Dolmar PS6400 - PS7900 ( PS-6400 - PS-7900 )

[wde_1978]

I could be wrong but I suspect you are going to find that the RPM on the 6400 is quite a bit less than you expect. Mine ran great before tuning and cut just fine. I was surprised to find that WOT was only about 11000 when not in wood. I couldn't leave that much RPM laying on the table so I had to mess with the saw. Without the tach I would have been just fine and happy with the factory settings.

Was actually surprised that it is about right!

Got the tachometer today , it's a cheap (possibly over payed) china product!
Even though the manual says to wrap the wire 3 times around the spark plug lead , it actually seems to work by just being leaned against the hood above the spark plug.
For apparent reasons , taking pictures , I opted to use the wrap around method!


Being impatient and curious I had to go play with my saws , at least a little bit.
Firstly I checked the rpm's on my fully stock , factory tuned Dolmar PS-6400:

Idle ~3300 rpm seemed a bit much so I turned the "S" screw about 1/3 counterclockwise and the PS6400 settled at a little below 3000.
WOT looks nice enough so I left it as is , especially considering that I really like how this beauty cuts wood set up like this!

Then I turned to my PS7900 , bare in mind that she is way out of tune.
Idle rpm was around 3000 rpm and WOT was meager 10900 rpm.
Considering I did a muffler mod on Monday (more about it in one of my next posts) I first re-tuned the "L" needle (had to go counterclockwise) which caused a rise in idle rpm.
After tuning idle I went for WOT - had to turn the "H" needle in (clockwise) quite a lot and called it a day when I reached about 13k.
Final results without actually cutting any wood are below , I think they are OK (idle / WOT).


Couldn't test rpm's under load as I have nothing to cut in nor around my yard , but as soon as I get an opportunity I will test that as well!

A tachometer like this is an interesting toy to have , I am already considering buying another one and permanently instal one on each saw - just for fun.

Now , for all of You to take a laugh at me if You want to - testing rpm's on my saws I encountered some unforeseen issue!
Checking the rpm's on my PS6400 went great , wrapped the tach's cable around the spark plug lead 3 or 4 times , put the hood back on and started her up. Let her warm a bit using the opportunity to tune idle down , then checked WOT - all fine.
Then I obviously turned to my PS7900 , set the tach up and gave her a start:
- choke+half throttle , third pull she sounds off
- pushed the choke in , automatic half throttle still on , she turns over blowing thick white smoke on the 2nd pull
- I give the throttle a tick to release automatic half throttle , she dies
- tried starting her up for about 5-10 minutes , got 2-3 sounds out of her but she didn't wanna run
- unscrewed the spark plug twice , thought she might be flooded , spark plug wasn't dry , but then it wasn't dripping wet either , checked whether there is a spark , yes there is
- @ that point I already got a little mad/furious , started to wonder what might be wrong and most things seemed highly unlikely considering the saw worked fine on Monday testing my muffler mod

Has any of You ever seen a light bulb glowing atop of Your head like it's being portrayed in cartoons or really stupid comedies?
Well , I did today when I thought "what if there is no fuel in the tank" , and guess what there wasn't!
750mL of fuel poured into the tank and the saw worked just fine!

Who knew a chain saw needs occasionally being re-fueled?! LOL

 

[Andyshine77]

All the 7900's I've owned liked to be tuned right around the limiter. Taking into consideration the saw will require more fuel when properly warmed up from cutting, I'd say you're in the ballpark.

 

[shootingarts]

Does sound like things are in the right neighborhood now. Funny thing about that gasoline. I bought a bunch of stuff, a half of a woodworking shop, and in my half was one of the little plastic saws. I forget that saw only holds about two teaspoons of fuel and curse it fairly often when just a little go juice solves the problem! I doubt anyone has not ran a saw out of gas without realizing it when working. Bouncing between saws gets me too.

I bought the trail tach. Like you I find it fun to play with and was tempted to go with permanent installs. My 6401 is often faster in the cut than I would expect it to be, don't remember just where now though, been awhile since I have played with the tach. Got a half dozen or so trees to take down, not really sure of the count. No monsters but I should get the saws warmed up. Especially on a little hickory that was hit by lightning or blight over the summer. A nice healthy tree that I see the bark splitting on now. Should be cured on the stem and may throw sparks when I try to cut it down!

