Volvo Penta 260a carbureted backfire under load (popping)

fish_guts

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Nov 17, 2008
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Hi all,

Seeing if anyone has experience with Penta 5.7 carbureted engines here. I'm having an issue of there being popping noise like a backfire and loss of power when accelerating sometimes under load only.

Idles perfectly and I can rev it up in neutral with no issues. Also after its been running for a long time it does it less and less. I can be going about 30mph and its running fine with only occasional popping once its been running for a few hours,

When I first start it up it takes a minute to idle without shutting off (have to keep it revved up) and then it will make the popping noise and loss of power every time it pops until has been running for a while (hour or more) Under load only.

The carb was recently rebuilt when I bought it and looks brand new inside. I filled it with 91 octane and fuel stabilizer also just in case every time I go out.

Anyone have any ideas or seen this problem before.

22.5 center console with I/O penta 260a 5.7 and Penta 280 outdrive.

FIrst 2 times I took it out it had no issues and then this started happening. (just bought it a few years ago)
 
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Jun 5, 2006
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Clyde
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252 GW Sailfish “Rosie”
Have you done a basic tune up yet? Spark plugs/wires, distributor cap/rotor and fuel filter?

Engines pop out of the intake because the spark is coming at the wrong time and the intake valve is not closed completely. This could be from bad wires or a distributor cap. As the engine warms up it is easier to deliver spark to the proper location. Electricity (including spark plug wires) follows the least path of resistance. Obviously there are other things that could cause an engine to pop out the intake. Also check to make your distributor is tight and that the timing hasn't changed.
 
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Bigeasy

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Jun 18, 2008
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Eric W.
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what he said! Sounds like a timing issue or spark plug wires or broken distributor cap!
 
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fish_guts

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Have you done a basic tune up yet? Spark plugs/wires, distributor cap/rotor and fuel filter?

Engines pop out of the intake because the spark is coming at the wrong time and the intake valve is not closed completely. This could be from bad wires or a distributor cap. As the engine warms up it is easier to deliver spark to the proper location. Electricity (including spark plug wires) follows the least path of resistance. Obviously there are other things that could cause an engine to pop out the intake. Also check to make your distributor is tight and that the timing hasn't changed.

Replaced fuel filter already, wires and cap were new when I got it but I'll check them again. Thanks for the response great tips. Ill have to check the timing also perhaps it did move.
 
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Weazel

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Jun 3, 2004
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skipjack
Also check to make your distributor is tight and that the timing hasn't changed.
This.... I had a similar problem years ago and this was the fix. I'm not entirely sure of your setup but I have to wire the distributor to lock out the electronic advance and then check the timing.
 
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fish_guts

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This.... I had a similar problem years ago and this was the fix. I'm not entirely sure of your setup but I have to wire the distributor to lock out the electronic advance and then check the timing.

Great! Im hoping its this ! Ill check it this weekend thanks everyone hopefully I can get this boat running great again.
 
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fish_guts

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Your risers are leaking water is going into the intake better get fixed asap mine did the same needed new heads and risers expensive

Actually one side of the risers is dripping from the freeze plug. I guess its time to change them. I have new ones just haven't got around to doing it yet
 
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fish_guts

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Update on this issue. I finally got a few minutes to be able to diagnose the boat. Went to do a compression check and found that one plug wire had come off!!! boat was running on 7 cylinders!!! plugged it back in and just tested it out on the water yesterday, problem gone!!!! but got 2 more problems when it was out on sunday!

Bait pump died and killed all the bait. and auto float bilge pump wasn't working and filled up the bilge enough to splash water on the distributor and shut down the engine. (used my backup bilge pump and emptied it then it was fine.

So got the main issue fixed now I just need to replace the bait pump and the Bilge float pump (or at least diagnose it. and replace the rusted out manfolds and risers and attempt to change the oil. the filter looks like its impossible to get to but theres some way to get to it. the previous owner changed it before I bought it so there has to be some way to get it done!!

Main thing is that the engine ran fine. power is back and hit cruising speed of 35mph at 4600 rpm. before I could barely get it to 25. (glad it didn't ruin anything else with that plug wire being off!)

Thanks for all the help here also.
 
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fish_guts

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What is causing enough water in the bilge to need the pump?

I figured it was normal. I have an old thru hull transducer maybe its leaking thru. And I get water in the boat from the rear drain holes inside the boat. When the boat isn't moving and people are in the back of the boat. Also have a thru hull for the bait pump. Ill take a look at all of it and see reseal them all.
 
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historic

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Oct 29, 2003
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Glad you got the engine misfire figured out and it was simple. The manifolds are a no brainier, replace them or you will be replacing the engine.

