454 Mag Stumble Issue
 

Looking for help diagnosing this problem. Boat ran fine yesterday right up until I was about to head to the ramp. Started it up, hit the gas, and it started stuttering and there was a loss of power. Probably should have shut it down then, but it was late and doubt anyone would see us waving for a tow. I putted (pretty much idling) over to where my buddies boat was 400 yards away, and let him tow me in. He was hanging out in my boat. I was having a steam issue prior to this, but it wasn't steaming at all, until shortly before this happened. I was getting white smoke, or excessive steam (couldn't tell), and the water out the exhaust was too hot to touch. The elbow was too hot to keep your hand on. I believe the manfiold/riser/elbows to be original and the boat is a '91.

I pulled the plugs and several if not all had an black residue on them...some more noticeably wet. not water, but either carbon & gas mix, or oil. When I pulled the plugs I did notice a gas smell.

Here are the results from the compression check::

8,7 - 145, 145
6,5 - 140, 147
4,3 - 135, 150
2,1 - 145, 145

My manual suggests 150 psi for the 454 mag. 135 seems a little low, but it's within 10 percent of the highest reading.

I gave the exhaust manifolds/risers/elbows a look over. Noticed rust on the outside near where it attaches to the motor. No noticable cracks. Didn't tear anything down. I was going to pull the oil, but couldn't find a dipstick oil extractor at any auto store nearby. I'll call some marine places around here tomorrow.

Could the Manifold/riser/elbow be clogged causing the gas/carbon mix at the plug??? Could this fouling of plugs then have caused a misfire results in running on 6-7 cylinders causing the stumbling issue I experienced?

I posted this over at checkmate-boats, but looking for all the help I can get.

Thanks.

did you over heat at any point? Is your choke stuck closed? Do you have soot on the transom?

Risers are most likely clogged from rust/scale and need to be replaced. You can soak them in muratic acid to clean out the rust but with thier age replacement would be wise. The stumble could very well have been water getting in to the cylinders from aged exhaust as well. This usually precedes an exhuast valve head popping off due to getting brittle from cold water spashing on it from leaking exhaust risers. Were the wet plugs in the center of the engine 3,4,5 or 6?

mr3dman: The boat never overheated, it was rock steady at 160 degrees. I don't think the choke is stuck closed, but if you tell me how I'll double check. It's been a little finicky to start...but nothing overly difficult. Usually takes a start or two and runs at low idle till it warms up. If you're asking if soot on the inside of the transom...no, neither on the outside. Before changing the exhaust tips I did notice they had a soot in them (had a internal flapper)

Blue Thunder - The plugs were dark in color. They all seemed similar, with a couple having more of a carbonized paste on them....if that makes sense. It was black in color and the consitency of a little less than shoe polish. Not a lot, but I could wipe it on my finger off the threads. I can post a pic. At first I thought this was oil....maybe it's carbon/gas mix....or carbon/water mix??

Do the compression numbers look ok?

Popped off the manifolds tonight. Looking at them I assume they are original which makes them about 21 years old. Posting some pics to get your thoughts.

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I would be looking at ignition for hard start and stuttering issues.....

Full Force, I agree with your post which is why I pulled the plugs. They were definitely carboned up, which made me think it was causing a misfire and I was running on 6-7 cylinders. I'm going to double check for spark at each plug tonight. What else should I look at??

I been having issues with my thunderbolt system on one engine it was the coil voltage was low and that was in the coil, the other seems to be a P/U in distributor as I did the coil and control box on that, I plan to do P/U this wekeend...

I am about to ditch the thunderbolt and go aftermarket... sick of issues due to older parts and new are insane expensive..

I can't tell in the pic, are there any water trails going down from the riser gasket surface towards the exhaust valves? The riser gasket was clearly leaking outwardly.

The water wont cause a miss, just break valves, I bet damn near anything its ignition...

