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No compression in one cylinder.. anything I can do before pulling head?

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kerb12
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No compression in one cylinder.. anything I can do before pulling head?

Post #1 by kerb12 » Wed Jul 10, 2019 10:08 pm

So I finally put the top end of my engine back together (a '66 200 with a later model head) after pulling the head to get a crack in the intake manifold fixed. While the machine shop was fixing that they also milled it a bit (it had previously been milled as well). I had asked the machinist to try to get to 48cc's.

When I finally went to start it, it actually cranked over but it idled really poorly, so I ran a compression test and cylinders 1-4 and 6 were all 140-150... BUT cylinder 5 has zero compression. It doesn't even bump the gauge.

From what I've read, this seems to point to a valve stuck open since if the head gasket didn't seal properly it would most likely cause low compression in an adjacent cylinder too. Does that sound right? Anyone have any advice on how I could troubleshoot this a bit more before pulling the head again?

I took the valve cover off and when I crank it, both springs on that cylinder compress and pop back, but I'm not sure exactly what I'm looking for. Nothing stands out as looking really wrong compared to the other cylinders though.

I know some people have shimmed their rocker arms after milling, which I didn't do, but if the problem was that it's too close and not letting the valve come back up all the way, it would affect all of the valves not just for one cylinder right?
'66 Mustang convertible 200ci/C4 - 2v converted large log, Autlite 2100, Duraspark II with MSD 6A, Comp Cams 260H, 2" dual exhaust

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Econoline
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #2 by Econoline » Thu Jul 11, 2019 3:32 am

Either way you're going to pull the head. Might be a valve problem, might be ring lands gone on the #5 piston.
It ain't gonna fix itself

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #3 by drag-200stang » Thu Jul 11, 2019 9:10 am

Sometimes when installing the rocker shaft the pushrod can get out of the lifter cup and ride at a higher location and hold the valves open ...Check that this is not your problem.
Shimming the stands is not the right way to fix too long pushrods, it will change the as designed rocker to valve geometry.
Your push rods should be fine, you milled the head but used a thicker gasket netting no real change that the lifter cannot handle.
Edit...I see that you have changed the cam and lifters from stock , it is possible that with those changes that the pushrod is to long now.
Try temporary shimming the stands to see if that helps ..Still check the pushrod out of proper location first.
Last edited by drag-200stang on Thu Jul 11, 2019 9:23 am, edited 1 time in total.
66 Mustang Coupe
200 turbo w/lenco 4-spd
stock adj. rockers, stock timing set, ARP studs
best 1/4 mile ET 9.85/best mph 139 on 8 lbs progressing to 15 lbs boost
Went 9's when 10's was fast.

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #4 by wsa111 » Thu Jul 11, 2019 9:21 am

Put air into the cylinder via the spark plug threads & listen for where the air escapes. Out the carb, intake valve, exhaust pipe exhaust valve, crankcase breather, piston or ring problem.
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #5 by kerb12 » Thu Jul 11, 2019 3:21 pm

Thanks guys, I think this is my plan - First loosen the rocker arm and wiggle the pushrods around and see if they seat better, retighten, and compression test again. If that doesn't make a difference I'll try the air in the cylinder test... maybe also loosen the rocker arm again and redo the air test and see if the valves close when there's no pressure on them from the rockers.

I just want to make sure I exhaust all the easy tests before tearing everything down again...
'66 Mustang convertible 200ci/C4 - 2v converted large log, Autlite 2100, Duraspark II with MSD 6A, Comp Cams 260H, 2" dual exhaust

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #6 by jamyers » Thu Jul 11, 2019 4:23 pm

wsa111 wrote:Put air into the cylinder via the spark plug threads & listen for where the air escapes. Out the carb, intake valve, exhaust pipe exhaust valve, crankcase breather, piston or ring problem.


