Totally stumped. Next stop is a real mechanic

[eavega]

Hello all

I have a 1988 Stingray 176 SBV with a 3.0L Mercruiser motor (same vintage; 1988). There was nothing obviously wrong with the motor last year. I did run it on the lake a couple of times before it got too cold to go out. Over the winter I changed over the points and condensers to a Pertronix electronic ignition. when I dewinterized two weeks ago, I initially had problems getting the motor to start. Found that I had the 3 and 4 wires crossed up, so I corrected that, and the motor seemed to be running smoothly on muffs (again, two weeks ago). I tightened the belt (it was loose and would squeal when starting up), and then it started smoothly. I rolled it back out today to try and set the idle speed and the idle mix. I set the screws to the initial settings (1 1/4 turn out from seat for the idle mix screw, and 2 turns out from contact for the idle speed screw), and tried to start the engine. The motor started, but was shaking like there was no tomorrow. It would also not idle down below about 1100 RPM by my optical tachometer, so I couldn't even set the idle speed screw correctly. I couldn't get the motor to idle smoothly. If I were working on a two-stroke, I would say that its not firing on a cylinder. I verified spark (good, blue spark) and compression (125 even across all 4 cylinders). I did a cylinder drop test but didn't notice any change pulling a single spark plug boot at a time. I figured that the carburetor is either delivering fuel or not for all four cylinders. Since the motor is running then fuel is not an issue. The symptoms I can see are as follows; won't idle smoothly (shaking). Blowing a lot of black smoke out the hub exhaust. The plugs look black and sooty (not wet) on all four cylinders. Won't idle down below about 1100 RPM. What I have done: I replaced the ignition with the Pertronix Thunderbolt II ignition module and Ignitor II coil. I had the motor running smoothly a couple of weeks ago, but idling high. The whole point of what I was trying to do today was to set the idle speed and idle timing. I did the initial settings for idle mix and idle speed, but again, I had to have the motor throttled up in order to keep it running, and the running was not smooth.

I am about at the end of my knowledge of this mercruiser motor, and am ready to take it to an expert to get it running right. Any help would be greatly appreciated, even if it is to recommend a real mechanic in the north Georgia area!

Rgds

 

[xjdriver]

Re: Totally stumped. Next stop is a real mechanic

Sounds like your carb float is stuck, try tapping on the float bowl with the handle of a screwdriver or something similar. Your carb is giving the motor to much fuel. May need to take your carb apart and clean it.

 

[achris]

Re: Totally stumped. Next stop is a real mechanic

Having verified the ignition system and mechanical state of the engine, then the only thing left is the fuel system. Since you have soot and black smoke we can assume it's getting too much fuel, so clogged filters and pick-ups are not going to be the issue.

What are we left with? Check the choke is opening fully, and the float is not stuck down, or sunk/flooded. I would also put all the screws back to where they were before you started moving them. Initial setting is only really used after a full strip and clean out, just to get the engine running so they can be adjusted more accurately.. If the engine is running, work with the screws from there.

Chris........

 

[Don S]

Re: Totally stumped. Next stop is a real mechanic

Sounds to me like you have the timing way way off. Even a 1000 or 1200 rpm, start getting the timing close to specs and adjust the idle speed and mixture each step till you get them all right.

 

[eavega]

Re: Totally stumped. Next stop is a real mechanic

So, you suggest I should set the timing at 6 deg at the current speed, then adjust the speed and re-adjust the timing until it all comes into spec? Or should I be setting the timing higher and dial down the speed and re-adjust the timing when I get the speed right?

Also, since I switched over to a more powerful coil (supposedly 45K volts vs the stock 30K volts *I think* coil), did I need to change the plugs or the plug gaps? I would think not, but I am wondering if more powerful coil would require a different gap or a hotter plug in order to keep the timing where it needs to be. If anything, I am wondering if somehow the spark is weak, and that might also be causing my problems.


Rgds

 

[matt167]

Re: Totally stumped. Next stop is a real mechanic

Electricity only flows what it needs, so with the hotter coil, you could open up the gap a little, but if you don't it will just use the spark it needs to jump the gap. Unless you bumped your distributor while changing the coil, you did not effect timing with that upgrade/ repair. However, I would agree. it sounds like timing should be checked, and with the petronix kit, even in the directions they say to recheck timing because the trigger position is sometimes different on the distributor than the points were, which effects the timing.

 

[Don S]

Re: Totally stumped. Next stop is a real mechanic

So, you suggest I should set the timing at 6 deg at the current speed, then adjust the speed and re-adjust the timing until it all comes into spec? Or should I be setting the timing higher and dial down the speed and re-adjust the timing when I get the speed right?

I don't know what your speed is right now, or the timing, but you have to adjust everything in steps so it will keep running until you get it all right. Obviously you can't get the rpm down to idle the way it is, so get the idle as low as possible and see where the timing is and bring it down. If you are over a thousand rpm, you have advance kicking in so you have to get it all adjusted at the same times.

Spark plugs will stay the same gap.

