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[Lee June woo]

Hello from South Korea.

I got pinking problem.

My Enfield is cast iron 612 conversion bullet with crank, cylinder, head and VM32 carb.
(no high performance cam)

and I`m Using 20T gearbox sprocket. 20t sprocket upward tendency gear change timing about 20km/h(12.4 mi/h)

so. problem is, when the speed reach 80km/h (49.7 mi/h) at gear 5, pinking is start when I pull the throttle more.

sometimes gear 4 also makes pinking sound when I pull the throttle hardly.

of course when I tried pull the throttle carefully, pinking is much less but there is a clear limitation.

because of these reasons, I use only 1/4 ~ 2/5 throttle at 4, 5 gear speed.

I tried retard ignition timing but engine getting unstable and loss power(or maybe my carb setting is wrong)

however, the 20t sprocket gear ratio can be the main problem I guess. because engine get much more load when

I changed 17t to 20t.

Should I change the gearbox sprocket smaller one for solve the pinking problem?

or is there a any options like ignition timing setting, carb setting, or others?

Please help me.

Thanks for read.!

[Count Johnny]

Hi Lee

Cleverer people, than me, will no doubt be along soon, but what is your compression ratio?

When The Bullet Whisperer built my 612 from our host's kit, even with all of the spacers the compression ratio was (around) 11.5:1.

His solution was to machine the top of the piston until we had a more sensible ratio of 9.5:1.

Cheers

Count 'Johnny'

[Lee June woo]

Hi Johnny!

Oh, I forgot write down my piston spec.

I`m using 8.5:1 piston and 3mm alloy gasket for spacer between crank case and cylinder.

but, I never measured combustion chamber pressure by gadget.

Should I check chamber pressure too?

and can you explain to me about relationship between compression ratio and pinking sound?

thanks for your reply.!

regards

Lee June Woo

[vince]

Hi, what sparkplug are you using? The reason I ask is that my co nnie ran ok on Champion plugs but pinked badly on NGK. Vince

[stinkwheel]

As above, I suspect your compression may be too high (or the fuel quality too low for the amount of compression you have).

Pinking is caused by predetonation of the fuel under compression or by a too far advanced ignition. The early detonation tries to force the piston back down the way it came slightly as it is on its way back up which is obviously a problem and risks severe piston damage. You can compensate for this by using higher octane rating fuel (which burns more slowly). This is why very high performance vehicles use high octane fuels to allow them to run with higher compression

I have 5.5mm of compression plates and two base gaskets on my 612 bullet (hitchcocks crank and forged piston, tuned head with performance cams) and I still get pinking unless I use premium fuel. I try to use shell v-power, it likes this but minimum 97 octane anyway. I also carry a small bottle of octane booster in case premium fuel is not available.

Riding style also has an effect. Keeping the revs higher so the engine isn't labouring at low revs is necessary. There is a tendancy to think these motors should be thumping along at low revs like a harley, this is not the case. The lightened valvegear componants etc mean they are much better purring along with a moderate level of revs. With a 21t rear sprocket, I don't hook top gear until I'm doing 60-65mph (about 100km/h).

If you are getting into the mechanics of it, with no compression plate and the forged 8.5:1 piston, I actually had a NEGATIVE deck height which would have given a compression ratio of 13.5:1. That would probably have tangled the valves and would need exotic fuel to run. With compression plates and extra base gasket fitted, the deck-height on my piston was 2.7mm to give a more sensible, but still high compression ratio of almost exactly 9:1.

The compression ratio for the piston is quoted for a standard crank. The 612 crank has a longer stroke so the compression ratio will be higher because it comes closer to the cylinder head (although the newest long-stroke cranks have a different crank-pin position to compensate for this).

Negative deck-height before fitting compression plate:



After compression plate and extra base gasket:

[Bullet Whisperer]

I would go down to 18 or 19t on the front sprocket and retard the inlet cam by 1 tooth. I built Count Johnny's engine and had to use both the 2 mm spacer and 3 mm spacer beneath the barrel, plus three gaskets obviously and I rounded the crown of the piston off, to get down to just over 9:1 compression on that engine. I can't advise regarding the Mikuni carb, as I don't do anything much with those, but raising the needle should help. DON'T keep retarding the ignition, you don't want to be any later than about 30 degrees BTDC @ full advance, any later than that and you will get overheating and possible damage.

[singleminded]

What ignition system are you using? Are you on points, as standard?

I have a 625 cc Bullet which, when I originally bought it, pinked very badly. I checked the ignition timing and it was as standard. I tried retarding it, but as you found, the performance was worsened (and it increased the exhaust noise, for some reason.) I fitted a compression plate but it still pinked. Then I noticed that Hitchcock's were selling stronger return springs for the auto advance. I fitted a pair and the pinking disappeared.

Apparently, the standard springs are rather weak and so maximum advance arrives at low rpm. This is probably ok on a standard engine, but a tuned engine with higher rpm capability needs to reach full advance at higher rpm. Hence the need for stronger springs.

[Bullet Whisperer]

The stronger springs which delay full advance may appear to do the trick, but will not give optimum potential engine performance and merely 'mask' a problem.
This is Count Johnny's machine enjoying life, with ignition timing set 'on the marks'. I can't comment on the Mikuni carb fitted by this time, as that was not down to me, but it seems to work well.
https://youtu.be/L3MJmT-BTI0

[stinkwheel]

Another way of dropping the compression ratio is to fit a thicker head gasket (can be cut out of copper sheet, standard is 1mm thick). Or two head gaskets. This is not without problems because it increases the gap between the spigot and the cylinder head.

[stinkwheel]

This may be helpful. Here is a video of me riding my 612. I'm sorry there is a lot of wind noise but you should get an idea of the rev speed I'm running the engine at.

Pulling under 50mph in top gear with a 20t sprocket is probably far too low a rev speed to be going to wide open throttle in.

From where the video starts, I'm in second gear. I don't select 4th (4-speed close-ratio gearbox but top gear is the same as a 5-speed) until I'm past the car and doing about 65mph.

https://youtu.be/rVI2w-DhdIk?t=162