In this episode, Weekend Mechanic Bruce Bonebrake diagnoses and repairs a severely damaged engine in a jet-boat. The engines in these boats are actually considerably simpler than the engines found in automobiles. There are fewer moving parts, and there is no transmission. The problem: The port-side engine on this jet-boat has simply stopped running. Listening to the sound when attempting to crank the engine suggests an internal engine problem. A compression test confirms a serious problem with at least one of the engine's cylinders. DIY Difficulty Rating for Repair: Because boat engines are less complex that auto engines, initially this job is given a rating of 3 on a 5-point intermediate-level scale. Subsequent investigation, however, revealed serious problems, raising the rating to a full 5 points. Important: Included below are the steps in this engine-repair procedure as shown in the episode. These steps are general guidelines that are applicable to most similar engines. With any particular engine, there may be procedures, specifications, settings, tolerances, components, etc. that are specific to that boat or engine. There are also variations according to the type and brand of repair components you select. Always consult your service manual when undertaking significant engine repairs, and read and follow the manufacturer's directions and precautions that come with your kit or replacement parts. Safety Alert: Always wear eye protection, and follow proper safety precautions, when working with power tools. Safety Alert: As a standard safety precaution, always disconnect the engine's negative battery cable before you begin work on any electrical or mechanical components. In addition to standard engine-mechanic's tools, some of the specialty tools (figure A) that will be needed for this project include a compression tester and pounds-per-inch torque wrench. An air-ratchet is optional, but will help speed the assembly process. Diagnosing the Problem - Cursory visual inspection of the engine (figure B) does not reveal any loose wires, hoses or other obvious problems. When the ignition key is turned, the engine makes a rough sound that indicates an internal engine problem.
- The next step is to conduct a compression test on the engine. The spark-plugs are removed, the engine is disabled by means of the boat's kill-switch and the compression-tester and gauge is hooked up to each cylinder one-by-one. In each case, the gauge is monitored as the ignition switch is turned. In a properly functioning engine, the compression gauge should read around 105 to 110 pounds per cylinder (figureC). The difference in compression between cylinders should be no more than 14 pounds.
- When the gauge is hooked to the third cylinder, the gauge reads zero (figure D), indicating a serious problem. Low compression typically indicates worn compression rings and/or leaking valves.
- For any further detailed diagnosis, the engine will need to be removed and inspected. The lid to the engine compartment is removed for easier access to the engine (figure E).
- Disconnect and remove the exhaust system first (figure F).
- Once the intake air-box is removed, the engine's flame arrestors (figure G ) can be examined. These screens prevent flame on a 2-cycle engine from being forced back through the carburetor. Close examination of these screens reveals a lot of loose metal particles -- an indicator of internal engine damage.
- The next step is to remove the carburetors. Unbolt and remove the carburetor plate (figure H) and the fuel pump (figure I).
- With the carburetor bank off, and the engine mounting bolts removed, the engine can be taken out of the boat for closer inspection (figure J). Engines like this weigh around 90 pounds, so you'll need an assistant and/or and engine hoist.
- Once removed, inspection of the engine reveals a large hole in the side-wall of the engine block (figure K).
- A broken fragment of a connecting rod (figure L) tells the story that this engine must have seized up during operation, and the rod fragment broke through the wall of the engine.
In the segment that follows, disassembly can begin to determine the extent and nature of the engine damage.
RESOURCES :
The Haynes Small Engine Repair Manual
ISBN: 1850106665
Author: Curt Choate, John H. Haynes
Haynes Publishing Group
Chilton's Small Engine Repair Up to 20 Hp
Model: 0801983258
Author: Kerry A. Freeman
Nichols Publishing
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