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GL1500 stall

9562 Views 78 Replies 18 Participants Last post by  toolcraft4100
I just bought a used 1991 1500 with 93,000 miles. It seems to be in pretty good shape, but the PO has no service records and the bike needs some attention. The price was right, but I should have looked her over better and test-rode her longer.
Last night I was riding with a passenger (maybe 350lbs. total). Temps were in the high sixties and we got into some traffic. Sat and idled for several minutes at a time. Went maybe a mile idling at lights and creeping along. Several times the bike stalled or almost stalled when taking off from a stop. Seemed to idle ok and ran ok once we got moving. Started right up when I killed it, but I'm not sure what the stalling means. The temp guage needle was pointed straight up when we were in traffic and dropped back down when we got moving again.
Where should I start troubleshooting?

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What RPM does the engine idle at and is it smooth with no surging? Check charging voltage at idle and 2k RPM.
The 1500 doesn't like to be idled on the stock alternator for long. The output from the alternator at idle is below the bike's demand and you'll prolly be discharging your battery if you idle long enough (even more so with the brakelights and fans running). Generally, the alternator can keep up if th bike is at or above about 1500 RPM (you should be able to tell this by watching your headlamp as you bring in throttle)
Agreed. Full output from alternator should be seen by 1500 RPM, 2000 RPM for sure.
Yup, idle adjuster is the silver colored knob under fuel door in right front corner. It might be turned down slow to prevent surging at normal idle of 850RPM. The bike should accelerate smoothly throughout the RPM range. If not a little more digging is in order.

The alternator appears fine with that output, battery should be OK for now.
There is a pump diaphragm which determines your amount of fuel discharged from the nozzles, that is the likely culprit.

That problem will not cure the surging of RPM at idle which is a lean condition caused by plugged carbs or vacuum leaks.
Someone did the Amateur Hour on your bike. The yellow hose is for your shot air system which is now disabled, other capped vacuum hose goes to the auto fuel valve which is missing and rounded screw heads in air horn top indicates someone has been in there before and judging by the other shoddy work would also be screwed up.
That is just what can be seen from above, it is probably equally bad under the carbs.

Plan of attack would be to pull carbs and check underneath for other problems. With carbs out go through them completely as we know it surges at idle and is caused by vacuum leaks or plugged carbs, with removal of pilot tamper proof caps. Reconnect everthing previously removed as originally installed testing components underneath where you used to have a mat, fire it up and diagnose more from there. Hopefully, testing of components under mat will not neccistate removal of carbs a second time. You will need a service manual and vacuum diagrams.

Different years had manifold vacuum lines on different intake runners and should be pinched off prior to pulling the carbs checking for downstream leaks one last time.
The 94 manual should be close enough and if questions arise you can always get them answered here. It will sound a little more difficult then it actually is, all the solenoid valves and controllers for the vacuum system should have the hose #'s stamped or decaled on them and is just a process of putting Humpty-Dumpty back together. OEM hoses are colored coded also according to what system they belong to. Study the diagrams and get to know where each valve is located and what the proper name of it is. Pictures help alot here when asking for help as other members can compare what a correctly plumbed system should resemble. Forget about the fuel valve for now as that can be done anytime after it is running correctly.

