Odd compression pattern
 

I'm trying to track down a hot start problem. My FD starts fine when cold but cranks without firing once it is hot. I checked compression and get 3 even bounces on each rotor of about 40 PSI. According to the Rotary Resurrection site this is normal (although other posts say it's low).

http://www.rotaryresurrection.com/3r...ion_check.html

When I put the valve back into my tester, both rotors pump up to exactly 60 PSI which is low according to all sources I have found. I cannot think of an engine failure mode in which all 6 rotor faces are equally low. I don't have a good way to check my compression gauge but I did test the air in a tire with it and it has the exact same reading as my tire gauge (30 PSI).

So is the engine truly bad? It has about 1000 miles on a fresh rebuild street port. I rebuilt it myself so I'm not expecting any surprises.

Any other ideas about hot-start problems in an FD? My fuel pressure solenoid has been removed for about 5 years so I doubt that is the problem.

Thanks
Alan

What was used in the rebuild? Better question is what was reused?

There's all kinds of ways to get generally low compression. Really only two or three ways to get low or 0 compression on two faces.

The only time I've seen 60 psi on a fresh rebuild is when damaged rotor housings were reused. If you built it yourself, you know what the main components were like. Assuming the rotor housings and irons were good, which would be main contributors to low overall compression, the only other causes would be improper clearances. High clearances would mean low compression regardless of temperature. Low clearance or tight spots due to previously impacted rotor faces may pinch side seals or corner seals causing the seals not to seal against the irons properly. This is usually magnified when the engine is hot due to thermal expansion. A freshly built and broken in engine should develop 100-110 psi in a snap.

To use a regular compression tester you only really need to worry about testing with the check valve at the end of the hose and without. If the bounces are uniform without the check valve, then you go on to the next test with the check valve. If you can watch the needle while an assistant is cranking, you should see it shoot past 90 psi evenly. Hopefully your tester is OK. You might want to confirm its function on another engine. I'd start there.

Previous failure was due to a bad corner seal on one rotor taking out the corners of all three apex seals. Surprisingly very little damage to the housing (once scrape just before the exhaust port, none to the rotor. Both were reused. Image available if desired.

Rebuild was with all new OEM 3mm apex seals and springs, new solid corner seals and springs, reused side seals and springs on both rotors. All clearanced to spec personally. Oil control rings were reused with new o-rings. All new OEM soft seals. Ported using Pineapple templates. Irons and housings reused, they were within spec.

Here is the damaged housing which was reused, the other was perfect with approximately 20K miles on it.

It also shows a fine example of what a Pettit street port looks like.

If I may ask, who milled the rotors? How well did the new grooves interface with the corner seal pockets? Did you use solid OEM Mazda seals or GSL-SE corner seals with the rubber plug? The solid ones are nice and strong but can be tricky if the newly machined apex seal groove is not asymmetrical or traverses on an angle.

I'm always leery of reusing rotors with side seal scrapes or potential corner seal pocket damage. Clearances are crucial and low by design; tight spots may not feel acceptable at ambient temps, but things may tighten up at operating temps.

That rotor housing is usable, though not ideal....it will result in slight power loss. Check PM.

have you ruled out the possibility that compression is lost into the cooling system?
maybe you need to check the coolant for C0 contamination to rule out the most obvious first

I assume they were milled by Pettit who built the engine I rebuilt.

Solid OEM Mazda seals.

Since the housing is damaged so close to the exhaust port, I didn't see how it would have much if any affect on performance.

Rotor images

Corrections were needed...sorry for the confusion....that's what happens when you type something and change your mind on the wording without checking the whole sentence structure :)

I would like to know what you found to be the cause of the low compression.

What was your engine speed during cranking? What was the battery voltage during cranking? what was your engine tempeture? were all the spark plugs out during cranking? does engine start right away now? dont get get caught up on one write up, has engine been fully broken in since rebuild? the more you drive it, the more the apex seals will wear into rotor housings making the compression number go up, hope this helps out...good luck