Engine high speed RPM problem

[Waterluvr]

All you need to do is place a channel lock on the opposing fuel supply hose to the other carb in order to eliminate that being part of the issue (I don't think it can be but why not seal it anyway just because) and place vacuum to the supply line you want to test for fuel flow. You should be able to draw fuel to it right away, if not your issue has nothing to do with the carbs themselves and you need to find out where the issue is or the boat won't run.

I use a brake bleeder with a hand pump and a recovery cannister to check for these type of issues, a few pumps to introduce vacuum on the fuel line and there should be gas drawn to the recovery cannister pretty quickly once it primes. I would suggest checking it with the fuel selector valve in all three positions to make sure the primary, shut off and reserve functions all work.

If you're not getting fuel that way, stop and remove the selector valve you'll find the problem there unless you have taken the fuel baffle out to service it or the float and the lines weren't put back in the proper location.

I'll be on the lake tomorrow and wanted to leave you some more info to work with if you needed it during the day, hope you get her running let us know how it works out for you.

 

[fzandome]

Great! I got both carbs on and running. So both fuel lines must be connected, or blocked off if you are going to run one carb. Next question. I'm going to do the lake thing to setting the low speed/ idle speed. Should I have the air cleaners on to do this, and if so, how do you adjust the carbs with all the stuff in the way. Also, same question on that high speed run check with the spark plugs..... Seems so me if you don't put the cleaners on until after you do that it will run rich. If they are on how do you adjust them.

 

[Waterluvr]

The arrester has to be on for the venturi's to function properly, I tuned mine by just leaving the air box off and just the arrester housing on with it sitting in place. It's a tight, painful fit to get down there but that's what needs to happen.

 

[Sportster-2001-951C-Stock]

Normally if your engines and air intake system isn't modified the factory settings work great thus go with correct factory settings. In the case of adjusting idle speed, it's trial and error, I shoot for 3000 on trailer and 1500 in water, you may find it necessary to readjust, reassemble, observe result, repeat if necessary (I think you'll be close to perfect 1500RPM in water by adjusting for 3000RPM on trailer).

Edit: Operating at low speeds is not as critical b/c the engine doesn't generate tons of heat at low speed but if possible, don't operate your engine with the airbox removed b/c the venturi's won't operate properly and probably wont run quite right that way, you will be running leaner than normal (caution/warning/danger - potential detonation hazard).

 

[Waterluvr]

Normally if your engines and air intake system isn't modified the factory settings work great thus go with correct factory settings. In the case of adjusting idle speed, it's trial and error, I shoot for 3000 on trailer and 1500 in water, you may find it necessary to readjust, reassemble, observe result, repeat if necessary (I think you'll be close to perfect 1500RPM in water by adjusting for 3000RPM on trailer).

Edit: Operating at low speeds is not as critical b/c the engine doesn't generate tons of heat at low speed but if possible, don't operate your engine with the airbox removed b/c the venturi's won't operate properly and probably wont run quite right that way, you will be running leaner than normal (caution/warning/danger - potential detonation hazard).

So, have you been into enough of these to notice what I call a *lean area* of the low circuit near where it transitions to add the high side at about 4-5200 rpm's under initial load from slow speed operation to get the boat on plane? That is the one thing that really concerns me about tuning these in the water and under load versus just what the book says.

Maybe it's just me being overly cautious? There was definitely a need for just a little more fuel on my low circuit and on the high pto side to get that cadence in that transition zone to a level that made me feel better about it.

 

[Sportster-2001-951C-Stock]

So, have you been into enough of these to notice what I call a *lean area* of the low circuit near where it transitions to add the high side at about 4-5200 rpm's under initial load from slow speed operation to get the boat on plane? That is the one thing that really concerns me about tuning these in the water and under load versus just what the book says.

Maybe it's just me being overly cautious? There was definitely a need for just a little more fuel on my low circuit and on the high pto side to get that cadence in that transition zone to a level that made me feel better about it.

Yes, I think you're not being overly cautious and I tend to agree, if anything the factory settings do tend to be slightly on the lean side of my preference as well. At least from my actual experience (not just Seadoo, others as well).

In the case of some motors, there are instances reported where the factory settings tend to be rather rich in the low end though, so I'm unprepared to say factory settings are ALWAYS too lean.

I'm in the camp though I like to hear some occasional off-idle 4-stroking, which is indicative of a mixture approaching too rich.