That's the Numba One answer right there.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #7 by Econoline » Fri Jul 12, 2019 12:03 am

kerb12 wrote:Thanks guys, I think this is my plan - First loosen the rocker arm and wiggle the pushrods around and see if they seat better, retighten, and compression test again. If that doesn't make a difference I'll try the air in the cylinder test... maybe also loosen the rocker arm again and redo the air test and see if the valves close when there's no pressure on them from the rockers.

I just want to make sure I exhaust all the easy tests before tearing everything down again...


I think that's a good plan. But outside of some problem you can see, find and fix within the valve train, you're pulling the head when you know the problem lies within. You'll see right away what the problem is when you pull the head. There's really no point in testing to figure it out beforehand imo with zero compression and no decipherable problem under the valve cover. If you have a good borescope you could stick it in the spark plug hole and it may tell you something. You can do a leak down and if it's a valve it may ease your mind, but you're still pulling the head.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #8 by drag-200stang » Fri Jul 12, 2019 9:37 am

It may take more than wiggling the pushrods, you have to knowingly feel that it is in the cup...The reason I know about this is It happen to me and it was also #5...Your problem may be something else but it does happen.
66 Mustang Coupe
200 turbo w/lenco 4-spd
stock adj. rockers, stock timing set, ARP studs
best 1/4 mile ET 9.85/best mph 139 on 8 lbs progressing to 15 lbs boost
Went 9's when 10's was fast.

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #9 by wsa111 » Fri Jul 12, 2019 7:39 pm

Take off the rocker assy & do a cylinder air check. If the problem is fixed then you have a pushrod & or a rocker arm problem.
Do you have adjustable rocker arms?? If not by milling the head you have a pushrod in that cylinder that is too long.
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #10 by kerb12 » Fri Jul 12, 2019 7:48 pm

Ok so here's what I just did... I loosened the rockers enough that I could move the pushrods around and made sure they were all sitting in the cups - no difference. Then I took the rockers off completely and put a straightedge across the tops of the valves and to my eyes they looked good, but I guess it could be off just slightly enough that I can't see it. Then I swapped pushrods from #5 (the bad cylinder) to #2 (which measured good) to try to rule out a bad pushrod. Torqued everything back down and #5 was still bad.

I tried the air test, but when I'm shooting air in it's pretty loud and I can't for the life of me hear if there is hissing from anywhere... but I got bad ears.

I guess my last try is to temporarily shim the rockers and see if it changes anything. What's a reasonable size? I have some hardened 1/8" washers leftover from my head studs. Is that too tall?
'66 Mustang convertible 200ci/C4 - 2v converted large log, Autlite 2100, Duraspark II with MSD 6A, Comp Cams 260H, 2" dual exhaust

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #11 by kerb12 » Fri Jul 12, 2019 7:54 pm

wsa111, I do not have adjustables, I have the stock unadjustable and stock pushrods.

By the way, the reason I'm avoiding just pulling the head is because I only have about 5k miles on the rebuilt engine, so unless something went really bad I can't imagine it's the rings/piston. And I just had a really reputable machinist rebuild the head before I put it on. I'm sure he could have made a mistake and not noticed a bad valve or seat, but I'm still pretty new to working on engines so I think it's far more likely that I just screwed something up on my end.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #12 by wsa111 » Fri Jul 12, 2019 8:44 pm

Did the engine run ok for the 5K??
Sorry about your hearing, but the proper way to put air into a cylinder is remove the check valve from your compression tester & put 100# plus into the cylinder.
Open the throttle to full & if its an intake you will hear it easily. Same with the exhaust & since you have the valve cover off you will easily hear a leak.
As a professional i could diagnosis your problem within 30 minutes.
You might need a second opinion.
Good luck.
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #13 by Econoline » Fri Jul 12, 2019 9:37 pm

