hey all,
I've had my 81 750c for about five years now, and it's always been a bit of a pain. my friends and coworkers say i need a better mechanic (true) but i'm determined to understand the mechanics and get it sorted on my own.
the bike is hard to start, sometimes requiring a bit of throttle to get her going, sometimes i make a decent adjustment (1 turn) on the idle screw. mixture screws are at 1.5 turns out. after about a minute sitting just at 1k rpm, i hear a loud PFFT and the RPM jumps to 2500, requiring me to undo the idle adjustment to bring it back down. seems to run fine, sometimes requiring further backing out of the idle screw over time. occasionally it will pfft again and then the idle will increase a bit, but nothing as dramatic as the initial period.
i replaced the spark advancer as the old one had all its index pins either bent or snapped off, and made sure to set the timing properly. rest of the electrical system is working fine, though it's worth mentioning i've been through two rotors in as many years. I had it running real hot before due to some poor valve adjustments (since fixed).
probably worth mentioning that it sure does smell like gas when it's running
so:
valves are in spec
timing is good
carbs are clean
spraying WD40 around boots and fuel rails has no effect. boots are new.
throttle cables are smooth and snap back fully
airbox is stock, filter is same part but replaced
prev. owner swapped to 4-into-1 exhaust
jets are stock sizes
vacuum petcock or whatever system affectionately referred to as the spawn of satan has been removed by previous owner
haven't resync'd the carbs since changing the advancer, so i'll do that today, but my confidence in that fixing anything is low since i've been through that a lot when trying to address this exact problem to no avail.
i've read a few hundred forum threads about this at this point on various engines and haven't been able to fix it, so i figured it's time to get my hat in my hands and ask for help. everything i've read is pretty confident it's a vac leak, and my current theory isn't that there isn't a vac leak, but that if there is one i haven't found it. should i keep it up with the WD40 or look elsewhere?
I've had my 81 750c for about five years now, and it's always been a bit of a pain. my friends and coworkers say i need a better mechanic (true) but i'm determined to understand the mechanics and get it sorted on my own.
the bike is hard to start, sometimes requiring a bit of throttle to get her going, sometimes i make a decent adjustment (1 turn) on the idle screw. mixture screws are at 1.5 turns out. after about a minute sitting just at 1k rpm, i hear a loud PFFT and the RPM jumps to 2500, requiring me to undo the idle adjustment to bring it back down. seems to run fine, sometimes requiring further backing out of the idle screw over time. occasionally it will pfft again and then the idle will increase a bit, but nothing as dramatic as the initial period.
i replaced the spark advancer as the old one had all its index pins either bent or snapped off, and made sure to set the timing properly. rest of the electrical system is working fine, though it's worth mentioning i've been through two rotors in as many years. I had it running real hot before due to some poor valve adjustments (since fixed).
probably worth mentioning that it sure does smell like gas when it's running
so:
valves are in spec
timing is good
carbs are clean
spraying WD40 around boots and fuel rails has no effect. boots are new.
throttle cables are smooth and snap back fully
airbox is stock, filter is same part but replaced
prev. owner swapped to 4-into-1 exhaust
jets are stock sizes
vacuum petcock or whatever system affectionately referred to as the spawn of satan has been removed by previous owner
haven't resync'd the carbs since changing the advancer, so i'll do that today, but my confidence in that fixing anything is low since i've been through that a lot when trying to address this exact problem to no avail.
i've read a few hundred forum threads about this at this point on various engines and haven't been able to fix it, so i figured it's time to get my hat in my hands and ask for help. everything i've read is pretty confident it's a vac leak, and my current theory isn't that there isn't a vac leak, but that if there is one i haven't found it. should i keep it up with the WD40 or look elsewhere?
