Topic: cb750 carb vent tubes?

[KRONUS0100]

what is proper routing?  i may have figured out my dumbass moment i been chasing for over a year.

[ekpent]

The 2 top ones go over the airbox and the 4 drain hoses are held by an oval band and go through a tab at the back bottom of engine towards the ground.

[KRONUS0100]

i friggin knew it.  damm im a dumbass.  had em pointing straight down in front of air box. 

[harisuluv]

it doesn't really matter how they're routed, as long as they're clear or not kinked.  If you're thinking that's a major problem, that's unlikely.

What is it you're chasing?

[KRONUS0100]

chasing a rev limiter type issues at around 65mph
have been for a couple years now since i put this F3 motor in.  starting to really think its ignition related.......i have verified and re-verified cam timing...and checked dwell, etc.....but points and condensors are ancient and kinda rusty.  everywhere i look TEC points are 30 bucks each and up.......thats frigging retarded.  bike runs like a beast to about 65....then feels like its hitting a rev limiter....just no power........any gear no more than about 4500 on tach.  stock airbox and filter and exhaust header.....glasspack muffler.   round top carbs 40 pilots and 120 mains  needles in middle.  fuel mileage is also a bit low at around 35mpg

[Pat_at_APE]

Fuel tank vent clogged maybe? 

Pat

[KRONUS0100]

nope. 

[Pat_at_APE]

Have you checked/cleaned/lubed the ignition advance?   Maybe it is sticking and you are not getting enough at cruise rpm.

Good luck!  I hope you can figure it out.

Pat

[DH]

Plug wire caps old/burnt out? Ohm readings?

Fuel levels in the bowls ok? not too low?

[KRONUS0100]

yeah advance works....springs might be a bit on loose side, but the advance works freely.  plug caps are old...but well within specs, i just put new taylor wires on her with the dyna green coils and hondaman resistor.  havent had a chance to ride in the last few days due to work and weather.........looking to do a plug chop at 65 in 5th

[evanphi]

Float level check. Your fuel level might be too low overall and then you're running lean at higher speeds, thus no power increase.

[KRONUS0100]

26mm correct for keihin round tops?

[guitarkev66]

What exhaust pipes are you running?  I had a very similar problem last year.  I had great power on the low and mid but would hit a wall at around 4500 rpm.  What my problem was the number 2 exhaust pipe was bent very slightly so when I would mount the exhaust at the rear foot peg bolt it would push the header away from the head creating a small exhaust leak up at the exhaust collar on the head.  You could see the 1/16 hole where the header has the slit over top of the collar.  I guess it would suck in enough air to kill the top end.  Other than that the bike would start up and run great no issues it drove me crazy took me a few weeks to figure it out.  I was running stock hm341 exhaust. 

[evanphi]

26mm correct for keihin round tops?

fuel level should be 2-3mm below the bowl's lip. Always good to check with clear tube.

[KRONUS0100]

stock f head pipe with thrush glassspack.  have no way to do the clear tube method to check fuel level.  i set the floats with a guage at 26 mm

[evanphi]

stock f head pipe with thrush glassspack.  have no way to do the clear tube method to check fuel level.  i set the floats with a guage at 26 mm

You can jam a piece of 1/8" ID clear tubing into the bowl drain hole when the plug is removed. You could also modify an old drain plug and screw in the tube, like I did.

Made some tubes for the clear tube method of checking float height. I have some new bowl drain screws coming in the mail (along with some other things!), so the two slot-most-hacked-up ones got re-purposed! Cut the heads off, then spread the end of a 1/8" ID vinyl tube to fit over the end with the holes in it using a pair of needle-nose pliers. Then added a bit of shrink tubing over it all just to be safe and have something for the threads to seal against when they get screwed into the bowls. Made two.

[KRONUS0100]

will have to give that idea a go this weekend

[PeWe]

Fuel tank vent clogged maybe? 

Pat

I had that with a tank last spring. Bike stopped 15km out of town. I was sure the fuel supply/carbs was clogged due to a dirty tank. I had just derusted it. I changed tank (from Freedom green to the Candy Sapphire blue set) and had the tank full with petrol on a shelf in my garage until a few months ago. I filtered the fuel thru double coffee filter when pouring it out into a can.
The flow stopped completely when I closed the tank fuel cap.... cap too corroded.

If I had known that when I got the stop I could have opened the cap and driven the bike home instead of calling a friend,  me in his car and the bike back on his trailer.....
- Open fuel cap one easy step when having engine problems.

[DH]

Try a plug chop at the rpm range where the trouble is, and when the engine is off, immediately turn the petcock off as well.
Un clip a bowl or two for an idea of whether or not the carbs are starving. Not the clear tube method, but could possibly
point to a fuel problem if bowl levels are low/ near empty for whatever reason.

[ekpent]

 I sold a 75 750F to a friend of mine and it would hit the wall at about the same rpm. He fiddled with the jetting,plug chops. other things etc even though it was basically stock and when he finally broke down and replaced the condensers the problem was solved.