1985 GT150 smoking like a banshee

[NoZero]

Hey all:
1985 GT150
Model: J150STLCOH
Serial:J6131805

Main symptoms: Excessive white smoke. slight sneeze at idle, bogs (with surging) starting at 5,200rpm.

Compression is on the lower end of the scale - but was even until this year. The symptoms have not changed from last year, so I don't think compression is the cause.

Starts like a champ. Idle is ~800rpm in the water, in gear, at idle throttle.

Smokes like a heck when started and when idling first off. If you come off a run at 5,000rpm, it smoke at all while idling into the marina.

Sometimes, after idling, it won't get rpms up to get on plane - (it will rev in neutral, and haven't tried choking it yet). It will eventually "clear" and run great up to ~5,200rpm.

It bogs after 5200rpm - MOST of the time. Occasionally, it will "catch" and fly to 5500rpm. It sounds like fuel starvation and it surges some in that range - so I don't really try to run up there - but it also makes me wary of timing.

In general, low end and Mid-range response is phenomenal.. torque for days.

The bulb: replaced last year - did not clear symptoms up. I can pump it up hard, but it always, always lets me pump just a tiny bit 1 more time... each time I hear the tiny bit of fuel going somewhere, but I don't see it at any fuel line connections. [all fuel line from the tank to the engine has been replaced] I replaced the fuel pump last year with the recommended pump from MasterTech Marine. - fuel is mixed 50:1 in the tank (was that way before I bought it 4 years ago)

Carbs: Carbs were rebuilt last year at a shop. Full kit was given to the mech. I hope he used it all. After he finished, it had the same symptom as before - in that it smoked and started to bog at 5200rpm (it surges a bit).


I de-carbed again yesterday, and changed the fuel filter.


I have an original repair manual that should be here this weekend. I have leak-down gauge, fuel pressure gauge, and carb sync gauges available.


I have (2) check valves on the way (all 6 could be bad for all I know) - but looking at it, it looks like I need to order carb gaskets and pull the throttle bodies to get to all of the check valves.


thankful for any help or guidance. - all ears

[billnorman1]

How about the fuel pump?

[NoZero]

How about the fuel pump?

Replaced last year with

# 06-200

at the bottom of this page.

- checking fuel vacuum pressure is on my list.

[omcforever]

What are the actual compression readings and what do you mean they where ok up til this year? What changed or is different? Recirculation valves can be changed without removing carbs but removal is so much easier. Why where the 2 being replaced? The manual will give exact details on how to service and check them.

[NoZero]

When I bought the boat 4 years ago, compression was 80psi dead even across all 6. The boat ran great - except, it had a little surging and bogging after 5,200rpm. (this engine HAS been rebuilt - not sure how long ago, but I've had 2 mechanics in the last 4 years tell me whomever did it, did a great job) The bogging and surging was why I replaced all fuel lines from tank to engine - checked the check-valve at the tank, and had the carburetors rebuilt. - didn't fix it.

After I bought the boat, when I de-carbed and all compression came up to 90psi even on the nose across all 6. Just before I de-carbed yesterday, 2 cylinders were at 82 and 84psi (1& 3, respectively). - the rest at 89 and 90. This is all on a cold engine. There IS some scoring in all cylinder walls - don't know if it's been there since I got it or if it is new. The heads of the pistons look pristine.

Check valves. I ordered (2) before I started digging in to them - so if one or two were bad, I wouldn't have to wait. That's it. I'm waiting on the repair manual to dig in to them.

[omcforever]

Since 1 and 3 are together and they are low,may have a bad head gasket or 2. Are those 2 spark plugs cleaner than the rest? May all be clean since you de-carbed yesterday....what do they look like after it has been run for awhile?

[omcforever]

Recirc valve being bad or clogged can cause bogging at take off and idle issues...has link and sync/timing been checked properly? (Pick up timing and max timing/TDC verified on flywheel per the manual)

[NoZero]

All of the spark plugs look the same. They were new last year. They are all oily and have some carbon on them. I have a new set waiting until I fix whatever issues can be fixed.

Link n' sync was supposedly done by the mechanic that rebuilt the carbs last spring. he came highly recommended, but repeatedly did not meet the time he told me it would take (weeks and weeks), and I finally had to call and let him know I was coming to get it in 2 days. I waited across the street while he finished. - so maybe he muffed it. I don't know.


Once I have the manual, I can check link n' sync and at least reference timing at idle.

[omcforever]

Dont worry about idle timing,,,if link and sync is right, that fixes itself. Worry about pick up timing and max timing. Was the bulb replaced with an OMC bulb or a junk wal-mart/sierra/attwood one?

[NoZero]

I first replaced the bulb with an OEM Johnson one. That one failed in under 1 year. It currently has an Attwood on it - For what it's worth, the fuel issues have been the same throughout ownership between the 3 bulbs that have been on it.

[NoZero]

It is definitely loading up at idle. It flat out will NOT get on plane. IF I run a long run in the 4300rpm range, then I can stop, put it in neutral, wait, and on take-off it jumps onto plane with no issues.

I don't want to change plugs until I get it to stop loading up at idle. Does this sound like a timing thing?

thoughts?

[316jughead]

What spark plug are you running?

[NoZero]

Champion UL77V

I think somehow the timing is off. I don't have a test wheel - shopping for one currently and waiting on that Johnson repair manual to get here.

[billnorman1]

Been along time since I had a similar motor. The older Johnsons had the VRO which always failed to the oil rich situation. Sounds like you two are already way past that but many FP's used crankcase pressure changes to pump and if the primary diaphragm failed they could suck oil right mightily.

[316jughead]

Champion UL77V

I think somehow the timing is off. I don't have a test wheel - shopping for one currently and waiting on that Johnson repair manual to get here.

I know that is the spark plug the OMC chart lists for your motor. We learned a long time ago to use Champion L77JC4 or QL77JC4 gap at .030, in these motors. I used this plug in all my customers motors and I ran this plug in all of my motors. You will solve most of your problems using this plug. The surface gap doesn't work good if you idle a lot, and not even at wide open throttle.

[omcforever]

I know that is the spark plug the OMC chart lists for your motor. We learned a long time ago to use Champion L77JC4 or QL77JC4 gap at .030, in these motors. I used this plug in all my customers motors and I ran this plug in all of my motors. You will solve most of your problems using this plug. The surface gap doesn't work good if you idle a lot, and not even at wide open throttle.

[NoZero]

Thanks, guys. I'll pick up some L77JC4s and gap them. I am going to try Reeves method for timing. Sticker on my airbox says 34 BTDC.

Also - with a buddy in the boat today able to put an ear to what I'm hearing... we both think it starts missing above 4500-4800rpm. It doesn't sound or feel like fuel starvation... it sounds and feels like it is missing.


It is ultra smooth in the mid range and has a lot of torque. - I'll address Timing first, then link n' sync.


Would a power pack cause only missing in the higher rpm range?

thanks-

[316jughead]

The max timing should be set at 30 degrees BTDC. We don't have the good fuel today, also when this motor was new my dyno testing showed no increase in power at 34 degrees BTDC. The miss you have is probably spark plugs. Yes the power pack could cross fire and cause a miss and loss of power.

[316jughead]

Here is some help on your max timing. This is for an earlier model but you have the exact same ignition system.img051.jpg omcforever recommends less than 30 degrees BTDC. I know you will not notice any difference in top end at 28 Degrees BTDC.

img051.jpg



Sorry about the double thing.

[ChampioNman]

If you don't figure it out swing by Riverside Marine in Old Hickory,