Runaway choke

[Appie]

Good day gents

I’m busy with an ‘81 Special (89000km) which I am slowly working on.

Ive finally got to starting it and having problems with idling
1. Starts immediately on full choke but after 30-60 sec, idle goes waaay up.
2. Half choke idling comes down but then also increases as the bike warms up
3. Off choke it bogs down and dies unless I fiddle with the choke again.
4. I managed a short ride, it manages a slow throttle increase but doesn’t idle without a little throttle.
5. Towards the end of the ride (2km), was increasing revs in between gearshifts.

I have had the carbs off to clean and fix a stuck needle float and have set the chamber fuel level as shown in the picture. I did have to set the floats lower than they were to get the fuel height correct….
New plugs and I’ve attached a picture one after this short ride.

Im Wondering if I’ve set the floats too low and it’s running lean? But I can’t explain the runaway choke.

Ill take the carbs off again but wondered if you legends could weigh in before I do so.

I have checked the fuel lines and filters and have good fuel supply to the carbs. At least there’s no float leak so I can scratch one thing off the list 😂

[Appie]

Sorry for the duplicated post. I’ve attached another carb pic showing the float level properly

[DEEBS11]

Did you sync the carbs?

[cajun31]

I agree with Deebs. What you are describing is out of tune carbs. Get them synched. Once you have that done the bike will run better and then you can fine tune everything else. Meaning you can then fine tune the pilot screw settings and after that double check the synch again. Once you know you have everything dialed in you then set the idle to 1100 rpm. There are articles in the maintenance section concerning the pilot screws and synching procedures. Various different methods employed depending on what you have available to you and how comfortable you are in using them.

[Radioguylogs]

Some increase is idle speed is normal with the choke on.

Normal choke behavior:

Start the bike with full choke. It idles around 1500 RPM. After 15-30 seconds, the engine starts to load up and it runs a slight bit more rough. Move the to half choke. The bike is idling at 1500 RPM until it warms up. As it warms up, the idle will increase to 2-3k RPM. When the RPM approaches 3k, turn off the coke and it idles at 1100RPM. Usually my bikes idle a little below 1100RPM until I ride a mile a two.

Sounds like you have more than one issue happening at the same time. Good luck.

[Appie]

Thanks guys for the advice, really appreciated. I’d be lost without your input.

Ok an update…
Deebs I did bench sync the carbs (thanks Crazy Steve, LarryM), all four carbs have the butterfly splitting the front hole in the upper carb throat.

I pulled the carbs and rechecked and all butterflys are equal and seem correctly positioned.
I did notice that in the upside down position my floats were all higher than the 25mm service manual measurement. I adjusted them all closer to spec and checked the float levels with the bowls back on….which are all about equal but higher now than before.

Put the carbs back on and she started right up on full then half choke. Off choke she rides beautifully, still a bit sluggish if I open the throttle too quickly but much better, way better than before. No rev jumps during gear changes, no having to mother the throttle the whole time. She still dies in neutral when I leave the throttle but I haven’t adjusted the idle screw at all yet. It’s fully screwed out.

So, I’ve never ridden this bike before today and I felt a little of the magic you all describe! I didn’t ride for long, just wanted to check it was running before I tackle valve clearances/camchain tension/ vacuum sync.

I’m left with the discrepancy between float height ( now correct) and fuel level which is less than the 3mm below the mixing chamber body edge.

[DiverRay]

You posted the update as I was writing.
Yes, SYNC THE CARBS! You MUST re-sync carbs every time you remove them. I use a set of four gauges to sync my '79 Special. I also set the four gauges to the same reading using a hand pump to draw vacuum. Three "T" fittings connect the 4 together.
The only other cause may be a vacuum leak someplace. Starting with carbs synced, AFTER checking the valve lash, is what I do on any "unknown" engine.

[Appie]

Thanks Diver Ray!

I think there’s no vacuum leak, I sprayed the (new) boots with carb cleaner before I removed the carbs and the revs didn’t increase.

You’ve all suggested a proper carb sync which I will definitely do and report back….as soon as my shim set arrives from the States and I can check the valve clearances!

[DEEBS11]

Be advised Appie, a bench sync is not a vacuum sync. You are guaranteed out of calibration. You can't proceed until you perform this correctly.

[Appie]

Deebs, I got my hands on these gauges. To confirm, do I check valve clearance THEN vacuum sync? Thanks for all the help - very tempting to open the cam cover and see what lies beneath. The cam cover bolts are not all original so somebody has been in there before but I doubt the valves have been checked in a long time.

[DEEBS11]

Yes, as Ray stated, first valve lash, then sync. Should run much better after calibration.

[DiverRay]

And don't forget to put all four gauges on one vacuum line to verify hey read the same! There should be an adjustment screw that you may need to turn one way or another, but in VERY SMALL increments to do this.

[cajun31]

Deebs, I got my hands on these gauges. To confirm, do I check valve clearance THEN vacuum sync? Thanks for all the help - very tempting to open the cam cover and see what lies beneath. The cam cover bolts are not all original so somebody has been in there before but I doubt the valves have been checked in a long time.

You will probably have to use restrictors with those to control fluctuation.

[DEEBS11]

..........

[Appie]

Thanks guys for the advice and help. Where in the circuit do the restrictors sit? Sorry, feel it’s a silly question.

My son is back home at the end of the month, I want to check the valves with him then because it’s a first for both of us 🫣. And he can run for the beers as well…

I don’t have a valve lifter but I have read the Valve Adjustment Pictorial (Crazy Steve) and am wondering about doing things this way. - but it does involve loosening the camshafts after checking clearances . Would you suggest as a beginner I do this or bite the bullet and get a valve lifter tool?