350 Mercruiser Backfiring Through Carbi

[Struggling Accountant]

Hi All,

I'm having a problem with my 1999 Skicraft. The engine is the 220hp 350 Mercruiser (220 is off the top of my head so I hope I remembered correctly).

I have been through a billion threads to try find this problem but I've been unsuccessful so I'm hoping someone here can help me.

When in gear (engine under load) around 3000 RPM the engine will seemingly starve of fuel, then when speed and revs are lost the engine most of the time will backfire through the carbi or die (suggesting fuel has been pumped back into the carbi just prior to the backfire). There are no problems when in nuetral, we can rev it to 4500RPM no worries. I have replaced the fuel filter and pump, and taken out the filter in the hose entering the carbi, and I have driven with the cap off the fuel tank to ensure there is no vaccuum created. We also loosened the hose going to the carbi at revs just before the back fire and fuel was definitely being delivered to the carbi still.

We have also checked that everything is firing correctly by taking a lead off the distributor and watching it spark. I recall when i had dodgy points in our old boat that it would backfire through the carby without dying first. This is why I am almost certain it is not spark.

We also had a compression test done when we had our last service... Probably 20 hours ago.

There are no cracks/leaks in the carbi, and we cleaned it out and pulled it apart before doing most of the tests mentioned above.

My mechanic is going to check out the needle and seat tomorrow. As I'm not a mechanic is this to do with the power valve? Or could the power valve be playing up. I am hoping I do not end up buying a new carbi and having the same problem.

Any thoughts or suggestions would be GREATLY appreciated?

Cheers

 

[SpinnerBait_Nut]

Re: 350 Mercruiser Backfiring Through Carbi

Welcome to iboats.
Let's get this to the Mercruiser, Volvo, and OMC I/O and Inboard Engine Problems Forum.

 

[JustJason]

Re: 350 Mercruiser Backfiring Through Carbi

are you austrailian?

 

[Struggling Accountant]

Re: 350 Mercruiser Backfiring Through Carbi

are you austrailian?

Hi.

Yes I am Australian. Do I type with an accent or are Skicrafts an Australian make only?

Sorry if I have posted this in the wrong spot. I was unaware until today that forums like this even existed... I will catch up one day...

 

[SpinnerBait_Nut]

Re: 350 Mercruiser Backfiring Through Carbi

Hi.

Yes I am Australian. Do I type with an accent or are Skicrafts an Australian make only?

Sorry if I have posted this in the wrong spot. I was unaware until today that forums like this even existed... I will catch up one day...

carbi may have been a hint.
J/K.

 

[ziggy]

Re: 350 Mercruiser Backfiring Through Carbi

ya could try running off a remote fuel tank. if the problem still exists, ya could safely assume it's not before the in side of the fuel pump.....

or heres a real good artical on how to test yer fuel system that dons wrote...

http://forums.iboats.com/showthread.php?t=167561

 

[Haut Medoc]

Re: 350 Mercruiser Backfiring Through Carbi

Yup, backfire through the carbi is usually related to a lean fuel condition.....
My guess here is either a clogged pickup in the tank, clogged tank vent or anti siphon valve, since it starves under load..........
If the remote tank works, that is where to look......
Also, have you changed the water separating fuel filter?.......
It might be wise to drain the tank to remove as much debris as possible...
Welcome to iboat, btw.....​

 

[bjcsc]

Re: 350 Mercruiser Backfiring Through Carbi

carbi may have been a hint.
J/K.

Oh yeah, throw another shrimp on the carbi...
or was it...ahh nevermind...

The fact that your boating means you're somewhere where it's summer!

A blown power valve usually results in an off-idle hesitation when throttling up...

 

[MikDee]

Re: 350 Mercruiser Backfiring Through Carbi

Yes, it could very well be the power valve, if you have a Quadrajet 4bbl (I assume) just behind the middle of the primary's is the power valve, it is a hollow brass piston with a spring under it, and it holds these 2 pointy metering rods in suspension, that go down into the primary main jets to meter the mixture above idle. The piston itself, sometimes get clogged up with carbon, or gunk, that prevents it from moving smoothly (sliding up, and down) in it's bore, while riding on the spring, creating all kinds of havoc! specifically symptoms as you describe, If your mechanically inclined you can take off the top of the carb, under the gasket in the front middle of the carb, there is a small plastic ring or keeper that holds this valve in place, just pop it out gently, and pull out the valve with the metering rods attached, and clean it, and it's bore out real good with carb cleaner, as well as cleaning the jet needles, then replace it, gently push the retainer ring back down, and while holding the retainer ring down, work the valve up, & down, to make sure it moves freely, once satisfied put the top back on the carb (the carb top holds this retainer ring down, once in place) and you're all set.

