1600 Kombi engine fitted with twin kadron carbs. Been running "as purchased" until time permits to get some stuff done on her. She's a daily drive that probably does around 7 to 8km of driving total each day at present.

What generally causes an engine to fluctuate in idle speed? When you rev the engine it will drop back down to idle and then start to climb and fall, does this a few times (much like a mower engine with an air operated governor does) and after a little while (30 seconds) it will level off and idle normally, but rev again and it will fluctuate again.

Any ideas what to look for? I'm guessing a carby issue or a jet or something similar. My plan was to do vaslve clearances, change the plugs and leads and dizzy cap etc then look into getting the carbs right.

I know nothing about twin carbs, how to tune them, balance them etc. It is probably one of the few jobs I would take the car somewhere to have done.

Vacuum leak

My guess is leaky bushings but could be many things

Air leak in manifold x 2 or Kadron linkage issues but not quite right for this by your description

Any easy way to test where the leak would be? It has a vac port on each carb manifold with a brass tee piece to run the brake booster, but uncertain when the gaskets were changed. Throttle butterfly and linkages seem nice, not loose or sloppy.

Do kads have any sort of auto choke? I don't see any wires so I'm thinking not.

Would I be likely to do any harm to the setting if I pulled each carb and manifold off and replaced the gaskets? I guess this can be done with the engine in situ as you would usually remove the carbs before dropping the motor. I need to take out my genny at some point which will need to have the linkage out anyway so maybe can be combined into the one job.

I use a vac pump and gauge to test for leaks in the booster lines. I've seen Kombis with half a dozen leaks in the booster system. A leak in the vac pipe is just like a manifold gasket leak. It must be fixed.

I have the Kadron manual as a pdf. If you want it, send me an email to beetleboyjeff@hotmail.com and I will send it back.

I reckon mats on the money with leaking vaccum hose for the brake booster I fitted twin carbs to my 1600 and could not get the idle or mixture right found all the hoses under car were porous/leaking replace and problem solved

I think the vac line underneath is a steel line with plastic pipes attached to it. Does that sound right?

Interesting though that when I hit a certain speed aka certain engine rev range, I can hear a whistling sound under the car which makes me think the possibility of a vac leak somewhere on the brake system is maybe it.

Is it ok to just renew the hoses with decent vac line or does it need the proper hard plastic hose?

Mine does this as well. but only with the lights on ?? 1800 type 4 with kadrons.

Lights on increases the load on the engine via the alternator. Increased load is increased manifold vacuum.

Squeek underneath is definately a place to start. Check all the joints. The factory plastic is great but the rubber seals required at each connection are often cracked or even missing. Good quality vac hose will do the job.

do you have a balance pipe fitted between the carbs? No balance pipe can cause idle issues.