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Old 05-30-2016, 04:56 PM   #1
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Unhappy 1997 mercruiser 4.3LH backfire/other issues

Hello all... I have been lurking on this board for a wile now and now its time for some help...
I have a 1997 1950 ME(sr1900) w/4.3LH and I bought the boat about a year ago and I love it but now I have some problems with it... when I got it I did plugs, wires, dist cap and rotor as well as water pump and engine / drive oil changes just to have a good starting point... and the 1st time out it was a little hard to start IMO and it wanted to diesel(it has all fresh gas in it) when shutting it off. well I chalked the hard starting up to being a carb engine and my other boat has a 350 MAG MPI in it and starts with a flick of the key... and for the dieseling after a little poking around found that these motors tend to do that when you don't let them idle for a wile to cool down after running.
so the next time out after running it I would let it idle for a minute or two and that did help a lot but did not completely stop it... so I check the timing and it looks like some one at one point tried to set it with out locking it into base mode, so I set the timing properly to 10 deg BTDC and take it out again and it was running good for a little wile then it started to back fire threw the carb when trying to make power... so now before I do anything more I would like some input from the people on here if I had done something wrong or what I should do next... BTW it does start way easier and rungs good up until about 2000 RPM +/- then backfires.... any help from will be great!!
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Old 05-30-2016, 09:05 PM   #2
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Welcome aboard

I am assuming you removed the jumper after setting the base timing.

When running on the water use your timing light to verify the ignition is advancing correctly past base timing.

Check the fuel for signs of water, pour filter contents into a glass jar and let it settle. Water will go to the top if any.
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Old 05-30-2016, 10:09 PM   #3
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Welcome aboard

I am assuming you removed the jumper after setting the base timing.

When running on the water use your timing light to verify the ignition is advancing correctly past base timing.

Check the fuel for signs of water, pour filter contents into a glass jar and let it settle. Water will go to the top if any.
thanks for the response... and yes I did remove the jumper... I'm not able to check the advance wile running as I have been alone... however I did check the timing after I removed the jumper and it was not at 10 degrees like when the jumper was connected... and it has new fresh fuel and a new filter/water separator installed (BTW water sinks to the bottom in gas) just to eliminate that possibility... might you have some other ideas? I'm open to anything rite now as I'm shooting blanks now.
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Old 05-31-2016, 02:30 AM   #4
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Of course water settles to the bottom it has a higher specific gravity.

Regarding the 4.3's dieseling I have heard a few folks install a AC soleniod set to engage when running and then disengage it to drop the rpms before shutting the engine down.

Typically backfiring is caused by timing or restricted exhaust. Maybe a stuck flapper that does not open and causing too much back pressure. How many hours are on the engine, exhaust?
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Old 05-31-2016, 03:59 PM   #5
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Of course water settles to the bottom it has a higher specific gravity.

Regarding the 4.3's dieseling I have heard a few folks install a AC soleniod set to engage when running and then disengage it to drop the rpms before shutting the engine down.

Typically backfiring is caused by timing or restricted exhaust. Maybe a stuck flapper that does not open and causing too much back pressure. How many hours are on the engine, exhaust?
thanks again for the reply... I have heard about the solenoid but don't know where or what part # I'm looking for... I was thinking that might be an option if I can find one... as far as backfiring I'm thinking it might have something to do with timing some how because it did not do it before I set the timing... but another thought was that it might be the carb... maybe whoever set the timing last time also set the mixture after the timing(that was incorrect) or possibly float level?? I think for trouble shooting purposes I will set the timing back about to where it was before I set it and see if the backfiring goes away... any other thoughts??

Thanks again!!
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Old 05-31-2016, 08:41 PM   #6
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The soleniod folks use is from the cars in the 70's, it was used to increase the idle when the A/C was turned on.
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Old 06-01-2016, 02:35 PM   #7
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The soleniod folks use is from the cars in the 70's, it was used to increase the idle when the A/C was turned on.
ahhhh... so any other ideas out there from anyone?

also in a side note that I forgot to add before I did run a bottle of seafoam threw it at a high dosage (1 bottle for 8 gallons of fuel) and it seems like it started backfiring about then too... but the other thing that I stated above where done about the same time as well so I thought I might add that info
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Old 06-19-2016, 05:11 AM   #8
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Default my maxum (2006) backfilres also ---help

did you fix your backfire on your Maxum?

my 2006 maxum backfires also, usually after about 30 min of running up and down the lake towing the kids.

I read the owners manual and it identifies this backfire/stalling issue (which is exactly what mine does).. .they say to replace or clean the filter on top of the carb (ram fire or something)?? I took it off today and its pretty oily, so maybe that's my issue...

wondering if you had a similar fix?

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Old 06-20-2016, 03:32 PM   #9
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I don't know if they used a vortech style distributor cap on that particular year, but if it is that would be my first thing to try. the vortech cap replacements have been known to have issues, even when almost new.
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Old 12-12-2016, 04:48 PM   #10
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UPDATE: got it all fixed... it was my error did not screw the cap ALL the way down... and after the new floscan install I have now confirmed my fuel burn #'s!! at low cruse of 23-25 MPH 4.6 MPG and normal cruse of 32-35 MPH 5.0 MPG and at WOT (54 ish MPH) 3.25 MPG... gotta love that!!!!

here is a video of it screaming along

https://youtube.com/embed/g8ncFoCen8Q?rel=0
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