zdude1967 Posted October 9, 2010 Share #1 Posted October 9, 2010 The other day I had my 260 out and the pedal got stuck under the floor mat. In any case it revved pretty high before I could release it. Now I have a loud valve tap and the # 5 cylinder seems to have lost compression. I did not do a compression test but rather a simple finger over the spark plug hole. Could I have bent a valve? It over revved for maybe 20 to 30 seconds. Can these Z engines typically have piston to valve contact. The head was rebuilt 500 miles ago, timing chain etc all new. Any ideas or suggestions would be appreciatedThanksGlenn Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/ Share on other sites More sharing options...
ktm Posted October 9, 2010 Share #2 Posted October 9, 2010 Valve float is a possibility if you pegged it past redline for that long. Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333018 Share on other sites More sharing options...
doradox Posted October 9, 2010 Share #3 Posted October 9, 2010 The other day I had my 260 out and the pedal got stuck under the floor mat. In any case it revved pretty high before I could release it. Now I have a loud valve tap and the # 5 cylinder seems to have lost compression. I did not do a compression test but rather a simple finger over the spark plug hole. Could I have bent a valve? It over revved for maybe 20 to 30 seconds. Can these Z engines typically have piston to valve contact. The head was rebuilt 500 miles ago, timing chain etc all new. Any ideas or suggestions would be appreciatedThanksGlennA lash pad could be cocked and holding a valve open. I'd pull the valve cover and take a peek.Steve Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333020 Share on other sites More sharing options...
LeonV Posted October 10, 2010 Share #4 Posted October 10, 2010 Yep, pull the valve cover and inspect the valvetrain for loose parts. I'm not getting where "valve float" comes into play here. Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333025 Share on other sites More sharing options...
rxsleeper Posted October 10, 2010 Share #5 Posted October 10, 2010 Pull the cover, I will bet a cup of coffee that you popped a rocker off, maybe even two. I routinely take my Z to 7k rpm, the rev limiter is set to 7100 and occasionally I pop a rocker off. FWIW, at my last race I spent most of the time between 6200-7000 in 3rd or 4th gear. While it is certainly possible you may have damaged something, it is unlikely.If you haven't driven it, the lash pad(s) is probably lying in the head.Re-adjust your valves, factory settings, and I will bet you will be fine. I ran 13 laps of a race on 5 cylinders, I popped #5's intake and exhaust rockers off. They were lying in the head and the lash pads made it to the pan. Replaced everything, re-adjusted the valves Compression test and leakdowns were good enough to race the same engine another 6 months.YMMV,Paul Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333026 Share on other sites More sharing options...
ktm Posted October 10, 2010 Share #6 Posted October 10, 2010 (edited) The other day I had my 260 out and the pedal got stuck under the floor mat. In any case it revved pretty high before I could release it. Now I have a loud valve tap and the # 5 cylinder seems to have lost compression. I did not do a compression test but rather a simple finger over the spark plug hole. Could I have bent a valve? It over revved for maybe 20 to 30 seconds.I said valve float could be a possibility. Pedal stuck, revved pretty high for 20 to 30 seconds, etc. A popped rocker would not cause a loud tap; a cocked rocker could but if the rocker is lying in top of the head if would not tap. If a rocker popped off the cylinder would build pressure as well as the valve would always be closed.Pull the valve cover and take a look. It could be a loose rocker, cocked rocker, etc. I would also run a compression test to verify your assumption. If #5 is low, run a leak down (or just run a leak down test first). Edited October 10, 2010 by ktm Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333033 Share on other sites More sharing options...
zdude1967 Posted October 10, 2010 Author Share #7 Posted October 10, 2010 The rockers are on... two were very loose and the lock nut was loose as well. Seems like the lash pads are in and correct. The retaining spring (not the valve spring) seems ok as well. Any suggestionsThanksGlenn Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333035 Share on other sites More sharing options...
ktm Posted October 10, 2010 Share #8 Posted October 10, 2010 Which two were loose? You really need to run a compression test or preferably a leak down test. Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333036 Share on other sites More sharing options...
Gary in NJ Posted October 10, 2010 Share #9 Posted October 10, 2010 A leak down test (aka diffential compression) is the tell-all method. Harbor Freight has a tester for around $30. Not professional grade mind you, but it is something you will not often use. Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333052 Share on other sites More sharing options...
zdude1967 Posted October 10, 2010 Author Share #10 Posted October 10, 2010 The number 4 intake and both # 5 were loose. I guess what I was/am hoping for is some issue in the head that can be corrected with the head on...bound springs or something. The lash pads seem fine, the rockers do not seem to be bent, I would have to remove them to make sure. My understanding is the leak down test will determine where the compression is going, exhaust, intake or rings/piston. My goal at this point is to hopefully determine the cause to be in the head and able to repair without head removal.Can these engines make piston to valve contact without snapping the timing chain.Glenn Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333076 Share on other sites More sharing options...
ktm Posted October 10, 2010 Share #11 Posted October 10, 2010 Can these engines make piston to valve contact without snapping the timing chain.Glenn Yes, which is why I said valve float. Google valve float for a better understanding. However, I am not saying you experienced valve float only that valve float can cause piston to valve contact. If the rockers were loose then that could explain the louder tap. When I had a hydraulic lifter collapse on my L28ET it was a very loud tap. Do a compression test or preferably a leak down test. We are only guessing at this point. A finger over the hole is not really a valid test. Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333084 Share on other sites More sharing options...
Diseazd Posted October 10, 2010 Share #12 Posted October 10, 2010 You probably did bend a valve if you've got stock valve springs. Just run a compression check......if you bent a valve, you'll read "0" in that cylinder. If you put your finger over the plug hole and it didn't push compressed air against your finger, my guess is you're running "0" compression in that cylinder. Link to comment https://www.classiczcars.com/topic/37341-over-revved-and-no-compression/#findComment-333088 Share on other sites More sharing options...
Yarb
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