Just got home the other day, and noticed a weird hissing noise coming from the engine, sounds like either a spark plug loose or a vacuum line come off, the bike is also running pretty badly, doesnt really want to rev cleanly until its warmed up, and even then has a massive flat spot from around 5-7k revs, and doesnt pull as hard as I think it should, and has a bit of a misfire at the top end. It did sit up for a couple of years before I got my hands on it, so I was kinda expecting to have a few problems.....
Any light shed on this would be much appreciated :)

shaneh711 wrote:Just got home the other day, and noticed a weird hissing noise coming from the engine

Check to see that a snake hasn't got caught up in your engine...?


The hissing noise could only be a couple of things. First thought would be a water leak releasing steam when hot, easy to look for (remove the fairings) and check to see if the coolant level's dropping. It could also be a blocked fuel tank breather (which will also cause running problems) open the fuel cap to see if the noise goes away.

After a couple of years sitting there's a good chance of running problems unless the carbs were drained before storage. I'd be starting with opening the carbs up and cleaning them, making sure all settings are correct and going from there. There's a V4 carb jetting guide here (for nc30 carbs - what bike do you have?) which might assist removing and cleaning the carbs: http://www.400greybike.com/docs.htm

It could also be an issue with the fuel tap vacuum hose. This is a hose that runs from the right rear cylinder (below the carbs) to the fuel tap, opening it when the engine's running. These often perish and result in running problems. When you take the carbs off for cleaning best to replace the hose even if it looks fine.

Another common issue with these bikes is problematic charging systems. The most simple check is a battery voltage check at 5000 revs (should be 12.5-14.5 volts), but there are many more checks.

Get stuck in and try to eliminate some of these things as the cause and report back.

Turns out it was the carb inlet manifold, there was a jubilee clip missing from the rear left carb. Put a new clip on it and bobs yer uncle the noise stopped :D Also noticed the pipe from the top of the rad to the expansion bottle was split and leaking, sorted that out, still got another few issues to sort out.....

Theres a bad enough leak out of the exhaust manifold on the right hand side beside the shock but all the bolts are rusted, gonna have to chop them off and put new ones in, would this affect the running really badly? The bike is running pretty badly at the moment, wondering if it could be head gasket, it doesnt have any coolant in the oil, or oil in the water, doesnt appear to be bubbling when im driving it, or heating up too quickly, but theres a massive flat spot around 7k revs, and it doesnt really want to rev past that, and when it does, it doesnt really pull very cleanly all the way up.
Is this likely to be the head gasket??
Any answers would be great :)
Cheeers!!!!

not likely to be the headgasket.

start with the simple fixes - make sure the carbs are on right - they will cause all manner of shit running

Get the rear stubby exhaust nipped up - clean off the nuts and either replace them or nip them up. They are spot welded into the manifold so you cannot chop them off and push them out...

there are two head gaskets on the bike...so if you think one is at fault, which one and why?>

Also have a look for your rubber flap in the front vee of the frame, my bet is its either missing or not fitted correctly which will make a big diffrence to how the bike runs. I will try and dig out a photo of how it should be fitted.


EDIT: This is the best photo i could find

when you get your tank and airbox off you should see that soft rubber over the frame bracing bar, the rubber should be attached to a piece of plastic that sits ontop of the radiator, and there will be a larger soft rubber piece that should sit on the inlet side of the front head and go down to touch the top of the inlet rubbers. Hopefully ive explaned that easy enough, bike is in bits at the minute but if i get time later i will dig out the whole rubber flap assembly and get a photo to post up.

Hmmmm, didnt see anything like that when I had the tank off the other day, ill have a look later on once im finished college, It runs really badly at the moment, vicious flat spot around 7k, and really doesnt want to rev past that, and once it gets past that it still doesnt rev cleanly to the red line.... Gonna have to whip out the plugs and see what way its running, probably best to do the carb slide mod whilst I have them off..... Looks like itll be christmas before i get a chance to rip it down completely.....

I had the same issue about 6 months ago, have a look at the pilot screws on your carburettors. Mine weren't adjusted properly and i had trouble getting it past 6k without it feeling underpowered. Hope that helps!

I have more or less the same problem. No rubber thing on radiotor. When cold misfire from 5000 to 7000rpm. Above 7 clean to 15500. When warm misfire from idle up to 7000. Total flat spot. Very bad!

jjgordon wrote:I have more or less the same problem. No rubber thing on radiotor. When cold misfire from 5000 to 7000rpm. Above 7 clean to 15500. When warm misfire from idle up to 7000. Total flat spot. Very bad!

Sounds like its running much too rich.

Do you have a dyno operater nearby.. get it on the dyno and see what it is doing.

Did you check your jets and pilots are set properly?