Cold Start Misfire.

Shadowjack

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Mar 15, 2018
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Re the stroke the engine's on when the throttle is blipped: if the crank is turning 3600 rpm (for ease of calculation) then it's turning 60 times a second. Or at 1800 it's turning 30 times. Pretty hard to catch it on a given stroke.
 

Otto1

New Member
Jun 13, 2021
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I agree that reciprocation is very fast. Just for the sake of argument, wouldn't that give many many chances to blip on the moment both valves are open during valve overlap after exhaust as the intake opens for the next cycle? And there are two cylinders, so twice as many chances. Just a thought.
 

Shadowjack

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Mar 15, 2018
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Well, my point was that in the time it takes for a shot of fuel to get into the cylinder, as in an old carbureted engine with an accelerator pump, the piston has done many complete cycles, and it won't make it through until the intake valve is open and engine is sucking, regardless. Fuel-injection ECUs won't shoot unless it's on the intake stroke in any case. Each cylinder has its own injector, BTW, so it doesn't matter how many cylinders there are.
 

Graves

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Well, my point was that in the time it takes for a shot of fuel to get into the cylinder, as in an old carbureted engine with an accelerator pump, the piston has done many complete cycles, and it won't make it through until the intake valve is open and engine is sucking, regardless. Fuel-injection ECUs won't shoot unless it's on the intake stroke in any case. Each cylinder has its own injector, BTW, so it doesn't matter how many cylinders there are.
I don’t believe that the AT fuel injection is sequential, I don’t see a high resolution tone ring in the parts diagrams.
 

Otto1

New Member
Jun 13, 2021
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I have been wrenching my own bikes for over 40 years and I am new to the FI world. I think what we are saying is a miss is also called a flame out on a single cylinder bike. I have had it happen on my 18 KTM 250 EXC-F (woods weapon) that is FI and on my older WR250 carb'ed bike. The way I see it is if you blip the throttle, not at necessarily high RPM, there is a momentary lean condition when that rush of air comes through. When the the Intake valve opens its just too lean and a miss or flame out happens. I think this could happen on a carb'ed or FI engine sequential or not. Just a lean condition. On my KTM, flame out was a big issue when I first got it. Mostly in tight low RPM quick throttle situations where I need power the most to stay in control. Being a single cylinder it would stall. The bike is set to run so lean and the back pressure on the super restrictive exhaust is crazy , all EPA reg's for noise and smog control. I did my research on that bike and changed the TPS, and opened up the exhaust some with a different end cap, need to have some restriction, and removed the EVAP system because it was just extra BS around the engine that got in the way and not relevant to my problem. NO more flame out/missing except when I blip the throttle on a cold engine, so i don't do that. The AT is pretty much the same as my KTM250. They are reg'ed by the EPA to run as lean as they possibly can with out melting the pistons, the exhaust is supper restrictive as well. When a flame out/ miss happens there is another cylinder to carry on the stroke so it doesn't tend to stall, just miss. So if you want to buck the regulatory authorities and reset the ECU to richen things up, and at the same time cool the pistons a bit, as well as make things a little louder by taking SOME of the restriction out of the exhaust, I think it will help the problem a lot. I don't think this is a willy-nilly job, there needs to be thought put into the fuel/air ratio and the restrictive need of the exhaust. I'm sure there are tuning kits and exhaust out there just ready to sell. Not all good either. Just my thoughts on this.

As for the original thought. Can a lean mixture happen and cause a miss on a running engine regardless of induction. I think Yes.

If, for that moment in time when both valves are open, can a rush of air cause ignition of unburnt gasses left form exhaust? possibly yes.

Will I do anything about it on my AT? Not at this time, but if it worsens I will figure it out. After all, its just an air pump with fuel, compression, and ignition.

There is a reason the AT's miss and the thing is to figure it out. Fun Stuff, when I'm done riding for the day.

Cheers!! Now lets go ride!
 

