Knocking after replacing fuel injectors

Kenneth K

Sustaining Member
Blogs Author
Ken, as I have recently half-watched a youtube video about diesels vs gasoline engines, I am now a certified online diesel mechanic. It could be that if something in the cylinder is already hot enough to start the combustion earlier than desirable, the very very first fuel coming out of the injectors might light before the entire spurt of fuel has been injected, is compressed, and ready to go. this would probably create a noisier/rougher shockwave than if everything was sub-second perfectly running.
Yeah, that makes sense. One article I read about gas-engine knocking talked about a "controlled burn/expansion" versus a "rapid explosion" in the cylinder--the latter causing knocking.

I think we're all (or, many of us) becoming youtube and EYO certified experts now. Makes me wonder how the hell the average guy knew how to maintain his boat back in the 80s-90s????
 

Tin Kicker

Member III
Really ought to set aside any talk of a spark ignition gasoline engine, as it it a completely different concept. The fuel is mixed with air become to become a turbulent mixture entering the combustion chamber, and when the spark plug fires the flame propagates through the mixture faster than the injectors are open in a diesel engine. The art in the gas engine design is to keep it from going supersonic, because then there is an explosion rather than a controlled burn and release of pressure on the piston. Carbon flake embers igniting a turbulent mixture can lead to a supersonic flame front detonation/knocking which is essentially like a sledgehammer hitting the top of the piston on every other revolution.

In diesel ignition, the fuel/air mixing doesn't happen till the air is being drawn into the cylinder and is initially too lean till the piston is nearing the top of travel. Then the mixture is compressed to heat until it finally ignites and iirc the flame propagation is roughly a quarter (?) that of a gasoline engine. The gas engine combination of pre-mixing, turbulent mixtures, a precise timing of the spark are reasons that gas engines can turn thousands of RPMs faster than diesels. On the flip side, the slower burn and push of the diesel mixture allows it to make more torque more efficiently to push sailboats on less fuel. Because of the need to reach stoichiometry, less turbulent conditions, and slower process to build pressure for ignition, a carbon flake can cause pre-ignition but it is harder to achieve & to create damage.

Diesel problems are more often related to fuel quality which can alter the stoichiometry, the slightest amount of air in the HP portion of the injection system since the air acts like a spring to absorb pressure which should be forcing injector springs open, Internally leaky HP pumps not delivery sufficient pressure to open injectors, and leaking or dirty spray nozzles.
 

Geoff W.

Makes Up For It With Enthusiasm
Blogs Author
Is there any reason I couldn't buy a length of 3/16 ID fuel hose to use as the jumper return lines between the injectors? Is there a specific reason it needs to be the universal part with the little fiddle clamps?
 

Kenneth K

Sustaining Member
Blogs Author
Because of the need to reach stoichiometry, less turbulent conditions, and slower process to build pressure for ignition, a carbon flake can cause pre-ignition but it is harder to achieve & to create damage.
Interesting discussion on diesels, and one I hadn't heard before.

So, is it the need for a "chemically balanced" fuel-air mixture that explains the old yarns about: 1) why diesels don't like changes in RPM and prefer to be run all day at a constant speed (presumably, at the high end of their torque & power curves), and 2) why over-propped engines (run at a low rpm) burn fuel poorly and develop soot?
 

Christian Williams

E381 - Los Angeles
Moderator
Blogs Author
buy a length of 3/16 ID fuel hose

I had trouble finding hose to match the Kubota diameter.

Also, if you replace the fuel [return] line from the tank, check the tank connection. Often it is a different size than the connection on the engine, which gives head scratching.
 
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Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
buy a length of 3/16 ID fuel hose

I had trouble finding hose to match the Kubota diameter.

Also, if you replace the fuel line from the tank, check the tank connection. Often it is a different size than the connection on the engine, which gives head scratching.
When installing my new Betamarine a couple of years ago, I had to adapt a metric hose on the engine (fuel return, IIRC) from metric to 1/4 or 5/16 (forget which), and finally ended up using a brass mender and filing off all the barbs on one side. Tedious work, but did not have a grinding wheel handy. The Betamarine is a marinized Kubota.
 

Alan Gomes

Sustaining Member
When installing my new Betamarine a couple of years ago, I had to adapt a metric hose on the engine (fuel return, IIRC) from metric to 1/4 or 5/16 (forget which), and finally ended up using a brass mender and filing off all the barbs on one side. Tedious work, but did not have a grinding wheel handy. The Betamarine is a marinized Kubota.
Here's a place that has a crazy big inventory of barbs and other bits of this sort. I recently picked up some banjo bolts here. It's a good one to bookmark for when you need it.
 

Geoff W.

Makes Up For It With Enthusiasm
Blogs Author
Well, mechanic came by. We started the engine, started fine. Ran in idle for a while, ran fine. A trace amount of fuel on the water but nothing he seemed concerned about. Put it in gear, ran up to 2500, ran fine - no knocking. He more or less shrugged and said it sounded amazing, and he wished his engine of this vintage sounded that good. He also said running it at 2500-2800 is overkill (despite my protests that the manual says run @ 80% of max). He also recommended trying to make a power output graph of speed over water at various RPMs to see where the "sweet spot" is.

So... that's good news, and bad news, because I'm not convinced it won't start intermittently knocking again. He said to go ahead and try to adjust my valves, because that could be the cause of some of the issues. Sounds like a complicated project, but a few other folks have said it's not that bad. He also said potentially putting a half cup or less of 30W oil in the fuel, or some sort of other lubricating additive, to try and get some more lube into the injector pump, cylinders, valves, etc.

On Seafoam, he went to the dock cart and pulled out his can of Seafoam to show me. Says he uses it all the time, on all sorts of engines. He said maybe I used a little too much of it by putting the whole can in my Racor, but didn't seem to have any concerns about it otherwise.
 

Tin Kicker

Member III
Glad to hear Geoff - Sounds like you are in good shape. Also glad he uses Seafoam because I do too, although there are some newer products that seem to work as well or better for keeping top ends clean. Asking which is best is like asking which girl is prettiest.

Interesting discussion on diesels, and one I hadn't heard before.

So, is it the need for a "chemically balanced" fuel-air mixture that explains the old yarns about: 1) why diesels don't like changes in RPM and prefer to be run all day at a constant speed (presumably, at the high end of their torque & power curves), and 2) why over-propped engines (run at a low rpm) burn fuel poorly and develop soot?
1) Sorta. When the throttle is advanced, the air fuel ratio leans out unless additional fuel is added. Gas engines with carbs had an enrichment valve or something similar and of course electronic injection does this by design. These old diesels have a pretty rudimentary HP pump that is slaved to the rpm and you won't get that extra bit of fuel when the throttle is advanced, hence the desire to advance the throttle slowly. Same with going too rich and smoking when you quickly pull the throttle to idle.

2) Yes. As Geoff's mechanic told him, the engine's designed for a sweet spot. Run it slow enough to not efficiently move the burning fuel air mixture out of the cylinder and a little of that inadequately burned hydrocarbon mix is going to stick. ALL engines need a good high rpm thrashing now and then.
 
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