Help building,tuning a Strato, Husqvarna 445

[Terry Syd]

Good point on the difference of the static POP (which can be tested) and the dynamic POP while running, they are two different things. As you said, the area of the diaphragm and the area of the needle seat are part of the relationship. So to is the barometric pressure working on the diaphragm. Basically, you have to see what works for the engine. That's why I came up with the testing of the two peaks to see if the metering of the fuel could become more uniform over the working powerband.

On my springs there are four collapsed coils at each end of the spring. The spring tension is produced by the open coils in the center of the spring. Essentially, each time I clip off one of the collapsed coils I am reducing the pre-load on the actual spring section. On this spring, each collapsed coil represents .2mm of pre-load to the spring. I am not shortening the working section of the spring, so the spring tension does not increase.

I tried to measure the static POP of the carb, but my guage only went up to 30psi. I agree the actual figure isn't meaningful in itself, however if I find a static POP that works for my carb, then I have a baseline I can use to work from, or to come back to when replacing a spring.

One of the things I have been doing after each mod to the spring is to lay the saw on each side and see if it continues to idle for about a minute - so far, so good.

Edit: do you agree that my way of testing of peak horsepower and peak torque is a valid way to tune the carb? If not, is there a better way?

 

[drkptt]

You can't say that barometric pressure is acting on the diaphragm because in the static condition ambient pressure is applied to both sides. Barometric pressure, of course, changes the mass flow through the engine and thus the magnitude of the carb signal.

You are correct about cutting off the inactive coils to change the free length of the spring; I didn't consider that.

 

[drkptt]

Edit: do you agree that my way of testing of peak horsepower and peak torque is a valid way to tune the carb? If not, is there a better way?

I've got a steady-state dyno running under speed control, a precision fuel-flow meter, and a 5-gas exhaust analyzer that resolves AFR (both overall and trapped for stratified-charge engines). I can't confidently make observations about carb tuning until I plot the data and study it. It seems to me that it would be extremely difficult to evaluate mixture at peak power and torque without those tools.

Typically I'd set the desired mixture at peak power and sweep through other speeds looking for little change in AFR. A flat response or enriching at lower speeds is desirable--when you dog a saw and slow it down you would not want it going lean. Playing with the size of the airbox inlet holes is the best bang-for-the buck to flatten the AFR/speed response.

 

[Terry Syd]

Yes, I was referring to the dynamic condition of the differential of the pressure on the outside of the diaphragm and the inside of the metering chamber, the static condition would be identical.

Your comments about setting up the POP is similar to the response I got from the representative at Zama. That is, they set the POP so that the saw will operate in various positions. There doesn't seem to be emphasis on setting up the POP for performance. Is that the situation?

I figure if the saw will idle while laying on either side, then I should be good. I can't ever remember using a saw upside down, so I wouldn't use that as part of my testing. Is there a specific reason that the saw is tested upside down?

What we have on this forum are people that are significantly modifying their engines, sometimes even using a different carburetor. They are trying to increase the performance of their engines and proper metering of the fuel would be part of their quest for better performance. In that regard, what is your opinion about the testing method I proposed for testing the two peaks?

 

[Terry Syd]

I missed your post while I was typing. - Already got the air box holes!

I agree that peak torque is where the greatest thermal loading would be and where the mixture should be the richest. However, we want to increase cutting speed also, the saw does the most work when it is at peak horsepower.

Since there is a desire to keep the torque peak rich, on a long flat powerband the engine may become excessively rich before it reaches the full potential of that powerband. Do you agree with the above comment about "the lower the (static) POP is set the richer the fuel curve is at the bottom"?

If so, then that would seem to favor a lower POP to keep the torque curve rich and help keep the top end from becoming excessively rich.

 

[drkptt]

I have never seen the metering chamber pressure used as a tuning tool to maintain constant mixture over a speed range. I once ran a series of speed sweeps with different metering spring weights and observed zero difference in regards to AFR vs speed. I can't comment on your tuning theory; it may work in one application but imo there are too many variables to apply it as a rule to all saw engines.

I won't use the term POP in this discussion because of my previous comments about the relationship of overpressurizing a carburetor to its performance. You might consider building a sealed diaphragm cover so you can apply pressure to the diaphragm and measuring when the metering valve opens. This data correlates better to the real operation of a carburetor and would make it easier to extend anything learned to other carburetors and engines.

 

[Terry Syd]

I've continued to use the term POP as it has become a 'term of art' of those who are are tuning their carbs in karting, aero and saw communities. There seems to be a fairly large body of research in those communities of using the POP to tune their carbs for various conditions. For example, the above post (#154) where I extracted various comments from the EC Birt forum, indicates they have established some theories of tuning which they have continued to use successfully.