I agree with the others that the water in the bilge should be a huge concern and your first priority to fix. There are numerous potential sources for water to enter the bilge. The ones already discussed plus don't forget about your Ujoint bellows, it it fails, a lot of water can come in quick enough to sink your boat. I have been on the water twice and heard of boats sinking due to leaking bellows. I change my bellows every third year whether it needs it or not, it's cheap insurance. Also it is good to store your drive trimmed down to avoid putting unnecessary stress on the bellows.

I recently replaced a shift cable conduit and exhaust Y pipe that were both leaking into the bilge, not a lot but enough to make my primary bilge pump run a few times per day while on the water. I hope I solved the "mystery" water that was getting in. If all systems and devices are in good shape and working properly, you should not get much water in the bilge during normal operation.

If your deck scuppers have the flapper valve outside on the transom, consider replacing them with ball type scupper valves. It's a quick, cheap and easy fix that will help keep water out of your boat.
 
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fish_guts

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Glad you got the engine misfire figured out and it was simple. The manifolds are a no brainier, replace them or you will be replacing the engine.

I agree with the others that the water in the bilge should be a huge concern and your first priority to fix. There are numerous potential sources for water to enter the bilge. The ones already discussed plus don't forget about your Ujoint bellows, it it fails, a lot of water can come in quick enough to sink your boat. I have been on the water twice and heard of boats sinking due to leaking bellows. I change my bellows every third year whether it needs it or not, it's cheap insurance. Also it is good to store your drive trimmed down to avoid putting unnecessary stress on the bellows.

I recently replaced a shift cable conduit and exhaust Y pipe that were both leaking into the bilge, not a lot but enough to make my primary bilge pump run a few times per day while on the water. I hope I solved the "mystery" water that was getting in. If all systems and devices are in good shape and working properly, you should not get much water in the bilge during normal operation.

If your deck scuppers have the flapper valve outside on the transom, consider replacing them with ball type scupper valves. It's a quick, cheap and easy fix that will help keep water out of your boat.

I already have the new manifolds so just need to replace them now. The bellows are new but I'll check them anyway to see if maybe its getting in that way.

The scuppers on mine are a pain. A small hole which gets clogged with anything small. I usually have to shove a hangar in there to get,them to drain when I wash the boat.

Thanks for the advice I'll definitely check all that out.

Oh and I store the drive up but with with a ratcheting tie down holding it up to take stress off it.
 
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historic

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Oct 29, 2003
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Sounds like you are on the right track. If the bellows are new, they should be fine. You can check to see if any water is coming in through the transom when on the water to make sure they are not leaking. I would store the drive down if possible, having it up really stretches the bellows into an unnatural shape, down compresses them and does not put a load on them at all.

My scuppers are a pain too, I use my shop vac to get any debris out before each trip. If you can make the ball valves work on yours you will have a drier boat. I used ones similar to the ones in this article http://www.boatus.com/magazine/trailering/2013/October/how-to-install-a-one-way-scupper-valve.asp

Good luck!
 
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fish_guts

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Dominic
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Sounds like you are on the right track. If the bellows are new, they should be fine. You can check to see if any water is coming in through the transom when on the water to make sure they are not leaking. I would store the drive down if possible, having it up really stretches the bellows into an unnatural shape, down compresses them and does not put a load on them at all.

My scuppers are a pain too, I use my shop vac to get any debris out before each trip. If you can make the ball valves work on yours you will have a drier boat. I used ones similar to the ones in this article http://www.boatus.com/magazine/trailering/2013/October/how-to-install-a-one-way-scupper-valve.asp

Good luck!

how do they do with draining out when washing the deck? do the ball valves get clogged easily?

Thanks for the link!
 
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historic

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Oct 29, 2003
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Jack
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2459 Trophy "Driftwood"
They will pass some pretty big leaves and sticks when I'm washing the boat at home. I usually clean them out from inside the boat with my shop vac first. Out on the water I don't have any issues washing blood and small chunks through them. They are always moving around in the water and stay pretty clean on thier own. Definetly one of the best bang for the buck upgrades I have done.
 
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fish_guts

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Nov 17, 2008
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Dominic
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22.5 foot searay center console SOLD
They will pass some pretty big leaves and sticks when I'm washing the boat at home. I usually clean them out from inside the boat with my shop vac first. Out on the water I don't have any issues washing blood and small chunks through them. They are always moving around in the water and stay pretty clean on thier own. Definetly one of the best bang for the buck upgrades I have done.

I'll definitely have to get some of those then. my current ones can barely pass water through them!
 
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