UK Karting - Tech Tips: Honda GX160 Tech Talk (Part 2)

Tech Tips
Honda GX160 Tech Talk: Race Preparation - Valve Gear

Honda GX160 Tech Talk:
Race Preparation - Valve Gear

RockerBox Components

Valve Springs...

The standard Gx160 valve springs are too soft for 5700 rpm so Gx140 springs are a better choice. These are still quite soft, with a free length of 1.34"/34mm, and so I would suggest you change them regularly.

We change them after every meeting. The difference in length is noticeable against new springs and subsequently does not control the valve gear as it should. (Note old springs adopt a longer length when left but are not as good as new springs.)

This is where problems start to happen. Worn springs allow valves to float and pop out of retainer and drop into the combustion chamber.

The retainer is another weak link. If they are examined regularly and changed when showing signs of wear they will behave themselves.

Push rods are very weak when over stressed so a few hints will help avoid problems. Remove any sharp edges from the guide that helps locate the push rod. Check pushrod for chrome lifting at the ball ends regularly. Discard any bent push rod.

Rocker arms are robust and do not cause areas of concern.

Cam followers are virtually trouble free if lubricated when assembled.

Changing valve springs without removing the cylinder head - Yes, it can be done if you work methodically.

Set the engine to TDC with both valves shut. Slacken the rocker arm lock nut and main nut, remove the rocker arm, compress the retainer against the spring and slide it towards the larger cut-out and remove it. Change the spring and reverse the procedure, setting the valve clearances to .004" and .008". The valve might move down the valve guide but it will not slip irretrievably into the engine if the piston remains at TDC.

Setting the valve clearances... Two settings need to be considered when studying this vital area of engine preparation for increased performance. They are hot and cold valve clearances.

The cold setting is easy to achieve and should be the starting point for setting up a newly built engine. Feeler gauges are used to check the clearance between valve rotator and rocker arm. ( be aware that it is only too easy to slightly compress the valve spring when using the feeler gauges so take great care to achieve accurate results).

Set the engine to top dead centre on the compression stroke(both valves will be closed and rocker arms should be slack). TDC can be checked accurately by taking out the spark plug and SLOWLY rotating the engine, in the direction of use and feeling for TDC with a long bladed screwdriver. An alternative is to measure vertical movement of the pushrods with a dial test indicator, mounted firmly at the rocker arm and directly above the push-rod.

When TDC is established the valve clearance can be set to .004" and .008" respectively for the inlet and exhaust valves. Take the time to check your work after setting the clearance and locking up the thin lock-nut on each rocker assembly. This area of the engine preparation will repay your efforts tenfold.

Take the time to learn what a true .004" clearance really feels like. It should be possible to slide the feeler in and out with slight resistance. Any other feel is incorrect!

Why is the exhaust valve set wider than the inlet valve? Because it is in the heart of the heat constantly as it exhausts the gases after the power stroke, and expands greater than the inlet valve. It is possible to melt exhaust valves with fancy fuels and... but that’s another story.

Now that the valves are set cold the engine can be started and warmed up. When the engine is at working temperature it can be stopped and the valves checked again for adequate, correct clearances. They will have ‘opened up’ to larger clearances. Let the engine cool down and check the valve clearances again. They should be the same as when you first set them cold.

Checking them hot is a very hit and miss event with so many variables and a lot of guess work. Only a dynamometer will show effects of different valve settings, apart from racing itself which can never be reproduced with the vast amount of variables involved.

To sum up...

Use new Gx140 springs, use new retainers on both valves, set them to .004" and .008" cold and the engine will rev to 5700, for short bursts, as in typical sprint racing, without problems.

Or...

Use old springs, use old retainers, use bent or scarred pushrods, set the valve clearances in a rush and you will either drop a valve or experience amazingly poor performance.

All the above presumes the valves and seats are in ‘good’ condition. What is ‘good’ condition when considering racing? ... Well that’s another article …

This article was produced by Brian Pollard, author of "Preparing the Gx160 for 'open' racing" which is available on CDROM, in multimedia format, as an e-book, and in paper form. All enquires should be sent to the above e-mail address.

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