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My 82 GSX750SZ Katana is well and truly run-in after the rebuild/resurrection and it makes good power in the upper reaches of the rev range, with the ass-dyno registering a definite giddy-up 7,500 - 9,500 rpm indicated. The 750SZ has slightly more aggressive cam timing and compression ratio is up a bit too (9.8:1) over the standard 16-valve GS750E (9.4:1) due to raised decks on the piston tops, which uprated claimed HP from 79bhp @ 9,200 rpm to 84bhp @ 9,500 rpm for the Kat. And claimed torque went from 46.7 lb-ft @ 7,000 rpm to 50.2 lb-ft @ 8,500 rpm in the Katana. So a peakier motor to be sure.
During the 750 engine rebuild I took 0.15 mm (0.006") off the barrels to close the squish areas up. With the head skimmed, the squish was measured cold at 0.8 mm (0.031") and I calculated the total reduction in chamber volume as giving a slight compression ratio boost to 9.9:1. Theoretically these changes should speed up the burn speed, by how much I do not know. While at stock timing the bike ran well and exhibited no detonation (except under high load at very low rpm), the power in the mid range felt.... a little flabby. Its not bad, but it feels a little anemic. Jetting looks spot on from the plugs, so I began to wonder if the timing was too advanced. I don't have anything to compare it to, apart from my 650G Katana, which are also deliver at rpm close to redline; peak hp is delivered at 9,400 rpm, max torque at 8,000 rpm. Its feels comparatively crisper/more lively in the mid-range.
I had checked TDC and indexed it and set the (stock) ignition timing to that. The manual states that full advance on the is 35° BTDC above 2,350 rpm. Which seems like a lot of timing given that my 1100 has bigger bores, the same TSCC head and stock it runs 32° of advance. The 8-valve GS750 ran 37° full advance (the 650G runs 40°), which is understandable because it is a slow-burning squish-free hemi. But a TSCC head (essentially a pentroof with a rib), a centrally located spark plug in a 67 mm bore, and a squish area of what I measured to be roughly 15% of the piston area, should be a comparatively quick burner.
I have decided to see whether I can tune the ignition timing via the ground strap/center electrode on the plug. Has anyone else done this? Essentially the theory is that as the spark is advanced the electrode temperature rises and this is witnessed on the ground strap and the tip of the center electrode by the amount of fuel film residue that is burnt off. More advance gives more localised heat and the more residue is burnt off (assuming correct heat range). Ideal for the street is when the 'tide' mark is on the bend of the ground strap and the film is only missing at hair's breadth from the top edge of the center electrode.
I didn't take a before photo, but after a days ride, the ground strap of a well-used plug was the same colour from tip to and including the weld. The center electrode was clean and uniform. I retarded the ignition a little did a high-speed(ish) plug chop and this was the result.
This is the same plug and colour has moved up the ground strap past the weld and fuel film has appeared to about halfway up the centre electrode. The bike feels to be running stronger in the mid-range. I will take out some more timing and report back. I will confirm with fresh plugs. Any thoughts?
During the 750 engine rebuild I took 0.15 mm (0.006") off the barrels to close the squish areas up. With the head skimmed, the squish was measured cold at 0.8 mm (0.031") and I calculated the total reduction in chamber volume as giving a slight compression ratio boost to 9.9:1. Theoretically these changes should speed up the burn speed, by how much I do not know. While at stock timing the bike ran well and exhibited no detonation (except under high load at very low rpm), the power in the mid range felt.... a little flabby. Its not bad, but it feels a little anemic. Jetting looks spot on from the plugs, so I began to wonder if the timing was too advanced. I don't have anything to compare it to, apart from my 650G Katana, which are also deliver at rpm close to redline; peak hp is delivered at 9,400 rpm, max torque at 8,000 rpm. Its feels comparatively crisper/more lively in the mid-range.
I had checked TDC and indexed it and set the (stock) ignition timing to that. The manual states that full advance on the is 35° BTDC above 2,350 rpm. Which seems like a lot of timing given that my 1100 has bigger bores, the same TSCC head and stock it runs 32° of advance. The 8-valve GS750 ran 37° full advance (the 650G runs 40°), which is understandable because it is a slow-burning squish-free hemi. But a TSCC head (essentially a pentroof with a rib), a centrally located spark plug in a 67 mm bore, and a squish area of what I measured to be roughly 15% of the piston area, should be a comparatively quick burner.
I have decided to see whether I can tune the ignition timing via the ground strap/center electrode on the plug. Has anyone else done this? Essentially the theory is that as the spark is advanced the electrode temperature rises and this is witnessed on the ground strap and the tip of the center electrode by the amount of fuel film residue that is burnt off. More advance gives more localised heat and the more residue is burnt off (assuming correct heat range). Ideal for the street is when the 'tide' mark is on the bend of the ground strap and the film is only missing at hair's breadth from the top edge of the center electrode.
I didn't take a before photo, but after a days ride, the ground strap of a well-used plug was the same colour from tip to and including the weld. The center electrode was clean and uniform. I retarded the ignition a little did a high-speed(ish) plug chop and this was the result.
This is the same plug and colour has moved up the ground strap past the weld and fuel film has appeared to about halfway up the centre electrode. The bike feels to be running stronger in the mid-range. I will take out some more timing and report back. I will confirm with fresh plugs. Any thoughts?
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