Unstable ST1100 vacuum gauge readings

[mbovino]

I'm in the process of restoring a 1991 that had gas left in it for a few years by the previous owner. I have completely gone through the carbs removed and flushed the tank, changed the fluids, filters, plugs, etc.

The bike starts and runs with a decent idle although when on the road the midrange is somewhat flat when rolling on the throttle until the main jets kick in. Opening the pilot jet screws 1 turn has helped as does partial choke. But it seems to be lacking power. The new plugs look like it is buring a bit on the lean side.

I put a vacuum gauge on each of the manifold ports in anticipation of synchronizing the carbs. But the needle fluctuates wildly at idle and at best only pulls 10 inches (I don't know what is right for the bike but that is low by auto standards).

The vacuum gauge fluctuations usually mean late ignition timing, late valve closing and/or a vacuum leak. The timing and valves are spot on. The carb boots are new and the clamps are tightened to bottoming. The carbs are seated correctly.

Even though the vacuum gauge appears to connect to independent intake manifold points, do they all interconnect somewhere because I find it hard to believe that they are all suffering from vacuum leaks? Perhaps there is a common line that is cracked or disconnected somewhere.

Is there a common fail point or some other test to look for intake leaks? The carb diaphrams are in good shape. I might try the unlit propane test near the boots, but of course things are a bit tight in there.

Any ideas or suggesions are appreciated.

Thanks,
Mike

 

[GitSum]

Sounds like something in the carb is still plugged up - where are you pulling your vacuum from?

I will see if I can dig up my vacuum gauge tomorrow and get some readings from my 98 ST

 

[BuddingGeezer]

A vacuum gauge on an individual cylinder will fluctuate wildly, which is exactly what you have 4 individual cylinders. It's not sucking when the valves are closed or on the exhaust stroke. On an engine with a common intake manifold the other cylinders will balance the vacuum gauge. The first time I put a vacuum gauge was on a V Twin and I thought the same thing.

I'm guessing the pilots are still clogged. Did you use a wire and rod the emulsifier tube holes out? I pluck a wire brush bristle to make sure all the holes are clean.

Ralph Sims

 

[saaz]

You need to dampen the vacuum hose for the guage to be stable. It either needs an inline tap or a restrictor. The idle vacuum won't be as high as a car, as you are taking readings from individual carbs.

 

[Highrider]

If you can find someone with a manometer, preferrably mercury filled, you will get better and more accurate readings. A needle gauge reacts too quickly and needs a restrictor in the line to smooth out the needle. I have used a small fuel filter inline successfully with a needle gauge.

 

[wjbertrand]

You need to dampen the vacuum hose for the Gage to be stable. It either needs an inline tap or a restrictor. The idle vacuum won't be as high as a car, as you are taking readings from individual carbs.

This is precisely the answer. Pick up a set of four in-line vacuum orifices at an auto parts store and splice them in. This should almost completely eliminate the jumping needles (or columns in the case of a mercury manometer set).

 

[mbovino]

All good suggestions thanks. And of course now that I think of it, the needle should fluctuate because it is only monitoring one cylinder at a time so it is measuring the change in vacuum on the intake stroke and is not balanced by others.

I could also be that the pilot jet bores are still gummed up. I ran carb cleaner through them several times along with compressed air but the suggestion of reaming with a stiff piece of bristle is a good one.

I guess it's off with the set one more time. I am getting so well practiced that they are off in less than 30 minutes.

Thanks, Mike

 

[Mark]

If you *ream* with wire you're going to change the jet size.
Use monofilament line.

If pulling to clean them I'd use solvents and the monofilament as well as compressed air...

 

[Mr. BR]

You need to dampen the vacuum hose for the guage to be stable. It either needs an inline tap or a restrictor.

and I'll confirm that as well. My old mercury filled unit has in-line plugs inside the tubes just upstream of where you connect them to the manifold. These are literally plastic plugs with little pin holes in them which damps the individual pulses. The mercury still dances around a bit but it's much less pronounced than what you'd get otherwise. AND, you can blip the throttle without the worry of drawing the mercury too far up the tube and into the engine.

Brendan

 

[GitSum]

I have the Morgan Carb Tune Pro which uses solid rods instead of Mercury. I tried connecting one of the lines from the Carb Tune, which has a restrictor in it, to my Sears Vacuum gauge, and the needle still fluctuates too much to do anything with it (between 12 & 18 at 2,000 rpms)

Here is a suggestion using some SeaFoam, maybe it will help clear things up:

Disconnect the fuel pump which is the + terminal in this pic

Run the engine until it runs out of gas and then disconnect the fuel line at the shut off valve or the back of the carbs and inject some SeaFoam directly into the line. I have used something like this before - it is made to inject two stroke oil into gas for your lawn tools, etc

About a half of can of SeaFoam should do. Start the bike and you will know when the SeaFoam has made it through the carbs - because it will start smoking. Shut it down and let it set over night or a couple of days and let the SeaFoam do it's thing. Then connect the fuel line back up and take it for a ride. A lot easier then pulling the carbs and a can of SeaFoam is around $6 to $9