Hu

Was actually surprised that it is about right!

Got the tachometer today , it's a cheap (possibly over payed) china product!
Even though the manual says to wrap the wire 3 times around the spark plug lead , it actually seems to work by just being leaned against the hood above the spark plug.
For apparent reasons , taking pictures , I opted to use the wrap around method!
View attachment 383821

Being impatient and curious I had to go play with my saws , at least a little bit.
Firstly I checked the rpm's on my fully stock , factory tuned Dolmar PS-6400:
View attachment 383822 View attachment 383823
Idle ~3300 rpm seemed a bit much so I turned the "S" screw about 1/3 counterclockwise and the PS6400 settled at a little below 3000.
WOT looks nice enough so I left it as is , especially considering that I really like how this beauty cuts wood set up like this!

Then I turned to my PS7900 , bare in mind that she is way out of tune.
Idle rpm was around 3000 rpm and WOT was meager 10900 rpm.
Considering I did a muffler mod on Monday (more about it in one of my next posts) I first re-tuned the "L" needle (had to go counterclockwise) which caused a rise in idle rpm.
After tuning idle I went for WOT - had to turn the "H" needle in (clockwise) quite a lot and called it a day when I reached about 13k.
Final results without actually cutting any wood are below , I think they are OK (idle / WOT).
View attachment 383824 View attachment 383825

Couldn't test rpm's under load as I have nothing to cut in nor around my yard , but as soon as I get an opportunity I will test that as well!

A tachometer like this is an interesting toy to have , I am already considering buying another one and permanently instal one on each saw - just for fun.

Now , for all of You to take a laugh at me if You want to - testing rpm's on my saws I encountered some unforeseen issue!
Checking the rpm's on my PS6400 went great , wrapped the tach's cable around the spark plug lead 3 or 4 times , put the hood back on and started her up. Let her warm a bit using the opportunity to tune idle down , then checked WOT - all fine.
Then I obviously turned to my PS7900 , set the tach up and gave her a start:
- choke+half throttle , third pull she sounds off
- pushed the choke in , automatic half throttle still on , she turns over blowing thick white smoke on the 2nd pull
- I give the throttle a tick to release automatic half throttle , she dies
- tried starting her up for about 5-10 minutes , got 2-3 sounds out of her but she didn't wanna run
- unscrewed the spark plug twice , thought she might be flooded , spark plug wasn't dry , but then it wasn't dripping wet either , checked whether there is a spark , yes there is
- @ that point I already got a little mad/furious , started to wonder what might be wrong and most things seemed highly unlikely considering the saw worked fine on Monday testing my muffler mod

Has any of You ever seen a light bulb glowing atop of Your head like it's being portrayed in cartoons or really stupid comedies?
Well , I did today when I thought "what if there is no fuel in the tank" , and guess what there wasn't!
750mL of fuel poured into the tank and the saw worked just fine!

Who knew a chain saw needs occasionally being re-fueled?! LOL

 

[cgraham1]

All the 7900's I've owned liked to be tuned right around the limiter. Taking into consideration the saw will require more fuel when properly warmed up from cutting, I'd say you're in the ballpark.

I recall putting a tach on mine and leaning it out until the tach went blank... I believe it ran right around 13,100 WOT, and ran good!

 

[Andyshine77]

I recall putting a tach on mine and leaning it out until the tach went blank... I believe it ran right around 13,100 WOT, and ran good!

That's what I found. However I have also found each saw/coil kicks in at a different rpm, some are right at 13,500 some like yours are limited at a lower rpm. This likely has to do with the manufacturing date, as the newest coils all seem to be limited to a lower rpm.

 

[7sleeper]

Here's are pic's (from a german thread) on how to not permanently mount a tach to your saw. This is one of those metal handle holders for rakes, etc you find very cheap in box stores. I did it even simpler and put a piece of foam between the tach and the holder and used zip ties through the holes of both to permanently fix both together. That way the vibration isn't transfered to the tach too much and you can read much easier.

