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Oldwrench 
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Joined: 12 Jan 2020
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 Topic: D17 IV Engine Run-on After Shutoff?
Posted: 14 hours 40 minutes ago at 7:43am |
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This is not a new problem, and has happened occasionally for as long as we have owned the tractor (30+ years?). The engine is in excellent condition and runs well under all conditions. The tractor is used only for light work. Once in a while, and it is hard to predict when, the engine will run-on or run backwards for a bit after shutting the key off. As a habit I always let it idle for a minute or so before shutdown, even if it has not worked hard or is not up to temp. It only happens once in a while, and I don't see any pattern to when it might happen. I put electronic ignition in a few years ago, but it did the same with points. It gets very few hours a year, and the plugs and oil are usually changed out long before they are even dirty. Is it carburation or electrical? How bad is it for the engine when it does this? It typically only does this for maybe 5-10 seconds before finally shutting off. As I said, it is not necessarily related to its being hot or after a long work session, although I always idle it long enough to cool down if it has been worked. Carbon buildup in the combustion chamber? I am curious as to what might cause this.
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Ed (Ont)
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Joined: 08 Nov 2009
Location: New Lowell, Ont
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Posted: 14 hours 35 minutes ago at 7:48am |
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Gas or diesel?
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steve(ill)
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Posted: 14 hours 21 minutes ago at 8:02am |
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points and plugs so it is a gas engine... Gas engine normally will not RUN with out a spark.. NORMALLY the compression is not enough to get ignition.. Carbon buildup in the top can be a cause..... another Possibility is the ignition switch is worn out and when you turn the key off, you still have an electrical path to the coil to give spark... and the vibration eventually causes the switch contacts to fall off.... I would try a KILL switch to the coil as a TEST to see if the switch is the cause.
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Like them all, but love the "B"s.
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DrAllis
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Posted: 13 hours 59 minutes ago at 8:24am |
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Slow idle speed of 400 RPM !!! D-17/G-226 engines with M-code on the serial number are 7.25 to 1 compression and normally aren't prone to doing this if the idle speed is truly 400 RPM and allowed to idle for 30 to 60 seconds. 170/175 engines with 8.0 and 8.25 to 1 compression are far more likely to do this. Higher octane gas will also help. A cranking compression test would verify if it has 7.25 to 1 pistons or not (145 psi). Surely it has been OH'd at some point in its life and maybe it has 170 psi compression.
Edited by DrAllis - 13 hours 57 minutes ago at 8:26am
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Oldwrench
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Posted: 13 hours 5 minutes ago at 9:18am |
It did get a overhaul at ~ 3,000 hrs by a very competent AC dealer before we bought it. It has < 1,000 hrs on it over the last 30 years, with light usage. Not sure what pistons are in it. I also forgot to mention that I checked the timing several times, and the advance is working, moving the Fire line up to the middle of the inspection hole at full throttle. I have never checked the idle speed with an accurate tach, but will do so and make sure it is at 400 rpm. I just set it to what sounds to be reasonable when I listen to it.
I'm curious. What causes this type of run-on that is related to the idle speed?
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Ed (Ont)
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Posted: 12 hours 31 minutes ago at 9:52am |
Oops sorry!! Missed that part. Lol.  Maybe check the timing if you have not done so already.
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SteveM C/IL
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Posted: 12 hours 12 minutes ago at 10:11am |
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Agree idle has to be SLOW! Wife doesn't quite close throttle on 45 sometimes and it wants to run on a bit and the knock is frightening. My mechanic dad said that's hard on brgs rings and crank.
Edited by SteveM C/IL - 12 hours 11 minutes ago at 10:12am
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Oldwrench
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Posted: 8 hours 43 minutes ago at 1:40pm |
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I followed up on Doc's advice, and checked the idle RPM. On a warm engine, with my trusty old Sears Dwell/Tach from the 60's, I am seeing about 500 RPM at idle, which seems just about right to my ear. I checked the manual, and it calls for 375-425, so 400 RPM. That sounds really slow to me when I bring it down that low. Barely running almost. I guess the main thing is to let it idle for a while at that speed, then shut it off. I can always leave it higher when just using the tractor. Anyway, apparently mystery solved, although I would still like to know the root cause of why a slightly higher idle speed at shutdown allows it to run backwards and sound like it is ready to throw the crankshaft out of the oilpan!
