Engine bogs at throttle up and cuts out at high speed.

[bigtexan99]

Two new issues have developed with my 2003 Sportster LE (carb) boat. It did not run much in the last year. Just finished a driveline rebuild on it.

It has new gas, and is pre-mixed, the oil pump was removed a few years ago. I cleaned and reinstalled the raves. Spark plugs look okay, maybe a little oily but the engine starts up very easy. Fuel filter was removed and cleaned, was not terribly dirty.

Issue number 1: Ever since I got the boat, it randomly cuts out at high speed. I can be cruising along at at 5-6k rpms and then it just shuts off like if the kill switch was pressed. Originally, it was pretty uncommon, I might drive continuously for 15 mins at 85% throttle and then for no apparent reason it cuts out. Recently, the cut outs have been happening much more frequently, and today it was only able to run for about 15 seconds before cutting out. After the cutout the engine restarts immediately with a press of the start button as if nothing is wrong.

Issue number 2: Engine bogs and dies when trying to accelerate from idle to full speed. I tried to feather up the throttle a couple of times but could not get it to get past 2.5rpm, it stumbles bogs and dies. I did find that while idling if I pull the choke full open for a half second and release while simultaneously advancing the throttle I can overcome the bogging and the engine revs up to speed but then Issue Number 1 takes over and it cuts out around 5500 rpm.

I've attached a video of the throttle up and engine cut out, but its not the best video. https://drive.google.com/file/d/0B3YepRZPJ3nLT2RtTThMdnIza3M/view?usp=sharing


Help!

 

[howie099]

Did you clean the carb and replace rubber parts as need be? You could have a loose ground cable. You could have a failing DESS post also. I am going to the cleaning of the carb first..

 

[Sportster-2001-951C-Stock]

You most likely have two problems:

1) Lanyard post is corroded internally and needs to be replaced. Moisture penetration tends to accelerate corrosion of the internal magnetic switch terminals.

2) Bogging on acceleration (I call this lean hesitation) is caused by lack of fuel and is easily identifiable by the engine falling on it's face and suddenly quitting due to running out of fuel. Pulling the choke very briefly at just the right instant, or "nursing" the throttle can often overcome this symptom but it's not a solution of course, just a mechanism to confirm lack of fuel is the likely culprit.

In your case there may be an air leak somewhere in the fuel line tubing that carries fuel from the fuel tank to the fuel pump mounted on the carburetor OR, the carburetors themselves are most likely in need of overhaul due to internal blockage of the small fuel filters inside the carbs and/or fuel passages.

So you can start by leak checking the fuel lines, assuming you haven't already. A quick and dirty way to do that is by disconnecting the fuel line from the carburetor and blowing backwards into the fuel tank using low pressure air (less than 5psi is good) and listening for the air bubbling up through the fuel pickup in the bottom of the tank.

BE SURE while pressurizing the fuel tank and lines, to remove or loosen the fuel cap else you might find after the test pressure within will cause fuel to shoot out of the disconnected fuel tubing.

I suppose you realize, running lean will quickly cause damage to the pistons and crank bearings of any 2-stroke engine.

The spark plugs center insulator should be a cardboard brown color, indicative of an acceptable fuel mixture, not aluminum(piston) grey. Here are a couple photos I took of mine recently.... The brown color indicates plenty of fuel is available during the combustion stroke, black insulators would indicate fuel soot. The brown indicates the insulator isn't overheating, that is not hot enough to completely vaporize the oil film off the insulator. it takes about 1/2 hour of normal running for a new plug color to stabilize, they start off yellow after the 1st 16 minutes or so then darken to brown. The brown color is actually from 2-stroke oil deposits.

Aside: When (not if) you eventually rebuild your engine, pay attention to the volume of oil puddles lying in the bottom of the crankcase can tell you if your pre-mix is too lean of oil. You want to confirm the puddle is enough to wet the crank wheels.

 

[bigtexan99]

Thanks for the feedback!

1. Is replacing the DESS post a matter of just splicing in the wires for the new post or do I have to get it programmed for the key?

2. I agree it sounds like a fuel issue, def will get the carbs rebuilt. Especially the bogg from idle to full speed. Maybe the accelleration pump is gummed up?

Is it possible the cut out at full speed after a few minutes is fuel related or is the consensus its the DESS post?

Update: I found this section in the shop manual on advanced diagnostics for the DESS post. I first cleaned the post and key with warm soapy water, then scraped the metal ring and contact pieces with some wet sandpaper and then scratched it shiny with a metal dental tool.

Then I ran the diagnostics. To my surprise, the system threw the 2 short beep error condition. I removed the key and tried the test again, this time there was no error condition and the engine started.

I repeated the test several times and sometimes the engine would start with no error condition and sometimes the system would throw the 2 short beep diagnostic message.

Thoughts?