@wde_1978

You are doing everything just fine! I also usually only mix 1 litre at once (but at about 25ml/litre full synthetic two stroke oil, also with a syringe), if I have a larger project I dump the mix into a larger container and remix again. That way I only use the freshest fuel mix. Although I go through about 15-20 L/year. All my two stroke equipment gets the same mix. I try to dump the old mix out of the equipment when not in use for longer periods. And them I pull the starter a few times, with turned off ignition, to get most of the fuel out of the fuel/carb system.

And yes about everyone I know who is honest admits to forgetting to refuel once in a while and relearns the age old lession! And let us not forget the chain on the wrong way around moments.... never happened to me....

What I find more important is that the fuel container should be certified for fuel! If you look on the bottom of the fuel container it should be marked HDPE. That is one of the very few plastics certified for fuel. For the small amounts I like using the 1 litre oil containers you can get at gas stations/car repair shops. I usually just go to their garbage in the garage and take out how many I want or whatever form I like.

Further fuel storage is important. No sunshine, ambient temperatures, shade, etc. That way everything should be fine.

Keep up your nice threads!

7

 

[wde_1978]

... how to not permanently mount a tach to your saw....

OK , that's somewhat wrong!
How is the operator PROPERLY holding on to the upper handle with that tach being mounted there?
If I end up permanently mounting a tachometer I'll try to mount it somewhere directly on either the cylinder or air filter hood.

...I also usually only mix 1 litre at once (but at about 25ml/litre full synthetic two stroke oil, also with a syringe)...

That's a 40:1 mix.
Should I decide to increase the oil ratio on my saws fuel , I'll go for 40:1 too , but for now I'll stick with 50:1 as it served me well.
In case I run out of my preferred Dolmar oil before I manage to get more , I would go 40:1 with any other oil just out of precaution.
I use an empty 1 Liter Shell oil container , I found it to have a perfect pouring shape and a nice see through window on the front.


Thanks for Your reply 7 !

Grüße nach Östereich

 

[wde_1978]

My mild Dolmar PS-7900 muffler mod - part 1

Last Monday I decided to mod my Dolmar PS7900's muffler!
FYI , my Dolmar PS6400's muffler will stay stock , at least for now.
That way I also have the opportunity to try stock/modded forth and back if I want to.

After taking the muffler off the saw I tried how to best un-crimp it and found that a side cutter works best.
I did not heat the crimp (fold) thus I bent back little by little.
As I am a lefty I worked my way clockwise around the muffler starting from the top where the only open spot was.
I folded the crimp back as less as necessary , about 90° , to reduce material weakening.


There isn't much inside this muffler , just a simple deflector.
Completely removing the deflector seemed like overkill , and I didn't like the thought of the exhaust gasses directly hitting the front of the muffler - that might get quite hot.
Instead I drilled the existing 5mm holes open to 7.5mm holes (50% larger diameter/radius) , I drilled 18 additional 4mm holes to the 4 slanted corner sides , and I slightly opened the deflectors mouth with a tungsten-carbide burr.
Estimated increase of breathing area (surface) of the deflector should be around the 125% range (!this has been edited after Hu pointed out my mistake in estimating the increase of surface , thanks Hu!).
Pictures below , stock followed by modded:




TO BE CONTINUED...

 

[wde_1978]

My mild Dolmar PS-7900 muffler mod - part 2

That been done I turned to the mufflers exhaust hole , it was rather small measuring about 14mm.
Considering that I wanted the muffler to stay slick I decided to just widen the existing hole as much as possible , while retaining the possibility to use the stock spark arrestor screen.
Again I used the previously mentioned burr and a drill.
My estimation is that I opened the hole at least 50% in diameter/radius resulting in about 125% more surface (!this has been edited after Hu pointed out my mistake in estimating the increase of surface , thanks Hu!) !
Pictures show stock/modded from the outside , and stock/modded from within:



That been done too , I cleaned the muffler thoroughly of shavings , put the two halves back together and hammered the crimp (fold) back up - not having bent it back too much helped a lot while re-crimping the fold.
There are some scratches all over the muffler , but I believe the result is satisfying:


I assembled the muffler back on my Dolmar PS7900 and did what anybody in my position would do , started her up ...
HOLY COW that was loud !!!
Bare in mind that I was inside a closed room and didn't go outside as it was raining , but then again I fired up my saws in closed rooms before and never got startled like that!

