possable cylinder scoring Dolmar 7910

[sixteenacrewood]

Hi Everyone
I need some input
My Dolmar 7910 started to run rough, hard to keep it idling but it would rev to WOT, then died and would not restart. It was smoking from around the muffle where some saw dust and canola had accumulated.
I decided to pull it off the mill, let it cool down, and cleaned it up, pulled the muffler and cleaned behind it.

I have this saw on a procut carriage mill, like a band mill but with a chainsaw. I run this saw pretty hard.
I had just finished milling a 24" dia. yellow pine log, when this happened.

I used a flash light and checked the piston and cylinder for damage while the muffler was off.

The piston looks new, but the cylinder on the intake side looks to be scored. this is from the exhaust side.

I decided to shoot some photos and get some input from some of you who have more experience. Does this look normal? or does it look damaged?

The plug looks to be light silver grey so I shot photos, does the plug look normal? Or does it look like it was running hot/lean?

I have searched here and another site and looked at a ton of photos, but still not certain of my next move

I have not pulled the carb or intake yet, I hate to pull things apart if some of you think this looks normal.

I am going to put it back together and see if she will start.

I am a good mechanic, just no experience with worn cylinders. I have fried a piston and cylinder on my 9010, but it looked a lot worse/ different from this, not hard to diagnose.

This saw is a money maker and I don't mind replacing the top end if its needed, plus, I need it running asap.
I don't have a compression tester but plan to get one later today, nearest town is 20+ miles from here.

Thanks in advance for any input.

 

[JonCraig]

Not normal. Running lean. Either impulse line (if applicable), cracked fuel line, or seal(s). If all you do is slap on a new piston & cylinder, you'll quickly have another roasted cylinder. Treat the cause, not the symptom.

Search this forum for tons of threads on pressure and vacuum testing.

 

[fordf150]

not normal. looks lean/hot to me which is about par for the course milling.

what oil and ratio are you running? what is the saw tuned to?

 

[sixteenacrewood]

Thanks!
Yes I'll vacuum test, I have one.

I use the 50/1 husky brand oil, all that's available here, I usually add an extra table spoon to the gallon as per suggestion on milling thread here.

This saw is three years old and I have not had to do anything to it, still running on factory tune. I've never had the plug look like this on this saw.

I had one of my helpers mix this last gallon of gas, I am wondering if he goofed.

I'll go ahead and pull the intake/carb and see what it looks like

or

Should I just go ahead and order the top end kit, and wait and tear it down when I have the parts?

 

[JonCraig]

Pressure/vac test it now before disassembly. That will tell you what all parts you'll need.

I will say this: I learned the hard way to clean the saw REALLY well *before* taking the jug off. Otherwise you wind up with crap in the crankcase.

 

[fordf150]

check it out before ordering parts that way its a one time thing instead of paying shipping twice.

if pulling the cylinder, replace the impulse line for sure

instead of just "add an extra table spoon", buy a ratio rite cup or go to a boat supply store and buy a 2T mixing cup and mix to a known ratio everytime. 40:1 is a great starting point but for milling i would be looking more towards 32:1 and tuned a few hundred rpm rich....say 12800(which is right at the limiter).

you may be able to get away with just a piston and clean up the cylinder but since a OEM piston is $125 and a cylinder is only $200 shipped....i say go ahead and buy the cylinder kit especially since its an income producing saw.

if your cylinder will clean up you can sell it later on to recoup some the money spend on the new cylinder.

 

[Bluefish]

Some great info here for OP. Good thread!
Russ

 

[sixteenacrewood]

Thanks for the input! I really do appreciate it.

I leak tested the case and it help vacuum, I left it for more than a half hour.

So I took everything apart
fried piston on intake side only same with cylinder
I'm about to order parts

My plan is to replace
cylinder and decomp valve, the old one stuck open several times last week.
piston
intake, it's rubber, so replace it.
fuel line, suction line, and the one coming out the bottom of the cab
cylinder and exhaust gasket

rather than skimp to save a few bucks, I'll replace what I can afford and get her back running. This saw has really out performed what I was expecting and I need her back in service on the mill.

