Cb500w 1998

[Cb500w98]

Have the above bike and its a nightmare trying to get it started. Have the following, new battery good spark fuel in carb bowls it's cranking but won't fire up. Compression test on each cylinder shows just under 125 for the left cylinder and 125 psi for the right. Cleaned the carb up and jets are fine. Am I right in saying this could be a compression problem I.e head gasket failure or is more fuel/air ratio mix problem that's preventing it from starting. Don't really know we're to look next don't really won't take the head of it when it might not be the issue and get into the problem of timing and new gasket. Pretty sure it's something to do with the carb as when I remove the plugs and squirt in some fuel in splutters into life and then dies. Anyone have any ideas?

[Krikkit]

125psi says it's not compression.

If the spark is good and timed properly then it must be the carbs. If you crank it over for a while then whip a plug out, is it fuel-smelling or OK?

[Cb500w98]

Hi

Cranking it over the sparks are dry when I take them out it's as though it's not getting fuel into the head. Checked the carb to head rubbers and there not split or brittle as I thought it could of been them or some sort of vaccum leak.

[Steve Bass]

Cb500w98 said:

Hi

Cranking it over the sparks are dry when I take them out

There's your clue.

Either the choke(s) aren't operating or you have no fuel coming through or water in the bowl(s)

Have you taken the carbs off and made sure the jets are clear and there's no crud in the bowls.
Check the choke cable is opening the choke slides properly.

[Cb500w98]

Choke appears to be operating normally when I pull the choke lever down it draws the choke plungers straight out. And the carb was cleaned yesterday with carb cleaner and jets were sprayed through.

[Onelastattempt]

Not being funny but does it have a petrol tap ? Could it be in the off position ?

[Cb500w98]

Yeah tap is in the on position and it does work as I've tried blowing through the vaccum hose and it does push fuel out.

[Steve Bass]

Well if the plugs are coming out dry or there's no fuel spraying out of the plug holes when you crank it over, there's no fuel getting into the cylinder.

Does it have a single or multiple carb's????. If multiple it's unlikely they both have the same problem.

[Cb500w98]

It's twin cylinders, took the carbs apart as I thought maybe in the screen filter underneath the float needle was gummed up with old fuel and maybe the floats weren't opening and letting fuel in, but it's clean inside at a loss as what to do.

[Rubin215]

Is there definitely fuel in the carbs?
Check they have fuel in them by opening the drain cock at the bottom of the float bowl and check to see if fuel dribbles out.

Turn the tap on the tank to prime and it should flow constantly.


If not, then take the tank off, block the vacuum hose with a pencil, then use a jug and a funnel to fill the float bowls and see if it starts.

To check the fuel tap on the tank you need to suck on the vacuum hose, not blow into it.

[srob]

Blocked tank breather?

[Cb500w98]

No prime on the tap just on off and reserve. Filled the fuel line with petrol from a syringe until it backed all the way up to the top of the hose and blocked off vaccum hose and it ran briefly but was very rough and still very hard to start on full choke. Getting a bit more and more back fire might change the carb to head rubbers boots sounds like to much air is getting in sum we're. Does blocking of the vaccum hose prove that the tap is failing in some way?

[Steve Bass]

Many carb'd Hondas have a petcock that is vacuum operated. If there's a small tube to the side of the petcock from the carb rubber then this is what opens the valve despite the on/off/reserve positions of the petcock.
When the engine cranks, the vacuum in the inlet draws a diaphragm in the petcock back and the fuel flows.
So simple sucking on the end of the tube will open the valve and fuel should begin to flow.
If it doesn't, that's the first thing you need to look at

[Cb500w98]

Yeah had the tank off just now and sucked on the vaccum hose with the tap in the on position and fuel come out.Tried it in off and reserve position and fuel still flows through I guess it shouldn't then in the off position.

[myvision]

I had a similar problem on my Hornet the vacuum was sucking the fuel pipe flat so no fuel going through replaced the old pipe then it was fine.
How old is the fuel pipework?

[Cb500w98]

It's a new vacuum hose and also a new length of fuel hose put them on in the summer.

[Rubin215]

From the sounds of it, you have bought a bike that has been sitting for a while; my money is on the carbs being blocked or gummed up.
Old fuel evaporates and leaves behind gummy, varnish-like residues which block jets and passageways in carbs; a proper stripdown and clean is a bit more than a scoosh of carb cleaner snake-oil and some frsh fuel; you need to actually take the jets out and have them cleaned properly, blown through with high pressure air etc.

[Tardigrade]

Fuelling blockage seems most likely.

But one other question: have you ever seen the bike run properly (at sale or during your ownership). If not, include wider possibilities, eg that ignition and compression timing are wildly out of synch.

[Cb500w98]

The bike ran very well when I bought it last September, rode it back all the way back up north from Coventry. Inspecting the petrol tap and carbs as a precaution the next day it was indeed full of old fuel like varnish. Replaced the the tap as well as the old filter that sits inside the tank as it had disintegrated and it ran fine although it did idle rough for a time it seemed fine. Its only lately I've had real problems with it even borrowed a mates set of carbs and his were pristine and it still won't run. I will try high pressure air on it next though and see what I can get if anything out of them.

[Cb500w98]

Mates just had a look inside my tank and says by the look of it someone has added petseal to the tank. Says that it's gone off and melted but none of appears to coming out from the petrol tap. as I understand it its a glue that seals any holes in your tank, could this have gotten into the engine, pistons valves etc looks like a nightmare to get out.