The saw is subjectively A LOT louder , but interestingly enough when measured with a decibel meter it is ONLY about 2-3dB louder then stock.
As I have a stock muffler at hand I might make a more detailed comparison at some point .
Pictures show stock/modded idle , stock/moded WOT:
- -

The measuring device is a Voltcraft SL-50 , pictures aren't the best as I had to hold the saw , the dB-meter and my phone.
The dB range at WOT might actually be louder now as I yesterday tuned the PS7900's WOT from 10900rpm up to 12900rpm.

That's my mild muffler mod , thanks for reading!

EDIT:
My sincere apologies regarding my estimation mistakes and thanks to Hu for pointing them out.
After correcting my estimation mistakes , this mod seems rather mayor after all.
I did this mod just for the fun of it and I am suspicious whether it makes any difference in performance of the saw or just causes more noise!
If desired the mufflers exhaust exit hole could easily be reduced by simply making a piece of sheet with a smaller hole that would go under the screen together with the spark arrestor.

Again , sorry for my writeup mistakes!
Thanks

 

[wde_1978]

I have been looking up the meaning of the term 4-stroking , thus far I found this:

Wikipedia - Four-stroking

ArboristSite - The Mechanics of Four-Stroking

ArboristSite - What exactly is 4 stoking??

ArboristSite - Please explain four stroking to me in layman's terms

Thought to post this here for safekeeping as I might want to re-read these!

 

[shootingarts]

A couple of things working from memory just "for your information" type stuff. Decibels are funny, subjectively I think the sound doubles every three decibels so it doesn't take many more decibels to get loud in a hurry. I grabbed my saw for a quick cut and just used a pair of wrap safety glasses without grabbing my hardhat and ear protection that were inside my house. Yikes, that saw is pretty loud before muffler mods!

The other thing is about holes. pi times the radius squared is hole area, the thing that counts. If my quick math is correct you opened the round holes up to over twice the area they were, over 100% increase in size. My finely calibrated eyeball measures the outlet at well over twice the original area too plus I think that reverse beveling was to deliberately create some turbulence and perhaps reduce flow and noise a bit. Not sure about either of those things being accurate though.

I learned long ago about holes, pipes, round things in general. A little bigger diameter goes a long ways towards increasing flow. Ran 600 feet of water pipe recently and it was amazing how much better flow each slightly bigger pipe diameter gained.

Hu

 

[wde_1978]

Yes Hu , I expressed myself wrong , i thought about diameter/radius of the existing holes but wrote surface!
My bad everyone , sorry for the confusion!

Hu is right with his surface estimation , I did a quick surface calculation and the deflector is now about 125% more open then before!
I didn't bother measuring the exhaust hole , but it should also be bigger about that amount of surface area.
That been said it is actually a mayor muffler mod , but still without fully eliminating the deflector and sticking to one exhaust hole.

And , yes Hu , to my knowledge the deflector is meant to slow down the expanding exhaust gasses causing reduced exit velocity and thus causing less noise.

Unfortunately I can "Like" Your post only once , does AS have other reputation giving options?

Going to fix those estimation mistakes now , thank You Hu!

 

[wde_1978]

More "keep 'em safe" threads , great reading :

Mastermind meets the Dolmar 7900

Mastermind meets the Dolmar 7910

also

MillerModSaws and the PS-7910
Excellent threads!

 

[shootingarts]

More "keep 'em safe" threads , great reading :

Mastermind meets the Dolmar 7900

Mastermind meets the Dolmar 7910

I am not entirely through those two yet , excellent threads!

Thank you for the links! I have the 7900 thread marked, don't think I have marked or read the 7910 thread. Anytime Randy meets any saw it is well worth the read, when he meets the saw you are using, always a lot to learn!

