sure you keep the steel spacer its important thats refitted 

refit the crimped cap and its done  no more silly mixture changes you dont want 

Pete

[cliff.b]

3 minutes ago, Pete Lewis said:

do the 1p (well on each you you need two 1P  )  and get rid of the wax capsule  

its an easy swap just pry the crimped cap of carefully  remove the wax capsule  add two 1p coins make sure you keep the steel spacer its important thats refitted 

refit the crimped cap and its done  no more silly mixture changes you dont want 

Pete

So you think it could be causing this, Pete?

[Nick Jones]

Very likely. Was a well known issue back in the day. Can’t be many waxstats left in the wild these days….

Nick

[cliff.b]

3 minutes ago, Nick Jones said:

Very likely. Was a well known issue back in the day. Can’t be many waxstats left in the wild these days….

Nick

I'm assuming it's not had the penny fix already, of course. Doesn't look like the jets have been tampered with though.

Only one way to find out I guess lol

[cliff.b]

11 hours ago, Pete Lewis said:

do the 1p (well on each you you need two 1P  )  and get rid of the wax capsule  

its an easy swap just pry the crimped cap of carefully  remove the wax capsule  add two 1p coins make sure you keep the steel spacer its important thats refitted 

refit the crimped cap and its done  no more silly mixture changes you dont want 

Pete

When removing the crimped cap, is there any spring pressure? 

[Pete Lewis]

you know i dodnt remember but guess there is a light spring in place to allow the wax to shift the jet base 

once the wax capsule is removed it all becomes static and doing all this wont affect the mixture setting which stays as is unless you wish to twiddle 

its cetainly a better move and vastly cheaper than the awful replacement kits that are sold and dont fit or work at all well (they are rubbish) 

you cant go wrong just lift the crimps with care dont butcher them 

as you have to undo the fuel /jet pipe you may need a new small rubber 0 ring/olive 

make sure the rubber olive is well on so the small pipe protrudes beyond the olive before you re insert the pipe and sleeve nut

Pete

[cliff.b]

14 minutes ago, Pete Lewis said:

you know i dodnt remember but guess there is a light spring in place to allow the wax to shift the jet base 

once the wax capsule is removed it all becomes static and doing all this wont affect the mixture setting which stays as is unless you wish to twiddle 

its cetainly a better move and vastly cheaper than the awful replacement kits that are sold and dont fit or work at all well (they are rubbish) 

you cant go wrong just lift the crimps with care dont butcher them 

as you have to undo the fuel /jet pipe you may need a new small rubber 0 ring/olive 

make sure the rubber olive is well on so the small pipe protrudes beyond the olive before you re insert the pipe and sleeve nut

Pete

Many thanks 👍

[cliff.b]

Did the Waxstat mod and noticed front capsule was black & the jet was sticking.

After re-assembly engine wouldn't run without choke until I made the front carb a lot richer. Remembered when I first got car running that carb was very rich until I adjusted it. Compared wax capsules & the one from the front is about 1.5mm thinner.

I'm thinking it has maybe leaked wax and this was the cause of the original richness which I compensated for previously.

[Pete Lewis]

let us know how this all works out 

Pete   ( sorry thats my 2penny worth  Ha!!)