Sunday 14 August 2011

Only running on one cylinder.

Several times a month I get a desperate phone call from somewhere in the world from a Ural owner wanting advice because their bike is only running on one cylinder or producing very little power. Fortunately most parts can be swapped from one side to the other. Following these instructions will establish why your bike is running on one.

To establish a bike really is running on one. Start engine and pull the throttle cable, or manually operate the throttle on one side then the other. If both sides pick up well then it really is only the tickover throttle stop or mixture screw that is the problem. However if one side picks up well and the other does little or nothing go to number 1

If the fault is only at idle (tickover) and adjusting the throttle stop or air screw does not seem to make much difference. Start the engine and hold the throttle open just enough to stop it stalling. Spray WD40 round the joint between the carburettor and the cylinder head on the side that is not working properly. If the engine note changes or it suddenly starts working you have an air leak between the carburettor and the cylinder head. If not, go to number 1

1. Remove and clean both plugs, make sure they are dry and then replace them. Leave the ignition off, flood or choke carbs equally and then give the bike 10 kicks. Remove both plugs and check to see if one is wetter than the other. If one is completely dry go to the fuel supply advice.

2. Fuel supply - remove the fuel pipe from carburettor and place a cup under it. Turn fuel on (or prime) and check there is a good flow. Then check the other carburettor. They should both be the same or similar. If not and you have a twin output fuel tap, remove it and give it a clean. If your tap only has a single output it could be a blockage in the fuel pipe itself. If there is similar flow to both carburettors go to float valve advice. (Don’t forget, never leave vacuum fuel taps on prime for more than 15 seconds)
3. Float valve - Drain carburettors completely and remove floats and fuel supply pipe, spray a little WD40 up through the hole where the fuel enters to clear any obstructions from the float valve. While you are there and the WD40 is handy also spray any other jets you can see. Use the thin plastic tube that comes with the WD40 to make sure the WD40 goes through the jet rather than just over the outside of it. Reassemble carburettors and connect fuel. Try starting the bike again. If it still runs on one side only, go to final fuel check.

4. Final fuel check - With the bike running on one, remove the pipe from the air cleaner to the non working carburettor. With the thin tube removed from the WD40 spray a little WD40 into the back of the offending carburettor from about 6 inches away. If this side suddenly cuts in but stops when you stop spraying then it still is a fuel problem as the bike is running by using the WD40 as fuel. In this case the carburettors will need to be completely removed disassembled, cleaned, adjusted and refitted. If this doesn’t help go to ignition checks.

5. Ignition checks - Remove both spark plugs and clean and dry them again. Put the left one in the right side and the right one in the left side. Restart the engine. If the fault has changed sides the problem is the plugs and they need replacing. If not go to coil checks.

6. Coil checks - Remove and clean the plugs and put them back in their original position. If they will reach, remove the plug lead from the left side at the plug end and place it on the right plug. Swap the other over as well. Restart the bike. If the fault has moved sides then it is either the coil, lead or cap which has failed on one side. So go to cap check.

7. Cap checks – Only do this if the plug caps can be easily unscrewed from the lead. NGK caps can, Ducati caps cannot. Put the plug leads back to their original position and then remove both caps by unscrewing them from the lead. Swap the caps from left to right and re-attach them. Re-start the engine. If the fault has moved one of the caps has failed and will need replacing. If not go to lead check.

8. Lead check - Put the caps back to their original position and if you have a coil where the leads can be removed at the coil end. Swap the leads over at the coil end, but leave everything else as it was. Re-start the engine. If the fault has not moved sides then it is the plug lead that is causing the fault. If it has moved it is the coil that has failed and this will need replacing.

NOTE - if non of this works start swapping major components from one side to the other, like the whole carburettor until you get the fault to move sides. Obviously if non of this works it could be something more serious like a bent valve or broken piston rings.

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1 comment:

  1. Thanks for your well described troubleshooting. Your description has already helped me once, thus I got same problem again now. I kept your instructions in my Ural M67 from now on and keep you informed :)