Engine starting and Idle Problem

[Rhino313]

I have a 99 Mercury XR6 150hp that is giving me fits on engine start/idle. It seems to have an intermittent problem with running smoothly when the throttle is in the neutral position. I say intermittent because two weeks ago I took the boat out, it started right up and ran smooth as silk on the water.

Here's the problem. Before taking the boat out today I decided to start the engine on the muffs. The engine cranked strong, but would not start with throttle straight up. I had to advance the throttle well forward to get the engine to catch and start. After warming up a bit, I tired to pull the throttle back to neutral. As soon as the RPM slowed below about 1000 RPM the engine started running rough and appeared to have a misfire (small belches of smoke out of prop as well). If the throttle was pulled all the way to neutral the engine would die. The only way to keep it running was to keep the throttle forward and keep RPMs at about 1000. I tried moving the fule line to a fresh gas container and had the same problem. Engine seems to run find as long as the throttle is pushed up.

About three weeks ago I had similar problems and I wen t through the electrical system as per the shop manual and did not detect any issues. This included voltage checks and spark checks and anything else I had easy access to. I also checked compression and all cylinders appeared strong and consistent. I also decided to change the primer bulb and low and behold, that was when the engine began behaving properly and I took it out on the lake as indicated above.

Seems to me I am having a fuel/air issue of some kind.

Any thoughts on where to start the search? I have tried looking through the engine manual to figure out where to focus, but I am coming up blank. I'm thinking maybe a fuel pump issue or - maybe obviously - a carb problem.

Any help would be appreciated. It has already been appreciated before when I have looked through the site to solve other boat issues.

Thanks in advance
Rob

 

[Texasmark]

Is that engine the EFI or carbed? I asked that question and while my answer was loading I see you said "it may be a carb problem". You are running lean. Adequate fuel not getting to the fuel air mix for proper combustion. Since the engine runs fine at higher rpms, I wouldn't expect a fuel line crack, filter clogged with pieces of fuel line, or hole in the fuel pump sort of thing. I'd go after the carbs and give them a good cleaning and a kit with fresh parts and setup your link and sync. Ensure that your carb work included a good blasting of compressed air directed at anything with a hole in it. Also a piece of wire run through tubes and flushed with cleaner and air is in order. Get that done and setup the low speed jets and 1.5 turns open for starters. Come back with the results......results mean an onwater test (not muffs) and hole shot data especially.

 

[Rhino313]

Texasmark

Thank you for the insight. I have already changed out the fuel lines and replaced the fuel filter. I was able to borrow a fuel pressure tester from afriend and was going to try that later this week although I see a fuel pump diaphragm kit is really not that expensive if I decide to replace the guts of the fuel pump. I've had the boat for about 4 years and I am unsure of the fuel pump maintenance history although I will say the bolts holding it on look like it's never been worked on.

The carbs were my next thought, but I was hoping to not have to pull all three of them off. But I guess you do what you gotta do. It may be a few days before I get to try all this out as it is now the work week.

Thanks again for your thoughts and I will get back with results when I have them.

 

[Texasmark]

Texasmark

Thank you for the insight. I have already changed out the fuel lines and replaced the fuel filter. I was able to borrow a fuel pressure tester from afriend and was going to try that later this week although I see a fuel pump diaphragm kit is really not that expensive if I decide to replace the guts of the fuel pump. I've had the boat for about 4 years and I am unsure of the fuel pump maintenance history although I will say the bolts holding it on look like it's never been worked on.

The carbs were my next thought, but I was hoping to not have to pull all three of them off. But I guess you do what you gotta do. It may be a few days before I get to try all this out as it is now the work week.

Thanks again for your thoughts and I will get back with results when I have them.

I put an electric fuel pump on a 115 tower due to problems with fuel line routing in the boat. I bought a variable and as I recall the pressure that worked best was 3 psig.

 

[Rhino313]

Well I had a partial success this morning. I rebuilt the fuel pump last night and put it back on the boat this morning ON the muffs the boat started on the first crank and ran very smoothly without the misfiring I was having last week. The tach on the panel shows the idle is running high at about 1100 rpm. I' see what I need to do get the idle back down. Can't take it to the lake today as my wife took the truck to horse show and I got no way to get to the lake.

Fingers crossed we're on the right track.

 

[Rhino313]

It was crazy October - Finally got the oat in the water yesterday. IN the last month I performed the following actions:

1. Removed and cleaned the carbs - they were incredibly clean. Only observable blockage was some dirt in the top carb backdraft vent; I sprayed seafoam, carb cleaner, and compressed air through all passages
2. Noticed that the #2 and #3 float bowls were both slightly below level when performing float check; I adjusted both floats to level.
3. Noticed that the air screws were all adjusted differently from about 1.25 to 2 turns. I set all the air screws at 1.5 turns.
4. While carbs were off I checked the reed valves. Three of the six reed blocks had reeds with gaps out of tolerance. I replaced the reeds with Carlson after market reeds.
5. Reassembled all components and then performed link/synch. Noticed the oiler connection was significantly misaligned; maybe this is just part of the synch procedure
6. Put the muffs on the engine and started it. It started on the first crank. Engine idle was little high at about 1000 RPM. Engine seemed to be running smooth other wise.
7. Took boat to lake and put in water. Engine was hard to start even though it started right up that morning on the muffs. Had to advance the throttle forward to get engine to start and then engine would stall when throttle moved back to idle. I watched the RPMs drop as I was pulling the throttle back.
8. I adjusted the idle screw to get higher idle speed. This allowed the engine to continue running when throttle at idle. The engine was now idling around 700 RPM.
9. Motored around at idle. The engine for the most part was pretty smooth although there may have been a little roughness in the idle (air screw setting?)
10. Went to WOT and the engine accelerated very quickly. The boat planed out as expected.


Bottom line - it looks like the engine is about where it needs to be. There is a little roughness that may be associated with the air screw, but I'm not sure how one would adjust 6 of them at once.

I do have a question out of curiosity. Why does the engine appear to idle lower in the water than when on the muffs? Is this an artifact of the muffs putting less drag on turning the water pump? Just curious.

Texasmark - thank you for the earlier advice. I think you helped me get on the path to recovery here.

 

[DavidMoore]

In the water you have back pressure on the exhaust, on muffs you don't.
It's why you do the adjustments in the water.

Idling @ 700 rpm in water sounds good, is it in neutral on in fwd gear? Idle speed is set with boat in water and engine in fwd gear, so if it is 700 rpm in neutral, then you could probably bump it up a bit to get a smoother idle.

 

[Rhino313]

Thank you David - Your comment about back pressure on the exhaust makes a lot of sense. The engine was not in gear when i set the final idle at the lake.

Darn - looks like there's a reason to take the boat back out to the lake this weekend.