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Spark Plug
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Introduction

THE Spark Plug has 2 primary functions:

  1. To ignite the Air-Fuel mixture
  2. To remove heat from the combustion chamber

The Spark Plug firing end must be kept COLD enough to prevent Pre-Ignition but HOT enough to prevent Fouling. This must be maintained at Varying RPM, Loads and Air-Fuel Mixture Ratios. The range of Spark Plug temperatures is known as the Heat Range.


Heat Range

FOR a 2-Stroke engine as used in the VeloSoleX S 3800, the spark plug temperature must be in the Heat Range of 500C to 850C (932F to 1562F). To check that the plug is within this Heat Range, a NEW plug should be inserted into a fully-warmed engine and taken out after a few minutes of normal running on a typical road. On viewing the plug the White Insulator Nose should appear COMPLETELY clean.

If the White Insulator Nose is fouled (oiled up) even slightly then the plug is too COLD meaning that too much heat is leaving the plug and returning to the cylinder head via the plug threads. As a result, the plug is not hot enough to burn off the combustion deposits of oil and carbon. If ignored, the fouling will worsen until the plug will not ignite the mixture reliably and misfiring of the engine will occur.

If the Electrode Tip shows serious signs of overheating (eg: turning blue, a straw colour or other discolouring) or the edges are rounded then the plug is too HOT. A plug which is too HOT does not mean it increases the engine temperature. There is simply too long a Heat Path from the Electrode Tip to the threads in the cylinder head to cool it sufficiently. A too long Heat Path means that the plug is less able to cool (due to greater thermal resistance) and the plug will warm to a higher temperature.

Changing the spark plug to one which is 1 Grade HOTTER means an increase in working temperature of about 70C - 100C (158F - 212F) and vice-versa for 1 Grade COLDER. Be aware that Spark Plug heat grades are not as evenly-spaced as the numbers suggest they might be.

HOTTER plugs are best for short stop-start journeys, low RPM or lighter loads. COLDER plugs are best for long journeys, high RPM, heavier loads and most highly-tuned race engines.

While manufacturers often recommend a COLDER plug for warranty reasons, the VeloSoleX S 3800 will foul plugs less with a HOTTER plug. A HOTTER spark plug has a longer White Insulator Nose within the plug but the nose DOES NOT project more into the combustion chamber.

A even better solution is a Hotter and Projected-Nose type of spark plug.


Projected-Nose

A Projected-Nose plug projects the Electrode Tip further into the combustion chamber than a normal plug. On a 2-Stroke engine this causes a temperature increase at the Electrode Tip of about 10C - 20C (50F - 68F) compared to a normal plug at lower speeds and less rapid increase of temperature at higher speeds due to the cooling effect of the mixture over the plug tip. Thus it tends to widen the Heat Range rather than change the operating temperature. This type of plug is better able to cope with varying engine conditions and loads.

Another advantage of the Projected-Nose plug is that it projects further into the combustion chamber to help ignite the mixture. There is no problem with plug clearance. With an NGK BP(R)5HS spark plug installed, there is a clearance of 5.5 mm between the Ground Electrode and the Piston Crown at TDC (Top Dead Centre).


Air-Fuel Mixture Ratio

THE correct Air-Fuel Mixture Ratio of the Carburettor can be checked by looking for a 0.25 mm wide Black Soot Ring around the extreme base of the White Insulator Nose. This is deep inside the spark plug and you may need a magnifying glass to view it. This indicates a mixture on the SLIGHTLY RICH side of the ideal stoichiometric air-fuel mixture ratio of 14.6 : 1 and tends to be the IDEAL setting for older, air-cooled 2-Stroke engines.

It is the safest setting as well since the mixture may lean out (weaken) while running because of other factors. For example the mixture will weaken slightly at lower altitudes or during drier or colder weather. Conversely, the mixture will richen slightly at higher altitudes or during humid or warmer weather.

