What can cause Vacuum loss?

Just to give you some pretext my carbs have been overflowing every now and then. So next time I start the bike it isn't running on all cylinders till the bowl or bowls that drained fill up again. I lived with this for about a week till I got tired of it. I have carb rebuild kits from the last times I worked on the carbs. They came with new needle and seat valves that are 1.0mm in diameter. I didn't realise this at the time but the original needle and seat in my carbs were 1.6mm(the 6 was worn and looked like a zero). I quickly realized that the carbs weren't getting enough fuel and swapped them back.

With the carbs still draining I them proceeded to adjust the float level(thinking it was the problem). Initially I set them at the seam for the bowl to the cast body of the carb which is incorrect. It is meant to be 3.5mm below this. The after some problems with the bike dieing at idle(presumably from too much fuel), I took the time to set the float levels correctly.

So after all that mucking around I now have lost about 10 Psi of vacuum across all cylinders :S , I realized this when trying to sync them. I know I have lost about 10 Psi because last time I hooked them up to gauge they all read about 20-21 psi.

I will also mention that while doing this I took the camshafts out to measure them to see if they were in spec. All valve clearances were check recently and I am the cam shafts are timed correctly(checked twice). I am also sure their are no problems with the coils or IC ignition.

I have no idea what I had done for this to have happened, and suggestions would be appreciated.

Good grief! Yet another testimony as to why folks should not buy kits to rebuild their carbs. I don't understand why folks feel compelled to do that. Ed

The actual vacuum reading is not particularly important. Having each carburetor pull nearly the same reading is the goal. That you have seen a 50% drop is an indication that something has changed. Are you sure that 20 is not too high?

Some common causes are--
Leaks between the carburetor bodies and the intake manifolds
Leaks between the intake manifolds and the cylinder head
Both of the above at the same time
Retarded camshafts - you have eliminated it as a cause
Valve lash far out of specification - also eliminated
Idle speed too low
Ignition timing way off
Badly worn rings - very unlikely in such a short time

Seems like I am forgetting something obvious . I'm sure somebody will point it out.

Yes, I never buy kits from anyone. Most are a one size fits all and frankly they don't care if they fit, just the sale. Most people will not return them. If 10% do, they get away with it 90% of the time. That's good money. AZ charged me $18 as a core for a new battery. Most people won't go back and get their money. Old game. :Plus you're depending on them knowing what your machine needs. Yes there were good company's out there supplying good kits but they are long gone or crowed by the scammers. I will go OEM or places like jetsRus. If they don't have them in stock, they will tell you, not take your money, wait a few weeks to give it back to you if ever. A major supplier was happy to sell me a item sight unseen but after a call, the SALESMAN reluctantly told me it was obsolete. NICE TRY.
Steve

BTW, check your carb holders where they fit to the head. If they are hard, get some new ones.

The carb holders are aged but quite flexible. The same goes for the carb boots. I conducted a leak test but didn't notice any change in idle.

Quotes from FSM

Then "Normal vacuum gauge reading is 19-24 em Hg, and the difference between any two cylinders should be less than 2 em Hg."

"If any gauge reads less than 14 em Hg
after synchroni zing the carburetors; check the points
listed in the end of the idli ng adjustment (Pg. 21)"

Page 21 points with notes
Engine Oil - Fresh Synthetic 10-40W JASO-MA rated
Spark Plugs - New and presumed all firing
Ignition Timing - Never touched electronic ignition presumed to be working
Throttle Cables - new and properly adjusted
Cylinder Compression - recently(week ago) checked read 180psi across the board usable is 140-220psi
Air Cleaner Element - Age unknown relatively clean
Air Cleaner Duct and Carburetor Holder Leakage - Minimal to none
Camshaft Chain - I assume it is most likely worn but still working for now.
Valve Clearance - Last set around 800km ago.

I sealed my carb holders where they fit to the head. They didn't seem to leak ether. I bought new ones and everything changed.
Best to replace them.
Steve

I use a Morgan Carbtune to synch my carbs and on two occasions have seen low vacuum readings like yours.

The first was an easy fix - I had the idle speed to low at first but once I turned it up to 1250 - 1500 rpm all was good.
The second time I had one carb so far out of synch it was dragging down the others. Not totally sure what I did but after fiddling about with the synching they eventually came right and vacuum readings went from approx 10 to approx 20 mm .

This was on my 750 with Keihin CV34 carbs. I don't know what carbs you have but if there is a way to bench synch to get them in the ballpark first it might be worth trying before using vacuum gauges.

Good luck

That's how it's done. Bench sync, get it to idle then sync them with gauges.
Steve

I don't know if you've seen my page on cam timing, but I made a clearer drawing for the timing. Some of the manuals are a little less clear on where the lines should be etc.

The links are in my signature.

I assume you mean cm Hg, correct? Vacuum is not usually measured in psi. When I tune 550 carbs, the vac level at idle, or actually 100 rpms or so above idle, tends to be in the 20 to 22 cm Hg range. 10 cm Hg across all four cylinders would be strange (for a 550). With no other major changes, I'd really be eyeballing that cam timing.

Loudhvx,

you were right on the money.
A compression check showed all cylinder were down from 180 to 160Psi.
I Then decided to take the valve cover off and check cam timing. I had the cams spaced correctly 43-44 links apart, but I was off by about a link in relation to the bottom end :S . I must have turned it slightly when I was positioning the cam chain and camshafts

With that corrected the vacuum went up to to 20, and I was able to sync the carbs properly and get it running like it was before.
Thank you for all your suggestions and advice.

Glad you got it figured out!