Engine

The CJ 750 motorcycle is equipped with a 750cc, twin-cylinder four-stroke, air-cooled engine. Depending on the model, the engine design is either a 24HP sidevalve (SV) a.k.a. "flathead" or a 32 HP Over Head Valve (OHV). Both of these engines use the original BMW "boxer engine" design principle. The horizontally opposed cylinder arrangement ensures proper cooling and easy serviceability. The M1/M1M CJ models use the sidevalve engines, the M1S CJ model uses the OHV engine.

During hot weather operation it is important to observe the engine, power train and the running gear to ensure that overheating does not occur. Under normal operating conditions overheating will not occur after the motorcycle has been run-in, unless however, the machine is fully loaded and operating under high engine RPM for extended periods. As a guideline, the temperature of the cylinder heads should not exceed 180C-220C / 356F-428F.

Engine Principles

You are most likely quite familiar with engines and how they work, otherwise you would not be at this point in this document. But, in case you don't, we would recommend sites like How Stuff Works: Motorcycle Engines or the more general resource Wikipedia: Internal Combustion Engine.

Engine Variations

Over the four plus decades that the Chang Jiang has been produced a number of different engines have been produced and used.

Sidevalve (SV)

Chang Jiang 750 Type I / Type II
CC	746 SV
Horse Power	22 (6v) or 24 (12v)
RPM	4200-4800
Compression	5.7:1
Electric	6 volts 45 watt or 12 volts 45 watt??
Kick Start or Electric/Kick Start
Size	610x620x440
Weight	75 kilo

Type I

This engine is almost an exact copy of the Russian M72's engine.

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Type II

Minor improvements/changes were made to the Type I engine to produce the Type II engine.

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Type II - 12v

Over Head Valve (OHV)

Model E

Chang Jiang 750 E Engine
CC	746 OHV
RPM	4500-5000
Compression	7:1
Electric	6 volts 45 watt
Horse Power	30
Kick Start
Clutch	double dry disc
Size	610x702x450
Weight	75 kilo

Model F

Chang Jiang 750 F Engine
CC	746 OHV
Horse Power	30
RPM	4500-5000
Compression	7:1
Electric	12 volts 280 watt
Electric and Kick Start
Clutch	double dry disc
Size	661x702x450
Weight	78 kilo

Installation of Engine Unit

If your kit was shipped with the sidecar assembled and attached to the motorcycle frame, disconnect the wiring to the sidecar and the two top bolts at the knife-and-fork joints. Then back off the locking screws and unscrew the clamshell/ball joints. Wheel the sidecar out of the way.

• With the frame sitting on the ground, remove the gas tank by removing the gasoline hoses and the four connecting bolts. (You can also do this with the wheels and tires on and the center stand down. If you do, block the from wheel so that it cannot roll.) The two bolts that connect the tank to the headstock are straightforward. The two bolts at the back are underneath the tank on either side. Remove these, taking care that two rubber spacers go above the bracket and one below the bracket on each side. Remove the tank and put it in a safe place.
• Remove the electrical package, which is located on a bracket beneath the seat. There are two 8 mm screws that hold it in place. Leave the wires attached and place it up where the seat goes. Remember that there is a brown ground wire attached to one of the bracket screws.
• Remove the air cleaner assembly from the engine, if attached, and put it aside. There are set screws on either side of the engine to hold it in place.
• Remove the coil and it's bracket from the top of the engine. Disconnect the yellow wire to the point housing. Put the assembly aside. If you are doing this with the frame on the floor and the wheels off, put a length of 2x6 lumber, 18'long next to the frame, and put some lengths of 1x4 or 1x3 on top of it. You can nail these blocks of wood together if you want to, but it isn't necessary.
• Place the engine unit on the wood blocks next to the frame and lined up with the frame on the left side (facing forward). (The next step is easier with two people, but one person can do it.) While raising the right side cylinder, slide the crankcase off of the wood blocks, over the frame rail and down between the frame rails. It goes easily. The top of the starter (or generator) may scrape the gas tank-mounting bracket, so be careful. You can always touch it up the silver paint later. The engine will slide back and forth about an inch. Slide it as far forward as possible (or pull the frame back as far as possible) and align the two drive shaft tips with the holes in the transmission output shaft rubber block. While they are aligned, pull the engine back in the frame until the tips are inserted into the holes. A little dish detergent will act as a lubricant. You can also use K-Y jelly. Do not use grease or petroleum jelly as it corrodes the rubber. Unwrap the padding from the frame rails.
• Lift the frame around the engine and put some wooden blocks in place underneath so that the mounting holes in the frame align with the mounting holes in the crankcase. The thicker spacers go on left side of the engine and the thinner spacers go on the right side.
• Run the forward mounting shaft through the holes and spacers and attach the lock washers and bolts. Do not tighten yet. Run the rear shaft through the engine, spacers and frame. The foot pegs go on the ends of the shaft outside the frame rails. The left peg is the short one and the right peg is the long one. Install by putting a lock washer and nut on each end. Tighten the front shaft nuts. Do not tighten the rear shaft nuts because the exhaust pipes have to be attached at the point as well.
• Re-install the coil assembly. Make sure that the brown ground wire at the front of the frame is attached to one of the mounting studs. Do not forget to re-install the yellow wire to the points unit. Connect the spark plug wires.
• While the electrical package is removed, install the battery cables. The red cable attaches to the upper right hand corner, where there are two other red wires attached. The black cable attaches to any ground, but a convenient one is the lower right mounting bolt on the back of the electrical package. It has a 10 mm nut. This is the bolt that holds the bottom of the starter solenoid in place. While you are at it, make sure all of the screw connections across the top of the electrical package are tight. Reinstall the electrical package.
• Attach the thick yellow wire to the lug on the side of the starter.
• Reinstall the air cleaner assembly.
• Untie the clutch release lever and attach the clutch cable. Adjust following directions in the owner's manual. On some bikes the lever is curved. On some bikes the lever is straight, which will requires some adjustments. If the clutch cable appears to be way too short, loosen the bolt on the ring clamp at the back of the engine to which the clutch release lever is attached. Now slide the clamp away from the engine just a little bit until you can get the end of the clutch cable into the fork on the lever. Making sure that the clutch lever does not touch the drive shaft drum. Tighten the ring clamp. After some use the clutch cable will stretch a little, and you will have to repeat this procedure for adjusting the ring clamp. Eventually you may be able to seat it all the way against the engine.

