Bourdon Tube -- An Elementary Example

This post is apropos of nothing; it's just something that I happened upon, and I thought I'd share it.

- - -

A Bourdon tube is a device that forms the heart of many pressure measurement gauges. A Bourdon tube is an arced, flattened tube, closed at one end. Gas pressure applied at the tube's open end will tend to 'straighten' the tube slightly. That 'straightening' results in a linear deflection of the tube's closed end. That deflection is linked to an indicating needle movement, and used to indicate the magnitude of the applied gas' pressure.

The linkages and needle movements can be pretty elaborate little mechanisms, but here's an example of what may be the simplest Bourdon tube gauge possible. The face of the gauge looks like this.

The mechanism at the rear of the gauge's face looks like this.

The hole in the white fitting at the centre of the photo is where air enters the tube. As the tube is 'straightened' by air pressure, the closed end of the tube cranks the gauge's needle across the face's scale to indicate pressure magnitude.

That's the simplest execution of the Bourdon tube principle I've ever seen, which makes it an excellent illustration of the principle.

# # #

FEEDBACK

# # #

Tecumseh TVS90 Governor -- Static, Baseline Adjustment

There are several adjustments associated with the TVS90's mechanical governor. There is one primary, fundamental adjustment that establishes the relationship between the governor's 'at-rest' control lever position, and the carburetor's wide open throttle (WOT) position. The adjustment is 'static' because it's made with the engine stopped; it's 'baseline' because it's the initial adjustment from which correct governor operation can commence and be further adjusted.

One simple fact about the governor's operation will help you understand the adjustment; i.e. an engine stopped condition must result in a WOT condition. Think of it this way -- the governor must respond to a slowing engine by opening the throttle to speed up the engine. There's no slower engine speed than 'stopped', so the appropriate governor response to that is WOT. (Even though a stopped engine cannot, of course, accelerate.)

Here's a view of the governor's control arm in its at-rest (engine stopped) position.

(The shroud, air cleaner and fuel supply hose have been removed for clarity. I've whitened the end of the governor's control shaft.)

The governor's control shaft swings the control arm via a short, rectangular clamp-arm. A 1/4" hex head screw on the clamp-arm permits adjustment of the control arm's angular relationship to the governor's shaft. Here's the adjustment procedure.

a) Throttle control cable disconnected. Clamp-arm loosened.

b) Clamp-arm/governor-shaft biased fully CCW. (i.e. the governor is held at its at-rest position.)

c) Throttle held at WOT.

d) Tighten clamp-arm.

That's it; the adjustment is made. The governor's at-rest (engine stopped) condition produces a WOT condition. The governor is ready to operate with the engine running. Governed operating speed and governed idle speed can be adjusted.

# # #

FEEDBACK

# # #

Tecumseh TVS90 Carburetor Teardown

Here's a view of a very grimy TVS90 carburetor just removed from its engine.

There's not a lot to it. As carburetors go, it's one of the less mystifying ones around. Proceed as follows:

a) Governor Link

Note the link's orientation, and which hole it hooks into at the throttle lever plate. It doesn't hurt to make a sketch of such details -- they can be surprisingly mystifying come reassembly time.

b) Fuel Supply Hose

Coax it off its nipple. A hose that's swollen or cracked ought to be replaced.

c) Primer Bulb

The primer bulb is held in place by a pushnut; it's not difficult to coax out the pushnut to free the bulb. See this post for more information.

A primer bulb that's no longer springy and supple should be replaced. Primer bulbs can be deceptive -- they can appear to be more-or-less ok, yet function poorly. If in doubt, replace it.

d) Float Bowl

The bowl's fastener is 1/2" hex. Unscrew it carefully so as not to damage its gasket. Pull off the float bowl and you'll have full access to the float chamber. Remove the float's pivot pin to remove the float and its valve needle. Remove the bowl's ring gasket.

That all brings you to here.

Still inside the carburetor's body is the resilient float-valve seat. Blowing compressed air into the fuel inlet will dislodge that item, but be careful not to launch the valve seat -- keep a thumb over the float-valve's opening to restrain the seat from leaving the carburetor entirely. I'm inclined to just leave the valve seat in place, unless I mean to replace it.

Clean all the parts in a parts washer, and blow out all the carburetor's passages. I never disturb a welch plug, or a throttle or choke butterfly, unless there's a pressing reason to. Should you need to dismantle a butterfly, be certain to note its orientation beforehand.

Adjustment -- Float Level

Float level is the only adjustment that can be made in this carburetor. It's easily gauged by eye with the carburetor upside down, so the float-valve is held closed by gravity.



