Engine Vacume readings

Just restored a 1981 XS650 Special 2. Like an idiot I only lapped the valves and didn't have them ground. I rebuit the carbs replacing diaphrams and ets and mixture screws etc. Cleaned out carb passages several times now. Problem is unsteady idle, unable to sycronize carbs, eratic carb performance. Replaced carb mounting manifolds with "made in Japan" ones. Hooked up vacume gauges and only get about 1-3" of vacume on both cylinders. I know on cars you should get around 15-20". I've seen no discussion on this subject on the 650's. My question is should I still see 15-20 " with the 650's short intake manifold run. Has anyone put a gauge on the carb hose barb and actually seen "normal values"? If so I suspect I have leaky valve seats., weak spring or other valve issues. I'm asking someone who has a good running 650 what their vacume reading is at 1200 rpm etc. I am running Uni pod filters

I only get 10-12” of vacuum at idle and the bike runs well. On my cars if vacuum drops below 20” at idle I start to worry. Lol.

Why are you unable to sync the carbs? I would start by readjusting my valves and then syncing the carbs. Out of sync carbs will cause erratic idle.

I've adjusted the valves and rechecked twice now and cam to crank timing and ignition dead on and replaced the cracked ignition coil (separate issue). I first made a carb balancing bottle set and no matter how I adjusted the carbs (or intenionally misadjusted them) I got fluid transfer from left bottle to right bottle, telling me the right cylinder vacume is stronger. After that I bought the vacume gauge set to get some numbers. Left is 1" right varies 1-3". Thanks Bosco for the numbers. I'll be putting back on the lift and give it a valve job/inspection, checking springs and and micrometer camshaft lobes. Was hoping not having to pull motor but a 30 second miracle instead but the writing is on the wall. This was my covid-19 stay at home project, love riding it.

If your getting erratic idle and cannot sync the carbs Id be looking for a vacuum leak. Check your intake boots, slide diaphragms, and vac petcock/line.

I would wait a while .. After a rebuild or even a winter storage .I don't expect the bike to run properly..In the startup
first 2 - 3 tanks
Mostly the float valves needing some flexing stabilizing
Here some re tightening of hose clamps and flanges against the cylinder would be expected
The diaphragms need to settle .. It is a 40 year old bike with its personality that no one can explain why and how
it protests at times.
A couple of heat cycles things usually changes
Then after a while if it not stabilized then more action might be needed.
A little misfiring and flowing gas . perhaps kickback Just normal

Lapping the valves should be enough I only did that the last time
Filling up with petrol for leak test stayed for ever.

Not exactly what you want, but ... I synchronised my Virago carbs in July using a vacuum gauge. At 2000rpm I measured 11.5" Hg for both. The carbs had been overhauled and the leaking intake boots replaced. I use 2000rpm because that's somewhere between idle and operating rpm ...

Note: If using a vacuum gauge not setup for syncronization, or without the glycol/thick oil added, then insert a blockage in the line with a 1mm hole drilled in it. This will dampen out the pulsing. A drilled 6mm plastic BB from a kids gun works well.

It might be worth mentioning that as you increase rpm vacuum drops. So if your bike is idling high due to an erratic idle vacuum will be low.

You might think about making, buying, or borrowing a leakdown tester to check for valve leaks before you pull the motor and open the top end. What do you see when you check compression?

ordered a leak down tester today

Pressurizing a cylinder on a twin cylinder motor with 100 psi of air is enough to make the motor turn over, and enough turn the wheel if the bike is in gear. The drill I follow is to park the bike on the side stand, set timing marks at TDC, put the tranny in first, set the rear brake hard by running down the adjuster, tie back the front brake hard, chock both wheels, and lock the crank with a socket, breaker bar, and cheater braced against the ground. Then check tappets to find the cylinder where both are loose (valves closed) and open the adjusters a few turns; that's the cylinder to be tested first. If the crank moves a little bit off TDC that's alright, as long as the tappets move, indicating closed valves.

Have you checked compression?