www.iFourWinns.com :: View topic - Loss of power after prolonged period of WOT... and backfire?

If you have a ‘98 I doubt you have a Quadrajet most likely a Holley 4bbl (model 4160) which was used on these engines after QJet production ended in ‘90 or ‘91...
I would:
Dump out the contents of the filter in a large mason jar and let it sit; this will tell you if you still have crud on the fuel. Don’t forget most carbs have a small inlet filter that can get clogged too. The valve referred to is the anti siphon valve on the gas tank. Follow the fuel line from the pump back to the tank and it’s the barbed fittings the line is attached to. These are a spring loaded valve that prevents fuel form siphoninn into the bilge if that fuel line ruptures. Inside is a spring and a check ball; they can get corroded and restrict fuel flow. I had to replace this on my boat when we first got it in 2001. The engine would start, run fine in neutral but would not plane out the boat.
If it’s not that I’d check the fuel pump pressure and output. The carb could be at the point where it needs a cleaning & rebuild. I seem to average 6 seasons between rebuilds on my QJet and the fuel is clean every time I dump out the fuel filter (change yearly).

I pulled the check valve off the tank line tonight. It doesn't seem obviously gummed up.. and if I press the ball bearing in with a screw driver I can blow through it fairly easily. However if I just try to blow through it it takes quite a bit of pressure to get air through it, and it kind of vibrates like a reed might... indication of it being bad?

Regardless i'm going to grab a new one tomorrow as well as a fuel filter, cross my fingers and head to the lake Sunday. Wish me luck!

https://www.holley.com/products/fuel_systems/carburetor_components/rebuild_kits/marine_renew_kit/parts/703-29

see the third set of instructions....they show a filter screen part # 33, but not sure if this is actually installed on the marine units because they usually have the large water separating filter installed on the engine. However, my Rochester Q-Jet for sure has a screen in the inlet. As a practical matter the screens the 2 times I've changed them have always been spotless but that's probably because the large water separating filter takes care of anything before the gas gets to the carb.

LouC wrote:

I will definitely check for that screen in the inlet fitting, thanks!

Author:	cdig [ Sun Jul 07, 2019 11:31 pm ]
Post subject:	Loss of power after prolonged period of WOT... and backfire?
Hey folks, after getting my boat back in the water last summer I took it to the brother-in-law's seasonal on Lake of the Woods in Ontario... fun was had by all until she developed a bogging, slow loss of power after prolonged periods of wide open throttle. After slowing down almost completely, as I was playing with the throttle it would backfire several times. After checking everything over we headed to the local marine shop and picked up a new fuel filter, and back on the water the next day. The problem seemed to be resolved after this! Unfortunately that was the last weekend we had the boat in the water last year. Flash forward to today, first time on the water this year and the same problem occurs.. several minutes at WOT, and it starts bogging down, followed by 'bang-bang-bang'... backfire. It never dies on me, and so long as I decrease throttle and let it idle for a minute I can continue boating, so long as I don't go full on with the throttle. Is my carb starving for fuel? (4.3L w/4bbl quadrajet) maybe the float is set too low, so it runs out of fuel after running at high speed for prolonged periods? I can't see it being fuel filter again.. unless they go bad after being stored for the winter? (I use stabil fuel treatment for storing over winter) Or is it possible the fuel pump is weak and can't keep up with fuel demands? If so, how would I test that? In-line fuel pressure guage? I've thought about pulling the carb and doing a rebuild on it... but if there's a simpler place to start I'd like to hear it! Thanks in advance, much appreciated for any advice and potential solutions. C.

Author:	babbot1 [ Mon Jul 08, 2019 7:28 am ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
I would try the simple things first. Replace the filter again. You may have gotten a bad batch of gas and need to get through all the "junk" in the tank. Maybe a good amount of water in there and it overwhelmed filter again. After you replace the filter empty it into a glass container to see what's in it.

Author:	cdig [ Mon Jul 08, 2019 10:45 am ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
Thanks, I'll definitely try a new filter. The most frustrating part about this is that we've had ongoing issues with the boat since getting it, it's hard to know where one problem ends and another begins... And we don't get enough time on the water each summer to be able to test the fixes. I feel like I need to take a week off work, grab my tool box and camp out at the lake until I've got it figured out once and for all!

