Tillotson Carb Question

[frankr]

OK, for all practical purposes, the fuel for starting the engine would be flowing through the high speed circuit, not the idle circuit. Generally speaking, that would be from the float bowl to the area of the high speed needle tip, from there to the main nozzle in the venturi, and out to the reed valves. Given the size of those passages, it would be pretty difficult to obstruct them so much as to not even allow enough fuel to even start the motor.

Besides, you just said you swapped carburetors and it didn’t help. I think you can lay the carburetor idea aside and look for something else.

Perhaps you should be thinking about crankcase sealing. Or for some reason the upper crankcase chamber is not isolated from the lower chamber (for instance reed plate gasket).

I have no clue what the actual problem is, just speaking in generalities.

Will it start out of the water? Yeah, I know, mind the water pump.

[billw]

Two ideas: First, make sure the choke linkage allows the choke to close all the way. Second, try new gas. If your gas is any more than a month or two old, it could have lost its poop. That drove me mental once, on my neighbor’s lawn mower. Gas looked good, smelled good….but it was dead. Proved it by spilling a small amount on a cement slab and lighting it off with a torch; then doing the same with new gas. The difference in flame was unreal. Before you say that it runs good when used on other engines, keep in mind that 80 psi compression, while okay on a small engine, really is getting down towards the border of not firing.

Long live American manufacturing!

[kevinrude]

quote FrankR:

OK, for all practical purposes, the fuel for starting the engine would be flowing through the high speed circuit, not the idle circuit. Generally speaking, that would be from the float bowl to the area of the high speed needle tip, from there to the main nozzle in the venturi, and out to the reed valves. Given the size of those passages, it would be pretty difficult to obstruct them so much as to not even allow enough fuel to even start the motor.

Besides, you just said you swapped carburetors and it didn’t help. I think you can lay the carburetor idea aside and look for something else.

Perhaps you should be thinking about crankcase sealing. Or for some reason the upper crankcase chamber is not isolated from the lower chamber (for instance reed plate gasket).

I have no clue what the actual problem is, just speaking in generalities.

Will it start out of the water? Yeah, I know, mind the water pump.

That’s interesting. Can you explain that to me a bit more? This is a new experience for me. Is it the gas can’t get through when the two chambers are not isolated because one piston is trying to suck the vapor in while the other is blowing the vapor out?

The gaskets "looked" good, but I did not replace them. What would be my better bet: (i) cutting new gaskets or (ii) applying some kind of non-hardening gasket sealer to the existing ones?

As for out of the water, I have only been attempting to dry fire it, figuring I could shut it off in a hurry if it caught and then transfer to the test tank.

Thanks for continuing to get your noggin’ working on this one because I was at a loss. Also, a buddy of mine is having a similar issue with a mid-50s Johnson.

[frankr]

One sucking while the other is blowing is one way of putting it….That is the charge is merely being passed back and forth between upper and lower crankcase chambers.

A couple of other ideas–has it ever been severely overheated? Could be the piston skirts are trashed but still have compression. Has it been apart? Did they put the pistons back in the right way (slopes in right direction)? Main bearing seals could be severely leaking?

[retiredoz]

One quick check : Squirt some of your fuel into the cylinders and crank away. If you don’t get a sign of life, I’d think BillW has it, try using new fuel.

[ronboonville]

Does it just pop with the starting fluid or does it try to run? reversed plug wires will pop. this has happened to me. I also spray fuel down the throat then spin the motor with my drill. and there is a passage at the bottom of the bowl that can be plugged.

[kevinrude]

Finally got back to this project (3 months later!!!). I re-installed the reed plate, gaskets and manifold, this time with gasket sealer on the gaskets. Same result — will start on starter fluid but not on gas/oil. I tried new gas too. So, based on what I have read above, it looks like I need to take the motor apart and see if it is a piston ring and/or main bearing seal and/or crankcase seal. Unless there are any silver bullets I have missed. Thanks again fellas!

[garry-in-michigan]

❓ Stale Gas? Weak choke spring? . ❓ . ❓

[green-thumbs]

A more serious rebuild involves removing lead plug and welch plug to
access and clean out passages using specified size number drills. Also
the very aggressive and toxic carburetor dip is a shop method.
Not many will want to get that serious. Usually spray and compressed air is
good enough.
The die cast zinc AJ series carburetor are sometimes corroded inside which
can plug or alter size of internal passages. Not much can be done about that.

It appears fuel is getting to the carburetor but not being vaporized.
The sure test is to swap in a KNOWN to be good carburetor
of correct type. KNOWN to be good is not same as ASSUMED to be good.

A friend had a small engine that would not start. He was a good mechanic who
knew his engines yet he could not get it to start UNTIL he replace the new
spark plug with one from an engine that would run. He had assumed a new
spark plug would be a good spark plug…usually but not always the case.
Louis

Tillotson AJ series has a screw at bottom of float chamber for draining fuel
and water as required or before storage. Curiously it is usually a 1/4" x 32tpi
super fine screw thread.Changed to 1/4"x 28 fine thread at
some point.

[cajuncook1]

Possible crankcase leak or poor crankcase negative pressure. The negative crankcase pressure shuttles the fuel mix charge into the cylinder for combustion.

Missing or torn carb to crankcase gasket?

Another consideration is obstructed exhaust ports with carbon or your exhaust housing leg is obstructed with trash…wasp or mouse nest or ant pile dirt from laying about.

If flow is obstructed or limited going out then it does not get sucked in.

Not saying that is your problem, but it needs to be considered when other usual culprits are ruled out.

[Author]

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