RESOLVED! Makita 7900 owners or Jay and other saw guros...I've got a question.

[Jutt77]

My Makita 7900 (Makita 6401 with Mahle 7900 P&C) has been running awesome until a few weeks back. It would start on the 2nd pull, every %$&* time for 5 months but now, I can’t get it to start. I took the cover off and when trying to start it, I noticed that fuel is lightly misting out of the decomp valve even when closed. So I got a compression tester yesterday and found I have around 109psi with the valve in the close position. I've pulled the plug, dried it, blew out P&C around 20 times so I don't think there is a problem with it being flooded. I've also changed the fuel with fresh 50:1 mixed gas.

Spark? Check: pulled plug, verified strong spark (on 2 plugs) and confirmed .5mm gap
Fuel? Check: Tons of fuel in P&C and end of spark plug after attempting start.
Compression? Check BUT only 109psi with valve CLOSED. Since fuel is leaking from decomp valve when closed, I think the decomp valve is bad so I ordered another. My theory is that this is causing the no start problem BUT why would it start when the valve is open under normal operation? Seems like open position under normal operation would create a similar low compression scenario but I’m pretty new to working on 2 stroke engines so I could be way off base…

Question for those who have the Makita 64xx with the Mahle 7900 P&C or a Dolmar 7900: What is your compression at with compression valve closed? What about open?
Any other suggestions, thoughts, comments, troubleshooting ideas or creative sarcasm welcome…

 

[smokinj]

First of all you sure your compression is correct.....You should be North of 150psi Could be head gasket blow or bad rings...It would never start at 109psi. Hearing any metal to metal sounds? What do you hear when pulling the cord?

 

[Jutt77]

First of all you sure your compression is correct.....You should be North of 150psi Could be head gasket blow or bad rings...It would never start at 109psi. Hearing any metal to metal sounds? What do you hear when pulling the cord?

Yeah I thought it should be around 150psi. I just picked up that compression tester yesterday so it could be off, plus it was cheap, I'll try testing using a friends tester this weekend. I removed the muffler to take a look at the piston and when I cycle it I hear a suction sound as it goes from the exhaust cycle to the compression cycle. The ring looks properly set, no score marks are noticable. No metal to metal sounds. When the piston moves north for combustion, I feel compression resistance and hear the suction sound. I haven't noticed any evidence of the head gasket being blown (fuel leaks in that area) but maybe I'll change it just in case.

Should fuel be escaping from the compression valve when its closed? I mean, its enough that I actually feel fuel spray the back of my hand when I crank it.

 

[smokinj]

First of all you sure your compression is correct.....You should be North of 150psi Could be head gasket blow or bad rings...It would never start at 109psi. Hearing any metal to metal sounds? What do you hear when pulling the cord?

Yeah I thought it should be around 150psi. I just picked up that compression tester yesterday so it could be off, plus it was cheap, I'll try testing using a friends tester this weekend. I removed the muffler to take a look at the piston and when I cycle it I hear a suction sound as it goes from the exhaust cycle to the compression cycle. The ring looks properly set, no score marks are noticable. No metal to metal sounds. When the piston moves north for combustion, I feel compression resistance and hear the suction sound. I haven't noticed any evidence of the head gasket being blown (fuel leaks in that area) but maybe I'll change it just in case.

Should fuel be escaping from the compression valve when its closed? I mean, its enough that I actually feel fuel spray the back of my hand when I crank it.

No it should not be and that could be the cause of the low compression. You need at least 135 in most cases to fire it.....That is the place to start!

 

[lukem]

Sounds like the decomp valve may have some crud in it that is preventing it from closing all the way. You def should NOT have raw fuel coming from it.

Is it easy to remove and inspect?

 

[HittinSteel]

I agree with lukem

And your tester needs a schraeder valve

 

[Jutt77]

Sounds like the decomp valve may have some crud in it that is preventing it from closing all the way. You def should NOT have raw fuel coming from it.

Is it easy to remove and inspect?

Yeah, it was really easy to pull out and check it. I blew it out with my air compressor and tried wiping it down best I could but its still blowing out fuel when closed. That saw was a HD rental and is 10 yrs old and the valve looks original, so hopefully, its just a bad valve.

 

[Jutt77]

First of all you sure your compression is correct.....You should be North of 150psi Could be head gasket blow or bad rings...It would never start at 109psi. Hearing any metal to metal sounds? What do you hear when pulling the cord?

Yeah I thought it should be around 150psi. I just picked up that compression tester yesterday so it could be off, plus it was cheap, I'll try testing using a friends tester this weekend. I removed the muffler to take a look at the piston and when I cycle it I hear a suction sound as it goes from the exhaust cycle to the compression cycle. The ring looks properly set, no score marks are noticable. No metal to metal sounds. When the piston moves north for combustion, I feel compression resistance and hear the suction sound. I haven't noticed any evidence of the head gasket being blown (fuel leaks in that area) but maybe I'll change it just in case.

