350 sputtering under load.

alk

alk

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Some of you may remember this motor in my pacemaker from last fall when a rusted spark plug shot out of the block. SBC with carb, electronic ingnition. I ended up replacing the head over the winter.

Motor starts right up and idles fine, and runs up to 3500 rpm or so in neutral with no issues. But I finally launched this morning - boat feels a bit sluggish up through 2000 rpm ( guessing, tach conked out on this motor) and if I pushed it further, the motor makes when I can best describe as a popping sound, perhaps shooting air back through the carb, with a significant loss of power. After the second attempt, I throttled back towards idle, and made my 20 mile trip at 5 knots, motors sounded fine at that speed.

So where do I start? Best guess is something I didn't do right when assembling the motor with new head. I thought I had the timing well set, 29 degree max advance, although at idle it was still at 12 degrees, I was expecting 7. Tach stopped working, I blamed this on a pretty fast idle and let it go. I'm pretty sure I got the valves right, nothing else I can think of that I would have gotten wrong. The cap and rotor didn't look great, but never had time to pick up new ones.

Is a sputter at this rpm under load more indicative of a fuel intake or ignition problem, or could it be either? Or something else entirely? Thanks.
 
Yeah, I suppose I should start there; having trouble finding the right replacements because I don't know much anything about these motors, not even the year.

One thing I just remembered; when I picked up the replacement head, it did not have the same casting number as the one that came off, and the combustion chambers weren't even the same shape. I thought it was pretty odd to run a motor with two different heads, but the machinist I bought it from said that it wasn't a concern; and the guy came highly recommended. Im also trying to remember if this was the motor where the electric choke was hot as hell as soon as the motor started.
 
You got tons of things that need to be clarified before any diagnosis can be started. You need to know that the engine is mechanically correct before guessing about anything else.

First thing, the head. Is the machinist a marine qualified machinist. I found out the hard way. Machinists think a motor is a motor...and not any head will just swap without a hitch.

What year is the engine? Also, the different combustion chamber size has me worried. Might be one of the older 'heavy' castings from the 'Muscle car' days with small combustion chambers.

Getting a rough idea of the year of the head. Is it a 'Heavy' or 'Light' casting. Look at the lower edge just below the spark plug holes. Is the edge straight or does it have a 'scalloped' look. The older 'heavy' casting will give you problems if a valve job is done on it. Ex seats aren't hardened.

Who set the valves? I've done valves from VW's to Honda's, Harleys to Chevys. I check them and re-check them and re-check them. Solid lifter are more critical but it's easy to F-up hydraulics, too.

Check and re-check the plug wires. I been doing them for many decades and I constantly check the plug wires and make sure the plugs wires are secure on the plugs.
 
Cap and Rotor are easy and a likely culprit. I've had three cases now where replacing them solved a miss/popping back condition. In two of those cases, the center contact of the cap (the one connecting Coil to Rotor) was jammed instead of moving on its spring.
But be sure, too, that you didn't get the firing order wrong. 1-8-4-3-6-5-7-2
 
Andy where are you currently keeping the boat? This is more curiosity than anything else. If it's close to my travels I could give you a helping hand if needed. I'm not a gear head but I'm pretty good with distributors and ignition circuits. Is it possible the main engine harness has a corroded connection/ connector?
Bill
 
The machinist for sure is marine qualified. While his shop is not exclusively marinw, its in a boating area, and was recomended by a mechanic at my marina. He told me he used phosphate valve seats or something similar on all his marine heads.

These are pre-vortec heads, the ones with the angled intake bolts. Center bolt valve covers, 14096217 was the casting number on the original, not sure about the replacement. Late 80's crAte motor. I looked up the new head, and it had same size chamber, 64cc, and same size valves, but shaped differently.

I'm pretty sure I got the plug wires right. First attempt I was not right, and the motor coughed when cranked. I redid it more carefully, and it idled perfectly. But I'll check again.

I did the eight valves myself, hydraulic lifters.

Boat is in Georgetown MD now.
 
Most likely your other head has the 72cc heads. 64cc heads is not the usual combustion size. Check that lower edge of the head to ID the era of it's manufacture. Though the motor will run with different size combustion chamber, they should be the same. Kinda like a bicycle with different length peddles.

Check them plug wires again and again. #5 and #7 not only fire sequentially, they are next to each other. The engine will start and idle fine but will not run up. Kinda like your problem.
 
I would check ignition too. They will idle ok, but if still points/condensers, under load it will do what you are describing.
 
With apologies for adding to your list, a clogged remote, on-engine or carb inlet filter( or other fuel or vent restriction) , or weak fuel pump or air leak into suction side of the fuel system can also cause the power loss and lean carb-popping you describe. As absolute suggests , you might visual & smell check a fuel sample too.
If the electronic ignition has a Timing mode , was it reconfigured back to Run mode?
 
If it is a Merc with a kill switch (shift interrupter switch), check it - my boat ran great with no load, but popped and backfired badly under load. $30 switch was issue.
 
Did you check firing order again? #1 on the cap is between 5 & 6 o'clock and count clockwise from there 1-8-4-3-6-5-7-2. It's easy to lose one of the plug wires as they tangle up. Pull all the plug ends and spread them out on top of the engine and do each one in order.
 
Thanks everybody, but I haven't had a chance to work on this motor yet. I have a new cap and rotor, as well as a new set of plug wires sitting in my car, just didn't have a chance the past few weekends. Fuel should be good, and just to be safe I ran both engines off one tank, and starboard side ran ok with same fuel, and through sme filter. While these are all great suggestions, keep in mind that I just removed the intake and head, bought a new head, and put everything back together. So it's a very safe bet, albeit not a guarantee, that the problem is related to something I got wrong when reassembling. If I ever get a chance to run it again after re-running the plug wires, I'll report back. Next I will probably check the valves again. I think I got them right, if not, would that cause my symptoms, or am I chasing a dead end there?
 
I have my heads off and out at shop getting re-conditioned.
One bad intake valve and I was advised all Sixteen valves should be changed/re-seated, etc and get new springs while at it. Five different respected mechanics told me to do BOTH sides at the same time so compression is equal.

There is no way I'd put on a different shaped head. If the void on the head is not the same, the compression comparison would be way off.

At this point, you should do a cylinder compression check. If they are the same, perhaps you are ok. However, I'd still worry about whether intake and exhaust valve sizes are the same.

My recommendation - compression check and report back.
 
Yes, compression and vacuum tests are well warranted. While the heads have different casting numbers and cylinder shapes, I've confirmed via multiple sources that the intake and exhaust valves as well as combustion chamber are identically sized. Looking back I should have replaced both of them, but I'm close yo done with this boat, which really isn't worth anything, so the cost of another head, and more importantly the amount of time to replace it wasn't something I was willing to invest. The machinist was adamant that the mixed set would work for me, and it wasn't just to make a sale, because he had the matching set which he broke up for me.

These motors were pretty new, only reason first head failed was a leaking manifold basket that the previous owner never fixed caused one of them to rust out. Valvetrain on both looked brand new. I'm leaning towards mechanic error (me) during reassembly.
 
When I'm done re-installing I plan on doing a compression test on heads as well as pressure testing the coolant prior to installing spark plugs. I think it's a valuable step
 
Walshie, depending on the type of head gaskets used, you might just fail that test. Some of them require going through a heat cycle before they seal off. Almost every engine I have built, I run it to full operating temp, hold it for ten minutes or so, let it cool, then test it, just to make sure all the seals have sealed off........D..........
 
Yep, I will do that too. And re-torque the head bolts after some heavy running.
 
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