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low compression has me stumped

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tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
The piston moving down provides enough force to suck in enough for a test. Besides, it would be the same restriction ( as already many times gone over as only a couple of pounds diff), to all four cylinders, not one as the OP is having an issue with. You leak enough air at idle to start and run the car. It WILL suck air. How many stories of rags stuffed in a manifold getting sucked in have you heard? Opening the throttle does not give false readings either, so it is not bad, just not much help. Ego for one or two more pounds?

I vote sticking valve, bad seat, ring upside down...

In reply to # 3696111 by Robert H When doing a compression test, there must be a way for air to get into the cylinder as the piston moves down for the intake stroke. Otherwise there is no air to compress on the compression stroke, so no pressure in the gauge. In some cases, maybe the clearance around the carb butterfly is sufficiently leaky, but my experience has been that the throttle must be open. I even prop the dashpot slides open.



Cogito ergo sum periculoso

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NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
In reply to a post by Scott S I vote sticking valve, bad seat, ring upside down...

The only one that would elude the leakdown test is the sticky valve, and OP claims to have somehow eliminated that possibility. A vacuum gauge reading would tell a lot since a sticky valve would both lower compression and cause an intermittent flutter to the reading. Not that the MGB engine produces the steadiest vacuum signal in the first place.

Bernd Kamenicky Avatar
Bernd Kamenicky Bernard Kamenicky
Altlengbach, Lower Austria, Austria   AUT
1966 MG MGB GT "The Red One"
1971 MG MGB MkII "The Blue One"
If leakdown test (at firing TDC, nowhere else: depending on cam exhaust opens roughly at 62BBTC=118 ATDC, inlet closes at 66 ABDC= 114BTDC, so both valves closed in this degree window only) shows same leakage and air blow-by sounds same at all 4 cylinders but compression test shows that much difference (forget throttle position for this test) it should be a sticking valve (valve stem, valve guide.....). No bedding in possible during operation!
Pls kep us informed about root cause!
Thanks
Bernd

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Norm58mga Norman Ellis
Hornell, NY, USA   USA
I've checked for a sticky valve as suggested with a feeler gauge inserted between tappet and valve stems. The feeler gauge remained tight through out - though I checked it while turning the crank by hand, maybe I should check again at cranking speed. I checked the valve lift using a dial indicator and #2 and #3 are both at or near spec at .355. Oddly enough, the exhaust valves on #1 and #4 are at about .280, but the compression is only compromised in #3. I did try the air pressure test. The first time I was sure that the air was coming out the exhaust. But I could never replicate it. The next time I tried and I thought the intake was leaking - but I could never get it to leak again. I'm just taking a couple of days off and I'll try the tests again when I'm fresh.

I'm expecting new fuel lines in a couple of days and I'll see what happens when I get it started. I've already pulled the head and pan once looking for this problem and am not mentally ready to do it again.

BumbleB74 Avatar
BumbleB74 William Milholen
Tidewater, Tidewater VA, USA   USA
Did this shop also fit valve guides? Some have stated on here that some valve guides need some amount of reaming for the MGB whereas the run of the mill Cheby or Ford don't. Still wonder if the valves are sticking slightly open and you just don't realize it...coupled with not being lapped? Or the valve seat could have a small burr on it not allowing the valve to fully seat?



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tigger00 Avatar
tigger00 Rick O
Creedmoor, NC, USA   USA
1974 MG MGB "Queenie"
I think it's time to take the head off and have another shop look at it.
Tell them what's going on, I believe they will find your problem. good luck !
p.s. I did mine 3 times before taking it to another shop (also reused the same gasket, just sprayed it with copper spray)

NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
In reply to # 3696461 by Norm58mga I've checked for a sticky valve as suggested with a feeler gauge inserted between tappet and valve stems. The feeler gauge remained tight through out - though I checked it while turning the crank by hand, maybe I should check again at cranking speed. I checked the valve lift using a dial indicator and #2 and #3 are both at or near spec at .355. Oddly enough, the exhaust valves on #1 and #4 are at about .280, but the compression is only compromised in #3. I did try the air pressure test. The first time I was sure that the air was coming out the exhaust. But I could never replicate it. The next time I tried and I thought the intake was leaking - but I could never get it to leak again. I'm just taking a couple of days off and I'll try the tests again when I'm fresh.

I'm expecting new fuel lines in a couple of days and I'll see what happens when I get it started. I've already pulled the head and pan once looking for this problem and am not mentally ready to do it again.

Disclaimer...There may be a more "proper" way to test for a sticking valve, but I dont know what it would be!

The only other thing I can think of trying would be to remove the intake and hold your hand over the port while cranking the engine. It should suck and not blow back against your hand.