Should fuel be escaping from the compression valve when its closed? I mean, its enough that I actually feel fuel spray the back of my hand when I crank it.

No it should not be and that could be the cause of the low compression. You need at least 135 in most cases to fire it.....That is the place to start!

Thanks as always for the quick reponse Jay, glad to hear I'm on the right track. I ordered the valve from Cassaga E&E yesterday so it should be here soon.

 

[Jutt77]

I agree with lukem

And your tester needs a schraeder valve

Would the schraeder valve allow me to test the decomp valve port?

 

[smokinj]

First of all you sure your compression is correct.....You should be North of 150psi Could be head gasket blow or bad rings...It would never start at 109psi. Hearing any metal to metal sounds? What do you hear when pulling the cord?

Yeah I thought it should be around 150psi. I just picked up that compression tester yesterday so it could be off, plus it was cheap, I'll try testing using a friends tester this weekend. I removed the muffler to take a look at the piston and when I cycle it I hear a suction sound as it goes from the exhaust cycle to the compression cycle. The ring looks properly set, no score marks are noticable. No metal to metal sounds. When the piston moves north for combustion, I feel compression resistance and hear the suction sound. I haven't noticed any evidence of the head gasket being blown (fuel leaks in that area) but maybe I'll change it just in case.

Should fuel be escaping from the compression valve when its closed? I mean, its enough that I actually feel fuel spray the back of my hand when I crank it.

No it should not be and that could be the cause of the low compression. You need at least 135 in most cases to fire it.....That is the place to start!

Thanks as always for the quick reponse Jay, glad to hear I'm on the right track. I ordered the valve from Cassaga E&E yesterday so it should be here soon.

That should bring the commpression in line.....No scoring and no funky sounds on the pull means the head gasket is good as well...Easy Fix. I know how much these thing cost to replace! :cheese: (i pucker a little every time I hear something new)

 

[HittinSteel]

I agree with lukem

And your tester needs a schraeder valve

Would the schraeder valve allow me to test the decomp valve port?

No, it wil just give you a "real" compression reading. If fuel is escaping your decomp, then it's leaking. Hopefully the replacement fixes it.

 

[CTYank]

Surprised nobody's yet pursued the gasoline gusher you seem to have.
What are you doing to try to start it? Full details, please. How quickly do you take the choke off?
What is the history of the carb? Last rebuilt when?
Sure sounds like a sticking needle valve, so you'd want the carb kit with one of same.

I hope you realize that well before liquid fuel gets to the combustion chamber, the crankcase is awash in liquid fuel. Stinkingly rich.

Professional guidance mentioned that 90 psi actual compression reading would be needed to start, not 120-something. Checking spark at atmospheric pressure is not the same as when threaded in- that's why they make inline testers.

My bet for problem source: fuel system (carb), operator.

 

[smokinj]

I agree with lukem

And your tester needs a schraeder valve

Would the schraeder valve allow me to test the decomp valve port?

No, it wil just give you a "real" compression reading. If fuel is escaping your decomp, then it's leaking. Hopefully the replacement fixes it.

No scoring and head gasket good this should be it! (leaking head gasket could have been fun though)

 

[lukem]

Surprised nobody's yet pursued the gasoline gusher you seem to have.
What are you doing to try to start it? Full details, please. How quickly do you take the choke off?
What is the history of the carb? Last rebuilt when?
Sure sounds like a sticking needle valve, so you'd want the carb kit with one of same.

I hope you realize that well before liquid fuel gets to the combustion chamber, the crankcase is awash in liquid fuel. Stinkingly rich.

Professional guidance mentioned that 90 psi actual compression reading would be needed to start, not 120-something. Checking spark at atmospheric pressure is not the same as when threaded in- that's why they make inline testers.

My bet for problem source: fuel system (carb), operator.

If you poured gas into the chamber, it should come out the muffler before coming out the decomp valve...my bet is on the valve.

 

[Jutt77]

Surprised nobody's yet pursued the gasoline gusher you seem to have.
What are you doing to try to start it? Full details, please. How quickly do you take the choke off?
What is the history of the carb? Last rebuilt when?
Sure sounds like a sticking needle valve, so you'd want the carb kit with one of same.

I hope you realize that well before liquid fuel gets to the combustion chamber, the crankcase is awash in liquid fuel. Stinkingly rich.

Professional guidance mentioned that 90 psi actual compression reading would be needed to start, not 120-something. Checking spark at atmospheric pressure is not the same as when threaded in- that's why they make inline testers.

