7.4 bogging down

[wileecoyote]

Driving me nuts. I'm having trouble figuring out exactly what the problem is. I'm sure in my mind it's a fuel issue. Intermittenly the port engine bogs down. It does it only on plane and either comes right back, or if you throttle back and then up again it comes back (sometimes for a while and sometimes for a few secs.) I thought I had it figured out running off SB tank it seemed to be OK. Today it did it on both tanks. Carbs rebuilt in off season, fuel filters (canister and water seperator) replaced prior to season. Plugs, cap, rotor, plug wires, all replaced(not coil though) Timing right on the money. I'm now unsure if it is possibly the coil, or fuel pump/water or junk in fuel system. Cruising at 21 Kts with sync in the middle you will see port dropping off, then it drops off to almost nothing. Sometimes it goes up and down, lately it's been just about to die. Never seems to do it coming up on plane. I'm hoping it will get more consistant so I can diagnose. Wan't very hot today, so I'm not looking at vapor lock issues (also if you come off plane it will come right back up) I don't have an inline tap to check fuel pressure and don't smell fuel or see any smoke (can't see fuel in clear line from pump to carb) Today it fell off and I throttled back and up again, go a backfire (lean misfire?) It's so random it's hard to figure without just swapping parts. I may swap coils just for piece of mind. Guess I should pull water seperator to check as well. Canister was just changed this past week, didn't seem to help. Another note, usually takes a good 10-15 min, but has taken as long as 30-40 min. Thanks for any suggestions/prior experience.

 

[PlayDate]

My bet is the Port carb has issues. Swap them and see if the problem moves to the Starboard engine. I know this is a pain but it is the only way to start solving the problem. The other possible cause is the Port fuel pump. You can make up a fuel tap with common fittings and have someone monitor the fuel pressure while you run the boat.

BTW I'm not a big fan of rebuilding Carters, Webers and Quadrajets. Depending on the process and experience of the re-builder....the results seem to create as many problems as they solve.

-John

 

[jjames002@socal.rr.com]

i understand the anguish and have been ther myself!!! check the fuel tank for a sticky anti syphon valve sometimes the ball gets old and sticky. then of course with a proper fuel pressure guage have a friend watch it to see if it drops when you start to bogg down on plane if it doesn't lose pressure and still boggs then the carb floats are probably adjusted wrong and you are running the fuel completely out of the bowls. as you stated when you throttle back it helps fo a short time this is because the bowls fill again then run out again might have to remove the bowl covers and slightly bend the float arm downward but not too much so more fuel is allowed into carb good luck, jim.

 

[wileecoyote]

My bet is the Port carb has issues. Swap them and see if the problem moves to the Starboard engine. I know this is a pain but it is the only way to start solving the problem. The other possible cause is the Port fuel pump. You can make up a fuel tap with common fittings and have someone monitor the fuel pressure while you run the boat.

BTW I'm not a big fan of rebuilding Carters, Webers and Quadrajets. Depending on the process and experience of the re-builder....the results seem to create as many problems as they solve.

-John

Hadn't thought of swapping carbs, it's actually very easy thing to do. I had thought I had a different issue. A few weekends ago I ran about 1 hour on the SB tank because it had more fuel than the port. It ran fine the whole way. On the way back I switched back to the port tank and it started to bog again. Yesterday port bogged down with both running from SB tank. So now I'm back to square one. I think swapping carbs will be my first choice. I didn't do the rebuild, because I was bsy at the time and a friend of mine is in his 50's and can basicly do a Qjet in his sleep. I looked at the metering rods and float level before they were put back together. It could still be something in there, but I'm pretty sure it wasn't a float setting, or missed seal.

 

[sbw1]

How new are the spark plug wiring harnesses? Sounds like you replaced all but the wires. My 454s did the same thing you desribed. Finally we ran the boat in the dark with the hatches off and there was blue lightening everywhere. Replaced the harnesses and all was well.

 

[douglee25]

I also had a problem like you described and it turned out to be the coil. Actually it was a bad ballast resistor which sent full voltage to the coil at cruise which caused the coil to heat soak and eventually fail (it actually leaked oil all over the intake manifold).

Ballast resistors are there to basically 'step down' the voltage from 13.6 volts to around 9ish volts during cruise. The coil receives full voltage at startup and that should be it. If the carb swap doesn't solve your problem, verify when the engine is running that you have only around 9 volts running to the coil. If that checks out OK, your ballast resistor is good, swap the coils anyway and see if the problem follows.

Doug

 

[wileecoyote]

How new are the spark plug wiring harnesses? Sounds like you replaced all but the wires. My 454s did the same thing you desribed. Finally we ran the boat in the dark with the hatches off and there was blue lightening everywhere. Replaced the harnesses and all was well.

All wires replaced with Merc "red" wires prior to the season.

 

[wileecoyote]

Swapped carbs, but haven't had a chance to go out and see what happens yet. I get 13+V on the + side of the coil with ign on or engine running. I get this on both engines. I don't see a resistor wire in the Thunderbolt IV diagram. Started both engines and ran them, will let you know after I can go out and cruise.

 

[douglee25]

Swapped carbs, but haven't had a chance to go out and see what happens yet. I get 13+V on the + side of the coil with ign on or engine running. I get this on both engines. I don't see a resistor wire in the Thunderbolt IV diagram. Started both engines and ran them, will let you know after I can go out and cruise.

Ahhh, you can ignore the info on the coil voltage. I was not aware that you had the thunderbolt ignition system. I believe they don't have points and the ballast resistor is used with a points system.

