13bbuggy

I am new to this forum and fairly new to rotaries. I have a sandrail that I just swapped a 1990 13b N/A into (I had an 86 N/A but it was overheated by the previous owner and is no more). I have a RB intake with a Dellorto 48 sidedraft and I run a distributor with the leading going through an MSD 6AL box and the trailing hooked up stock. When I get it up to 5500 RPM's it starts to backfire randomly for some reason. I have checked the timing, run a fuel pressure regulator set at 3 lbs. and pre-mix my gas (I eliminated the OMP when I did the swap). I have tried new wires, coils, plugs, tightened all nuts and bolts again, re-grounded the motor, and I am stumped on this one. I have dual 2 1/2 in exhaust into single and it's probably only 18" long before it enters the muffler (Flowmaster 2 1/2 in and out). Any ideas would be greatly appreciated. I also know rotaries backfire if you accelerate and let off and mine does that also, but this is on acceleration.

13bbuggy

I've had a couple of people tell me it may be my MSD box, but it worked fine with the other motor for the 2 trips it kept running before giving up the ghost, so to speak.

13bbuggy

I just checked the timing again and with the leading at 5 the trailing seems to be at 25 or so, instead of 20 like it should be and it idles a bit rough. If I line up the trailing then it smooths out. Neither way eliminates the backfiring but I know that the 2 of them should be set at 15 degrees apart from what I can find. Is there any way to adjust one or the other to get it back to 15 degrees apart?

13bbuggy

Nevermind the last comment, I just looked it up and the distributor is the N249 from a 12a and it is set at 24.5 degrees apart, which explains the timing difference. Unless I'm wrong this also means that it is idling better with the leading at 0 or so and the trailing at 24.5. If I set it at 5 then the trailing would be 29.5 I think and it doesn't run nearly as well there. When I revved it up it goes nice and smooth and sounds ok until 5500 or 6000 when the backfiring starts. Also, the backfiring is kind of sporadic, not rhythmic or regular at all. There was some gas sitting in the tank the last 6 months, but I added 5 gallons of fresh gas a while ago so I would think it would've mixed up by now and be ok, and the carburetor isn't gummed up at all either.

mazdaspeed7

The spark plug location changed between the 12A and the 86-88 engines, and changed again for the 89+ engines. The different engines have different timing requirements due to the change in the trailing plug location. I believe you can play with the vacuum and/or mechanical advance to change the timing split. Maybe fix one or both the leading and trailing to a certain position to reduce the timing split.

13bbuggy

I checked the spark plugs today and 3 of the 4 are clean and moist (gas). The spark plug on the rear leading plug is darker and seems dry. Any ideas on this, and could the backfiring have anything to do with this?

13bbuggy

I solved the mystery. Apparently the guy who sold me the engine ripped me off. Only 1 of the rotors is firing and there is no compression on the other one. I hadn't bothered checking for compression because I was told it was a good motor and it did start right up after I installed it. I guess that ends it and I'm looking for a new motor since the rebuild kits are apparently over a thousand bucks, unless I'm missing something.