Please help! I can't figure this one out!!!!

[Sleepybud]

I've just got my '81 CB 750 Custom up and running after a four year hiatus. I had a spill where I cracked the left side crank cover(both pieces: the small circular one with Honda on it and the cover it screws to) and a few cosmetic issues like blinkers, mirrors and clutch & brake handles.

I R&R'd it and it looks nice. Besides the crank cover(s), I pulled the carbs, cleaned out the bowls, jets, replaced the float bowl gaskets, new wires and plugs... I just wanted her to be right.

Here's the fudged-up part: It cranks up, runs good... a little spit and sputter from the intake/carb area, but when I shut it down, I took a water-soaked cloth and touched the exhaust headers the second cylinder is cool.

So, I pulled the plug to ensure spark, and it was, removed the carbs again and the floats are all fine and the jets are all clear. Everything looks fine. Put it back together...Same Stuff!!! What The Heck, man???

It was backfiring on the side I would expect, but after I took it apart the second time, like I said before, maybe a spit or sputter(that I can audibly hear coming from where the boot connects to the carbs), but no visable or audible backfire!

I want to sell this bike this year, but I don't want to sell it until it's right!

Please, any ideas would be hugely appreciated!
SB

[Sleepybud]

When I say second cylinder I mean, as I'm looking at the bike, standing in front of it and I'm counting left-to-right 1, 2, 3, 4.

#2

[TOOLS1]

I would start with looking for a vacuum leak at the carb intake boot there.
TOOLS

[shinyribs]

Just for future reference, the cylinders are numbered as seated on the bike. Left (clutch lever)- 1,2,3,4 - Right (throttle).

Your cylinder that is cold while idling, if your bring the rpms up a tad (say 2,000-2,500) will that pipe start to heat up? If so, it's a plugged idle jet or circuit. It's dead/cold at idle 'cause the circuit isn't flowing fuel, but it will heat up when you rev it 'cause it is then fueling off the main circuit.

It's possible to be a vacuum leak, but vacuum leahs typically cause a lean condition which will result in an overly hot cylinder rather than a dead/cold one.

[Sleepybud]

Thanks gentlemen! Ok, I had the cylinders out of order. Got it!

I did clean the jets out when I had the carbs off. It doesn't appear to heat up when revving, but I can't be 100% certain, as I'm sworn off riding(plus, its not registered, nor is it insured...).

I will check the carb/boot seal, as i said, I can hear some kind of, what I thought was a backfire, but it actually could be sucking air hard(god knows something sure is sucking hard about this whole deal).

I think I could get a better idea if I could take her down the road myself.

Thanks again Gents! I'll keep you posted!! And Please, if anyone has had this issue and has more to add, I am very open to suggestion here!

SB

[shinyribs]

You don't need to ride it to see if revving warms the pipe up. Better done at home. Put your hand on the cold pipe,  raise engine speed, if it starts warming let it return to idle. If it goes back cold its a plugged idle jet. Nearly impossible to clean without running a wire through them.

[TOOLS1]

The popping, and he also said that he could see it popping out of the rubber boot, made me think that it could have a vacuum leak so bad that it would not draw enough fuel to run.
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[LukeM]

Moving this thread to a more appropriate location.

Luke M

[LukeM]

Also thinking about the vacuum leak issue:

There are some very thin rubber diaphragms in the CV carbs.  One of the rebuild procedures is to check them for pinhole leaks. Handle them VERY delicately.  They are expensive to replace.

Hope this helps.
Luke M