2 Stroke Detroit Diesel needing help

2 Stroke 671 Detroit Diesel wont turn freely!
Those of you who have experience with the old 2 stroke Detroit's may have an idea of what is happening here. I own a '54 GMC Greyhound Coach refurbished into an RV, It has sat for many years with being started only infrequently notably, without problem or hesitation with the exception of the fuel filters needing to be replaced. We decided to ger her back on the road and spent ample time testing before an actual "journey". After about 35 miles in the neighboring area, the engine began to struggle and then refuse idle at all. Upon restarting, soon the starter motor would no longer turn the engine. I've gone through several tests coming to no solid conclusion. After installing a new starter Ive not had any improvement over the original one. This engine has always been very reliable and starts very easily even using a single 12v automotive 900 CCA battery. Now, throughout roughly half of the crankshaft rotation, the amount of effort to bar it over is quite high. Any thoughts and ideas are greatly appreciated. I've been unable to locate knowledgeable locals. Hopefully your experience is exactly what will help. John

Did it have plenty of coolant in it? Temp gauge works?
Overheating can score liners and break piston rings
which makes it hard to turn. One 900 CCA battery is
pretty low for that application.

Something is wrong with it if it's hard to roll over by hand. You're need to dig farther into it to find it. It does sound like a piston if it gets tight once every revolution.

i would say its time for an engine job. them 2 cycle engines are something else. or maybe the blower piled up. we had a couple of them
671's on the mud pump on the drilling rig.one would get warm and just seize up and stop. then let it cool down and it would fire up and pour
the coal to it again. then had the mechanic come out and put in a set of sleeves and pistons. one day and it was running again. lol.

Had that happen to a grain truck with an 8V71. Two cylinders seized. ran fine when cool but went tight when it warmed up.

Thanks to everyone for their reply, I have much information that I didn t include because I don t want to scare anyone away before really getting windy. This bus has sat for roughly 18 years until we decided to get her back on the road. We took a short trip to town to buy fuel. On return home, the engine died for the last time and we rolled her into our horse pasture in the back. While I drove the bus during my trial run, I didn t notice any indication of excess heat either at the instrument panel or engine compartment, but this strikes me as a plausible cause. At any rate I don t know where to concentrate my efforts in making a repair. Can t be hydro-locked and haven t been able to insert a small enough scope into the cylinder head to observe the condition of individual cylinders walls. The part that is very interesting to me is that I've found that while baring the engine, throughout the 360 degree rotation of the crankshaft, by counting the number of teeth on the ring gear, I find that the resistance is at intervals and those intervals repeat themselves each revolution. For the life of me I cant think what would influence the engine rotation in this manner. From the time I began checking her over until the moment she refused to run further I had driven about 30 miles. Thinking back about it, I was having trouble starting and thought my problem was an old, undersized battery but instead I eventually learned it was that the starter was having to exert more effort each time and that the engine was struggling to continue to idle or run without additional throttle.

I know the firing order is 156324 and I found TDC after removing all the injectors to assure there is not any compression influencing my efforts. At TDC, I painted a corresponding crankshaft tooth and counted all teeth on the ring gear coming up with 116 total. Then I marked each 10th tooth, so I'd know where in the rotation I am when my effort changes. What I find is that starting from zero, #1 TDC, the engine turns easily until I get to 10. From there it is hard to turn until the 35th tooth. It then turns freely until around 45 and then stiff again until 75. During this area the effort increases significantly. The rest of the rotation back to TDC it turns easily. I've created a diagram showing the location of the pistons as they each achieve the top of their stroke in order, each 19 teeth apart. Im thinking possibly adding lubricant to each as they begin to come up on compression stroke but haven't done this just yet. I realize that the lubricant will be lost as soon as the piston lowers sufficiently but perhaps the rings and cylinder walls will benefit. In the beginning I thought my starter was bad and spent $400 on a new one without improvement at all. The mention of the battery is only to demonstrate that previously it didn t take any excess effort. Now, the starter will only move the engine when it is free for about rotation.

Any ideas on where to find the culprit and what may be causing the tightness during rotation? Don t know what to do next. John

I can't help other than to suggest the best place I've found to get Detroit info is from marine sites with a forum like boatdiesel.com. It has a dedicated 71 series forum. If it's hard to rotate with the injectors pulled it definitely sounds mechanical. There are many possibilities including the blower shaft, etc. Obviously spitballin-, so good luck.

Have you taken off the air box covers to look at the rings as they pass the ports? I'm no expert but I did work on a 2-71 in an AC crawler.

If only my old buddy that worked for Harper diesel when the old 2 strokes were in there glory years was still alive. In the little I learned
from him My guess is its likely to be a worn out blower. either from drive gear wear or bearing wear or both lobes could be too close to case
or each other causing the repeating tight spots you are referring to. Its a bit of a long story but I had a 453 with every top ring broken
that still ran o/k when warmed up so I am doubting rings. If it was me I would take the intake cover off of blower and check blower
clearances and backlash to see if thats your problem? Or maybe just pull entire blower assembly as its not that hard and see if problem goes
away?

If you scored the pistons on a couple of cylinders
, the aluminum will pile up on the cylinder walls and make it tough to rotate.

Just remembered. Back when I worked at Lincoln they used 2-71s on the engine drives, occasionally 6-71s on the Nelsons, and did in-house rebuilding.
The old-timer there told me it was common for a wire from the air filter to go through the blower and score the rotors and case.

Dont know where you are located but there is a diesel
shop/parts house that kinda specializes in Detroit
diesels in new Iberia Louisiana called canal diesel.they
knew a lot about a 453 Detroit we were having
problems with and had the parts in stock.had us going
the same day!337-369-7232

Spun a bearing?

Thank you very much to everyone with your thoughts. We're thinking this is the result of over heating although it's not confirmed yet. I'll post back when its decided for certain. Looking like its going to need some new cylinder sleeves.

Thank you very much to everyone with your thoughts. We're thinking this is the result of over heating although it's not confirmed yet. I'll post back when its decided for certain. Looking like its going to need some new cylinder sleeves.