Engine knock

My engine started knocking a couple of weeks ago so I took it in to see what is going on. Mechanic told me it was spark knock. So he replaced spark plugs and adjusted the timing. He told me he put the timing back to factory specs but it still knocked some so he retarded the timing until it stopped knocking. I took the boat out today and it was not knocking but I think since he retarded the timing so much the engine has started to lose power. I want to adjust the timing back to factory specs so I gain the lost power back, but e engine will begin knocking again. since retarding the timing solved the knocking problem it is a pre-detonation problem. What else could be causing this problem. It is not bad gas I burn a tank of gas every week and I always put 89 octane in it. Could the carburetor need to be cleaned and adjusted? Is it the distributor? I don't want to take it back to the mechanic he has pissed me off so I was looking for some advice on where to look next. The engine is an Indmar assualt 310 it is carbureted. Any advice would be welcomed.

What fuel are you running? Try a higher octane? We had a similar issue and started running premium fuel and it stopped. When summer ended we did a full tune up and haven't had an issue since

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Not all of this is relevant, but there's definitely some possibilities for you look into...


Spark knock (detonation) is an erratic form of combustion that occurs when multiple flame fronts occur simultaneously inside a combustion chamber. Detonation occurs because fuel is subjected to either too much pressure, too much heat or both. It usually happens during acceleration when the engine is heavily loaded and cylinder pressures are at their peak.

Instead of a single flame front growing outward smoothly like an expanding balloon from the point of ignition, multiple flame fronts are generated spontaneously throughout the combustion chamber as the fuel automatically ignites from heat and pressure. The multiple flame fronts collide, creating shock waves that produce a sharp metallic pinging or knocking noise.

Mild detonation can occur in almost any engine and will not cause damage. Prolonged heavy detonation can crack pistons and rings, blow out head gaskets, damage spark plugs and valves, and flatten rod bearings.


Any of the following can cause detonation:

Too Much Compression: An accumulation of carbon deposits in the combustion chambers, on piston tops and valves can increase compression to the point where it exceeds fuel octane rating. If a top cleaner fuel additive fails to remove deposits, a new alternative is to blast the deposits loose by blowing crushed walnut shells through the spark plug hole. Otherwise, the head will have to be removed so the deposits can be scraped off.

Overadvanced Ignition Timing: Too much spark advance causes cylinder pressure to rise too rapidly. If resetting the timing to stock specifications does not help, retarding timing a couple of degrees may be necessary to eliminate knock.

Engine Overheating: A hot engine is more likely to suffer spark knock than one which runs at normal temperature. Overheating can be caused by low coolant, a defective fan clutch, too hot a thermostat, a bad water pump, etc. A buildup of lime and rust deposits in the head and block can also reduce heat transfer

Overheated Air: The thermostatically controlled air cleaner provides the carburetor with hot air to aid fuel vaporization during engine warm-up. If the air control door sticks shut so that the carburetor continues to receive heated air after the engine is warm, detonation may occur, especially during hot weather. Check the operation of the air flow control door in the air cleaner to see that it opens as the engine warms up. No movement may mean a loose vacuum hose or a defective vacuum motor or thermostat.

Lean Fuel Mixture: Rich fuel mixtures resist detonation while lean ones do not. Air leaks in vacuum lines, intake manifold gaskets, carburetor gaskets or fuel injection intake plumbing downstream of the throttle can all admit extra air into the engine and lean out the fuel mixture. Lean mixtures can also be caused by dirty fuel injectors, carburetor jets clogged with fuel deposits or dirt, a restricted fuel filter, or a weak fuel pump.

The air/fuel ratio can also be affected by changes in altitude. A carburetor calibrated for high altitude driving will run too lean if driven at a lower elevation. Altitude changes are generally compensated for on computer cars by the barometric pressure sensor.

A lean fuel condition can be diagnosed by watching for lean misfire on an ignition scope, or by using a four-gas infrared analyzer and watching exhaust oxygen levels. A reading over about 3% to 4% oxygen would indicate a lean fuel condition.

Spark Plug Too Hot: The wrong heat range plug can cause detonation as well as pre-ignition. Copper core plugs are less likely to cause detonation than standard spark plugs.

Loss of Exhaust Gas Recirculation (EGR): EGR keeps combustion temperatures down, reducing the tendency to detonate. If the EGR valve is inoperative or someone has disconnected or plugged its vacuum hose, higher combustion temperatures can cause pinging.

Low Octane Fuel: Burning cheap gas may be one way to save pennies, but switching to a higher grade of fuel may be necessary to eliminate a persistent knock problem.

Defective Knock Sensor: The knock sensor responds to frequency vibrations produced by detonation (typically 6 - 8 kHz), and signals the computer to momentarily retard ignition timing until detonation stops. A knock sensor can usually be tested by rapping a wrench on the manifold near the sensor (never hit the sensor itself). If there is no timing retard, the sensor may be defective.