Author:	captkevin [ Mon Jul 08, 2019 10:48 am ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
Might also try opening the gas cap or loosening it next time it starts bogging down. Sometimes there are issues with the vent lines that can cause problems.

Author:	cdig [ Mon Jul 08, 2019 10:50 am ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
Something else I just read, is there a check valve on the fuel system of these boats that might be clogged and starving the fuel pump? If so, where would it be located?

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Author:	LouC [ Mon Jul 08, 2019 11:14 am ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
If you have a ‘98 I doubt you have a Quadrajet most likely a Holley 4bbl (model 4160) which was used on these engines after QJet production ended in ‘90 or ‘91... I would: Dump out the contents of the filter in a large mason jar and let it sit; this will tell you if you still have crud on the fuel. Don’t forget most carbs have a small inlet filter that can get clogged too. The valve referred to is the anti siphon valve on the gas tank. Follow the fuel line from the pump back to the tank and it’s the barbed fittings the line is attached to. These are a spring loaded valve that prevents fuel form siphoninn into the bilge if that fuel line ruptures. Inside is a spring and a check ball; they can get corroded and restrict fuel flow. I had to replace this on my boat when we first got it in 2001. The engine would start, run fine in neutral but would not plane out the boat. If it’s not that I’d check the fuel pump pressure and output. The carb could be at the point where it needs a cleaning & rebuild. I seem to average 6 seasons between rebuilds on my QJet and the fuel is clean every time I dump out the fuel filter (change yearly).

Author:	cdig [ Fri Jul 12, 2019 10:40 pm ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
I pulled the check valve off the tank line tonight. It doesn't seem obviously gummed up.. and if I press the ball bearing in with a screw driver I can blow through it fairly easily. However if I just try to blow through it it takes quite a bit of pressure to get air through it, and it kind of vibrates like a reed might... indication of it being bad? Regardless i'm going to grab a new one tomorrow as well as a fuel filter, cross my fingers and head to the lake Sunday. Wish me luck!

Author:	cdig [ Fri Jul 12, 2019 11:17 pm ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
LouC wrote: If you have a ‘98 I doubt you have a Quadrajet most likely a Holley 4bbl (model 4160) which was used on these engines after QJet production ended in ‘90 or ‘91... I would: Dump out the contents of the filter in a large mason jar and let it sit; this will tell you if you still have crud on the fuel. Don’t forget most carbs have a small inlet filter that can get clogged too. The valve referred to is the anti siphon valve on the gas tank. Follow the fuel line from the pump back to the tank and it’s the barbed fittings the line is attached to. These are a spring loaded valve that prevents fuel form siphoninn into the bilge if that fuel line ruptures. Inside is a spring and a check ball; they can get corroded and restrict fuel flow. I had to replace this on my boat when we first got it in 2001. The engine would start, run fine in neutral but would not plane out the boat. If it’s not that I’d check the fuel pump pressure and output. The carb could be at the point where it needs a cleaning & rebuild. I seem to average 6 seasons between rebuilds on my QJet and the fuel is clean every time I dump out the fuel filter (change yearly). You are correct sir, it's a Holley not a Qjet. Do you happen to know if these carbs have an internal fuel filter like the Qjets do? The fitting at the fuel inlet line seems too small to have a filter in it...

Author:	LouC [ Sun Jul 14, 2019 12:22 pm ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
https://www.holley.com/products/fuel_sy ... rts/703-29 see the third set of instructions....they show a filter screen part # 33, but not sure if this is actually installed on the marine units because they usually have the large water separating filter installed on the engine. However, my Rochester Q-Jet for sure has a screen in the inlet. As a practical matter the screens the 2 times I've changed them have always been spotless but that's probably because the large water separating filter takes care of anything before the gas gets to the carb.

Author:	LouC [ Sun Jul 14, 2019 12:27 pm ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
cdig wrote: I pulled the check valve off the tank line tonight. It doesn't seem obviously gummed up.. and if I press the ball bearing in with a screw driver I can blow through it fairly easily. However if I just try to blow through it it takes quite a bit of pressure to get air through it, and it kind of vibrates like a reed might... indication of it being bad? Regardless i'm going to grab a new one tomorrow as well as a fuel filter, cross my fingers and head to the lake Sunday. Wish me luck! the real test is can the fuel pump suck gas through it...in the old days we used to siphon gas via sucking on a fuel line but that's a hazard to your health so I'm not going to tell you to do that...lol...if you want to test it by blowing thru it you'd have to blow on the end that was screwed into the tank fitting, not the fuel line end.... https://www.dropbox.com/s/qrlzm4eqsfg08 ... e.png?dl=0 take a look at this...this makes it clear how it works....