As far as a testing method, if I built a sealed diaphram testing rig as you proposed, the large area of the diaphram and the leverage ratio of the lever would have the needle lifting off the seat at such low pressures that it may be hard to differentiate between subtle changes to the POP. Perhaps that is one reason that those communities of enthusiasts have choosen the fuel POP method, as the pressures are high enough to observe the subtle changes.

I don't understand your comment "I once ran a series of speed sweeps with different metering spring weights and observed zero difference in regards to AFR vs speed." That statement seems to contradict your previous statement of "A flat response or enriching at lower speeds is desirable--when you dog a saw and slow it down you would not want it going lean." (post #163)

A 'speed sweep' may be a familiar term of art for you, but I don't know what it entails. Perhaps you could expand on what is being done.

Good discussion, thanks for contributing.

 

[drkptt]

I've continued to use the term POP as it has become a 'term of art' of those who are are tuning their carbs in karting, aero and saw communities. There seems to be a fairly large body of research in those communities of using the POP to tune their carbs for various conditions. For example, the above post (#154) where I extracted various comments from the EC Birt forum, indicates they have established some theories of tuning which they have continued to use successfully.

What quantitative measure of AFR have you seen from the body of research of these hobbyist communities?

I don't understand your comment "I once ran a series of speed sweeps with different metering spring weights and observed zero difference in regards to AFR vs speed." That statement seems to contradict your previous statement of "A flat response or enriching at lower speeds is desirable--when you dog a saw and slow it down you would not want it going lean." (post #163)

A 'speed sweep' may be a familiar term of art for you, but I don't know what it entails. Perhaps you could expand on what is being done.

Speed sweep was described earlier as: set mixture at max power, measure mixture over a range of speeds. I repeated this procedure with different metering springs (thus changing 'POP') and there was no significant difference between the runs. For this one engine I observed that 'POP' could not be used to tune the speed response of the carburetor.

 

[Terry Syd]

"What quantitative measure of AFR have you seen from the body of research of these hobbyist communities?"

Obviously things like throttle response and lap times could be indicators, However, the best I could do for objective readings of insturments would be their tuning by EGTs and cylinder head temperature. Some of the kart fellows are monitoring the head temps and looking for certain temps, say at the end of a straight.

"Speed sweep was described earlier as: set mixture at max power, measure mixture over a range of speeds. I repeated this procedure with different metering springs (thus changing 'POP') and there was no significant difference between the runs. For this one engine I observed that 'POP' could not be used to tune the speed response of the carburetor."

Thanks for the clarification. So, what you're saying is that the tension of the metering spring is irrelevant for tuning purposes across different speeds/rates of airflow. OK, what can be used to change the speed response of the carburetor? From your post #163 you indicated you did see some change in AFR across some carbs speed ranges.

 

[tdi-rick]

Tillotson used to include specific pop-off tuning in the service brochures for their karting carburettors.

..........If you find that the engine will not chop off clean or is slow to come back from high rpm's when you lift the throttle, this is a sign that the pop-off pressure in the carb is too high. The cure for this is to lower the pop-off pressure. If you experience loading-up problems, this is a sign that the pop-off pressure may be too low. The cure for this is to raise the pop-off pressure. As a general rule of thumb, the pop-off pressure will be from 8 to 12 lbs, and is a tuning factor that you can adjust for the altitude of your racetrack and the type of racing that you do.

Terry, I have no data on CHT, EGT's and pop-off pressure, data acquisition came in after I 'retired' from karting in the late eighties.

 

[tdi-rick]

Just found this on the Tillo racing site for the current generation HW kart carburettor.

Contents:
1pcs RK-1HW (Repair Kit)
2pcs DG-1HW (Diaphragm & Gasket Kit)
1pc Inlet Tension Spring 24-B323X 26g (10 Pack)
1pc Inlet Tension Spring 24-C296X 31g (10 Pack)
1pc Inlet Tension Spring 24-B299X 37g (10 Pack)
1pc Inlet Tension Spring 24-C298X 42g (10 Pack)
1pc Inlet Tension Spring 24-C297X 46g (10 Pack)
1pc Inlet Tension Spring 24-B345X 48g (10 Pack)
1pc Fuel Pump Diaphragm 237-223X (Mylar) (10 Pack)


Inlet Tension Springs:
The range of inlet tension springs will allow you to test various springs and help achieve a better set-up. The lower gram springs give a lower pop-off pressure hence faster acceleration / mid range.

Fuel Pump Diaphragms:
In cool weather the Mylar Fuel Pump Diaphragms delivers better performance due to the density of the air.

