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Collection, Preservation and Display of Old Lawn Mowers

Qualcast commodore not running under load

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Hello everyone

 

This is my first post here. I recently bought and restored an old 60s qualcast commodore mower. I've stripped every piece apart and painted. I've cleaned the motor and carby and tuned it. But it seems to struggle under load sometimes. Even on thinner grass. Just wondering if anyone's had a similar issue and could give me a few ideas to check out.  I have sharpened the blades and adjusted to cut paper. Fresh fuel and spark plug. I cleaned the carby just the other day and resealed the gaskets and tuned per manufactures instructions. It runs fine until it hits the grass. Bearings are all good with no hesitation. 
 

any help would be appreciated. Thank you 
 

Forums

Richard Sun, 20/10/2019

I've played with the jet quite a bit without any success. Tried another 1/8 turn and another with the same results. 

Richard Sun, 20/10/2019

I used valve lapping paste and manually spun the blade backwards with a socket wrench. Works quite well and valve lapping paste is much much cheaper and easier to get hold of. 

wristpin Sun, 20/10/2019

Valve paste is oil based and not recommended for blade back lapping as it’s difficult to remove all traces. If not removed any benefit of the operation risks being undone when the cylinder is rotated in the working direction.  Will it cut paper all the way along on each blade?

That said the cylinder will have to be pretty dire to stall the machine. Another half turn on the main jet should produce a heavy and black smokey exhaust. If it doesn’t it is still suggesting weak mixture caused by a blockage. The other possibility is an incorrectly assembled / set up throttle / governor linkage not reacting to the load.

Chris G Sun, 20/10/2019

Given (IF) your cylinder spins freely increasing revs without drive, and the rear rollers spin freely also, then struggles when drive is engaged I would say its down to carburation as above.

I had a similar issue not making enough power under load which was down to the carb mating surface / poor gasket creating a lean mixture. Might be worth going over the carb again

hillsider Mon, 21/10/2019

Whilst valve grinding paste may not be ideal I have used it myself for one off jobs and as wristpin says the cleaning after lapping is the important bit.

Getting back to your issue with the struggling engine you seem to have tried various carb adjustments that have not worked. This leaves the question could the mower be losing power due to over tightening of the cylinder, I have seen this happen if the cylinder is wound down tight enough to cut or rather tear paper despite being blunt. 

Another possibility is that the air vane governor is not set up or working as it should be so that as load is applied the engine is bogging down rather than the governor arm keeping the revs up.

 

 

 

hortimech Mon, 21/10/2019

Sorry, but back lapping does not equal sharpening. Your cylinder and bottom blade have to be already sharp before you backlap them, it is a maintenance procedure, not a repair.

Also using valve lapping paste isn't recommended, you probably didn't use enough paste (not unless you buy it in industrial quantities) and you would have to use a solvent to remove it.

Examine the cylinder to bottom blade when it stalls, I think you will find they are 'jammed' with grass.

hillsider Mon, 21/10/2019

Agreed I should have pointed out in my post from earlier that back lapping (with any paste) will not cure a blunt cylinder.

I was however trying to make the point about valve paste not being ideal but a case of using what you can get your hands on.  

 

 

 

wristpin Mon, 21/10/2019

Just a thought. With the chain removed does the cylinder spin smoothly and freely?  If it feels a bit stiff and lumpy , it’s possible that when  you reassembled the cylinder the races went in the wrong way round,  and the back of the cages are running against the cones* and dragging. *Edit, should read cups.

A peculiarity of the old Qualcast Commodores is that at some point in their production there was a change from the bearing cones being on the cylinder shaft to being mounted on the chassis carriers and the cups fitted to the cylinder. No idea why as on both versions the removable carrier on the drive side allowed the cylinder to be removed without splitting the chassis.

 

 

Richard Tue, 22/10/2019

Thank you everyone for the replies. The valve lapping paste I used is water based and has been washed off thoroughly. But I believe you may be right about the bed knife not being as sharp as is should be. I may look at replacing it but it is hard getting new 14' ones of these without having to modify one to suit the qualcast mounting holes. 
 

I may also redo all the gaskets on the carby too as there is still one original one on there between the carby and the engine. 

 

wristpin Tue, 22/10/2019

I may look at replacing it but it is hard getting new 14' ones of these without having to modify one to suit the Qualcast mounting holes. 

For what its worth the part number was DR69 ! Your model Commodore was known as a DRC - no idea what that stood for. If its serial number starts J onwards it will have conventional "cone on cylinder" bearings.

Garfitts will make you one which, unless you already have a suitable blade to modify, may not work out any more expensive than having to buy a near match and modify it. **

Unless your blade is worn out, save your self time and angst by having it professionally skimmed on both the top and front faces. While you're about it have the cylinder properly ground. If you are having the bottom blade skimmed do NOT remove it from the sole (carrier) plate. However, depending on what equipment is used to skim it, the curved brackets at each end may obstruct the wheel head. If so carefully remove just one rivet from each and fold the bracket out of the way. When the job is done, return the bracket to its original position, and replace the rivet. With the assembly reinstalled into the chassis the rivets will be held in place.   

All goes to show that back lapping a blunt cylinder is no substitute for proper grinding. Once ground, maintain the edge by back lapping if you are so minded.  

** Edit. Whoops, just seen where you are!!

 

hortimech Wed, 23/10/2019

Two things spring to mind, if I remember correctly, you can adjust the the cylinder bearings, so, are they adjusted correctly, perhaps a bit tight ? The other thing is, just sharpening the bottom blade will just be like sharpening one half of a pair of scissors, you need to regrind the cylinder as well.