R80 ST / Lumpy Running

Just recently and progressive, but the ST does not like start ups from cold with it running very lumpily (is that a real word).

When hot tickover is still lumpy but not as much as when cold.

Once the revs are up >1800 / 2000 rpm plus then it runs absolutely fine and accelerates just like it always did.

So, I am suspecting a slow running or pilot jet problem - blockage maybe.

Going to have a look tomorrow and drop the float bowls.


What should I be looking for?


Thanks

I don't know the answer, but my money's on blocked pilot jets. I reserve the right to change my opinion based on further evidence coming to light.

Do you ride the bike often or has it been standing for a while?

I'd be looking at the pilots too.

As you may know, on these bikes the coils can create running problems that are heat related (although in my case and I believe most others, it's a cracked coil casing that starts to perform badly when hot, rather than the other way round). Anyway, if the carbs don't provide the answer - which I am sure they will - maybe consider coil issues.

tuftywhite said:

I don't know the answer, but my money's on blocked pilot jets. I reserve the right to change my opinion based on further evidence coming to light.

Do you ride the bike often or has it been standing for a while?

Click to expand...

1000 miles since early July MOT.

See if you have any air leaks in the inlet system.

Sounds like it needs a tune-up with gauges, etc. Problems on start can be down to the start fuel circuit (choke to you and me) with air leaks or crap getting in and blocking up stuff. Also if they are poorly adjusted and don't close properly when the engine is warm then that makes life tiresome.

Also make sure all the throttle and choke cable have free play.

barny said:

Also make sure all the throttle and choke cable have free play.

Click to expand...

A good call. One of my bikes was behaving just as described with nothing in between the last ride and ‘ the problem’ to give a clue as to the cause. I then remembered I had lifted the tank for some reason. I had dislodged a throttle cable in doing so and that caused an imbalance. Flicked it back into place and smooth running again.

I’d put money on an air leak. Could just be dirt in the pilot jet or the tiny passages at the front of the carb or serious imbalance due to tight throttle cables.

This modern fuel is pants and it does gunk things up, check the chokes are going off by pushing the cable arms down a spring may have broken, check the end of your silencer pipe, if it's black & sooty your running rich, and that's where you should start.

Fuel is Shell V Power Unleaded E5, and fresh.

Tail pipe is a little sooty

In line fuel filter checked. Its clean. No dirt
Timing checked
Valve clearances checked
Throttle and choke cables are all correctly seated
About 2mm play in the adjusters
Choke is off and fully home
Float bowl removed.
Inside of bowl is clean, with no obvious dirt
Fuel level is same in both at 24mm
Carb cleaner sprayed into pilot jet and left to soak before blowing out with the airline
No obvious air leaks on inlet side or outlet side
Throttle stop screw just kissing the butterfly lever - fag paper gap, then wound down 1/2 turn
Mixture screw wound in lightly until it bottoms, then wound out 1 and 1/2 turns



Now, this is where some confusion sets in. According to the Haynes manual, page 5.3, it says 3-4 turns. Does that mean in total, and the turns out should only be 1 and 1/2 turn, or should it be 3-4 turns out??

So, set the mixture at 1 and 1/2 turns out
Started 1st time
Ran the engine until 'hot'

Better already
Adjusted mixture screw in until l/h side faltered, then backed it off a 1/4 turn
Did the same on the r/h side
Then adjusted the throttle screw up on the l/h side until it just faltered then down 1/2 turn
Did the same for the r/h side

Now attached the vacuum gauges to the take off points
At tick over both gauges had the same needle position
Then increasing the revs, 2000 rpm the r/h needle went down and stayed steady, and the l/h needle also went down but fluctuated quite a bit, and adjustment of the in line damper did not make a lot of difference to the needle stability
At the moment no further adjustmnt to the throttle cables. So I cant really tell if both carbs are synchronized

Overall its better already. Going to use it tomorrow.

Doesnt appear to be an ignition coil problem..at the moment

4 observations.
- Position of mixture screw?
- Why does the gauge needle flutter? Is that pointing to something?
- Are vacuum gauges OK to use, or is there anthing better?
- By the fact that the harmonizer is no longer available I hunted down something else - The Carbmate, sold by Optimate UK. Anybody using this?

More to follow no doubt

1.5 turns out is a good starting point for the mixture screw, and you’re doing it right by what I remember.

I’d also check the plug caps are absolutely fully seated, and that they have the same resistance across them on each side - they do degrade over time.

Sorry not clever enough to work out why one side has fluctuating gauge and other doesn’t but that does sound odd. Mebbe an air filter side leak?

Did you try spraying carb cleaner on both before carb and after carb joints while it’s running? When you do that an increase in revs = leak.

Check your diaphrams, ensure there are no holes, pin holes will cause probs, also ensure they slide freely with the spring inside. Further inspection of the main jet holder would prove worthwhile, remove all the gubbings attached to each inc the brass insert that guides the needle in the slide, crud sets like concrete in there so loads of carb cleaner, I just had both carbs sonically cleaned and rebuilt all the O rings etc noticable difference. Kits from Motorworks well worth the overhaul.

Carbmate is good.
Swap your vacuum gauge hoses over to see if it’s the gauge or the bike.

CHeck choke cable is coming off completely……I had a time when one choke didn’t completely come off….ran like shit until I made sure choke was completely off

Gelandestrasse said:

Carbmate is good.
Swap your vacuum gauge hoses over to see if it’s the gauge or the bike.

Click to expand...

Are you using 'Carbmate'?

I'm considering he investment.

dr nosh said:

Are you using 'Carbmate'?

I'm considering he investment.

Click to expand...

Sorry, senior moment - I am using Carbtune, like this one https://www.ebay.co.uk/itm/22379068...kdpbP7nMW8CR38CRVCXM17wBQ=|tkp:Bk9SR_CEsP3WYA or at Motorworks https://www.motorworks.co.uk/pro-vacuum-gauge-toa14026-2.html

Easy for Bings and Dellortos if they have vacuum ports.

Carbtune work OK just make sure the rods are clean and occasionally cross the tubes over right to left to see if your readings are consistent.

Have you got secondary air injection on your bike? If so I would get rid of it.Caused all sorts of idle issues with mine.Now idles and picks up pefectly.No popping at idle or on over run now.

Checked choke. It is coming right off when the choke lever is rotated clockwise. About 1 - 2mm free play on the cables.

No secondary air injection.

Spent a bit more time fine tuning mixture/throttle stop/cable adjustment. Much better. Getting results now.

It always starts first press on the starter button, but when its started from cold, like first thing in the morning I have to hold the reves up to 1000 until its warm, otherwise the revs drop and it stalls, then it will tick over correctly at 1000. Choke in the first click position is generally too much, and it just needs a few seconds at 1/2 of the first click.

On the road it runs absolutly fine. Accelerates well and smoothly as it always did.