Starting a CMB 90/91

The best way to start a CMB to prevent damage to the big end needles is,

Remove plug, open carb fully and needle valve to full rich. Place a cloth over plug hole and place finger over carb. Make sure there is fuel in the float or in the fuel line then turn over motor with the starter until fuel is coming out of plug hole (lots of fuel) Then close the throttle and spin over motor again to remove some excess fuel. Put the plug in loosely and start motor slowly rev up and down the motor and then tighten the plug.

Oiling up after running CMB Motors

The best way to prevent corrosion is to stop the motor, remove the fuel line and glow plug. Then with your starter motor turn over the engine, with the carburettor open and a cloth over the plug hole, while pouring through the carburettor a good quality light oil. When the oil only is coming out of the plug hole stop, I then leave my motors with no plug in and the carburettor slightly open until the next use.

How to take CMB 90 engine apart and general maintenance tips 

                     Photos

 

14mm spanner

7 mm spanner

slotted screw driver

2.5 mm allen key

3 mm allen key

small jewellers screw driver slotted

circlip pliers

bar with 5 mm thread

flywheel holding tool

puller

piece of steel modified to turn the circlip

 

Same instructions apply to the CMB 45 but the sizes for some of the tools may vary.  It is good practice to use a box keeping all the parts together belonging to the engine.

 

 

  1. Remove carburettor with 2.5 mm allen key undoing the two screws at the top and bottom of the carburettor.  Make sure that you don’t loose the o- ring on the back plate.

 

  1. To remove flywheel use special tool with pins, to hold flywheel and the 14 mm spanner to undo the coupling. Use flywheel puller on the back of the flywheel to remove it. To remove flywheel collet, ease open the grove and pull it off.

 

  1. To remove water-cooling jacket undo head bolts with 3mm allen key. Head bolts should be removed regularly and oiled to prevent rusting.  Lift off you water-cooling jacket and wipe the red o-ring seal with dry cloth.  Make sure you don’t get oil on the rubber seal as it will swell up and not fit the engine properly any more.

 

  1. To remove head button.  If you can, rotate the head button by hand and try to lift it off.  If you cannot rotate it, be very careful because the liner is held in with a pin. If you try to turn it with too much force you may snap this pin.   To help, replace a glow plug into the head button and with a pair of grips try to ease it out of the liner.  If the liner comes out with the head button just remove complete assembly and with a piece of wood or something soft tap out the head button.

 

  1. To remove the piston and liner.  If the liner came out in the previous stage, replace it back into the engine and with small jewellers screwdriver flick out the circlip through the exhaust port. The reason the circlips are sold in packs of ten is that the one just removed probably flew in the unknown corner of the workshop.  Try to remove the liner by putting you finger down the liner and into the exhaust port.  If the liner is too tight and you can not pull it out just with your finger replace the coupling on the crankshaft, place the end of the tie wrap into the exhaust port or the side of the tie wrap into one of the transfer ports and rotate the crankshaft with the spanner to ease out the liner.  Now the liner is loose put it back into the engine rotate the piston until the wrist pin hole lines up with the exhaust port, screw into the end of the wrist pin a 5mm threaded rod and pull the wrist pin out.  You should be able to remove the piston and liner together. Make sure that when you pull the piston and liner out you do not lose the small needle roller pin holding the liner in position. This pin is very important as it stops the liner rotating and it keeps the ports on the liner properly aligned to the case. A tip is to regularly remove the water-cooling jacket and oil the area of this pin to stop it corroding away.  In many engines, particularly if run in salt water, the pin can completely corrode away.  As the pin is made from a needle roller once broken it is too hard to drill out.

 

  1. To remove back plate undo the 4 screws holding it with at 2.5 mm allen key and lift off the back plate.  Carefully remove paper gasket, which if damaged should be replaced.  On the earlier series of CMB’s the disk can look the same on both sides, to make sure you get it back in the right way around mark it with a waterproof marker pen.  On the latest series of CMB’s the disk is lighter and goes in with the smooth side facing outwards.

 

  1. To remove front housing.  Remove the 4 bolts with 3mm allen key.  If you cannot rotate the front housing with your hand and just pull it off, gently push on the back of the crankshaft and you should be able to push the front housing slightly forward and pull it clear of the engine.  If the front housing is tight on the crankshaft use a puller to gently pull it off.  Note that the front housing has a flat section, which goes to the top of the casing.  In the front housing, you will find a steel spacer.  This is the front seal and you should check once you have pushed it out, that the inside of the housing still has a grove around it to retain the oil.  If this grove is no longer there in any part of the hole you should replace the front housing as your engine will not tick over or throttle at all without the grove.

 

  1. To remove the crankshaft rotate it until the crankshaft is at top dead centre, pull crankshaft forward from the needle roller and jiggle it about until you can lift it out the front of the engine.  Check the crankshaft doesn’t have too much play in the needle bearing at the big end. There will however be some backwards and forwards play in the big end to allow for the free running and oiling. 

