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post #1 of 4 (permalink) Old 08-21-2019, 08:57 AM Thread Starter
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Runs Rich with Air filter

Hello all. I recently picked up a old Generac Model 1647-0 Power Boss 8000 watt generator on Let Go. I needed another hefty genset and couldn't pass it up for $40. It had been sitting outside(for years) with no wheel kit, missing air cleaner, recoil starter etc. It was mess but I just wanted to tinker with that Twin cylinder 18 HP Briggs. Cleaned it all up and pumped the water out of it. Yes, cylinders were full. Oil looked OK but changed it and the filter anyway. Cleaned the carb, intake and head up and re installed the exhaust valve pushrod on the cyl 2. It just came off the lifter and was sitting in the engine. Adjusted the valves and cleaned up the tank and fired her up. She had a miss in both cylinders so I changed out the plugs and mags. This was with no air cleaner mind you. I loaded it up with a couple space heaters and flood lights for about a 1/4 tank and she ran strong. Now, here is the kicker. As soon as I installed the air cleaner and filter she loaded up and started missing fierce. No smoke but you could smell it. I had previously disassembled the carb and cleaned everything I knew, including removing the main jet and float/needle. I did not remove the mid range jet/nozzle. I didn't have a screwdriver that size and didn't want to damage the body.


My question is, is it possible the Mid range jet/nozzle is clogged and the engine is pulling fuel through the HS jet instead when I install the air filter and in essence, choking the engine with HS fuel metering at midrange speed? This has been a blast and that Briggs is smooth, so far. Just can't run it without the air filter. Thanks for any insight.
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post #2 of 4 (permalink) Old 08-21-2019, 04:25 PM
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It's surprising the rings werent rusted to the cylinder walls. That happens often when a motor has sat in a moist environment for a long time. Every time the metal cools, it becomes a moisture magnet as soon as the air warms..


You need to clean the carbs properly. Some engines are airfilter dependent to run properly. Some Honda Shadow650cc motorcycles for example. If you remove the air filter it won't run properly.
So re do the carbs and definitely clean ALL the jets and ALL the passages.


Only then can you expect it to run properly. (provided there is no corrosion damage inside the carb passages)


Is what you asked "possible"? Sure, it's possible. The vacuum is there regardless. That vacuum will pull on the fuel through whatever passage it can.


Also, be sure the air filter is not restrictive. Don't forget to clean the small transfer ports located on the inside wall of the venturi.

To clarify - I'm not a Predator, but one of my Generators IS
HarborFreight Predator 3500 Inverter
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post #3 of 4 (permalink) Old 08-21-2019, 08:41 PM Thread Starter
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Thanks. When I went to look at it the first thing I did was turn the crank by hand. When it moved, I said I'll take it. I Pulled that other jet tonight. It had a little crud at the tip. I also pulled the idle mixture screw. That passage was clear all the way to the throat. Now the idle mixture screw actually effects something.

The air cleaner plate and cover came with a used filter. It may be too dirty. Has a grey tinge to it. New one is in the way.
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post #4 of 4 (permalink) Old 08-23-2019, 01:49 PM Thread Starter
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New air filter arrived and that was the problem. Runs smooth now. Just waiting on the pull starter and she is fully resurrected.
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