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

[wde_1978]

Hello everyone !

I am quite new here and my name is Wilhelm.
I have been browsing this great forum and read a lot of useful and interesting posts.

Some might have noticed by my few posts that I am a Dolmar user , not necessarily by choice but definitely by chance.
As such I have specifically been looking up any Dolmar related Threads and spent hours browsing through a fraction of them.

Now, the reason for creating this Thread.
I own both saws mentioned in the title and have some questions and might like some opinions.

The Dolmar PS7900 ( PS-7900 ) I purchased recently , used , supposed to have low hours.
Everything seems to confirm that:
-clutch drum is in mint condition
-the rim sprocket is the factory one (does not have wear indicators like the replacement sprockets have) and it is just slightly worn
-the exhaust port of the muffler has very little discoloration/soot on it
To additionally check for wear I decided to remove the muffler and take a look at the visible part of the piston , ring and cylinder (see pictures) - I would dare to say that they seem to look great.
What do You think?



I bought the Dolmar PS6400 ( PS-6400 ) new back in 2006. , served me well without any issues or hick-ups and hopefully will do so for many more Years.
Point is , after inspecting the PS7900 I got curious and removed the muffler on my PS6400 too - shouldn't have done that!
Upon removal of the muffler I immediately notice 2 significant score marks on the piston as it was in the up-position.
Slowly pulling the starter line I moved it half way down and damn it there are score marks on the piston ring as well.
I do have to say:
-I mix my own fuel (EuroSuper95 and Dolmar 2 stroke oil @ 50:1 - per manual instructions)
-the mufflers exhaust opening is always showing a slight amount of oil (but it is not dripping)
-I never used no-name oil on my saw
-I never straight gassed it
-other than me , only my dad used the saw 2 or 3 times (and I didn't like it at all each time I found out)
-the saw has great compression and excellent power (can't be stopped with a 20" bar buried!)
By looking at the pictures , what do You think - should I be concerned that my saw might seize up any time soon?
Should I consider replacing any parts , ring , piston , cylinder - worst case all three?



Knowing now that my PS6400 has score marks I intend to add more oil for future usage for both saws.
Is 40:1 going to make enough of a difference or should I go fatter?

In short:
1) do You think/agree that the PS7900 seems to have low hours
2) should I be concerned about the damage showing on the piston and ring of my PS6400 , should I consider replacing the damaged parts although the saw works rock solid
3) should I fatten my fuel mixture up a bit , is the specified 50:1 insufficient for adequate lubing

 

[w8ye]

That might have been carbon related on the 6400.

I would be thinking about a disassembly, clean up, and new ring for the 6400?

 

[cgraham1]

That might have been carbon related on the 6400.

I would be thinking about a disassembly, clean up, and new ring for the 6400?

Exactly.

 

[wde_1978]

That might have been carbon related on the 6400.
I would be thinking about a disassembly, clean up, and new ring for the 6400?

Exactly.

You might be correct with that assumption , the exhaust port had a quite thick layer of soot deposit all around the opening!
I cleaned it out before reassembling the muffler.

I'll post any progress when I decide to break down the PS6400 to asses the situation!
I hope the cylinder didn't take significant damage.
If I'm lucky replacing the piston ring might suffice to prevent further damage , even a OEM one from Dolmar shouldn't cost much.

Thanks for Your reply's guys!

 

[cgraham1]

An OEM ring should be about $7. I recently bought one for a 7900.

If I were you, I wouldn't run the saw again until you pulled the jug and confirmed that there is no aluminum transfer. If there is, it needs to be completely cleaned up and you might as well install a new ring before put it back together.

 

[cgraham1]

I would recommend between 32:1 and 40:1 on the fuel/oil mix. Just be sure to tune your saw accordingly.

 

[Treespotter]

Just leave it as it is. Stop using car gasoline that you mix yourself and take the leap of faith and fuel your saw with full range two stroke fuel from Aspen.

 

[wde_1978]

If I were you, I wouldn't run the saw again until you pulled the jug and confirmed that there is no aluminum transfer. If there is, it needs to be completely cleaned up and you might as well install a new ring before put it back together.

I agree , I won't use her anymore until I asses the damage!

I would recommend between 32:1 and 40:1 on the fuel/oil mix. Just be sure to tune your saw accordingly.

