Turd Polishing

[fishinwrench]

3 hours ago, MrGiggles said:

 

I back the idle screws out until it starts to run crappy, and then give them 1/4-1/2 turn in. They're air screws, backwards compared to many.

Adjustable Idle Air jets are a japanese concept that, to me, is ignorant design.   Basic carburetor design is that the Venturi is your "air jet".    Why anyone would design a carburetor such that instead of metering FUEL you are metering the amount of air pressure passing over the emulsion tube.....is beyond my comprehension.   Nevertheless it does seem to work IF all the other functions of engine operation are doing THEIR job.   

But....and this is a huge BUT..... If someone in this motors past life FORCED those idle air screws too tightly closed, and deformed the opening that the needle seats against... .then that carb body is JUNK, and you'll never get it to idle properly.  

Just something to consider.  

[MrGiggles]

7 hours ago, fishinwrench said:

Adjustable Idle Air jets are a japanese concept that, to me, is ignorant design.   Basic carburetor design is that the Venturi is your "air jet".    Why anyone would design a carburetor such that instead of metering FUEL you are metering the amount of air pressure passing over the emulsion tube.....is beyond my comprehension.   Nevertheless it does seem to work IF all the other functions of engine operation are doing THEIR job.   

But....and this is a huge BUT..... If someone in this motors past life FORCED those idle air screws too tightly closed, and deformed the opening that the needle seats against... .then that carb body is JUNK, and you'll never get it to idle properly.  

Just something to consider.  

One of them is bent a little bit.

Would there be a visible indent around the needle if that were the case?

I've been watching Ebay for a set of DT85 carbs, those and the intakes are the only big difference between the 85 and 75. Haven't seen any for a decent price though, and I may not get anything that's better than what I have. 

There is a whole DT85 in Springfield, but the guy wants way too much for it, and he won't part it out.

[fishinwrench]

1 hour ago, MrGiggles said:

One of them is bent a little bit.

Would there be a visible indent around the needle if that were the case?

Well then, There you (quite possibly) have it !    

There might be a ring around the needle if the needle is brass, but the lack of a ring doesn't indicate that it wasn't messed with.  The fact that it's bent tells you that it indeed was.   The carb body is very soft pot aluminum, and the hole the needle goes in is typically a very thin section (easily distorted).   The needle is either brass or stainless steel.   

If it was a FUEL jet then a little damage there wouldn't be quite as critical, but distorted air flow over the emulsion tube definitely is a big deal.  Most repair kits for air metering style carbs include a new stainless needle for that reason. 

[MrGiggles]

On 2/19/2021 at 8:51 AM, fishinwrench said:

Well then, There you (quite possibly) have it !    

There might be a ring around the needle if the needle is brass, but the lack of a ring doesn't indicate that it wasn't messed with.  The fact that it's bent tells you that it indeed was.   The carb body is very soft pot aluminum, and the hole the needle goes in is typically a very thin section (easily distorted).   The needle is either brass or stainless steel.   

If it was a FUEL jet then a little damage there wouldn't be quite as critical, but distorted air flow over the emulsion tube definitely is a big deal.  Most repair kits for air metering style carbs include a new stainless needle for that reason. 

I believe the wobbly one is bent along the outer portion, not the actual needle, but it's really hard to tell. I order a set and see what happens. I figured someone got between the needle and the carb plate with a screwdriver while trying to adjust them through the airbox, you can't see anything through the little holes that they give you.

Looking down into the hole, it looks like the tapered seats are fine, but without having anything to compare them too, I don't know.

I've got it all together and will give it a rip when the ice melts, see what it does after it's had a good run. Looks like finding another set of carbs may be a dubious task, there's a couple sets on Ebay for $300, and they look like they've been fished out of the bottom of a lake. 

[fishinwrench]

4 hours ago, MrGiggles said:

Looking down into the hole, it looks like the tapered seats are fine, but without having anything to compare them too, I don't know.

You can lightly seat the needle, then count the number of turns until the needle comes free from the carb body.   Compare by doing the same to the other 2 carbs.   

If differences are more than 3/4 turn you can confirm that it has been "jacked with".    

I put a little drop of nail polish on one side of the screwdriver-slot on the needle, that helps with the counting.

[MrGiggles]

After running it for a good long while tonight, it is definitely better, but still not 100%. 

I'm going to pull the plugs in the morning, see what they look like. It was also idling high, higher than it ever has, I'm going to check the sync again, it never occurred to me until now that since I had the reed plates and everything apart things may not have gone back in the exact same place. 

Watching videos on Youtube of these motors, half run perfectly smooth, and the rest are lumpy and rough like mine. 

[MrGiggles]

Sync was set pretty good. Quick adjustment to the linkage got the idle down.

Just for shits I removed the carb linkage and tried opening each carb one at a time. They all responded pretty much the same, speeding up slightly and eventually choking out if you went too far. 

Plugs are looking better, bottom is still just a touch cleaner, they're oily because it's essentially running a 25:1 oil mix, the oil system should be bled, so I can fill it up with straight gas. 

Not sure if it was the Boyeson reeds, or just the removal of a big vacuum leak from the crank seal, but it picked up 2mph on the top end as well.

I disconnected the electric pump, the mechanical pump seems to lose prime, think the plastic check valves may leak, need to prime it up if it sits for more than 30 minutes. Those are NLA so I'm not sure what to do about that. Wonder if a Briggs impulse pump would work.

I'll keep an eye out for a set of carbs on Ebay, until I find a decent deal it runs well enough as is. 

I did try counting turns on the pilot air screws, they're so long winded that it's hard to keep count, and worn enough that the screwdriver just wants to slip out, then you're back at square one. 

They do all bottom out with the screws oriented differently (meaning, one may have the slot horizontal, the other vertical, and the other in between), but that may very well be just variance in the way those needles are manufacturered. I guess I could take one needle and try it in every carb.

 

[fishinwrench]

2 hours ago, MrGiggles said:

They do all bottom out with the screws oriented differently (meaning, one may have the slot horizontal, the other vertical, and the other in between), but that may very well be just variance in the way those needles are manufacturered

Yeah you can't pay any attention to the slot orientation.   The tapping of the threads isn't that precise.   That's why you always count the number of turns from the fully closed position.

[MrGiggles]

The adventure continues, thanks Mike.

Go ahead and get it outta your system, Wrench.

[MrGiggles]

Started off by verifying BPS's diagnosis. Compression test showed 175, 175, 175, and 60. Borescope down #4 showed cylinder damage, to what extent wasn't readily apparent.

So I got it all stripped, powerhead is still sitting on the leg but everything is undone to lift it off when the time comes. Kept looking for the head bolts until I realized the girdle bolts are like 18" long and secure the head from the bottom.

The bores are magnetic, so the block has liners. In theory a quick hone to knock the glaze off and a new piston would put it back in service, if the bottom end is decent. The cam bearings and top end were beautiful. The scuffing in the cylinder is very, very minor.

There is also the cause of this mess in the first place, which I'm not sure of. #4 must have been really lean to break up the piston like that. Plugged injector?

When I get some more time, I'll yank the flywheel off and pull the crank girdle, and push the #4 piston out of the bottom and see what the bearings look like. Regardless, I think the powerhead is coming off, since I think it would be next to impossible to get everything timed right as-is.