Tecumseh L-Head question

[creia]

1970 Tecumseh HS40-55282B (Horizontal)
Full adjustable diaphragm carb with both a idle/low speed screw and a main/high speed screw
Here is my question: On a cold start, when using the choke (lever) does the fuel draw from (and is controlled by) the low speed or high speed screw? That is to say, which one would I adjust to effect how much fuel is drawn during choking?
Thank you,
Michael

[bgsengine]

Trick Question. Just like with choke open, where fuel draws from is gonna largely depend on where throttle plate is at - but technically with engine not running, both high and low will feed fuel , with choke closed since both ports are exposed to vacuum behind the choke plate.
Engine running, however, generally fuel is fed the same way whether choke is open or closed - it'd depend on throttle position - assuming no load, and choking, generally that extra fuel is coming from low speed passage

[Skywatcher]

Hi Michael and a Happy New Year to you

As BG says, on full choke, fuel is being drawn from both circuits. If the carburetor is adjusted for the engine to run correctly when warm, no further adjustment should be required. If the engine is reluctant to start when cold, maybe a few other things should be checked into. As this is almost a 50 year old engine, the carburetor will have been serviced, messed with or even monkeyed with at least a couple of times during the engine's life. Valve adjustments should also be checked as some wear will have taken place in 50 years. Both valves .010" ± .002". Things I would want to look at in the carburetor are: clear fuel passage from connector to metering needle, diaphragm flexibility, diaphragm orientation, diaphragm and gasket assembly order, trueness of diaphragm plate, correct location of high and low adjustment screws for starters.

Some of those diaphragm carburetors require the diaphragm against the carburetor body and some require the gasket. The metal disc of the diaphragm assembly always goes towards the metering needle regardless of diaphragm/gasket order. The high speed screw is a single taper, whereas the low speed screw is a stepped taper. 1½ turns out on both screws will get you going so the carburetor can be tuned from there. If the diaphragm plate is warped, it will pinch the diaphragm/gasket on the corners and may leak along the edges between the screws.

If the engine is on a winter application engine, it will likely have a primer hose attached to the diaphragm plate via an elbow fitting. Make sure the hose is not cracked and the vent hole in the primer bulb is not blocked off, this hole is the atmospheric vent for the diaphragm. Again, if the diaphragm plate is warped, you may be losing priming pressure around the edges of the plate. If it doesn't have a primer assembly, make sure the hole in the center of the diaphragm plate is clear, and you should be able to see the center rivet of the diaphragm assembly through the vent hole. Hope this helps point you in the right direction,

Sky

[creia]

Many thanks gents for your replies and info- very much appreciated (as always).
Full disclosure up front: While my son and I have worked on and restored some old Tecumseh L-Heads, they are not "in our wheelhouse" like the Briggs of the same era (mid 1960s-late early 1980s). I think that we are "fairly" competent and experienced with them, however.

Here is why I posed the question...
I am getting contradictory results with the idle and high speed screw settings as they relate to the performance of the engine.
I adjusted both the idle and high main screws to get the best overall performance as per the Tecumseh manual.
Idle screw is opened about 2/3 turn out and high speed is opened about 1/2 turn out. (My Tecumseh 3-11 HP L-Head manual states that 1 turn out for both is the "starting point" on these diaphragm carbs to get it to run, then adjust in or out from there, which we did)
1. On cold start the choke seems kind of lean- taking 3-4 pulls to get it to fire (I'm in So Cal- not THAT cold here!).
2. After the engine has started and warmed up it runs GREAT- nice smooth low rpm idle and good performance throughout the upper rpm range.
3. If I shut the engine off and try to restart after a couple of minutes it will start and run on only one pull.
4. If I wait much longer than that (engine still very warm to the touch) I will need to choke it to get it to fire- THAT seems like lean condition to me? If I try turning out (opening) the high speed screw at all performance really degrades.
5. Only 1/2 turn out for the high speed screw seems odd to me? Every other Tecumseh I've owned was 1- 1.25 turns out

