xbr 500 running probs. plz help

[gellis8]

hi thumpers. i got a prob with my fathers xbr500 its 1987 model. its runs fine at low revs but even when revving up at standstill it struggles to reach 7k and when out on road barely gets close to 7k. weve got new cdi unit. checked spark/plug. carb been stripped and cleaned and set to factory setting. new air filter. crank breather cleaned and were coming to the end of our tether now. please help if you know of any points weve missed or any ideas. many thx and keep on thumping also valve clearances all done and ok.

[boze]

are the exhaust, air filter and carb all factory standard parts? any mods at all? is the piston in good condition? gaskets all sound?

keep in mind when i ride my sr500 its rarely touching 6k, even if im hammering it....

[gellis8]

hi there. everything seems to be standard. exhausts are brand new. air filter new. new air shut off vavle and idle speed diagphram thing new. carb stripped and cleaned out. were both struggling on what else it could be. thx for replying

[guest27]

Hi tere are better people than me to answer this one (any really I guess) Is it different under load than on the stand?  You have checked the spark plug - have you replaced it?  You say it is struggling - is it just going flat bleeeerrrrgggggggg or is it popping and banging, or is there a good clatter coming on?

R

[themoudie]

MMMMMmmmm, like rog says, take it for a run and warm it through steadily, once it's warm then spin it up through the gears but keep the engine loaded. See what happens, as long as the boys in blue haven't felt your collar!

Revving the nuts out the engine with no load won't tell you much and what about this "air shut off valve" thingy? Have you rebuilt any other bits. There just seems to have been a lot of changes with no new spark plug, plug cap or HT lead mentioned. Change one thing at a time and you may have a better idea of what may be at fault.

Finally, how many miles has the motor done and has it been lying afore the new bits went on? Did you check the valves seating or is the exhaust valve burning out or sitting off the seat due to carbon build up. Although the afficienados claim no loss of performance have you got an open crack between the valve seats and the spark plug hole inside the head?

Questions, questions, I'll get shot! See what 002 suggests or 003 when they next log in, I only run an SRX!

Hope you find the beast soon, Bill

[GB500nz]

Wozzat? It doesn't sound like a standard part. Try removing it.

[gellis8]

hi guys.

well where do i start?.

right the spark plug shows good spark when turning over, new cap fitted and coil checked. not sure on age of spark plug though.

when running on road it makes little pops when off the throttle. up to 4k in low gears it feels quite strong but only when full choke. but after 4k ish it goes dead flat and really struggles in any higher gears. no splits on inlet side of things.

new Air cut off valve fitted to carb and new internal diagphram (nightmare to get new)

all electric components tested as says in workshop manual.

engine done 40k and was last on road in 1996. oil and filter changed.

looking down spark plug hole, top of piston doesnt look very clean but not excessive in my eyes.

redex fuel treatment run through tank in into carb and straight into head.
 
thx for all responses guys. really appreciate it

[guest27]

Pops off throttle - would suggest to me a leak in the inlet or exhaust letting in air.

Runs up to 4K but only on choke - suggests to me a blocked jet or jets - if it has been stood for some time theyre may well be gum in the jets, a bugger to get out unless you strip the whole lot down and carefully clean each jet and tube to jet etc (recognise this eh Damo?).

SPark plug of uncertain age, when it is about £5 for a new plug, even me on benefits will try a new one as a first option - old plugs can break down with age and the environment in the chamber under pressure is very different to outside.

I would - put a new plug in - then you know - and would have that carb apart on the kitchen table.  I have used carbon tet to clean out the gum, there is probably something better - like carb cleaner, the gum is real sticky stuff and difficult to get it all off the jet etc with just a prod and a poke.  I have a MK2 Amal that had been run on a stroker and then left for ages and the gum had gone so hard in places I was resorting to drilling a lot of it out.

Hope this helps

R

[guest7]

Put a new plug in for goodness sake

Yeah it has to be said that a bike that last ran in '96 is probably going to have crap in the carb, despite a clean out.

As for factory settings, there was a factory mod to the jet sizes and tappet settings, althouhg this shouldn't actually affect the running (it was aimed at the dreaded XBR stalling at tickover problem).

I'm not too technically minded so I can't help much. I know it's no consolation, but you really have to mess the carb up to get them to run badly. The only time I've ever had a problem is with one carb that would rev when the bike was stationary, but wouldn't pull under load. Jethro did a little test with the carb off, he blew down one of the carb's little tubes and the slide should have risen but didn't... can anyone clarify?

For club members I'd have suggested that I sent over a carb that I knew worked. Can't offer the same service to non-members though, no offence intended.

Cheers
GC

[guest27]

Ahh- the CV thingy was having a paddy - in this case the diaphragm has been replaced - it is seated correctly etc?

R

[guest7]

As a sure sign of my technical failings - what is an air shut off valve?

Give me the part number and I'll have a look at the microfiche so I can see which bit you mean.

Cheers
GC

[pigafetta]

'Valve set air cut' thing part number appears to be 16048-KRO-003. I don't know what it is but I'm not good with carbs. I'm having similar troubles to Damo with an old CD175. Anyway, in this case, after a plug change I'd be having the carb to bits. Sounds like a possible blocked jet, don't you think? Its a place to start anyway if the bikes been lying up for ages.

Dave B

[gellis8]

right, the latest is.......

a brand new correct spark plug was fitted a very short while ago along with a new battery.

carb had been stripped and cleaned a few weeks ago. quite thoroughly im assured.

as im not near bike and my father is, he is going to remove carb and inspect inlet chamber into head and ensure its resealed properly as well as checking the rubber trumpet for any splits.

i'll keep you informed on his progress when he's done it. any other suggestions welcome.

many thanks again   G

[002]

Strip and clean the carb again and check the diaphragm just to make certain.
Check for holes in the diaphragm and when back together make sure it rises and falls easily.

Jethro

[guest27]

I am kind of unwilling to accept my own assurance that I have done a job.  A through vclean of carb could be to wash the outside with a bit of Gunk, or thave stripped it to the very last part, saoked all the parts over night in an ultrasonic cleaner filled with suitable solvent and then rebuilt with all new gaskets in a silicone wafer production plant.  Then tested on a custom to ensure it is all working.

As a kid (ahem - what do you mean last time I did the job) I was prone to pinching innertubes when I put the tyre back on my push bike - I could claim that I had fitted a new tube so the hissing sound and deflating tyre  was nothing to do with the proper job I had done.  It may be that the carb has been cleaned and overhauled - but...


R