Sound like a good plan?

 

[JonCraig]

Strange that only the intake got scored. How's the air filter?

 

[fordf150]

Thanks for the input! I really do appreciate it.

I leak tested the case and it help vacuum, I left it for more than a half hour.

So I took everything apart
fried piston on intake side only same with cylinder
I'm about to order parts

My plan is to replace
cylinder and decomp valve, the old one stuck open several times last week.
piston
intake, it's rubber, so replace it.
fuel line, suction line, and the one coming out the bottom of the cab
cylinder and exhaust gasket

rather than skimp to save a few bucks, I'll replace what I can afford and get her back running. This saw has really out performed what I was expecting and I need her back in service on the mill.

Sound like a good plan?View attachment 585095 View attachment 585096

order this cylinder kit....it comes with the decomp vs the actual 7910 cylinder just including the piston/cylinder and you have to buy the $16 decomp valve seperate.
https://performanceoutdoorequipment...cts/cylinder-and-piston-kit-ps-7900-038130910

 

[fordf150]

Strange that only the intake got scored. How's the air filter?

intake is usually last to score.

 

[fordf150]

intake scoring only is a little puzzling. if there were some exhaust scoring to match it would be a pretty easy diagnoses.

looks pretty dry and lean. milling+lean oil mix+lean tune+heat+air leak(stuck open decomp)= scored.

 

[jmssaws]

It scores on the intake from lack of oil,50:1 killed it.
Tune it slower and run 32 or 24:1 and it will thank you for it.
50:1 on a mill with a small saw and it lasted 3 years!
Dolmar should buy it and have it bronzed and on the ceo's desk.
Tough ain't the word.
Clean up the cylinder and put a new piston in and don't use 50:1 anymore and tune it fatter

 

[blsnelling]

Milling with 50:1 on a factory tune is an almost for sure recipe for disaster. I'm amazed that it lasted as long as it did. You should be running 32:1 and tuned rich for a milling application.

 

[astnmacgto]

Hi Everyone
I need some input
My Dolmar 7910 started to run rough, hard to keep it idling but it would rev to WOT, then died and would not restart. It was smoking from around the muffle where some saw dust and canola had accumulated.
I decided to pull it off the mill, let it cool down, and cleaned it up, pulled the muffler and cleaned behind it.

I have this saw on a procut carriage mill, like a band mill but with a chainsaw. I run this saw pretty hard.
I had just finished milling a 24" dia. yellow pine log, when this happened.

I used a flash light and checked the piston and cylinder for damage while the muffler was off.

The piston looks new, but the cylinder on the intake side looks to be scored. this is from the exhaust side.

I decided to shoot some photos and get some input from some of you who have more experience. Does this look normal? or does it look damaged?

The plug looks to be light silver grey so I shot photos, does the plug look normal? Or does it look like it was running hot/lean?

I have searched here and another site and looked at a ton of photos, but still not certain of my next move

I have not pulled the carb or intake yet, I hate to pull things apart if some of you think this looks normal.

I am going to put it back together and see if she will start.

I am a good mechanic, just no experience with worn cylinders. I have fried a piston and cylinder on my 9010, but it looked a lot worse/ different from this, not hard to diagnose.

This saw is a money maker and I don't mind replacing the top end if its needed, plus, I need it running asap.
I don't have a compression tester but plan to get one later today, nearest town is 20+ miles from here.

Thanks in advance for any input.View attachment 585072 View attachment 585073 View attachment 585074 View attachment 585075

@fordf150 will have everything you need at a good price and even better service.

Even though it's been said before I run all my stuff at 32:1 ratio for regular cutting let alone milling. I've pulled the jugs off some of my saws that have been run on 32:1 and they all have a thick shiny film of oil on everything and a puddle in the crankcase, they are happy that way.