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Timing skipped while working on my car?

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88BB8B

Proven Member
179
18
Oct 14, 2016
Coldstream, BC_Canada
Hey guys, so I'm pretty perplexed by this. I double and triple checked my timing and tension as I was putting my motor together. Turning the motor many times. The tensioner was such that the pin slid in and out freely. (And yes I removed it before installing the cover.)
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I checked it again after I got the cover and crank pulley on and remember those marks lining right up perfectly because it was really satisfying.
Fast forward to today and I have the motor in the car and almost ready to crank and I go to quadruple check my timing and it's off by 2 teeth. It seemed that the crank had skipped clockwise by 2 teeth. The only time I could see this happening was when I was hammering the crank bolt tight (my impact has bit the dust) pulling things apart I found that the tensioner had changed its position, it was now a little more extended and I could only get the pin in the first 2 holes, but I reset it with a vice and installed the pin properly.

I was worried about the valves, I turned the motor super super slowly and carefully when I checked the timing. I only turned over 2 times before I realized it was off. I didn't feel any obvious hard spots that would indicate hitting a valve. And I felt really good compression (should've had the spark plugs out) I did a quick check by blowing down into the sparkplug holes while playing with the cams. I could hear when valves were open and which valves were open, and I could hear when they were closed and things seemed as sealed as I would expect a never run set of rings to seal. To further my luck I'm running forged pistons with big valve reliefs, but stock valves, and cams, and most of my lifters are really floppy right now. So hopefully we escaped that without damage.

the belt could have been slightly loose, but it wasn't like the it was slack or anything I was getting like a mid G when I plucked between the cam gears, LOL. And I've been continually checking while I work on the car to ensure tension hasn't released for some reason.

I obviously want to make sure this is put together all correctly and this doesn't happen when the car is running. I've read the VFAQ on timing and watched a bunch of videos, and I'm doing everything correctly as far as I can tell.
Or was it just that I put barley not enough tension on the belt and that was enough to allow it to skip by hand?
 
If anything too much torque. I checked against my photos before I loosened it and it hadn't changed angle. I'll torque it properly this time for sure though. I did turn it backwards a little bit, to realign the gears if I went past, maybe just in messing with the pulleys and belts. Through all my reading I missed that, that could cause the timing to jump. That could be it.
But would that be more likely to cause the crank to slip counterclockwise?
 
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I've never thought about this exact issue but I think dwb's suggestion that the problem was caused by turning the crank counter clockwise makes sense.

When you turn it CCW the hydraulic tensioner compresses. The crank pulls a loop of belt over to the (normal) tension side -- the engine's left side. What happens when you turn it forward again is probably luck -- depends on the set in the belt from packaging, stiffness, etc. -- but the slack could move down under the crank pulley allowing it to skip a tooth or teeth before taking hold and pulling the slack out. Since the actual skipping occurred while going forward the crank would be clockwise from the proper place.

Some Mitsu engines have fixed tensioners -- they're spring adjusted and then locked in place. Good news is that if correctly adjusted and tightened you usually get away with turning them backward a bit. Bad news is that belt wear is not taken up automatically -- you must manually retension the belt periodically and if you don't you will get the dreaded on-the-road tooth skip.
 
I routinely turn it backwards a little however I immediately turn it forwards after. I have done this to just remove slack from the firewall side and not have to turn the motor all the way around again.
At the end if the day only 2 things matter. All the marks are aligned and tensioner protrusion is correct. I have preached this for literally decades. No matter what methods you use to get a belt on etc you cannot properly tension unless all the slack on the firewall side is gone. Occasionally I will use a wrench on the exhaust cam gear and turn it back just a few teeth CCW then turn it forward at the crank. This allows me to line marks up again AND ensures there is no slack. All the slack will be on the tensioner side so you can set the pulley correctly. If you turned it over 6 times afterwards and marks were still good and tensioner was still good I'm guessing something had to have happened while doing crank bolt. I tighten crank bolt before I do the belt with a low setting on the impact then I follow up with a torque wrench wrench after belt is installed.
As far as valves are concerned you should be fine.
Hope this helps.
 
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