Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Greebo on April 18, 2016, 12:56:54 PM
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Hi guys,
me not being a expert tinkerer, i am looking for advice on setting the timing on my CB750k6, when the build was complete with the Dave Silvers electronic ignition, iridium plugs, the timing was set 10% advance, the reason I was given by the guy who built the bike was because the carbs were re-jetted & needles adjusted to compensate for the K&N's & straight through 4into2 pipes, the builder thought it ran better advanced 10%, but I a not sure, I have had a couple of issues sometimes, not always, when starting up it can miss fire on a cylinder usually cylinder 4
On cylinder 4 the spark plug porcelain in the centre of the plug where the spark is has gone brown & the electrode spark arm is greyish, I know its not rich on that cylinder because the rest of the plug is the right colour but this cylinder 4 is the one that sometimes does not fire-up, I have changed the plug just in case I had a bad plug & it was ok for a couple of runs but then the new plug electrode porcelain changed to brown, same as the first one did , the centre porcelain on the other plugs are white as they are from new & all plugs look to running a good mixture.
so I am thinking it could be the timing.
Whats the best way for me to re-set the timing to TDC, I don't have a timing light, I do know the builder of the bike marked the original TDC position on the ignition plate, so can I simply move the position back to where it was ? to try it ?
any advice appreciated
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If the problem was timing, it would affect at least 2 cylinders, not just one. Have you tried replacing the plug cap on the affected cylinder? Just because they LOOK okay, it doesn't mean they are.
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hi harry
No I have Not tried changing the plug cap at this stage, mainly because all the leads are brand new from Silvers, the bike has only done 970 miles since complete nut & bolt rebuild. but it might be worth a try
Thanks for the advice
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It wouldn't be the first time a new part has been defective in my experience. Many years ago a mate changed plugs, points, condenser and dist cap on his Corrina, and had a misfiring problem, so bad that a test drive round the block blew the silencer apart with an almighty backfire. Turned out the new condenser was duff. So never assume brand new parts are always okay!
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You should be able to reroute the leads of one and four plugs completely if they will reach (both out of the same coil) then see if the problem transfers also, this should check out both the cap and ht lead function in one go.
If problem stays on number four, then you can look elsewhere.
Timing is usually a result of combustion chamber burn time (efficiency of the mechanical design) so without a change to some fundamental internals you'd have to question why the builder decided on that spec change. But it wouldn't just affect one cylinder though so you may have to revisit that decision after you've got to the bottom of one odd cylinder performing differently to the others.
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Fuelling or not ?
if I hold the throttle in one position in top gear & cruise at 60mph constant I notice I bit of hesitation as though its lacking fuel, if I accelerate past this its not noticeable at all, & if I hold it at slower constant cruising speeds 40 mph it seems OK...
accelerating through the gears it does not miss a beat, its only when holding the throttle in a fixed position as mentioned above.
Any ideas on what this maybe ?