Honda 85 VF500C suddenly running poorly

I have a 85 VF500C with about 18K original miles on it. Carbs
overhauled and valves adjusted by me a year or so ago ~ 1,500mi. The
carbs were clean and all internal parts looked to be in as new
condition. I used the Honda revised clearances on the valve lash. Bike
ran fine until the other day when I noticed the exhaust note had
suddenly changed (sounds a little burbley, maybe on right side). I put
the bike away running fine, next time out I noticed the problem.

Acceleration is now weak and uneven from idle to about 5K. If you open
the throttle too fast the engine dies immediately. Above 6K it smoothes
back out and pulls well. Symptoms are getting progressively worse.
Plugs are fairly new, look even, are not fouled, and are gapped
properly. I have checked for vacuum leaks, clogged air filter, etc. The
bike has always smelled like it was running rich at idle. When I
balanced the carbs after the overhaul I leaned out idle mix which
raised the rpm and lowered the smell. Smell maybe a bit richer now.
Fuel filter is original (I have a new one, but it looks like a pain to
change).

If this was a car I would suspect the carburetor, maybe accelerator
pump, or idle circuit passageway clogged. It was a real pain getting
the carburetor out this thing and checking the valve lash, so I don't
want to do those things again unless I have to.

I ruled out the fuel pump, filter, and ignition because at higher RPMs
it smoothes back out when fuel and voltage requirements increase.

Any ideas where the problem might be? Could it be in the ignition
system - wires coils, etc.? If I take the carbs off again, where should
I focus inside? Thanks in advance.
Ken

 

I have a 85 VF500C with about 18K original miles on it. Carbs

First off, I would check the fuel filter (if you have one) to see if
there is anything clogging it up. If you see any tiny particles in there, I
would consider redoing the carb. I have a 79 where the tank started to rust
and every now and then I would find tiny particles in the fuel filter. I
eventually protected the inside of the tank because it was a pain to clean
up the carbs every time....
You may also have a fouled plug and just need to change it or if you
don't wanna spend alot, sandblast it.
Other than that, maybe someone else has a thought or two....

Fwed

 

That's it. Clean the idle jets and passages out, and reset the throttle
butterflies to cover up the transition ports at idle. There is probably
a pattern of three transition ports. Typically, only half of one
transition port (the furthest downstream) will be uncovered when the
butterflies are set to idle at the specified RPM in the manual.

 

wrote:

Why do I think this might be at the root of the problem?

 

Well, it did run fine for a year or so after that. I tried riching up
the mixture the other day and that did not help. It changed so suddenly
I suspect one of the four carbs has something plugged, or a valve
adjuster loosened up. I find it odd that the exhaust note has changed,
that is what made me think of valves. It does not sound like it is miss
firing. If the fule filter was clogged I would think it run run poorly
at high RPM/Speed.
Thx.

 

The suddenness of the problem makes me think of particulate debris
lodging somewhere, OR an electrical problem. Either way, is it possible
that you're only running on 3 cylinders?

Are all four cylinders firing at idle (it sounds like they're all firing
at higher rpms, but that doesn't guarantee that all four are doing so at
lower rpms)? If not, does swapping coils or spark plug leads move the
cylinder that's not firing?

fwiw,

--
Later,
John



'indiana' is a 'nolnn' and 'hoosier' is a 'solkk'. Indiana doesn't solkk.

 

Ah! That's different, then!

 

If I pull any single spark plug wire the engine dies promptly at idle.
The engine does idle well enough so I could be getting a lean miss fire
at off idle. I would expect the engine to die if the cylinder was
completly without spark at any time so maybe it is just weak combustion
due to lean mixture. I will try the plug pull again at off idle and see
if the engine dies.

Based on exhaust note I would say the problem is with one of the two
right side cylinders. I will swap wires and coils and wires and see if
it move to the other side - good idea. The plugs all appeared even in
color - not black, not oil fouled when I checked them.

I am thinking that if it was valve related it would run poorly at all
times.

If it was a plugged fuel filter it would misbehave at high speed as
fuel needs increased.

Ditto if it was the fuel pump.

I will check the ignition as best I can and then pull the carbs. I will
put on the new fuel filter at the same time in case it let something
through.
Ken

 

wrote in @e3g2000cwe.googlegroups.com:

Did you use a feeler under both valves for a given rocker? If not, they're
most likely too tight.

You should hear some valve tap at idle.

 

I did use the feeler gauge under both method and there is some valve
noise at idle, more after setting the clearance to the revised specs
which makes sense since they are larger. I also bought the special
honda tool for holding things level. I am not sure they had ever been
checked before and may have been a little tight at the time. If I end
up taking the carbs off I will check the valve clearance again as well.
I will check out the ignition system as best I can before tackling the
carbs.
Thx

 

I have an 86 VF1100 that runs like crapola when the idle circuits on the
carbs are even slightly gummy. I've gone to great lengths (fuel filters,
tank coating, etc) to clean the fuel and keeping the air filter and intake
boots new (ish) to keep the air clean. Those damn carbs are so hard to get
out of there, a little insurance has gone a long way with mine (going on 70K
miles btw) my 2 cents...

 

Over the weekend I ran the Ohm meter tests on the coil units. Both
tested identically. The resistance between the primary was about 1 Ohm
over. I think the spec was something like 2.5 and they were both 3.5.
The secondary to ground spec was 25K to 35K, but both coils read
infinite from secondary posts to ground. This did not surprise me since
the housings are plastic. At any rate both units tested identically. I
pulled a plug and laid it on the head cranked it with the cable on and
got a spark, but it looked kind of weak - not the strong blue spark the
manual was looking for. It still starts well and high revs are OK so I
kind of doubt it is the coils both failing at the same time all of a
sudden. I am also leaning toward the carbs being gunked up. Thanks to
all that have offered help.

Ken