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MGB-GT Engine Problem 74MGB GT

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jimz26

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I,ve just redid the rings on my Gt and put it back together and first off it wouldn,t start......I've checked the spark etc and fuel and still nothing.....I checked the compression looking for some answers and found i have only between 15 and 30 pounds...rechecking the tappets i got it up to 20 and 40......I've did the valves and seats and they are new rings.....i removed the ridges off the tops of the cylinders and thats about all.....I'm at a loss to figure out what is wrong so i'm asking for any and all answers.....

Baffled in Oshawa
confused.gif
 
It has to be cam timing!!! Put the #1 piston on TDC and check it with your timing mark!! Onless your distributer is 180' out and there is a little grit or something under a valve, but I really doubt that!! GOOD LUCK!!!!!
 
Jut pulled head off again doublecheck head gasket,was ok Valves decarbonized lapped and in good cond. When#1 onTDC rotar is at app, 2 o'clock Timing chain or gears were never touched before pulling motor were OK. Rings are staggered on pistons,honed walls So??????????????????
JIM
 
OK, let me see if I got this straight. You just removed the head, pistons and rods and timing cover. Not the crank shaft, cam or timing chain. Did you remove the distributor? Well, that wouldn't cause the low compression. The only thing I can think of if you didn't touch the cam timing is maybe the possibility the pistons are oversized from a previous owner and you put in standard rings. If this is true the ring gap would be rather big and could possibly allow leakage. Did you check the ring gap by slipping it down in the cylinder without the piston. Also, did you decarbon the piston grooves so the rings rotate easily in the piston grooves. If they are jammed, the rings wouldn't expand out to the cylinder walls enough.
Since you are getting low compression on all cylinders, it seems remote that a couple of valves are causing it. For the valves to cause it they would all have to be set wrong.
 
Not blocked intake,I thoughtof that,took off the carbs,same result. Really without touching the timing chain,tentioner or timing gears,how can the cam timing be out,HOW?????PLEASE-----HELP
JIM
 
Ring grooves cleaned and free of carbon;; Piston rings were all checked in cylinders for gap before installing, Bore and pistons are standard I measured them before ordering rings. I was needing a new clutch, and it was burning a bit oil,so I decided to do both,can't blame this on the Prince of Darkness. Halloween maybe???????
JIM and "Murphy" Thats the car. Murphy's
LAW
 
Have you tried a different compression gage?

Check the valve sealing by pouring kerosene into the ports & look for seepage into the chambers.
D

[ 11-10-2003: Message edited by: Dave Russell ]</p>
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>I checked the compression looking for some answers and found i have only between 15 and 30 pounds...rechecking the tappets i got it up to 20 and 40......I've did the valves and seats and they are new rings.....i removed the ridges off the tops of the cylinders and thats about all<hr></blockquote>

Let's focus on things that affect compression...you say the rings are the proper size for the cylinders, the grooves in the pistons were cleaned & that you removed the ridge at the top of the cylinder

...are the ring gaps staggered? did you cross-hatch the cylinders

...whatever is affecting all 4 cylinders is in those 4 cylinders
 
Is the head flat? (and uncracked?) Did the car overheat prior to this work? (warping block or head).
Did you torque the head bolts properly?
Is the drive key to the cam gear sheared?
Or the cam cracked at the drive end (I saw this in a Spit...the valves still moved, but all out of phase).
Are you adjusting the valves in the "nine-total" method?
Check the ring end-gap...I'd guess it should be well under 0.010" in an MGB.
Correct size pistons for the bores?
I think a lot of us still suspect a problem with the cam timing. When the number one piston is all the way up (TDC) and the distributor rotor is pointing towards the number one plug wire lug, both valve should be closed, with a bit of lash (looseness) in the rocker arms. Is this the case?
 
Another thought is the possibility of "egg" shapped cylinders. They normally wear like this but would have to be really bad to cause that much leakage. You might try putting some oil in the cylinders and see if the compression came up. Then there would be a probability of the valves causeing it if the compression didn't come up any. It still might not come up to full value if the pistons are the cause but it should come up some.
 
Here's what I would do in trying to work through it logically.
1 Try another compression gauge. You need to know if it really is low first.
2 If still reading low, remove the pushrods or adjust the valves so they don't open. Test again. If the valves are staying closed, cam timing shouldn't affect compression.
3 Still low readings, squirt a little oil in one cylinder. Test that one. If it's still low, squirt some in the others as well. If it improves to around spec then, you got leakage between cylinders.
4 Still low, see if you can feel air movement at the intake or exhaust ports of the head when cranking. Remember, the valves should be staying closed from step 1.
5 No air movement, well, I think it needs to come back apart. check for holes in the head and pistons, badly oval bores, undersize pistons, etc.
 
Hi, Thanks to all you guys that are giving me feedback on "Murphy" One thing I seethat I did I gave rings more gap than you recomend 10 thou or less??? well I have probably made it 25 or 30.If after trying your suggestions I don't rectify problem, I'll tear it apart and try again.
Again thanks to all of you for your assistance
Sincerely Jim Robertson
 
<blockquote><font size="1" face="Verdana, Arial">quote:</font><hr>Originally posted by MikeP:

2 If still reading low, remove the pushrods or adjust the valves so they don't open. Test again. If the valves are staying closed, cam timing shouldn't affect compression.
<hr></blockquote>

How does any air get into the cylinder to compress if the intake valve does not open?
D
 
I'm confused....did you reuse the original pistons? Did you mic the entire cylinders - top to bottom? Did you install the correct sized rings in the proper manner?
 
The compression guage is ok checked out on my other car 135 ok. Took off timing cover and cam timing is ok,marks line up,#4 is on TDC Rotor at *8 o'clock. Next,look for leak.
Yes same pistons
JIM
 
You might also try a leak down tester, this will dirrect you as to where the lack of compression is coming ie: the valves or the rings. As you pressurize each cylinder you can listen for air excaping to the head or into the lower end. Are you also sure you replaced each piston and tappet back into its original position and that the tappets are not hanging up?
 
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