That is another problem I will have to address in this thread.
The previous owner of my PS7900 messed around the carburetor screws and it runs worse then my PS6400.
The saws manual requires one to have a tachometer to adjust the carburetor , so I am waiting for a cheap tachometer that I ordered to arrive.

Tried to adjust the PS7900 by sound and behavior comparing her to the PS6400 - she runs better now then when I bought her - but , believe it or not , the PS6400 still out-cuts her meaning that I am far from optimal settings!

 

[wde_1978]

Just leave it as it is. Stop using car gasoline that you mix yourself and take the leap of faith and fuel your saw with full range two stroke fuel from Aspen.

I don't get that kind of stuff here - Croatia/Europe

The saws manual says to use unleaded 91 ROZ or higher octane in this saw!
That been said , I didn't use any wrong fuel considering that I can't buy gasoline with lower octane than Super95!

 

[Treespotter]

Too bad, because you referred to the fact that there was a lot of soot build up. Adding more oil will only make matters worse.
My Dolly's have virtually no carbon build up anywhere.

 

[wde_1978]

Too bad, because you referred to the fact that there was a lot of soot build up. Adding more oil will only make matters worse.
My Dolly's have virtually no carbon build up anywhere.

Indeed , its a devils circle - adding more oil might ensure better lubing but will cause even more soot deposits on the piston and exhaust port!

I'll have to make it a habit to remove the muffler on a one or two years basis and if soot deposits are showing consider disassembling the cylinder as well to clean it up properly.

 

[angelo c]

That looks like it's being run "rich" . How do you tune it?...if I remember that is a limited coil. Pretty much has to be tuned in the wood. The rev limiter sounds like 4 stroking it it ain't .
The good news is everyone has a 6400 piston sitting around from upgrading to the 7900 or BB so you should be able to source a cheap one...get a good look at what caused that and the condition of the cylinder .

 

[Andyshine77]

Looks like I see carbon buildup in the ring groove, and some blow-by. I'd remove the ring and clean the groove. I doubt you seen any cylinder damage. Keeping the fuel fresh and saw tuned properly, will keep carbon buildup down.

Maybe one of these videos will help you with carb tuning.

 

[erikvr]

Adding more oil makes the combustion in fact leaner, as less % fuel is in the mixture.

 

[wde_1978]

1) That looks like it's being run "rich" . How do you tune it?...if I remember that is a limited coil. Pretty much has to be tuned in the wood. The rev limiter sounds like 4 stroking it it ain't .
2) The good news is everyone has a 6400 piston sitting around from upgrading to the 7900 or BB so you should be able to source a cheap one...get a good look at what caused that and the condition of the cylinder .

1) I don't think that my PS6400 is running rich (fat) considering that upon inspecting the spark plug the electrode had a nice even light brown color - as opposed to my PS7900's spark plug that is dark black with flakes!
To date I NEVER re-tuned my Dolmar PS6400 , she is running factory tuned for the last 8.5 years. I never even considered meddling around the L/H/S screws because she was running rock solid and pulls the chain strong as a bull with 20" guide bar fully buried into oak and beech tree trunks (even with that damage on the piston and ring) - it was a "don't fix what isn't broken" thing.
Ever since I started browsing this Forum I encountered the term "4 stroking" , but I still don't quite understand its meaning related to 2-stroke engines. Is it when the chain saw starts "farting/puttering" at WOT?
A tachometer is on its way , should arrive soon - it is primarily intended for tuning purposes of my out of tune PS7900 , might as well check the rpm's on my PS6400.

2) I am not that lucky! Dolmar is less then popular in Croatia , as such Croatian people don't have spare PS6400/7300/7900 P&C's lying around their basements or sheds!
I am not quite "Croatian people" , thus I have ONLY Dolmar chain saws.
I already looked up OEM P&C parts and it would set me back about 200 Euro - that is twice the price then what a cheap BB kit would cost on E-Bay (w/o shipping though!)
A piston and ring costs about 80-90 Euro , haven't looked up what just a ring would be.

Looks like I see carbon buildup in the ring groove, and some blow-by. I'd remove the ring and clean the groove. I doubt you seen any cylinder damage. Keeping the fuel fresh and saw tuned properly, will keep carbon buildup down.
Maybe one of these videos will help you with carb tuning.