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tvrgeek Avatar
tvrgeek Silver Member Scott S
Hillsborough, North Carolinia, USA   USA
1965 MG MGB
Leakdown tester and a stethoscope? I like the idea of pulling the manifolds. Listen for air in the ports and in the oil dipstick with the aid of a stethoscope.



Cogito ergo sum periculoso

MadMax133 Avatar
MadMax133 Max Fulton
Durham, NC, USA   USA
Mr Ellis,

Nice puzzler! :-)

Some random editions to the thread:

1) YES, WOT throttle makes a difference in compression, especially with Sidedraught Webers. Customer brought a dual carb Volvo 122 to the shop and said another shop did compression and it was "fine, about 100 each cylinder". (This seemed awfully low to Me.) I did it-- 170, pretty much across the board. I then left the throttle closed-- 85!!! Yes, THAT big of a difference. I've seen this many times, but only with sidedraughts. (Don't ask me to bother to explain WHY.....)

2) I also like the sticking valve theory. Get car running and a) see if it feels/sounds like it's running on only 3 cylinders and b) take the vacuum reading.

3) You've checked so many cylinders, this is probably NOT it, but I had a friend who was getting "low" cylinder readings... and then he tapped his compression gauge and it jumped from 120 to 160 (as expected). So, faulty TOOL?

4) Machine shop is wrong. Enough people have said it. This fact alone puts a LOT of suspicion on the valve job.....

5) Are they silicone bronze guides? Famous for sticking valves. (Poorly made, they swell when hot). Also, many machine shops use a MODERN clearance when redoing guides, which assumes fuel injection, thinner oils, different materials, etc., and is MUCH tighter (.0015" on exhausts). Our engines want .0025" on exhausts, and even looser on the older heads. (Which, interestingly, meant they wore out LESS. The sloppier the initial guide clearance, the longer the guides lasted. The later heads--- with emissions concerns and tighter clearances-- wore out their guides faster. Go figure....)

6) To get a low reading from a worn camshaft, it would have to be the 6th lobe (#3 intake). And it would need to be VERY worn (so that the engine is gasping for air that it can then "compress" to make compression. Same idea behind the closed throttles on the carbs-- if it can't breath, it can't compress! However, you've said those lobes seem healthy (though your exhaust lobes definitely are NOT. I'd allow anything .320 to .350" for a "stock" cam, some wear and manufacturing tolerances (in cam as well as rocker! Some rockers have anything from 1.38 to 1.44 ratio.) But lower than that-- cam is going flat. Sorry.)

Good luck!

Cheers,

M

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Captain Dave Avatar
Captain Dave Silver Member David Blakey
Clinton, TN, USA   USA
1967 MG MGB GT "Lil B"
Camshaft timing perhaps.

spikemichael Avatar
spikemichael Platinum Member Michael Caputo
Ocean Shores, WA, USA   USA
1973 MG MGB "Freebie"
1973 MG MGB
1974 MG MGB "Spike"
1979 MG MGB "MegaBeanie"    & more
Your comment on the lift of #1 adn #4 exhaust make me think you need a cam anyway so things are coming apart for that job soon, might as well be now.

thought #2 Is it possible you replaced piston rods? Is it possible one is an incorrect rod maybe a half inch too short? It's out there but hey... When the head comes off (and it must) wind the short block and confirm that piston #3 travels the full distance of 1, 2 and 4.



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Norm58mga Norman Ellis
Hornell, NY, USA   USA
Here is the latest update as of this morning. I took the head and the #3 piston and rings to a different machine shop. He did a vacuum test on all the manifold ports and found the intake for #3 and #4 cylinders was leaking and #1 and #2 was coming up to full vacuum but at a slower rate than he thought was right. All the exhaust valves were well sealed. He wanted me to also check the end gap of the new rings which I can easily do with the engine sitting on the bench once again.

I feel much better now and hope to have the engine back in next week! Thanks for your help. Norm

NOHOME P P
O, ON, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
In reply to # 3699863 by Norm58mga Here is the latest update as of this morning. I took the head and the #3 piston and rings to a different machine shop. He did a vacuum test on all the manifold ports and found the intake for #3 and #4 cylinders was leaking and #1 and #2 was coming up to full vacuum but at a slower rate than he thought was right. All the exhaust valves were well sealed. He wanted me to also check the end gap of the new rings which I can easily do with the engine sitting on the bench once again.

I feel much better now and hope to have the engine back in next week! Thanks for your help. Norm

How did this not show on the leak-down gauge? Pretty much exactly what the tool is meant to detect no?

Good to hear some progress on this, fingers crossed that this solves everything.

Pete

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Sounds like some progress!

Norm58mga Norman Ellis
Hornell, NY, USA   USA
The second machine shop that I had dropped my head off to called today. The first shops machining of both seats and valves is very rough and causing the incomplete seal in #3. They are going to fix it - so I think the problem will shortly be solved.

Thanks for the Help! Norm

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