can't get hi RPMs down to spec.--air leak?

[Megunticook]

I'm tuning a 2004 Husqvarna 55 Rancher. Just rebuilt the carb and am adjusting the mixture. When adjusting the H screw I found I could not get the RPMs to drop down below 13,000 (spec. is 12,500). At one point I had the H screw backed out 4 complete turns but the RPMs were still over 13,000. The L adjustment seemed to work fine.

So am I looking at an air leak somewhere?

If that's the case, how do I go about this?

Thanks.

 

[Justin Taylor]

I'm tuning a 2004 Husqvarna 55 Rancher. Just rebuilt the carb and am adjusting the mixture. When adjusting the H screw I found I could not get the RPMs to drop down below 13,000 (spec. is 12,500). At one point I had the H screw backed out 4 complete turns but the RPMs were still over 13,000. The L adjustment seemed to work fine.

So am I looking at an air leak somewhere?

If that's the case, how do I go about this?

Thanks.

Pressure and vac test @Spoon Carving With Tom knows stuff about that but have you tried tuning out the low screw 1/2 turn

 

[Megunticook]

OK, thanks, I ordered that fitting from Husqvarna that threads into the spark plug hold and lets you apply pressure or vacuum.

As far as the L screw, I started off by adjusting that to get the best RPMs but a little bit on the rich side. If I back it out more RPMs drop and it will start to stumble.

I'm scratching my head as to why backing out the H screw is having any effect. Seems like it should be smoking like crazy and burbling if it's backed out 4 whole turns.

Seems like it would have to be an air leak somewhere, or else the carb. is not able to keep up with the fuel demand? I just rebuilt the carb though and the fuel filter and fuel line are brand new.

This saw was so dependable for so many years, now it's just a complete PITA. Wish I could get it back to where it was.

 

[Slackjaw]

Can't help your saw issue but I like your screen name. Been 35+ years since I've heard that name. Mooselook....

 

[Megunticook]

Grew up in Maine, huh? There's a few native names here I STILL can't pronounce...

 

[Deleted member 117362]

Grab a can of carb cleaner and spray around each bearing side, around base gasket, by carb block, but be careful not to spray into intake or by filter. If saw stops you have an air leak. You can also make that tool from an old spark plug and piece of tube. That model is easy to block off, gasket of duct tape behind muffler and duct tape between carb and block (make sure to cover inner impulse line.

 

[DaddyFlip]

Did you leave off any gaskets during install?

 

[Deleted member 117362]

Did you leave off any gaskets during install?

On muffler, put duct tape covering entire exhaust port or make a solid gasket, use muffler and gasket on duct tape and bolt it down. Intake side, cover intake, install carb and bolt down. Pull vacuum from spark plug. Other models must be done differently, reason for blocking plates and gaskets I made.

 

[DaddyFlip]

On muffler, put duct tape covering entire exhaust port or make a solid gasket, use muffler and gasket on duct tape and bolt it down. Intake side, cover intake, install carb and bolt down. Pull vacuum from spark plug. Other models must be done differently, reason for blocking plates and gaskets I made.

Sorry, I was asking the OP to check all his gaskets were in place. I should'a quoted.

 

[Deleted member 117362]

My bad.

 

[Megunticook]

Gaskets are installed. Is the muffler gasket one-time use only? I removed the muffler to inspect the cylinder, then replaced the muffler using the same gasket which looked fine (was new 2 years ago when I installed an aftermarket muffler). I didn't happen to have any new muffler gaskets on hand, but since then I've ordered some. Should I replace that? Could that be causing an air leak?

Waiting on the tool that threads into the spark plug hole so I can apply some pressure/vacuum. Sounds like it should be arriving this coming week.

 

[Huskitoter]

No problem using an old muffler gasket that is in good shape.

You could try cleaning the carb again, making sure to remove the mixture screws and blowing some cleaner in there. Have you tried adjusting the lever a tad higher? Just throwing out ideas.