I always make my own fuel mix and only as much as I need!
EDIT: I have seen the Dolmar PS7900 video before - I don't find it very conclusive!
Now the Echo tuning video is good , nice tutorial - thank You!
But I have the same problem as that guy has , I have no really big wood to cut until next year , thus I can't bury the bar to get the saw running at full load.
My Dolmar PS6400 sounds like this guys Echo @ about 6:50 , once under load the engine gets a significantly different sound and starts pulling trough like crazy. At that point there is literally no , to the eye , visible exhaust smoke.

As already has been suggested several times , I will asses the damage first and then go from there!

Thank You for Your reply's , much appreciated

 

[angelo c]

Post up in the "want ad" for the 6400 piston. I bet you get a few hits.
Post up some pics when you have the top apart. We'll get a better idea then.
Just as a rule of thumb...for me...I don't go near a wood pile without my orange STIHL screwdriver.
I tune for almost every wood, fuel, altitude or anytime I hear the saw telling me to.
Could not imagine never tuning a saw in years...glad it works for you though.

 

[shootingarts]

First a warning, I am not a two-stroke mechanic and I am nearly deaf as a post, makes hearing four cycling tough.

First thought, both of your saws are probably tuned rich from the break-in tune. I suspect someone has the 7900 tuned very rich.

I own the Makita 6401, AKA Dolmar 6400. I got the tach like you did, largely because my hearing if far worse in some frequencies than others so I wasn't sure what I was hearing.

First I tuned for optimum RPM, sneaking up on it from too rich since the limiting coil is going to start capping RPM when you are too lean and you can burn the saw up. My tach bounced around a good bit but I was able to get 13000RPM or a bit over without the tach going squirrelly like they do when the limiting coil kicks in.

Then I put it in the wood, I wasn't thrilled with performance. Always adjusting from the rich side, when you are in a fairly heavy cut the saw should sound duller when you lift the bar slightly and take the load off of the motor, this is four stroking. If the saw sounds shriller when you lift slightly, you are running lean, too lean for a working saw.

I took a couple pieces of Velcro hook and loop and attached the tach to the outside of the saw so I could see speed in the cut. For best speed in the cut and four stroking when I lifted I wasn't getting but 12500 to 13000 maximum rpm according to my tach, sometimes 13200. That 13500 is higher than Dolmar tunes to, it isn't a "standard" RPM, it is maximum safe RPM. Don't insist on getting 13000 running free when another setting works better in the cut and gives you four stroking. Later I tested again and my RPM had climbed a little on that day, I was getting more like 13200 with it bouncing to 13500.

Tuning right on the ragged edge you get into all kinds of issues. The fuel companies are constantly slightly changing their gas formulation, not just the loosely defined summer and winter mix. What is fine on a cool day might destroy your engine on a hot day with the same gas too. If the saw cuts well and gets around 12000 to 12500 free and four strokes when you lift slightly that might be what you get with your particular saw and tachometer. Don't get obsessed with numbers. If you really want to play with a maximum number, work with speed in the cut while four stroking when you unload the bar, not maximum free RPM.

Now a final caution and I will close this book. Do the math to see the difference in 50-1, 40-1, and 32-1 with the amount of fuel you are mixing. When mixing small amounts a tablespoon of oil changes your mix a lot. I assume you deal in liters but mix at least about a gallon of gas at a time and don't fall prey to a little extra oil doesn't hurt. With two-cycle oil it can! Some two-cycle oil partially burns, some doesn't burn at all for practical purposes. Every time you change between synthetic, semi-synthetic, and mineral oil you need to recheck your tune. Best to choose one type of oil and one brand of oil and stick with it. You obviously care for your saws, choose a good synthetic and stick with it. The cost per day for a nonpro isn't that big of a deal.

Others will no doubt comment on this tome I wrote, hope it helps!

Hu

 

[angelo c]

Good to see ya Hu.

 

[wde_1978]

I tune for almost every wood, fuel, altitude or anytime I hear the saw telling me to.
Could not imagine never tuning a saw in years...glad it works for you though.

My little Sachs-Dolmar 105 is a very sensitive one , always requires tuning.
But , honestly , my big PS6400 never made any notions she might require tuning!

... I own the Makita 6401, AKA Dolmar 6400 ...
... hope it helps!...