Shadowjack

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Mar 15, 2018
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Graves, I believe you're right about the non-sequential injection. I was getting used to thinking of automotive systems, which are not batch-fire much anymore.
Whenever the intake valve opens, there's going to be fuel with it. If the EFI is properly designed, there's just the right amount of fuel, squirted in at about 42PSI, and there's never a period where the intake valve is open without something to burn. It's not like a carb, where the act of air rushing past will draw the fuel into the port, and hopefully it's all sized properly to let it run without problems. Accelerator pumps were added to bike carbs in the early 80s because the idle mixtures were set very lean to meet emissions specs. Without them you have your scenario where there's a flow of air without enough fuel in it to combust. Cold starts are particularly hard, since the engine wants more fuel than warm running. Which is why there were chokes and enricheners, but the EFI does all those compensations now. So if inspection and analysis says that it's lean, either it's purposely set "on the edge", or there's a fault. Some designs are nearer the edge than others, and the engines need to be fairly well warmed up before they'll run right. Not getting into the "unintended consequences" scenario of end-user changes to the various systems.
Valve overlap has such a big impact on emissions that most engines are designed with as little as possible these days. We've all heard the rump-rump sound of muscle-car cams from the olden days, but that was a direct result of too much overlap. At low speeds, those engines ran like crap, and some of the intake charge was either going straight out the exhaust valve, or burnt gasses were getting pushed back up into the intake and diluting the next incoming fuel charge. On a big V8, this could be endured, but on a single or dual-cylinder engine, the idle speed would have to be set very high just to keep it running.
And just FYI, on my particular bike, in 12K miles over 3-1/2 years, I've heard it cough once (only at startup) about 3 times. So they don't all do it. I've not changed anything in the fuel/exhaust areas.
 

Graves

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Graves, I believe you're right about the non-sequential injection. I was getting used to thinking of automotive systems, which are not batch-fire much anymore.
Whenever the intake valve opens, there's going to be fuel with it. If the EFI is properly designed, there's just the right amount of fuel, squirted in at about 42PSI, and there's never a period where the intake valve is open without something to burn. It's not like a carb, where the act of air rushing past will draw the fuel into the port, and hopefully it's all sized properly to let it run without problems. Accelerator pumps were added to bike carbs in the early 80s because the idle mixtures were set very lean to meet emissions specs. Without them you have your scenario where there's a flow of air without enough fuel in it to combust. Cold starts are particularly hard, since the engine wants more fuel than warm running. Which is why there were chokes and enricheners, but the EFI does all those compensations now. So if inspection and analysis says that it's lean, either it's purposely set "on the edge", or there's a fault. Some designs are nearer the edge than others, and the engines need to be fairly well warmed up before they'll run right. Not getting into the "unintended consequences" scenario of end-user changes to the various systems.
Valve overlap has such a big impact on emissions that most engines are designed with as little as possible these days. We've all heard the rump-rump sound of muscle-car cams from the olden days, but that was a direct result of too much overlap. At low speeds, those engines ran like crap, and some of the intake charge was either going straight out the exhaust valve, or burnt gasses were getting pushed back up into the intake and diluting the next incoming fuel charge. On a big V8, this could be endured, but on a single or dual-cylinder engine, the idle speed would have to be set very high just to keep it running.
And just FYI, on my particular bike, in 12K miles over 3-1/2 years, I've heard it cough once (only at startup) about 3 times. So they don't all do it. I've not changed anything in the fuel/exhaust areas.
It’s all about getting around the limitations of the camshaft isn’t it. I thought that we’d see a mass production cam-less engine someday, but everything is pointing to the direction of motors. You’re right about the muscle cars of the “old days”, they made all the right noises, odors, vibrations… but modern stuff is so much faster.
 

Otto1

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Jun 13, 2021
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Graves, I think cam-less engines are an interesting concept but how much of a future will they have with the EV/and other alternative markets expanding and the beginning of bans on ICE, like the one in the EU banning all ICE by 2035. I don't think they will succeed with the ban as Italy is already seeking exceptions from the ban. Exceptions or not it is coming. Will 100% EV/alternative vehicles become the norm? Maybe, not totally in my lifetime, but definitely in my children and now G-children's lifetime.

Just to throw a wrench into the pot. Shadowjack, all very good points. There is a reason these engines miss and as a youngster, I was taught that if there is an engine not running right, the first place to look is the weakest part of the process, electrical. This could all come down to an ignition issue like a weak coil, worn plugs, incorrect plug temp, or plug gap. Just a thought.

I went for a 270 mile, mostly back/dirt/baby heads/rutted roads, ride yesterday and I had one miss. It is an intermitent issue for me and as long as it doesn't interfere at critical moments, like when stuck on a hill sideways, then I'll leave it as it is.

As I am thinking about this it seems the misses happen more at lower rpm like when a quick throttle is used to get the rev's up to pop the clutch to climb a hill.
 

Graves

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Graves, I think cam-less engines are an interesting concept but how much of a future will they have with the EV/and other alternative markets expanding and the beginning of bans on ICE, like the one in the EU banning all ICE by 2035. I don't think they will succeed with the ban as Italy is already seeking exceptions from the ban. Exceptions or not it is coming. Will 100% EV/alternative vehicles become the norm? Maybe, not totally in my lifetime, but definitely in my children and now G-children's lifetime.