In the case of my 951, I went up two jet sizes on the low speed "pilot" jet in order to eliminate a nagging mid range lean hesitation, this also dramatically improved low end throttle response and introduced a hint of off idle 4-stroking (around 3000 RPM with cool engine) indicative of a rich condition that clears out completely with additional throttle and disappears with fully warmed engine. I can only say it runs a heck of a lot better than the factory calibration did, IMO.

Arguably, I may have gone slightly richer than necessary by going up two jet sizes though, and out of curiosity plan to go back one size next/if the carbs need to come back off but this is an extremely low priority b/c it runs great with this calibration and hot restarts are effortless so I like where it is.

Adding fuel - I've always felt another benefit of adding some fuel to mid range occurs when coming off WOT, I feel the extra fuel helps to avoid a potential partial lean-sieze that might occur due to the heat pistons pick up under WOT conditions. LOL, I rest my argument on confirming reports circulating in cyberland of lean-siezures occurring coming out of WOT.

Lastly, my basic rule of thumb for 2-strokes is to keep going rich until drivability issues appear(rich 4-stroking) then go back towards lean and stop at the point where the drivability issue resolves.

 

[Waterluvr]

Absolutely agreed, in my book when it comes off of a high side load I want to hear that 4 stroke burble or it's not right yet. And you know I was really tempted to increase the low speed jet one size on mine but that water tuned exhaust makes it really challenging to tune these Rotax motor's just by ear and my throttle response is just *almost* perfect, or maybe it is and I can't hear it right anymore but it sure feels good on the way up and coming back off the high side.

To many years of V-6 powerheads on the shaft dyno had it's way with my hearing a time or two, I almost have to feel for it anymore if that makes any sense.

 

[fzandome]

Ok carbs set to factory setting 1 1/4 for low speed 0 for high speed. Runs at 2500 RPM out of water. And the air boxes installed. Hopefully it runs at these settings as I sure don't want to take it down again... LOL Anyway, what is off-idle 4-stroking. What does that sound like???? Or feel like. And when do you hear it.. Idle, mid-range, WOT, or coming off WOT? To hear it is good/bad? If you hear it what do you do about it?

 

[Sportster-2001-951C-Stock]

Ok carbs set to factory setting 1 1/4 for low speed 0 for high speed. Runs at 2500 RPM out of water. And the air boxes installed. Hopefully it runs at these settings as I sure don't want to take it down again... LOL Anyway, what is off-idle 4-stroking. What does that sound like???? Or feel like. And when do you hear it.. Idle, mid-range, WOT, or coming off WOT? To hear it is good/bad? If you hear it what do you do about it?

4-stroking is actually an engine miss that begins to occur at the point where there's too much fuel, ie: too rich for the plug to reliably fire every compression stroke. In order to avoid melting our pistons the goal is to adjust for an air/fuel ratio that avoids detonation and one way to accomplish this in a 2-stroke engine is by adjusting progressively richer until the engine begins to 4-stroke miss then back off until the 4-stroking stops. At that point the engine is slightly on the rich side of maximum power capability and safely out of the detonation danger zone.

Detonation is a piston eater, we want our tune to keep the aluminum remaining in it's original shape as long as possible.

The late Gordon Jennings explained detonation this way, although he was talking about motorcycle engines which aren't so heavily loaded as a marine engine. Marine engines don't benefit from frequent or even occasional coasting periods like a motorcycle engine would normally experience, thus maintaining a sufficiently rich fuel mixture is even more critical for a heavily loaded marine engine and we must pay close attention to the finer detail by erring on the safe side when possible:

"the air/fuel ratio that yields maximum power is only a shade richer than the one that is most detonation-prone; fortunately, the plug will tell you when there has been even slight detonation inside your engine. The signs to look for are pepper-like black specks on the insulator nose, and tiny balls of aluminum concentrated mostly around the center electrode's tip. Severe detonation will blast a lot of aluminum off the piston crown, and give the plug a gray coating-which is a portent of death for the engine. A few engines will show just a trace of detonation when jetted and sparked for maximum power, but that never produces anything more than a few miniscule spots of aluminum gathered on the center electrode's sharp edges. If you see more aluminum and an extensive peppering evident on your plug, you're in trouble."

http://www.strappe.com/plugs.html

 

[fzandome]

Went to the lake, same problem as before. Took the carbs out again and found out I have a problem with the mag side body pump PN.270500223. Now the problem is to find one of these. Nobody seem to have this part in stock.

Edit Found two pairs of parts carbs on ebay for $79.00. A lot of $ for junk parts but where could I find this specific PN for a boat this old. Will have two parts to pick from..... Anybody need parts off these, let me know.... LOL

 

[Sportster-2001-951C-Stock]

Went to the lake, same problem as before. Took the carbs out again and found out I have a problem with the mag side body pump PN.270500223. Now the problem is to find one of these. Nobody seem to have this part in stock.