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(from the shop manual)


ADVANCED DESS DIAGNOSTIC
The mode must be manually activated. Proceed as explained below:

1. Remove safety lanyard from boat DESS post and set shifter in NEUTRAL.

2. Press 5 times on the boat starting button (any on twin-engine models). • 1 short beep and 1 long beep must be heard. They validate beginning of diagnostic mode.

3. Install safety lanyard on boat DESS post.

4. Press the boat starting button again.

If everything is correct, engine will start. Otherwise, refer to the following chart.

NOTES: • If you need to listen again the coded beeps, remove safety lanyard and repeat the procedure to activate the diagnostic mode. • If many problems are encountered, the MPEM will send only one error code. When the problem is solved, the MPEM will send a second code and so on until there is no problem.

ADVANCED DIAGNOSTIC MODE NUMBER OF BEEPS CAUSE REMEDY

No beep • Engine actually starts. • Everything is correct.

1 long and 1 short • No safety lanyard has ever been programmed in boat MPEM. • Use programmer and program a safety lanyard. This code can occur only when you receive a new MPEM from the factory and no safety lanyard has ever been programmed.

2 shorts • MPEM cannot read the electronic circuit in safety lanyard or the magnet is defective in safety lanyard. • Check safety lanyard condition with the MPEM programmer. Replace safety lanyard if reported defective. • Mixed wires at switch connectors or wrong connections. • Check DESS post wiring harness.

2 longs • Wrong safety lanyard or wrong connection of the DESS wires. • Use the safety lanyard that has been programmed for the boat. If it does not work, check safety lanyard condition with the MPEM programmer. Replace safety lanyard if reported defective. Check wiring harness connections.

3 shorts • Wiring harness of DESS post is grounded or there is a short circuit. • Check wiring harness and DESS post.

 

[bigtexan99]

Update:

1. I got the new DESS post in today and installed it.


2. Took the carbs off the boat and the accelerator pump nozzles were clogged shut. Applied carb cleaner and compressed air and now they are flowing. Re-installed carbs, started and sounds good. I can now visually see the squirt from the accelerator nozzles when throttling up.


I'll be hitting the lake tomorrow to see if either or both issues are resolved.

 

[bigtexan99]

Update: FAIL.

1. DESS post replacement made no difference in the cutouts at high speed...usually above 5500 rpm, the engine just shuts down as if the kill switch was pressed. Restarts instantly without having to remove/replace the lanyard on the post.

2. Engine is still boggy from idle to full speed. I visually verified that the accelerator nozzles are spraying fuel when throttling up.


Ideas? Suggestions????

 

[Mike99]

When you had he carbs apart, how did the internal filters look?

Mike

 

[howie099]

The accel pump just gives you that shot of gas to stop the stumble the first time you hit the controls. If the carbs were fully lean and whole pull system looked good maybe you have a week magnet in your DESS key.

 

[bigtexan99]

Taking the carbs off the engine and cleaning out those nozzles is all I did. In retrospect, I should have at least checked the internal fuel filters, but I did not want to open it as I don't have a rebuild kit on hand and with my luck I would have torn a ring or diaphragm.

I guess I'll order 2 rebuild OEM mikuni kits.

 

[bigtexan99]

maybe you have a week magnet in your DESS key.

Is there a test I can do?

Even with the new dess post, when I tried the advanced diagnostics again the system still threw 2 short beeps indicating an error condition. But it starts just fine, I don't get why it says there is an error but it still starts.

 

[howie099]

Usually the diaphragm doesn't come a part. It is the internal pumps gaskets that usually mess up when you take a part. I don't think there really is a test for magnet in the key. I am guessing you only have one key for the boat.

 

[Sportster-2001-951C-Stock]

The DESS key magnet in mine is pretty stout, can't lift a screwdriver but you can definitely feel the magnet with a screwdriver.

I think while the carbs were off, you should've carefully disassembled the covers on bothe sides and cleaned them out, remove the mixture adjuster screws also, so you can shoot carb cleaner down through the passages and get the junk out of there. 2 cans of carburetor cleaner will be more than enough.

Beware, these carbs being made of aluminum sometimes dont respond to a gentle cleaning of those passages, the three pilot holes corrode inside and close up slightly so it takes some persuasion to get them totally clean. Mine were still a bit restricted after cleaning them tice, compressed air 120psi and all.

What worked for me was increasing the pilot jet orifice size by two numbers larger. I didn't try using phosphoric acid to clean them out although I think that might've worked without changing the jets.

The Mikuni owners manual is a document you might want to review, beware of aftermarket parts (use OEM mikuni parts) and no need to replace anything that isn't obviously worn or damaged (I think new needle and seats are a good idea, the needle/seat size for 951 engine is 2.0).

Keep your original factory metering springs, don't change them to the wrong ones that often come in the kit.

On the 851, the pilot bypass circuit has a large effect up to 70% throttle, so any lack of fuel from this circuit you'll notice lean hesitation from off-idle to way up past 5,000 RPM.

I'd bet the small fuel filters in the fuel inlet side of the carbs are full of debris, especially the PTO (rear) carb. But don't stop there, make sure while they're off to clean the metering side as well.

