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Mercruiser 30 Backfires

metago

New member
"Hi Everyone,
I recently had


"Hi Everyone,
I recently had some boat work done at a local shop. They replaced the valve cover gasket and the boot around the u joints because i was taking a little water in. When i went to pick it up he also made mention that my rubber exhaust tube was old and cracked and should be replaced with the new upgraded one that Mercruiser suggests. The new rubber exhaust tube does not need to be connected on both ends and has a few slits in it to allow some exhaust out, for what ever reason. So they put it on. Now, i took it out and it was backfiring out the exhaust and had a general lack of power. I took it out a second time thinking it was just simply due to the kill switch not being properly secured, maybe it was causing some issues, but it did it again. It only seems to backfire when under a load. If im sitting at the dock i can rev the motor in neutral and its fine, if i put a load on it or if i try to run at a smooth steady RPM for to long it will backfire. I never had any problems with how the boat ran before i took it to the shop and had this work done.

I read another post where someone had a similar problem and the end result was that his exhaust was plugged. Could that cause this to happen? Should i take it back to this mechanic or did he steer me wrong on that exhaust?"
 
"ok, small update. I hooked u

"ok, small update. I hooked up the water and ran it in front of my house. The problem seems to happen if i have the stern drive is up or down, load or no load. I think it just becomes more obvious with a load and also becomes way more obvious if the stern drive is in the lowered position.

My first thought was that maybe when they replaced the valve cover gasket they had put the spark plug wires on 1 wire off on all spots thus changing the timing. I confirmed TDC and firing order and this is not the case.

I did also however notice the points appear to be very tight, if not closed. Im doubting this is what is causing the problem because of how good it was running before its shop trip and how bad it ran after and i'm sure they did not touch the points."
 
"Check your spark plugs wires,

"Check your spark plugs wires, I bet they got the firing order wrong.

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"Update:

I removed the plu


"Update:

I removed the plugs, the burn looks good on them, i also checked the firing order just to be sure, they have it right. It still had a backfire. This morning I ran out and got some new spark plugs just because they looked a little old, I also rechecked those points. They didnt appear to ever open. I gapped them to .022. Between the spark plugs and the points gapping it seems to be resolved, at least out of the water. So this is some kind of improvement. I will confirm this fixed the problem this afternoon when I take it back to the lake.

I find it odd if it really was just the points, its pretty coincidental that this happened right after i got it out of the shop and it was fine before i took it in. But never less i will keep my fingers crossed and hope it was only that.

Thanks KGhost for that firing order, i was curious about that.
And also thanks Guy for mentioning to recheck those points."
 
"Check the timing? Was the lo

"Check the timing? Was the lockdown screw tight on the points???? Loose screw and they'll move. Tight screw; had they been moved?
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"Well it ran GREAT, so i assum

"Well it ran GREAT, so i assume that was the problem. It ran better than it ever had since i've owned it ( only two months mind you). The screw for the points was tight, I dont think they could have moved by themselves so i assume either the spring broke down and gave a little or someone moved them. The scary thing is the oil link didnt get fixed properly so i have to take it back there to the mechanic again and have him fix what didnt get fixed the first time. I'm almost willing to do the repair myself just so i know whats happened to it."
 
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