Edit Found two pairs of parts carbs on ebay for $79.00. A lot of $ for junk parts but where could I find this specific PN for a boat this old. Will have two parts to pick from..... Anybody need parts off these, let me know.... LOL

I must be missing something, didn't your carb kits come with the parts for rebuilding the fuel pumps?

 

[fzandome]

I must be missing something, didn't your carb kits come with the parts for rebuilding the fuel pumps?

Simple, I screw up when I reassembled the mag side carb. put the top cap in backwards and dimpled the plastic disk and body... my bad... What happens when you are tired and in a hurry to get the boat up and running before a two week camping trip with the little ones. stuff happens....

 

[Sportster-2001-951C-Stock]

I see, then I guess you need a replacement FP body plus disk(s) & stuff. The valve disks have an imperceptible curve in them that's easy to overlook and not sure if I sent you the valve disk install instructions but here's one:

 

[fzandome]

Yes the kit I received had a blue marking on the bowed side. So that was properly installed. The monkey proof metal stem is what did the damage. Put a nice little dimple on the metal valve seat where the plastic disk seals. If it can be put in backwards somebody will find a way to do it...... Murphy strikes again

 

[Sportster-2001-951C-Stock]

Yes, and the blue marking kinda freaks me out honestly, so I lay the disk on a flat surface so I know for myself.

 

[fzandome]

The Saga continues. Latest problem is on the mag side carb. When the engine is running at idle, the engine backfires through the carb.... only on that cylinder. Move the throttle forward, the engine dies. The first time this happened, I noticed there was a lot of fuel dripping down the carburetor throat during idle so readjusted the needle valve seat and removed all the jets and re-cleaned and tested and passed all the pressure setting. The fuel dripping down the carb was fixed and although it backfired with the factory settings I was able to adjust the idle adjustment. On the water the engine ran well for the first run. After I brought it back to idle, the backfiring started again and no amount of adjustment would make it stop. (No fuel was dripping down the carb throat) The rear carb works fine and, I think, the only reason this engine is starting is because of that cylinder. I did replace the needle valve stem and seat during the rebuild and verified it was not damaged.

 

[Waterluvr]

Providing there is adequate cylinder compression a 2 stroke backfiring is caused by either a severe lean condition on the fuel side of the equation starving the cylinder of fuel on it's intake / compression stroke and not allowing it to fire on every cycle. This allows an accumulation of unburned fuel in your exhaust and when the cylinder does finally ignite on a compression stroke that also ignites the residual fuel in your exhaust.

The other issue can be ignition related if the plug isn't firing consistently allowing unburned fuel to pass into the exhaust and subsequently be ignited.

Timing issues can cause backfiring without a lean condition being present but on a rotary valve engine you would expect issues on both cylinders so no matter how convinced you may be the problem lies in the carb you need to check cylinder compression, ignition and plugs first before you go any further into the fuel system and if you can get it running again I would verify correct timing.

When you observed fuel dripping from the carb was it from the gasketed mounting area on the intake itself? I would have some concerns about the correct adjustment of the metering arm height and also the condition of the o-ring on your needle seat.

 

[Sportster-2001-951C-Stock]

If pop-off pressure is fine yet the carb is dripping fuel and flooding the crankcase with fuel, the mylar film main venturi check valve may be bent or leaking for whatever reason, or the gasket under the valve body: Items 14 & 13

 

[fzandome]

In answer to the two posts below. When I took the carb apart the last time, when dripping fuel was visible in the venturi, the mylar main check valve was off center a little, so I repositioned it. Also verified the needle valve was flush with the body surface. Every thing else appeared OK. When I reinstalled the carbs, the fuel in the venturi was not visible but the carb was backfiring THROUGH the carb, not the exhaust. Adjustment of the idle screw stopped the backfire. Later when it came back, no fuel was visible in the throat and adjusting the idle screw did not work
Compression is at 140 and 145 on that engine. Plugs are new set, at .21, per my manual. Good strong spark... (Don't ask me how I know.) This engine was the better running of the two before I rebuilt the carbs so I sure whatever the problem is, it carb related. Reordered a fresh rebuild kit and will start from scratch when that comes.

 

[Sportster-2001-951C-Stock]

I'm concentrating on the backfiring symptom. Is this a single ignition coil setup? In that case the plug will fire twice per revolution.

Is the spark plug of the problematic cylinder wet with unburned fuel or dry as a popcorn fart?

