Backfire through carb 5.7 mercruiser

[swampfx914]

Ok guys here it is... I have a 1994 Sea Ray 250 EC with the 5.7 mercruiser. There are less than 300 hrs on the motor. The boat sat unused for a year. When I got back to it there was water in the cylinders and in the carb. I drained the cylinders I ran ATF fluid throught the intake until it was coming out of the cylinders until it was all out of the cylinders. I rebuilt the carb. I have run the boat several times with no problems. I was at the lake and decided the gimble bearing was making to much noise for me so I changed it. The next time out the boat started back firing through the carb. I pressure washed the engine making sure to stay below the carb and away from the distributor. I have changed the wires,plugs,dist cap, rotor and coil. (thunderbolt IV HEI) I have put another carb on it for testing purposes and have the same problem. I have removed the valve covers and looked for bent push rods, broken springs, etc. When I accelerate from idle fast, the engine "loads up" and will not run I can accelerate slowly and it will rev fine. once I get above about 1500-2000 rpm I can rev up and down with no problems. I have checked and rechecked wire routing. I have not checked the timing yet but i plan to tomorrow. This engine has not been run that much so I don't see how timing could be an issue yet. please let me know what you think. I have read every post on here reguarding backfiring and none seem to quite fit my problem. I will thank everyone in advance. I appreciate the help. Brandon

 

[Northern Eclipse]

Re: Backfire through carb 5.7 mercruiser

Swamp, did you ever find out how water intruded the motor?? could be bad valve due to rusted seats,,,But it sounds more like water in the fuel, change your fuel/water separator filter and see what happens.

 

[swampfx914]

Re: Backfire through carb 5.7 mercruiser

Water entred through the carb while sitting. There was water in the carb as well as the cylinders. I have taken fuel samples and checked the fuel/water seperator all is well.. Well almost!

 

[magster65]

Re: Backfire through carb 5.7 mercruiser

The back-firing sounds like a carb problem to me (accelerator pump).

 

[swampfx914]

Re: Backfire through carb 5.7 mercruiser

Hey Magster, I replaced the accelerator pump during the rebuild. I have also tried another carb that I know is good. Still have the same results. let me know what you think from here. Thanks,<br />Brandon

 

[Northern Eclipse]

Re: Backfire through carb 5.7 mercruiser

Swamp did you check ignition advance ??

 

[swampfx914]

Re: Backfire through carb 5.7 mercruiser

Northern,<br />What advances the ignition on the thunderbolt IV? <br />I assume it is electronic and no I have not checked it yet. How does the module inside the distributor cap work and does it ever need to be replaced? That is where I was going next but I wasn't sure how it worked.

 

[magster65]

Re: Backfire through carb 5.7 mercruiser

I'd do what NE suggested. Check your ignition base timing and advance. I think that particular ignition system has a magnetic pickup in the distributor. Yes they wear out but they usually just stop working. On the ignition module there is a number molded in to the plastic casing, that is the total number of degrees of advance.

 

[akriverrat]

Re: Backfire through carb 5.7 mercruiser

http://www.marinemechanic.com/site/page19.html

 

[Northern Eclipse]

Re: Backfire through carb 5.7 mercruiser

Swamp Mag is right they are magnetic pickup, and the cap should be vented, to prevent condensation within the distributer, Someone put the wrong cap on my 5.7 thunderbolt IV and I had some light corrision on the pickup, which caused some high rpm missing, Anyway I would like to tell you how to time it right but I don't have my Merc manual with me, Maybe Magstar can lend his Knowledge..

 

[magster65]

Re: Backfire through carb 5.7 mercruiser

My manual's on my boat but the base timing for my 7.4 is 8 btdc and I have the 24 degree module. I think it said the 350 has the same 8 btdc but with a 22 degree module. Undo 2 of the 3 bolts and rotate the module slightly, you should be able to see the number then. A couple of degrees more/less total advance may not cause any problems but any more than that will.

 

[magster65]

Re: Backfire through carb 5.7 mercruiser

What would cause a backfire?<br />- cross firing in the distributor cap<br />- bad spark plug leads<br />- bad coil wire<br />- too lean of fuel mixture <br />- water in the fuel<br />- incorrect timing<br />- sticky intake valve<br />- overheating causing pre-ignition (longshot)<br />Anyone think of anything else?

 

[swampfx914]

Re: Backfire through carb 5.7 mercruiser

Thanks mag I am hot on the trail. I will let you know what I find today. I am going to start over as I feel I have over looked something simple! I'll let you know.<br />Brandon

 

[swampfx914]

Re: Backfire through carb 5.7 mercruiser

Ok guys, I checked the timing today and if their is nothing special about doing it ie. disconnecting vac. hoses etc. this is what I have.<br />At 900 rpm the timing mark is around 12 deg btc. when i accelerate to 2000 rpm it goes completely off of the scale. I loosened the dist and brought the mark to 8 deg btc and the engine ran rough. even then it would backfire through the carb under rapid acceleration. I am running clean gas straight from a gas can. I am anxiously awaiting your suggestions! Thanks,<br />Brandon<br />

 

[bubbakat]

Re: Backfire through carb 5.7 mercruiser

usually backfiring through carbs pertains to ignition system if backfires through the exhaust pertains to valves or timing. check to make sure the wires are in the right hole one out of place will make it do what you are describing

 

[swampfx914]

Re: Backfire through carb 5.7 mercruiser

Hey Bubba thanks for the tip. Unfortunately I have checked the wires over, and over, and over. meticulously. I have come to the conclusion that they are right. I wish i would look at them and find one wrong I would happily report my embarrassment to the group! Keep the ideas coming please!

 

[bubbakat]

Re: Backfire through carb 5.7 mercruiser

ck for small crack in rotor button or in dist. cap

 

[magster65]

Re: Backfire through carb 5.7 mercruiser

No luck eh? Rather than relying on my numbers, is there a sticker anywhere that tells you what your timing should be? (They're usually on the flame arrester cover or on the rocker cover). It may be the case that your engine does require 12 btdc at idle.<br />When the timing light's on does the timing bounce around or is it steady?<br />Does it backfire continuously or just occasionally?<br />You may be doing a compression test soon.

 

[swampfx914]

Re: Backfire through carb 5.7 mercruiser

Hey Mag, I am going to work on it again today. I will check the distributor for stickers today. the timing mark is steady at idle and advances with acceleration. I think i will check compression today and see what happens. The backfire only comes under heavy acceleration after the carb seems to load up. the carb loads giving you the "Waaaaah" sound and then it backfires and sometimes after the backfire it will catch up and the engine will rev normal. Sometimes when I accelerate hard it will not backfire and the carb will load up enough for the engine to shut off. It does this with 2 different carbs, one which I know is good because it came off of a car that was running fine!

 

[swampfx914]

Re: Backfire through carb 5.7 mercruiser

Hey Bubba, When it started doing it I replaced the cap and rotor and it does the same thing. I have replaced everything! The complete list is in the original post if you can think of anything i missed let me know!