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Cylinder timing question - base milling

benjpi

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hello. I've got a cylinder timing question about a Husky 51 rebuild. I would like to reset the squish to 0.015-0.020, but I measured the aftermarket cylinder at 0.050" squish with no base gasket. (this matched the OEM cylinder, scored & crap.) I measured the stock, compressed base gasket at 0.010". I'd like to use a base gasket because on this saw it looks like the impulse port relies on the gasket thickness; I don't want to cut a groove in the cylinder cases.

Cutting 0.040" from the base seems like it may significantly effect port timing. Is that amount large enough that I should consider raising the exhaust, intake, and transfers at least 0.040" to match?

ben
 

benjpi

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The saw has been assembled now for a while, but it still doesn't run right. I did raise the exhaust port about 0.04" which correspondingly made it a little bigger, but that was it for work inside the cylinder. Because of the goofy setup on this saw for the impulse port, machining the 0.040" off the base meant that the impulse port became larger than the area it was "assigned" under the base gasket. I tried a first JB Weld repair, which didn't work - the saw had an air leak. So since I bought the saw to learn on, I bought a MightyVac and took it apart again, including changing the crank seals (which were still fine). I pulled the cylinder and filled the impulse port (again) completely with JB weld, and then modified it. I put the cylinder in a drill press, with the face of the impulse port perpendicular to the bit, and re-drilled the impulse grommet port 0.040" higher up the cylinder, figuring that the little tube might seat better in the grommet if it was higher up. I connected it to a new small hole in the bottom of the cylinder, where the old hole was.

After re-assembly, the saw would pass a vacuum test but would not hold pressure, only about 1.5 PSI before the impulse tube would bypass: The impulse tube extends from the carb mount plate into a little rubber grommet at the cylinder base, and the tube is tapered at the end -- by design it doesn't look like it would ever hold pressure so I didn't really worry about it. I didn't see how a little positive pressure leak there would do anything as bad as a vacuum leak. The vacuum held great though.

But it still didn't run right. So I rebuilt the carb, using an aftermarket Walbro kit (saw has a WT 170, I think). That immediately wouldn't work, could barely get the saw to start. So more reading, then I pulled the carb (again) and put the old pump diaphragm and gasket back in place. Saw ran then, but still pretty bad, and seemed to guzzle fuel. It ran okay up top, but if the saw was under load, it would bog and seems to run out of fuel. If I released pressure and let it rev up again, it would cut again for a few seconds and the repeat the mess. If I just pulled it out of the wood it would rev okay, but seemed like it really wanted to stall. If I stopped revving, that's what it would do. Nothing I did to the low screw seemed to have any significant impact.

More reading, and I think maybe I set the metering lever too high. So I open up the carb, and the lever is actually a bit too low, so I raised it up just a hair. Now the saw runs a little better, but has the same run-out-of-gas feeling.

So its back on the shop floor. I've ordered a cheap aftermarket carb to try that, but I'm still not convinced that it's a carb problem or the air leak is still there. The trick here is that the carb mounting plate - the part that attaches to the intake port - can't easily be made part of the pressure/vac test. I used an old one where I filled it with silicone (read that here somewhere) but that obviously can't be used on the running version of the saw. I'm wondering if I somehow damaged that goofy little grommet when I pulled the blocking plate off and replaced it with the (new) open one.

Does anyone have any ideas?
 

Al Smith

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I'm trying to make sense of all this but I got lost around the first corner .Concerning the impulse ,stop and think .It's the rise and fall of crankcase pressure .It has nothing to do with fuel transfer or exhaust .It has nothing to do with engine timing .
If the fuel delivery is off,starving under power, it would make sense if the impulse tube or channel is not leaking the problem most likely is in the carb .What you are trying to do with JB weld I won't even try to guess .
 

huskihl

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We're all lost. We need pics. .040" off the base didn't affect your impulse
 

huskihl

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Al, this should clear things up a little bit. That passage angle lets it open up wide as the base gets thinner. View attachment 141656
Well why in the hell was I thinking it was an 044? I was way off. I could see how that might move the impulse opening. Thanks
 

Al Smith

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So now let me get this straight .Evidently the impulse hole meets the crankcase at an angle? Why would that make a difference if the hole in the base gasket seals it up ?Punch it bigger if it doesn't line up .
 

benjpi

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Al, this should clear things up a little bit. That passage angle lets it open up wide as the base gets thinner. View attachment 141656
that's it exactly. I had to fill the opening entirely with JB then drill the impulse hole about 0.040" higher up, to line up with the tube that comes off the carb base and sticks into that hole. (there's a litter rubber grommet that goes in the hole too)

it isn't a hose barb, or some other fitting that would accept a lot of pressure without leaking. it's just a rounded end on a plastic tube that fits into the hole.

