OMC 5.0 305 wot lag and power surge

nportwine

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Jul 18, 2017
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Having an issue with my 5.0 305 OMC in my 1988 215 four winns sundowner. When i slowly give it throttle I can get it close to Wot but once I give it full throttle it looses power and speed. If I leave it at 3/4 throttle sometimes itll surge forward with more power then lose power and it keeps going back and forth like that. It has been getting a bit worse and now I can never really get it to wot no matter how slow I give it more throttle. It seems like a carb issue because of the random power surges and how it runs fine until 3/4 throttle. It is worse after running the boat for awhile, doesnt do it as bad when cold. The boat has had the points replaced with a pertonix kit and has had a carb rebuild within the past few years. It hasnt been run for two years but I have burned a full tank through it now and have ran seafoam in it as well. I am suspicious that maybe the secondaries arent opening up like they should but really lost as to where to start to try to get this solves. Thanks in advance.
 

southkogs

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Welcome aboard: how 'bout the basics? Fuel filter, spark plugs, compression?
 

kenny nunez

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Start by checking the filter at the fuel inlet on the carburetor. You will need a 1" open end and either a 1/2" or a 5/8" flare nut wrench to keep from damaging the fitting. Then drain the contents of the large filter on the engine into a bucket to see if there is any water present. That may be all that is the problem. Inspect the hose from the tank to the fuel pump to be sure it has not collapsed internally. Some fuel tanks have a small filter inside of the suction tube that can get restricted.
 

nportwine

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Welcome aboard: how 'bout the basics? Fuel filter, spark plugs, compression?
Sparks plug were replaced not too long ago, compression is good. My grandfather owned the boat about 4 years ago and he put the electronic ignition new plug wires in and a carb kit when he had it. He sold it to a guy who used it maybe twice and then I bought it back. I haven't checked fuel filter is there more than one to check? I know there's the screw on one which I'm assuming I can get a new one at any parts store but is there an inline one as well?
 

Bondo

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and a carb kit when he had it.

Ayuh,... A carb is rebuilt only until more crap is pumped into it,...

I'm guessin' ya got a spin-on canister type filter, 'n probably another in the carb's inlet fittin', as mentioned above,...

As for the spin-on filter, some parts stores might carry 'em, but it's an OMC part, the threads are different than the Merc equivalent,...
The filter screen in the carb's inlet is more likely to be found in an auto parts store, as rochesters used 'em for years,...
Just be forewarned, there are different lengths,....

When ya pull the canister filter, pour it into a clear container to Inspect it's contents,....
 

nportwine

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Ayuh,... A carb is rebuilt only until more crap is pumped into it,...

I'm guessin' ya got a spin-on canister type filter, 'n probably another in the carb's inlet fittin', as mentioned above,...

As for the spin-on filter, some parts stores might carry 'em, but it's an OMC part, the threads are different than the Merc equivalent,...
The filter screen in the carb's inlet is more likely to be found in an auto parts store, as rochesters used 'em for years,...
Just be forewarned, there are different lengths,....

When ya pull the canister filter, pour it into a clear container to Inspect it's contents,....
I pulled the spin on filter and drained it into a jug. Had a lot of particulates in it and the gas was green which I thought was odd. Didn't seem like it had water in it but the filter definetly was full of junk. Haven't had a chance to pull the carb one yet.
 

nportwine

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Ayuh,.... There's probably more of the same in the tank,....

Any thought as to why the gas would be green? The guy who my grandpa sold it to said he ran like 100 octane fuel in it or something would that be dyed green? Going to change both filters as well hopefully that'll solve the issue. But I have ran probably close to 40 gal of new gas through it already so I don't think the condition of the fuel itself would be a factor at this point
 

kenny nunez

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Av gas is colored as well as racing gas, some fuel additives will also change the color if you put enough in it. 100 octane is actually "over octaining" the fuel. The compression ratio on this engine does not need it and it will perform better with 91-93 non ethanol.
 

nportwine

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Never did get this problem figured out yet, changed fuel filter again and tried taking gas cap off to see if it was a tank vent issue. Was going to order carb filter but not sure what one to order. It also does seem to be a little overfilled on oil after it sits for awhile, may change it as well then try it this weekend. Would like to get it figured out
 

nportwine

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I pulled the inline filter and this is what it looked like? Could be the issue?
 

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Lou C

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Are you using e10 gas?
which carb does it have, the Rochester 2 bbl or 4 bbl?
 

nportwine

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Are you using e10 gas?
which carb does it have, the Rochester 2 bbl or 4 bbl?

Yes, Its running normal 87 octane e10 gas, my grandfather ran it in it for years before I bought it. It normally gets a bottle of seafoam every tank as well. And it is a Rochester Quadrajet 4bbl
 

Lou C

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Well I see you have a pertronix electronic ignition system so normally I'd say look at your points, but in your case that would be pointless lol. So you might check the total timing advance before tearing in to the Q-Jet. I have one also, and I decided to try a new Holley 4bbl instead of rebuilding this 30 year old carb again, but I still may get it done, because changing over to a different carb, brings with it detail issues (fuel line, throttle linkage etc). In the case of carbs, there is just so much more local support for Holleys than for the old Q-Jet.

Your timing should be for the 5.0:
10* BTDC
centrifugal advance at 3200 rpm = 12* so your total should be 22* at 3200 rpm. From my '88 OMC Shop Manual.
 

pantaloonz

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Hi There!

Well I've been a way for a while but I had some similar throttling issues on my 1992 OMC 5.0 L.

I went through a lot of troubleshooting down the path your going. Mine was sporadic and "surge like sometimes" as you described.

it got a little better as I checked inlets and hoses and such.

Strangely enough, as it turns out.. my air filter was caked.. It never even dawned on me to look at that..

Pressure washed it, cleaned it with carb cleaner.. ran great.

Worth a quick check . :)

-Pantz
 

Bondo

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Yes, Its running normal 87 octane e10 gas, my grandfather ran it in it for years before I bought it. It normally gets a bottle of seafoam every tank as well. And it is a Rochester Quadrajet 4bbl

Ayuh,..... Yer skirtin' the issue,.... The fuel system, includin' the tank needs to be cleaned out, 'n the carb needs to be Rebuilt,.....

Somebody used the wrong filter in the carb, so I imagine piles of junk passed it by, 'n are now inside the carb,,...
 

Lou C

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while the Qjet does have a filter like that in the inlet most OMCs I have seen have the normal cannister style water separating fuel filter which should be filtering out the crud. Does your engine have one?
 

nportwine

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Yes ive changed the filter in the inlet too the carb and the spin on filter. I took the cover off the top of the carb to see what happnes when it bogs, and as i push the throlttle down the last 1”, the secondaries open up a tad more thsn they already are and that is when the bog occurs and the rpms go down. Highest i have got the rpms is 4300 but its hard to get it there consistently. Maybe possible that the secondaries are opening up too much, or getting too much air and not enough fuel. Any way i can hook it up to an outboard tank to see if my fuel tank or anti siphon valve is the issue?
 

Lou C

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Yes you can do that, just be careful with the connections and lash it down some way. I'm not sure what rpm the secondaries should open at but at that throttle opening, they should be open, keep in mind the throttle blades are mechanical but the air doors above them are vacuum operated. There is an adjustment, you'd have to look it up....
 

nportwine

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I checked the timing yesterday and it was good, took the boat out and got it up to 4300 rpm again, but after 45 minutes of pulling tubes, it progressively got worse until it was tough to even plane it out. After sitting for an hour, it ran good again on the way back to the launch. My buddy thinks it’s the fuel pump but I hate to go replacing things that may not be the issue.
 
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