Ford 9n surging/running lean

A couple weeks ago I couldn't resist a '41 9n for sale at a good price down the road. It started and ran at all engine speeds, but surged (especially under driving loads) and popped a bit. I bought it expecting to have to replace the governor, which I did. But while that helped, it still surges a bit. I read a thread here from early 2018 about this and tried all of the adjustments to the carb (which is a new TSX-241B the previous owner put on). It definitely acts and sounds lean, but I can't enrich the idle or main screws any more than I have. I checked the fuel screen into the carb (which seems quite a bit shorter than others I've seen), and secured the elbow joint (which seemed a little loose) to make sure no air was getting around it. I don't know how to check for a vacuum leak, but that's about all I can think of. Any tips on how to do that? Also, I saw somewhere (one person) that aftermarket carbs sometimes ship with an oversized jet that can let air in from about the bowl. Is that real?

To check for a vacuum leak one can either use a propane torch not lit and wave it around the carb/manifold or spray carb cleaner and if it smooths out then you found the area of a leak. You can also pull the air tube off and put your hand over the air intake and make sure you get a good suction there and have gas on your hand. If choking it helps then yes it is running lean if it hurts it is running to rich

Thanks - I'll give those a try!

It may be lean, but it may also have weak spark.

Check the spark first. Pull one wire at a time from a spark plug, it should jump a minimum 1/4" to ground.

Check the point gap, check the plugs, gap, clean, if any doubt change them.

Check the air cleaner. There is a wire mesh inside the canister. It is often overlooked and needs periodic cleaning.

To check for vacuum leaks, get a oil pump can with some gas in it, or use the flammable type carb cleaner. Squirt it around the gasket areas, anywhere you suspect a leak. The engine will suck it in and change the way it runs.

If that is good, move on to the fuel system.

Before trying to adjust the carb, locate the drain plug in the bottom of the carn bowl. Have a clean glass ready, engine off, fuel valve open, remove the plug and catch the flow.

It should have a full flow, slow some as the bowl empties down, but continue to flow a steady stream. If it slows to a drip or stops, there is a fuel restriction, not enough fuel getting into the bowl.

Look at what was caught. If there is water, dirt, rust, then the same will be in the carb, even if it is new. The tank will need to come off and be cleaned. The carb may need to be eased apart and cleaned, simple easy job. Inline fuel filters do not play well with gravity flow systems. There should be a factory screen in the inlet fitting of the carb, a screen in the sediment bowl, and one inside the tank. If the tank is flaking rust it may be time for a new one. Fighting a rusted tank is a loosing battle.

Once everything is clean and the carb has plenty of fuel, then it can be adjusted. Make adjustments with the engine up to temperature.

Start with the idle. The engine must be idled down to 400-500 RPM. Turning the idle screw in richens the mix, backing it out leans it. You are adjusting air bleed, not fuel flow. The adjustment is minimal. Adjust it for best idle. If it is ineffective, either the idle is too fast or there is a clog in the idle circuit.

Next adjust the main jet. Gently turn the screw in until it bottoms, then back out 2 turns.

With the engine at idle, suddenly open the throttle, listen for the response. It should take sudden throttle without hesitation. If it does, turn the main screw in 1/2 turn. Repeat the test. Keep leaning the mix until the engine stumbles on sudden throttle. Then back the screw out 1/4 turn at a time, repeating the test until it will respond without hesitation. A puff of black smoke is where you want it.

That should get the carb where it should be. Do these things and get back with us, see what happens.

like Steve said if it has an extra inline fuel filter remove it

> Before trying to adjust the carb, locate the drain plug in the bottom of the carn bowl.
> Have a clean glass ready, engine off, fuel valve open, remove the plug and catch the flow.
> It should have a full flow, slow some as the bowl empties down, but continue to flow a
> steady stream. If it slows to a drip or stops, there is a fuel restriction, not enough fuel
> getting into the bowl.

Well, I checked this at lunch break. No crud came out of the bowl, but the stream fizzled to drops. The seller had replaced the fuel line, and I foolishly figured nobody would do that without tending to the sediment bowl and petcock. MY MISTAKE! I removed the line and it's clear, but the sediment bowl had no screen above it, and a little crud in the bottom. When I put it back together it isn't flowing at all.

So tomorrow (when I'm off) I'll borrow a sediment bowl from a 2n I've got waiting for other work (it was new a few years ago). If that shows me a decent flow to the carb then maybe that was the fuel-starving culprit. If not, hopefully at least I'll be able to start it and try the propane test for vacuum leaks.

By the way, the post about incorrect jets was on the YT board this April in a thread about 8n surging. I can't see the whole thread, but this part caught my eye: "suggest measure the economizer jet. Spec for the stock, Marvel Schebler carb is .046. The jet in my aftermarket, silver carb was .065. The bigger hole in the jet lets engine pull to much air from the float chamber, giving your symptoms."

No inline filter present.

If you have a compressor blow air into the fuel line and if possible look in the tank at the same time and you might see some dirt/rust come out of the sediment bowl area. Last year my Oliver S88 would not run like it should and I found a small leave in the sediment bowl assembly blocking off fuel flow

frysinger, rather that telling what things seem like, it is much more helpful to tell what you are seeing, hearing, feeling, or tasting that tells you that something "seems". for instance surging would probably be interpreted as the engine speed varying up and down without changing the setting of the throttle lever. Popping would be interpreted as back firing. So far as acting and sounding lean I am at a loss as to what may be telling you that. As a result people are trying to help you with a lean condition without knowing if that is the unlikely cause.

> for instance surging would probably be interpreted as the engine speed varying up and down
> without changing the setting of the throttle lever.

That's correct. I assumed most folks would be familiar with that term and its meaning, and I didn't want to waste peoples' time with long descriptions.

> Popping would be interpreted as back firing. So far as acting and sounding lean I am at a
> loss as to what may be telling you that.

That's the characteristic sound of lean mixtures that I've been familiar with for about 50 years of tinkering on various engines (cars, trucks, motorcycles, and tractors). After enough time you learn to recognize the sound of it. It seems like most folks here got the correct impression of what I was observing.

Surging usually indicates a blocked idle circuit. That you've opened the idle screw as far as possible with no effect also indicates the circuit is plugged. Disassemble the carb and clean out all the passages with a fine wire.

> Surging usually indicates a blocked idle circuit.

Thanks! I was thinking that should be my next thing if cleaning up the fuel supply doesn't solve the problem. Given the missing fuel screen at the sediment bowl and a little crud I found in the carb filter I guess it's possible that even a new carb got something in that main circuit.

As I mentioned before, the carb filter is noticeably shorter that my others, almost like it was cut down. I wonder if that could restrict fuel flow as well (I have a new one coming).

I'm still curious about the issue of the economizer jet possibly being oversized. If I do end up removing and disassembling the carb I wonder if I should check the jet sizing, but I'm not sure how to do that (or where to find a correct jet, short of buying a whole deluxe rebuild kit).

> If you have a compressor blow air into the fuel line and if possible look in the
> tank at the same time and you might see some dirt/rust come out of the sediment bowl area.

Good idea! I blew through the line when completely disconnected, but your trick might clear stuff at the top of the sediment bowl. I'll have to get my wife to do the compressor part so I can watch the bowl, but she'll likely go for that. Thanks!