A chuckle about measuring, when I was quite small living on a farm we sometimes had to share a treat. One thing was an ice cream novelty. It has many names but is a cone shaped ice cream cone. Round and evenly getting bigger from the point on bottom to the round top. Even at three or four I knew that I was getting the short end of the stick and started a fuss so that my older brother didn't get quite all of the first half. It wasn't until many years later that I learned that if you draw a line across a cone half way from top to bottom, the volume of the top of the cone will be eight or nine times the volume of the bottom. I'm owed a lot of ice cream!

Hu

 

[wde_1978]

Thanks to @computeruser , who sent me a pair , one of my saws got the big 3-point USA style 1 spikes.
I opted to mount them on my Dolmar PS-7900 as I intend to fell and buck 2 pear trees this weekend (a third one already went down last Saturday).

Some pictures of the saw with her new teeth , European inner spike included for comparison in the first picture for those who still don't grasp the ridiculousness of the Europe-style spikes size:





NOTE: The tach mount is temporary as I wanted to see what rpm she is holding in the cut.
She holds something between 9500 and 9700 rpm dogged in heavy in fresh pear while bucking , ~3/4 the length of the 20" bar.
My PS7900 4-strokes quickly as soon she isn't under full load , as such I might try to lean her out a little more next time I use her.
The tach mounted at that spot is acceptable while bucking , but it interferes with work annoyingly when felling and limbing.
I intend to mount the tach atop the air filter hood , there it should be pretty much out of the way.

The one pear tree that went down , and the other two that will follow:

 

[wde_1978]

Got to work with my PS-7900 today again , felled and cut up a big pear tree.

As my PS-7900 didn't sound and feel right last weekend I leaned her out a little more and she currently runs WOT@13200rpm (sorry no pictures).
The tune made quite a difference!
Wearing a fresh filed aggressive chain (rackers are too low , it's the chain that came with the saw when I bought her) she holds impressive 10500rpm in the cut while self-feeding (very low to no forced pressure).
While dogging her through a cut she holds about 9750 to 10000rpm.
Again , sorry no pictures as it's kinda difficult to take pictures while bucking at knee level.

In addition to WOT I raised idle to ~3050 rpm as she was idling a little uneven at ~2700rpm.

I like how my PS-7900 sounded and cut today , I think I might have hit the sweet spot!




NOTE:
Those USA style felling/bucking spikes (Type 1) are awesome!
Due to their teeth length the PH no longer come in contact with the trees bark.
The spikes overall height and the fact that there is now a second spike row on the clutch cover improves grip and cutting stability.
I can't thank @computeruser enough for sending me these - thanks B. !

My PS-6400 will be granted bigger spikes as well , custom made ones are in the works!

Dolmar should be ashamed to sell such awesome saws with these ridiculous European-style spikes , and on top of that providing only the inner spike and stating that the outer one is accessory!

 

[wde_1978]

Does anybody know for sure what the part number for the new air filter/carb cover with the mesh is?
I mean this one:


Nate , any insights?

EDIT:
I gathered these part numbers , but am not 100% sure about the part descriptions:
- 038 118 022 - hood red
- 038 118 024 - hood matte black
- 038 118 221 - hood glossy black , should feature the pre-filter mesh

Any confirmation or corrections are welcome!

 

[Poleman]

I would go with the HD filter....better performance and a 10+ for looks!!!
And Nates the "MAN" on this!!!!!

 

[wde_1978]

I would go with the HD filter....better performance and a 10+ for looks!!!
And Nates the "MAN" on this!!!!!

I agree on the looks of a PS-6400/7900 with a HD filter kit!
But for my purposes the kit is not needed as I don't have issues with wood dust passing the flaked stock filter as I don't cut dry hard wood at all.
And honestly I find the HD filter kit to be priced way too high.

I am eyeballing the standard hood in matte black , should look nice as well.

 

[computeruser]

I think you will want to double-check things - I believe that the matte black hood set only works with the newer cylinders with the decomp moved to the right side of the saw. At least that's what the 6400c project saw in my basement had going on; an earlier 7900 P/C swap required the older-style covers (I had some, in red, waiting around for this) and a new inner flywheel shroud with a provision for the left-side decomp.

The shiny black top covers and, I believe, the HD setup, will work with the earlier cylinder where the decomp is on the left. My preference is for the early red-top covers, but I appear to be in the minority on that. Now if they would come out with red HD setup, that's something I could get behind!