I've got an aftermarket carb I'll try later this week.
 

benjpi

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I'm trying to make sense of all this but I got lost around the first corner .Concerning the impulse ,stop and think .It's the rise and fall of crankcase pressure .It has nothing to do with fuel transfer or exhaust .It has nothing to do with engine timing .
If the fuel delivery is off, starving under power, it would make sense if the impulse tube or channel is not leaking the problem most likely is in the carb .What you are trying to do with JB weld I won't even try to guess .
Thanks. My thought here was that if the impulse tube leaks under vacuum, it allows in more air = lean condition & running problems. But when the piston is falling, before the transfers open, the case pressure might go positive enough to push air out of a slightly leaky connection at the impulse tube. With that said, I find it really hard to believe that ENOUGH fuel/air mix would leak out to cause an issue, but I thought I'd ask.

I'd like to get the Walbro working, if it is the carb, but I'll try the aftermarket carb first to see what it does.
 

benjpi

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Saw is toasted again, burned the Piston with lean running. The JB weld repair and move of the impulse port came apart at the thinned section of epoxy at the bottom of the relocated hole. The ring rotated too. I really don't like this impulse tube design, but I guess it works okay if the cylinder base isn't cut .

IMG_20181007_195616.jpg IMG_20181007_195557.jpg

I had some ideas about refilling the hole again and embedding a small hollow threaded insert, and then a small hose barb and trying again, but I'm not sure I want to fight with cylinder any more .

I'm now thinking about getting a closed-port cylinder, doing a gasket delete and widening the exhaust port and seeing how that goes . Fooling with the squish on these just seems to be a big pain in the ash .

The saw was fun when it ran, with so much gap in the stock squish it really seemed to like the reset to .020" but getting rid of the vacuum leaks from the impulse tube with an
040" mismatch seems too much for me .

It's been fun though, I've learned a lot so far, and by now I can get the saw apart down the crankcase in 10 minutes . Live and learn.
 

Al Smith

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I see said the blind man to his deaf wife as picked up a hammer and saw .So does the gasket extend to the flat area in the intake side .If it does ,does it have another flattened area on the crank case to match with ?
 

benjpi

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This picture shows where the impulse tube goes into the cylinder. There's a $5 rubber grommet that goes in that hole, then a tube extends off the carb into the grommet. When the .040 was taken from the base the offset from the carb mount is too large and the tube won't seal to the grommet .IMG_20181007_195612.jpg
The part at the cylinder to case seals fine. There is a flat on the base to match, but it does drop away.

You can see in the top of the photo that the hole was redrilled off center after it was epoxy filled, but that the thin section of the epoxy at the top of the hole came apart .
 

Al Smith

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If it's a tube of sorts it might be such a thing as blocking off the hole and drilling the case and installing a small hose barb .You can get barbs down to around a 10 32 screw size.
 

benjpi

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If it's a tube of sorts it might be such a thing as blocking off the hole and drilling the case and installing a small hose barb .You can get barbs down to around a 10 32 screw size.
I tried something similar, I tapped the hole out to 1/8 NPT and got a hose barb to fit . Even after filing the flats off the fitting it still hit the intake bell. Even if I manage to get it threaded all the way in, the hose itself interferes with the intake boot .

I've ordered a closed-port cylinder and Piston, I'll try a simpler build with just a base gasket delete. Best case with this saw and my skills is a 3.5 hp saw anyway, which is nice, but I think I'm going to look for and old 61 for the next project .You can put some big cylinders on those, and I can stop using my 391 for milling.
 

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benjpi

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So I got the closed port cylinder and started assembly. The Hyway cylinder seemed a bit nicer than the last one, same with the Piston and ring. So far so good .I ground a small divot in the case to provide the impilse since I was assembling with no base gasket. I measured the squish - awful at 0.050, but better than with the gasket in.

Took a few minutes to file down the base of the carb mount, not taking too many chances on the tube not lining up. Got it all together and prepared it for a pressure test.

Fail . WTH?! It all looks good, but maybe I missed something. Break out the soapy water and spray the usual suspects - nothing. But won't hold at the spark plug hole. Okay, just tighten up the test fitting and get the o-ring to seat, right?

Nope. After a bit more head scratching, turns out that the spark plug hole is not fully tapped. :mad:

Ah well, there is always tomorrow, after I purchase a spark plug tap, remove the cylinder, chase the hole and try again .
 

Al Smith

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On that tap you'll better of to spend a few more bucks and buy a three fluke spiral point than a 4 fluke bottoming tap .Those damned 4 flukes especially on aluminum can walk off and cut a double thread .
 

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My guess is that after lowering the cylinder the intake partition was bottoming out before it could get lined up with the impulse opening.
 

benjpi

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My guess is that after lowering the cylinder the intake partition was bottoming out before it could get lined up with the impulse opening.
Yes, that's pretty much what it was doing. On the rebuild without a base gasket I took.02 off the bottom of the partition base with a small block plane to try to make sure it lined up well.

Seems to have worked. Tapped the spark plug hole with a grease covered tap, that worked well to keep the chips from falling into the cylinder. I cleaned the few stragglers with a small screwdriver with grease on the tip. Vacuum test passed nicely this time.
 
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