Help weird compression problem
 

Hi
Jason from New Zealand here, I have a very big problem I am hoping you guys might have come across before or might have an idea.
I have recently bought a s5 13b pp the guy I bought it off had fully rebuilt it ALL new seals and bearings. Housings appear mint no chrome peel, cracks ect plates are within spec.
Engine done 100 odd kms, to cut a long story short I got it compression tested it, reading 55 psi even on both rotors, so I rebuilt engine myself with new apex seals and springs.
Compression tested before start up reading 120 psi even both rotors. Then I started it, it started straight away ran for a minute turned it off then wouldn't start. I compression tested again and it's back to having no compression at 55 psi on both rotors???? Any help would be very much appreciated.

Regards Jason Mcguire

How are you testing compression?

After 100KM, the engine is still breaking in.

Both leading plugs out ect, I kind of thought the same even with compression so low was a bit skeptical, so I done a further 500 km still no gain. After rebuild and before start up compression was 120psi

What's the history of the engine? What all internal work was done? Irons lapped? What was the measured e-shaft runout/endplay?

Engine was a factory 13b 6 port, with rx4 3mm na rotors that's it just the perferial ports were done. Plates haven't been touched the wear was with in spec and end play is done to spec

Idk if it makes a difference but uhm. Are you taking out the plugs one at a time and testing ports one at a time? Or did you remove the all plugs at once?

Take the bottom plugs out and test, always have, it tested 120 psi before I started it up

Of course it tested 120 PSI before you started. Because it was still sapping wet with assembly lube, which artificially increased compression. Compression will then drop after it is started, and gradually increase again during breakin.

Maybe it will increase a little not 65 psi more?? I blew out all Vaseline before tested. Isn't 55 psi extremely low, hav you seen a rebuilt motor reading 55?

I done 500 odd kms an compression didn't change at all

Depends on the seals and housings.

Or if your measurement technique is faulty.

An engine with 55 PSI of compression would not run very well.

Does the engine run well? AFRs where they should be? Smooth? Decent vacuum (for a p-port)? Start up well? If yes, then the compression is fine.

Housings apear mint seals are brand new factory Mazda. It's very very hard to get started when it does start sometimes It will idle mint an sometimes it will idle like crap but will rev out well, I thought this might hav been a carb problem, not because of compression? Sorry what are AFRS? Thanks for your help

Air Fuel Ratio.

Do you have no way of monitoring AFRs?

the P port can be tricky to tune because a wideband o2 sensor will give you false readings sometimes. they also have a smaller window of mixtures where it is happy.

if its got a carb on it, presumably an IDA, then you should try all the idle jets you have or can borrow. the IDA is meant for a piston engine, so while it flows enough air up top for a P port, the idle hole is not in the optimal place for a rotary. the rotary generally requires the throttle stop, which is mistakenly called the idle speed screw turned in further than it is supposed to be. this opens the throttle plates enough so that the engine actually idles on the 1st transition hole and not on the idle hole.

this matters because it makes the idle mixture more controlled by the idle jet than it should be. so the idle jet choice is more important than it would be if the carb was on a piston engine. so you should try a few idle jets and see what works best. from memory i think the two P port engines i've got running around are about a 65 idle jet.

you should be able to tell fairly quickly if the engine is happy or not, if you get the mixture right, it'll basically run like a stock car, if you're off it'll be a bucking snorting monster.

the idle jet on an IDA supplies fuel based on throttle position, so it works up to around 25% throttle, after that the main circuit, which is based on airflow, should be working. transition is managed with the air corrector.

Does it start and run? If so then ignore compression readings for time being.

If you are not using the Mazda compression tester then your readings can be wildly inaccurate. I can get a 20-25psi difference between two gauges, or the same gauge on two different hoses, just with a 30-40cm difference in hose length!

I had a 12A that ran fine despite testing at "50psi" on a compression gauge. My current 13B tests at something like 80psi front and 60psi rear, it has been a long time since I checked it.

I've heard of people having low compression that builds up after breakin, but that has never been my experience. Best compression has always been immediately after being rebuilt, with a gradual decline thereafter, barring major component injury. On the other hand, i also smooth out the rotor housings and I am critical for measuring apex seal end-gap. If you just throw in new seals without clearancing them for length, compression is going to suffer when new.

Hey thanks a lot pal that's some very helpful and useful info. Cheers

[QUOTE=peejay;12021331]Does it start and run? If so then ignore compression readings for time being.

If you are not using the Mazda compression tester then your readings can be wildly inaccurate. I can get a 20-25psi difference between two gauges, or the same gauge on two different hoses, just with a 30-40cm difference in hose length!

I had a 12A that ran fine despite testing at "50psi" on a compression gauge. My current 13B tests at something like 80psi front and 60psi rear, it has been a long time since I checked it.

I've heard of people having low compression that builds up after breakin, but that has never been my experience. Best compression has always been immediately after being rebuilt, with a gradual decline thereafter, barring major component injury. On the other hand, i also smooth out the rotor housings and I am critical for measuring apex seal end-gap. If you just throw in new seals without clearancing them for length, compression is going to suffer when new.[/QUOTE

It's very very hard to get started and when it does start sometimes It will idle mint an sometimes it will idle like crap but will rev out an drive well.

I don't clearance end gap on apex seals but i will be from now on with future builds.
I have always tested with a normal piston compression tester, I know what yr saying tho really do need a Mazda one. You only have to turn motor over by hand hand an you can notice straight away the compression is terrible.