Just to throw a wrench into the pot. Shadowjack, all very good points. There is a reason these engines miss and as a youngster, I was taught that if there is an engine not running right, the first place to look is the weakest part of the process, electrical. This could all come down to an ignition issue like a weak coil, worn plugs, incorrect plug temp, or plug gap. Just a thought.

I went for a 270 mile, mostly back/dirt/baby heads/rutted roads, ride yesterday and I had one miss. It is an intermitent issue for me and as long as it doesn't interfere at critical moments, like when stuck on a hill sideways, then I'll leave it as it is.

As I am thinking about this it seems the misses happen more at lower rpm like when a quick throttle is used to get the rev's up to pop the clutch to climb a hil
That’s what I meant by saying everything is pointing to the direction of motors. I know that motor and engine are often interchanged, but technically speaking motor refers to electric motor. Electrically controlled valves are more than a concept they exist, just not in mass production. Think of it, computer controlled valves, you could throttle through the valve train, control turbo boost eliminating the waste gate, control exhaust gas recirculating without a valve, drop cylinders, total control of engine braking… the possibilities are endless. But the focus these days is on improving electrical storage, so the internal combustion engine is taking a back seat, if the metal battery works out the engine is doomed.

The miss we’re discussing in this thread is a cold start misfire. I Looked up spark plugs for my twin yesterday to order for replacing and not only did I find out that they are expensive, I found out that they’re laser iridium plugs that should last well longer than 100k miles. Now I doubt that plugs are the cause of my misfire, but they could be, it just depends on how much current they’re exposed to, I’ve seen super plugs like these go off in as little as 40k in cars. Inspection is warrented. You do have to ask yourself why Honda put two plugs in each cylinder, do you know? I know why manufacturers have in the past, don’t specifically for the AT, but I’d guess the reason is the same.
 

Otto1

New Member
Jun 13, 2021
11
5
3
Maryland
You are correct, motor refers to, well motors not engines. Brain fart on my part.

Two plugs could be for several reasons:

Reliability if one fails with twin magneto. More of an aircraft thing than road vehicles.

A twin flame front for better and more complete ignition.

And better ignition. Instead of a single centerfire, that burns top-down from the center, with two on opposing sides of the cylinder, you have burn from the edges to the center helping to prevent hot spots and a more even distribution of pressure, which means more power.

My guess is Honda did this to have a better burn on a large bore to please the EPA numbers game.

Those plugs are expensive. Mine were replaced at 38K miles and I now have 46K
 
Last edited:

Graves

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Aug 14, 2016
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You are correct, motor refers to, well motors not engines. Brain fart on my part.

Two plugs could be for several reasons:

Reliability if one fails with twin magneto. More of an aircraft thing than road vehicles.

A twin flame front for better and more complete ignition.

And better ignition. Instead of a single centerfire, that burns top-down from the center, with two on opposing sides of the cylinder, you have burn from the edges to the center helping to prevent hot spots and a more even distribution of pressure, which means more power.

My guess is Honda did this to have a better burn on a large bore to please the EPA numbers game.

Those plugs are expensive. Mine were replaced at 38K miles and I now have 46K
From what I’ve read the reason for two plugs per cylinder is to remedy an otherwise unsolvable problem. Modern era engines have a problem at low RPM, the problem is a lean combustion charge that becomes stagnant in the cylinder at the point of ignition. Ideally you would want the combustion charge to be mixing, or spinning, when the ignition sequence is initiated, so that the burn pattern remains predictable and complete. In modern engines with their relatively higher compression and lean combustion charge, gaps are created between the hydrocarbons of the combustion charge when the mixture stops mixing due to the compression by the piston just before ignition. When correctly timed, two plugs in the cylinder can help to mitigate the problem of incomplete charge combustion and prevent unburned hydrocarbons from leaving the cylinder through the exhaust. Two flame fronts as you suggest are not desired as they would tend to cause a knock. This “knock, ping, detonation…”, from whatever source, also creates an extraordinary amount of heat, heat that not only damages metal surfaces but also creates NOx (oxide of nitrogen), which is one of the components of photochemical smog. Side note: Do you know what the exhaust gas recirculation valves’ (EGR) function is in your automobile? It’s sole function is to introduce and inert gas back into the combustion charge to cool the combustion process to prevent the formation of NOx. The more I learn, the more I realize that a lot of the engineering involved in things is to work around the imperfect nature of things, everything is a compromise to reach a more desirable outcome. At least that’s how I understand it.
 

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