 

[DaddyFlip]

No that's not it. Reuse that gasket unless you have oil blowing out over everything. Most muffs are gutted anyway not giving any backpressure.

Maybe pull everything apart and start over is usually how I roll, even on motorcycle. I usually figure out my error that way.

 

[Old2stroke]

You could just be looking at a restriction in the fuel supply. Is your fuel filter free flowing and does the fuel line fill with fuel with no air bubbles (crack or pin hole in fuel line)? Possibility that the metering lever is set too low? Which fuel pump diaphragm did you use? The acetate (stiff plastic) one will pump the least amount of fuel, the black rubbery one the most and you might have to use it. If the saw was running well before you started working on it, it's not likely to have suddenly developed an air leak.

 

[Megunticook]

I changed the fuel line and filter so doubt that's the issue. I checked the metering lever with a straightedge, referring to a Zama carb. guide I found online, seemed like I had it right but I suppose I could re-check that. The carb. kit I bought came with a stiff black plastic diaphragm.

It does kind of seem like maybe rather than an airleak there's a fuel delivery issue at high speeds. Otherwise you would think that with the H screw backed out 4 full turns it would be smoking like a chimney, even if it had an air leak.

Well, I guess I may as well do a pressure-vac. test just to be absolutely sure, then proceed from there.

 

[1hander]

that would be a massive air leak with the H needle ou that far and not slowing down rpm...and rollin coal LOL
any chance the fuel filter pickup is the kind that comes apart and you can check the inside..
i vote fuel starvation issue

1. i have a stihl 08s that the filter looked good but i could not see the inside had a huge fine sediment wood plug built up in it.

2.sometimes when you get a rebuild kit you gotta pay really close attention to the length of the replacement needle..sometimes the replacement metering needle will be longer and sit taller in the needle housing...the actuating lever will not open it enough to feed enough fuel for high rpms ...so you have to set the actuating lever higher to get the metering needle to open enough.

3. that stiff black diaphragm sounds very culprit...if that diaphragm doesnt have enough range of motion or is too hard... the metering lever will not have sufficient up and down action to get in a sufficient amount of fuel for high rpm.

4. when buying a carb rebuild always compare the parts . alot of times i find that unless i buy a genuine tillotson or walbro kit, the metering needle, the actuating lever and the actuating lever spring will different (i still compare the parts though) and that puts you in a position where you basically have to guess on the lever height, and then open the carb and readjust maybe several times to get it right.

3 and 4 have my interest peaked

 

[rupedoggy]

I will say if your saw idles and does not race or die it probably does not have an air leak. Just because you rebuilt the carb does not mean you did it correctly. Mike

 

[Northerner]

Check for leaks around the cheap rubber intake boot that doesn’t have clamps. Just pressure tested a 55 today, wouldn’t even hold 1 psi, leaking so bad on the rubber/plastic side.
Very poor design.

 

[drf255]

Another vote here for no air leak.

Air leaks generally manifest with bad idle/off idle issues. At WOT the path of least resistance is in the wide open carb bore. It’s unlikely to be pulling air from a small leak also.

Gotta check entire fuel delivery system. Tank vent, the filter and lines. The carb-make sure you did it correctly. Check metering height, gasket install vs. diaphragm.

Did you use an oem kit on the carb?

 

[rupedoggy]

One more thing to check on the 55 rancher. There is a little prick that sticks out in the plastic piece nearest the cylinder. This goes into a hole in the cylinder. There is a little piece of rubber in there and it must be aligned perfectly and the rubber in good shape and not distorted. This is the pulse line hole and if it leaks a little it could still idle but run out of signal to pump the larger amount of fuel at high speed.
I have seen this piece broken off and cracked also. The fuel line goes under this plastic piece but has to aligned with a space at the bottom or it could pinch it and restrict flow. Throttle rod goes under thr front piece. Mike