Could very well be ignition just due to the fact he says the plugs are ball balck. He was talking about steam in the original post so that sounded like water getting in a cylinder. Plugs would be clean though if they were seeing any water. I have seen where the hall effect sensor in the dist will have this type of problem when hot. You might check that.

Took some pics of the plugs tonight, along with a paper towl after I wiped one of them off. Seems like most of the oily/carbon residue is around the threads, but no oil was on the part of the plug outside of the block, so it didn't come from above. All the plugs were similar. Also, took about a pint of oil out of the oil pan, it was not milk shaky.



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I know this is an old thread....well, my old thread, but wanted to add to it. I sent it to a local shop and they said they couldn't get it to do what I described. Went for a ride....was perfect. Next time out, the issue was back. It's usually after a hard run....then shut down for say 30 minutes. Start up and she idles a tad rough, and stumbles when given gas. However, last time out I just nailed the throttle and she came out of it and started runnning fine. ANy ideas?

change the fuel filter/water separator,you may have got some fuel with water in it.

Id look into a issue with the carb. Float sticking, needle and seat malfunctioning, well plugs leaking. The old quadrajet carbs were no exceptions to these issues.

Pull the engine oil dipstick. Does the engine oil smell like gasoline? Did you notice a strong smell of fumes from the exhaust when experiencing the issue?

fixx
 

if it is a carb model check the choke pull off to make sure the choke is not snapping shut at idle..also the pick up in the distributor are common to go out and will cause a misfire..also check the alternator voltage at the back of the alternator,,if its below 12.8 its junk,,should be 13.2 or higher..

I honestly couldn't tell you, this took place late last year before I put her into winter hibernation. To clarify, it was stumbling trying to get on plane, then I hammered the throttle...and kept it there. After about 2-3 seconds the motor came alive and the stumble was gone. I'm pretty sure the water/fuel separator was replaced as part of winterization, so if that's the problem it should be fixed. I'd been considering getting the carb rebuilt in the offseason because I'm pretty sure it's never been done (boat came with a ton of maintenance history). Anyone inparticular to send this carb to for rebuild?

Hall effect sensor, and they show problems hot and clear up cold often.

Are these old q-jet carbs? If so, the float bowls may be leaking.

A number of years back, I worked in a tune-up shop. Whenever a carb needed a rebuild, we would keep the car overnight to verify the cold start was OK the next morn. The q-jet guy showed me where the casting plugs were located in the float bowl. Over time, the plugs in the casting holes would loosen allowing fuel to leak from the float bowl into the manifold. He would smear some JB weld over the casting hole plugs to stop the leak, let dry overnight and re-assembe the carb the next morn.

The fuel leaking into the manifold would flood the eng unless it sat long enough for the gas in the manifold to evaporate. This situation would also cause longer than normal cranking times the next morning due to the float bowls having to re-fill. Just a thought..

I believe the carb is original to the boat which is a '91.

Yep.

And the fact you hammer the throttle and eventually it clears up and takes off and runs ok, sounds like a carb/flooding/fouled plug issue. If you cant or don't wanna mess with it yourself, id probably just call these guys up and get a rebuilt carb and exchange yours. Less downtime and bs.

http://www.allcarbs.com/catlist.php

91 was abt the timeframe I was working at the tune-up shop, so we mainly were working on 70's and 80's models. Q-jets didn't change very much so you most likely have the casting plugs I was talking abt.

Sounds like you are running very rich at start-up. The choke as prev mentioned, would be the first place I would start.

When you go to start it after it's been sitting overnight, do you have to crank the starter for a while before it fires, or does it fire up immediately?
If you have to crank for a while, the float bowls are dry and having to re-fill.

fixx
 

if that were the case then he would have a long crank time if the boat sat for a period of time..those plugs you guys are refering to the secondary ones are sandwiched between the throttle plate and the main body and the other plug which is open the the primary throttle blades..i have never had one or seen one cause a stumble from sitting for a few hours,,just long cranking time if the boat sits for weeks..

