Mercruiser 5.7 stumble/bogging issue - Quadrajet

[M.A.CCruiser]

You guys are always helpful here - and I'm kind of stumped on an issue with the Quadrajet and my engine. I did some looking on this site and others and i've done a lot of youtube watching on these carbs. Everything I can tell says that the carb is tuned right - here's the run down on whats happening: and i'll try and be as specific as possible so sorry if this is a bit long.

Leave the marina and slow speed (1000 or so RPM) this lasts for about 10 minutes and the engine is almost to normal operating temp. Then i hit a normal operating zone. Get up on plane and ride for a bit - and after the last bridge I go WOT. Engine takes off - I hear the secondaries open fine, 4800 RPM no issue. Stay at this speed until i get into Tampa bay (5 minutes or so tops) and throttle back to cruising speed, 3000 ~ 3200 RPM. Still no problems. Maintain cruising speed for i would say an hour or so (small slow speed zone). Loop around and get back into the ICW and give it WOT again. The boat takes off, RPMS increase, hit 4100 and then the engine dies. No stumble, pop, smoke or anything. It just falls flat on it's face.

I back off the throttle immediately when i feel/hear this happen. The engine springs back to life and can maintain cruising speed without an issue. I decide to slowly increase the throttle to see what happens. Accelerates fine until about 3800 then it kind "jumps" a bit between there and 4000. Vrroom... vrrroom... VROOM.. vroom.. - like it wants to go but can't. Again, no smoke, popping sounds or anything. This starts to frustrate me. I don't drive at WOT often at all. but knowing that i can't bothers me. So we get into Sarasota Bay and I open the engine hatch, take off the cover and flame arrestor. I tell my girlfriend peg the throttle, but if the engine starts to die, back off the throttle. She does this as i watch the carb. At WOT i see the vacuum diaphragm release. The secondaries begin to open and make it to about 50%, i see fuel spray going in as they begin to open. And then, it's almost as if the fuel spray stops. The secondaries start to flutter, the vacuum diaphragm starts to flutter and the engine stumbles - She pulls back on the throttle, secondaries close back up and the engine cruises. We do this a few more times. A few times i pushed the secondaries open more, no change to engine running. And on a few occasions i hold the secondaries closes longer. No real change either.

My guess at that time is the secondaries are too loose (opening too quick, too much air, dead engine) so I kill the engine and tighten them up. Get the G/F back on the throttle. Peg it from idle to WOT. Engine takes off again, makes it to about 4000 and then same result. I reversed my attempt and loosened up the secondaries. No real change either except more of a stumble when the throttle is pushed to WOT. So I put them back where they were and we continue on the day at nothing more than cruising speed. Not going to ruin our day without the kid just because i can't make the boat go fast!

The part that's stumping me is the fuel flow stopping when the stumble begins, and the vacuum diaphragm for the secondaries fluttering as the engine begins to die. This is not the first time this has happened. But the first time i've really begun investigating. The last time it happened i figured it was a maintenance issue, or bad fuel. So i did cap, rotor, plugs, F/W sep filter, marine Sta-Bil and ran the tank down LOW. Fresh 30 gallons of gas on top of into an almost empty tank, and still have the issue.

The carb was re-built along with a new long-block about 45 hrs ago. The WOT issue started about 5-10 hrs ago - but again; i don't run WOT a lot, so i didn't really notice. I did run some WOT during the last 5 hrs of break in as per suggestions and online reading.

I read about it potentially being the fuel vent line. But, on one attempt towards the end of the day Saturday i removed the fuel cap before trying the WOT and it didn't resolve the issue.


I'm going to be checking the fuel pick-up tube tonight to see if it's clogged up. Thinking maybe the extra vacuum on the pick-up after the tank gets stirred up is causing a blockage and restriction. Coming back on the throttle lets it out and then lets it run normal again? I'm also going to pull the in line fuel filter at the carb to see if there is anything in there. But, considering the recent re-build i would assume that was replaced. Granted, never good to assume. While checking i'm going to give the fuel lines a squeeze to see if they feel flimsy. Maybe the extra vacuum is causing a fuel line to collapse?

Any other suggestions, or testing procedures would be REALLY appreciated. I've put on here before, i'm not a mechanic by any-stretch, but i can follow most instructions if they involve basic tools and not too much electrical work (wiring diagrams give me a headache). Thank you again!

Technical details that may help:

1993 260 HP Mercruiser 5.7, quadrajet
New Long block with stamp 880 - 96 or newer roller style
Oil pressure a bit high- 40 at idle/cruise - 60ish at WOT when the engine gets there - Could this factor into the issue?
Alpha 1 Gen II
60 gallon fuel tank - only run at about 1/2 tank when issue arises - never attempted with full fuel (excessive suction creates a whirlpool when the tank is low allowing air in the system?)

