Throwing in the towel

[CobiaXL]

1986 Cobia monte carlo 188xl
3.0 140HP
Stringer 400 with (D3)13x21 prop
1977 gm short block
1988 mercruiser head and intake
1986 orginal rochester 2bbl

I bought this boat 2 yrs ago and have been fighting a lean issue at 3/4 to full throttle.After complete overhaul of fuel system,try 2 differant fuel pumps(Mechanical and electrical)Changed all filters,water separator,replaced screens in pick up,replaced the valve with ball and spring on outlet side of tank,checked to make sure vent was venting,rewired entire ignition,replaced ignition switch,tachometer,even swapped out distributor from another running boat.Changed plugs,wires,coils,points,condensers,ballast resistors,eventually switched to electronic ignition.Still same issue,maybe a touch better...Winter comes around and i find broken piston ring in cyclinder#3,So I decided go with a complete rebuild from oil pan to rocker cover.Rebuilt the carb and didnt find much but none the less its fresh..Guess what,same exact issues....Getting frustrated at this point and ready to throw in the towel and give in.My thinking is if i set throttle not to open past 3/4 throttle,would this be hurting me?A little more then half throttle i'm at 25mph (Gps)and 4000 rpms(New Tach)My boat won't keep plane anything under 3900 rpms, I can run all day at 4000-4100 but anything past it starts popping popcorn out the carb.Any suggestions are greatly appreciated at this point.

 

[Raystownboater]

Re: Throwing in the towel

I ran into almost the same issue and it turned out to be the wrong Prop. I went from a 21 to a 19. I could be wrong but you should be at full throttle at 4200rpm and until I changed the prop, I couldnt go above 3800.

 

[CobiaXL]

Re: Throwing in the towel

seems to me going down on pitch my issues would get worse?

 

[Raystownboater]

Re: Throwing in the towel

How does your boat do out of the hole? Does it come right up and plane out? Mine took a long time to get up on plane and it coughed and sputtered the whole time. Im not saying this is your problem but I had to think "out of the box" to fix it and after trying several props, the 19 was the one that made the boat perform like it was suppose to

 

[CobiaXL]

Re: Throwing in the towel

I ran into almost the same issue and it turned out to be the wrong Prop. I went from a 21 to a 19. I could be wrong but you should be at full throttle at 4200rpm and until I changed the prop, I couldnt go above 3800.

When you say you couldnt go over 3800rpm,was that at wot or before it would start popping?

 

[Raystownboater]

Re: Throwing in the towel

No. It would start popping around half throttle and wouldnt stop until I throttled back to just over idle

 

[CobiaXL]

Re: Throwing in the towel

How does your boat do out of the hole? Does it come right up and plane out?the 19 was the one that made the boat perform like it was suppose to

sluggish i would say,maybe 6-7 seconds.I never was on a boat till i purchased mine,so im not sure how its suppose to act yet.

 

[CobiaXL]

Re: Throwing in the towel

No. It would start popping around half throttle and wouldnt stop until I throttled back to just over idle

Sure does sound similar...

 

[Raystownboater]

Re: Throwing in the towel

You're in Lewisberry? Did you ever take your boat to Long Level? They will let you change props and test them to see what works for your boat, but im thinking most marinas would do the same. It's atleast worth a try.

 

[CobiaXL]

Re: Throwing in the towel

Ya we've been down there quite a few times.I have a buddy who is a member at SYC.We usually launch from there.I'll stop in at long level next time we get down there and hope they have my prop.My outdrive uses a 13 spline and used props are hard to come by,Thanks for the info,I never knew they allowed you to try props before you buy,I should probably be obligated to purchase from the marina as well?Oh how the wife frowns when she sees the marina prices,lol

 

[infideltarget]

Re: Throwing in the towel

Ok...confused a bit here. Help me to understand how a prop change can help a timing issue? If you are backfiring out of the carb, you have too much advance on the timing, as the plugs are firing before the intake valve has time to fully close. That would mean either your timing marks are off (bad balancer), your distributor was put in one (or more) teeth off, you were not truly at TDC when originally timed for base timing, you have too much advance dialed in, your distributor or ignition box is advancing too much, or your cam has too much duration on the intake side for your set up. I think a prop change which is basicaly just bogging the engine and not allowing it to rev too fast, is just masking a timing problem.