Pull the valve cover and turn the engine over with a wrench, cw, until the you are sure both valves on #5 are closed. Like right after the exhaust valve on 5 closes. Then you can pressurize the cylinder for a leak down test like Bill is describing. If the test proves a major valve leak, then pull the rocker arm assembly and retest. If the problem goes away, you know there is something wrong with the valve train on #5. There are only two things I can think of(outside of a long pushrod) that would cause a valve to hang open on your engine, one would be if the pushrod is not in it's cup in the lifter, the other is a problem with that lifter causing it to somehow act like a solid lifter and have it's plunger stuck fully extended. You should definitely check out the valve train. Don't get me wrong, if you can avoid pulling the head you should. Nothing like taking a major step that was totally unneeded and find it was just the seating of the pushrod. You may be able to see with a flashlight if the pushrod is seated into the lifter properly though the little hole. Easy fix. Unfortunately, if it's a lifter problem you'll have to pull the head, but at least you will be pointed in the right direction in the unlikely event that that is the case. Do the leak down as described above, get some help if you need to and it will help guide you into the inevitable solution. Hopefully it's the pushrod. It's all fixable no matter what the problem turns out to be unless there's a hole in the cylinder wall ;) I'm joking of course.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #14 by wsa111 » Fri Jul 12, 2019 9:47 pm

Seth, it may well be a hardened exhaust valve seat coming loose in the head????
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #15 by Econoline » Sat Jul 13, 2019 1:27 am

Yeah there's another one. If it's had seats installed. That's always kind of worried me about exhaust valve seats. Maybe it's irrational but it kept me from doing it. Well that and the added expense. Especially after my machinist said not to worry about it anyway b/c I didn't need hardened seats in his opinion. Heads gotta come off for that one to. I suppose he could have a bent valve as well. Maybe the pushrod wasn't in it's cup and it bent the valve stem?
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #16 by kerb12 » Sat Jul 13, 2019 9:03 am

Thanks for the more tips. I'm going to see if I can borrow an actual leak down tester from someone and try it with the rockers on and off. Will let you know what that shows.
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No compression/one cylinder.. tests before before pulling head?

Post #17 by chad » Sat Jul 13, 2019 8:53 pm

"Can't think of(outside of a long pushrod)..."
bent PR?
"Big thing is only make one change at a time. Change 2 or more things at a time it becomes difficult to figure which change helped or hurt" turbo2256b » 1/16/2017
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #18 by wsa111 » Sun Jul 14, 2019 9:10 pm

If you bent a valve, pull the spark plug & shine a led light onto the piston top. You will see a mark or dent in the crown if so. I wish you the best.
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #19 by kerb12 » Mon Jul 15, 2019 8:43 pm

Alright I have some updates. I was not able to get a leak down tester but I was able to get a hold of a mechanic smoke tester... I already had the rockers off, so I pumped some smoke in and did not see any smoke come out of the carb or exhaust. Good sign since they should be as closed as they can get. Just as a sanity check, I rotated the engine until one of the valves was open and tried again, and saw smoke come out of the carb. Then rotated until the other valve was open, and saw smoke come out of the exhaust... so far so good.

Put the rockers back on and rotated until both valves appeared to be as closed as I could get them, and... still no smoke out of either carb or exhaust? I pulled the oil dipstick and no smoke out of that either.

One thing I did notice was an odd gurgling sound but I couldn't tell where it was coming from.

Could it be that my head gasket didn't seal properly and that the leak is towards the cooling channels and my air is pushing into the coolant?
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #20 by Econoline » Tue Jul 16, 2019 12:41 am

You can get a tester on loan from your local parts house for a test of a leak from the head to the cooling system. I think they call it a coolant system pressure tester or head gasket coolant pressure tester... something like that. Basically you screw it on in place of the radiator cap and follow the instructions. If the head gasket is leaking into the coolant it raises the pressure and I think there's a chemical test to that can be used while performing the test that's even more conclusive. There's no way that would cause a dead cylinder in a compression test. Or you'd already know the head gasket is a huge problem.