If the Black Soot Ring is over 1 mm wide then the Air-Fuel Mixture Ratio is rich enough to be safe but TOO RICH for maximum power output. It is important, before viewing the Black Soot Ring, that a NEW plug is inserted into the fully-warmed engine and taken out after a few minutes of normal running on a typical road:

Air-Fuel Mixture Ratio

A RICH mixture will tend to make the plug get COLDER. A WEAK mixture will tend to make the plug (as well as the cylinder) get HOTTER and possibly lead to Detonation or Pre-Ignition.


Detonation

THE effect known as Detonation ("knocking", "pinging", "pinking") occurs when unburnt Air-Fuel Mixture ahead of the flame front ignites BEFORE the flame front arrives. This is due to the mixture being ignited too early and the resulting, partially exploding mixture creates an excessive cylinder pressure before the piston reaches Top Dead Centre. Under these conditions, the combustion of the mixture becomes uncontrolled and a "knocking" or "pinging" sound results because of the shock waves on the piston crown.

Normally the Air-Fuel Mixture on the VeloSoleX S 3800 is ignited at 23.5 BTDC (Before Top Dead Centre) to allow a few milliseconds for the flame-front of burning fuel to expand throughout the mixture so that maximum pressure will occur just after the point of maximum compression (or Top Dead Centre). In fact, the mixture should be completely burnt by about 15 to 20 ATDC (After Top Dead Centre). This flame-front moves at around 335 metres per second (1100 feet per second).

If the ignition timing is set too far retarded (the setting is changed from say, 23.5 to 13 BTDC), the mixture will burn late causing a lack of power on the Power Stroke and excessive heat and flame in the exhaust pipe.

If the ignition timing is set too far advanced (the setting is changed from say, 23.5 to 33 BTDC), the mixture will begin burning EARLIER than usual. The piston which has not yet reached TDC (Top Dead Centre) will have the force of an exploding mixture above it and the force of the Flywheel (via the Connecting Rod) below it. In the end, the piston rattles from side to side and the resulting shock waves are heard as audible "pinging" sounds.

Advancing the ignition timing by 10 results in a rise of temperature at the spark plug Electrode Tip by 70C - 100C (158F - 212F). Most of the engine damage caused by Detonation is from extreme temperature peaks inside the cylinder that can reach 1650C (3000F) above the piston head. These extreme temperature peaks can cause severe damage to the piston top or spark plug insulator nose. The associated cylinder pressure peaks can blow the Head-Gasket and bend or break the Connecting Rod.

When detonation first occurs, the spark plug insulator nose may have tiny fused black specks from the oil shaken from the piston rings, specks of fused carbon soot from the combustion chamber or shiny specks of aluminum (like tiny diamonds) that have been removed from the piston crown. Alternatively, these specks may be found on the central electrode itself or just under the tip of the ground electrode. If detonation is severe, the spark plug insulator nose itself may break up into pieces. Detonation will lead to overheating and overheating will encourage detonation.

The main factors encouraging detonation are:

  1. Too low Octane Fuel - less than 95 RON fuel
  2. Too heavy Load particularly when travelling up hills
  3. Too high Cylinder Temperature due to overheating
  4. Too weak Air-Fuel Mixture possibly due to faulty Fuel Pump Diaphragm or too much oil in pre-mix
  5. Too high Compression Ratio possibly due to carbon build-up in the Cylinder Head
  6. Too far advanced Ignition Timing due to incorrect adjustment of the Contact Breaker

It is important to note that detonation will lead to engine overheating and engine overheating will encourage detonation.

The main cause of Detonation on the VeloSoleX S 3800 is Excessive Carbon Build-up (leading to a too high Compression Ratio) and/or Over-Advanced Ignition Timing.


Pre-Ignition

IF detonation occurs BEFORE the plug has sparked, an even more damaging effect known as Pre-Ignition can take place. This is caused by excessive heat in the cylinder Air-Fuel Mixture.

It may be caused by the Compression Ratio being too high (possibly due to Carbon build-up) or because the engine or intake manifold is too hot for some reason. It may also be caused by a hot carbon surface of an unserviced piston crown or cylinder head causing Pre-Ignition of the mixture.