 

[CobiaXL]

Re: Throwing in the towel

Ok...confused a bit here. Help me to understand how a prop change can help a timing issue? If you are backfiring out of the carb, you have too much advance on the timing, as the plugs are firing before the intake valve has time to fully close. That would mean either your timing marks are off (bad balancer), your distributor was put in one (or more) teeth off, you were not truly at TDC when originally timed for base timing, you have too much advance dialed in, your distributor or ignition box is advancing too much, or your cam has too much duration on the intake side for your set up. I think a prop change which is basicaly just bogging the engine and not allowing it to rev too fast, is just masking a timing problem.

Dist being off a tooth?I can drop mine in anywhere and spin the base and cap to #1.Ive tried 1 btdc,4 btdc 6 and 8 btdc.
Mechanical advance freely moves and springs were replaced.

 

[infideltarget]

Re: Throwing in the towel

You can (typically) only spin the dist so far around. I have seen in most every OEM manual for any engine I have worked on (car, bike, boat, etc) that you mark where the dist is when you pull it so you can put it back in the same way. I have gotten them off by 1 tooth before and not been able to time the engine for crap...until I pulled the dist and put it back in right AT TDC...not btdc...@TDC with the #1 cyl, and intake valve closed. You may have to base time it with the rocker cover off, so you can actually see when the #1 cyl intake valve closes.

 

[Don S]

Re: Throwing in the towel

Sounds like you have thrown enough parts at it. Might want to try this test and see what you have.

http://forums.iboats.com/showthread.php?t=283269

 

[CobiaXL]

Re: Throwing in the towel

Sounds like you have thrown enough parts at it. Might want to try this test and see what you have.

http://forums.iboats.com/showthread.php?t=283269

Thanks don,I'll do this test soon as the rain stops and i can get out on water.Ill post results....Its real hard to believe its fuel after a complete overhaul of the fuel system but at this point i'm willing to try anything.Thousands spent with no results tends to make for one hell of a parts bin.

P.S. Forgot to add I already did run fuel line straight from 2 different pumps to carb to a can of gas with same results.I will add that My fuel pressure is 5-7lbs till I hit the popping then pressure gets very erratic,jumping back and forth from 3-7 lbs.

 

[Don S]

Re: Throwing in the towel

Rebuilding a carb on a marine engine only has about a 50% chance of working. So just because you rebuilt it, doesn't make it good.

 

[CobiaXL]

Re: Throwing in the towel

Rebuilding a carb on a marine engine only has about a 50% chance of working. So just because you rebuilt it, doesn't make it good.

Ive rebuilt over 200 carbs for race cars, Busch series,late models and street stocks.Im wondering if bowl is not venting properly?Also just discovered from a pm that there is a exhaust shutter in the stringer down deep and the intermediate needs to come out to service it....Could this be defective?

 

[CobiaXL]

Re: Throwing in the towel

Wanted to add that Ive used 2 different rebuilt carbs with same results.

 

[sschefer]

Re: Throwing in the towel

I'd have to go with Don S on this one. A sand hole in the casting is the typical culprit. Over time the fuel flushes a tiny piece of grit out of the hole it was plugging and the carb goes south on you.

The other possibility is in the heads. Weak valve springs or pitted valves/seats will cause these symptoms also. At higher RPM's the valves will start to float and never fully close. In boats, there is a lot more moisture in the air and after shutting down a hot motor, that moisture will settle in the intake and eventually on the valves. If your motor does not have stainless steel valves you could have pitting at the seats.

Valves can be tested with a vacumm gauge although weak springs won't show up until the higher RPM's. If you're looking to rule out possibilites, I'd pull the heads and have them done by a shop that knows the difference between an automotive and marine head. I'd also think about replacing the carb.

 

[Don S]

Re: Throwing in the towel

Have you had the outdrive off ? Looked for restrictions in the exhaust, like broken shutters or something?