1936 4F/6 OHC Square Four - spark plug caps

[orabanda]

Hi everyone,
Almost finished a friend's bike.
It has been fitted with NGK spark plug caps, and the engine backfired from 1/3rd throttle. Putting the choke on reduced the backfiring, but it still occurred. I richened the pilot mixture and lifted the needle. This was an improvement - the backfiring  firing has reduced but it is not eliminated.

The NGK plug caps have a suppresor fitted (5 KOhm).

before I try a larger main jet, am considering replacing the suppressed caps; opinions sought.

I have also ordered four new plugs (B7HS).

Cheers,

Richard

[groily]

My cent's worth: If it's still running on the magneto, I'd replace them Richard. Might make no difference at all to this particular problem, but as a general rule suppressors and/or resistor plugs aren't ideal and make the mag work harder than it needs to. If the ignition system has been upgraded to something more modern, then I'd leave them.
I know suppression is a legal requirement in some places, but with digital radio and all that I doubt it really matters any more.

[Triton Thrasher]

As a test, you could remove the plug caps and secure the wire cores to the plugs with the little nuts on the plugs.

Failing under throttle load can be an ignition HT problem: insulation cracks, tracking through dirty surfaces, distributor cap, faulty ignition coil.

Can also be restricted fuel feed, water in carb, main jet fouled.

[muskrat]

G'day Richard.
TT's suggestion of HT wires direct to the plugs should eliminate it if that is the problem.
I found it hard to find non suppressed plug caps.
Cheers

[Adler]

Hello Richard,
is the backfiring through the carburettor or through the exhaust? Or have you already found the root cause for the problem? Would be interesting.
Best regards, Manfred

[fffcycles]

No suppression caps for plugs definitely. I have a few Ariel’s and can say watch out for fake NGK’s.
Go to the Ariel forum and there should be lots there on this.
Frank

[orabanda]

Thanks everyone,
Fitted non suppressed caps.
The valve block in the brass bodied 76 carby was loose. Air for the pilot circuit is drawn in via four holes drilled in the side of the body. Because the body was loose, air was also getting past and leaning out the mixture in general.
40 minutes of the brass jet ./ valve body in the home made zinc plating bath, built up the body by 0.008".

Refitted (tight/ firm fit into the body) and now the carby is working as it should. Mixture can be adjusted via the pilot needle, banging eliminated, and engine feels clean & responsive.
Cheers,
Richard.

[orabanda]

See pics of carby "rebirth".

[fffcycles]

Well done. Lovely bikes.
I have two 1932 4f/600 engines sitting waiting for lots of attention. Not going to happen it seems. I really have to get on with my second ‘48 A7 and a quite rare 1940 Ariel VA500 SV

[orabanda]

Good on you Frank; more power to you!
The VA500 would be worth seeing.
I have three Ariels as work in progress: 1927 Model A, 1938 4G and 1957 VH.

Also on the burner are: 1976 RD400, 1975 GL1000, 1951 G80S, 1954 and 1955 A7SS, 1948 A7, DT360A, DT175C, 1969 A50, CL350, CB350 and XL175, so better stop typing and head down to the shed!

Unfortunately other peoples machines (ie the 1936 square four), have been distracting me.

Cheers,
Richard

[chaterlea25]

Hi All,
Richard,
How did you mask the pilot drillings  before plating the jet block?

John

[fffcycles]

I see a ‘76 RD400 I have one of those and a ‘39 4G basket case, ‘37 VH500 twin pipe as well, but I will never get that far now. Set to begin chemo again in December.

[fffcycles]

I have five Norton’s left as well but they are proving hard to move on

[orabanda]

Hi John,
I sealed the pilot hole with wire that neatly fitted (after thinning the wire by pulling on it in a vice).

See the wire in the pic below.
Cheers,
Richard