 

[Struggling Accountant]

Re: 350 Mercruiser Backfiring Through Carbi

Thanks for your help guys I really appreciate it... Hence my frustrated username

I will try running an alternative fuel tank to see if it something from before the fuel pump. Unfortunately the water fuel separator most likely isn't the issue as I replaced it two months ago.

I also have the carbi off so I will follow your instructions MikDee to check the power valve. I will also read that article when I am in struggletown this afternoon at work so thanks Ziggy.

I will let you guys know how it goes this afternoon.

And that was gold re carbi... Hahahaha.

Cheers

 

[billbayliner]

Re: 350 Mercruiser Backfiring Through Carbi

Unfortunately the water fuel separator most likely isn't the issue as I replaced it two months ago

Do not discount the fact that the fuel seperator can be a suspect. I had a new seperator do it's intended job and become clogged after a couple weeks. I at first though the same thing as it was fairly new.

Very easy to bypass the filter to absolutely rule the water fuel seperator from the long list of possibilities.

 

[John_S]

Re: 350 Mercruiser Backfiring Through Carbi

Being this is a 1999 Mercruiser, it is probably a 2brl Mercarb.

 

[Don S]

Re: 350 Mercruiser Backfiring Through Carbi

This thread should help narrow down the problem.

http://forums.iboats.com/showthread.php?t=167561

 

[FreeBeeTony]

Re: 350 Mercruiser Backfiring Through Carbi

Could it simply be water in the fuel?
Maybe try some dry gas............

 

[Flukinicehole]

Re: 350 Mercruiser Backfiring Through Carbi

Could also be valve float have your mechanic check your valve seat pressure. This is cheap and easy.

 

[Struggling Accountant]

Re: 350 Mercruiser Backfiring Through Carbi

Thanks for all the help guys.

I took the boat for a run this morning after replacing the water fuel separator, draining the tank (which turned out to have some water in it... Grrr), and pulling the carbi right down and cleaning it. Ran a lot better up top... I was able to rev it right out without any problems, however when I put the foot right into it to take off it laboured at first, and then would take off (occasional backfire as well). I went back to my mechanic who adjusted the air fuel mix whilst runnning, so hopefully tomorrow when I take it out to test it again under load it will be better

I will keep you guys posted.

Thanks heaps for all your help so far you guys have been a great help

 

[Struggling Accountant]

Re: 350 Mercruiser Backfiring Through Carbi

Thanks for all the help guys.

I took the boat for a run this morning after replacing the water fuel separator, draining the tank (which turned out to have some water in it... Grrr), and pulling the carbi right down and cleaning it. Ran a lot better up top... I was able to rev it right out without any problems, however when I put the foot right into it to take off it laboured at first, and then would take off (occasional backfire as well). I went back to my mechanic who adjusted the air fuel mix whilst runnning, so hopefully tomorrow when I take it out to test it again under load it will be better

I will keep you guys posted.

Thanks heaps for all your help so far you guys have been a great help

Also I just noticed I said carbi again... Haha. I think I saw it referred to as carb???

Cheers.

 

[oops!]

Re: 350 Mercruiser Backfiring Through Carbi

hey accountant.....

welcome to i boats.......

achris lives on that continent...

any hoo....

the guys that have answered you really know their stuff.....
read it twice.....also...read the link don's gave you...

if you do you wont be struggeling any more..

cheers
oops

 

[Struggling Accountant]

Re: 350 Mercruiser Backfiring Through Carbi

Hey guys,

Just letting you know that after that final tune my boat ran sweet!

I spent all day out yesterday (good weather too) and there was not one hiccup.

Appears it was a combo of bad fuel plus blocked valves.

Thanks guys for all your help. I will be sure to keep checking out these threads to hopefully be able to return the favour for someone one day

Cheers.