 

[Terry Syd]

I thought I might take one more loop off the metering spring, but it looks like I have the fuel metering where I want it. I did two, back to back cuts in a big log with the saw pulled down low in the torque range. The plug came out showing a slightly rich condition, which is where I want it under those conditions.

I had the 20" bar on the saw and had the cutting angle down around 4.1-4.2 degrees. With the bar buried, the saw clearly wanted more cutting angle, I could load the saw and it was cutting better. It looks like I should be able to run the 8 pin with the 20" bar.

Although I was really pleased with the saw's performance, I admit to a bit of a let down - I can't think of anything else to do to it to raise the performance. After two years of tweaking this thing, I've come to the end of the road.

All that is left to do is to drill out the oiler a bit more (I think a 2mm hole will do it) and mount the 8 pin.

 

[steve316]

more left on the table

there is alway more, but it is usely too costly to go after.Fantastic post, I look forward to reading more on your accompaniments.steve

 

[Terry Syd]

Steve, yep, there's always more left on the table. For example, I could have a pop-up piston made for it, or have it built up with plasma spray, or I could find an unlimited coil and then create some sealed bearings to bump up the base compression. That would allow me raise the cutting speed and also allow for some extra transfer port area to be added. It's all do-able, but there's a limit on cost and access to machine tools, everything I've done so far can be done in a shed with hand tools and some GB Weld.

However, this is neat little wood saw that weighs 10.8lbs and thinks it is a 60cc saw. The modest compression will allow me to continue using 6 month old regular gas in it and it should give me many years of enjoyable cutting with a lightweight saw.

I've got a chainsaw class in a couple of weeks in order to get qualified for our Local Fire Brigade. Hopefully, some other guys will show up with their iron and I'll get a chance to compare the power characteristics with some of the more popular models. - Hmm, maybe I should find a 445 flywheel cover before hand to create a real 'sleeper'.

EDIT: One other mod that I think I forgot to mention was using a Champion RCJ6Y spark plug. The Champion plug is a half range colder than the NGK plug, has a thicker base for the threads and a slightly longer nose. It raises the compression a few psi, especially with a thinner base gasket.

 

[tdi-rick]

[/snip]

I've got a chainsaw class in a couple of weeks in order to get qualified for our Local Fire Brigade. Hopefully, some other guys will show up with their iron and I'll get a chance to compare the power characteristics with some of the more popular models. - Hmm, maybe I should find a 445 flywheel cover before hand to create a real 'sleeper'.

[snip]

I'll take this to the dribble thread Terry, but I missed our course a few months back but pretty sure you'll be using the brigades saws, and FWIW the one in our tanker is an 064.

 

[Terry Syd]

I just got the letter today, and yes, they mentioned bringing the Brigade's saw. I'll have to take another look at it, but I believe it is an early Husky of around 80cc. I have the first theory class tonight. The practical should be this Saturday.

The chainsaw course I did at Richmond TAFE had us cutting up trees for two days. My arms and back were rooted from limbing with a 365 and 24" bar. It made such an impression on me that I'm sure it helped me decide to purchase a 50cc saw and mod it.

 

[Terry Syd]

The other Brigade member and I balked at taking the Husky 181 with the worn out chain to the course, so we took my saw. Everybody else was using Stihl 036s. The little 450 didn't give anything away to those 61.5cc motors. In fact, one of the instructors went 'chainsaw racer' with my saw to show the troops what you could do with a saw. - I've got to get a re-match with my mate's 365.

 

[ausneil 1]

terry syd,
what an impressive thread, i will read again to pick up what i missed.
I am new to this forum and was directed to this thread by tdi-rick who i met in person on the weekend.
I own a 576 piped race saw, and are still learning all the go with x-torque engines.
If i may ask mate, where are you from,
later neil

 

[tdi-rick]

terry syd,
what an impressive thread, i will read again to pick up what i missed.
I am new to this forum and was directed to this thread by tdi-rick who i met in person on the weekend.
I own a 576 piped race saw, and are still learning all the go with x-torque engines.
If i may ask mate, where are you from,
later neil

Fark, I knew it was a bad idea sending you to this thread, I wanted to pick terry's brains on building a strato saw for the u77 class

Terry isn't too far from you mate, I think he uses the same dealer (Gary ?)

 

[ausneil 1]

Fark, I knew it was a bad idea sending you to this thread, I wanted to pick terry's brains on building a strato saw for the u77 class

Terry isn't too far from you mate, I think he uses the same dealer (Gary ?)

haha, i'm no worries to you guys, just a bushman. it was worth reading