Seat the bobbin & adjust tension on Singer 9018

Could be that your bobbin thread is wound too tightly and having a problem pulling thread from it. Try setting the pressure on bobbin a bit less and that may cure it.

adjust the top thread tension and see if that helps the bobbin tension
stitches.

here is a link for a free manual for the machine
http://www.singerco.com/uploads/download/545_7470-instruction-book-engfrespa.pdf

try this link for a manual, it should show how to adjust the top and
bobbin tension.
http://www.singerco.com/uploads/download/466_7462_efs.pdf

try adjusting the top thread tension-one number or letter less, then
sew on a piece of the scrap fabric if you have any. to see if that helps
try changing the needle
make sure the bobbin is turned the right way in the bobbin case, I
know this may sound crazy but it can hinder the sewing, I was sewing
a skirt the other night & I turned the bobbin the wrong way & inserted
the bobbin into the bobbin case & it didn't work good,, I took the bobbin
out of the bobbin case & had multiple threads. so I had to clean the mess up & turn the bobbin the opposite way & the machine worked
just fine

Try adjusting the top tension dial to tighten up the loose stitching on the underside. Generally if you have loose threads underneath, its the top tension and visa versa.

However, if you are using a thread heavier than normal sewing thread through the needle and bobbin (which you probably need for this canvas) then the machine may struggle. I've sewed vinyl seat covers and gave up with my domestic machine and purchased a second hand industrial Singer machine 20U to cope with this heavier weight textile. Generally for anything heavier than upholstery fabric a domestic machine may not be strong enough.

Insert a new needle, (flat side facing rear of machine), now totally rethread. Try this, with presser foot up, pull top thread, it should pull smoothly, now lower presser foot, the thread should barely move. This should seat the thread in the tension discs. If that doesn't work, call Singer at 1-800-4singer.

I went to the singer website & typed the number in 6218 & it came up with nothing, is that the correct model number?
the tension problem try adjusting the top thread tension to 3-5, & see if that helps the loose tension

need needle threading instructions singer 3116 simple. pictures would help. thank you

Needle Faces Where?  
I Need to Tighten the Tension....  
Changing a Needle  
Need Help  
Basic Questions-  
Bobbin Assembly Popped Out!  
Do You Think My Machine is Fixable?  
How to Change a Broken Needle  
Needle Bar Adjustment  
Oiling an Elna 3007  
Getting the Timing Belt Off?  
What Size Needles for My Machine?  
How to Oil my Machine?  
Where to Oil??  
Stitch Length Knob Woes  
Can't Seem to Put the Bobbin in Correctly  
Timing Belt Tension  
Adjustments  
Running Very Slow-  
Oiling Points on Singer 257  
Oiling My Singer Touch & Sew  
Singer 457  
Featherweight Case Odor  
Brother XL2010 Q's  
Does My Machine Have More Than One Belt?  
Cannot Seem To Adjust the Tension  
What to Do About a Gumming Up Needle  
How Tighten My Bobbin Tension, Please  
Is it Covered Under Warranty?  
Proper Threading Instructions?  
Just a Foot Pedal?  
Simple Threading Instructions for a 1036  
Threading a Singer 600  
Replacing a Blade on a Johnson Ruffler  
Changing the Needle  
Lubricating a Singer 1036  
Thread Balling Underneath  
Warranty Info  
Machine Covered in Hardened Oil  
Spindle Stuck?  
Do Motors Die?  
Singer 457  
Brother Sewing Machine  
Servicing My Machine-  
Belt Replacement  
Lubricating My Sewing Machine  

check out this. good luck. thanks.

Needle Faces Where?  
I Need to Tighten the Tension....  
Changing a Needle  
Need Help  
Basic Questions-  
Bobbin Assembly Popped Out!  
Do You Think My Machine is Fixable?  
How to Change a Broken Needle  
Needle Bar Adjustment  
Oiling an Elna 3007  
Getting the Timing Belt Off?  
What Size Needles for My Machine?  
How to Oil my Machine?  
Where to Oil??  
Stitch Length Knob Woes  
Can't Seem to Put the Bobbin in Correctly  
Timing Belt Tension  
Adjustments  
Running Very Slow-  
Oiling Points on Singer 257  
Oiling My Singer Touch & Sew  
Singer 457  
Featherweight Case Odor  
Brother XL2010 Q's  
Does My Machine Have More Than One Belt?  
Cannot Seem To Adjust the Tension  
What to Do About a Gumming Up Needle  
How Tighten My Bobbin Tension, Please  
Is it Covered Under Warranty?  
Proper Threading Instructions?  
Just a Foot Pedal?  
Simple Threading Instructions for a 1036  
Threading a Singer 600  
Replacing a Blade on a Johnson Ruffler  
Changing the Needle  
Lubricating a Singer 1036  
Thread Balling Underneath  
Warranty Info  
Machine Covered in Hardened Oil  
Spindle Stuck?  
Do Motors Die?  
Singer 457  
Brother Sewing Machine  
Servicing My Machine-  
Belt Replacement  
Lubricating My Sewing Machine  
check out this. good luck. thanks