Vacuum gauge indicates late valve timing- help needed XJ750P

Hi, the bike (seems) to run fine, i have the vacuum gauges hooked up and across all 4 cylinders it reads 7 inches.
Vac gauge
My problem is scenario 10.

I have checked for vacuum leaks around the manifolds to no avail.
Because i am getting the same reading for all cylinders makes me wonder out loud.
I have the eBay YICS tool inserted, but the length of the tool is shorter than the YICS passage, i just made a guess at centering before screwing the bolt back in, could this be the cause?

I checked the ignition timing to find it a little out, and the vac reading went from 10 to 7 after using the strobe.

Also, i took the valve cover off, and checked the cam dots to the crank at TDC, it was perfect so i rule out incorrectly installed valve train.

But, this is a ex- police special, and i still don't know if they installed XJ900 cams at all, Ill post my photos tonight, but if anyone can post some XJ900 cam photos it would be a help.
I was thinking that different cams would show different behavior in the valve timing.

Does anyone have experience with vacuum gauge troubleshooting, this is a new motor and shows no other problems.

It now starts easily and runs fine, i would just like to put my mind to rest about the weird reading, a good reading is between 15-20 inches.

Thanks, Rob.

 

A good reading on that gauge is for a car with a single plenum manifold.

Single cylinder readings on a small engine will be different. I may have to fire up mine and get a reading to put your mind at rest.

My vac gauge is at work so it may take a few days though.

Personally I wouldn't stress if its running fine.

 

According to scenario 3, "high performance engines... will show a normal vacuum reading lower than "stock" engines..."

I wouldn't go by the actual numbers, the whole purpose is to get the same readings as you sync the carbs, matching them is the most important task.

I've never paid attention to the actual numbers; so I went and looked at my CarbTune, and the cylinders are graduated in HG/CM. So I'm useless for comparison data...

 

Rockerto says all 4 cylinders read 7 inches and looking at th link to th gauge pic that = 20cm, so maybe you can discuss.
I use a 4 tube water manometer and get 'em equal, aint got no #'s to help you with.
And who or what says a good reading is between 15-20 inches, just wonderin' what th basis is....rings, valves, shims, timing?

 

Agreed.

10" Hg (scenario 10) is right where they vacuum reads on a well synced, good running engine.

 

Re: Vacuum gauge indicates late valve timing- help needed XJ

Thanks guys for the reply's, i have them consistently matched so sync is right.

Bushy- Some info varies This site shows 18"-20", but also says
"Steady Extremely low"

Up to 15" drop,
Warped or cracked intake manifold.
Bad carburetor-to-manifold gasket.

I made a set of gaskets with good paper for the startup a year ago, but will swap them out for the set that came in the rebuild kit tomorrow and see if it helps.

When revving, the gauge displays symptoms consistent with a healthy motor, but at a lower reading.

I missed the lower reading bit for performance engines bigfitz and i take the point there, the police special has,
Wiseco big bore kit, 4 into 1, Pods, Bigger jetting.

And here i am expecting normal readings :?

Here's the cams, i have a non tacho head?





Cheers

 

Re: Vacuum gauge indicates late valve timing- help needed XJ

I thought everything was ok with my carb boots, but i took them off to replace the gaskets and the inside of the boots have some cracks on the inside of them, possibly leaking air though the boots, am trying to get a new set now.
Will be interesting to see what the readings are after putting a new set on, will let ya's know..

 

Um, got new intake boots, no improvement and the readings are the same across all pistons.
My mechanic suggests if it's running ok then don't fret it.

I had a thought that my vacuum gauges are not calibrated, i'm at 650m above sea level, and i have the gauges set to 0 vacuum inches.

 

your trying to compare your bike readings to a chart that was made for car. bikes don't work that way. as you've only got one cylinder. where as cars have ALL cylinders working on one port.

one cylinder only has vacuum 25 percent of the time.

some posters have said if the timing is off one tooth. the bike won't run. and bent valves will result.

since your bike is running. don't worry about what the vacuum gauge says.

if you've got good compression that's what matters.

 

I get 7 inches too, it's normal for a 1 carb - 1 Cyl set-up.

 

Are you using a Restriction Device with the Vacuum Gauge hooked-up to the Manifold?

Without a Restriction Device the Gauge will fluctuate so rapidly and wildly, that an accurate reading is difficult to get displayed.

With a Restriction Device, the Gauge will display an an accurate reading, without the indicator being affected by inertia against it spring and fluctuating so wildly that an accurate reading is very tough to get.

Restriction Devices for Vacuum Hoses.
Carb Cleaner Red Tubes Epoxied in Hose Union Splices.