sjwarden

I have an 85 GS and I decided to check the compression. I used a standard compression tester(not a rotary specific tester) and installed it into the lower plugs of the rotors when I did the test. I think this was incorrect but I am not sure if that makes the readings false or not.

I have linked below to two videos of the test that I have uploaded to YouTube. The first is the front rotor, the second is the rear rotor.

The test was done hot as you can see on the video, the plug wires were removed from the distributor and the fuel fuse was taken out for the test. Accelerator was pinned and the engine was turned over for several pulses while the valve on the tester was held open, and then also for several pulses with the valve closed for a highest compression reading.

I am having trouble deciphering these values and I was hoping to get opinions on whether or not this engine is healthy given these results, or if the results are meaningless because I goofed up the test somehow. Please let me know any help is appreciated.



Thanks again.

sjwarden

Thought I would post some additional info about the car that might help. The car is totally stock running a stock Nikki carb. It starts hot no problems, and I have never had any issues with a hot start. However if it sits for longer than a week it takes a bit longer to get it to start but I have never had a real problem starting the car. The engine runs well I think, but has a slightly lumpy, not smooth idle, at about 800 rpm. If I gas it a little while idling the engine will get very smooth and idle for about 2 minutes at 1,000 rpm, then drop back into the lumpy not so smooth idle at 800 rpm. I can repeat that cycle over and over again while idling.

I checked compression in the videos above just because I had never done it before, and I was not sure of the car not starting right up on a cold start after sitting a few weeks, and the lumpy idle were symptoms of a bigger issue.

Any thoughts on the health of the engine are appreciated. If the engine compression is good I will look to tune up the carb for the lumpy idle.

j_tso

iTrader: (2)

With your finger off the button, you're going to see the highest number from the 3 combustion chambers on the rotor.
Opening the valve by holding down the button is not going to give accurate numbers, but you're looking for even bounces on the needle.
Both rotors look like they have even bounces, the rear rotor looks like it's hitting lower numbers than the front.

Is the problem just idling and starting? How does it drive?

sjwarden

The car drives great, smooth acceleration, no misfires, pulls nicely. Seems to have all the power it is supposed to have. Just a slightly lumpy idle as described above, and when it sits for a week or so it takes a little time to get it to fire up. Hot starts great, and if it sits for just a few days it starts right up with the choke.

If the rotors are good then I will look to ignition and intake to get the car tuned up. I was hoping that the readings with this cheap compression tester would tell me that. Not looking forward to spending 500 bucks on a rotary specific tester, and I cannot find a shop to help me. As soon as they hear rotary they turn me down.

Ckforker

Have you checked your float bowls after it sits for a few days? If they drain down, it makes it hard to start.

t_g_farrell

iTrader: (1)

How many miles on the 12A? As they age its normal for this to happen. Mine does this and it has a lumpy idle as well sometimes. I think a seal gets stuck after sitting for many days or a week. When I start it I will see some of the usual smoking which clears up fairly quickly. If I start it every few days I don't see this issue. Mine actually has a hot start issue if its really hot and turned off and not left to cool for 30 or 40 minutes, then it can be hard to start. Again, I attribute this to age more than anything, cold and warm starts are fine. I don't have an AFR meter setup but that would probably resolve my mystery hot start issue.

sjwarden

It has about 41,500 miles on it. I have not checked the float bowls and will do that after I let the car sit a few days. I might be being too picky here, is the idle supposed to be really smooth, or is some lumpiness to be expected? I don't know anyone else that has a first gen, so I have nothing to compare against in person.

Maxwedge

iTrader: (1)

I don't have the rotary comp tester, but with a similar tester to yours, my '85 12A reads 95..95..95..95..95..95..95..95..95.. on each rotor.

It has a lumpy idle on the stock block but revs smoothly with light throttle input.

t_g_farrell

iTrader: (1)

Thats not true, depending on where you are in Ohio you have one of the best known rotary experts in your state. I'm sure Dave will come along at some point and help out (mazdaverx713b).

sjwarden

Thank you all for the replys, sounds like a little lumpiness to the idle is to be expected and no big deal. I would love to meetup with some other rotary enthusiasts here in Ohio, I am in the Cleveland area.

I am going to check the float bowls and see what they look like after a few days.