Carb problem

[jetsam]

Short version: How do I turn that screw on the right? Difficulty: I have to be able to turn it with the saw fully assembled so I can tune the carb. Right now I can only see the damn thing because the starter assembly is off. You are looking at a flat round piece of steel, not a plastic limiter cap.



Long version:

My daily driver saw is a Dolmar 6400 with a stock Zama carb. Use it the most, like it the best. (Someday when bad things happen to the piston or cylinder it will become a Dolmar 7900.)

I lost several bolts out of the saw and the muffler was fixing to fall off, so I pulled the muffler and dealt with that. While the muffler was off, I thought it would be a shame if nobody drilled a couple strategic holes in it, so I did that.

Now I want to tune the carb as the airflow situation has changed. I got my trusty limiter-cap-pullin' drywall screw out and *clink* hit something metal. Hmm.

Can't see a thing, so I took the side of the saw off to get the rubber carb adjuster cover thing out.

Hmm, it looks like I am missing a part?



Or maybe the bastards did that to me on purpose.... argh



The part shown above is bad enough, mine is even worse and has a completely round smooth head.


I thought of reassembling it without the adjustment cover and tuning it, but that stupid screw feels completely smooth and round. I can grab it with hemostats but I can't turn it a whisker.

Even if I pull the carb it doesn't look like I can get a dremel wheel in there to notch it.

So... drill it out and tap it for a tiny screw? I worry about this because I think the drilling or the tapping will turn it hard enough to damage the carb, and I have no way to hold it while I drill.

Rip the top of the saw apart, pull the carb, and try to cut a tiny notch in it... maybe with a burr point dremel?

I know two or three of y'all have a better idea than that!

 

[huskihl]

You need a D shaped tuning screwdriver.

This is a pretty solid kit it looks like and will cover most small engines

https://www.ebay.com/itm/184706908575

 

[gurwald]

Looks like the high jet screw adjustment knob/screw head is gone?

 

[jetsam]

You need a D shaped tuning screwdriver.

This is a pretty solid kit it looks like and will cover most small engines

https://www.ebay.com/itm/184706908575

The parts diagram shows a D- or rectangular shaped head on the H screw, but that ain't what is actually there. It is smooth and round.

I gave it some gentle yanks in case it was a cap, but I don't think it is.

 

[jetsam]

Looks like the high jet screw adjustment knob/screw head is gone?

Agreed, but how would a seperate adjustment head even attach to the shaft? It is round without splines on the side or any indentations on the face. Unless they epoxy them on or something? That sounds like a bad idea...

You might also think that maybe the screw is broken off, but the face of it is smooth and machined. A sheared fastener is going to have some rough stuff somewhere on the shear plane.

My current theory is that this is a stupid new style of screw-you adjustment screw. They probably have a special thin wall nut driver with a slightly crooked shaft, or splines inside, meant to turn it at the factory.

I think at this point I will cut a notch in it. I also ordered some carb adjustment tools as huskihl suggested, I'll use them elsewhere if they don't work out for this (I have too many small engines).

 

[SOS Ridgerider]

Even if it looks round, I bet a Single D driver will take care of it, like @huskihl said. That’s the only kind I’ve ever seen on these. I’ve been into a few of them, and Kevin has been into many.
Worst case, if it’s hard to get a hold on it with a D for some reason, use a pair of pliers to unscrew it, then slot the head with a Dremel and a wheel.

 

[jetsam]

Even if it looks round, I bet a Single D driver will take care of it, like @huskihl said. That’s the only kind I’ve ever seen on these. I’ve been into a few of them, and Kevin has been into many.
Worst case, if it’s hard to get a hold on it with a D for some reason, use a pair of pliers to unscrew it, then slot the head with a Dremel and a wheel.

Ahh, can I just back it out and remove it? I don't know how the stop mechanism to limit adjustment range works, and I don't want to turn this into a carb teardown if I can avoid it.

I'm imagining broken little pieces of the stop wandering merrily around the inside of the carb...

I did order some thin wall D- and spline drivers too. (Also a promising looking "pacman" driver that is round with a tiny sharp wedge inside.... that might be good also.)

 

[huskihl]

Ahh, can I just back it out and remove it? I don't know how the stop mechanism to limit adjustment range works, and I don't want to turn this into a carb teardown if I can avoid it.

I'm imagining broken little pieces of the stop wandering merrily around the inside of the carb...

I did order some thin wall D- and spline drivers too. (Also a promising looking "pacman" driver that is round with a tiny sharp wedge inside.... that might be good also.)

There are no limiters on them. The limiting factor is the screw head.

Once you get the tool you’ll realize how easy they are to adjust while running. I’ve slotted the screws in a couple 7900s before and you can’t keep the screwdriver in the slot when the saw is running. The proper driver is a much better idea

 

[jetsam]

And the completely round shaft is a normal thing? I see two styles of that adjustment screw in the parts diagrams (there's a pre-2012 version that is slotted with a plastic limiter cap (looks like the same screw they use for the L adjustment), and a post-2012 version that is either rectangular or D-shaped.

 

[huskihl]

And the completely round shaft is a normal thing? I see two styles of that adjustment screw in the parts diagrams (there's a pre-2012 version that is slotted with a plastic limiter cap (looks like the same screw they use for the L adjustment), and a post-2012 version that is either rectangular or D-shaped.

I suppose it’s possible that yours was an early version and the head is snapped off or something similar. I’ve actually never taken the jets out of a newer one to see what the head actually looked like. I do remember looking in there and not being to identify a shape in the head so I went through my selection of drivers in the kit and found one that would turn the screw. I didn’t pay attention to which driver it was as the white identification ink on the handles has long since worn off. I found one that worked and engraved “7900” on the handle

 

[legdelimber]

Any chance they've slipped in a press-fit-fixed-jet ?

To use the pressed in jet, the hole where a screw would normally fit is not threaded at all.
They just press in that smooth plug like thing with a hole drilled in the small end (down inside the carb body) and a cross drilled hole to create a metered ( by the fixed diameter hole) flow.
Seems like I recall seeing them on a few mower carbs and a couple of string trimmers a few years ago.

 

[huskihl]

Any chance they've slipped in a press-fit-fixed-jet ?

To use the pressed in jet, the hole where a screw would normally fit is not threaded at all.
They just press in that smooth plug like thing with a hole drilled in the small end (down inside the carb body) and a cross drilled hole to create a metered ( by the fixed diameter hole) flow.
Seems like I recall seeing them on a few mower carbs and a couple of string trimmers a few years ago.

Not on any 7900 that I’ve seen

 

[jetsam]

It does rotate. My fancy tools get here tomorrow, but I have wood to cut today.



Found a galvanized spike bigger than the peg in the carb, drilled the same size hole in it, then mashed it up until it grabbed.

It will work until tomorrow!

I feel a little dumb for even making the thread now, but I just couldn't believe that they would design an adjustment screw, which is there for adjusting, to be a plain piece of steel rod on the end...

 

[legdelimber]

Nothing wrong with learning something or refreshing our minds!

 

[jetsam]

Well, it turned out okay. Now I just gotta pull the recoil back off and put the rubber carb adjuster cover thing back.



(And look how CLEAN the saw got while it was on the bench! That top cover with the air pre-filter went through the ultrasonic cleaner even.)