Thanks that is exactly what I was looking for a list of things to trouble shoot. I was already running 89 in because that is what was recommended.I figured running 93 in it would help but that would just be like a band aid for now and I would really like to find the real issue or issues

Check your timing curve, ensure the timing always returns to base and advances smoothly throughout the RPM range. Check timing at idle, 1000 RPM, 1500 RPM, etc. until you get full advance. Should be all in by 3000 RPM. Do this 2-3 times to verify it is consistent and always returns to whatever base timing you set it to.

If the timing advance is at all inconsistent or erratic then remove, clean, and lubricate the advance mechanism. You should do this every few years regardless, as the lube gets thick and since it's always near water the mechanism rusts eventually. If you notice timing advancing more quickly than it should it's possible one of the springs in the distributor is broken and it's advancing too quickly. You can also adjust the rate of advance by bending the tabs the springs attach to to increase spring tension slightly. Also stronger and weaker springs are available to tune the advance curve.

Ensure you're not running lean as a lean running engine will detonate more easily.

Ensure you're not running hot, hot engines detonate more easily. Check manifold and riser temp on each side of the engine and temp of each head if you suspect a temp problem. IR temp guns are cheap and very useful.

If the detonation occurs when loaded up more heavily but not so much at lower loads then you may be bogging the engine. Reprop to get the engine more in it's torque curve if running very heavy when this occurs. You can also jet richer to cool the mixture and lower cylinder temps.

It would help if we knew what type of engine this is in, what boat, and under what conditions the detonation occurs.

it is a 2001 Mobius with a carbureted 5.7 indmar. The first time it happened we had been out riding for about an hour and a half then stopped for a about 15 minutes. when we started to pull up the next rider it started knocking real bad at about 1800 rpms and about 15 mph. Once the boat planed off and most of the load was taken off the knocking stopped, but if you accelerated again it would knock again. I don't believe has to do with the engine running hot because we took it out the next day with a friend of mine so he could listen to it and as soon as we got it off the trailer and throttle up it began knocking. Engine has 840 hrs on it

Check your timing at 1500, 1800, and 2000 RPM if that is where it detonates most easily. Also what is your initial timing set at? Makes sense as 15 MPH you're not quite on plane yet and load is highest at a fairly low RPM. Does this engine use a standard distributor like a Mallory or Prestolite? If so pull the cap and rotor and the ignition pickup/points plate and check the advance mechanism for rust and broken springs. If one spring is broken it will advance more quickly and can detonate at lower RPM and high load from too much timing.

Also I'd suspect a 2001 Indmar may have a knock sensor and electronic ignition box. Replace the knock sensor if it has one. Typically they're mounted in one of the block drain plug holes down low, though they may be mounted in the end of one of the heads.

The engine should not knock when set to specs...

for some reason I doubt you are actually having preignition.

Make a video and let us hear this knocking

Checked the timing tonight and it was set at 10* ATDC. I called the mechanic that made the initial adjustment and he said they set the timing to the factory specs I believe he said 8* BTDC and it was still knocking. So they retarded it until it stopped knocking. I am thinking since it knocks only under load at higher rpms it has to be something wrong with the distributors advancing. Tomorrow I going to take the boat out on the lake and advance the timing back to factory specs then retard it till it stops knocking. I will also check the total advance. Does anyone know what the total advance is supposed to be? On these est distributors, do they still use springs or weights to control the advance or is it electronically controlled

Not sure on total advance for that engine but you should be in the ballpark of 30*-36* by 3000 RPM, probably toward the lower end of that in a marine application. I'd shoot for 32* total.

As I mentioned before I suspect your timing is advancing too quickly. Also note that severely retarded timing such as 10* after will make your engine run hot and increase deposits on plugs and chamber. Are you certain the plugs were replaced with the correct ones? If the mechanic put in a hotter heat range plug it could be the ground strap heating up too much and igniting the fuel prior to the spark. Verify the plugs are what it ran well with previously and what Indmar calls for.

If you have a mechanical advance distributor you should be able to pull the cap and twist the rotor by hand. It will twist by hand to advance and when released return to original position... Unless of course the advance mechanism is rusted and not advancing. If so it may move but not freely or not return to base timing without assistance.

Regardless, here's a timing document you may find useful. Note that for electronically advanced timing you need to short a jumper or apply 12v to a wire similar to most newer cars with electronically controlled timing advance. If not done the timing will not be correct. http://www.bakesonline.com/images/Me...tionTiming.pdf

Yea the timing shunt was in place on the distributor and when I hooked it up to 12 volt power it placed the engine in base timing mode which is where I shot the timing. The old spark plugs were in plac whenit orginally began knocking i will check what is recommended and what the mechanic plqced in there. AlsoI am not 100 percent sure but I believe this is not a mechanical distributor. I think the ignition module is responsible for advancing the timing and if it fails the engine typically does not run at all... Any thoughts

