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The carburetors on the ACE have what is called Pilot Screws that set the air/fuel mixture for when the engine is at idle. These screws are set at the factory and then covered with brass plugs and normally, no adjustment is necessary on them. However, experience has taught us that a very large number of ACE’s were delivered to customers with these screws incorrectly set and it is highly recommended that every ACE owner does this procedure to make sure these screws are set correctly. Also, any changes to the exhaust system may require you to re-adjust the Pilot Screws, such as the Stage 1 mod where the stock baffles are punched or Stage III where an aftermarket exhaust is used in conjunction with larger main jetting.
The reason why these screws need to be adjusted right is to prevent a condition known as "lean misfire". If the screws are set too lean (as is often the case for new bikes) the mixture at idle may become too lean and sometimes it won’t completely ignite in the engine. This unburned fuel them collects in the exhaust and ignites there, causing a backfire. The rider would notice this as rapid "pops" or "bangs" on deceleration, sometimes called the "Rice Crispy Snap Crackle and Pop". This condition is NOT normal and NOT healthy for the engine. It creates serious back pressure in the exhaust and can cause damage not only to the exhaust but also the engine and valves.
Now the service manual has a detailed procedure for setting the Pilot screws that involves making changes to obtain a specific RPM drop at idle but this requires you to have a shop with an accurate tach and also, you can’t properly adjust the forward cylinder with the air filter housing installed and running the bike without the air filter is not recommended. So I’ve created this "simplified" procedure that is more then adequate to meet the needs of the average ACE owner.
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Step 1:
The first step is to remove the air filter housing. We are taking the whole housing off, which is easier then removing just the filter. Remove the two allen key bolts that secure the back of the housing and then loosen the screw on the clamp that secures the housing to the air box. Then just pull the housing off.
Caution: After each adjustment, reinstall the air filter before running the bike to test it. The bike will not run properly under power without the air filter attached. |
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Step 2:
Remove the plug covering the pilot screw on the left carb (rear cylinder - see Pic). A good way to do this is to drill a small hole in the plug (be careful that you only drill just enough to get through the plug and not into the screw behind it!). Then take a PK screw (or any sheet metal screw - the kind with a point that is self tapping) and screw it into the small hole you drilled until it grabs. Then just pull the plug out using a pair of pliers on the PK screw head. NOTE: If you bought the bike used, it's possible that the plugs have already been removed.
You will now see the small pilot screw under where the plug was. Take a small flat screwdriver and gently turn the pilot screw IN (clockwise) until it seats, counting the number of turns it takes. Use gentle force as this screw is very thin and has O-ring seals on it.
Now turn the screw OUT (counter-clockwise) the number of turns in the table below.
- Stock exhaust, no mods… 2 and ¼ turns (What was supposed to be the factory setting)
- Stage I - Punched exhaust baffles, no new jets though … 2 and ½ turns
- Stage I - Punched exhaust or aftermarket pipes with baffles and Stage 1 Jets … 3 Turns
- Stage III - Aftermarket exhaust and Stage III jets … 3 and ¼ turns
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Step 3:
Repeat the above procedure with the right carb (forward cylinder). Use the same settings as above. Some people choose to make the rear carb slightly more rich as it runs hotter, so if you want, make the back ¼ turn richer then the front (the back is the left carb, also known as the BASE carb)
Now reinstall the air filter housing and take the bike for a test run. Make sure to do lots of fast decelerations while downshifting to see if the exhaust pops on you. An occasional pop is not uncommon, especially when the bike is cold or the weather is cool and damp. If, however, you are still getting lots of the misfires, then go back and readjust each screw ¼ further out.
Caution… do not go more then 4 full turns out as the screw may come loose at about 4 and ½ turns. If you are unsure of where you left off, then reseat the screws and count out again to get the right setting.
For bikes with extreme set ups, such as open pipes and modified air intake systems, you MAY have to go one size up in the pilot jets to get rid of the lean idle condition. This is rare though and normally, you can adjust the Pilot Screws to get rid of those pops. If you have a normal Stage I or III and you can’t seem to get rid of them, then look for another cause. Exhaust leaks are a common one. Also, over time, the jets in a bike’s carbs can get clogged up. When I last did my Stage III jetting, I found my pilot jets almost half the diameter they were supposed to be so I cleaned them with some fine wire and the bike idles much better afterwards.
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Final Step:
Once you have the screws set to where you want them, I would suggest sealing the screw holes with a dab of silicone rubber. This will keep them weatherproof and act as a thread locker. If you need to get back to them for another adjustment, just pry the silicone off with a small screwdriver.
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