twostick
Senior Member
Control racks. Stuck injectors. Couple videos, one just a couple minutes, and the other about 12 minutes. Both vids feature 8v71's, but different vintages.
There's a "green" engine and a "blue engine. Plus the "blue" engine (8V71TA Turbo charged with After-cooling) has a "Jake Brake" system (didn't know that's on the heads!).
My buses do not have Jake Brakes. but the Whisperer tells me its something to consider for both my buses later..
Doesn't remotely make me an "expert" but even a noob like me can appreciate the "engineering" that went into these old DD's. But a bunch screws, nuts, cotter pins, etc ... seems like maintenance of/checking that stuff is key.
Yeah, they (diesel valve train/injector designs particularly) have probably advanced a lot in 70 years. But what people keep saying -- there is NO good reason why these "old" systems WILL misbehave IF well-maintained.
For me, I am learning what stuff is supposed to do, how it looks, the names of key systems, etc. I am on the road to being a better owner -- hopefully.
Anyway, the short vid is a guy going through a running engine that had NOT run in years. He's focused on the "control rack" for the injectors looking for any that are "stuck".
We know the principle runaway cause in these old DD's are "stuck" injectors .. stuck "open" could be bad.
He's trying to figure out the "what fer" on the control racks before he started the engine. Lo and behold, he had stuck injector(s) on the first side he checked.
He also said something I found interesting about control rack design changes in later vintages.
The older vintage meant ONE stuck injector had the mechanical effect on the control rack of ALL the injectors being stuck (even if there were NO OTHER injectors actually stuck on that side).
Later designs, if ONE injector was stuck, there was some "breakaway" feature that did NOT result in all being stuck.
This could be why control rack design improvements MAY .. MAY .. be why runaway mitigation with the damper-based ES systems was LESS necessary in later 8V71 vintages.
The longer video is a guy going into the "box" .. where injector control/governor mechanisms are located. I recognize the "box" on my bus .. that had my hands on it in panic mode when my bus would not shut off.
The obvious outward appearance of the blue "box" is MUCH DIFFERENT than mine, but its in approximately the same relative position on the engine .. to the upper left of the blower above the head on the outboard side of the an installed bus engine.
Tolerances in the "box" are "thousandths of an inch". Seems to me, only the most experienced people should be muckin' around in there.
I talked to the prior owner of 4903 (he sold it to the guy I bought it from, three months before I bought it). The prior owner, a carpenter/electrician by trade, is the one who up-fitted (actually remodeled an older upfit) 4903 into an RV, and had owned it for almost 2 decades.
View attachment 582925
He said something yesterday that only today makes fuller sense to me.
He told me when I have 4903 looked at for this shut-off thing, do NOT let anybody other than a professional go into the "box". The longer vid narrator/expert shows why, I think, this is true on perhaps any of these 8V two-strokes.
We'll see how she goes ...
NEWS FLASH
The fella in the long vid above gives an 8 minute tutorial (using a 6V71) with NON-spring (the kind that "breakaway" if one injector is stuck), or "safety" control racks.
The DD two-strokes' injector "default" position (goes there are shutoff, remains that way until start up) for injectors is "open". Likelihood, therefore, if gummed up/not started for a year or longer, of runaway goes up with poor maintenance/disuse.
He goes on to state affirmatively the "safety" control racks, added by GM in the 1970's he thought, eliminated the need for damper-based ES systems. He also had a few seconds on the ES damper-system and placement/operation on top of the blower.
And, now I see more clearly why on one of bus forums, the guy that wanted to DIS-able his ES damper system was gettin' sh*t from the members in #1,436.
First, he never said what kinda racks he had. And, second, nobody asked him the question about his racks when they were jumping on him.
Maybe it come up in other threads, maybe I missed it, but they were just arguing ... politely .. but totally UN-productively. Never seen that happen in a forum.
I conclude it IS likely when the "safety" control racks were added to the DD 2 strokes, GM said "something" -- in manuals/bulletins/safety rack kits, whatever -- why they thought the damper-based ES may not be needed.
I still dunno IF GM also said you could use the "safety" racks on OLDER designs. If I had to bet, and of course now that I know what to ask, is BOTH my buses (even after contemporary rebuilds) do NOT have "safety" control racks.
And even if you could retro-fit older DD's, it doesn't seem like a smart thing for them to tell customers to disable their existing ES damper system .. unless they were VERY sure the safety control racks could PREVENT (or significantly reduce the risk of) runaways.
They ("safety" racks) were denying FUEL to the engine, but near as can tell they were NOT denying a DD air - the best way to stop a rare but potentially catastrophic runaway.
Regardless of my vintage of control racks, I still kinda like the idea of a WELL-FUNCTIONING damper-based ES, maybe with a manual pull-cable (to close the damper) that can be easily/quickly accessed in an emergency?
Now I'm done with topic for now.
Agree it's still a good idea to have the emergency shutdown functional. Even with the breakaway rack control, if it had in the unlikely event, 2 or 3 stuck injectors, it would keep running. It not likely would run away but it would rev up past idle and unless your bus has a manual trans, you have no way to stall it. Then you have the failed blower seal scenario where it can inhale engine oil and it will for sure run away on that regardless of what the fuel rack is doing. Another one I've experienced on a tired out one is fuel slobber at extended idle when it's really cold. Unburned fuel will slobber out the exhaust but a lot of it winds up running back into the airbox when the piston is coming up and it just collects there until the first time you rev it up and that big rush of air hauls it back into the cylinders. It doesn't go completely nuts but it will make like full throttle for a second or two, enough to make 2300-2400 rpm. Not fun if you have just put it in gear to pull out of a spot in the morning after idling all night. Wheels up isn't strictly airplane terminology... lol
Kevin