In your pics I did not see any vacuum lines going to the bottom of the air box for the hot air door of intake? or are they just out of the way. What exactly is the engine doing again (backfiring, stalling, rough idle,popping thru carb, hard start, skipping, flat spots, etc.) to sum it up and do yourself a favor and DIAGNOSE what the engine is or is not doing prior to taking it apart. This gives you important clues as to what systems need to be zeroed in on for a successful repair. A vacuum pump and gauge will be essential here. Start with ign. system and then to carburation, some problems may be found and corrected before the real work starts such as the simple connecting of the yellow hose. A compression test of the engine might also be advisable if you can't get it to run decently during the diagnosing stage. During the diagnose stage you leave the air box off and vacuum lines to it plugged and if needed road test it around the block that way.
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I found another yellow hose right next to the first one that was plugged off. I have studied the sparsely labeled diagram of the shot air system in the manual and am having difficulty seeing where the hoses go and matching what I see on the bike to what I see in the book. Hopefully I can get these hoses reconnected, ride it, and record what I experience. Maybe a miracle will occur. Is it important to test drive with the airbox off and hoses plugged? It does not take much to reinstall it.
I discovered the air pre-filter that filters the air supply to the carb-jet solenoids (I believe all that terminology is correct). The filter element was dry, brittle and had a hole in the middle. It turned to powder when I handled it. Could unfiltered air or particles of filter media contaminate the solenoids and effect the carb performance?
If I buy new bulk vacuum hose at the auto parts store are there key words I need to use to assure I get the right product?HoganJr
AICV solenoid valve is on left tube above horn in front of fan, Decel compensation valve (Shot) is under left pocket area, Air injection control valve is between the intake manifolds under carbs, shot air valve is under carbs left front, shot air solenoid valve (neutral) with hoses #3 & 4 front of carbs under coils, air jet controllers under right pocket. AFAIK air jet controllers work on the slide circuits of carbs and should not affect idle. Yellow is decel compensation, green for air injection, carb and air jet controllers use white. The above from my 88, YMMV.

If you want to take the box on and off each time started or ridden you might as well put zippers on it,:) you aint in a Okie dustbowl and for your testing purposes will not change the mixture or running quality to a hill of beans. Gear shift sensor affects operation of the above valves, did your neutral and OD lights work, if so you can give that a cursory pass for now.

Pic was from Tim IIRC regarding vac hose sizing.


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From the manual for Shot Air System:


Due to the large 1500 cm3 engine displacement and the fact that the valve timing has a small intake/exhaust overlapping, the
compression braking effect during deceleration is strong.
The shot air system has been incorporated to moderate compression braking and improve riding comfort.
The shot air system supplies air to the intake manifolds during deceleration to prevent sudden induction of strong manifold
As illustrated, the intake manifold vacuum acts on the lower diaphragm chamber of the shot air valve through No.1 and No.2
tubes. When vacuum rises during deceleration, the diaphragm is pulled down to allow air from the air cleaner to enter the intake
To ensure a natural deceleration feeling when the engine is revved up and down with the transmission in neutral, the solenoid
valve (Neutral) opens the vacuum route to the upper chamber of the shot air valve when neutral is detected by the gearshift
sensor. Under these conditions, the shot air valve remains closed.
Engine rotation per minute is detected by the pulse generator and signals are transmitted to the carburetor control unit via the
ignition control unit. The solenoid valve 2 opens when engine speed is above 2,000 min-1 (rpm) to increase air supply. In addition
to air through No.7 tube, air flows through No.6 tube, solenoid valve, and No. 10 tube.
The one-way valve is provided in the three-way joint near the intake manifolds. The one-way valve prevents manifold
pressure interference.


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The pre OBD carb equipped vehicles in the beginning of the emission requirement era where a bear to work on with the miles of plumbing. But each sub system can only affect certain parameters of the running characteristics.

And the usual question it boils down to is it running rich or lean with a given problem, ignition problems aside.
Look under the left pocket.


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Apparently different years had components in different locations (great huh) When in doubt look at the component for the hose #. That valve looks like it should belong there and the valve you are looking for is indeed missing.
1. Where is the Air Injection Valve on a 1990 or 1991 and what does it look like?
2. Where are the One-Way Valves and what do they look like?
3. The #4 Green Hose: Is that the large-diameter hose that comes out of the front-right corner of the Air Box?
1) The main valve is between the 2 intake manifolds and under the carbs, it looks like a EGR valve from a car. The solenoid valve looks like a mini hand grenade and is attached to left frame tube above horn (my ride) and is nothing more than a barometric pressure sensor.

2) One way valve is more commonly called a vacuum check valve, usually white small half inch diameter it allows vacuum to flow in one direction only. This would be towards the device so at times of low engine vacuum device does not get starved for vacuum. One way valve of air injection is larger and can be seen by main valve from above.

3) #4 hose would be the one that goes to RF of box.
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