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502 Backfires at WOT

Old 07-26-2021, 04:06 PM
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Default 502 Backfires at WOT

Hi All,

New to the forum (and boating for that matter). I've got a 502 with an Edelbrock carb and MSD 6M-3L ignition (new) as well as a new distributor. Boat runs good for the most part, but it has a couple of issues:

- It doesn't like starting on the trailer at the boat launch. It will run, but as soon as I put it in gear it dies. Push it off the trailer into the water and its fine
- At WOT it will run fine but every couple of minutes I'll get a single backfire through the carb. No power loss when it happens, but it's loud. I've refrained from running it WOT until I can figure it out. Yesterday I was out and once when I tried to get on plane it backfired 3 or 4 times, power loss and then it cleared and ran fine. Other than that it ran the entire day no problem.

Interestingly enough I just did a large trip through Washington state putting about 400 miles of boating on her without a single backfire, but the lakes I went to were between 1,500 and 2,500 feet. These backfires so far seem to only happen at home which is sea level.

I've adjusted the idle mixture screws to try and combat the first issue, turned them to get the highest idle and then dialed back the idle screw to 800. Didn't seem to fix it.

Could this be an overall mixture issue that's causing that backfire at WOT? Seems strange that I only get that single bang, but it runs fine other than that. I originally thought rev limiter so I dialed up the MSD a few hundred more, didn't seem to fix it.

Thanks in advance

CC
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Old 07-26-2021, 04:28 PM
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If itís the marine 750 , it is calibrated for your typical marine low hp 310-330hp 454.
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Old 07-26-2021, 06:50 PM
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That sounds like a lean "pop" you have going on there. Thus when in higher elevation it disappeared.
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Old 07-26-2021, 09:32 PM
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Originally Posted by SB View Post
If itís the marine 750 , it is calibrated for your typical marine low hp 310-330hp 454.
So find the Edelbrock manual and start setting it for the 502.
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Old 07-26-2021, 09:57 PM
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Quick search 502 Edelbrock 750 found me this:
Edelbrock 750's ? How big of a motor?

Bet there are a few more.

Hope others that know these carbschimr in. I sure donít. Lol.
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Old 07-26-2021, 11:15 PM
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Interesting tips. Thanks guys. I think it is a 750, it could very well be set up for a 454. I'll look into this and report back.

CC
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Old 07-28-2021, 07:39 AM
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Originally Posted by chopper_chris View Post
Hi All,

New to the forum (and boating for that matter). I've got a 502 with an Edelbrock carb and MSD 6M-3L ignition (new) as well as a new distributor. Boat runs good for the most part, but it has a couple of issues:

- It doesn't like starting on the trailer at the boat launch. It will run, but as soon as I put it in gear it dies. Push it off the trailer into the water and its fine
- At WOT it will run fine but every couple of minutes I'll get a single backfire through the carb. No power loss when it happens, but it's loud. I've refrained from running it WOT until I can figure it out. Yesterday I was out and once when I tried to get on plane it backfired 3 or 4 times, power loss and then it cleared and ran fine. Other than that it ran the entire day no problem.

Interestingly enough I just did a large trip through Washington state putting about 400 miles of boating on her without a single backfire, but the lakes I went to were between 1,500 and 2,500 feet. These backfires so far seem to only happen at home which is sea level.

I've adjusted the idle mixture screws to try and combat the first issue, turned them to get the highest idle and then dialed back the idle screw to 800. Didn't seem to fix it.

Could this be an overall mixture issue that's causing that backfire at WOT? Seems strange that I only get that single bang, but it runs fine other than that. I originally thought rev limiter so I dialed up the MSD a few hundred more, didn't seem to fix it.

Thanks in advance

CC
i am no expert,,,and there some pretty sharp people in here,,,,,but..sounds like carb issues...
is compression good??..good place to start,,make sure there no internal issues..
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Old 07-28-2021, 09:26 AM
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Agree carb issue, and my guess is float adjustment
Sitting on the trailer the bow is high and fuel level is angled way back. Once its floating the fuel level is enough to keep it going. At high rpm the motor goes lean and it backfires
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Old 07-28-2021, 09:30 AM
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agree float level, wasn't this addressed in another post ?
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Old 07-28-2021, 09:37 AM
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Yes, he started 2 threads by mistake.

See this one for more info and responses:
502 Backfires at WOT
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