Cs490 goes lean when hot

[Nutball]

Any ideas why it is going lean when it warms up? Only 3 quick cuts into a 24" log from cold and it starts to bog at full throttle. Several seconds of idle time helps it run better for a few seconds. I tried loosening the gas cap, but no change. It's been ported if that makes any difference, very low run time and clean. The H screw is one full turn richer than normal. I'll tear into it next week.

 

[Ketchup]

I’ve had a lean overheat on several 2511t. In that case I think the fuel lines overheat and start to vapor lock. I vacuum tested several of them and not one ever lost vacuum. But they didn’t bog. They cut like hell but wouldn’t return to low idle. Blipping the throttle helped them calm down sometimes.

Yours sounds like a true air leak. Or a bad coil.

 

[Nutball]

I had a 355t do what your 2511s do. I need to try running it again someday to see if it still has the problem. Last I ran it it seemed better. Top handles, especially Echos, don't cool well in my not scientifically tested opinion.

I haven't had a 2511t ever fast idle from overheating...or overheat for that matter. But on future builds I'll be sure to focus on improving cooling since I recently saw one I modded wearing a 16" bar. I've read about stock 201t's fast idling from overheating by working too hard.

My John Deere does something similar to this 490, but it goes a step farther and pumps gas out the carb even after it is shut off. All I can think is somehow dirt is in the carb, or the cylinder base or transfer cap has a poor seal when hot. The JD has 518 while the Echo has 1184. Kinda hard to pressure test a hot saw before it cools too much.

 

[Kerfed]

I’ve had a lean overheat on several 2511t. In that case I think the fuel lines overheat and start to vapor lock. I vacuum tested several of them and not one ever lost vacuum. But they didn’t bog. They cut like hell but wouldn’t return to low idle. Blipping the throttle helped them calm down sometimes.

Yours sounds like a true air leak. Or a bad coil.

What did you end up doing for a fix? Work partner’s 2511 does that sporadically. Re-tuned a few times now, runs perfect some days and then won’t others, annoying and unsafe aloft.

 

[huskyboy]

Did it run normal before the portwork?

 

[Ketchup]

On the 2511s, I double insulate the engine compartment to keep the fuel from overheating. In general, I just try not use the saw that hard. Most of the cylinder failures I‘ve seen on those saws seem related to excessively hard use and lack of cleaning. But I’m not convinced the problem is totally solved. I’m considering a tiny heat sink on the metering side of the carb, but haven’t tried it yet.

I agree, it is basically impossible to pressure test a saw before it drops temperature. Sometimes vacuum will indicate leaks that pressure won’t. Also pressure testing to 12 or 14psi can reveal heat leaks sometimes. That has worked for me on some subtle intake leaks (346, 200t).

@huskyboy asks a good question too. I had my ported 192 do a similar WOT bog. Turned out it was free porting very slightly.

I also had a 394 that fell flat at WOT. Turned out the choke butterfly wasn’t opening all the way. That one gave me fits until I found it.

 

[Nutball]

I just found the carb heat shutter in the winter position when i was running it in 60deg weather if that makes a difference, though I did have the same problem last winter, but thought I'd tune it out being from the really cold air.

Free porting where @Ketchup ? Exhaust side free porting under the piston doesn't seem to cause problems.

It holds pressure, but there's a huge vacuum leak. Maybe a bearing seal.

 

[Ketchup]

On the 192 I freeported the intake trying to put windows in the piston. Seal is where I would look.

 

[Nutball]

The vacuum leak is through the exhaust port. I will assume that is not the problem because the piston should be a tighter fit when hot and freshly lubed with oil, and any leak there would suck exhaust which if anything will make it run weaker instead of leaner. Not being used to pressure testing, I easily forget to plug the exhaust port during the test.

I tweaked the metering lever slightly to let more fuel in, I'll see if that helps.

 

[Nutball]

Seems to be running fine now. I was able to make 9 cuts in the 24" log. Ran it up to 380degF with no issues. I'm thinking it was more of a cut time issue, than heat, causing it to run low on gas under the diaphragm.

 

[dahmer]

Seems to be running fine now. I was able to make 9 cuts in the 24" log. Ran it up to 380degF with no issues. I'm thinking it was more of a cut time issue, than heat, causing it to run low on gas under the diaphragm.

Still can’t rule out that winter/summer diaphragm being reversed. Big difference between hot air and HOT AIR.

 

[Nutball]

Still can’t rule out that winter/summer diaphragm being reversed. Big difference between hot air and HOT AIR.

I don't know much about how that feature affects performance.

Does the hot air off the engine make it run richer, leaner, hotter, or cause the gas to boil in the lines?

I think it was the metering lever since it wouldn't respond to the H screw.

 

[smokey7]

@Nutball. Am I understanding this correctly that you pressure and vac tested it while exhaust was uncovered? Also testing like that jt only leaked under vac and not under pressure?

 

[Nutball]

Yes I did at first. It held pressure well with an open exhaust port. With vacuum there was a sudden leak which sometimes remained until sufficient pressure was applied, then it's like a valve popped shut again and it would hold pressure.

My assumption is that oil caused that valve effect. I imagine after a long enough wait pressure would have leaked too.

 

[Piston Skirt]

I don't know much about how that feature affects performance.

Does the hot air off the engine make it run richer, leaner, hotter, or cause the gas to boil in the lines?

I think it was the metering lever since it wouldn't respond to the H screw.

The winter shutter on Echoes works way more effective than on other saws due to reversed flow of g-force.
Once you leave it open it is very effective to prevent carburettor icing.
However it is very important to shut the winter mode above +5C.

If you leave it open, the carburettor body heats up which brings the fuel to boiling temperatures. This in turn creates bubbles in smaller carburettor passages and decreases fuel flow. (This and following is obviously valid for all makes and models)
As it was noticed - blipping the throttle or quick choking may help clearing the bubbles but generally the most important part is to cool down the carburettor body.
If you have troublesome starts on a hot day on a saw or a trimmer even pumping cooler fuel through the carburettor with a bulb helps to cool it down a bit.

Some stuff above about bad thermal insulation of fuel system is pure BS as Yamabiko focuses a lot on fuel/mixture temperatures from the tank to the combustion chamber, sometimes even sacrificing some peak performance for maximum startability.
Some competitors don’t, hence they have notoriously poor hot startability.

 

[Novice4Rent]

Having what seemed like tunability issues with an Echo CS590 which got a MM and LRB popup piston. It worked well enough until warm, made very good power, then needed the choke to fire even when warm. Running an AM 620 carb as well. Suspect vapor lock when cutting the bigger stuff. Other have a similar experience- IE increased compression causes tunability issues when warm?

 

[Ketchup]

Having what seemed like tunability issues with an Echo CS590 which got a MM and LRB popup piston. It worked well enough until warm, made very good power, then needed the choke to fire even when warm. Running an AM 620 carb as well. Suspect vapor lock when cutting the bigger stuff. Other have a similar experience- IE increased compression causes tunability issues when warm?

“Needs choke to start” is either L way too lean or an air leak. Try turning out your L 1/2 to 1-1/2 turns. Check your carb and cylinder bolts. If that doesn’t find some improvement, pressure/vac test.

Vapor lock is also possible. Make sure you’re air box is closed off from the cylinder and check the fuel in the tank. If the fuel is boiling in the tank when you’re having problems you have winterized fuel. Get rid of it.