backfire - omc

[cimy3]

I have a 1984 OMC 140hp, 4 cylinder. I just had a tuneup. My problem: engine will not start and it backfires.<br /><br />I am suspecting a bad tune up job...on the invoice I just see labor...not itemized by service area.<br /><br />My guess is the dwell and timing were not set properly? <br /><br />Thanks for your suggestions.

 

[rbezdon]

Re: backfire - omc

if the wires were replaced (which is a good idea) it could be the timing order is wrong. There is also a possiblitiy it is not related to the tune up. Maybe you can give us more info or history. Was the boat running fine before tune? If the cam timing were off from a damaged cam drive (timing chain) or a valve is stuck open, this could also account for this condition. If the engine was running poorly and backfiring before the tune, it is possible the condition simply was not "cured" by the tune up.

 

[cimy3]

Re: backfire - omc

Before the tuneup, the engine lost electrical power...getting no spark. Furthermore, year after year, it has been a solid running engine. No problems to report.<br /><br />So, this is the reason for the tuneup. For the tuneup, I had the ignition coil, points, condensor, rotor & cap replaced.<br /><br />How would I correct(or better yet check the wire order to see if that has been put together wrong<br />)?

 

[rbezdon]

Re: backfire - omc

Crank or rotate the engine by hand till the crank is in the top dead center position for #1 cylinder. This can be identified by the timing marks on the crank harmonic balancer and the timing case cover. Be careful to make sure you are at TDC for the power stroke of #1 and not between the intake and exhaust cycles. BEst way I have to know is to do the rotation with the vavle cover removed. Then watch the valves for #1 as you approach the timing marks. If the exhaust valve just dropped and the intake starts to open, you are on the wrong cyle. Some others may have a simpler way to know. Once you have the engine in the right position, remove the distributor cap and see which terminal the rotor is pointing to. Now you know which terminal on the distributor cap is number one. Trace the wire in this terminal and make sure it is connected to #1 spark plug. Look up the firing order for you engine in your manual (I do not know your firning order off of the top of my head but it is not hard to find, in fact I bet many on this site do know it off the top of their head). Since the distributor usually rotates in the clockwise direction looking down on it, the next terminal better have the wire from the next cylinder in your firing order in it. Then check the next and the last. If all wires go to the right cylinders in the right order, your in good shape here and look elsewhere. Many of the other items mentioned earlier like timing, dwell etc could also be the issue.

 

[lowshovel1]

Re: backfire - omc

did you tell the shop that you were having a electrical problem when dropping boat off for service? or did you tell them to do a tune-up and assume that would fix your elec. problem?

 

[pine island fred]

Re: backfire - omc

When you say backfiring do you mean blowing out the exhaust ( big bang) or flaming back out the carburator? I agree with others, wires crossed or cap or rotor in incorrect position. Get it fixed, you are missing so nice days on the water. regards fred