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TOPIC: 1971 rt1 run away engine. Can you tell me why?

1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 12:31 #1

  • Scottddove
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I at first thought my throttle had hung wide open. But after further investigation I had the pepcock in the on position and it was running out of gas resulting in a wide open lean condition I am assuming that’s what happen anyway. Question is when it was running out of gas it was wide open and even when I reached and turned off the key it had no affect just kept on going at wide open throttle, why is that?

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1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 12:59 #2

  • FrankC
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Lean air fuel mixture burns easier. The bowl probably got just a little gas to draw from, and this expanded hot fuel mixture(from under crown in crankcase area) goes along way in filling the cylinder got the sparky hot enough or piece of carbon to glow keeping the chain reaction going. Just a guess on what could be going on.

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1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 13:32 #3

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I bought 2 RT3 360 Enduros from a guy who restored them using aftermarket cylinder base gaskets. I ride them often. After 6 months one of them went into the red zone RPM with the throttle at idle. Turned the key off and it still screamed. Had to pull in the clutch, put in in gear and let out the clutch to stall it. The cylinder head bolts were loose causing an air leak. Checked the other one and its head bolts were also loose. Found out that the gaskets were too thick, they expand and contract with the heating up and cooling down of the engine resulting in the head bolts loosening up. Pretty scary. No permanent damage. Replaced them with Yamaha OEM.

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The hours spent riding my Enduros is not deducted from my life span.

1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 14:55 #4

  • RT325
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In theory "my theory" if the throttle is shutting right off it shouldn't be able to happen. If on the other hand you're running a super rich idle mixture and because of 'that' you've had to screw the idle speed slide screw up to stop it stalling then say you start running dry in the carb as in out of fuel or a blocked tap letting a dribble through then it'll climb in the revs for sure as it leans off. Would have thought though that if the slide is hitting the bottom with the screw backed off then it must 'stop'--in theory lol.

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1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 14:57 #5

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The two stroke "runaway engine" is a classic symptom of an air leak...

I always thought it was caused by a "hot spot" in the combustion chamber that would ignite the mixture without the spark. Apparently in a two stroke what actually happens is a complex chemical reaction coined "Activated Radical Combustion". Honda actually built a race bike and then a production bike that controlled the "Activated Radical Combustion" and used it to get better emissions and power. From an article:

The ARC phenomenon was observed by many owners of two-stroke-powered motorcycles, generators, and other products, whose engines would continue running after the electrical ignition was shut off. This auto-ignition was generally attributed to a pre-ignition caused by hot spots in the combustion chamber, and engineers' attention was focused on eliminating it. Honda pays due homage to the researchers who had earlier discovered the role of active radicals in the auto-ignition phenomenon: Yakov Zoldvitch in the 1930s, and more recently notable efforts by Toyota's Masaaki Noguchi and his team (The Toyota-Nippon-Soken combustion, SAE 790840), and Shigeru Ohnishi of the Nippon Clean Engine Research Institutes who had actually constructed a stationary engine for generators on the active radical principle in the late 1970s.

Full article here: dwolsten.tripod.com/articles/jan97.html

The production bike was the Honda CRM250AR...
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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 15:00 #6

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Could be that a super lean fuel mixture also self ignites under compression and latent heat in cylinder/head/ piston crown and needs no hot ember or spark. Just a theory.

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1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 15:04 #7

  • MarkT
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P.S. the reason it's a classic air leak symptom is that whether the engine keeps running due to a "hot spot" or "Active Radicals", it needs enough air to be able to rev that high.

It is possible to have the idle circuit too rich which will require a larger than normal slide height to idle... and when you ran it out of gas it leaned out and was able to get enough air to hit super-high rpm because the slide is open more than normal.

It still would be good to check for air leaks.

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 15:27 #8

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I have to agree with Mark and RT that there was some air required more than slide (if it was at bottom) to cause a super runaway. We have all shut off the petcocks while running the engine or have run out of gas and it will high rev in a(hard to explain) bucking sort of fashion. Not a clean rev up at all, and not sustained for more than a 1/2 minute or so while the bowl empties.

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1971 rt1 run away engine. Can you tell me why? 12 Feb 2018 16:11 #9

  • MarkT
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Yup... RT325's response snuck in while I was typing slow... so I repeated the same thing he said. :Ugh

I do think the ARC stuff is pretty cool... it acts like diesel compression ignition but it really isn't the same thing... sounds like some complex chemical reaction that I believe relies on residual exhaust gas to work.

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1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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