Hi all,
I've got an 82 CB750 that I picked up over a year ago not running and have been slowly rebuilding and converting to a cafe racer. The project is around 85% complete and have had a few attempts at getting the engine to run properly with not a lot of success.
I'll start with a bit of history on what has been done. (I've done all the work myself except for the engine machining).
Engine, new bearings, rings, seals, gaskets, cylinders honed. Head machined, valves lapped, valve shim gaps set between 0.10 – 0.15 mm.
Carburettors, fully dismantled (except throttle shafts/butterflies), cleaned, new jets, needles, float valves, seals. Bench synced each carb to #2 carb. Needles 2 ½ turns out.
New carb to intake boots, new coils, spark plugs and wires. (Stock airbox)
First start up, fired up straight away, idle was all over the place and slow to decelerate and running rough. Spraying carb cleaner around various spots to find a leak revealed vacuum leaks around the throttle shafts. Ordered a set of throttle shaft felt seals and off with the carbs again to replace them. Again, bench sync and paid special attention to ensuring no binding and crisp spring back to idle position. Also took the spark advancer off for a good clean and lube which I hadn’t yet done which now seems to move a lot smoother. Checked compression, carbs fully open, spark plugs out using a car battery to turn the engine over. Compression was around 140psi on all cylinders a bit low probably due to my untrustworthy gauge and or new rings still to bed in however they were consistent which I think is the main thing.
Next start up, seemed better but not great, idle reasonably consistent. Loosened off and retightened the timing chain adjusters (one at a time) to tension the chains. Noticeable reduction chain in noise after doing the rear tensioner (2 acorn nuts). Confirmed spark and combustion in all cylinders with a Gunson colour tune plug which seemed to indicate mixtures were in the ballpark. Checked timing on cylinder #1 which was ok.
Still runs a bit rough and seems to not rev that happily, like there is a bit of hesitation and an odd pop sound here and there (not as loud as a backfire) whilst revving.
When at idle, it’s pretty stable now if left alone but sometimes blipping the throttle makes it not return to back to the same idle. I’m yet to vacuum balance the carbs but don’t believe this can cause it to run like crap but I could also be wrong. So running out of ideas on what else to check and was hoping for some assistance please.
Check it out on instagram @fiorellogalluzzo if you're curious.
Cheers
FG
I've got an 82 CB750 that I picked up over a year ago not running and have been slowly rebuilding and converting to a cafe racer. The project is around 85% complete and have had a few attempts at getting the engine to run properly with not a lot of success.
I'll start with a bit of history on what has been done. (I've done all the work myself except for the engine machining).
Engine, new bearings, rings, seals, gaskets, cylinders honed. Head machined, valves lapped, valve shim gaps set between 0.10 – 0.15 mm.
Carburettors, fully dismantled (except throttle shafts/butterflies), cleaned, new jets, needles, float valves, seals. Bench synced each carb to #2 carb. Needles 2 ½ turns out.
New carb to intake boots, new coils, spark plugs and wires. (Stock airbox)
First start up, fired up straight away, idle was all over the place and slow to decelerate and running rough. Spraying carb cleaner around various spots to find a leak revealed vacuum leaks around the throttle shafts. Ordered a set of throttle shaft felt seals and off with the carbs again to replace them. Again, bench sync and paid special attention to ensuring no binding and crisp spring back to idle position. Also took the spark advancer off for a good clean and lube which I hadn’t yet done which now seems to move a lot smoother. Checked compression, carbs fully open, spark plugs out using a car battery to turn the engine over. Compression was around 140psi on all cylinders a bit low probably due to my untrustworthy gauge and or new rings still to bed in however they were consistent which I think is the main thing.
Next start up, seemed better but not great, idle reasonably consistent. Loosened off and retightened the timing chain adjusters (one at a time) to tension the chains. Noticeable reduction chain in noise after doing the rear tensioner (2 acorn nuts). Confirmed spark and combustion in all cylinders with a Gunson colour tune plug which seemed to indicate mixtures were in the ballpark. Checked timing on cylinder #1 which was ok.
Still runs a bit rough and seems to not rev that happily, like there is a bit of hesitation and an odd pop sound here and there (not as loud as a backfire) whilst revving.
When at idle, it’s pretty stable now if left alone but sometimes blipping the throttle makes it not return to back to the same idle. I’m yet to vacuum balance the carbs but don’t believe this can cause it to run like crap but I could also be wrong. So running out of ideas on what else to check and was hoping for some assistance please.
Check it out on instagram @fiorellogalluzzo if you're curious.
Cheers
FG