In April we pulled the heads on my 502 due to low compression on two cylinders and had them refreshed. All cylinders now read 150+ PSI. While it was apart I had the injectors bench flow tested and cleaned. We put new rotor, cap, wires, and plugs on the motor. Bolted everything together and added new fluid and filters. Set the engine to top dead center on cylinder #1, installed the distributor with the rotor pointing to #1, put new cap wires and plugs in and went to fire it up. All we are getting is back fires and nothing else.

Things we have checked two and three times:
-checked the firing order
-checked that the wires are going to the correct cylinders in the correct firing order
-replaced and checked coil with a known good one
-replaced and checked distributor with a new billet msd distributor and new rotor
-we are getting fuel in the cylinders
-we are getting spark
-we have tried turning the distributor
-oil pressure is good
-checked that the injectors hooked back up correctly
-checked kill switch
-checked for loose connections

Can anyone else suggest anything to look for? Someone mentioned to me that maybe the computer lost it memory due to be unhooked for a few months from the battery. Suggested to bump the throttle to get it to start and then hold it higher than idle until the computer relearns. So far we cannot get it to attempt to start.

Any suggestions would be appreciated.

Sounds like very advamced timing. Ive always marked my distributors and the block on teardown so i can get them close enough to get it started..

Sounds like you have the wires 180 degrees off.

Your positive your getting fuel in the cylinder? I had pretty much the same thing happen to me after re doing my top end on my 502 also. The cleaning solution injector service companies uses causes the pintle to get stuck closed if there not put into service right (1-2 weeks) after cleaning. Take a screw driver and tap on the side of each injector a few times. Then try to start it.

I pulled my hair out (2 weeks worth of troubleshooting, buying tools, noid testing kit, Rinda computers, gauges, ect ), trying to figure out why mine wouldn't start and would just get a back fire every once in a wile. The whole time it just took 1 min and tap on the injectors to free the pintle. I got the idea after talking to another fellow boater that said (if I remember correctly) it was common on the Merc 1050, 1075 or something like that, were the pintle would constantly get stuck and had to tap on the injecotrs before starting the engine.

That is what we thought originally but we pulled the distributor and set the motor to top dead center on the number 1 cylinder and reinstalled the distributor with the rotor pointing to number one on the cap. We have pulled the distributor out twice and tried this, as well as, advancing and retarding the timing to try and get it to fire.

Yeah we are positive we are getting fuel, it was seeping out of the plug holes when turned the motor over during the compression test. We are going to try hooking up a fuel pressure gauge tonight to check that. I will also try out your tapping on the injectors suggestion. At this point, we are willing to try anything.

BIGSLOW, are you sure you are at TDC on #1 (compression stroke, not exhaust) when installing the distributor?

Call me tonight if you want to chat.

slow... sounds 180° off to me as well. also I just had a bad control module in my distributor give me fits.. find the testing procedure for coil - module.. ect and rule that out.
didn't know you been doing this boatin thang.

swing by the marina and say hello - I'm on b house.

Casey Rickard. (449-5237)

U are either 180 out or you adjusted the valves to tight. When you have a big backfire its because you have ignition while the valves are open.

If you are having trouble figuring out compression stroke on #1 pull #6 plug put finger over #1 and #6 at same time and have someone spin it over when 6 is on exhaust 1 is on compression you will be able to feel the difference.

When you say you're turned the distributor, how have you done it? If you're off 180 degrees (that's what it sounds like) you will have to turn the crank one revolution (with the distributor out) and drop the distributor back in, lined up on #1. Another method is to leave the crank lined up as you have it, pull the cap off the distributor, raise it and MOVE THE ROTOR 180 degrees... that would get you close on timing if you're off 180 degrees. I've made the same error myself...