Here's the background on this engine;
This was a full tear down, cleaning, inspection (measure for any reject sizes) and rebuild:
Rebore with new .020" O/S piston/rings set
New rod
New valves- they were lapped and lash was set at .007" for each (My Tecumseh manual states to set from .004-.008)
New head
New crankcase oil seals (both sides)
Carb was taken apart , all orifices probed and cleaned in ultrasonic, followed by carb spray. Carb was fully rebuilt including new diaphragm (put on the correct way!)
New points/condenser
Engine re-timed (Thank you Brian! )
New NGK spark plug
New air cleaner element
Fuel tank cleaned and in-line fuel filter installed

Does this added information give anyone some other/additional ideas?
Michael

[Skywatcher]

Hi Again Michael

Sounds like you've pretty much created a clean slate to start from by rebuilding this engine. Here are my next thoughts. From what I understand, these diaphragm type carburetors do not give fuel as readily as the float type carburetors so take a little more vacuum to get the fuel flowing through the low speed ports and the emulsion tube. This would likely explain the need for a little choke even when starting a warm engine.

Now onto the mixture screw settings. If the high speed mixture screw is only ½ turn out, I would start to question the high speed circuit air bleed gallery not supplying enough air into the mixing chamber around the emulsion tube. If anyone else has some ideas, please chime in.

My next question is what is the micron rating of the inline fuel filter? Anything finer than 75 micron filtration can create too much resistance to flow on a low drop gravity feed fuel system. Please keep us informed with your progress,

Sky

[bgsengine]

Yeah, what Skywatcher said. Also, is the throttle shaft a nice fit (no wobble or side play?) But as Skywatcher said, it would be good to wonder about air supply to the idle mixing chamber (the air cleaner case screws - are they the correct size and length? is the correct gasket being used? There are more than a couple different ones..) see if you can find where the air goes (from the outside side of the venturi) to get to the idle mix chamber (typically covered by a welch plug, and 3 differently sized idle ports in a row drilled through to where the throttle plate closes in carb throat - transition ports and idle ports) Also I assume you checked those little ports also to be clear, no?

[creia]

I appreciate the continued interest and help....
As to your follow-up comments (all excellent questions!):
1. Fuel filter- It is OK. I always check fuel flow from tank through petcock, filter, and fuel hose BEFORE I connect it to the carb.
2. Throttle shaft- It is OK. I know all to well about the problems with worn ones working on the old Briggs Pulsa -jet carbs.
3. Air cleaner case screws and gasket- Are all the correct ones.
4. The 3 tiny idle ports on the inside of the venturi- I cleaned them "the best I could" by probing with some tag wire from inside the venturi, HOWEVER, I did NOT take off the welch plug on the outside to inspect or clean. Also, there are also 2 more (smaller) welch plugs on the base/bottom of the carb next to the inlet needle- I did NOT remove these either. Possibly, this could be part of the problem?
Michael

[bgsengine]

Any one of those could very well be source of problem - Ultrasonic cleaner does not always get rid of corrosion beads (tiny bits of metal that have flaked off after oxidation) and also corrosion blooms (that haven't fallen off) can block passages , and of course if there happens to be some stubborn pieces of varnish blocking passages, those need cleared out too... so probably worth getting new welch plugs for those spots (I always check if I can get new ones before removing old, or find a source of identical size ones from other brands) and go through them I'm well aware that you're not "time sensitive" so you can spend more time on the cleaning and re-cleaning but you may well have to get a new carb (those are expensive little suckers, and hard to find too) or unfortunately have to consider possibly refitting with a float carb.

[KE4AVB]

Definitely agree that some carburetors are hard to clean no matter what cleaner you use. I have seen vanish so smooth that UC just can't even start to clean it until it is roughen up a little. I have had the old pulsa jet carburetors with towers where the metering passages so clogged that nothing would clean them without drilling out the steel balls. Mainly because how rusty the fuel tanks were. Sometime you get lucky using vinegar to remove corrosion and rust. I have clean some carburetors to back to working order that most of us probably would have just gave up. I wouldn't have blame them either as I thought it was a lost cause too at times.

The next time I get a chance I going try electrolysis to remove the rust since I am now setup to do small pieces. Just got to be carefully and remove the brass. The last attempt was a cube carb and it came out with golden finish on the aluminum as some the brass transferred via electro plating.

[creia]

It looks like I'll be pulling the carb and removing the welch plugs...
Michael