Note: BMW manuals suggest a different method for engine installation. If you don't have a helper, you may want to try this.

• Remove the four nuts that attach the engine to the transmission and separate the engine and transmission.
• Rotate the engine front down and pass it into the middle of the frame, then rotate it back to the upright position and put it on the frame rails. Slide it all the way forward.
• Drop the transmission in behind the engine and mate the output flange to the rubber-biscuit drive shaft coupler.
• Re-attach the engine and transmission. This takes a few more steps, but is a lot easier on the back.
• Unscrew the two screws that hold on the alternator/generator cover at the front of the engine. Attach the wires to the generator or alternator. Follow the diagram provided. If you don't have the diagram, the three white wires (on some bikes these three wires are red) go to the three connectors in a row at the top of the alternator. The blue wire attaches to the 'J+' connector, and the black wire goes to the 'N' connector. There is one connector left over (A+)which is not used. Reinstall the cover. There is a diagram at the end of the instructions for the alternator and for the whole bike. The alternator diagram was drawn by Ted Smith. The wiring diagram for the whole bike comes from 'Butch,' of the Shanghai Bikers.
• Install the throttle cables in the carburetors. Unscrew the ring nut at the top of each carburetor. Remove the top unit. Carefully remove the slide and needle, taking care not to get it dirty. Run the end of the throttle cable through the ring nut and the pipe through the top of the carburetor. Take the throttle return spring and put it around the end of the throttle cable. Carefully compress the spring so that the end of the cable with the ball end sticks out. Slide the end of the cable under the bottom of the slide and lace the cable through the slit in the throttle slide. The throttle slides go with the cutout away from the cylinder head (the test unit was shipped with the slides in backwards). The spring will then sit in it's proper well. Turn the twist-grip to make sure that the cable is free and moves the slide. Insert the slide back into the carburetor body, taking care to get the needle into the main jet orifice. This is made easier if you place a finger on top of the needle and wiggle it around a bit until you find the hole. Put the top in place (there is a locating lug) and tighten up the ring nut. Repeat on the other side. Adjust the carburetor cables according to the instructions in the owner's manual. This completes the engine installation.

Cylinders

The engine cylinders are identical in design and interchangeable. A gasket and o-ring are fitted between the cylinder and the crankcase. When mounting the left-hand cylinder, see that the holes in the gasket are in line with the oil feed and drain back holes crankcase.

Piston, piston rings and piston pins

M1 / M1M: Piston, Rings, etc.

M1S: Piston, Rings, etc.

The piston rings seat into the milled grooves of the pistons. The sidevalve (SV) piston has four grooves/rings. The over head valve (OHV) piston has three grooves/rings. The piston is connected to the connecting rod by means of a floating gudgeon pin, which is retained by circlips. When installing new piston rings the ring gap clearance must be checked and if necessary adjusted to 0.3mm to 0.4mm. Piston to bore clearance should be set to 0.07mm.

Crankshaft and Connecting Rod

M1 / M1M: Connecting Rod

M1S: Connecting Rod

The engine crankshaft and connecting rod assembly are of a multi-piece design with the crankshaft and the big end of the connecting rod running on roller bearings. The piston gudgeon pin runs on a brass bushing in the connecting rod small end. Servicing of the crank is not recommended unless the correct equipment to verify crank accuracy is available. It is generally cheaper to replace the crankshaft and connecting rods as a unit in the cases of severe damage.

