carb not getting fuel

[cookies]

The inlet valve is only supposed to be opened by the diaphragm operating the control lever in response to the engine fuel demands. If the control lever is set too low, it has to be adjusted and tank pressure should never be able to open the valve.

Thats why I said what I said, also the spring strength will increase or decrease the duration of time the needle remains open as can the lever height meaning both can and will directly effect the length of time fuel is drawn in and in what amount based on needle height from the seat . It can also act as a pressure blow off valve on one way vented gas tanks to protect the carburetor , vents, case and fuel hoses from over pressure damages. Even the diaphragm gasket thickness can effect duration of opening.

 

[Automender]

When I rebuilt my TS420 concrete saw I put in a rebuild kit but the first thing I noticed is that the needle was slightly different and I didn't install it. If your carb was rebuilt with a aftermarket kit it could have a different needle which you will never be able to determine without the original to compare. I thought that you had two carbs and both don't work properly, what are the chances both are set wrong. Like I said before I tried to get the carb to pop with pressure but I replaced it due to a bad internal check valve and the new one wouldn't pop either.

Not sure how your carb never lever should be set but the couple of carbs I did were level with the face of the carb body.

 

[Vintage Engine Repairs]

I don't believe your Stihl Tech unless he can come up with test data to support his claim. There isn't enough variation in spring tensions to have much affect. I've never heard this theory provided by the carb manufacturers.

I honestly can’t say either way what’s right or wrong, because I haven’t tested it, but see no reason he’d make it up.

I guess the best way is to set pop off at about 7 psi, warm the engine up, set the rpm with a tach and get idle and WOT readings. Take the carb apart and set pop off to 30 psi and then see if rpm’s are different.

I just make sure it holds at least 10 psi. Less than that and it’s likely just a bit of debris or a dry needle needing some fuel to help sealing. I have had pop off be above what my mityvac can produce and it’s run just fine.

 

[MikeRock]

On a fire line once, cutting snags and stuff, it was hot and very sunny. Old Mac of some vintage..... didn't notice the flame until I stood the saw upright and my shirt started on fire. The damned tank was hot enough it was venting, and burning.....a cool little torch. Kept an eye on it, putting it out when it started up again. Geez.......that was 1971...

 

[Old2stroke]

Thats why I said what I said, also the spring strength will increase or decrease the duration of time the needle remains open as can the lever height meaning both can and will directly effect the length of time fuel is drawn in and in what amount based on needle height from the seat . It can also act as a pressure blow off valve on one way vented gas tanks to protect the carburetor , vents, case and fuel hoses from over pressure damages. Even the diaphragm gasket thickness can effect duration of opening.

I don't believe duration is very critical, as long as the control lever is within a good operating range and the diaphragm is flexible enough they will work together to reach an equilibrium where a fairly constant volume of fuel is maintained in the fuel cavity. These items respond to fuel demands of the engine and it's not like they operate on every stroke of the piston.

 

[Old2stroke]

I honestly can’t say either way what’s right or wrong, because I haven’t tested it, but see no reason he’d make it up.

I guess the best way is to set pop off at about 7 psi, warm the engine up, set the rpm with a tach and get idle and WOT readings. Take the carb apart and set pop off to 30 psi and then see if rpm’s are different.

I just make sure it holds at least 10 psi. Less than that and it’s likely just a bit of debris or a dry needle needing some fuel to help sealing. I have had pop off be above what my mityvac can produce and it’s run just fine.

Some mechanics are good a generating myths and misinformation based only on their thinking and not on factory training. Awhile ago a "echo trained" tech claimed that primer bulbs worked by pressurizing the fuel tank and forcing fuel into the carb.
So how does one change the pop-off pressure? It's easy to reduce it by cutting a coil off the CL spring but increasing it, not so easy. You can set up a jig to measure spring strength and select one that's stiffer but there isn't that much variation in springs that are the same length (I've done it). You can also search through your parts and find a longer spring that will fit or maybe try stretching the original (messy trial and error). There are reasons why carb manufacturers don't recommend fooling around with the spring as most of them will fall within the range of holding off at least 10psi.

 

[singinwoodwackr]

Just got a $20 OE carb off ebay...blew off the crud, threw it on...saw started on 3 pulls

 

[Automender]

Used OEM or new aftermarket?

 

[lone wolf]

Just got a $20 OE carb off ebay...blew off the crud, threw it on...saw started on 3 pulls

Why ain't I surprised damn Ethanol ruins so many of them. Good to hear it runs.

 

[singinwoodwackr]

Used OEM or new aftermarket?

Oem

 

[singinwoodwackr]

I believe the previous owner used the same ebay kits that I did. Can you see the problem?

 

[Woodslasher]

You mean the open hole, or the fact that it looks like the impulse port will be blocked off when the gaskets are lined up with their holes?

 

[singinwoodwackr]

You mean the open hole, or the fact that it looks like the impulse port will be blocked off when the gaskets are lined up with their holes?

The little gap not covered by the gasket. Impulse port is on the intake to boot side of the carb. What you see is the vent.

Note: new AM carb...completely different gasket and diagram

 

[Aknutter]

The little gap not covered by the gasket. Impulse port is on the intake to boot side of the carb. What you see is the vent.

Note: new AF carb...completely different gasket and diagram View attachment 984179

AF ?

 

[singinwoodwackr]

Oops, AM. After market

 

[singinwoodwackr]

Or, only works before noon?

 

[MikeRock]

so the gasket has been on backward all this time....? And what's with the two missing holes on the AM unit?

 

[cookies]

Should look like this, i'd send that kit back and leave a review to warn others

 

[singinwoodwackr]

so the gasket has been on backward all this time....? And what's with the two missing holes on the AM unit?

No, not backwards...wrong parts.
Look at the pics

 

[MikeRock]

I see it......little slow this time of night.. Thanks for an interesting thread. I have a pile of ethanol destroyed or damaged carbs to replace, a few 041's and a dozen 031's and 032's. The three 075's never saw ethanol, thank goodness.
Good night