Pressure Check a 2-Stroke Engine

Reader Contribution by The Motorcycle Classics Staff
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Owners familiar with 4-stroke motorcycle engines often find themselves at a loss diagnosing poorly running 2-stroke engines. A 2-stroke engine is simpler (no camshaft, valves or valve gear), so by extension that should make it easier to work out issues with poor performance. The frustration comes when you can’t get your oil burner to run right even after confirming adequate compression, proper carburetion and correct ignition timing.

Recently, we hit a wall trying to get a 1973 Yamaha RD350 2-stroke running properly, so we turned to Brad Obidowski of HVCcycle in Lincoln, Nebraska, for help. A 2-stroke fan and RD350/400 fanatic in particular, Brad has acquired a great deal of knowledge about Japanese 2-strokes. After we described our RD’s running problems to him, Brad offered to spend a few hours showing us the steps he goes through diagnosing 2-stroke performance issues.

Beyond checking compression, carburetion and ignition, Brad says the single most important test to run on a 2-stroke is a leak-down test. A 2-stroke engine requires a properly sealed block to run right because it uses the crankcase vacuum created from the rising piston to pull the incoming charge into the crankcase; the downward stroke of the piston pressurizes the charge, pushing it into the intake ports and up to the combustion chamber. If there’s significant leakage anywhere in the block, the fuel/air charge won’t be adequately drawn or pressurized to transfer to the combustion chamber. The engine will also run lean from drawing in extra air, which can lead to piston seizure. Leakage points include the cylinder head, cylinder base, intake manifolds, and crankshaft seals. Repairing cylinder head and intake leaks is generally fairly simple, while replacing crank seals typically requires a full teardown.

A full assortment of test fittings is nice, but you can make your own from readily available bits.

The test involves pressurizing your 2-stroke engine block to see how long it holds pressure: If it loses any pressure over 2 minutes, you have a problem. Although there’s a bit of prep time involved (carburetors and exhaust manifolds have to come off for the test), it’s actually a surprisingly straight-forward process, and in our case it returned immediate results that explained why we couldn’t get our RD350 to run properly.

When testing, Brad suggests 7psi for pressurizing. Never exceed 10psi or you’ll risk blowing seals. For testing, Brad has a comprehensive kit with different sized aluminum intake and exhaust port plugs, but you can make up the parts you need fairly easily. Expanding rubber freeze plugs available at any auto parts store are perfect for sealing the exhaust ports, and you can make intake plugs from PVC with an air fitting adapted to one of them for pressurizing the engine. Alternatively, the intake can be sealed with a rubber gasket sandwiched between the intake manifold and the engine, with an air fitting fitted at the spark plug to pressurize the engine; just make sure the pistons aren’t completely covering the ports.

As always, have a good shop manual on hand for proper torque specs.

1. To begin, remove the carburetors (and the gas tank, if necessary) and the exhaust system. We only had to remove the exhaust headers on our RD350.

2. Next, plug the exhaust ports. For Yamaha RDs Brad uses a thick rubber gasket under a flange to seal the port. You can sometimes use the existing flange, but an expanding rubber freeze plug works just as well.

3. Exhaust ports sealed (with the right side port barely visible behind the frame downtube). Slide the rubber gaskets over the exhaust mounting studs followed by the flanges, then tighten securely using the stock flange nuts.

4. Next, plug the intake side. We used aluminum plugs from Brad’s kit, making the job easy. The plug in the left intake is threaded and has an adapter for an air fitting for pressurizing the crankcase.

5. To pressurize the crankcase we used Brad’s handheld pressurizer. It’s nothing more than a squeeze bulb attached to a pressure gauge. Simply pump the bulb until 7psi shows on the gauge, then wait for 2 minutes to see if the engine loses any pressure.

6. In our case it was immediately apparent we had a problem. The pressure in the engine dropped almost instantly from 7psi to 5psi before continuing its downward spiral.

7. To find the source of the leak you need to supply a steady flow of air to the engine. To do this, we first attached a pressure regulator to an air hose off our compressor. Before you start, make sure you have the regulator dialed all the way down to zero to avoid over-pressurizing the engine. Slowly bring the pressure up to 7psi.

8. Following Brad’s advice, the first place we checked for leaks was at the intake manifolds, the cylinder heads and the cylinder base. We sprayed those areas liberally with water, mixed with a little bit of liquid dish detergent.

9. Spraying down the intake manifolds returned immediate results, with soapy bubbles showing that our RD350 was leaking around the right intake manifold where it seals to the cylinder.

10. Working our way around the engine with the spray bottle, we found a second leak, this time at the rear of the right hand cylinder head. We continued checking, but fortunately found no more apparent leaks.

11. Next, we removed the intake manifolds. Yamaha RD350s have a reed valve sandwiched between the manifold and the cylinder. Ours had been removed at some time and reinstalled with silicone but no gasket. It had probably been leaking for years.

12. Use a scuff pad and a scraper to remove all traces of old sealant and gasket from the cylinder. Be careful using scrapers as it’s easy to gouge the soft aluminum. In this photo the left intake sealing surface has been mostly cleaned while the right is as found.

13. Clean both sides of the reed valve plate, and the sealing surface of the intake manifold. Smear a thin layer of fuel-resistant sealant on new gaskets for the manifold and reed valve. We also opted for new intake manifolds, readily available from HVCcycle. Gaskets were $6.95 each and new manifolds $50 each.

14. Next, we removed the right cylinder head. It’s held by four bolts easily located around the spark plug. Removing the head showed tiny marks on the combustion surface, suggesting something had been in the combustion chamber striking it.

15. Although hard to see, the front edge of the piston has been losing material — probably burning up from running lean — explaining the marks on the inside of the head. This engine will need more work, but we installed a new head gasket so we could check if we’d found all our leaks.

16. Reinstall the cylinder head, making sure it’s centered as there’s some play around the cylinder studs. Next, torque the cylinder head down to proper specs, working in a cross pattern. Our manual shows 18ft/lb.

17. With the intake manifolds and cylinder head back on, we ran another pressure test. This time our RD held 7psi without wavering, showing we’d found the source of our leaks. If it wasn’t for the piston damage we found, we’d send the RD out on the road.

18. As a final piece of the process, we checked our RD’s compression pressure. Both cylinder showed 100psi cold, which is perfectly adequate. Coming soon: Rebuilding a Yamaha RD350 engine! Stay tuned!

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