It may also be caused by an excessively HOT spark plug electrode tip igniting the mixture. If the Air-Fuel Mixture Ratio is correct then a spark plug 1 Grade COLDER is worth trying.

Although a HOTTER spark plug such as the NGK BP(R)4HS has been used on a VeloSoleX S 3800 with no problem at all, it is now recommended to use the NGK BP(R)5HS together with Regular Unleaded Fuel (eg: UK 95 RON, US Pump 89 AKI).


Spark Plug Gap

FOR low energy Magneto Ignition systems, a Spark Plug Gap of around 0.5 mm is recommended (but not less than 0.4 mm).

For Electronic Ignition (Transistor-Assisted or Capacitor Discharge Ignition), a Spark Plug Gap of  0.6 mm is recommended.


Spark Plug Table

Spark Plug Manufacturer

COLD

MEDIUM

HOT

AC / Delco
US

Standard Nose

42F
43F

44F
45F

46F
47F

Projected Nose

42F
43FS

44FFS
45FFS

46FFS
47FFS

Autolite
US

Standard Nose

414
2653
2655

415
416
2656

417

Projected Nose

273
274

275

276

Bakony
HU

Standard Nose

F85(C)
F75(C)
F70(C)

F65(C)
F50(C)

F40(C)
F20(C)

Projected Nose

F85(C)P(D)
F75(C)P(D)

F65(C)P(D)
F55(C)P(D)

F45(C)P(D)

Beru
DE

Standard Nose

14-6A(U,S,P)
14-7A(U,S,P)

14-8A(U,S,P)
14-9A(U,S,P)

14-10A(U,S,P)
14-11A(U,S,P)

Projected Nose

14-6B(U,S,P)
14-7B(U,S,P)

14-8B(U,S,P)
14-9B(U,S,P)

14-10B(U,S,P)
14-11B(U,S,P)

Bosch
DE

Standard Nose

W6A(C,P,O)
W7A(C,P,O)

W8A(C,P,O)
W9A(C,P,O)

W10A(C,P,O)
W11A(C,P,O)
W12A(C,P,O)

Projected Nose

W6B(C,P,O)
W7B(C,P,O)

W8B(C,P,O)
W9B(C,P,O)

W10B(C,P,O)
W11B(C,P,O)
W12B(C,P,O)

Brisk / Halo / PAL
CZ / NO / SE

Standard Nose

N15(S,C,P,PP)
N16(S,C,P,PP)

N17(S,C,P,PP)
N18(S,C,P,PP)

N19(S,C,P,PP)

Projected Nose

N15Y(S,C,P,PP)
N16Y(S,C,P,PP)

N17Y(S,C,P,PP)
N18Y(S,C,P,PP)

N19Y(S,C,P,PP)

Champion
Federal Mogul
UK, US

Standard Nose

L7(J)
L10(S)
L82(C,CC,P,PP)
L85(C,CC,P,PP)
L86(C,CC,P,PP)

L87(A,B,C,CC,M,P,PP)
L88(A,B,C,CC,M,P,PP)
L288(A,B,C,CC,M,P,PP)
L89(A,B,C,CC,M,P,PP)
L9(J,C)
L90(C)

L95(A,C,CC,M,P,PP)
L14

Projected Nose

L66Y(C,CC,P,PP)
L82Y(C,CC,P,PP)
L87Y(C,CC,P,PP)

L12Y(C,CC,P,PP)
L89CM
L92Y(C,CC,P,PP)

L14Y(A,C,CC,M,P,PP)
L15Y(A,C,CC,M,P,PP)
L95Y(A,C,CC,M,P,PP)

Cyklon
HU

Standard Nose

43

-

-

Projected Nose

-

-

-

Nippon / Denso
JP

Standard Nose

W20FS(-U)

W16FS(-U)

W14FS(-U)

Projected Nose

W20FP(-U)

W16FP(-U)

W14FP(-U)

