Pressure Check a 2-Stroke Engine

| 4/13/2015 1:57:00 PM

Tags: March/April 2015, Yamaha, How-To,

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.

BikeMasterBeyond 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.

6/17/2016 5:11:16 PM

Checked with my friend today and he commented that your bike is likely pre-oil injection, which could mean the seal might be different than the traditional Suzi isolating seal. I did check an online parts fiche it appears to me the seal is likely a normal oil seal. Event the 68 version, which the diagram is for, does not appear to be injected. You might try the Vintage Suzuki motorcycle forum on Yahoo to see if some of the real experts there might know the configuration of your seal for sure. The group is not very active, but queries do seem to be monitored and answered. Cool little bike. I'd have one if one could be found... JonC

6/17/2016 11:26:17 AM

Hi hotrodjohn71, I'm not familiar with the S32, but as far as I know, all Suzis have a normal (non-labyrinth) center crank seal. I'll check with my Suzi expert friend today and let you know if I'm wrong. Assuming a normal seal, yours is apparently leaking and bad. Your description sounds like only the center seal is bad, i.e. minimal leakage to the outside of the engine. Still, if you're going to repair the center seal, you should take the time to find out where your small leak(s) are occurring, thru the tranny, or side seals using the soapy water spray method. Good luck, JonC

6/17/2016 9:28:50 AM

Hi Jonc, Regarding your comment about the Suzuki twins, would that apply to a 1965 Suzuki S32 150cc? I am having problems with it having no low end power whatsoever and difficult to start and keep running. I pressure and vac tested it but only on one side. I was getting pressure and vac simultaneously at both spark plug ports when testing. On the pressure test, it lost a half pound in 7 mins starting at 6 psi. on the vac test it lost a pound in 5 mins.

5/21/2015 1:27:19 PM

Very nice article. Note that with most Japanese 2-strokes, each cylinder can be isolated and tested separately, since the center crank seal doesn't allow pressure to flow back and forth. Yamaha twins, however, have a labyrinth center seal that allows pressure to flow back and forth between crankcase bottom ends. Each cylinder is sealed by power pulse from the opposite cylinder, when running. That's why this engine needed to have both cylinders sealed and tested together. If you use the same procedure used in the article to test a Suzuki twin, you'll only be testing the side you've pressurized. I just assembled a kit for testing my Suzuki Stinger. It consists of a Mityvac MV8500 that can be used to check vacuum or pressurize. It's a bit expensive, but a multipurpose tool. I purchased inexpensive rubber plugs from Amazon in various sizes to seal the intake & exhaust ports. Example: A small hole was drilled in the center of one of the plugs and a fitting from the Mityvac kit inserted and sealed to allow pressure to be introduces. Worked a treat. Also note some 2-strokes were prone to imperfections and leakage in the castings. Kawis were famous for this. JB Weld time. Some great info on 2-stroke air leaks is available from 2-stroke guru Harry Klemm at:

4/30/2015 3:03:24 PM

Thanks for that fantastic article! And, for the HVCcycle link.

4/30/2015 11:34:47 AM

Has there been any other comments on this articule?

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