Starting from the begining

[FreeBeeTony]

Guess I will have to start from the begining.<br /><br />Boat: '84 24' Grady White Offshore.<br /><br />Engine: according to the S/N it is a '92 Mercruiser 350 Magnum. It has been rebuilt/replaced (longblock?), the sticker on the valve cover says Dealer Marine.<br /><br />Symptom: engine runs smooth from idle to ~3200, then it seems to "bog down". I don't know how else to describe it. I give it more throttle and maybe it will go to 3500 but has little power. If I get to 3200-3300 w/o the bog and run like this for a few minutes sometimes this "bog" will appear on its own w/o changing the throttle position. I have to back off the throttle to "clear it, then come back up on it. It doesn't even appear to be load sensitive, it will happen at the same RPM with 2 peaple or 5 people on the boat. When the "bog" appears you can see the RPM drop on the tach 100-200 RPM's and the engine sounds different, almost like a pinging or I am loosing a cylinder. It's like someone threw a switch. Comes on quickly. I have checked the timing, could it be the distributor itself?<br /><br />Here is a list of what has been done thus far:<br />1) Replaced cap, plugs, wires, coil, sensor in distributor.<br />2) Replaced carb with Edelbrock 1409 (600CFM).<br />3) Replaced fuel pump, checked fuel pressure at carb: 7-8lbs.<br />4) Bypassed fuel tank by using a seperate gas can right at input to fuel pump.<br />5) Performed vacum tests, ~20" at idle and steady.<br />6) Checked timing, 8*BTDC. I tried to check the advance by taping a piece of paper to the damper but it kept falling off at the higher RPM's. But it did indicate that the timig was advancing.<br />7) Swapped out ignition modules with no change in performance.<br />8) Pulled outdrive to check for exhaust restriction, exhaust is clear and bottom is clean.<br />9) Oudrive is proper ratio (1.5:1) and prop is 15" pitch.<br />10) Checked compression, 140-170lbs.<br /><br />If I forgot something I will add it to the list.<br /><br />Thanks for sticking with me........

 

[trog100]

Re: Starting from the begining

"almost like a pinging".. have u tried some octane booster or better gas.. ??

 

[ottawamerc]

Re: Starting from the begining

Hey Freebee your not alone in this I too have the same problem, and I have tried all solutions that you have less the carb and pump ( overhauled both though). Does yours act up more once it is hot after 30 min. run time? Mine is a 85 Merc 228hp 305 chev with a replaced motor also. I have heard that cracked exhaust manifolds could cause this? but you would think it would show more at idle than high speed? If I figure out whats wrong with mine I will pass it on to you, let me know if you figure it out too, this problem is driving me nuts and my bank account into the ground!!<br /><br />Good luck

 

[asgrinne]

Re: Starting from the begining

We are in about the same "boat" Under posting of New 350 Long Block post.

 

[newport dave]

Re: Starting from the begining

Is there a drop in fuel pressure when the bog occurs?<br /><br />Dave

 

[Ralph 123]

Re: Starting from the begining

SBC 350 Cam specs<br /><br /> http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=24;t=012378#000001

 

[Buttanic]

Re: Starting from the begining

Is the timing advance controlled mechanically or electronically. If it is electronically are you sure you are following the correct procedure to set base timing. Some systems require the electronics to be disabled when setting base timing. If it is mechanical is the mechanical advance working correctly, not sticking. no broken or missing springs or fly weights, correct advance curve, etc.

 

[FreeBeeTony]

Re: Starting from the begining

The ignition advance is electronic (Thunderbolt IV) which as far as I know does not require anything special to set the base timing.<br /><br />I am really at a loss here.........<br /><br />How would I measure the cam lift? The difference in hieght of the rocker arm when the cam turns? How far off would it have to be in order to create a problem such as this?

 

[Buttanic]

Re: Starting from the begining

If there is a difference of more than .020 between actual valve lift and the spec there is excessive ware. Another possibility is the outer ring of the harmonic balancer can slip on the rubber bushing in between the outer ring and the hub on the crankshaft which will cause the timing mark to be in the wrong place.

 

[FreeBeeTony]

Re: Starting from the begining

I adjusted the timing pretty radically in either direction to check if the balancer had moved.....didn't get any improvement in either direction......actually got worse which kinda indicated that I was timed just about right to start with.

 

[Reel Poor]

Re: Starting from the begining

Just another thought.<br /><br />Unhook your tach from the coil and and run the boat without the tach hooked-up.<br /><br />

 

[FreeBeeTony]

Re: Starting from the begining

Tried that already........see other post: " FOUND SOMETHING"

 

[waterone1@aol.com]

Re: Starting from the begining

Tony, you have pretty much beat the engine thing into the ground. What about looking in the other direction...the drive train ? what drive, ratio, prop do you have ? when did you last pull the drive and inspect the gimbal bearing ? Has the prop been checked or replaced ? A few months ago, I was loosing my mind on a twin, one engine was several hundred rpms behind the other, did all of the engine checks and came up with nothing....finally, in desperation, found out one prop had bent blades....no vibration, etc. fixed the problem. Several years ago, had a 7.4L that would not hit top rpms or speed...turned out to be a loose/bad gimble bearing. Recently had a Carver that would not do much of anything, as far as speed goes, pulled boat and found serious growth on bottom...cleaned and put back in, ran like crazy.

 

[FreeBeeTony]

Re: Starting from the begining

see other post......... here

 

[ottawamerc]

Re: Starting from the begining

FBT I hit on something with my similar problem today. I was told by someone that base timing was 8*,,we all know that, but make sure there is no advance kicking in. So I tried again set my timing light and reduced engine speed low and behold timing dropped to 4* @ 600rpm or so. I set it to 8* again set rpm back to 750rpm and tried it out. Well it has never run so well, out of the hole it jumped and ran strong to 3900rpm. I still have a full advance problem (fine tuning) but it is way better. Just a thought yours might have similar problem?<br /><br />Scott