Spark plug on race engine

[redcarandco]

Dear all
I would like to have advises on which way to solve a very small misfire in a race engine...... engine is (really) a full race specification and all electrical are correct and have been reniewed 3 times and to be not too long spec are : compression ratio 12.3 and pressure of compression(not sure english correct) is 16 kg it revs perfectly to 8400 rpm but we play around 8000 in race condition and every thing is alright and no trouble in any sector ..... we use NGK B9 ecs and even when lamba is perfect we always have a very very tiny misfire at max rev and sparks are always a little bit to rich (very dark brown)The questions are :
1)can I use NGK B 8 ECS without a chance of "drilling" a big hole piston head.....or
2)as engine builder (for me a 50 years friend thanks to whom I have won +/- 80 race before cobras and griffith became "reliable") says just fit a little bigger air corrector (190/195 in place of 185)???
I know thar RGH is somewhere there and his experiment is impressive so ....if the problem was yours what would be your choice ? .....
Many thanks for any experimented or lived advise
Best regards to all
Roger

[nmauduit]

Bonjour Roger,

I would not attempt answering for Rohan, but as a matter of curiosity would ask for extra detail : do you have a lambda sensor on the car or when the engine was dynoed, then what were actual values of richness at high revs under load ?

keep it on the track !

[rgh0]

I would and have used a BP8ES in an engine of this specification. I presume you are using race fuel with the 12.3 comp ratio?

What dizzy and coil are you using with points ? What plug leads ? as these could also be a source of the problem.

My dyno guy also likes the engine a little rich at 8000 rpm and full throttle to minimise the risk of overheating a piston or exhaust valves.

Also make sure your fuel pump is keeping up in these conditions. I had a misfire on a long uphill straight when the pump was just not delivering enough fuel and the level in the carbs was dropping

cheers
Rohan

[redcarandco]

Dear All
To answer to first question of nmauduit the engine builder has a splendid portable lambda gauge we fit on car ....then he sits on the battery box and we go for a road test on the mythic Chimay old race track which is now a public road and then we "play" until the gauge gives us the correct values being if I remember (not sure at all)something like 12,5???? and then we make an acceleration final test with AIM mini computer and we verify the 4 sparkplug color which are always correct and we go to dynotest just to have a power idea which is always around 169/171 hp (measured in DIN )
Now to answer to Rohan we do not use race fuel just because french and belgian regulations (which are copy of FIA one ) prohibit to use something other than road 98 super in period F race car but we are """allowed"" to mixt with an octane booster such as Bardhal or any other giving +/-101/102 octane As to the distributor and coil we are obliged to keep 23D4 with points but we are allowed to modify it with an adaptator which permits the use of Mini cooper S points which never fails at very high revs and coil is always the lucas to suits this points ...as to the pump we use a red top facet which gives normaly twice of the engine needs and we have a Aéromotive pressure regulator to achieve +/- 2,5 psi ....In conclusion I must add that with NGK 8 ECS or BPS things seems to come to a correct functioning As you say you use these plugs with similar engine I think I am going to try very carefully this solution in a track day before a real race and we 'll see ....no ? any way I'll keep you informed of results
Roger and many thanks to take time for me

[rgh0]

I used a facet punp and found it did not deliver enough through the standard fuel line to . I put in a larger pump and larger fuel line and regulator in the engine bay.
But I believe the first step like you say is try BP8ES and maybe try a little richer at full throttle

cheers
Rohan

[prezoom]

Take a look at your carbs needle and seat. On my last race car, with Webers, the jet was too small to allow enough fuel on long runs at high RPM. On a dyno, the engine is not held long enough at high RPM to significantly reduce the level of fuel in the float bowl, as when a long run can empty the bowl. Try increasing the jet to the next larger size.