Author:	cdig [ Sun Jul 14, 2019 12:32 pm ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
LouC wrote: https://www.holley.com/products/fuel_systems/carburetor_components/rebuild_kits/marine_renew_kit/parts/703-29 see the third set of instructions....they show a filter screen part # 33, but not sure if this is actually installed on the marine units because they usually have the large water separating filter installed on the engine. However, my Rochester Q-Jet for sure has a screen in the inlet. As a practical matter the screens the 2 times I've changed them have always been spotless but that's probably because the large water separating filter takes care of anything before the gas gets to the carb. I will definitely check for that screen in the inlet fitting, thanks!

Author:	LouC [ Sun Jul 14, 2019 3:17 pm ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
cdig wrote: LouC wrote: https://www.holley.com/products/fuel_systems/carburetor_components/rebuild_kits/marine_renew_kit/parts/703-29 see the third set of instructions....they show a filter screen part # 33, but not sure if this is actually installed on the marine units because they usually have the large water separating filter installed on the engine. However, my Rochester Q-Jet for sure has a screen in the inlet. As a practical matter the screens the 2 times I've changed them have always been spotless but that's probably because the large water separating filter takes care of anything before the gas gets to the carb. I will definitely check for that screen in the inlet fitting, thanks! look back at my previous post, I put up a pic of the anti siphon valve in my dropbox….

Author:	cdig [ Sun Jul 14, 2019 10:38 pm ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
So limited success today on the lake. I changed the anti-siphon valve and fuel filter prior to leaving home. I dumped out the old fuel filter into a jar, looks clean - no crud or sediment, however it is fairly yellow. Given that I filled the boat last weekend for the first time this year, I'd say it's a 50/50 mix of fresh and last year's gas.. I do run Stabil fuel conditioner in it at the end of every year, so I'm hoping last year's gas is still ok? Out on the lake, after several minutes at WOT (~4000 RPM) she again bogs down, losing power and slowing until I let off the throttle to avoid backfire again... after a minute of running at a lesser speed, almost idling, I can jam on the throttle again and away we go. I then tried loosening the gas cap (thanks CaptainKevin) to see if that would allow it to vent, in case the vent tube is clogged... again, bogged down after a few minutes at WOT. So I dug out the wrenches and pulled the fuel inlet line off the carb, and yes indeedy there is a screen in there. However it looks pretty clean (I was able to see through it fairly well when held up to the sun anyway). I pulled it out to test run without it, and this time the boat seemed to run quite a bit longer at WOT... so long in fact that I was about to declare it 'fixed'... and then it bogged down again. The only difference after pulling the screen out was that I didn't have the family in the boat this time, so roughly 450lbs less cargo in the boat... not sure if that was the difference or if pulling the screen was. At any rate we continued boating for the rest of the evening just keeping it at a lower speed. Longer stretches at 4000 RPM caused the issue to return. At this point I think the next step is to pull the carb off and rebuild it, as the marine shop said fuel pumps are around $350. Not sure if that's going to happen this year, but we shall see. I do have a used fuel pump around here somewhere, be dammed if I can find it right now though, and it may be just as bad. I may also try putting a fuel pressure gauge in-line as well and hopefully get back to the lake again soon. LouC - do you happen to know if those carbs are easy to rebuilt? I just did the Qjet in my 68 Chevy, so I've at least had some experience.

Author:	LouC [ Mon Jul 15, 2019 2:45 am ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
I've done both the QJet and a Holley and I found the QJet a bit easier to take apart because the air horn does not get stuck on so much as the float bowls on the Holley. Other than that, I'd say neither one is difficult to rebuild, you just have to make sure to get the right kit and clean all the passages with carb cleaner and use a good air compressor to blow out the passages.

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Author:	babbot1 [ Mon Jul 15, 2019 6:59 am ]
Post subject:	Re: Loss of power after prolonged period of WOT... and backf
Sounds like you are getting close. If you didn't see water or crud in the filter and all the screens are clear then seems like a delivery issue. I had a similar issue but different engines. It was the booster fuel pump. It was getting enough to run the engines from the main pump but not enough for full throttle. The inline fuel flow gauge should help determine that issue.