BCM Community
Bus Discussion => Bus Topics ( click here for quick start! ) => Topic started by: silversport on August 27, 2021, 12:54:36 PM
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Looking for input, attempting to adjust the buffer screw on a 8V71 that has fast idle & buffer switch for Jakes. After a few attempts to have the fast idle work but couldn’t get the Jakes to shut off at idle, only at fuel off, would the buffer switch activate. Then it happened, the next start RUN AWAY, pushed the emergency stop, nothing, ran to the back manual trip flapper, guessing 3000 RPM by that time. When the flapper engaged, it slowed the engine down to around idle (with lots of black smoke) but would not shut down. Ended up unscrewing the fuel filters, finally slowly the engine can to a stop. Today reprimed back off buffer screw (to be safe) and she firered up normally. Still can’t get the Jake buffer switch to engage at idle. I have all the books, GM manual, Detroit service manual, and Jacobs instruction manual, am I missing something, says tighten buffer screw to no more than 15 RPM increase, I stopped at 10 RPM increase. So why did the emergence stop not work, well the solenoid was bad. In 10 plus years with this coach, I never thought about checking that solenoid. And why did I have run away? The injectors were not stuck, the engine is not weak (inframe around 15 years ago, low mileage).
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No idea on proper buffer adjustment, but I know the SOP anytime you're adjusting anything inside the governor system is to have a pair of vice grips on the rack for manual control if things go screwy. Emergency shutdown solenoids should be on everyone's to-check list at least yearly, I like to check mine anytime the bus has been sitting for a few weeks.
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Pull the wires from the switch and see if it engages,if not check the switch,diodes and adjustment be sure you haven't lost a roller and the switch is not stuck in the closed position ,If it engages with the wires disconnected then you need to change the 3 o-rings on each solenoid,that buffer will get your attention :P
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The "emergency shut down" will not necessarily stall out an engine that is being fueled, it's job is to stop the run-away, which it handily did.
We had about 1500 of them, long ago, and would periodically find one that wouldn't seal, just chugged the air full of black smoke.
You really want to get intimate with the fuel shut-off lever on the governor, all you had to do was give it a push and hold it in until it stopped.
Thanks for sharing your excitement, someone else will learn something by reading this!
Happy coaching!
Buswarrior
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A design of a lot of shut down flappers is to just control runaway. Our flapper is smaller in circumference than the seat it sits on otherwise the blower oil seals would fail during a total intake air starve so the design is to slow the rpms not suck the seals out of the blower.
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You really want to get intimate with the fuel shut-off lever on the governor, all you had to do was give it a push and hold it in until it stopped.
Thanks for sharing your excitement, someone else will learn something by reading this!
Happy coaching!
Buswarrior
I held it for what seemed like 5 minutes, that is when I took off the fuel filters, took about 2 more minutes to stop. Since this coach is an automatic didn't think about putting it in gear.
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The 92 series don't have the air shut off I saw a 8v92 go into orbit a guy was working on his Jake buffer cost him a engine before he could cut the fuel off, it does no good to push the shut off when the buffer is in control
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Fortunately it didn't drop a valve. Easy to have one come apart if buffer screw is misadjusted or injector sticks causing full fuel runaway. Always check emergency stop that it works from front and rear.
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You know Detroit issued a service bulletin in 1982 with a parts list doing away with the flapper and installing the spring loaded fuel control rods if your manual is up to date the page will be there.
I always toss those and remove the shaft and plug the holes and install the newer fuel control rods.They were always bad when people never maintain the systems vibration would wear the shaft and the tiny seals would go bad letting dirt and dust enter the engine.
The 92 is basically the same engine as the 71 and you never see one on the transportation 92 series engine
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When your rolling all that black smoke out,, you will also badly foul all your spark plugs,>>>Dan ;D ::)
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When your rolling all that black smoke out,, you will also badly foul all your spark plugs,>>>Dan ;D ::)
I will pull and clean them tomorrow, I was wondering why it was running so rough, thanks for the advice, >:( Dan
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check the points too.
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When your rolling all that black smoke out,, you will also badly foul all your spark plugs,>>>Dan ;D ::)
Not to mention all that excessive smoke from the turbo will clog the carburetor!
;D BK ;D