Runability issue... any suggestions would be appreciated

So I tried to upload a quick video to aid, but I'm not sure if it worked or not...

What's happening is all 4 carbs are backfiring at random times and at random rpm's... this would indicate to me a lean condition or incorrect timing normally.
Heres what I have done...

-compression tested 200psi on all 4 cylinders
-leakdown is zero
-valve lash is in spec on all valves
-timing belt is good and a pretty new oem Honda belt
-static timing is good, and dynamic timing is correct with a timing light
-intake boots weren't bad but I replaced them with brand new ones anyway
-the fuel pump is putting out well within the spec (and bike does the same thing wether the fuel intake is coming from the tank or being bottle feed)
-I took the cut off valve diaphragms out and inspected them (they and the sealing o-rings are all in great shape)
-took all 4 carbs completely apart, inspected and cleaned them out, all 4 slide diaphragms are in good shape and have vacuum (and function properly when you push them up with your finger and let them drop)
-carbs have been synced
-all floats are working correctly, and with the carb on a bench when I feed the inlet tubes with a bottle all 4 fill up and don't overflow
-I have completely removed the system that breathes fresh air over the exhaust ports and blocked the holes over the exhaust ports
-I have an oem Honda manual and went through the ignition troubleshooting setup, all points at the icm for the sidestand switch, neutral switch, and pulse generator check out good

i have done a ton to this bike, including a 40amp alternator upgrade (it's a 95'), race tech gold valves and springs in the front, works piggyback in the back, fresh paint on all the plastics.... but this running issue has me at a loss at this point... any suggestions would be much appreciated

ignition coils. connectors to ignition coils. ignition pulse generation module?

sounds like you did the static tests on these items, but its also possible there's an intermittent failure issue that wouldn't show up in a static test.

Also, its normal for the 1100 to backfire a little on deceleration, at least on the models with the PAIR valves, not sure about the ones without it. But I'm reading between the lines that yours is backfiring (missing??) at normal running speeds, right? Describe the symptoms a bit more.

I know you probably checked these things but....
Air Cleaner, is it other than stock and clean?
Spark plugs, Iridium are good if it's time
Fuel filter replaced?
Battery Terminals good and tight, also common grounds checked and cleaned?
Main fuse clean and tight?
Maybe throw some Seafoam in the fuel to clean up moisture.

Try running some Seafoam ( 2 oz / 1 gallon of gas ) for a few tank fulls and just for S&G's check the resistance of the spark plug caps and coil.

A quick & easy check is to measure the resistance between the two caps on each side of the bike. Should read about 22.65K Ohms. That includes the resistance of the two caps ( 5K Ohms each ) plus the secondary coil resistance.

Seems like a vacuum leak likely. If it happens only when you let off the throttle I would also check the accelerator pump

I'd think that a bad coil or spark plug wire or boot would lead to misfiring of that cylinder - i.e running rough because you've lost one cylinder under load. Backfiring thru ALL the carbs at random has me stumped. Maybe the gremlin is contagious?

Maybe something in the ignition switch circuit, keyswitch, kickstand switch, handlebar kill switch anything that interrupts the ignition. worth checking out.
Backfiring randomly in all cylinders is kind of a tip off.

Dave.David said:

Seems like a vacuum leak likely. If it happens only when you let off the throttle I would also check the accelerator pump

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There's no accelerator pump on the ST1100, CV carburetors, such as the ST's don't generally need an accelerator pump.

The bike will run poorly with the air cleaner removed (or modified) or if equipped with a K&N, unless the foam sock is removed from it. Double check you don't have a vacuum leak from the fresh air system you removed?

Double check that side stand switch... it is very temperamental when it starts to go. you can bypass the switch by jumping the two green wires. When I had the issue of backfiring and the engine shutting down, that was the cause. The other time I had a backfiring issue the red connector at the main relay had melted. These are quick items to eliminate in the course of troubleshooting your issue.

I had this same symptom on mine. I had removed the p.a.i.r. system, but didn't execute the plugging of the vacuum vents properly. The hoses I installed would come off and the bike started backfiring like that. Totally random, any RPM.

Frank

When I acquired my '99, it had sit for quite a while and the carbs were dirty. After disassembling and cleaning and re-assembling, they were still not 100%. Ran Seafoam for several tanks to clean the last bits of residue I obviously must have missed. Worked fine. Good luck with it and let us know the final outcome.

vinny said:

When I acquired my '99, it had sit for quite a while and the carbs were dirty. After disassembling and cleaning and re-assembling, they were still not 100%. Ran Seafoam for several tanks to clean the last bits of residue I obviously must have missed. Worked fine. Good luck with it and let us know the final outcome.

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A Merry Christmas and a happy upcoming New Year to you and the misses!

Brant said:

A Merry Christmas and a happy upcoming New Year to you and the misses!

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Thank you! Merry Christmas to you and your lovely wife!