Vacuum leak from torn carb diaphragm?

LCBear350

Joined
May 19, 2009
Messages
9
Location
singapore
Bike
ST1100V
Hi guys, need help to solve a problem on my 97' ST 11.

Situation:
Bike starts normally without choke.
Runs normally until 4k RPM and the bike starts to run rough, sputtering or misfiring.
The RPM is still able to climb up despite the rough running.
The coolant temperature also rises to mid position during normal riding speeds of 90-100km/h. (Previously was 1/8 of the operating range)
All the above happened one fine day on this daily ridden bike without any major service done.

Bike:
Side stand switch removed and wired accordingly.
Vacuum shut off valve bypassed and removed.
Engine oil and oil filter changed.
Radiator flushed and new coolant in the system. (Did this first as the temperature of the bike was running higher than normal)
Thermostat was already removed and the coolant flow is strong.
Nothing helped to fix the temperature problem.

That is until I opened up the air box and stuffed each air intake tube with a rag while the engine was running.
Sitting on the bike, the right hand tube openings had zero response to it being stuffed with the rag.
But the engine cut off when the left hand tube opening were covered by hand only, even without the rag.
Given the right hand tube openings means left hand carbs, the left hand carbs must have a vacuum leak since it does not depend on the tubes for air.
This is also consistent with the higher temperatures due to a lean condition in the left bank of the engine.
I have sprayed WD40 around the boots and the idling did not change. So my best guess is the left carbs are drawing air through a torn diaphragm.

Is there anyway to test if the diaphragms are leaking apart from taking the carb out as I still need the bike to go to work daily.

Or is there anywhere else I need to check that could be leaking air?

Or is there other things I should check before diving into the carbs?

Sorry for the long winded post. I am hoping to fix up the bike by this weekend as I am going for a ride next week.

Thanks in advance for all advices rendered. Your input is greatly appreciated. If not for this forum, I wouldn't have gotten to test for vacuum leaks and keep trying to diagnose the temperature problem.

Bear
 
Generally, what you just did by "choking" the intakes one at at time until it effected the engine implies that the insulator attached to the carb that was unaffected by the test is leaking. The engine had to be getting air from somewhere. Might just be a loose clamp. The bottom clamps on the insulators are notorious for being loose. Mine were loose when I removed my carbs.
 
Slasher said:
Generally, what you just did by "choking" the intakes one at at time until it effected the engine implies that the insulator attached to the carb that was unaffected by the test is leaking. The engine had to be getting air from somewhere. Might just be a loose clamp. The bottom clamps on the insulators are notorious for being loose. Mine were loose when I removed my carbs.
Click to expand...

Thanks for the quick reply. By bottom clamps, do you mean the lock plates that fasten the air intakes to the carbs or the boots that attach the carbs to the engine?

Bear
 
Im talking about the ones on the bottom side of the boots. I don't know if you can get to them with the carbs in place. I've only ever messed with them when the carbs were off the bike. You might be able to get to them if you pull the gas thank. It is relatively easy to pull the tank. There are four bolts on top of the tank, one hose going to the fuel filter, two or three electrical connections on the top and one on the left side to the front of the battery. My bike ran fine when these clamps were loose but my boots were in pretty good shape, I imagine I'd they were older and harder they could be a source of a leak.
 
You can *almost* get to all 8 of the carb boot clamp screws if you really work at it with the carbs on...
 
Replaced fuel/gas filter yesterday but hadn't tested it on the road.

Road test today confirmed the running was much better and smoother, with better power from idle up. The temperature gauge was also reading 1/8 of operating range.

But if you run it around 4k RPM and up, there is a hint of misfire/pinging/rattle before going further until the red line. (tested for a short time only)

After that, the temperature gauge is at 1/2 of the gauge, can feel higher heat from the engine plus the rough feeling is back. The temperature also rises much more quickly in stop and go traffic. The temperature didn't went down much even after a period of time while cruising at 80km/h at 2.5k RPM. (Usually temp gauge would read about 1/8 of gauge)

I came home and smell a hint of petrol/gas when parked. Opened the tank shelter and discovered that the smell only came from the left hand rear carb area. No smell from the rest.
There was no signs of petrol leak or anything on the carb or engine that I could see. The smell went away when the bike got started and running.

I also did a smoke test to see if the sub filter hoses were sucking in air but both hoses don't.

By the way, the CDI/ECM is leaking resin too. No way to test this unit except to swap another one in though.

Any thoughts?

Bear
 
I find it relatively easy to just pull the carbs. Easier to work on the rest of stuff under there. Also, the only way to actually check the diaphragms. To me, the biggest PITA is locating the screw heads on the boot clamps. Strong, focused light helps. Other threads have lots of details, but use plumbers grease to ease reinstallation, and a little heat in the valley to soften up the boots.

REMINDER: any time ya have the carbs off, check the clamp on the PAIR valve hose under the main coolant hose at the front-right (near carb #1 boot). I had a clamp there punch thru the coolant hose at 105,000 miles.
 
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