Looking at the colour of cylinder #3 plug, clearly it was running way richer than the other cylinders.... more on this later.
Anyway, last weekend I did a compression test (using a brand new tester) and discovered that all 4 cylinders were "low", ranging between 60 and 82psi.
What to do? Off with her head!!!
On Friday, having just taken delivery of a full gasket set and new cam chain guides, I dove right in and set about overhauling the top end.
Upon removal of the cam cover, I was suitably surprised to find everything in pristine condition.
I carefully measured and tabulated the cam clearances. All were towards the upper end of the tolerance - good news to be sure!
I removed the cylinder-head and dissembled it down to its constituent pieces, taking care to keep each cylinders "bits" together in a group, using an egg box!
I used "oven cleaner" to de-grease and clean off 40 years of carbon build-up, aided by a brass bristled brush.
The most stubborn deposits were remnant of the leaded petrol era. (tetra ethyl lead was added to petrol as an upper cylinder lubricant for valve guides and seats) Removing these deposits required judicial brushing with a brass wire brush in my cordless drill.
I must say, lapping each valve is quite therapeutic, as one imagines returning the seal to it's original glory!
I was quite happy with the result - a dull grey continuous line around each seat and valve.
Ensuring that absolutely no valve lapping paste remained) I masked up the mating faces and studs, ready for a coat of satin black high temperature paint.
After curing the paint (in my kitchen oven) for 6 hours at 150 degrees C. (302 F) I set about re-assembly, not forgetting to install new valve stem seals.
I installed a new head gasket and torqued up the head nuts according to the prescribed spec. and pattern.
Next, I installed the exhaust cam, followed by intake cam, being very careful to line up the "T" mark (Top Dead Centre) on the crank right hand side (visible through a slot in the timing plate behind the right hand side lower crank end cover.
I was very careful in performing the cam timing as per the manual, as one tooth out could spell disaster!
Once I was done, I rotated the crank a couple of times using the 17 mm bolt head on the right hand side of the crank, feeling carefully for any "resistance" which would indicate incorrect timing... all good!
I re-checked the cam clearances.
I was surprised to find that all but cylinder one's exhaust valve clearances were now almost dead in the middle of the clearance spec.
I recalled that cylinder one's exhaust valve required just a bit more lapping to provide a good seal pattern.
The resulting clearance was 0.1 mm, which is 0.05 mm under the lower limit of 0.15 mm. So, not having any spare shims, I carefully surface ground 0.05 off each side of the shim, which should put the resulting clearance at exactly 0.2 mm, bang in the centre of the tolerance.
On the 550, the shims reside UNDER the cam-followers, which requires cam removal in order to perform any "adjustment"
I subsequently re-assembled the exhaust cam into the head: again, carefully timing the cams. I re-measured the clearance: spot on: 0.2 mm!
Time to "close up" the open heart surgery!
Application of new cam cover gasket, including a bit of silicone sealer applied to the cam end half-moon rubbers: I closed her up...
After having done all the above, I turned my attention to the flaky black paint on the carbs:
I started by removing the float bowls to inspect...
Lo and behold, I found cylinder #3's main jet languishing in the bottom of the float bowl: ....
Ahaaaa, the source of the misfire and evidence of rich mixture by sooty black spark-plug! duh!
Everything else was in order - float levels were damn near 4.5 mm below the float-bowl joint line. Sweet!
I re-installed the carb. bank and pod filters, followed by ignition coils and then the 4 into 1 exhaust (with new copper sealing bushes)
Upon application of the compression tester, I rejoiced that the compression was now up near the 130 psi mark on each cylinder
with very little variance between cylinders!
Plugs in, ignition on, choke on..... deep breath - crank....
"pop, pop" she sounded as fresh fuel permeated the carburetors for the first time in a few weeks...
On the second application of the starter motor she fired right up to a very pleasing idle at 1500 rpm. Success!
I stood back, admiring my handiwork, letting her warm up for a good few minutes... eventually setting the (off choke) idle at 1000 rpm,
with my previous carb. syncing efforts clearly evident!
A couple of blips of the throttle showed pick-up to be good, and the idle rpm returning after a second or two.
Using the tried and test "screw-driver in the ear" method of "stethescoping" unnatural engine noises, I inspected the upper cylinder...
none could be heard except a very satisfying "tic tic tic" of closing valves and cam lobes leaving their followers! Nice!
I checked for signs of oil leaks....
None!
I suited up and took her for a ride...
I am so pleased, the bottom end is crisp, with meaty grunt (for a half-litre) low down in the revs.
Out on the freeway I opened her up and was properly surprised at the surge of power as revs pass 7000!
Marvellous!
Next on my list is a full paint refresh....
Thanks everyone for valuable contributions to this very satisfying endeavour!