Ran good, popped and now not running good

Boat, '73 Aristocraft with a Mercruiser 165. Took the boat out for the first time this year and was running fine. Was cruising around 3500 rmp's and the motor popped/ backfired then started running real rough. Was able to slowly (just above idle) make it back to the launch. It would rev up, but was running real rough. Back at the house on muffs, it starts up fine with no problem but runs real rough. Have re timed 6 deg BTDC, new points, cap, rotor. Going to do new plugs/ wires. When timing, I put the plug wire lead on all the wires and a few were not make the timing light light up. I'm going to check compression when I can find a gauge to borrow/ rent. The other things I'm considering are bad fuel, checking fuel filter. What else should I be looking at or testing. Thanks in advance, and any questions just let me know.

Re: Ran good, popped and now not running good

Bump

Re: Ran good, popped and now not running good

Holley carb?If so check the power valve.

Re: Ran good, popped and now not running good

When's the last time you had a tune-up? Rotor, distributor cap, plugs, etc? If it's not firing on all cylinders, it ain't the fuel .....

Re: Ran good, popped and now not running good

Have re timed 6 deg BTDC, new points, cap, rotor. Going to do new plugs/ wires.

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Ayuh,... Where was the timing when it Popped,..??

When timing, I put the plug wire lead on all the wires and a few were not make the timing light light up.

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But are the Sparkplugs Firin',..??

Re: Ran good, popped and now not running good

pull a plug cap at a time and see if tone of engine changes to prove spark but i would start with cap,points and condenser and check your timing and get yourself a manual.

Re: Ran good, popped and now not running good

the timing was still on 6 deg. BTDC, no change there, thats when I noticed the timing light wasn't firing everytime. I'm going to do new plugs and wires then check for spark, just wanted to get any other ideas I could befor I started working on it. I guess I mis spoke when I said it "popped", it kinda coughed/ sneezed, and backfired then started running rough. I did take off plug wire #1 with it running and no change, then removed plug wire #2 and it immediatly died. This is what got me thinking spark problem, but was just confused on what causes an instant spark problem. I did a cap, rotor, and points this spring. Thanks again!

Re: Ran good, popped and now not running good

Anything you put on a boat new or not can fail I had mine out this past weekend just tuned up with new cap button and points missed above 3000 rpm put old condenser back in and ran like a dream.

Re: Ran good, popped and now not running good

just a little update. I now have put in new points, cap, rotor, plugs, condensor. Waiting to put on new plug wires (they had to order them) and its still running rough. Using a timing light, I'm not gettting the timing light to light up on plug wires 2,4, or 5. When I pull these wires off the distributor cap (one at a time) with the engine running, there is no change (still just runs rough/bad). When I pull the other wires off the cap, the motor imediatly runs a lot rougher and almost dies. When I switch plug wires around it's still #'s 2, 4, and five that aren't lighting up the test light. I tried putting on the old distributor cap and had the same thing going on. Still wating to do a compression test once I locate a gauge I can borrow. Any other ideas, I might surrendor and have to take it in to a shop. Thanks in advance.

Re: Ran good, popped and now not running good

73Aristocraft said:

Still wating to do a compression test once I locate a gauge I can borrow.

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Autozone will loan you one for free.

Re: Ran good, popped and now not running good

I know, but all we have here is Carquest and Napa and they don't rent tools.

Re: Ran good, popped and now not running good

Wow, it seems almost impossible that three cylinders would go bad at the same time, unless something was injested through the carb while you were running. The only thing I could think of would be bent valves. But, only running on three would require the idle to be stepped up quite a bit just to keep it idling.

A quick compression check can be done with a piece of dowel slightly larger than the sparkplug thread diameter. Pull all the plugs and remove the coil lead to the distributor. Press the dowel into each sparkplug hole and give the engine a short crank (enough so that you know that it went through at least one compression stroke. A good cylinder will blow the dowel back forcefully. A weak one not so well. Try it on the known good cylinders first so you know what a good one feels like.

Re: Ran good, popped and now not running good

Yeah, I thought of bent valves. I know it's just weird to have three go at a time. I don't know if it's electrical or something broken. It still idles at normal speed. I think I'll check spark, and check the wires again (w/ DVOM) and then I'll surrender to taking it to a shop. It's finally starting to get warrm here, lol. Thanks again!

Re: Ran good, popped and now not running good

73Aristocraft said:

weird to have three go at a time.

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Always look to the simplest explanation. Yeah very unlikely 3 at once, however it is very easy for 2 to go, blown gasket between 4 and 5. If 2 was already weak, you have your 3, and all of a sudden a poor running engine. Just wild speculation.

Re: Ran good, popped and now not running good

stick to basics first he said no timing light responce from multiple plug wires timing light doesnt descriminate... im a auto mechanic not a boat mechanic but alot can be looked at the same and throwing parts at it is never a surefire way to fix a problem theres alot of suggestions on what to check or try but the fact u have multiple wires w no fire is first concern dont get me wrong a compression check to make sure all is well is never a bad idea if u fear damage but bad compression wont cause no spark as for your no spark condition 2 4 and 5 if i googled correctly are not sequential in firing order im on the assumption u have a single coil ignition and the wires w no spark are not changing we can assume timing is not moving it is possible the wires are at fault but the fact that it happened in a short order suggests otherwise but one thing bothers me is earlier u said u pulled number 1 wire and it did not affect the engine so that brings the question why then not now would love to help save u the repair bill but i need more info about what cyl are missing and are u sure number one missed before and not now

Re: Ran good, popped and now not running good

Thanks for the responses. Plug numbers 2, 4, and 5 are not firing per timing light not working. When I remove any of those wires there is no change, but when I remove one of the other wires the engine begins to run really rough and dies. I was thinking of the compression test because it came on all of a sudden on the lake and when it did it kinda backfired and/or coughed out of the carb, but I'm still not getting spark on #2, 4, and 5. I don't really know what's going on now, but I may or may not have 2 different problems? I can switch spark plug wires around and still get #'s 2, 4, and 5 not firing. Thanks again!

Re: Ran good, popped and now not running good

well no spark on three cylinders would be where i start with that all the mechanical issues your concerned with would not cause no spark on 3 cylinders sounds to me like you have a base ignition problem that needs sorted out but i would recommend verifying no spark with a tester or even lay a plug on the manifold or block and see if it is firing just to make sure we dont go in circles for a overlooked concern if you still have no spark look closely inside your distributor make sure the lobes for the points are not abnormaly worn if it is new enough to have a module i would conciter replacing it and make sure if it has a white paste with it to apply a thin layer to metal surface before assembly and clean the surface on the distributor its mounting too well

Re: Ran good, popped and now not running good

Think I got it fixed. Started at the begining and went to re-check the point gap and the points were'nt opening when bumping the engine. The little fiber bushing on the points had worn a little and/or they just slipped. Either way, gapped them again and running good now. Going out on the lake today to double check, but should be good to go. Thanks again for all the advice. Feel kinda dumb it was something so simple, oh well.