 

[Rick Stephens]

Check the fuel tank vent and the anti-syphon check valve. Fuel filters new? And fuel pump pressure.

Doesn't sound like a carb problem. Sounds like a fuel supply problem.

Rick

 

[M.A.CCruiser]

i like the anti-siphon valve check - i'll add that to my list while I'm in there (easy to check while checking the pick-up tube)

fuel filters are new except for the one in-line at the carb. The fuel pump pressure is going to have to hold until i get another day off and can get the parts needed to run a pressure test.

 

[harleyman1975]

Sounds like a possible float issue to me. Starving for fuel top end. Fuel pick up in the tank? Possibly sucking air someplace? Needle sticking or not opening enough?

 

[M.A.CCruiser]

Ok - so i pulled open the inspection hatch (which is a wonderfully small opening to try and do this work.) And, here is what i see...

pretty ugly - but most of what was there wiped away - just crud built up from washes/water it appears.

But that Anti-siphon valve and the fuel line was pretty ugly

So after NUMEROUS itty bitty turns i pulled it out and here is the valve (from the fuel line side)

Pretty well closed up with crud. The check valve side wasn't bad. But then again, that side doesn't half fuel sitting it in most of the time

[FONT=verdana, geneva]So whether or not that was causing my issue or not, i dont know. But, I'm sure it wasn't helping the situation. Blowing through the check valve side did open and allow air through. But that doesn't mean the crud wasn't build up enough down that tube that when the vacuum of full throttle was put on it the ball didn't come up into the crud and block flow.[/FONT]

This added to my concern for the pick up tube. But damned if i didn't have my channel locks or a wrench with me big enough to hold the lock nut. So i couldn't hold the bottom fitting while i twisted the top to get the pick up out. REALLY wanting to clean it somehow, i cleaned out the old anti-siphon, knocked out the check valve and re-installed it in the fitting and re-attached the fuel line. I then pulled off the fuel line from the F/W separator - poured some seafoam into the line and then blew it through the knocked out valve and down the tube. Hopefully it did something. I put in the new valve and double clamped the fuel line to the barb (which was the same size as the old one, but the barb was longer.

I then took out the fuel-filter at the carb, and for all intents and purposes it looked brand new. Considering the carb is a recent rebuild i wasn't really surprised.

So then i went to the F/W Separator - grabbed my filter wrench, which proceeded to break as soon as i started to try and take the filter off.

[FONT=verdana, geneva]The mechanic gods were not in with me last night obviously. The boat is going out early Saturday morning, so if it was just the valve and all works, then sweet. Otherwise, still a few more checks.[/FONT]

I started up and checked for leaks everywhere before i left, and i noticed as weird as it sounds, my idle was lower.... and it seemed like i was running rich. Some black smoke out of the exhaust. On the hose i was kinda bouncing at maybe 600ish. Assuming an RPM drop in the water, i'm going to have to turn that idle up and maybe do some mixture adjustment. We will see what happens. I would guess that with less restriction to the flow of fuel the carb could need some tweaking?

More to follow!

Oh - and yes I intend to replace that fuel line

 

[Rick Stephens]

Wow, great pics. Amazing what ends up in a reasonably clean fuel system.

 

[NHGuy]

I wouldn't sweat that idle speed change. My 89 5.7 qjet changed idle quite a bit. Might be from carb, choke or timing. Just set it to 675 hot neutral idle and you'll be happy.
That poison in your fuel line is bad stuff though.
When you DO get the fuel water separator off, don't reuse it. Contaminants get around the element during the removal process. Pour out the contents into a clear container and check em for junk. Also save the separator, you may want to split it later for inspection.
There's a chance there's stuff in the needle/seat, from before the new separator. but after the new carb. And there's a chance the float isn't at the perfect height. These things can both be seen by removing the top section of the carb. If you get a qjet book it's in there. Or if you need it I can walk you through.
Another possibility, and it is real. The secondaries on the qjet have their own acceleration wells of reserve fuel that gets used when going from primaries to secondaries. Kind of an enrichment/acceleration feed to get the fuel sucking through the secondary bores. If those wells aren't filled before you go wide open, you might get the issue as described.
To test my theory try running to wide open after your 1000 rpm warmup. Then back off in a minute or two. Now run it back to wide open again. Does it fail or get to 4800 on later attempts? What I am asking is once the secondary reserves have been used are they empty or low and unable to get you over the hump to wide open on subsequent pulls?
If it's that, there are some tiny fill orifices that may be clogged. Or your float height could be wrong, your fuel pump could be poor, or your carb could be from a wrong application. There are dozens of qjet combinations. You want the one for your exact motor. If you want the part numbers they are in the Mercruiser guide. If the float is too low the fuel can't flow into the secondary wells as designed.
Pushing a boat like that uses all the fuel system's capacity, no kinks in the steel line, are there?


