RZ350 compression test

[mcherubini]

Hello everybody! I'm new forum user and from Brazil. I have a RD350/92 - (CKD in Brazil).

I already downloaded many RZ/RD service manuals and I couldn't find any information about compression test.

It seems that by the time (80 and 90s) that information was not given at all.

I found some information that can help me already, but none of them was from the service manual.
RD350 - Compression rate 5:1 (Brazil Spec) - It goes from 85 to 100 PSi (It seems to be about right - that's what I have)

Does anyone have that information from Yamaha Shop Manual to help me out?

Many thanks, Folks!

That's my bike... Almost completed!

[kpke]

I am unable to help with compression specs. Someone will come along here with the numbers.

Did you rebuild the engine with new pistons?

I would like to compliment you on your unique build. The bike looks fantastic.

Hopefully you'll get it out on the road and enjoy some miles soon. Looks like it.

[MK]

I've bought a new compression test kit from China as I needed something for the small thread of my Pitbike.

That new gauge showed some 7 bar on my RD engine whereas the old one had more realistic 11 bar as the engine was freshly rebuild and the head had increased compression ratio.
Instead of throwing it to the dustbin, I'll keep it for one purpose: Next time I'm going to buy a bike, I'll bring that crappy kit and buy the "damaged" engine for a few pennies

So first thing I'd do is cross check the readings with other engines known to be "good" or even measured with different gauges.

85 psi (5.8 bar) to 100 psi (6.8 bar) is a bit on the low side, if it turns out to be a correct reading.
New and stock RZs should have some 8.5-9 bar.
On the other hand Brazilian models are known to have reduced compression and a different CDI, so maybe that's still fine for your fuel and riding style.

BTW: Ensure you measure with PVs open as the closed position adds around 0.5 bar to the values.

[Yamavarna]

You definitely have a good eye for style! The whole package fits together well and looks like a polished piece of art.

[mcherubini]

Thank you, folks.

(kpke) - I rebuilt that engine back in 2014 with Yamaha DT 180 pistons. I bought it back in 2018 with the same tires, the guy didn’t use it. The bike wasn’t running good by that time (2018) so I left it at my shop for the last 4 years until complete the project.

(mk) – I’m after the information from the manual because I’ve been arguing with the mechanic that 60 psi is too low pressure. I´m having difficult cold start and misfire of the left cylinder for a few seconds.

Today the mechanic took my gauge (old craftsman gauge) and we measured the pressure of a good running bike. Guess what!? 60 PSI. I had to accept that, because the problem was a bad ignition coil.

“On the other hand Brazilian models are known to have reduced compression and a different CDI, so maybe that's still fine for your fuel and riding style.” You are correct Brazil is 5:1 and the carburetor has no power valve also.

I’m 4 stroke car “mechanic”, I know about two stroke but YPVS confuses me. That’s why I’m looking for shop manual information.

“Ensure you measure with PVs open as the closed position adds around 0.5 bar to the values.” Good information – thank you.

Do you have the information from the shop manual even for RZ specs?

Yamavarna – Thank you – I took the style from this guy - DESIGNER'S CUT Café Racer Projects Oberdan Bezzi (https://www.behance.net/gallery/4424201 ... r-Projects)

Many thanks and sorry about the English :(

[mcherubini]

That's how it was before my second renovation.

[MK]

I´m having difficult cold start and misfire of the left cylinder for a few seconds.

Now that smells like the usual left right float bowl swap.
Only the left one has the choke jet at the bottom of a narrow passage. The right one has the passage, but is missing the jet and the small bore at the bottom of that region.
The symptoms are exactly what you describe and I guess because Brazil is a rather warm place, you weren't too bothered, yet, to check why that happened...

[mcherubini]

Thank you, MK!

Now I understood what happened! I took two coils to the shop and both were condemned by the mechanic with the same problem. One of the coils was new, not OEM but new.
During the diagnosis, assembling and disassembling, he might have swapped them back (or realized) – about float bowl. Because the bike is running fine now.
But I’m still intrigued about the compression, how can it be so low!?
My bike doesn’t have OEM YPVS valves no more – that’s another issue to lower the compression (fact!).
I’m still after the Yamaha Manual compression test information.

Thanks again.

[brrrappp]

There is a procedure for measuring compression. The reading will vary wildly if you measure incorrectly. Compression testing is really just a guideline test for diagnosing a non-starter. If over 100psi look for other problems. Under 100psi you have a top-end issue. 100 psi is generally the magic number for a running engine. 120-130 is a good number for a working engine.
* It's important that you hold the throttle WIDE OPEN when testing. At least several strong kicks. You could secure your gage to the bike to keep it steady, put the bike in 2nd gear and do a bump start from a fast run to get a good read. Be sure the kill switch is OFF or the opposite plug is also out so you don't fire up on one cylinder.

[mcherubini]

Thank you, Brrrappp.
I’ll follow your procedure to measure the compression.
Thanks again!

*** To Moderator - this topic can be closed. Many thanks! ***