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500efi issues

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500efi issues

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Old 07-21-2019, 06:41 AM
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Default 500efi issues

ok experts, need some advice, 2001 Efi is running poor under load after 2800 rpm. Runs great up to 28 and free revs fine, it will also run fine for only a minute or so at higher rpm once on plane, then it falls flat, looses power and back fires. A complete tune up was just done, including all new distributor parts. Heads/valves/springs etc complete and all fine. Thoughts please!
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Old 07-21-2019, 07:13 AM
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Does it back fire through intake or exhaust? And was this a Existing problem before all the work
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Old 07-21-2019, 07:32 AM
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Originally Posted by Velocity Tom View Post
Does it back fire through intake or exhaust? And was this a Existing problem before all the work
existing issues, backfires thru intake.
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Old 07-21-2019, 07:40 AM
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Did u check fuel pressure.?
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Old 07-21-2019, 08:34 AM
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monitor fuel pressure when running,could have a gas line collapsing.
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Old 07-21-2019, 09:28 AM
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Def do a pressure test first...should be 38-43 psi. We recently had issues and wish we started by checking fuel pressure. Might also have to replicate the issue to see if pressure is still there.
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Old 07-21-2019, 11:38 AM
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fuel pressure test should be done at the start of every year and especially when anything has been done to the engine and or taken off on the fuel side. Scan the engine as well should be done once per year and logged to compare for years and years down the road to see if any of the parameters has changed. Kind of like a blood test every year persay.

And for the record PROPER fuel pressure spec for the HP 500 EFI is a very steady 39 psi at the correct idle rpms fully warmed engine and 41 psi at WOT very steady fuel pressure is needed. That is the factory OEM spec

Next is the injectors whether one thinks so or not - if it was a carb engine how many times would have the carb been clean and rebuilt on a 20 year old engine? Both are fuel delivery items both get get crap in them for a jist.

and proper ignition coil - also ohm out plug wires and look at the differences. Alot of cheap wire sets out in the market
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Last edited by BUP; 07-21-2019 at 12:26 PM.
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Old 07-22-2019, 03:00 PM
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All great advice.

Backfire through the intake occurs from one of the following:

1. Incorrect spark event (ignition wires wrong or crossfire).
2. Extremely lean under load causing fuel to ignite in the intake tract.
3. Intake valve not sealed.

My experience has proven option number 3 to be the most likely culprit. Has a compression and/or leak-down test been done? How long has it been since the springs were done? 500 and 525 EFI engines need periodic spring maintenance. A lifter could also be an issue (had this problem on one of my MPI engines).
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Old 08-07-2019, 09:40 PM
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Unfortunately back at it. So...swapped the fuel pressure regulators (just to rule out), re-checked timing (dead on 8 deg base), fuel pressure readings are as follows...on the hose - 35lbs with ignition on, 39 lbs at idle & 33-39 at 2k (pretty erratic) on the lake it varies 20-41 lbs at 4K (very erratic) then it starves for fuel. Pretty obvious it needs a pump, so which one? Now by swapping the regulators it does seem to run a little better (still not right, but better) and when you give it a quick free rev itíll stall as it comes back down, you all know the 500 efi will stay put at 1500 for a few seconds after a quick free rev. Iíd assume this is the electric pump thatís bad, but just not positive.
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Old 08-07-2019, 10:28 PM
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Originally Posted by BYFS View Post
Unfortunately back at it. So...swapped the fuel pressure regulators (just to rule out), re-checked timing (dead on 8 deg base), fuel pressure readings are as follows...on the hose - 35lbs with ignition on, 39 lbs at idle & 33-39 at 2k (pretty erratic) on the lake it varies 20-41 lbs at 4K (very erratic) then it starves for fuel. Pretty obvious it needs a pump, so which one? Now by swapping the regulators it does seem to run a little better (still not right, but better) and when you give it a quick free rev itíll stall as it comes back down, you all know the 500 efi will stay put at 1500 for a few seconds after a quick free rev. Iíd assume this is the electric pump thatís bad, but just not positive.
merc pump provides 100-110 psi. The fact it drops to 20 at high rpm tells me regulator isnít regulating correctly or your fuel pump canít keep up.
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