If you have to jump a wire to set timing it's electronic advance not mechanical. In that case set initial to where it needs to be and watch the advance curve, specifically what the timing is at 1800 RPM where your detonation is most prevalent. I am not familiar with how electronics fail, only that they are less reliable than mechanical systems and much more time consuming and expensive to troubleshoot and repair. I'd suggest checking your knock sensor wiring and replacing the knock sensor.

alright so I had a chance this afternoon to look at the boat again. Here goes...initial timing set at 10* ATDC and timing at 2000 rpm and 3000 rpm was 6* BTDC. I advanced the timing back to factory specs of 8* BTDC and at 2000 and 3000 rpm it was 26* BTDC. . I don't know what the total advance is supposed to be so I don't know if this is correct. Oreilly's can check the ignition module but it will only check to see if it is working not if it is advancing correctly. Called indmar about the knock sensor and carbureted engines do not have knock sensors. Right now I am out of ideas...other than carbon build up so in the mean time I have been running seafoam through the carb and letting is sit in the cylinders for about 30 minutes.

26* at 2000 RPM is fine. Seems a bit low for total advance at 3000 RPM but not unreasonable. It should not detonate. If it's detonating with 6* total timing then you have other issues than timing. Could be running lean. What type of carburetor? Maybe rebuild it and check jetting. Check your plugs to see if they read lean.

I'm surprised you can run it with initial timing set to 10* after. It would run like crap, have very little power, idle poorly, be loud, run hotter, probably have stumbling and hesitation on acceleration, maybe smoke from exhaust. Plugs are probably fouled if you ran it with timing that far retarded for long. If it actually ran reasonably well with timing set that far retarded I'd check to see if the balancer has slipped the outer ring. They are typically rubber and will break down over time. Use a piston stop to determine true TDC on #1 and compare to the TDC mark on balancer. If they don't match, replace the balancer. You could think you're setting timing to 8* but it may actually be 18*. Though anything above 14*-16* initial and engines get pretty hard to start as they'll kick back against the starter.

I am going out on it this morning and I will record the idle and the knocking sounds. When timing was set to 10 after and total advance was at 6 it still idled fine but when throttling up there was a noticeable loss of power.

Went out and did some water test today and this what I found. I advanced the timing back to factory specs of 10* BTDC to see if the seafoam cleaning made any changes. Also I was running 93 octane. No changes engine still began knocking around 2000 rpms. I have video but don't know how to post it. Next I retarded timing until engine quit knocking...all the way down to 10* ATDC for it to stop. Next I adjusted the carb and made the fuel mixture richer and by doing this I was able to advance the timing back to 0* without knock. So I think I am on the right track now. I am going to take the carb off and do a full cleaning and possible rebuild. I believe there is only one vacuum hose to the manifold and that is from the PCV valve and I checked it for leaks so that is not causing the fuel mixture to lean out. the first and second pictures are of the rear left side spark plug. The area the knocking sounds like it is coming from. the third and fourth pictures are of the opposite side the front right cylinder. I don't know if you can see it in the pictures but on the rear left cylinder the plug is white one side of the electrode and black on the other while the front right cylinder was a tan ish color all the way around.

Pure white insulator can indicate lean. The ground strap looks hot, hotter on the left rear plug. This can indicate too hot of a heat range. You want the color change on the ground strap to be right on where it bends. Too hot heat range cna cause this or also too lean mixture. The second plug looks more normal.

Fuel distribution to the end cylinders will typically not be as good as the center cylinders, they can run leaner. Sometimes you'll need to jet a little rich for the center cytlinders to get the outside corners mixture right, this is ok. Going through the carb is a good idea, hopefully that helps this problem, I suspect it may since you richened the mixture and it helped.

Also look more closely at that first plug that looked lean with the clean white insulator. If you see dark specs on the insulator that is a dead givaway for detonation. It is specks of aluminum from the piston getting melted to the insulator of the plug. This is caused by the extreme heat and violent shock to the piston when detonation occurs.Typically detonation sounds like a rapid rattle, or shaking peddles in a can. It can also sound like a slower rate pinging sound, almost like tapping a chunk of metal with a metal rod.

So before I began tearing down the carb I thought I would check the compression on all the cylinders, since I checked everything outside the engine and was getting nowhere. Had almost no compression 30 psi on 5 cylinder and 43 psi on the 3 cylinder. I never thought about it before because the engine idled fine and ran good aside from the knock, but looks like the head gasket is blown. I hope this helps someone else down the line a compression test takes 30 minutes and I probably spent 6-8 hours troubleshooting everything else

Thanks for the update. Good to hear its an easy(ish) fix and maybe a chance for some upgraded heads and an intake.

Its posts like these which make me feel like a mechanical idiot

Hah I know how you feel all you need is a basic understanding of how an engine works, a good manual, and some free time to get in there and start tinkering around. Oh yea it also helps to have an engine without all the computers and complicated electronics of today's engines

Hah I know how you feel all you need is a basic understanding of how an engine works, a good manual, and some free time to get in there and start tinkering around. Oh yea it also helps to have an engine without all the computers and complicated electronics of today's engines