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AQ131A with backfire and low compression

M

marktrux

New member
I have an AQ131A that developed a slight backfire when under heave throttle. A freind told me the timing was probibily off so I replaced all the ihnition parts (coil, points, rotor, cap, plugs and plug wires) then sent it to a local shop to have the timing set. He told me the engine wouldn't start or run due to low compression in two holes (I don't remember which two) and that it wouldn't be worth fixing. He was nice enough to offer me $500 trade on a boat he had for sale, but not being one to give-up, I declined the offer. I found some new rebuilds online this week and called another local shop about installing one for me. Not knowing the history of my engine he asked why I wanted to replace it, and I told him what the other shop had told me. He told me that both the low compression and backfire could be caused by blown headgasket (I dont think this is it) or by worn, pitted and sticking valves, possibly caused by ethanol in gas, and that it may just need head reworked. Could this be true or is it just wishful thinking? Any thoughts on this would be greatly appreshiated(sic). Thanks.
 
The AQ131 is an OHC engine with a camshaft and counter-shaft drive belt. These drive belts are wear items, and require routine replacement.... along with the belt tensioner system.
If the drive belt has slipped a cog, the camshaft timing and ignition timing will no longer be correct.
You should verify this before throwing the towel in.

.
 
The answer "it wouldn't be worth fixing" is, in my opinion, very premature without further testing.

A leak down test will help to determine the nature of the low compression.

If the low compression is in two consecutive cylinders, chances are the head gasket is blown. Replacing it won't be expensive.

If compression is very low in all cylinders, chances are the timing belt could have jump one or more teeth. Again, not an expensive fix.

If the problem is valves, examining the head will give you an idea if they are just gummed up or what is causing the problem.

If the problem is worn piston rings but the block is still good, re-ringing won't be expensive either.

And if the problem is a worn-out block, you can always buy a car engine from a wrecker and marinize it.

None of the options is terribly expensive, especially if you are the one doing the work.
 
If the gears are pitted , it is better to replace them, especially if the boat has been running in salt water. Pitted gears can chew-up a new timing belt in no time.
 
I'll check the timing marks A.S.A.P. if they're in sync, I'll look into the other options. I just couldn't believe it was an all or nothing deal.
 
I got word from the shop that I've lost compression on #1&4 cylinders. I'm thinking about popping the head off myself and having a look at the bottom side of the head and cylinder walls. If I decide to re-ring it, can this be done without pulling the engine and replacing the main and rod bearings? I wouldn't mind replacing the rod bearings but I don't have a place(or the knowlage) to pull engine. It had good oil psi when it quit running.
 
I would do a leak down test and try to pinpoint where the leak is taking place. If the air goes to the crankcase you have a bad piston or bad rings which could also mean scored cylinder walls (that would be bad news). If it goes to intake or exhaust manifolds that would be valves, which would not be difficult to fix.
 
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