Depends on how fast fuel was leaking into the intake and how long it sat.
If the eng was off for a relatively short period (like an hour or so) and fuel was still puddling in the intake and there was still some fuel left in the bowl, it could fire up immediately but run extremely rich untill the puddled fuel was sucked thru. Opening the throttle would expedite this process; which would match the symptoms he described.

After a few hrs (again, depending on leak rate) the fuel would evaporate and you would just have the long crank time.

Yes, the plugs at the bottom of the main body (in float bowl) are the ones I'm refering to.

In all likelyhood, this isn't the problem, but it is possible. IMO, the most likely cause would be some crap got caught between the needle and seat or the needle/seat assy is worn and can't close completely..

That's why I asked if he experienced long crank times after eng sat overnight. If it fires right up the next day, it's probably the needle/seat. If there's a long crank time the next day, the fuel drained from the float bowl.

Regarding starting, here's my run down:

Per mercruiser manual I start the boat (if memory serves correctly) - 1/2 throttle if cold, 1/4 throttle if warm.

Completely cold start hasn't run that day = Will usually start first try, but I gotta keep the rpm over 1000 to keep her running for a minute or two or she will die. Occassionally she will start stumble then die, but I can keep her running by stayin above 1000 rpm on second try.

Has been run that day = Will kick over on first try....if sitting long time have to keep above 1000 rpm, if not will idle fine.

Run for a while, shut off for under an hour = Sometimes fine, sometimes start right up but stumble at idle and stumble on takeoff as described as the problem in this thread.

If it were a distributor / electrical issue I don't think I would be able to gas my way out of it. You guys are convincing me I need to replace/rebuild the carb.

I agree.

I haven't had many ignition problems that got better with increased Load/Throttle. They normally would get worse with Load/Rpm.

If the plugs wreaked of gas when you pulled them, that also would lead me to a carb issue. Most of the time ignition systems misfire, but still fire the plug. Just at the wrong time.

I'd be looking at the carb being that its 23 years old. Good thing about ignition systems is normally, you can diagnose the problem, by testing individual parts. Rather than throw parts at it. When I worked as a tech at a GM dealer, we couldn't throw parts at things. We had to diagnose driveablity problems step by step. We were able to recommend certain things, but at the end of the day we had to fix the part that caused the problem. If the car had a bad coil pack, crank sensor, pickup coil, or module, we tracked it down and replaced the faulty part. We didn't send the customer home with a entire new ignition system because of one part being bad.

Since its starts ok after sitting for a extended period, then yes, you can pretty much rule out the carb well plugs leaking like Mrfixxall said. It could be a choke malfunction like he said, internal issues in the carb. However, its really not something you can diagnose over the internet. We can sit here and give suggestions, but we aren't actually there seeing whats going on.

Theres a couple ways to handle a situation like this. You can bring it to a shop, and pay someone labor hours to track down your issue. Which can get expensive.

You can try to narrow it down yourself.

You can do preventative maintainence and replace things that may be overdue for replacement. Things like Spark plug wires, cap and rotor, plugs, carberator rebuild, water separator and fuel filter at the carb, sea pump impeller, exhaust riser gaskets, water tstat, and clean all your electrical connections. These are things that after 23 years and 500+ hours, just need to be done.

Plugs, water separator, sea pump impeller, exhaust riser gaskets are all new as of last year (water separator as part of end of year winterization). I'll rebuild the carb or just buy a reman. before the season. I'll also take a look at the maintenance records to see what I can find on the fuel filter, plug wires, cap & rotor, and tstat. The previous owner was anal and documented everything. If it's not listed I'll just replace them so I know where I'm at. Also thinking I should replace the risers and exhaust manifolds since they are original. (fresh water boat). If I'm missing anything from your post let me know.

Thanks to all who've contributed!!!