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How To

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Personal Water Craft Carburetor Rebuild

Our 1999 Polaris SLH-700 Personal Water Craft struggled to perform during its first test run of the season in 2011.

When we driveway tested the boat everything seemed to be in working order and it started and idled just as well at the boat launch. Throttle response was great with no trouble noted. Problems revealed themselves when the rider would stand on the gas to get up on plane after we launched.

Hitting the gas left us with the impression that the motor was fuel-starved. A dramatic bogging affect occurred every time the rider applied aggressive throttle. Lift off the throttle and the engine would return to a normal idle. If you feathered the throttle it was possible to get up on plane and the boat ran well at high RPM. But anything less than aggressive riding and it just wouldn't perform.

Here are a couple of videos to orient you to the problems and what we're doing about them. Be sure to check out the pictures that follow; they show the details as we progressed through the repair.

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We're reviewing the parts and pieces we purchased in order to rebuild the carburetors as well as replacement of gray fuel line with black fuel line.

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We were able to get the boat back on track but began experiencing huge vibration problems with the engine – the engine was shaking itself apart and that was another problem that had to be dealt with.

white board project plan

We work to a plan. We document symptoms and what we believe may be causing the problem and we order the steps we're taking to resolve those problems. We like doing this type of work but we only want to do it once if possible. Again, we work to a plan.

carburetor kit carburetor kit close up

Here's the kit we purchased to rebuild the carbs. This kit as well as most carb kits we've encountered come with parts and pieces that fit many different types of carburetors – they may not all apply to the carb you're rebuilding.

carburetor throat carburetor throat close up carburetor gasket damage

If you watched the second video you already know we have a vibration problem and the engine is shaking itself to pieces. There was evidence of this even before we started disassembly for the first time. The air cleaner has been removed and looking down into the carb throat you can see bits of gasket all the way around the flat surface that encircles the butterfly.

damaged carburetorgasket

While we're on the subject lets make a new gasket since we had to make a number of these during this project.

new carburetor gasket material

Obtain sheet-gasket material from your local parts supplier.

new carburetor gasket

If there's any life left in the old gasket use it as a pattern, if not, do your best and start drawing and cutting until it looks like it will seal with no gaps.

new carburetor gasket completed

We used a box knife to make the cuts – like anything, if you want an acceptable end product take your time and do it right. Holding the knife still and moving the gasket material to perform the cuts rather than moving the knife around on the gasket is a much easier way to get this taken care of.

carburetor gasket test fit

Test fit the gasket and tweak it until it's right. Save any large pieces of the old gasket until the project is completed. We had to disassemble the carbs again 6 weeks later after the engine shook itself apart…and cut all new gaskets again.

carburetor assembly

The carbs for this dual-carb setup are not symmetrical, the rearward carb (right side in the photo) is physically larger than the forward carb and disassembly is somewhat different for both, just keep all of the bits and pieces neat and clean and maintain the order of disassembly.

dirty carburetor fuel inlet screen

As soon as we removed the metering cover from the carb it was evident that this internal fuel screen was caked with dirt. It was so dirty we thought we'd found the problem. In the end it was probably a contributor but didn't solve the problem.

Remember that at the time of this writing this boat is 13 years old and everything is subject to breakage. This screen is held on with a plastic skirt and was gently removed and cleaned.

carburetor diaphragm

2-cycle engines like the one in this boat typically have a diaphragm-style carburetor rather than a float-bowl style carb found on 4-cycle engines. The diaphragm is the black cloth-like object on the right side of the photo. Engine vacuum is used to alter the shape of the diaphragm and this in turn alters the gas flowing into the carb. There are pros and cons to both styles of carb but one notable difference is that a diaphragm carb can run in any orientation including up-side-down. A float-bowl carb can't. 2-cyle lawn equipment are equipped with diaphragm carbs.

A torn diaphragm might cause the symptoms we're seeing with this boat but this diaphragm had no visible problems – we replaced it anyway.

carburetor needle and seat assembly

The needle and seat are underneath the diaphragm.

  1. Seat
  2. Needle
  3. Metering lever
  4. Metering lever pivot pin
  5. The diaphragm applies or releases pressure to this point, the rocker arm pivots and the needle either allows or restricts gas flow through the seat.
carburetor needle and seat removal

Backing this Phillips-screw out releases the assembly from the carb housing. The metering lever is spring loaded – be aware of what is happening with the spring at all times during the disassembly. The spring is calibrated for this carb set up and if it becomes stretched, kinked or damaged in any way you will encounter problems with fuel delivery.

Our kit came with 3 different versions of replacement springs (light, medium and heavy). We weren't able to determine what weight spring was present in the carb before disassembly and ended up reusing the original spring during reassembly.

carburetor metering lever spring

(F) The spring is located under the metering lever.

carburetor needle and metering lever

The whole assembly lifts out of the carb body.