 

  1. To replace the front bearing.  It has a retaining clip that should be lifted out with the circlip pliers.  Then using a hot air gun, warm the housing avoiding heating the bearing directly and when it is hot enough you should be able to tap the housing on a block of wood and the bearing should drop out.  Make sure you wear protective leather gloves when holding front housing and tapping it – it will be very hot at this stage.   Bearings removed this way should not be re-used. 

 

  1. To remove the rear needle bearing.  Again heat the area around the needle bearing with hot air gun, making sure that the heat is not directly on the bearing.  When it is hot enough again using a protective leather glove tap the housing against the wooden block and the needle bearing should drop out.

 

There is no actual advised time scale for replacing bearings in the 90 but I remove my engine after every weekends racing and oil it with very thin oil.  This allows me to feel the condition of the bearings in the engine.  I personally replace bearings twice a year and always before a world championships.

 

One additional part to check is the carburettor.  The parts of the carburettor that need to be checked for wear are the pin and grove.  If the carburettor has been vibrating, you will have a worn pin and possible damage either side of the grove.  Grove can be smoothed with a hand wet stone and the pin needs to be replaced.   One way of stopping the vibration is to put a small spring to hold the barrel firm in the housing.  Some of the later engines may already have this.  One of the major parts that wears out on the CMB carburettor, and should be regularly replaced is the spray bar.  After a couple of seasons racing the hole down the inside of the spray bar will be worn out due to the slow running needle touching it during throttling.  If your engine is having problems with stopping during the middle of the race whilst throttling check this part.

 

 

REASSEMBLING THE ENGINE

 

The method I use to replace the bearings in my engine.  I warm the housing up with the hot air gun and taking a cold bearing drop it back into the casing until it is fully seated.  In the CMB factory the bearings are pressed in cold using a press and special tooling.  Next replace the front bearing circlip making sure it is fully seated.  Lightly spray the needle bearing and ball bearing with light oil.

 

To insert the crankshaft use the reverse procedure as for removal.  Gently push the front housing over the crankshaft making sure the flat part is upright and with hand pressure push the housing fully home.    Replace the four bolts.  Loosely do up all four bolts and finally tighten them firmly.  They need to be tight but make sure you do not over tighten them to the point of stripping thread.  I do not use lock-tight on any part of the engine.  Check that the crankshaft rotates freely, after front housing is in place, spray some oil into the front bearing hole and replace the steel spacer / seal.

 

To replace piston and liner make sure the piston is pushed into the liner so that the wrist pin hole is showing through the exhaust port.  Make sure you have the exhaust side of the piston to the exhaust port.  The exhaust side of the piston has the circlip hole with a notch in it.  Make sure the liner and the bore of the engine are lightly oiled and slide the liner back into the case aligning the notch in the top of the liner with the pin in the top of the case.  You may have to rotate the crankshaft to get it to go up the inside of the piston. 

 

Now for the hardest part of the reassembly.  You have to line up the small end of the conrod with the hole in the piston and push the wrist pin back thought the whole assembly.  Once the wrist pin is fully home, remove your threaded shaft and you are ready to replace the circlip.  To make this easier put back the flywheel and cone onto the front of the engine and push the flywheel on firmly so you can rotate the crankshaft.  I put the wrist pin circlip back in the engine the same way as they do in the CMB factory.  Position the piston so that the wrist pin is facing the exhaust, drop in a circlip, than using slotted screwdriver, making sure it is not magnetised, rotate the circlip until one of the ends is pointing into the small notch at the bottom of the hole, rotate flywheel so that the piston starts to go upwards and with the help of the slotted screwdriver and the angle on the top of the exhaust port you should be able to pop the circlip into place.  If you have succeeded and circlip is now in the hole, check it is fully seated with the help of the slotted screwdriver. 

 

Now to replace the back plate.  Put the disc back onto the crankshaft ensuring the markings are facing outwards or the smooth side faces outwards.  Replace the back plate doing up the screws ensuring they are not over tightened.  A useful tip for preventing over tightening is to use T-handled allen key and only tighten using the longer allen key.  This will spring slightly which will prevent over tightening.

 

I always put the head on after the back plate. Spray some oil onto the top of the piston.  Make sure you refit the shims taken out, replace the head button and rotate it to ensure it is fully seated.  Replace the red seal in the water jacked, ensuring it has not swelled up, moisten the black seal with water then turn engine upside down and push engine into the water cooling jacket. Oil the head bolts then replace the head bolts and loosely do each one up.  Slowly with the long end of the allen key work your way round diagonally tightening the bolts, keep going round until they are all tight. 

 

Reassemble the carburettor, checking that the o-rings on the needles are not perished and replace the carburettor ensuring that the o-ring seal is in place on the back plate. 

 

Refit coupling using the tool to hold the flywheel.  Give the engine a good spray of oil and you should be ready to refit it in the boat.