Rough running '85 Honda VT700C

I would appreciate any advice on this.

I have an '85 VT700C with about 32K miles. When I got the bike about 4 years
ago there were acceleration and top end deficiency issues. Otherwise the
bike ran fine. I had not previously done much work with motorcycles, so I
ignored the problems for about a year before deciding to do something about
it. After many diagnostic tests (hoping I wouldn't have to get into the
carburetors), I found the source of the performance issues - two torn carb
diaphragms. I replaced the diaphrams and regained the missing power. But,
as luck would have it, another problem revealed itself. The bike would
hesitate when quickly opening the throttle. I let that issue go for another
season and dug back into the bike to find the cause. After another series of
tests, I ended up checking the carb synchronization and found them off.
While the carbs were off the bike, I did a thorough cleaning of the jets and
ports. I reassembled and synced the carbs and that did the trick. Rapid
throttle response, good acceleration, etc. The bike ran wonderfully that way
for about 3 months. Now problems again.

After about a month of not riding, I took the bike out. After about 20
minutes, I was on the interstate going around 70 mph. I accelerated to pass
a truck and the engine suddenly tripped over itself, cutting out instead of
accelerating. I took an exit and managed to get back home, but the bike
sounded bad and wouldn't break about 45 mph.

Now the bike will start fine and idle alright, but it is stumbling again on
rapid throttling. It also runs roughly as the rpms climb. There is a lot of
hesitation and backfiring. I have checked the fuel, fuel filter, diaphragms
and slides, compression, and sychronization. All appear good. The plugs are
fairly new, but I did check them out also. With the bike running, I removed
each of the 4 spark plug wires (each cylinder has two) one at a time to see
if one of the plugs was perhaps not firing. I noted decreased performance
when each plug wire was removed. I figured that meant that each one was
actually firing alright.

One other thing happened today as I was testing compression. The battery
seemed suddenly drained. This is odd because the battery was new as of the
spring. It has not given me any trouble at all. Then, as I was starting to
crank the engine for the compression test, it weakly turned once or twice and
gave up. I used a set of jumper cables to get through the test and I am
charging the battery with a charger overnight. I'm not sure what to look for
next on this bike. I also don't know what the deal is with the battery, and
the timing of the battery failure seems a very odd coincidence.

Any suggestions?

Thanks,
Darren

"drsuaveiguana" <u36331@uwe> wrote in message news:761fa4b1870f6@uwe...
>I would appreciate any advice on this.
>
>
> After about a month of not riding, I took the bike out. After about 20
> minutes, I was on the interstate going around 70 mph. I accelerated to
> pass
> a truck and the engine suddenly tripped over itself, cutting out instead
> of
> accelerating. I took an exit and managed to get back home, but the bike
> sounded bad and wouldn't break about 45 mph.
>
> Now the bike will start fine and idle alright, but it is stumbling again
> on
> rapid throttling. It also runs roughly as the rpms climb. There is a lot
> of
> hesitation and backfiring. I have checked the fuel, fuel filter,
> diaphragms
> and slides, compression, and sychronization. All appear good. The plugs
> are
> fairly new, but I did check them out also. With the bike running, I
> removed
> each of the 4 spark plug wires (each cylinder has two) one at a time to
> see
> if one of the plugs was perhaps not firing. I noted decreased performance
> when each plug wire was removed. I figured that meant that each one was
> actually firing alright.
>
> One other thing happened today as I was testing compression. The battery
> seemed suddenly drained. This is odd because the battery was new as of
> the
> spring. It has not given me any trouble at all. Then, as I was starting
> to
> crank the engine for the compression test, it weakly turned once or twice
> and
> gave up. I used a set of jumper cables to get through the test and I am
> charging the battery with a charger overnight. I'm not sure what to look
> for
> next on this bike. I also don't know what the deal is with the battery,
> and
> the timing of the battery failure seems a very odd coincidence.
>
> Any suggestions?
>
Fuel. Spark. Air. Need all three.

I went through this a few years back, also on a '73 CB, a CB750K. Few
things to check:

Fuel - A restricted or poor quality fuel supply will let SOME gas into the
carbs, but any restriction will show up at higher RPM's when fuel can't be
supplied fast enough. Change the gas. Empty the tank and refill it, and
change your fuel filter. Changing the fuel and filter (you mention you
"checked" them so you may have already done this) is a cheap and easy way to
eliminate SOME fuel supply issues. Does this bike have a vacuum-controlled
fuel petcock? Check the vacuum line from the petcock to the intake. You
could take off the hose and put some tape over the fitting on the intake and
try running the bike with the petcock in the "prime" position after checking
that fuel flows freely from the tank with this setup.

If that fails to fix it, start thinking Electrical problem.

Coils - check the resistance. You may want to pull and have tested if you
can't find anything else wrong. As the bike's RPM's go up, the coils need
to recover and fire faster and faster. A weak coil will show itself as
problems with high RPM firing. This generally happens over time, not all at
once as you have indicated.

**** Wiring - especially to the coils. If the coils aren't getting full
voltage, they're not going to be happy. Your comment about the battery
suddenly going dead indicates a potential ground fault. Look for wires with
insulation worn off them or discolored insulation due to high current
(temp). Take out all your fuses except the ignition system and see if the
bike runs better. Depending on where the fuse is in the circuit, you might
luck out. Otherwise, ground fault issues are usually a WHORE to track down.
I'd look really really closely at the power supply wires to your coils.

Plug leads - if the leads are the type which can be removed from the coil,
remove and test by hooking each end up to an ohmmeter. Wiggle them around
and see if the needle moves (analog meter is best here). My bike had
integral coils/leads with cracked insulation on the leads. I sheathed the
leads with clear plastic tubing as they would arc to the frame at high RPMs.

What was the weather when you were having the problems?

Dave S.