It sounds like maybe it was the pushrod. You tested it w/ the rockers off and all was good. Put the rockers back on and got nothing out of the crankcase. I've got my fingers cross for you.
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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #21 by drag-200stang » Tue Jul 16, 2019 9:33 am

It has been suggested before , you need to pressurize your cylinder with 120 lbs or air.
Get a fitting or modify what you have, I knocked out the center out of the spark plug and welded a air chuck that fit my hose.
Fill the radiator flush to the top...If you have a leak to the water passage you will see bubbles.
Valves have to be closed when testing.
66 Mustang Coupe
200 turbo w/lenco 4-spd
stock adj. rockers, stock timing set, ARP studs
best 1/4 mile ET 9.85/best mph 139 on 8 lbs progressing to 15 lbs boost
Went 9's when 10's was fast.

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #22 by kerb12 » Tue Jul 16, 2019 11:37 am

Econoline, I didn't see anything out of the valves or crankcase with the rockers off, but I also didn't see anything with the rockers back on, and I still had no compression. So I'm thinking pushrods are fine, and valves appear to be fine too.

drag-200stang, I'll try to get a hold of a leak down tester so I can pressurize the cylinder. With the gurgling I heard when I was pumping smoke into the cylinder though, I'm pretty sure I'll see bubbles in the coolant when I do that.

I guess the big question is whether a new head gasket fixes the problem, or if I have a crack somewhere...
'66 Mustang convertible 200ci/C4 - 2v converted large log, Autlite 2100, Duraspark II with MSD 6A, Comp Cams 260H, 2" dual exhaust

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #23 by kerb12 » Wed Jul 17, 2019 9:41 pm

Ok well I feel kinda dumb, I didn't realize that the check valve in the end of the compression tester hose can just unscrew out... so did that, blew air in through the hose and even at only 40 or so psi I could hear air rushing out somewhere. Just to make sure I had the engine turned enough, I pulled the rockers off again, and same thing.

I used a stethoscope and could hear air flowing through the headers, and then I could feel it coming out of the tailpipe. No bubbles in the coolant.

So yeah, I guess head gasket was sealed fine and it's a bad exhaust valve or seat? I guess the head is coming off this weekend. Is there anything basic I can try to repair myself? Or am I better off just bringing the head back to the machinist and have it looked over again?
'66 Mustang convertible 200ci/C4 - 2v converted large log, Autlite 2100, Duraspark II with MSD 6A, Comp Cams 260H, 2" dual exhaust

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #24 by drag-200stang » Thu Jul 18, 2019 9:01 am

I do not think that you will know till you take the head off and look at it .
Let us know what you find .
66 Mustang Coupe
200 turbo w/lenco 4-spd
stock adj. rockers, stock timing set, ARP studs
best 1/4 mile ET 9.85/best mph 139 on 8 lbs progressing to 15 lbs boost
Went 9's when 10's was fast.

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #25 by kerb12 » Mon Jul 22, 2019 10:50 pm

Ok, got the head off tonight. Honestly, nothing stands out to me.. Attached are a few pics of cylinder #5. The head gasket didn't look like anything blew out or was burnt up.

I'll probably bring it back to the machine shop tomorrow and have them look it over.
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'66 Mustang convertible 200ci/C4 - 2v converted large log, Autlite 2100, Duraspark II with MSD 6A, Comp Cams 260H, 2" dual exhaust

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #26 by drag-200stang » Tue Jul 23, 2019 6:05 pm

It is puzzling.
Please let us know what they find.
66 Mustang Coupe
200 turbo w/lenco 4-spd
stock adj. rockers, stock timing set, ARP studs
best 1/4 mile ET 9.85/best mph 139 on 8 lbs progressing to 15 lbs boost
Went 9's when 10's was fast.

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No compression in one cylinder.. check w/o pulling the head?

