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#3 carb losing vacuum on 84 aspy 1200
bike will pop and miss below 2500 rpm. add a little choke and it smooths right out. runs great above 2500 rpm. did a carb sync at 1000 rpm idle all is good but add a little throttle and # 3 carb vacuum goes to 0 " hg. the other 3 carbs maintain an equal vacuum.

#1 12-21-2011, 04:36 PM,
forgot to mention spark plugs and caps are new ran seafoam through intake and gas tank numerous times
#2 12-21-2011, 04:50 PM,
Before you dig too deep into things, you'll first want to have a good look at all the vacuum lines / hoses. Dry rot, brittle, cracked, loose, pinched or unplugged. After all, they are now 28 years old. However, with the symptoms your describing, it sounds like you may have a pilot jet, air jet or an idle circuit is becoming plugged. Ever had the carbs out for a good cleaning / overhaul?
'96 ST1100
Brewerton, NY
#3 12-21-2011, 05:52 PM,
i have not removed nor cleaned the carbs, to the best of my knowledge the last owner didnt either
#4 12-21-2011, 06:27 PM,
i'd have to 2nd the vacuum hoses and/or loose intake runner or gasket on # 3

could even be a bad plug,cap or sparkplug wire,try one thing at a time till you fix it,i've had bad wires effect bike below 2000 but above that ran fine,guess because the coil output was higher
1987 Aspencade 129K
1986 SEI 93K
2014 Tri-Glide HD 17K

#5 12-21-2011, 11:17 PM,
thanks for your help guys i'll check those things first then maybe carb rebuild
#6 12-22-2011, 06:41 AM,
hvac994 Wrote:bike will pop and miss below 2500 rpm. add a little choke and it smooths right out. runs great above 2500 rpm. did a carb sync at 1000 rpm idle all is good but add a little throttle and # 3 carb vacuum goes to 0 " hg. the other 3 carbs maintain an equal vacuum.


The symptoms you describe sound like partially blocked jets and a malfunctioning needle jet.
But, I don’t understand how the engine could be running smooth if number three carburetor has a vacuum reading of 0.
Starting with a visual inspection is always best. Getting a good look at the vacuum hoses under the carburetors will require the removing of the carburetor assembly.
Once they are out, might as well give them a good cleaning or rebuild.

Before doing your rebuild, take a close look at their function:
do the butterflies open and ‘snap’ shut?
do all the choke plungers travel the same distance? (this circuit actually adds more fuel, not decrease the air, like a choke, but the effect is the same.)
do the needle slides slide up and down smoothly and without binding or hesitation?

This is very important: these carburetors operate using a difference of atmospheric pressure through the use of the diaphragm mounted in the top of each carburetor and attached to each throttle needle.
As air rushes through the throat of the carburetor, there is a difference of pressure created below and above the diaphragm causing the slide (which is attached to the diaphragm) to glide up.
In doing so, the needle jet which is attached to the slide gets pulled out of the main jet tube. Because the needle is precisely tapered, as the needle goes up, the area at which fuel can pass through the jet gets larger, thus allowing the engine to run faster.
This is being done by four different carburetors which is the reason why we must synchronize the carburetors, to make sure they are all passing the same amount of air.
Image if one of those carburetor sliders got stopped or slowed in its raising travel due to either the slider walls having crud on it, the cylinder wall in which the slider is sliding has crud on it, or if the diaphragm is damaged in such a way as to not allow a difference in atmospheric pressure?
The engine cylinder with that carburetor would not function like the other three.
So, one of the areas to check when doing a carburetor inspection/rebuild is the sliders found in the top portion of the carburetors.
Make sure the sliders ‘glide’ up and down smoothly. Raise them using a non-metallic probe (so as not to nick them, thus changing their characteristics) and retract the probe quickly so the slider can ‘snap’ back into place.
If they don’t glide smoothly it may be because of crud built up on either the slider walls or the walls of the cylinder the slider glides in. Use gas to clean these areas as carburetor cleaners may damage the rubber parts of the carburetor.
If it slides up smoothly but doesn’t ‘snap’ shut, maybe the diaphragm is torn or not seated properly in the carburetors body. It may also be that the return spring is either damaged or not seated properly in the cap of the carburetor or not seated properly in the body of the slider.
Bottom line here is, the jets and passages in the bottom of the carburetor are only part of the job when doing a rebuild. Take a look at the top of the carburetor as well and be extra careful when reseating the diaphragm as they are fragile, easy to damage and pinch and can be expensive to replace. Take care.

Good luck and let us know what you find.

enjoying the view from the saddle....... due mainly to the people and information found within this site
#7 02-29-2012, 04:37 PM,

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