Hey everyone, so I've been riding my bike regularly (CB750 K5) commuting and it has ran next to perfect, save a few idle issues that I planned on fixing with a vacuum sync (was waiting for parts to come in to make my own kit). My commute is never more than 20min and San Diego is a pretty cool place, even in the summer, so the bike hasn't gotten that hot. Well this weekend I did a road trip down Baja Mexico with some buddies for a bachelor party and it got really hot.
We had to lane split for almost an hour to get to the border in dead stopped traffic (which gets the bike really hot), after a little while the RPMs started getting really high whenever we stopped.... like 3k! The engine would be racing, but if I blipped the throttle they would drop back down to 1,200-1,400-ish and then slowly rise back to around 2k-2,500ish, I would then blip the throttle and they would go down again, but the longer we we were driving slowing between shops the harder it would be to get them to go back down. I had the exact same problem when on the way back to San Diego, which was compounded by the insanely slow lane splitting to beat the 6hr line at the border (no joke, the Border Agent told us we skipped a 6hr line). So now, this week, I have switched campuses and have a 20min commute through traffic that I have to lane split the whole way and the engine is doing the same thing now every time I go to school.
I've been scouring the forums and internet and have come up with a few ideas. I don't think it's a Hung Idle because it doesn't "come down slowly" when revved, it will rev like normal and drop like normal, it would then just start going up slowly, or a little more quickly the hotter it got. I don't feel like I have any loss of power anywhere in the powerband, it pulls like a freight train in all situations EXCEPT for a slight hesitation going from 4th to 5th gear when I'm fully pinned at WOT coming onto the freeway.
I found the carb cleaner test, and tested the carb boots and found that the #1 & 4 definitely have a vacuum leak, but also found that the airbox/Carb boots also leak. I've also found that my actual airbox is deformed and does not seal completely on the long straight side nearest to the carbs.
My bike have MAC 4-1 with a Cone Engineering shorty 12" cone muffler, stock air filter and stock jetting, and I have the idle setup so that when it's cold I have to have the choke wide open and once it starts I need to close it immediately and hold my throttle open a crack until it's fully warmed up. I sit for a little bit before riding and then even if I stop at a light for the next few minutes I need to keep the idle cracked or it will die. Once warmed up good, it sits around 1k-1,200 and sometime dips a little even when warm, so I usually set it to 1,200 when warm so I never have an issue.
My question is this: I realize I need to replace all of these boots, but would just these leaks cause this much havoc on my idle? And if so, why does it only affect my idle when the bike is extremely hot, and not when it's cold or fully warmed up?
Any help is always appreciated!
