Hitchcocks Forum

Been out on my 612 bullet this weekend. I noticed it pinking slightly when under load at low revs. I've already tweaked the ignition a touch more retarded.



On the way home, I filled up with Shell V-power fuel (99 octane). The pinking went away entirely and the bike pulled noticeably more strongly up hills from low revs for the remainder of the journey. Suggests it's probably best to stick the higher octane fuel in it.



The trouble is, I would need to go out of my way to get V-power locally and it's not as commonly stocked at the small, rural filling stations I tend to stop at. The solution would seem to be an additive? There are so many out there though. Any recommendations of a brand? Easy dosing for motorcycle tank volumes would be a plus.

You could retard your inlet cam by 1 tooth off the marks, this will reduce the cylinder pressure on the compression stroke and reduce / remove any tendencies towards pinking. If you try this, I doubt you will want to put it back on the dots.

I went down that path trying to eliminate the pinking on my 612 and following my experience, would NOT recommend it! When I stripped the top end in order to add the second (2mm) compression plate, to my horror I found the barrel extensively scored on one side (not the fore and aft thrust faces). After eliminating possibilities like a bent conrod, too tight ring gaps or piston clearance, mixture, timing, etc, Allen Hitchcock (to whom I sent photos) drew my attention to the excessive amount of carbon build-up on the piston crown and areas where this had flaked off. He also noted how the scoring was consistent with pieces of carbon having been trapped between the ring lands, the assumption being that this (particularly hard) carbon was caused by the octane booster that I had been using. Having covered thousands of trouble-free miles since re-assembly and no longer using octane booster (I smoothed the scoring with emery) I have no reason to dispute Allen’s theory. Along with the 5mm of compression plates and 3 base gaskets, I filed off all the sharp corners from the Acralite piston crown. I don’t know my precise ignition timing - I have just adjusted it “on the road” to the exact point where it no longer pinks. I run it on Shell V-Power and it’s happy. I suspect, by the way, that when you eliminate the pinking on yours, you will also see a reduction in that vibration (as I did). The brand of octane booster I used was NOS “Racing Formula”.

Shell V-power and similar high-ron (octane) petrol will also keep the engine cleaner, that`s why it is recommended for engines with injectors. My Mazda 626 handbook says `do not use additives in the fuel`.

I always use V-power. Thumper loves the stuff. Better by far than the usual weasel piss we get over here...

The problem is I can't really get V-power (or any of the high octane fuel brands) in the kind of places I go for protracted summer rides such as in the Highlands of Scotland. They tend to have 2 pumps. One petrol, one diesel. If it was available, I'd use it.



So not NOS racing formula...



I used to use silkolene pro FST in my bikes in winter to prevent carb icing (before I got ones with heated carbs). It's supposed to have a mild octane increase as well as stabilising fuel.

Octane boosters can be a reasonable short term option on a high compression 535 or 612, but the correct method is to rework the advance unit within the 'distributor' housing. Shortening the slot width or widening the pin, will add less centrifugal advance to the timing curve. This in turn allows more initial timing which large cams respond well too, but still maintains a total timing figure around 30-32 degrees. The motor will no longer need multiple base plates or stacked barrel gaskets, and will perform well not only at the top end of the rev range, but at low rpm as well. Regards Paul. PS, no high performance motor will reach its potential without the correct octane fuel- the more mismatched the effective compression ratio vs the required octane supplied, the more bandaids need to be applied.

I always used to use Castrol Valvemaster in my 612, and NEVER use supermarket petrol, always get "branded" fuel - IMHO supermarket fuel is pretty crap. Never had any scoring problems with mine, and like Barry N I set the timing by ear / feel just back it off until the pinking stops. Mine has a 3mm compression plate and composite head gasket.

‘Supermarket’ petrol does not differ in its essential specification as fuel from branded petrol; both must conform to the European Standard EN228, a very comprehensive standard specification for all petrol sold in the UK. Where they may, and most likely do, differ is in the additive content of the fuel.

I can't comment on what fuel is used, as 'Slo Poke' usually has a near full petrol tank when he comes to visit. what I can tell anyone who wants to know is - both barrel spacers and three base gaskets are used, the piston crown has been rounded off, compression ratio is just over 9:1 and the inlet cam is retarded by 1 tooth. Ignition is set 32 degrees BTDC @ full advance. The engine is made up of the full Hitchcocks 612 kit ... https://youtu.be/Fm-Nmpz18fI