Crankcase

M1 / M1M: Crankcase

M1S: Crankcase

The crankcase accommodates the cylinders, the timing gear and auxiliary mechanisms, as well as serving as the oil reservoir with the oil pan. The engine is provided with forced ventilation of the crankcase. The breather is located in the timing gear cover. The crankcase ventilation system is routed to the air

Valves and Components

Valve / Ignition Timing

The valve timing gear controls the intake of the fuelair mixture into the cylinders and waste gas exhausted into the atmosphere. The intake and the exhaust valves are interchangeable on sidevalve engines, but not on OHV engines. The camshaft rotates in two bearings inside the engine crankcase, a ball bearing at the front and a bronze blind bushing at the rear.

Correct valve timing is obtained by aligning the mark grooves on the timing gears. This must be carefully observed during disassembly and reassembly of the engine.

Steps for adjusting timing:

• First adjust the points at maximum opening to between .016 and .020 inches. This is easy.
• Now take the rubber plug out of the flywheel cover on the left side of the engine.
• Rotate the engine with the kick starter until the '6' is in the center of the hole. Loosen the hold down screw on the points assembly (M1M and M1S). Hook up a 12V test light from the point wire to ground. Rotate the body of the points cover until the light just goes out.
• Tighten the lock nut and replace the rubber plug. You can also do this with a timing light. With the engine at minimum RPM the timing should be at 6 to 8 degrees BTDC, and at maximum RPM it should be approximately 30 degrees BTDC.

Either way, test drive the bike and make sure it is running well. The compression is low on these bikes, so it is possible to get the timing advanced way past where it should be and still not have then engine ping. The best test is to balance the mid-range torque with the idle speed. By retarding the spark you can get a nice even idle, but mid-range acceleration will suffer. If you adjust the spark for good mid-range acceleration, the idle will be a little ragged. I prefer the latter.

Valve adjustment

It is very important to adjust the valves properly. Valves are adjusted with a cold engine.
The clearance should be 0.1-mm cold or 0.07mm hot. In service, the clearances will change due to beddingin
of valve train components.

It is important to readjust the clearances after grinding or partial disassembly of the valve mechanism. For this purpose, put a pan under the cylinder head, take off the head cap and drain off accumulated oil. Turn the crankshaft using the kick lever. When the intake valve begins to close, set the clearance for the exhaust valve, and when the exhaust valve begins to open, set the intake valve clearances. Check the clearance between the larger end of the rocker arm and the valve stem. If the clearance happens to be larger or smaller than specification, slacken off the lock nut and by turning the adjusting bolt in or out, set the required clearance with a feeler gauge. Lock the adjusting bolt with the lock nut and then check the clearance again. In the course of running in, check valve clearances after 500 km and again at 3000 Km. Readjust clearances on the cold engine only.

Oil

Filtration

The CJ750 is not fitted with the modern disposable oil cartridge type filter but utilizes a oil slinger type centrifugal filter which is integrated with the crankshaft. The oil slingers will only require servicing at approximately 70,000 km. The servicing of the slingers requires the removal and disassembly of the crankshaft. This is a specialist procedure and should preferably be carried out by a motorcycle dealer with the necessary expertise.

Oil Consumption

If oil consumption exceeds 500 ml per 500 km, replace the piston rings.

Petrol engines depend upon oil to lubricate cylinder walls, pistons and piston rings. With each stroke of the piston a thin film of oil is left on the cylinder walls and on the combustion stroke it is burned away by the combustion flame. If an engine burns just one drop of oil on every firing stroke, it would use approximately 200ml oil per kilometer. Consumption of this amount is unheard of in the internal combustion engine but all engines do however use some oil to prevent rapid engine wear.

The rate of oil consumption depends upon the quality and viscosity of the oil in the crankcase, the engine r/min, the temperature and the amount of dilution and oxidization in the crankcase. High speed operating conditions will greatly accelerate oil consumption due to the inability of the piston rings to scavenge the oil completely at higher r/min and high engine temperatures. It has been proved in tests that an internal combustion engine uses seven times the quantity of oil at 100 km/h than it does at 60 km/h.

Modern engine manufacturers consider oil consumption of up to 2,0 liters per 1000 km during the first 10000 km to be considered normal, as it can take up to 10 000 km for the piston rings to bed in to the cylinder walls. Consumption should then drop to approximately 1 litre per 1000 km.

Bearing in mind that the design of the CJ 750 has its origins in the pre-war era, similar oil consumption to this is could be considered to be normal for a CJ750.

It must also be borne in mind however that the total crankcase capacity of the CJ 750 is a little less than 2 liters and it is therefore imperative that the oil levels be checked on a regular basis. Recommended that a daily check on the oil level be performed, and that the oil level be checked during every fuel stop on a long distance trip.

Oil Pump

Being aircooled, the CJ engines are prone to over heating when ridden hard or when idling. As stated elsewhere in this manual, it is also not recommended to let the bike idle for more than five minutes because the engine cannot sufficiently lubricate itself at idle RPM. To solve these problems many have installed high pressure oil pumps and deep sumps. Insert photos, links, etc. here, as well as a better discussion about this topic.

Engine Seals

Need to add information about suitable sources for seals or material to use to trace/create seals.
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Copyright 2006. All Rights Reserved. Chang Jiang Collective / Besiex / B. 'Quincy' Cabell