My Diploma

I just completed my Dan's online motorcycle repair course and I made this achievement certificate to post here as a remembrance and a reminder that whatever we want to achieve, we can get it online, in the internet.
If you want to be an airline mechanic, then Go and don't waste time!

How carburetor works

Motorcycle carburetors works on similar principles: A passageway (Venturi) is connected to the intake side of the combustion chamber. A vertical tube (jets) runs vertically from the passageway to the float chamber. As the piston executes its intake stroke, a great vacuum is created in the combustion chamber and at the same time the intake valve opens that creates a suction pressure in the venturi and the jets. A simple explanation for this is like sucking the liquid inside a bottle with a straw while the remaining space inside the bottle is kept at atmospheric pressure.

The amount of fuel flow is controlled by the needle and the needle jet. The higher the needle is lifted, the greater the fuel flow. When the needle is at its lowest, it effectively shuts off the jet. At the highest position of the needle, gas flow is controlled by the smallest tube which is the main jet. The needle, needle jet, and main jet can be change to smaller or bigger size to adjust the fuel-air mixture ratio.

Airflow is controlled by a valve. Either a butterfly valve or an airslide. The airslide works hand in hand with the needle in such a way that when the needle is lifted allowing more gas flow, the slide opens allowing more air to flow. Airslide usually have a cutaway at the bottom to allow more or less air through at throttle openings.

The pilot jet with its own airway is built to provide a trickle of fuel for tick-over.  It is adjusted by a screw which opens or closes the size of the airway. This allows a smooth transition from a tick-over to faster running.

The fuel in the float bowl is kept at a constant level by the float. If the level is raised, it would cause too much gas to pass through the jets causing a rich mixture. If it is too low, it would cause a weak mixture. The fuel enters the bowl in an opening with a needle valve connected to the float.  When the float is lifted, it closes the entrance at a prescribed point and as the gas is being used it will go down with the level of the liquid gas, thereby opening the entrance to allow fuel to come in.

Carburetor tuning

 

To do this the motor need to be run at speed and in a straight road with no traffic and if possible run it uphill or against the wind to ensure that the engine is puling at the greatest load. The engine must be in perfect condition and at its normal working temperature.

1. 1.       With the throttle wide open, the mixture is controlled only by the main jet. Test the motor at top speed on full throttle then cut the engine quickly and check the spark plug. A medium brown color around the insulator is ideal. A darker color means a rich mixture. A lighter color indicates a weak mixture. A slightly rich mixture is better than a weak one. A weak mixture could cause overheating and may damage the piston.

1. 2.       Throttle openings from one fourth to one fourth are governed mainly by the needle and needle jet.

1. 3.       Beyond the one fourth open position; the airslide cutaway has the most effect. It provides for a smooth pick-up with no ‘flat spots’ when the machine is driven away from a standstill.

1. 4.       When the throttle is closed, the pilot jet supplies the fuel. A second pilot outlet increases the fuel supply for a clean, smooth engine pick-up.

1. 5.       The carburetor should be tested in these positions. Get the engine running smooth and clean then check the spark plug.

1. 6.       To richen the mixture, fit a jet with a larger number. Go one stage at a time and in any case, opt for a slightly rich jet.

1. 7.       Adjust the tickover to get it reasonably slow and even.

1. 8.       Test the motor in the one fourth to one fourth open positions. It should run evenly at a steady throttle opening and pick up quickly and smoothly when accelerated. Check the spark plug.

1. 9.       If the mixture is seriously amiss, a larger or smaller jet will be needed. A larger means richer on all jets. It might be possible to fit a needle with a different taper. It the motor runs good on half throttle but misbehaves below this, change the airslide. Here a larger number means a weaker mixture.

1. 10.   Test the motor at low speeds and for a clean pick-up when gently accelerating. This is affected by the airslide and the needle. When OK, check for full-throttle acceleration.