Doug

 

[wileecoyote]

Ahhh, you can ignore the info on the coil voltage. I was not aware that you had the thunderbolt ignition system. I believe they don't have points and the ballast resistor is used with a points system.

Doug

No, I do not have points, there is a hall effect switch in the dist and a reluctor wheel on the rotor button. I do have a ign module, I may swap that later. It was interesting to me to see the problems JLevine was having (I think it was him) with the loose plug wire overheating his coil. Who knows. I tell you one thing, I will check all the plug wires! RonDDS also talked me into replacing them, my first conversation with him is where it all turned for the worse!

 

[sprink56]

Wiley,

Do your Q-Jets have the filter in the fuel line intake on the front ? If so, check them both. Based on what you said about your tank suspicions, it's possible that some crud got sucked up int this filter or worse into the carb if those filters aren't present. It sounds like fuel pressure or volume. When those big block Chevys pull those secondaries open they are asking for a bunch of fuel. If there is a restriction...you know the rest.

One other Q-Jet ism comes to mind. The secondary air valves have a diaphram "dashpot" that controls the opening and closing of the air valves. If there is a leak anywhere associated with that thing (cracked hose, split diaphram, cracked housing) the air valves will not stay open. Talk to your rebuilder and he will remember if the rebuild kits came with a new dashpot and associated vaccum hose. The vaccum can build up and hold during low speed operation on the up stream side of the diaphram. Once you are up and running and a crack or split allows enough vaccum to bleed off, the secondary air valves will close. Just a guess but worthy of consideration.:thumbsup:

Good Luck.

 

[wileecoyote]

Wiley,

Do your Q-Jets have the filter in the fuel line intake on the front ? If so, check them both. Based on what you said about your tank suspicions, it's possible that some crud got sucked up int this filter or worse into the carb if those filters aren't present. It sounds like fuel pressure or volume. When those big block Chevys pull those secondaries open they are asking for a bunch of fuel. If there is a restriction...you know the rest.

One other Q-Jet ism comes to mind. The secondary air valves have a diaphram "dashpot" that controls the opening and closing of the air valves. If there is a leak anywhere associated with that thing (cracked hose, split diaphram, cracked housing) the air valves will not stay open. Talk to your rebuilder and he will remember if the rebuild kits came with a new dashpot and associated vaccum hose. The vaccum can build up and hold during low speed operation on the up stream side of the diaphram. Once you are up and running and a crack or split allows enough vaccum to bleed off, the secondary air valves will close. Just a guess but worthy of consideration.:thumbsup:

Good Luck.

Mine do not have the inline filter at the line connection. I know on car versions there is a long neck where the filter inserts. On mine the line goes straight into the 90 deg elbow out the front of the carb. I had thought of that, but I would think that the water seperator would get it first. If the carbs make no difference it will be back to looking at the rest of the fuel system and if it does I'll be looking at Ign and mechanicals.

 

[wileecoyote]

Bad news! Swapped carbs yesterday after about 10 min on cruise and port engine bogged down again. Today I went out to see if it changed to SB or stayed on port. I ran almost an hour and nothing! Ran great the whole time:smt021. Now I don't know what could be the cause or when it will come back. If for some odd reason I moved something and by shear miracle it fixed itself, thats OK. It never happens that way!

 

[douglee25]

Doesn't it figure? That darn Murphy guy is at it again!

Doug

 

[sprink56]

I hate situations like that. I would be "sucking up the seat covers" now every time I went out wondering when it was going to happen again.

How about swapping coils? :huh: If you still get the port engine problem that rules the coil out...but if it follows the coil...:thumbsup:

I had a collapsing fuel line once on a car that did what you describe and a fuel line to close to a source of heat can cause fuel percolation.

Just brainstorming.

 

[rondds]

my first conversation with him is where it all turned for the worse!

My sinister plot to be the only boat in the world running properly is taking shape! :lol: (emoticon laughing evil-ly)

 

[wileecoyote]

My sinister plot to be the only boat in the world running properly is taking shape! :lol: (emoticon laughing evil-ly)

Just wait until we all curse you at once! They'll find that old gal at the bottom of some NJ bay with all the needles and syringes floating around it! I can almost guarantee that Sat the motor will start to act up again, just my luck.

 

[wileecoyote]

Ran it all weekend, and it was fine. Topped off with fuel and ran about 65 gallons of fuel on Sat/Sun. No problems at all, never even missed a beat:smt021. My SB engine has always used a bit more fuel, according to FloScan, and thats still the same. Not sure if I dislodged something, moved something, or haven't met the proper situation to cause it to happen. If it acts up again, I will get back with you.

 

[sprink56]

Ran it all weekend, and it was fine. Topped off with fuel and ran about 65 gallons of fuel on Sat/Sun. No problems at all, never even missed a beat:smt021. My SB engine has always used a bit more fuel, according to FloScan, and thats still the same. Not sure if I dislodged something, moved something, or haven't met the proper situation to cause it to happen. If it acts up again, I will get back with you.

Maybe you stepped over the right crack in the side walk or some other thing to make the Power Boat Gods smile on you.:thumbsup:

 

[lovestocruise]

I was going to ask the same question and then saw this post. I have had the same problem with the port engine. My mechanic re-built the carb and it still happend then we change the relay and still the same problem. We then changed the coil and it worked great. We took her out the next day and it ran awesome. After spending the day on the hook she would not start. kept cranking and would start for a sec or two then died. I could smell fuel coming from bilge. Like it was geting too much fuel. Limped home on one engine mechanic coming tomorrow. He is in the dark. Says it could be the ignition module. This is driving me nuts.....