Hobbyist Forums banner

1 - 20 of 25 Posts

·
Registered
Joined
·
15 Posts
Discussion Starter #1
I have a Generac portable generator gp7500 series that is a couple of years old. I used it for 4-5 days almost continually (converted to natural gas). I shut it off and then put it away. Several weeks later I tried to start it and i get virtually nothing-- just an occasional backfire. It has about 100 hours of usage. I have tried to solve the starting issue but with no luck. I have done the following.

1. Changed oil.
2. Checked for spark
3. replaced spark plug and checked the gap
4. Disconnected low oil shutoff sensor
5. Tried to start with carb cleaner as opposed to the natural gas
6. Opened up the valve cover to make sure either one of the valves was not stuck (I have not checked the valve clearance yet)
7. Replaced the fuse that I found.

Would a suddenly mis adjusted valve be the cause of a complete lack of any ignition? Does anyone have any ideas with what might be the root cause of the failure to start.

Any suggestions would be greatly appreciated.

Thanks.

Marten
 

·
Registered
Joined
·
1,433 Posts
That is unusual, could be mouse damage to ign. kill wire and is grounding out at times. First thing that comes to mind given it ran so recently, and that it runs on NG. Fuses on GN units are for cranking/charging circuits.

Using carb cleaner as a spray prime is a great idea, but if the NG is flowing you may be flooding it out. It sounds like you have a good aptitude. Can you tell us how you tested spark?

Doubtful valve problem would totally prevent starting, but that doesn't mean you should overlook them. A compression test of any engine with a compression release still has a minimum of 60-80 lbs. If you have a tester, it will tell you if the valves are seating properly. Otherwise...since you know how to take the valve cover off, you can check to see if either valve has excessive clearance, and if so see if the spring retainers are at the same height. If one is sitting low, a valve might be sticking a open a bit from corrosion or foreign matter stuck under on a seat.
Depending on the engine version, lash would be 0.002" - 0.004", or 0.004 - 0.006", for both. I have seen generac engines become somewhat difficult to start, and run a little rough when lash needs to be set - usually around the 50-100 hr. mark for initial adjustment. As for their spark plugs...get an NGK BPR6ES or 5ES (5 is hotter that 6).
 

·
Registered
Joined
·
15 Posts
Discussion Starter #3
That is unusual, could be mouse damage to ign. kill wire and is grounding out at times. First thing that comes to mind given it ran so recently, and that it runs on NG. Fuses on GN units are for cranking/charging circuits.

Using carb cleaner as a spray prime is a great idea, but if the NG is flowing you may be flooding it out. It sounds like you have a good aptitude. Can you tell us how you tested spark?

Doubtful valve problem would totally prevent starting, but that doesn't mean you should overlook them. A compression test of any engine with a compression release still has a minimum of 60-80 lbs. If you have a tester, it will tell you if the valves are seating properly. Otherwise...since you know how to take the valve cover off, you can check to see if either valve has excessive clearance, and if so see if the spring retainers are at the same height. If one is sitting low, a valve might be sticking a open a bit from corrosion or foreign matter stuck under on a seat.
Depending on the engine version, lash would be 0.002" - 0.004", or 0.004 - 0.006", for both. I have seen generac engines become somewhat difficult to start, and run a little rough when lash needs to be set - usually around the 50-100 hr. mark for initial adjustment. As for their spark plugs...get an NGK BPR6ES or 5ES (5 is hotter that 6).
Paul,
Thanks for reply. It is possible that a rodent damaged a wire. Does the wire run to the panel? I will look around the unit.

This is the spark plug I bought. Champion N9YC (300) Copper Plus Spark Plug, Pack of 1
It is supposed to be compatible.

I did turn off the natural gas when I tried to start with carb cleaner.

I tested the spark by taking out the plug and then reinserting in the boot and grounding it against the valve cover.

I will check the spring retainers.

Marten
 

·
Registered
Joined
·
1,433 Posts
A spark plug not subject to compression doesn't provide enough load to test the coil. You need a gap of about 0.150". Cut the ground electrode off an old plug.
 

·
Registered
Joined
·
15 Posts
Discussion Starter #5
A spark plug not subject to compression doesn't provide enough load to test the coil. You need a gap of about 0.150". Cut the ground electrode off an old plug.
Paul,

So just cut the bottom part of the spark plug off and then see if it sparks? Not sure I follow you

Marten
 

·
Registered
Joined
·
1,433 Posts
If a 4-stroke engine having good spark and a shot of spray prime won't fire, either a compression or ignition timing issue exists. On such a new unit, seems like something stupid like a hanging valve. I'd check on the compression aspect first before considering ign. timing.
 

·
Registered
Joined
·
15 Posts
Discussion Starter #11
If a 4-stroke engine having good spark and a shot of spray prime won't fire, either a compression or ignition timing issue exists. On such a new unit, seems like something stupid like a hanging valve. I'd check on the compression aspect first before considering ign. timing.
So the fact that I can see the valves go up and down when I take off the cover does not preclude the fact that the valve could be hanging?

Marten
 

·
Registered
Joined
·
15 Posts
Discussion Starter #12
So the fact that I can see the valves go up and down when I take off the cover does not preclude the fact that the valve could be hanging?