1. 11.   Go over the slow-running adjustments carefully to get the best tickover. These tests often have to be repeated several times to get the best results.

Carburetor - slow run

 

In order for the carburetor to get a smooth, even and consistent tick-over, frequent adjustment will be the slow run. It should be made when the engine is at its normal working temperature and when the rest of the machine is in good condition.

1. 1.       Make sure that the throttle cable adjuster has enough slack before starting and running the engine.

1. 2.       Usually, the carburetor has two adjusting screw; the throttle stop and the pilot adjusting screw.

1. 3.       Turn the pilot screw clockwise until it meets light resistance and then turn it counterclockwise one and a half to two turns.

1. 4.       If the motor is running fast, turn the throttle stop until it runs as slow as possible without faltering.

1. 5.       Turn the pilot screw to speed up the engine then bring the speed back down with the throttle stop.

1. 6.       Repeat until the tick-over is right. Adjust the throttle cable to the right slack.

Open-Top-SEE

My funds are limited but I can’t just sit down and leave my bike park in the garage so I decided to do a top overhaul by myself and see what happens.

·         I managed to dismantle the whole engine down and put it on a table.

• ·         Removed the chain and sprocket cover.

• ·         Unbolted the sprocket from the camshaft.

• ·         Release the chain tension rod.

• ·         Took out the overhead camshaft cover. The rocker arm is built in the cover.

• ·         Removed the cylinder head.

• ·         Lifted the camshaft and check the bearings. It’s OK. 

I took the cylinder head to a motorcycle repair shop for a ‘grind-valve’ job. And there it is the main culprit - A broken valve guide and probably a dented valve.  It has to be taken to a machine shop for repairs.

I checked the piston, piston rings, and the cylinder. No visible signs of wear and tear. It’s a still reasonably tight fit. I didn’t bother to check on the gaps because I think that it’s still good. I just check and clean the lubricating holes. I also checked the connecting rod and crankshaft bearings. There’s no perceptible rocking movement or up-and-down play.

I cleaned and change the broken gaskets and use the same old metal gasket because I’m planning to do a complete overhaul pretty soon.

I carefully refit everything back and follow the tightening sequence and feeling the right torque I set the intake and exhaust valve clearances.

After a few try the engine is running again. I tuned the carburetor and set just the minimum tickover.  

I’m back on the road again. The road test make me suspect that the exhaust valve needs more regrinding. Well that will be taken care of the next time.

The hard climb

My motorcycle engine misfires at over 500RPM so I dismantled the air cleaner and start the engine. I positioned my fingers to block the carburetor intake side where the air cleaner was connected and twist the accelerator handle to speed the engine. It didn’t misfire so I put the air cleaner back and positioned the choke lever in the middle so the airflow valve is now blocking the air and allowing only less air through. The engine runs good or so it seems. Riding my motorcycle with the choke lever in the middle position is unnatural and I suspect that something is wrong.

When I used my motorcycle going up to Baguio, it was overheating and I have to cool it off 5 times before reaching the top and the cool weather of Baguio City made it run better. We (me and Norma) barely made it to the top because the engine has a weak pull and I actually decided once while we were on our mid-climb to go back down to Laoac, but realizing that we had already spent most of our money and our best option is to pray that our ride will bring us to the top, we continued the climb.

God heard our prayers and we’re safely backed home. I took the motorcycle to an expert mechanic for assessment and was told to prepare a big sum of money as he suspect that the engine needs a major overhaul. I need time to think as my funds are limited.

Back on the road again

   Finally! My motorcycle resurrected after more than 2 years and my driver's license renewed and registration of motorcycle taken care of. I'm back on the road again. Ooops! I manage to bring it up to Baguio City and realized that I have to do something about the engine. It's weak. Not the same pull that it has 2 years ago. It's a gl 125Honda that I bought 2002 (9 years ago). This is the first time to bring the engine down and check.
   I just hope its not a major major problem.

I'm just wondering what would the problem be?