My bet for problem source: fuel system (carb), operator.

I'm using the same method to start it as I historically have when it has started after 2 pulls following cold start procedure verbatim per page 25 of the manual: (broken link removed)

I don't have an inline tester but I will look into rebuilding carb if the valve is not cause of problem as I purchased the saw at a pawnshop last year so I'm not sure of carb rebuild history. Maybe I'll just rebuild the carb why I'm at it. Thanks for the idea.

EDIT: Carb rebuild kit ordered from Baileys...also noticed that the cheap compression tester I purchased from the Toolking retail shop is the same one Baileys sells.

 

[amateur cutter]

Put a plug in place of the decomp valve & pull it over on the tester. Don't know the thread pitch & diameter on the dolkita saw, but any parts or hardware store should have a plug. If comp comes up problem found, if not look further. While many say that a saw will run @ 90 to 100 psi I've found they start hard/or not at all & don't idle. For my purposes, anything below 135 psi gets rings & a piston.

P S you can run the saw with the plug if you need it. Just watch your grip on the rope handle, they'll kick ya occasionally. A C

 

[smokinj]

Put a plug in place of the decomp valve & pull it over on the tester. Don't know the thread pitch & diameter on the dolkita saw, but any parts or hardware store should have a plug. If comp comes up problem found, if not look further. While many say that a saw will run @ 90 to 100 psi I've found they start hard/or not at all & don't idle. For my purposes, anything below 135 psi gets rings & a piston.

P S you can run the saw with the plug if you need it. Just watch your grip on the rope handle, they'll kick ya occasionally. A C

He has already order a new one and its on its way!

 

[Jutt77]

Well, I replaced the compression valve and same problem, no start. Compression is reading the same 120 PSI so maybe this gauge isn't accurate because starting takes a heckuva pull so it feels like there's proper compression.

I also rebuilt the carb put it back together, same problem, no start...LOTS of fuel being sent to cylinder though. I adjusted my H & L to all the way in 1 turn out on H & L...same problem. 1 1/8 out on H; 1 out on L...same problem.

Maybe its not getting enough spark? I tested that by gapping an old working plug to over 1/8" and a strong spark was able to gap it easily each pull. The last thing I'm going to do is look at the ignition magnet gap...should be around a credit card thickness right?

If all else fails, I'm taking her to the shop.

 

[lukem]

Is fuel still coming out of the decomp valve now?

 

[wendell]

If you're getting fuel, it probably isn't the impulse line but I'd check it, especially where it comes off the engine on the clutch side.

 

[Jutt77]

Is fuel still coming out of the decomp valve now?

No, that is fixed now.

 

[smokinj]

Is fuel still coming out of the decomp valve now?

No, that is fixed now.

Hows that beast running?

 

[HittinSteel]

Well, I replaced the compression valve and same problem, no start. Compression is reading the same 120 PSI so maybe this gauge isn't accurate because starting takes a heckuva pull so it feels like there's proper compression.

I also rebuilt the carb put it back together, same problem, no start...LOTS of fuel being sent to cylinder though. I adjusted my H & L to all the way in 1 turn out on H & L...same problem. 1 1/8 out on H; 1 out on L...same problem.

Maybe its not getting enough spark? I tested that by gapping an old working plug to over 1/8" and a strong spark was able to gap it easily each pull. The last thing I'm going to do is look at the ignition magnet gap...should be around a credit card thickness right?

If all else fails, I'm taking her to the shop.

I like to test by bending the electrode on a new plug straight up. Ground the plug, turn out the lights and pull the saw over. Does a nice blue spark jump the gap?

 

[smokinj]

Well, I replaced the compression valve and same problem, no start. Compression is reading the same 120 PSI so maybe this gauge isn't accurate because starting takes a heckuva pull so it feels like there's proper compression.

I also rebuilt the carb put it back together, same problem, no start...LOTS of fuel being sent to cylinder though. I adjusted my H & L to all the way in 1 turn out on H & L...same problem. 1 1/8 out on H; 1 out on L...same problem.

Maybe its not getting enough spark? I tested that by gapping an old working plug to over 1/8" and a strong spark was able to gap it easily each pull. The last thing I'm going to do is look at the ignition magnet gap...should be around a credit card thickness right?

If all else fails, I'm taking her to the shop.

I like to test by bending the electrode on a new plug straight up. Ground the plug, turn out the lights and pull the saw over. Does a nice blue spark jump the gap?

Just caught the spark thing.....+1 and I use a match book cover.

 

[Jutt77]

Is fuel still coming out of the decomp valve now?

No, that is fixed now.

Hows that beast running?

She aint running yet, possible iginition problem...I'm stumped. What's the ignition test with a match book you mentioned below Jay?