Test my theory by repeatedly running at 1000 rpms for the same time it takes to exit your no wake zone before hitting the gas.

Also - The reason the vacuum diaphragm flutters as the engine stumbles is cause the vacuum changes during that time, not an issue.
When the vacuum drops under acceleration the diaphragm releases the secondary doors to open. That is working correctly.

The secondary spring should normally be 3/4 turn from loose.

Hope you get it.

 

[M.A.CCruiser]

thanks NH! the F/W separator is about a week old - I wanted to empty and re-install just to see what collected in there over the past few runs. The one on before that lord only knows how long it was in there. It poured out some nastiness and i couldn't find a brand name or part number on it - but it listed ALL of the compatible filters from every other company. was strange.

No, there are no kinks in the lines anywhere - but i like your test of the acceleration wells. I thought the acceleration pump provided that extra fuel when the throttle was pumped - i didnt know there was an extra set of "bowls" (for lack of a term).

But considering that, wouldn't an extended idle refill the reservoirs? or start up or something of that nature?

What I need is a weekend out on the boat with a buddy and some beers as opposed to my two girls - so i can tinker and examine without having a 5 year old (and 35 yr old) getting bored.

 

[M.A.CCruiser]

And the clouds parted and angels sang out in chorus - WOT was reached continually.

Leaving this morning I could barely break 3800 RPM... It was killing me. So once we beached and the little lady was playing and my lady was getting sun I did 2 things (in hindsight I wish I had only done one at a time)

I emptied the F/W separator again.... And I changed the fuel filter at the carb. What was in there was a paper style filter with a small rubber "nipple" where it would meet the fuel line. What spec calls for is the "basket" style filter.

Swapped... Enjoyed a few beverages... Left the beach and hit the normal zone, crossed my fingers, pegged the throttle lever and 5 seconds and 4800 rpms later I was cheering as I blew past a pontoon boat.

Now, the F/W filter is only 2 weeks old, so I have my doubts to that being the culprit. But, this gave me enough cause to say I'm switching it with a racor style with the clear bowl.

I didn't take pictures - but this is what I put in
http://www.marinepartssource.com/new...0Fuel%20Filter

And this is the style that came out

http://discountmarinesource.com/p-52847-sierra-in-line-fuel-filter-7855.aspx

 

[NHGuy]

I'm glad you found the issue with the entry filter. But I'm serious, next time you pull off that separator trash it. It ain't worth the 7 dollars for all the grief you can suffer.

And after that recheck the entry screen filter again. And as long as I'm lecturing like an angry grampa, don't forget to use line wrenches on the fuel filter fitting. Bending up a new fuel line really blows if you have to buy the bender and flaring tool to do it. I've got the t- shirt on that dumb play!

 

[Boat Doctor1]

That 2nd paper carb filter is best- filters better!! That also could have contributed to the problem yet was doing its job!! That check valve being coroded up badly was the main culprit!1 The paper filter -likely was catching the crap from that corosion!! Install a new paper filter & run again!! See if all is same then?

 

[M.A.CCruiser]

I'm glad you found the issue with the entry filter. But I'm serious, next time you pull off that separator trash it. It ain't worth the 7 dollars for all the grief you can suffer.

And after that recheck the entry screen filter again. And as long as I'm lecturing like an angry grampa, don't forget to use line wrenches on the fuel filter fitting. Bending up a new fuel line really blows if you have to buy the bender and flaring tool to do it. I've got the t- shirt on that dumb play!

Are you saying run without the separator? Like bypass it? Or get a new one?

Didn't go to the boat today but I do intend to replace it next time out. I also realized why I can't cross reference the filter for one with a clear plastic bowl - apparently they aren't CG approved for I/Os and inboards.

I'm sure the paper filter is great - but i just put in the part number for what I'm supposed to have.

 

[Bondo]

Are you saying run without the separator? Like bypass it? Or get a new one?

Ayuh,.... He's sayin', carry a spare, never dump out a filter, 'n reuse it, that just allows crud to get to the clean side,.....

 

[M.A.CCruiser]

Ayuh,.... He's sayin', carry a spare, never dump out a filter, 'n reuse it, that just allows crud to get to the clean side,.....

Gotcha - I most certainly will! And i'll be carrying a spare from now on.