what happened?

Hello,

I'm a first time poster with a '72 CB350. I've had it for a few
months with no issues - carbs were cleaned, runs/idles like a champ,
starts right up, etc.

Last night I was riding and had to gun the engine at one point to
avoid a sticky situation. I had the throttle wide open only for about
three seconds but the bike got mad at me. It responded well but then
felt like i had hit a modern rev limiter - the power just cut off. I
was able to make it home by driving it gingerly but it felt like the
more I opened the throttle, the more bogged down the engine became. I
have plenty of fuel and no known fuel delivery issues. HELP!

Thanks,
Brian

 

Maybe holes in your pistons hehehe
gun the engine to avoid a stick situation?
sounds like you need lessons

 

Rather than try to figure out what happened by asking questions
on the internet, I would just start to investigate without jumping
to any foregone conclusions.

A good starting point would be to pull the plugs. Are they all
a good color ? Are they all the same ?

With the plugs grounded to the engine, do they all spark when
you crank it ?

Do all the cylinders have good compression ?

Do the carbs all open and close in sync with
intact return springs ?

Do the float bowls all contain about the same amount
of fuel ?

Much better sometimes to just run down a list of stuff
to check without making too many assumptions.

 

thanks for the help, everyone. looks like I have a nice list of
things to look at this weekend...

 

a torn diaphram is not good under any circumstances.

 

Good afternoon, Brian.

One question for you... Does the engine run smoothly when you're running
at a steady speed, and does it just misbehave when you accelerate? If so,
I would think that would tend to point away from things like bent valve
stems or bad plugs since those would tend to make the engine run rough in
all regimes.

I don't know anything about CV carbs but "torn diaphragm" (whatever that
is) does sound like something that perhaps could cause what you're seeing.

I agree with Rob about taking it step by step and not jumping to any
conclusions. However asking for suggestions on the Internet, as you go
about compiling a list of things to check, does seem like a good way to
start as long as you don't get it into your head that we're some kinds of
experts here.

C.J. <who can change oil and check the battery but that's about it...>

 

Maybe so. I'd start with the plugs though 'cause they're
the least intrusive thing to check and might point you to
one particular cylinder that was having problems.

 

You've never had a sticky situation while riding? Sounds like you need
to get off the computer and ride the bike more.

 

Nah. I've had punctured diaphramgs and what happens is that the bike
starts and runs perfectly up to maybe half-throttle and then simply
doesn't go any faster, because the, er, the vacuum is leaking through
the holes (how about that for a lovely unscientific explanation!? and
the slides just can't lift any more.

A Z400 twin I ran a couple of years ago had holed diaphragms, and ran
beautifully up to about 65-70, but that was your lot.

Actually, I used "flexible superglue" to seal them, which worked, and
got me on a 500-mile trip to FDrance and back, but then new holes
started appearing and I got new diaphragms fitted.

Wow! 90mph and more!

 

Considering all the advices, it comes down to step-by-step analysis:
1. Pull plugs & examine them for color & condition.
2. Now that the plugs are out, do a compression test. This will tell you if
you've got to dig deeper for valve or piston problems.
3. Carbs. (Judging from various posts, the #1 cause of weird problems in
bikes...)
4. When all else fails, electrical/electronic...

 

well put

 

Um, boys? 1972? I really do doubt they're CV carbs. They're likely old
Keihin slide carbs if they're anything like the 70's-era CB750's I've owned.

Dave S

 

A surprising number of early 1970s bikes had CV carbs, including the
CB350.

 

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Learn something new every day. Wonder why they put them on the 350 and not
the 750 of the same (or even later) year? I've had '73's and '75's both of
which had slide carbs... I think even the 750F may have had slides. One
would think they'd put them on the big flagship 750-Four first...

 

If you mean the SOHC one, yes, it did. All the SOHC Honda fours had
slide carbs. Every single one. CVs were fitted to the 250, 350, 360 and
450 (later 500) twins surprisingly early.

Mind you, they were pretty crappy carbs.

 

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Taking a page from ol' Albrecht and his bikebandit schematics, it looks like
the CB350K4 (twin) had CV carbs, but not the 350F of 1972. The 350F was the
350-Four though so you must be right about the SOHC fours.

Thanks

Dave S.

 

I just said it did. Really, really early Honda twins had slides.

I am.