New 4.3L Mercruiser Wont start, backfire through carbs

[leglesscrow]

I just finished putting together and installing a refurbished 4.3L long block. Got everything wired up and installed now im attempting to start it and it wont start. Ive got fuel and spark and Ive checked and rechecked my firing order and distributor location vs TDC and timing. It ALL seems to be correct. I even went as far as making sure to have the proper AC Delco plugs in it. The motor turns over fine but doesnt even try to start. Then about every 5 or 6 revolutions itll backfire through the carb (4bbl). I am a technician by trade and this is a customers boat but this is completely blowing my mind. Ive never seen this before and I just cant figure it out. Any help or comments would be greatly appreciated. THANKS!!

 

[Volphin]

Re: New 4.3L Mercruiser Wont start, backfire through carbs

Double check the distributor. Wire hookups should be approx 10:00 position. Ensure you are on TDC, rockers not moving on #1. Sounds 1 tooth off. Also, check that rockers, if using adjustable ones, are not too tight.

V

 

[leglesscrow]

Re: New 4.3L Mercruiser Wont start, backfire through carbs

Ok! Are you saying that it sounds like the distributor is 1 tooth off?

 

[stonyloam]

Re: New 4.3L Mercruiser Wont start, backfire through carbs

That sounds like it was TDC on the exhaust rather than compression stroke. If it is backfiring through the carb a intake valve is open when the plug is firing. Agree double check the distributor at TDC.

 

[leglesscrow]

Re: New 4.3L Mercruiser Wont start, backfire through carbs

I used my compression gauge hose and felt for the compression stroke to make sure I was at compression TDC.

 

[stonyloam]

Re: New 4.3L Mercruiser Wont start, backfire through carbs

Backfire through the carb would be from the timing being way advanced (assuming all of the wires are correct, and it sounds like they are) Right? So what I would do (you have probably done already). Very carefully mark the side of the distributor at the #1 plug wire. Bring up to TDC and pull the cap off and the rotor should be pointed at the mark +- a few degrees. If not turn the distributor so the rotor points at the mark. Should be close enough to start. Good luck.

 

[leglesscrow]

Re: New 4.3L Mercruiser Wont start, backfire through carbs

Well you guys got me second guessing myself so I went and checked it one more time. This time, IDK HOW, I found that I was 180 degrees out. Thank God! Problem SOLVED! She fires up and runs like a champ. But now I have a new minor problem. I got a "ping" coming from my exhaust. Its steady. Almost like a metronome keeping a beat. Motor runs fine and sounds fine on the inside of the boat but the "ping" is coming from the drive on the outside.

 

[Volphin]

Re: New 4.3L Mercruiser Wont start, backfire through carbs

Well you guys got me second guessing myself so I went and checked it one more time. This time, IDK HOW, I found that I was 180 degrees out. Thank God! Problem SOLVED! She fires up and runs like a champ. But now I have a new minor problem. I got a "ping" coming from my exhaust. Its steady. Almost like a metronome keeping a beat. Motor runs fine and sounds fine on the inside of the boat but the "ping" is coming from the drive on the outside.

Glad it started! That ping would have me concerned as well. You don't have a sticky valve do you? Is it running smooth? What happens when you put it in gear on the muffs? Does it change with RPM?

V

 

[Kaplooi]

Re: New 4.3L Mercruiser Wont start, backfire through carbs

I take it this sound is different from the back-flow valves flapping in the Y-pipe? If there truly is a detonation problem, you can try dialing back the distributor base timing some and see if it goes away but I can't imagine why you'd be getting detonation if the base timing is set correctly. You could try higher octane fuel but that's no replacement for timing being set incorrectly.

 

[Bt Doctur]

Best to start you own post . Please descbibe what work you have done.