Enker / Bosna
HU

Standard Nose

F85(C)
F75(C)
F70(C)

F65(C)
F50(C)

F40(C)
F20(C)

Projected Nose

F85(C)P(D)
F75(C)P(D)

F65(C)P(D)
F55(C)P(D)

F45(C)P(D)

Eyquem
FR

Standard Nose

775(M,MOTO)
755(M,MOTO)
750(M,MOTO)
75(M,MOTO)
700(M,MOTO)
70(M,MOTO)

50M
500M

200

Projected Nose

775S(MOTO)
755S(MOTO)
750S(MOTO)
705S(MOBY MOTO)
700S(MOBY MOTO)

600S

580S
550S

Flashpoint
HU, UK

Standard Nose

FP12(C)

FP11C

FP10C

Projected Nose

FP5C(R)

-

-

Hitachi
JP

Standard Nose

M44-W

M45-W

M47-W

Projected Nose

M44-PW

M45-PW

M47-PW

Iskra
PL

Standard Nose

F85(C)
F75(C)
F70(C)

F65(C)
F50(C)

F40(C)
F20(C)

Projected Nose

F85(C)P(D)
F75(C)P(D)

F65(C)P(D)
F55(C)P(D)

F45(C)P(D)

Isolator
DE

Standard Nose

M14-175

M14-145
M14-125

M14-95

Projected Nose

PM14-175

PM14-145
PM14-125

PM14-95

IVP
IN

Standard Nose

-

BV

-

Projected Nose

-

-

BP6C

KLG
HU

Standard Nose

F85(C)
F75(C)
F70(C)

F65(C)
F50(C)

F40(C)
F20(C)

Projected Nose

F85(C)P(D)
F75(C)P(D)

F65(C)P(D)
F55(C)P(D)

F45(C)P(D)

Lodge
UK

Standard Nose

HN

CN

BN

Projected Nose

HNY

CNY

BNY

Marchal / Valeo
FR

Standard Nose

35(B,D,M,P)

V36A
36(B,D,M,P)

37S
88N

Projected Nose

5N(CYCLO)
GT34/5(D)
35/1(B,D)

6N
GT35(D)
V6N-SOLEX

GT34(D)

Marelli
IT

Standard Nose

CW7N
CW6N

CW5N
CW4N

CW3N
CW2N

Projected Nose

CW7NP
CW6NP

CW5NP
CW4NP

CW3NP
CW2NP

Motorcraft
US

Standard Nose

A(E,V)2(C,U,X)
A(E,V)3(C,U,X)

A(E,V)4(C,U,X)
A(E,V)5(C,U,X)
A(E,V)6(C,U,X)

A(E,V)7(C,U,X)
A(E,V)8(C,U,X)

Projected Nose

A(E,V)22(C,U,X)
A(E,V)32(C,U,X)

A(E,V)42(C,U,X)
A(E,V)52(C,U,X)
A(E,V)62(C,U,X)

A(E,V)72(C,U,X)
A(E,V)82(C,U,X)

NGK
JP

Standard Nose

B6HS

B5HS

B4H

Projected Nose

BP(R)6HS

BP(R)5HS

BP(R)4HS

Splitfire
US

Standard Nose

SF409C

SF265F

SF265F

Projected Nose

SF412C

SF412C

SF412C

Unipart / Surefire
UK / US

Standard Nose

GSP4256
GSP560
GSP550

GSP540
GSP530
GSP4266

GSP520
GSP510
GSP4276

Projected Nose

GSP4356
GSP561
GSP551

GSP541
GSP531
GSP4366

GSP521
GSP511
GSP4376

BLUE = ORIGINAL          RED = Recommended

 

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Copyright 2008 BRIAN'S VELOSOLEX. All rights reserved. This is a personal website containing information for and by VeloSoleX enthusiasts. It is not intended to replace any official information and whilst every effort is made to ensure the accuracy of all published information no guarantee is offered nor liability accepted for any loss or damage that may arise from any errors or omissions.