1982 R80ST Problems

[Macheta]

Hi there,

I have bought a 1982 R80ST a couple months ago, as my first dig into airheads. I would like to share a problem that arose lately and see if any of you have any ideas what might be happening. I have posted this in other forums to try and see if any of you have been in my same situation.

The bike was in good condition but wasn't ridden more than 800 miles in three years, so I did a major service, including all oils, filters, spark plugs, valve adjustment, ignition timing, greasing and carbs rebuild (inc float needle) and sync. After this it was riding well. The only thing was the idle speed, which I never managed to get below 1,500 rpm (possibly butterfly throttle valves?)
It all went well, and then did a 2,500 mi roadtrip in europe.

The problems started here, while after many hours under the pouring rain the bike started loosing the engine idle, so I had to keep revved to get home. After that, I thought that it probably was some debris in the carbs so I took them apart again and cleaned them thoroughly, but now the bike behaves very badly.

The symptoms are:
- It starts relatively well when cold. Not great but miles better than warm.
- The rev's get stuck at around 3-4k rpm once you accelerate the bike past them (it doesn't rev down to idle). This is the most relevant and awkward issue.
- If I force the bike to rev down when it is doing this, it soon dies
- Once the bike is hot, it starts very badly, having to wait between attempts if it dies.
- Power is not great, but much faster when is still cold than warm
- I just realised that my carbs don't have the bowl needle clip that they should. They never had it and I just discovered in the forum that the hole in the needle is for that. Even though I am not sure that this is the issue, as I think I have ridden the bike the past 2 months without them and did not have the problems.

I daily ride the bike and use it for everything (commuting, trips etc) and need to make it work somehow. I have cleaned the carbs another 3 or 4 times since it started having the problems. The floats are at the right level, and have 24mm fuel in them. Fuel comes down the float needle, so no apparent issues with that. All jets have around 3,000 miles since the rebuils, as well as the O-rings, diaphragms etc. The vacuum is there as I have tested the bike without the pipes and could see the barrel going up when accelerating.
I certainly don't know what could be. Maybe the needles are bent? They both are in the 3rd groove and come up the barrel around 38/39 mm. It certainly sounds like the fuel/air keeps getting in.
The throttle cables look fine, I can move them around and the engine doesn't rev up. They don't come out of the sleeves or so.

I know loads of lines have been written about airheads, but I just haven't found the solution to my issue yet, so maybe some of you have experienced similar troubles. I have certainly read many threads and I have the Haynes manuals but still struggling to understand. The trees quite possibly don't let me see the forest.

I was thinking of buying new floats, float needle, float needle clips and main needles. Diaphragm maybe too? Any ideas would be appreciated!

Thanks in advance!
Cheers

 

[ChasF]

Sounds like a diaphram problem to me. May be something to do with the choke - it is possible to get them the wrong way round.

 

[tadhgocuilleain]

- It starts relatively well when cold. Not great but miles better than warm.
- The rev's get stuck at around 3-4k rpm once you accelerate the bike past them (it doesn't rev down to idle). This is the most relevant and awkward issue.

I don't know the airhead engines, but my first thought when I read the above is that the choke is stuck open. I'm also wondering if it's as simple as your throttle cable fraying and not letting the carb fully close?

- I just realised that my carbs don't have the bowl needle clip that they should. They never had it

I'd be starting by replacing the missing part. I know you rode for two months without it, but you couldn't get it to idle below 1,500rpm for that two months, so it still wasn't right.

 

[Macheta]

Sounds like a diaphram problem to me. May be something to do with the choke - it is possible to get them the wrong way round.

They seem to be okay, following Snowbum's webpage.

I don't know the airhead engines, but my first thought when I read the above is that the choke is stuck open. I'm also wondering if it's as simple as your throttle cable fraying and not letting the carb fully close?

I'd be starting by replacing the missing part. I know you rode for two months without it, but you couldn't get it to idle below 1,500rpm for that two months, so it still wasn't right.

The cables go back, they seem to be okay. Choke mechanism looks alright.

The diaphragms have 2 months and 2,500 miles. I just replace them with the ones that I had just to see if that was the problem, but nothing has changed.

 

[Kenny]

The ignition coil on this model has a tendency to crack ( it is a hairline crack not really visible to the naked eye and usually occurs in the very wet - when the initial damage is done - and cannot be repaired ). Could need a new coil. Also, you seem very capable re maintenance so it won't be some obvious. The flot needle clip is just to secure it in place and help pull it back down ( obviously if it isn't returning then that will cut the fuel off ) ..will still run OK.
Always have approx 2mm free play in all the cables as these do tighten up with engine heat etc.

 

[Captain Black]

Something as simple as the tubes to the air box and heads maybe a small gap/leak could expand when the engine is warm ?

Maybe spray something like GT85 or WD40 around the area when the motor is running see if it changes the revs ?

Could it be head gasket ? Valve issues maybe they are on their way out ?

Loose head studs ?

Sorry I'm just throwing a few ideas you're way.

Seems you've done all the carburettor stuff, maybe do a leak down test ? Could be rings etc ?

How many miles on the motor ? ?

 

[Captain Black]

The ignition coil on this model has a tendency to crack ( it is a hairline crack not really visible to the naked eye and usually occurs in the very wet - when the initial damage is done - and cannot be repaired ). Could need a new coil. Also, you seem very capable re maintenance so it won't be some obvious. The flot needle clip is just to secure it in place and help pull it back down ( obviously if it isn't returning then that will cut the fuel off ) ..will still run OK.
Always have approx 2mm free play in all the cables as these do tighten up with engine heat etc.

I had a cracked coil on my PD Kenny a few years back. Seem to recall you or Rob pointed me towards that. Turned out it was a hairline crack in it, only obvious when the coil was removed and looked at close up in daylight.