CV740 27hp bogs severely under average load

[Sockmonkey]

Check the governor linkage/settings.

 

[STEVES]

do both intake valves have the same 'deposit' on the stem?

 

[LMPPLUS]

Sounds like you have a compromised head gasket.

 

[biggertv]

Sounds like a Fuel/Air Delivery problem.

 

[CWatters]

I know you suction tested the fuel feed and changed the filter but twice now I've had insects cause my mower to bog down under load. In both cases I disconnected the fuel line after the filter and because fuel drained out I assumed it was all OK. In one case it would run for an hour and cut 1.5 acres before bogging down! I would drain your fuel tank and dry the inside with rags. Vacuum out any debris. In one case the insects partially blocked my fuel switch which on my mower is before the filter. So my filter looked OK, some fuel drained out but the line was still blocked just enough to be an issue under load. Idle and unloaded it ran fine.

If not already checked I would also remove the deck belt and turn the blades by hand to check they aren't partly binding.

Compression check both cylinders.

 

[OzPete]

Worn valve lifters will give that symptom. They are Hydraulic Lifters, and they wear out.
We had a similar issue on a CH740 in a Walker. Replacing all four Hydraulic Lifters resolved it straight up.

 

[doug9694]

Worn valve lifters will give that symptom. They are Hydraulic Lifters, and they wear out.
We had a similar issue on a CH740 in a Walker. Replacing all four Hydraulic Lifters resolved it straight up.

I agree!

 

[TobyU]

Can anyone help here?

I'm absolutely baffled by this issue. I am now 100% out of ideas. The problem presents itself in two ways.

1. Within the first few moments of average load mowing, the motor RPMs dip significantly (about 5-10 seconds after engaging everything ). The greater the load, the more the RPMs dip. I hooked up at tachometer and it shows 1200 at low idle, 3600 "high idle", and during mowing it will usually sit right around 1700 RPMs depending on load (barely enough to mow). If it's a downhill, it'll go to 2500 or so, and small uphills dip to the point of basically being at low idle speed. If at any point I pull back the sticks to a stop (blades engaged), rpms will slowly pull back up to 3300+ RPMs (in 3-5 seconds). Push the stick forward and it dips back into the 1700 range.
2. Using clean plugs, cylinder #1 plug is always turns sooty black (carbon fouled) after a ten minute mow. Cylinder #2 is white to very light brown (varies). But never rich like #1.

I've put together a spreadsheet to track this problem. Just because after checking EVERYTHING I wanted to make sure I didn't miss anything. I know it's a lot, but it will show that I've tested all the typical problems.
The mower is a 2005 Gravely 260z (zero turn) that I bought a month ago for cheap.
I'm completely at my end here and would be happy for any experienced ideas.

View attachment 66771

One thing that caught my attention in your other post was that you said when it's running with no load it's about $3,600 RPMs which I feel is a little bit high but not terrible. Most of these mowers are designed to run no more than 3,400 today but again, I forget what it's actually on and I think it was a larger gravely but don't remember if it was commercial residential but regardless 3600 is not terribly fast.
But when you said you engaged the blaze in it bogs down and then comes back up that is a classic sign of one of two things...

Either the engine power is low and it's weak for whatever reason or the governor is crazy out of adjustment or not working at all and not allowing the carb to do its job.

I I find the quickest diagnosis for this is to actually watch the carburetor throttle plate from the top and to first verify that it can open all the way and then when you pull the blades on watch to see if the carburetor throttle plate is opening all the way during that entire time that it's bogging down and coming back up too freewheeling speed.
I have seen more than one engine that sounded like it was lawn power etc when in fact it was just a governor adjustment.
Other times, the governor has been busted and they have basically adjusted the throttle so it will go no faster than somewhere around 3,600 RPMs and of course it will bog when you put a load on it because the throttle cannot move on its own.

 

[Savage3]

Try spraying carb cleaner around intake where carb mounts. If RPM changes, there is an air leak.

Sounds like two possibilities worth investigating:

1. Intake leak causing AFR to be off.

2. I've seen the same symptoms when one coil is not producing proper spark. It sparks, but, not enough spark on one coil. Just because they are new, doesn't mean they work properly. Check the resistance on both coils (static). See if they are about the same.
If you don't have a spark tester or proper ignition tester, pull plugs and test dynamically while cranking engine. See if spark is weak or red on each plug. Also, check coil and kill wire for short to ground.

 

[smallenginesmd]

Can anyone help here?

I'm absolutely baffled by this issue. I am now 100% out of ideas. The problem presents itself in two ways.

1. Within the first few moments of average load mowing, the motor RPMs dip significantly (about 5-10 seconds after engaging everything ). The greater the load, the more the RPMs dip. I hooked up at tachometer and it shows 1200 at low idle, 3600 "high idle", and during mowing it will usually sit right around 1700 RPMs depending on load (barely enough to mow). If it's a downhill, it'll go to 2500 or so, and small uphills dip to the point of basically being at low idle speed. If at any point I pull back the sticks to a stop (blades engaged), rpms will slowly pull back up to 3300+ RPMs (in 3-5 seconds). Push the stick forward and it dips back into the 1700 range.
2. Using clean plugs, cylinder #1 plug is always turns sooty black (carbon fouled) after a ten minute mow. Cylinder #2 is white to very light brown (varies). But never rich like #1.

I've put together a spreadsheet to track this problem. Just because after checking EVERYTHING I wanted to make sure I didn't miss anything. I know it's a lot, but it will show that I've tested all the typical problems.
The mower is a 2005 Gravely 260z (zero turn) that I bought a month ago for cheap.
I'm completely at my end here and would be happy for any experienced ideas.

View attachment 66771