Marten
Also, I took off the valve cover again and really watched the valves more up and down. They both seem to be moving the same amount I could smell the carb cleaner smell and hear the woosh of the valves. Could it really be hanging if I am seeing the movement like that?
 

·
Registered
Joined
·
1,433 Posts
Yes. That is why I suggested you see if the retainer heights in closed positions are relatively the same. Even so, had a Generac pressure washer that had corrosion (oddly enough they covered it under warranty) in which at least one valve wasn't seating causing a no-start situation. A compression or leak-down test can verify. Given you've cut off the NG and tried with spray prime only, have tested spark with an adequate load imposed, I figure you're down to a) compression problem, be it valve not seating properly or cam related issue, or b) ign. timing issue such as sheared flywheel key.
Again, a "known good" 4-stroke should fire on a spray prime unless an ignition or compression problem exists.
 

·
Registered
Joined
·
1,433 Posts
P.S. 'Cept for something totally stupid like a mud wasp cell blocking the exhaust...not common on 4-strokes though.
 

·
Registered
Joined
·
15 Posts
Discussion Starter #15
Did a compression check. Virtually no compression. When I opened up the valve cover again this time I noticed that the valve rod on the intake side was totally bent. I have attached a photo. Do you think it is just that Rod that is bad and can I replace it without disassembling the entire motor?

Thanks.
309405
IMG_20200802_141355683.jpg

Marten
 

·
Registered
Joined
·
1,433 Posts
Bent push-rods are usually caused by: valve seized in guide at one point (may not be seized after cooling down); cam timing issue allowing piston to strike valve.

In my first reply I eluded to a compression (valve) issue, although it is a rare event. Can't help but try to instill going back to basics when diagnosing, else you're assuming.

What you have to do now is a thorough inspection. Doubtful there is a cam timing issue, but easy way to check:
Before doing that, visually inspect the intake valve guide (inside the valve spring), try to see if it appears as if it's moved. Use the exhaust guide as a reference. This can occur from valve seizure. Corrosion, pick-up damage, over-heating can cause this, the point is if the guide moved the head (in commercial repair shops) is considered junk. Yes, I know some hammer it back into place and clean up the tolerances...but by the book it's junk.

In any case, you MUST replace that bent push-rod and set lash.

Cam timing may be an issue - outside chance, but for now...

If the guide didn't move, you may be able to lube the valve and coax it back into service. I have had some success just by spraying the crap out of the valve stem in the intake port and pulling the engine over repeatedly (and re-spraying, you know, rinse, repeat). Had some push-blowers that would seized a valve after sitting for months, but sometimes the valve has to come out and things have to be cleaned up properly. Things like a narrow wire brush on a drill or a reamer etc....

If you are able to free up the valve to your satisfaction, as I stated in an earlier post look at the retainer heights to see if they are the same...you're looking to see if the valve is now closing fully.

Cam timing...

Remove spark plug.
Remove pull starter.
With a probe (something non-metal, such as a plastic straw (unless you're skilled at this as a metal probe could be bent and jam or do damage)), rotate the engine to TDC after the exhaust valve closes, while using the straw on top of the piston to check for TDC. This will be TDC on the exhaust stroke, a period that has both valves open a tad, called valve overlap. At that point if you rock the flywheel/crank to and fro, the valves/lifters should be opening/closing in tandem, opposing each other.
 

·
Registered
Joined
·
15 Posts
Discussion Starter #18
Thanks for all your help. It sounds like there is a good chance I need to replace the entire head. Frustrating. Bought generac as opposed to a cheap alternative and it fails after 100 hours. They told me that since I did not check the valve adjustments that it was my fault (already out of warranty as I did not have a need to use it for the first couple of years as I did not lose power).

Marten
 

·
Registered
Joined
·
1,433 Posts
You're welcome.
Again, the only reason you should need to replace the head is if the guide moved. And that would definitely not be caused by lack of lash maintenance, let alone any lack of maintenance at 100 hrs. save for extremely harsh conditions. It could however have been caused by corrosion from damp storage conditions, or even short run time resulting in condensation. As a field rep, I once had to critique a failed engine on a generator for LifeStar set on a 10 story building roof-top. The repeated 5 minute run time combined with the harsh weather conditions resulted in so much corrosion from condensation, the piston seized and the piston literally broke in half. Not warranty I told them.

Be careful on warranty, as regardless, and I've had this argument with Generac and won, EPA emissions warranty mandates any OEM cover certain components for 2 yrs. against defects. One could argue valves would be one of them, but they're not itemized as such in their emissions warranty statement.
 

·
Registered
Joined
·
15 Posts
Discussion Starter #20
The guide moved unfortunately. I am debating whether to replace the head myself. I have already removed the head but I need to remove the valve springs and it seems like I might need a tool to do that. I have posted a picture of the guide. I will post some pictures of the top of the piston and bottom of the head. Curious if they look right to you.

I will try to call generac again and see if I can get them to provide some warranty coverage even though I am passed the 2 year point.

Thanks.
309423
20200803_144024 (1).jpg

Marten
 
1 - 20 of 25 Posts
Top