Description

Caterpillar 4G63, 4G64, 6G72 Service Manual – LP-Gas Supplement

I’ve seen guys burn through a whole Saturday trying to tune an LP-gas carburetor with a generic online guide. The mixture goes lean, the lift truck stalls, and they end up chasing vacuum leaks that aren’t there. That’s why you need the factory Caterpillar 4G63, 4G64, 6G72 Service Manual – the actual OEM supplement that covers the LP-gas fuel system on these engines. Not a reprint, not a scan of a scan. This is the 48-page document Caterpillar issued for 4G63, 4G64, and 6G72 lift truck engines equipped with liquefied petroleum gas systems.

This manual starts with the basics – what LP-gas is, how to select the right grade, and why olefins damage your fuel system. It then walks through every component from the tank to the converter. If you’ve ever had a pressure relief valve stick open or a fixed liquid level gauge give a false reading, you’ll appreciate the section on structure and function starting on page 5. Pages 14 and 33 are the ones you’ll dog-ear first: troubleshooting on page 14, inspection and adjustment on page 33. That’s where the real work happens.

LP-Gas Knock Rating and Octane Number – Why It Matters

You might think octane is only for gasoline, but LP-gas has its own knock rating. The manual spells out the relationship between propane/butane mixtures and vapor pressure. Ignore that and you’ll get detonation under load. The table on page 4 of the TOC section shows vapor pressure vs. temperature for propane and normal butane. I keep that page taped to my toolbox. When a customer brings in a lift truck that pings on propane, this is where I start.

Converter and Carburetor Disassembly

The converter (vaporizer) and LP-gas carburetor are the heart of the system. The manual gives separate disassembly procedures for the 4G63/4G64 (section 1) and the 6G72 (section 2). You need to know that the 6G72 uses a different converter diaphragm spring. Page 17 breaks down the main components. I’ve watched techs try to swap parts between the two – doesn’t work. Stick to the factory procedure. And for god’s sake, mark the orientation of the diaphragm before you pull it apart. The manual shows the correct reassembly order for the converter. Use it.

Troubleshooting the LP-Gas System

A 48-page manual doesn’t waste space on theory. The troubleshooting section on page 14 is a checklist of symptoms: engine won’t start on LP-gas, rough idle, lack of power, gas smell. Each symptom points to a specific component – filter blockage, hydrostatic relief valve stuck, converter regulator diaphragm torn. No guesswork. You go down the list, check the part, move on. That’s how you bill hours instead of burning them.

Inspection and Adjustment Procedures

Page 33 kicks off the inspection schedule. You’ll find the gas leak check procedure here – the one that uses soapy water and a regulated test pressure. I’ve seen guys skip the hydrostatic Relief Valve check and then wonder why the tank vents at 300 psi. The manual also covers idle speed adjustment and output power adjustment for the LP-gas carburetor. The periodic inspection schedule lists intervals for every component. Follow it, and you’ll keep that lift truck running for years. If you need to pair this with engine service info, check the Caterpillar C175, 3600, C8.7, C12.9 2016 Service Manual – Factory Tool Catalog for additional tooling specs. For other engine families, the Caterpillar 3500B 2002 Service Manual – Factory Specs covers larger power units, and the Caterpillar C27 C32 DWB1 SXC1 WDR1 2008 Service Manual | OEM Factory Specs applies to the 2008 generation.

Why OEM Documentation Beats Aftermarket Copies

Aftermarket “service manuals” for LP-gas systems often lift diagrams from the internet and miss critical torque values or valve adjustments. I once had a tech bring me a printout that showed the converter adjustment screw in the wrong location. Cost him a day and a new diaphragm. This manual is a direct Caterpillar publication – part number 99719-84130. The page numbers match the original factory layout. No third-party reprint, no condensed version. If you’re working on a lift truck that also has a Manual Transmission, the same principle applies: get the factory data, not a summary. And if you’re dealing with driveline issues on a different vehicle, the Manual Transmission resource covers the basics, but for LP-gas, this is the only manual you need.

Does the manual cover the entire engine or just the LP-gas system?

It covers only the LP-gas fuel system supplement. For base engine specs (torque, timing, valve lash), you need the separate engine service manual for the 4G63, 4G64, or 6G72.

What is the gas leak check procedure on page 36?

It specifies applying 15 psi of regulated air to the system, brushing soapy water on all connections, and watching for bubbles. Do not exceed 20 psi. The hydrostatic relief valve must be tested separately according to the inspection chart on page 37. Skip that and you risk a tank rupture if the valve fails under pressure.

I keep this manual within arm’s reach on the bench. When a job calls for an LP-gas converter rebuild, I don’t memorize steps – I flip to the page. You should do the same. Access the Factory Manual now and get the OEM data that makes the difference between a job done right and a job that comes back leaking.