In the above photo, note that the float is parallel to the bowl flange; that's correct float level. Float level is adjusted by bending the tang that presses on the end of the float-valve's needle.

Model Number

Should you need to obtain any replacement parts, you'll need the carburetor's model number. Typically, that will be stamped on the output flange of the carb's body casting, like so.

The model number may appear elsewhere, but it will be there somewhere. Even when you have the model number, it's not a bad idea to take the actual carburetor with you when you go to buy parts.

Reassembly

Lubricate the bowl fastener and its gasket with WD-40 prior to screwing it in and tightening it. Also lubricate the bowl's ring gasket. Here's the cleaned up carburetor back together.

Clean inside and out and ready to run.

# # #

FEEDBACK

# # #

Tecumseh TVS90 Kill Switch Operation

The kill switch on a Tecumseh TVS90 lawnmower engine resides under the engine's flywheel, where it works in conjunction with the engine brake. A closed kill switch grounds a point in the ignition system, killing engine operation. Here's a view of the switch in its closed/off/kill position. (I've whitened the switch's 'pigtail' contact to make it easier to see.)

And here's a view of the switch in its open/on/run position.

The switch itself is rugged and adjustment-free; its only real enemy is rust. (More on that later.) Following are the likely failure modes/mechanisms:

a) Engine Won't Start -- No Spark

A stretched/binding control cable can cause this when the cable develops insufficient travel to open the switch. The best solution is replacement of the control cable. In a pinch, it's possible to 'adjust' the switch to accommodate the impaired control cable by bending the switch's wire 'pigtail' contact. That's not a method that I recommend, but I have done it and gotten away with it.

b) Engine Won't Stop When Bail Is Released -- Binding Control Cable

This is an unlikely failure mode because of the switch's design -- the switch doesn't need much control cable travel in order to close. However, it is a possible failure. The only practical solution is replacement of the control cable; there's no good work-around for a cable that's binding that badly.

c) Engine Won't Stop When Bail Is Released -- Rusty Switch Contacts

I've actually seen this failure happen -- a kill switch with contacts so rusty that it couldn't make, even though it would physically 'close'. The solution was to scrape/file the mating switch contact surfaces. That got the switch working again.

# # #

FEEDBACK

# # #

A Broken-Off Tecumseh Oil Drain Plug

This is what happens when one overtightens a plastic oil drain plug in a Tecumseh TVS engine.^[1]

The hex is gone, and the remaining straight-threaded plug remnant will seep oil forever.

Fortunately, the plug remnant is not difficult to remove -- jab it with the tips of a pair of long-nose pliers, and you can coax it out. Here's the plug outside of the engine, along with what I mean to use to repair it.

That disc in back is a steel knock-out from an electrical box.

Key to this repair is squaring off the ragged end of the plug in the lathe, like so.

With that done, I can bore the plug remnant and the steel disc through 11/64" diameter to accept an 8-32 screw. With the disc lapped flat, the whole affair can go together with CA adhesive as a threadlocker, like so.

Add the o-ring, and the plug remnant is a functional plug again.

That made my day.

- - -

Note:

[1] It wasn't me who overtightened the plug -- honest. The engine shown is one that I salvaged from an old wreck of a mower.

# # #

FEEDBACK

# # #

Noma Gran Prix Snow Blower -- Carburetor Service

* * *

[To get to removal of the carburetor from the engine, see this post.]

* * *

[For a detailed carburetor overhaul narrative, see this post.]

* * *

Here's the carburetor off the engine.

Carburetor service is an art and a science that I'm by no means a master of. This post is only intended to give a few tips and hints that may be helpful for dealing with the subject carburetor.

Note the following:

• If you're familiar with the carburetor from Tecumseh's TVS lawnmower engines, this carburetor is similar. The big differences are the presence of adjustable idle and high speed needle valves, and the absence of a primer bulb.
• Before removing either needle valve, observe and record the number of turns required to gently close it. That will give you an initial adjustment point to get you going after re-installing the carburetor. Tecumseh's default setting is 1 1/2 turns for the high speed needle; 1 turn for the idle needle.
• The float bowl's fastener is 7/16" hex.
• I never disturb a welch plug, or a throttle or choke butterfly, unless there's a pressing reason to.
• Blowing compressed air into the fuel inlet fitting will dislodge the float valve's resilient seat. That's helpful when you mean to replace the seat; it can be disastrous if you don't have a spare seat on hand, and the seat takes off for parts unknown.
• The fuel filtering at the fuel tank's outlet nipple is fairly effective, and there shouldn't be any big particles inside the carburetor that could plug an orifice. A good flush and rinse with solvent in a parts washer, and a blow-off with compressed air, are all that a carburetor usually needs in the way of cleaning.
• Float level is easily gauged by eye. If the float is level/parallel with the upper casting's lower flange when the float valve is closed, that's close enough.
• Lubricate all threads at reassembly with WD-40.