Some needles have a small hook on the upper end and hang from the pivot arm. This needle hangs from a U-shaped hole on the end of the metering lever.

carburetor needle

Here's the needle after separation from the metering lever.

carburetor seat

The seat (G) can now be inspected for dirt build up and replacement. Looking down into the seat there should be a small, clean hole at the bottom. If you attempt to clean this hole you cannot alter the bore size and remember that the brass material will be very soft. If you accidentally alter the bore size fuel delivery to the carb will change.

Some seats can be unscrewed from the carb body. This one appeared to be pressed in and we left it in place rather than risk causing damage during removal.

carburetor fuel inlet removal

We were concerned about dirt build up in areas of the carb we couldn't see without further disassembly. This fuel inlet is pressed into the carb body and was removed with a very dull chisel and some light tapping until is separated from the body. Again, it's made of soft material and will become damaged if you strong-arm it.

carburetor fuel inlet installation

We used rags to protect the carb body and positioned a C-clamp over the fuel inlet (H) to press it back into place.

carburetor metering cover

The rebuild kit comes with a number of gasket-like pieces including this clear plastic covering. Removing the plate on the same carb opposite the needle & seat side reveals these pieces – they were inspected and replaced.

carburetor metering cover rubber gasket

The surfaces on this side of the carb are ringed with an O-Ring style gasket (I). All of the passages were inspected, cleaned, and the gasket was replaced.

carburetor mixture screw

Both carburetors are equipped with mixture screws that are adjustable after the carb has been reinstalled. A cup-style seal and O-ring are present on the screws and these pieces were replaced.

carburetor mixture screw installed

This mixture screw is shown after installation (J). The screws are buried in the carb housing and the head protrudes just far enough for a flat-blade screw driver to reach the slot.

To "zero-out" the initial mixture screw setting after rebuild we installed the screws and turned them in (tighten, clockwise) until they stopped turning. Go easy and don't over tighten the screw. We then backed the mixture screws out on each carb 1 ¼ turns (loosen, counter-clockwise). From this baseline we make adjustments in ⅛ turn increments, in or out.

Evan and I count mixture screw turns aloud when each of us is doing carb adjustment work and we do this for three reasons:

  1. We don't want to lose the count within our own minds.
  2. We can hear each other count as a form of validation of the other man's work and to make sure we haven't mis-communicated the direction we're headed in.
  3. If the mixtures are off the engine won't run well or it won't run at all. We need to be bullet-sure that we've got the settings right or we might try changing mixture screw adjustments to get dialed in only to find the base settings were wrong in the first place.
carburetor fuel jets

We made a controversial decision to change fuel-jet sizing during the rebuild. The reason it's controversial is that by changing the jets we introduced another variable to a list of diagnostic variables. In diagnostic work we're always trying to isolate known working parts and systems from unknown parts and systems. Diagnostic work is the most important thing we do on this site and we've committed a big diagnostic no-no by re-jetting without fully understanding whether the rebuild alone would solve the problem.

Our rebuild is a best-guess at fixing the throttle response problem and if it doesn't work properly we won't be sure what the root cause is. Potential problems include the rebuild, the new jets, some other undiscovered problem or a combination of all three.

carburetor fuel jets top view

However, to re-jet after the rebuild would require more disassembly and more carb-specific gaskets being purchased since they're not reusable and it could become very expensive (we can't make these gaskets ourselves). We took a gamble and it seemed to have paid off.

Here are the jet specs:

  • Original jets = .68 and .142
  • New larger jets = .70 and .150

We went with a larger jet since the boat seemed to be starved for fuel.

carburetor fuel jet installation

Use a flat blade screwdriver to remove the jet (K), check to make sure it's clean and install the new jet. Flip this carb-plate over and perform the same step on the other side.

carburetor intake manifold stripped mounting hold

Fast-forward 6 weeks and we've got a PWC engine vibrating itself to pieces and poor performance has returned. We removed the carb assembly and learned that the front manifold carb-hold-down screw hole is stripped and would need to be rethreaded with a tap. The carb was so loose that air was introduced at the intake manifold flange and caused the poor performance.

helicoil kit

We took one of the carb hold down bolts with us to our auto parts house, performed some measurements with the help of a sales person and purchased a Helicoil kit that includes the tap, thread installation tool and new thread inserts.

helicoil thread insert

After tapping a Helicoil was installed on the insertion tool and threaded into the stripped hole.

helicoil thread insert tool
finished helicoil thread insert repair

Here's the repaired hole with the new thread insert. It worked perfectly and saved us a lot of money.

To finish the project out we cut new gaskets (again), added blue lock-tite to the carb mounting screws exactly as described within the instructions and haven't had any more problems with the boat.