Post #27 by chad » Tue Jul 23, 2019 6:11 pm

that's not the wrong sidea da head 2 look @?
"Big thing is only make one change at a time. Change 2 or more things at a time it becomes difficult to figure which change helped or hurt" turbo2256b » 1/16/2017
Chad - '70 LUEB on '77 frame (i.e. PS, D44, trapezoidal BB 9", 4.11), 250, NV 3550 & DSII to B transplanted, "T" D20/PTO, 2" SL, 1" BL, 4 discs, 33"X15", tool boxes, etc. Seeking: Hydraulic gear motor for Koenig pto. chrlsful@aol.com (413) 259-1749

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #28 by kerb12 » Wed Jul 24, 2019 2:59 pm

Well I stopped by the machine shop today, and he showed me that the back of the valve had a thick coating of film on it, and starting to go up the stem. He said it was caused by bad gas and this was preventing it from closing all the way. He also saw evidence of it building up on some of the other valves too, but not as bad.

The gas was only about a year old, and I usually keep a bit of stabil in it too, but he said he's seeing this more and more every year with gas getting worse and recommended I clean out the fuel tank and blow out all of the fuel lines. He said once the old gas hits the heat of the combustion chamber it gets a consistency almost like Permatex #2.

He's going to give the head a good cleaning, reassemble it, and pressure test it before giving it back, but he seemed pretty confident that was the problem. Luckily I had only idled it for maybe 5-10 minutes, so it wasn't long enough to start burning up the valve, so it's looking like no parts need to be replaced.

What do you guys think? Is the shelf life of new gas really only less than a year now for these engines?
'66 Mustang convertible 200ci/C4 - 2v converted large log, Autlite 2100, Duraspark II with MSD 6A, Comp Cams 260H, 2" dual exhaust

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gas evap residue on new head rebuild

Post #29 by chad » Wed Jul 24, 2019 3:52 pm

"...shelf life of new gas really only less than a year..."
was it ever?
I'd say U get some probs in under/close to 3 mo.

I usta leave my bronk out doors, run it Oct- March every yr w/o problems (6, 7 mo of gas souring in the bowl'n tank) & she'd still start w/a lill tromp to the floor (set the carb linkage on the choke finger - linkage) and a few sec of "key hold-over".

Around Y2K (with the noticeable climate crisis round here) the gas here got more'n more alcohol init. Required same w/either a spry of starter fluid or a drizzle of a few teaspoons of gas in the throat.

I think I C 'that residue' in ur pic. Surprised that's the problem.
"Big thing is only make one change at a time. Change 2 or more things at a time it becomes difficult to figure which change helped or hurt" turbo2256b » 1/16/2017
Chad - '70 LUEB on '77 frame (i.e. PS, D44, trapezoidal BB 9", 4.11), 250, NV 3550 & DSII to B transplanted, "T" D20/PTO, 2" SL, 1" BL, 4 discs, 33"X15", tool boxes, etc. Seeking: Hydraulic gear motor for Koenig pto. chrlsful@aol.com (413) 259-1749

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Re: No compression in one cylinder.. anything I can do before pulling head?

Post #30 by wsa111 » Tue Jul 30, 2019 9:13 am

I think your machinist screwed up & the head has too little clearance in the valve to valve guides.
I would tell him to let you look at the head dissembled & take some photos in the suspected areas.
67 mustang,C-4, with mod. 80 hd, custom 500 cfm carb with annular boosters, hooker headers, dual exh.-X pipe, flowmaster mufflers, DSII dist. MSD-6al & MSD-Blaster 8252 Coil. Engine 205" .030" over with offset ground crank & 1.65 roller rockers. 9.5 comp., Isky 262 cam.
2003 Ford Lightning daily driver. Recurving Distributors. billythedistributorman@live.com
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drag-200stang
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Posts: 1106
Joined: Sun Oct 19, 2003 6:10 pm
Location: Michigan

Re: No compression in one cylinder.. anything I can do before pulling head?

Post #31 by drag-200stang » Wed Jul 31, 2019 9:02 am

I also agree, gas is not your problem...Stuff happens.
Let us hope that it works out this time. :beer:
66 Mustang Coupe
200 turbo w/lenco 4-spd
stock adj. rockers, stock timing set, ARP studs
best 1/4 mile ET 9.85/best mph 139 on 8 lbs progressing to 15 lbs boost
Went 9's when 10's was fast.

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