 

[Waterluvr]

I'm concentrating on the backfiring symptom. Is this a single ignition coil setup? In that case the plug will fire twice per revolution.

Is the spark plug of the problematic cylinder wet with unburned fuel or dry as a popcorn fart?

That's a single capacitor discharging to a single coil with dual wires, separate ignition generator coil under the flywheel on his motors.

Probably wouldn't hurt to clean up the wire end at the boot on the problem cylinder or swap leads and see if it changes cylinders.

 

[Sportster-2001-951C-Stock]

I apologize actually, my earlier post was incorrect. If the mylar check valve was not sealing it would allow air in the fuel chamber via the main venturi and the metering needle would not pop with vacuum as it should, thus the carb would not fuel properly (lean).

Incorrect: "If pop-off pressure is fine yet the carb is dripping fuel and flooding the crankcase with fuel, the mylar film main venturi check valve may be bent or leaking for whatever reason"

I have to think at this point your metering needle is not functioning correctly, difficult to know if you're experiencing a lack of, or too much fuel.

Backfiring from carb - The single coil responsible for firing both plugs means the coil must ring twice per revolution. Thus, the backfiring you detect is most likely the fuel being lit while the piston is at the bottom of the stroke. Normally this won't happen if the fuel mixture is correct, so my feeling is the carb might not be fueling, only partially.

One way to check and see if a lean mixture is the reason the cylinder isn't firing correctly is by adding fuel manually. I keep a squirt bottle of pre-mix on my bench just for this purpose, by squirting fuel into the carb of the dead cylinder while the engine is running if the dead cylinder firing normally then you will confirm the carb is dry and not fueling the cylinder.

Remember, oil is important for lubrication so a dry carb means the oil is not being distributed properly by the fuel and extended running under these conditions (with inoperative carb) will lead to cylinder damage due to lack of lubrication.

In conclusion, it's my feeling something is wrong with you carb, or the fuel system delivering fuel to the carb. Most likely the metering needle isn't functioning properly but it's possible the fuel return and fuel feed are swapped in the wrong positions as well, so check and make sure you didn't accidentally get them crossed.

I tend to think the backfiring is a result of a lean condition, and confident a fueling problem of some sort (either excess fuel or too little fuel) is the cause. A fuel-wet spark plug will often mean the carb is likely passing too much fuel, assuming the plug isn't already fouled with carbon.

 

[fzandome]

Finally got to work on the boat today. I did the spray the carb with gas trick and that stopped the backfiring. All the fuel line are correctly installed. I have a new OEM kit here but if it's something that's not in it, recommendations on what to focus on. PS, I did change the needle valve previously, so will relook at that again to see if it's damaged... Did pass the pop test... .

Edit

Before I started the motor, I exchanged a clean plug from the other motor... Noticed the plug from the effected cylinder was NOT "dry as a popcorn fart"

Exchanged the spark plug wires. No joy.

I also decided to move the port engine carbs over to this engine to see if the problem moves with the carbs when I finish reworking them. Going on vacation this coming week so I'll will report back then its done.

 

[fzandome]

The problem with my MAG side backfire problem has now resulted in a fried piston. When the problem started it was backfiring through the carb. Numerous attempts of fixing, what i thought was a carb problem did not fix the backfire. Recently I moved the carbs from the right side to the left engine and that engine worked great. So it was not the carbs. I think the last run at the lake did it in. So the question is. what is causing the problem... fuel to lean or oil lubrication.... The crack in the piston is on the exhaust side of the cylinder along the edge of the piston. The last time I check the cylinder pressure was last month and it read 140-145. The PTO engine reads 150. I don't not want to put the $ into this if I can't figure out the root cause.

 

[Sportster-2001-951C-Stock]

Oh no, that's not good... My first thought was the crankshaft might be twisted and caused the cylinder to fire too early before top dead center, or perhaps the flywheel key is sheared, but this would also be the case for the PTO as well (firing the fuel charge out of time, ie: too early).

Now I'm thinking you must have a leaking outer crankshaft seal causing a lean cylinder, specifically the crank seal under the MAG flywheel.

So best case the motor should be leak checked to confirm the seal is leaking, replace the crankshaft seal and repair the cylinder or perhaps go with a reman engine exchange with no fault warranty.

I dunno why earlier we didn't consider your crank seals might be damaged.....

EDIT: As far as confirming lubrication goes, you can confirm the oil injection nozzle is not clogged on the MAG cylinder.

Also, if there's ever any question about cylinder/crankcase lubrication then oil can be added manually either by mixing with the fuel or even temporarily by squirting some into the carburetor.