Hello Hu
Quite a post You made , thank You!
I really wouldn't wanna jump the gun but I estimate that my Dolmar PS6400 is tuned just about right considering how well she runs throughout a wide band of rpm's , as I am not used to run her WOT without the bar being buried in wood.
Also , as mentioned before , the spark plug has a really nice brown color - not silver/white (lean) , nor black (rich).
The PS7900 on the other hand is totally out of tune as the previous owner screwed around the carburetor.
Once I get my tachometer I'll roughly set the PS7900 to match idle and WOT rpm's of my PS6400 , then I'll try to fine tune her in wood.
I don't really care about maximal rpm's , I want torque/power!

When it comes to the fuel I use , here is how I mix:
- I have a windowed 1 Liter canister with 100 mL (1/10) markings
- I have a plastic 20 mL (0.020 Liter) syringe
- I strictly use Dolmar 2-stroke oil , 1 Liter package due to better pricing
- I use normal car gasoline , EuroSuper95 (NO i can't get special ready to use 2-stroke mix , using car fuel is common sense here)
- I take 1 Liter (1000 mL) of gasoline and add 20 mL of oil - that's my 50:1 chain saw fuel
- I never mix more then 1 Liter of fuel (the PS6400's tank holds 0.75 Liter)
- throughout a year I burn an average of 10 Liters of fuel in my PS6400

Note:
1 Liter = 0.264172051 Gallon [Fluid, US]
1 Liter = 0.219969248 Gallon [UK]

 

[shootingarts]

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.

The evil that the people on this forum cause , I had to file on my flywheel key and clean up the case and cylinder so I could run without a base gasket while tuning for that extra thousand or so RPM. Then I turned my attention to my chains and did some tweaking on them. No timed cuts to tell but I'm sure that I got some gains, the saw cuts through wood fast enough to put a grin on the face of everyone that uses it. Cost a friend down the road some money, he figured he could get by with one of the under $200 plastic saws until he cut with mine. First cut, he looked up and said he wasn't getting a hundred dollar saw.

Tweaked or just kept set at a good factory tune the 6400 and 7900 will work well and give years of satisfactory service. I planned to bump my saw up to 79cc or so when I got it but I'm going to put some hours on this piston and cylinder first. This thing runs too nice to have half of it sitting on a shelf from now on.

You shouldn't hurt a thing with short bursts of wide open throttle out of the wood with the rev limiter. Warm the saw up and have the tach on it first time you do it, make sure that limiter is working! I have tuned lean enough to bounce off the limiter but there is no reason to. Once I got there just to see what happened I went back rich and snuck up on 13000 again. I never have liked rev limiters and how an engine runs on one so I try to keep it out of the picture.

Hu

PS Angelo, nice to see you too. I sneak through pretty often, not enough time to play on the 'puter lately so I had to get in conversations.

Hu

My little Sachs-Dolmar 105 is a very sensitive one , always requires tuning.
But , honestly , my big PS6400 never made any notions she might require tuning!


Hello Hu
Quite a post You made , thank You!
I really wouldn't wanna jump the gun but I estimate that my Dolmar PS6400 is tuned just about right considering how well she runs throughout a wide band of rpm's , as I am not used to run her WOT without the bar being buried in wood.
Also , as mentioned before , the spark plug has a really nice brown color - not silver/white (lean) , nor black (rich).
The PS7900 on the other hand is totally out of tune as the previous owner screwed around the carburetor.
Once I get my tachometer I'll roughly set the PS7900 to match idle and WOT rpm's of my PS6400 , then I'll try to fine tune her in wood.
I don't really care about maximal rpm's , I want torque/power!

When it comes to the fuel I use , here is how I mix:
- I have a windowed 1 Liter canister with 100 mL (1/10) markings
- I have a plastic 20 mL (0.020 Liter) syringe
- I strictly use Dolmar 2-stroke oil , 1 Liter package due to better pricing
- I use normal car gasoline , EuroSuper95 (NO i can't get special ready to use 2-stroke mix , using car fuel is common sense here)
- I take 1 Liter (1000 mL) of gasoline and add 20 mL of oil - that's my 50:1 chain saw fuel
- I never mix more then 1 Liter of fuel (the PS6400's tank holds 0.75 Liter)
- throughout a year I burn an average of 10 Liters of fuel in my PS6400

Note:
1 Liter = 0.264172051 Gallon [Fluid, US]
1 Liter = 0.219969248 Gallon [UK]