Edited by Oldwrench - 8 hours 42 minutes ago at 1:41pm
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DrAllis
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Posted: 6 hours 3 minutes ago at 4:20pm |
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What octane gas ?? If it's 85 it needs to be 86 or 87.
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steve(ill)
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Posted: 5 hours 40 minutes ago at 4:43pm |
Anyway, apparently mystery solved, although I would still like to know the root cause of why a slightly higher idle speed at shutdown allows it to run backwards and sound like it is ready to throw the crankshaft out of the oilpan!
YEA... i wonder what is SPECIAL about this motor.. Plenty of auto motors with 9:1 compression ratio, running crap gas and idle at 700 rpm... ?????????
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Like them all, but love the "B"s.
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DrAllis
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Posted: 5 hours 24 minutes ago at 4:59pm |
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Automotive has had fuel injection of some sort for 50 years?? So when the key is shut off there is no spark or fuel to fire when the engine coasts to a stop. Carburated engines are still drawing in an air/fuel charge as the engine rotation winds down. Enough heat from something inside the combustion chamber can still ignite it. The result sounds something like a sledge hammer hitting an anvil.
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DrAllis
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Posted: 3 hours 14 minutes ago at 7:09pm |
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Brings back some old memories. A John Deere 2520 had an electric shut-off solenoid screwed into the main jet passage on its Marvel Schebler carburetor. This was to prevent run-on by shutting off the fuel when the key was shut off. IH 574 also had the same attachment. They must have felt they had a problem by adding that solenoid.
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Les Kerf
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Posted: 1 hour 14 minutes ago at 9:09pm |
 steve(ill) wrote:
...
YEA... i wonder what is SPECIAL about this motor.. Plenty of auto motors with 9:1 compression ratio, running crap gas and idle at 700 rpm... ????????? |
Dynamic compression.
You won't find ANY automotive engines running camshafts with the low overlap period and early intake valve closing that these stock Allis-Chalmers engines have. The automotive camshafts will bleed off a lot of compression even at 700 rpm compared to what a tractor camshaft will do. Of course, the tractor camshaft also runs out of breath long before 4000 rpm; any engineering endeavor is a compromise.
Back when I came of age in the 1970's a lot of cars had ant-dieseling solenoids designed into the carburetors so as to slam the throttle butterflies completely closed when the ignition switch was powered off.
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Oldwrench
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Posted: 52 minutes ago at 9:31pm |
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Thanks everyone for the insights. I run only 90 octane non-ethanol. I remember now as well hearing about the anti-dieseling solenoids on some of the 70's era motors. I learned something and understand it better now. I will set the low idle to around 400, and let it settle there before shutting off, even if I don't let it idle that low when I'm using it. Thanks to all the collective wisdom here.
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SteveM C/IL
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Posted: 32 minutes ago at 9:51pm |
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Bet a dollar it will pull itself around the lot in 3rd gear at that low idle....
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AC7060IL
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Posted: 4 minutes ago at 10:19pm |
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A warm engine that when pulling will probably register 800-1200f EGTs(exhaust gas temps)? If it is immediately reduced to idle, it will take 30 seconds or so to register 450-475f EGTs? Then If left to idle another 1-2 minutes, it’s EGT will drop to 350f or lower?
For many gas engines, these higher EGTs plays an important role in quickly atomizing raw carburetor liquid gas & intake air into a gaseous cloud status which burns best in cylinders. To accomplish this, most exhaust manifolds run under or adjacent proximity to carb &/or intake manifold for best EGT heat transfer.
So 400 rpms not only reduces raw gas / intake air flow, but also greatly reduces EGT~intake manifold heat transfer. AND the reduced intake temp drastically reduces piston/cylinder pre ignition temps of any hot spots found around any edges / carbon deposits.
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