Hello everyone,
I am new to online forums and don't know where or how to post so I hope someone reads this and can help me out...
I am hoping for some help... I am a long time reader of this forum but just became a member as I am beating my head against the wall trying to figure out what is going on with my bike.
I have a 1978 Honda Cb750f
History of the bike: I really wanted to go all out with this build, I'll try to figure out how to post a picture and a vid to show whats up.
I put an 836 kit in it with a mild cam, wiseco pistons, cryo treated chains n what not. Did a full motor tear down and restore with new bearings and what have you. I am running the stock carbs but with an air box from cognito motor.
First time around: when build was finished everything was great, did a break in period and bike was happy and health, very good throttle response and very snappy. Well one day just cruzing around it seemed to have lost a little bit of power, was not as responsive and I could not figure out why. Upon further inspection I had a leak from the head gasket, nothing serious but it was weeping which seemed to be a common thing. Be being a bit of a nut said ok gonna lets tear it down and find out why.
Instead of pulling the entire motor I opted to cut the frame and put one of the weld in kits for the cross bars, since I was only getting in the top end I thought this would be a little easier. So after I get the valve/cam cover off I realize two two things: 1 an oiler for one of the rocker arms was clogged and I had fried the cam, bummer but not the end of the world. I also noticed that some of the studs were loose! well shoot. They are the kibblewhite studs but with regular stud bolts. So I pull the head make sure all the bolts are tight and re-torqued, put a new base gasket on, put the cylinders back on put decided to reuse the piston rings. bike has less that 2k miles on in and thought that would be fine... maybe not??? I reinstall the head but this time with kibblewhite nuts, expensive nuts but hopefully worth it. followed a lot of info from this site and made sure to seal those pucks up. (it also had the drain hole modified and a valve job done the first go around by cycle x). none of this is really the problem but just giving some history.
So get the bike all buttoned back up but the bike is not that happy at idle. its will change and then also suffer form a hanging idle. all of which seem to indicate a vacuum leak. so I take the carbs off and give them a clean, put them back together and no difference. I check with a vacuum gauge and while the needle flutters it stays towards the bottom of the gauge ON ALL FOUR CARBS. I think the gauge works as the first time I synced the carbs it went as expected, they are the alpha moto ones and when I suck on the tube the needle moves. I have hosed the boots down with carb cleaner to find a leak but nothing. What could cause the gauge to be low on all four carbs? did I miss something inside the head? I even changed out carb body one for a spare I had and same thing. just for fun I adjusted one of the slides and sure enough I get more vacuum but I can't base it off of number two. I took them off and bench synced them and same story, very very low vacuum. The boots are fairly new, well new when I did the rebuild, I even pulled out the stock clamps to tighten them down and nothing. I rechecked valve lash and same thing.
The bike does run fine, throttle response is not as crips as the first time I built it. but the bike feels like it makes some good power and can really go. I am not sure what to try next except new boots and clamps, hell at this point maybe just put some mikuni rs34 on it or something but I dont really just wanna throw a part at it and never find out what was really going on...
It has a cycle x four to one stainless steel exhaust.
all symptoms point to a vacuum leak that I can not find to save my life, I have read every post on the forum and others to try to find a solution.
Has this happened to anyone else?
again I am new to forums and a not sure how or where to post stuff.
please let me know if anyone has any insight, thanks
Sammy