 

[Silvertip]

Re: Totally stumped. Next stop is a real mechanic

Mess with a distributor and restting timing is mandatory. Black exhaust and sooty plugs is a clear indicator of a carb problem. Choke stuck shut and/or float/needle & seat issue are the likely suspects.

 

[eavega]

Re: Totally stumped. Next stop is a real mechanic

Okay, so the low hanging fruit is the choke. Is there a way to bench test the choke without running the engine? I opened up the cover plate and see that the electric choke is a spring that will expand and allow the choke butterflys to close. I wanted to eliminate it as the culprit of the rich running, but don't now how.

Next step is to start tearing into the carbs.

Thanks for any further information.

Rgds

 

[xjdriver]

Re: Totally stumped. Next stop is a real mechanic

Hook a jumper wire from the positive battery terminal to the electric choke, the butterfly should open within about a minute or so. Im still betting this is a stuck float. When you take the carb apart make sure the float and needle valve move freely. Oh, and make sure the choke is getting 12v when motor is running. Dont forget to time it once you get rid of the rich condition.

 

[achris]

Re: Totally stumped. Next stop is a real mechanic

If you suspect a stuck float be very careful when removing the carb from the engine and transporting it to the bench. You want to see the problem, so don't make any violent movements of the carb or tip it too much. When you remove the screws holding the top section on and start to lift the top off the body, also be gentle, the float is attached to the top cover. Again, when you look at it, try holding it high and looking up, so you don't have to tip the cover over.

Good luck.

Chris.....

 

[eavega]

Re: Totally stumped. Next stop is a real mechanic

So, will I need to replace gaskets if I remove and disassemble the carbs for inspection? If that is the case, I should just tackle a full-on rebuild of the carb? I know the carburetor was serviced last year in March when the previous owner de-winterized the boat.

Rgds

 

[xjdriver]

Re: Totally stumped. Next stop is a real mechanic

As long as you dont rip the gaskets you should be ok, but i know others will disagree. As far as rebuilding, I wouldn't, just give it a good cleaning while you have it apart.

 

[achris]

Re: Totally stumped. Next stop is a real mechanic

As long as you dont rip the gaskets you should be ok, but i know others will disagree. As far as rebuilding, I wouldn't, just give it a good cleaning while you have it apart.

I agree. I have re-used carb gaskets quite a few times, as long as you inspect them and make sure they are in good condition, nothing bad will happen....

 

[fat fanny]

Re: Totally stumped. Next stop is a real mechanic

A good tip is to coat them really good with vasoline on both sides has always worked in my favor!

 

[achris]

Re: Totally stumped. Next stop is a real mechanic

A good tip is to coat them really good with vasoline on both sides has always worked in my favor!

Not a great fan of greasing/vaselining any gaskets unless instructed to by the GENUINE manual. And they usually say to use a drying type gasket sealer, like Perfect Seal. Grease/vaseline makes them slippery and on tightening they have a tendency to slide out of position. Seen it a lot of times. Usually on leg gaskets, the very thin section on the starboard side by the shift slide.... Gasket slides out, slide cavity fills with water.... Not good.

Chris.....

 

[eavega]

Re: Totally stumped. Next stop is a real mechanic

Okay, its not the choke. That is working as expected. A question about the idle speed screw, it is supposed to hold the choke in position when the shifter is in the neutral position? It seems to me that by setting the idle speed screw in the initial position (two turns after contact) it will hold the choke butterfly in whatever position it finds iteslf in until the shift/throttle is moved into gear, or at least out of neutral in the case that you disengage the shifter with the button at the base of the shift/throttle.

I'll tear into the carbs tomorrow to see what I might find there. Thanks again to all of you for all your help.

Rgds

 

[GA_Boater]

Re: Totally stumped. Next stop is a real mechanic

The choke and gear position are totally unrelated. Choke and throttle are. When the motor is cold, advancing the throttle from idle (or neutral if the throttle is not disengaged) to full throttle and back to idle, will set (close) the choke. You may have a fast idle cam which cracks the butterfly open a bit for a higher idle until the choke opens and the throttle is opened. My brain might be crossing over into cars on the fast idle bit.

Without having the carb in front of me, can't answer about the operation of the idle speed screw.

 

[xjdriver]

Re: Totally stumped. Next stop is a real mechanic

Yep, thats the fast idle cam, it comes in contact with the idle speed screw when the choke is closed, it will hold the chokes position untill throttle is advanced. Normal. I've worked on one of those carbs before, pretty basic and simple to work on. Remember to set idle speed when motor is fully warmed up.

 

[eavega]

Re: Totally stumped. Next stop is a real mechanic

When the motor is cold, advancing the throttle from idle (or neutral if the throttle is not disengaged) to full throttle and back to idle, will set (close) the choke.

Its a parlor trick! Advancing the throttle with the motor cold simply moves the idle speed screw off of the cam which attaches to the choke, allowing the spring in the electric choke to close the choke butterfly. Doesn't matter what you do with the throttle after that, the spring will hold the choke shut until the spring warms up, expands, and moves to open the choke. Moving the throttle to full advance then back will work the accelerator pump and squirt fuel into the carb.