Carburetor trouble

viperv10

Well-Known Member
Joined
Oct 7, 2016
Threads
45
Messages
171
Hi guys. I recently picked up a 1980 R7070 model pre compliant mower with a F 100 series engine. It was hard to start, taking at least 20 pulls to get it going.
So yesterday I took the carb off and cleaned it. I checked the exhaust ports. They were clear. I took the engine off and changed both crank seals. They were the old metal ringed ones. Cleaned off the cooling fins, put in a new plug and put her back together again. After six or seven pulls it started but won't stay running. It runs a few seconds and quits. If I hit the primer bulb it will run till that gas is gone and die down then I hit the primer again and it run a little more.
So it sounds like it isn't getting good fuel flow. Does anyone have any ideas what might cause this or what I might look for when I pull the carb off again?
It is a plastic carb with really no adjustment screws. It did have a jet screw on the side but I'm pretty sure it is supposed to be screwed in snug. Here are two
other things going on with this mower. Sometimes the pull rope will fly back real hard pulling the rope out of my hands, I never really knew why mowers did that and also the crank has probably 40 to 50 thousandths play up and down. I have never had a mower that had that much play. Any help would be appreciated.
Thanks.
Jerry
 

unclelee

Well-Known Member
Joined
May 5, 2014
Threads
22
Messages
337
Make sure your gas cap is venting probably. You may try loosening it and see if anything changes.
If not remove the main jet and make sure all the little holes in the side are clean.
Lee
 

jp1961

Well-Known Member
Joined
Jul 22, 2013
Threads
39
Messages
785
Hi Jerry,

Have you checked the compression?

Regards

Jeff
 

Dixieboy

Member
Joined
Jul 19, 2017
Threads
3
Messages
26
I don't think there is much you can do about the crank end play issue unless you what break the case down and check for wear and add a thrust washer. I think it will continue to run ok regardless for a long time. Sounds like a fuel flow issue. First thing I would check is fuel flow out of the tank, pull the fuel line from the tank and see if you are getting flow when fuel valve is on. If not you can pop off the clip and clean the valve out. Next would be to remove the carb and check float operation and clean needle and seat. Like Jeff said above if you can check compression to verify good or bad would be a good idea. Carbon buildup can cause high compression making it hard to start. One more thing that came to mind is I think that model has a small filter inside the carb float bowl that could be plugged
 

viperv10

Well-Known Member
Joined
Oct 7, 2016
Threads
45
Messages
171
Thanks for your responses. I just checked the compression and it only showed 75. That's awful low. The funny thing is it ran good the couple of times I used it.
It was just hard to start. One of these days I'll look at the carb again. The Lawn boy master parts viewer does show a filter inside the carb but this one doesn't have one, I don't know any history of this mower. I got it from a guy that his wife's grandpa and dad used to have a lawn boy repair shop years ago. Taking the gas cap off didn't help any. It won't start now. Maybe I'll put it aside until mowing season is over and take it apart and see what it looks like inside. If its not too bad I might re ring it. Thanks again guys. If you have any more thoughts, shout them out.
Jerry
 

lewb

Well-Known Member
Joined
Jun 4, 2013
Threads
40
Messages
275
I believe you need at least 90 psi to start engine. You can try and use transmission fluid to loosen up the rings, just flip motor up and fill up the spark plug hole, let sit for a day and see if you get more compression.
 

Dixieboy

Member
Joined
Jul 19, 2017
Threads
3
Messages
26
If you feel confident in the 75 psi reading that is too low. Since it was trying to run you may have scoring cylinder walls and rings. You know anyone with a bore scope to look and see without a teardown? Another thing is when you rock the engine by hand do you feel any slop? like rod or wrist pin bearing failure? I have seen wrist pin bearing failure and it didn't make much noise so it was hard to tell.
 

viperv10

Well-Known Member
Joined
Oct 7, 2016
Threads
45
Messages
171
Well I took the tank shroud off and I had good gas flow out of the tank. The float bowl was full. I cleaned out every little passage in the carburetor again. Then I noticed the mounting surface area of the plastic carb was warped a little bit, so I used 2 new gaskets just in case it was sucking air. Still would not start. Then I tried Lewb's idea. Turned mower up on its side and put some ATF in the spark plug hole for awhile. Every now and then I would move the blade back and forth some to move the piston up and down a little bit to try to soak the rings. The next day I checked the compression and it only showed 60 psi. That's going the wrong direction. Some of atf drained out of the carb so i'll probably clean it again and put it away for a later project.
Several years ago I noticed a Lawn Boy setting in garage across the alley from a place I mow in town. I always wanted to ask about it but there was never anybody around. Finally the other day when I went to mow there there were people there. So I went over to ask about it and they said they would let it go.
They couldn't get it started. It turned over ok so I took a chance on it for $40. It is a 1989 L21ZSNA, capstan drive, steel deck. It didn't have an air filter or cover and the carb was very dirty. The plug was fowled real bad and wasn't firing. I got good spark with a new plug. Compression showed 90 psi, but still would not start. I'm sure the carburetor is a mess. I have never had this type of shaft drive before so this will be a new experience for me. I'm going to start looking into this one in my spare time. It might work out ok or it might be a heap of scrap. Only time will. I just wanted to rescue this old mower and try to bring some new life to it. I have really enjoyed tinkering with these mowers this past year. I will probably have some more questions for you guys soon so put on your thinking caps. Thanks guys, over and out.
Jerry
 

jp1961

Well-Known Member
Joined
Jul 22, 2013
Threads
39
Messages
785
FWIW, G.M. top engine cleaner (available in spray cans from any G.M. dealer) removes carbon very well.

It used to be available as a liquid back when cars had carburetors.

Regards

Jeff
 

lewb

Well-Known Member
Joined
Jun 4, 2013
Threads
40
Messages
275
Well I took the tank shroud off and I had good gas flow out of the tank. The float bowl was full. I cleaned out every little passage in the carburetor again. Then I noticed the mounting surface area of the plastic carb was warped a little bit, so I used 2 new gaskets just in case it was sucking air. Still would not start. Then I tried Lewb's idea. Turned mower up on its side and put some ATF in the spark plug hole for awhile. Every now and then I would move the blade back and forth some to move the piston up and down a little bit to try to soak the rings. The next day I checked the compression and it only showed 60 psi. That's going the wrong direction. Some of atf drained out of the carb so i'll probably clean it again and put it away for a later project.
Several years ago I noticed a Lawn Boy setting in garage across the alley from a place I mow in town. I always wanted to ask about it but there was never anybody around. Finally the other day when I went to mow there there were people there. So I went over to ask about it and they said they would let it go.
They couldn't get it started. It turned over ok so I took a chance on it for $40. It is a 1989 L21ZSNA, capstan drive, steel deck. It didn't have an air filter or cover and the carb was very dirty. The plug was fowled real bad and wasn't firing. I got good spark with a new plug. Compression showed 90 psi, but still would not start. I'm sure the carburetor is a mess. I have never had this type of shaft drive before so this will be a new experience for me. I'm going to start looking into this one in my spare time. It might work out ok or it might be a heap of scrap. Only time will. I just wanted to rescue this old mower and try to bring some new life to it. I have really enjoyed tinkering with these mowers this past year. I will probably have some more questions for you guys soon so put on your thinking caps. Thanks guys, over and out.
Jerry

I would say that if the compression got worse than there must be scoring or something else going on. If you could find a short block that would be the easiest fix to get the same mower running again. Good luck,
 
Top