# # #

FEEDBACK

# # #

Noma Gran Prix Snow Blower Maintenance

My son recently acquired an old Noma Gran Prix snow blower for very little money.

The thing starts and runs, albeit a bit roughly, and doesn't appear to have been abused or badly neglected. The plan is for the snow blower to be a big time-saver this winter for my son at his house. For that to come true, I'll have to get the machine into absolutely flawless good order.

With that in mind, I'll embark on a little TLC journey here, and document what I do and what I learn. Here goes.

The Engine

The engine is an 8hp Tecumseh HM80.^[1] The compression feels adequate, so the machine can be taken to be basically sound. I'll first attend to the spark plug and the fuel system, the fuel system being the most critical item of all. In my experience, fuel systems are the source of most small engine trouble. If an engine's fuel system is sound and immaculately clean, the engine will start and run reliably. If not, the engine will be balky and troublesome.

The Spark Plug

One can't complain about the spark plug's accessibility.

The plug was in barely finger-tight; that may explain at least part of why the engine didn't seem to run all that enthusiastically.

The plug  is a Champion RJ19LM, which is the correct type for this engine, according to Tecumseh's parts listing. Here's a view of the plug.

It's a gasketed type, with a 13/16" hex. The plug appears to have been replaced recently.^[2] I'll clean and file it a bit, even though it really doesn't need it. The gap is a little excessive; I'll set that to the correct figure of 0.030".

The Fuel Tank

I'll dismount the tank, empty it, back-flush and rinse it with solvent and leave it to dry thoroughly in sunlight. That should ensure that there are no particles or water in it whatsoever.

The tank is attached by two 3/8" hex head screws up top.

Underneath, there's a single spring clamp securing the fuel hose to the tank's output nipple.

The tank perches on a hook at the side of the engine. Just tip and lift away the tank to remove it.

The Heater Box

At the left side of the engine there's a rectangular cover that conceals the carburetor; that's the heater box. It's there to trap heat from the muffler so the carburetor and intake pipe will be warmed as the engine warms up.

Note the rod that runs closely alongside the heater box. That's the control rod for the snow blower's output chute position, and it will interfere with getting the heater box off. The rod's front end is fastened by a cotter pin, like so.

Remove the cotter pin and washer, and the rod can be disengaged and swung out of the way without disturbing the adjustment of the big 'screw'. Then, pull off the choke knob and remove four screws, and the heater box can come away. You'll be left with the heater box dangling by the kill switch wire.

The kill switch is easily popped out of its mount to free the heater box entirely.

Note that there is no air filter. It appears to be taken as a given that snowy driveways are not very dusty places.

The Carburetor

Here's a view of the carburetor in place at the left side of the engine.

And here's a better view of the governor linkages.

NOTE the position of the link end in the throttle lever-plate. You'll want to be certain of getting that back in place as it was.^[3]

To remove the carburetor, you'll need a 7/16" wrench and a No. 3 Phillips screwdriver. (You may need an offset screwdriver for extra leverage.) Slip the primer tube and the fuel hose off their nipples. Undo the two 1/4"-28 fasteners at the intake pipe, unhook the governor link and the carburetor is free of the engine.

And here it is inside on the workbench.

And with that, this post has gotten long enough. I'll end it here before Blogger or I can blunder it out of existence. For some further information on the carburetor, see this post.

- - -

Notes:

[1] The 'HM' stands for "Horizontal Medium Frame". I believe the '80' stands for 8.0 horsepower.

[2] Probably every small engine that goes to the scrapyard goes there with a brand new spark plug in it. Replacing the spark plug is conventional wisdom's answer to all small engine troubles. In fact, the spark plug is one of the least likely items to fail in a small engine, and it can almost always be salvaged by cleaning, filing and gapping.

[3] Most small engines will present you with governor linkages to be unhooked when the carburetor is to be removed. Always photograph or sketch their installation before dismantling them. Unless you have a photographic visual memory for such details, and I certainly don't, linkage connections are liable to be mystifying when it comes time to reconnect them, if you haven't taken a picture or made a sketch.

# # #

FEEDBACK

# # #