And pressure leak check your fuel lines, to be sure!!!!
http://www.mikuni.com/pdf/sbn_manual.pdf

 

[Sportster-2001-951C-Stock]

Oh, and new fuel lines are a must if you still have the grey ones made by Tempo, those will fill your carbs with junk as they corrode internally and decay.

The test for the metering diaphragms is they shouldn't have any pinholes in them (hold up to bright light) and should be nearly as soft as rose petals (not stiff, or they'll oil-can).

Carb cleaner will eat many rubber parts and definitely ruin a metering diaphragm, so don't hose them down with anything other than diesel fuel or gasoline, compressed air.

The Mikuni original "thimble shaped" filters in mine washed out fine using carb cleaner (acetone) and compressed air with no visible damage.

 

[bigtexan99]

Thanks for all the tips! I'm throwing in the towel and taking it to a pwc mechanic to rebuild the carbs, test the fuel pressure, set the idle, popoff test, etc.

I just don't have enough experience with carbs to know what I'm doing or to be able to spot something that doesn't look right.

At least my driveshaft rebuild went great, no sign of cavitation at all.

 

[Sportster-2001-951C-Stock]

I can be cruising along at at 5-6k rpms and then it just shuts off like if the kill switch was pressed. Originally, it was pretty uncommon, I might drive continuously for 15 mins at 85% throttle and then for no apparent reason it cuts out.

it cuts out around 5500 rpm.

My thoughts, FWIW...

5500 RPM and less, is still dependent on the low speed (pilot bypass) circuit. (less than 70% throttle)

I recommend not driving continuously at 85% throttle, I feel the 951 isn't robust enough for that. Short blasts even full throttle for a minute or two are fine but prolonged 85% throttle this is pushing it IMO.

I cruise my boat around 5200RPM, about 1/2 throttle to 5800RPM or so, mostly around 5200 just above where the RAVES are open constantly.

If you need more speed, something more robust is in the cards IMO.

Maybe you're forced to run 85% throttle just to keep it running on the large venturi b/c the pilot circuit isn't adequately providing fuel (lean hesitation, engine falling on it's face and quitting).

With a 2-stroke, we don't try running them lean as possible, we try to run them rich. That is, rich enough we don't foul plugs and the throttle response is crisp. Go rich until some 4-stroking occurs then go back lean just enough for that to clean up.

When my engine is fairly cold it tends to 4-stroke a tiny bit off idle 2,000RPM up to about 3,000RPM than cleans out above that. Once well warmed up after a 10 minute run at 1/2 throttle I don't detect the 4-stroking under 3,000RPM

 

[Sportster-2001-951C-Stock]

Thanks for all the tips! I'm throwing in the towel and taking it to a pwc mechanic to rebuild the carbs, test the fuel pressure, set the idle, popoff test, etc.

I just don't have enough experience with carbs to know what I'm doing or to be able to spot something that doesn't look right.

At least my driveshaft rebuild went great, no sign of cavitation at all.

In this case, consider replacing the carbs with new ones. The hard part IMO is taking them off and putting them back on, sounds like you were able to do that already so once you've done it once it gets easier.

Say for instance you took it to a mechanic, would he rebuild your old carbs and maybe still have problems (might happen) or replace with a new set?

I spent a lot of time getting mine right, going to larger jets worked for me. I have no doubt a new set would've been the quick route but I'm not that way b/c I'm used to tuning carbs, it's one of the things I do, being a gear head.

 

[Sportster-2001-951C-Stock]

When I place my lanyard on the DESS post I get the normal two short confirmation beeps telling me it's ready to go. I won't get the two short beeps if the shifter is not in neutral. In fact, if the shifter is not in neutral there aren't any beeps.

Also, in case you weren't aware, if the lanyard is installed on the DESS and the engine isn't running the computer times-out and goes to sleep. It's not necessary to remove and reinstall the lanyard, instead all you have to do is press the start button to awaken the MPEM computer than press on the installed lanyard and you should hear the two short beeps informing you the computer is ready to go.

 

[bigtexan99]

Final Update: All Systems Go!

Picked up the boat from the mechanic today and we went to the lake for a quick test. 10 minutes of perfect running. No hesitation, no cutouts. He rebuilt the carbs and adjusted the idle.

Here is a pic of the old carb parts. On a scale of 1 to 10, with 10 being the worst...how do the old parts rate?

 

[racerxxx]

11.

That's the first thing that should be done on any 2 stroke ski or boat, before hitting the water. Just make sure it fires and sounds good, then complete overhaul/inspection of the fuel and oil injection system. A carb that isn't correctly flowing fuel and nursing cut outs and bogs is the worst thing you can do to these engines. You run the risk of leaning out and seizing the top end.

Sent from my SGH-T889 using Tapatalk

 

[Sportster-2001-951C-Stock]

If you're paying the labor, no reason not to install a full kit. I'm surprised he didn't replace the needle and seat though.