CV740 27hp bogs severely under average load

[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

You're overthinking the issue. If it runs you have to look at the basics. KISS(keep it simple stupid)
1) check cyl compression Should be close in both cylinders.
2) Spark: Solid blue visible
3) Clean fuel, no contaminates : water, dirt, old
4) Good constant fuel flow: disconnect line at carb (into a container) remove plugs and crank engine
5) Check that throttle cable is not slipping/moving under load (very common)
6) Perform all the above with belt(s) disconnected from deck/clutch: If engine runs strong connect belt(s) and run again.
If problem returns look for issue(s) with mowing/drive system.
PS: Remove top engine cowling/cover. Make sure no rodents have built a nest on top of your cylinder head(s) Another common problem that causes engine to run hot / poor performance
Retired owner of small engine shop for over 15 years .
Marc

 

[STEVES]

Line 30 and 31 of my OP checklist sort of verified that at least the intake seal isn't leaking (and this motor has no seal on the exhaust valve by design). But I removed the #1 valves yesterday and they don't seem loose in the guides. Not tight, but not loose either. Guides are positioned vertically in the heads correctly too and haven't slipped. And leak down confirms that the exhaust valve is closing properly.

At this point I just think my motor is possessed by a demon. Nothing but a slightly out of spec cam is the only thing I see even remotely 'wrong'.

My thought was piston on intake stroke sucking oil by intake valve stem seal. Not the static testing.

 

[lefty2cox]

This is awfully reminiscent of a fuel starvation issue under load. I'd be curious to find out how TobyU's suggestions panned out. Whether or not that throttle plate was actuating when under load. Could it be as simple as a loose or poorly positioned throttle cable anchor point? I read through all of the posts and didn't see anything relating to eliminating that. I apologize if I missed it or if this is off base.

 

[Turbodriven]

This is awfully reminiscent of a fuel starvation issue under load. I'd be curious to find out how TobyU's suggestions panned out. Whether or not that throttle plate was actuating when under load. Could it be as simple as a loose or poorly positioned throttle cable anchor point? I read through all of the posts and didn't see anything relating to eliminating that. I apologize if I missed it or if this is off base.

In my OP checklist I put a string on the governor lever (which is of course attached to the throttle) and under the rpm drop under load I pulled on the string to no effect. Full throttle has the throttle plate fully open (verified). So that tells me the throttle is full open under load. Also says to me that the governor isn't weak and backing things down under load either. When I got into the motor this was confirmed. Governor lever isn't binding, and the gear/flywheel assemble operates as it should with no defects. Also, fuel starvation wouldn't be my number one answer because I'm getting thick carbon fouling on the #1. Which at a glance and from other tests doesn't appear to be from oil. So I can't be rich and lean at the same time. That said, when I put this engine back together with a fresh rebuild I'm going to tear down my carbs piece by piece and make sure everything is right in there too. Might switch back to the (cleaned) OEM carb even.

That said, there have been a lot of people suggesting things that have already been covered. Like head gaskets. They are new. So I apologized if I didn't respond to everyone.

 

[toolman034]

Have you checked valve stem clearance that is very important for peak power? I did a repair not too long ago that had zero clearance the motor would still run but had very little power. That was due to wear in the seat and causing the valves to open too early and closing late alowing fuel to escape. I ground the stems to regain the clearence and the motor straightened out an run as it should with plenty of power. The almost same would happen if you had too much clearence it would cause the valves to open late and close early. Just a expierance that I had that may help.

 

[Turbodriven]

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'm replacing the one with wear on it (see post #52), and I'm disassembling the other three and cleaning them. That said, I've pulled one apart already and it looked super clean with no issues.

On that note however, and I talked with someone else about this too, but I'm not 100% sure what kind of oil this had when I bought it. It looked like new oil, but maybe someone put some crazy wrong oil in it. Not totally sure how even the worst oil weight would cause this, but I know these lifters require a certain type to operate correctly for sure. So in the rebuild, I'm definitely going to eliminate this by confirming that the correct oil is in it.

 

[lefty2cox]

This one is a stumper. What about the safety circuit? Can that end up intermittent and cause this? Or would that sound more rough rather than the smooth drop you have in RPM. Both the hand levers and the blade require bum in the seat. Have you tried to disable the seat safety temporarily? I apologize if this has been covered.

 

[Turbodriven]

This one is a stumper. What about the safety circuit? Can that end up intermittent and cause this? Or would that sound more rough rather than the smooth drop you have in RPM. Both the hand levers and the blade require bum in the seat. Have you tried to disable the seat safety temporarily? I apologize if this has been covered.

It has not. However the seat is already disabled. I'll double check but I'm pretty sure it's just the two seat switch wires twisted together and insulated. If there was something "off" in that I'd feel like it would cause problems in all levels of operation though.*shrug. And certainly wouldn't cause carbon fouling on only #1.

That said, I've got this gut feeling that #1 carbon fouling might be a red herring of sorts. One thing brought up to me was that neither side runs well with the other unplugged. It's not like #2 runs great when #1 is unplugged. They both struggle to idle when run independently. That said, whatever is wrong is affecting both cylinders. Just affecting them a little differently. Perhaps just because of the design of the motor. #2 is lower with a slightly longer intake runner (being a cv motor). In any case, seeing #1 black and sooty and #2 white/tan initially made me think this was a problem with only #1. Now I'm not so sure.

In the end here soon though I'll have a freshly rebuilt and inspected motor. And if that doesn't fix it then I'll know that it HAS to be something else besides the motor.

 

[lefty2cox]

Gotchya.

Again, this was a long one so sorry if this was also covered and I don't remember, but did you do a leak down 1/2 way down and at the bottom of the stroke?

 

[Turbodriven]

Gotchya.

Again, this was a long one so sorry if this was also covered and I don't remember, but did you do a leak down 1/2 way down and at the bottom of the stroke?

No, I didn't. Just TDC. I guess it's a moot point at this stage since I just honed both cylinders for new rings. However I will say that both cylinders looked really good. A bit glossy but with visible cross hatching in both still. No scoring or anything. I don't have any bore measuring tools but I don't see any reason to suspect anything wrong.