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MGB Worrisome compression test results on my MGB 18GK

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wkilleffer

wkilleffer

Jedi Knight
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Anything to avoid doing homework...

Anyhow, my car's a 1974 MGB with an 18GK engine. The engine was rebuilt just over six years ago, and if I've counted correctly, it has less than 15k miles on it. The HS4 twin carbs were rebushed and shafted shortly thereafter.

In the last year, I've changed the manifold gasket, added insulation to the heat shield, put new jets and jet tubes on the carbs, and installed a Pertronix dizzy and matching coil. On the scale, the timing shows up right where it's supposed to. Any higher, and the car will knock some even with 93 octane.

The car seemed to be idling roughly the last few times I've had it out, so I decided to check a few things. The spark plug tips are grayish-white except one. If #1 cyl. is near the radiator, and #4 is by the bulkhead, #2 is the one I'm talking about. It seems to use more gas than it should. I can't get it to break 20mph city or highway, and people tell me that 23 mpg city isn't unusual even with spirited driving.

I've never done a compression check on it, so I was curious what would show up. I ran the car till the temp gauge was at NOT, and started with the #1. I didn't squirt oil into the cylinders or anything like that. Just a warmed-up engine. Here are the results:

#1 1st- 149lbs 2nd- 149
#2 1st- 145lbs 2nd- 145
#3 1st- 107lbs 2nd- 107
#4 1st- 130lbs 2nd- 130

Bentley says that an 18GK should be at 160lbs. Since the engine is rebuilt and not brand new, the 149lbs and 145 might be ok, but I really don't know.

Seems strange otherwise. I've always tried to adjust the valve lash correctly, but am willing to admit that may be a weak area for me. The rebuilder told me to use 10w40 oil in synthetic if possible, and I've always done that, or synthetic blend when I couldn't get full synth.

I've never abused nor raced the car, though I do like to go up to 3k between shifts if possible. The only thing that might be questionable is that I let the coolant stay in for two years one time.

Is this a blown head gasket or misadjusted valves? I've never seen coolant in the oil nor the other way around, but know that other things can happen. I'd hate to think that I've killed my baby.

Any advice or suggestions would be most appreciated.

Thank you,
-Bill
 
I'd do it again 'wet' (squirting just a little oil in the spark plug holes)...there's a bit of difference between #1 & #4, but #3 is the one that bothers me.
 
Hello William,


I would certainly double check the valve clearances, but before you do, just torque the head down.

As the engine has only a few miles on it, I wouldn't expect too many serious problems although the plug colour sounds as though it's set on the weak side.

The next thing I would do is take it for a good run on open roads and give it some throttle for many miles. Being too light on the engine can be worse than driving full bore everywhere. (as long as the engine is warmed fully first)
This should blow a lot of rubbish out of the engine. It may be that the engine is coked up due to light driving.

If after this the figures show no improvement, you can either lift the head or do a leak down test to see where the leakage is?

I don't know about America, but the general opinion in the UK is to use 20w50 multigrade non synthetic oil in these older engines.

Alec
 
Ok, I torqued the head down to 50 lb/ft, but all but three of the bolts were already there.

I ran another check with a few squirts of SAE 30 in each cylinder:

#1 1st- 155 2nd- 158
#2 1st- 155 2nd- 155
#3 1st- 126 2nd- 125
#4 1st- 143 2nd- 143

Not quite as bad, but not too good either. I may double-check the valves on the back cylinders while the engine is still warm.

As far as engine oil viscosity goes, I was and still am considering switching to 20w50 at the next change. Won't be too much longer. But I have a hard time with the idea of giving up synthetic. Most, but not all, of the people in the club I used to belong to used full synthetic. I know the rocker arms always look clean when I take the cover off.

Thank you,
-Bill
 
DRY:

#1 1st- 149lbs 2nd- 149
#2 1st- 145lbs 2nd- 145
#3 1st- 107lbs 2nd- 107
#4 1st- 130lbs 2nd- 130

WET:
#1 1st- 155 2nd- 158
#2 1st- 155 2nd- 155
#3 1st- 126 2nd- 125
#4 1st- 143 2nd- 143

I'd redo the dry tomorrow morning while the engine's cold....if #3 is still low, you've got a problem. IMHO, 15psi difference between cylinders on either check (either dry or wet - not from dry to wet) is acceptable; anything greater warrants looking inside the engine. IMHO.

But, lemme ask a question first: when you first start it in the morning, does the wife see any smoke out of the tailpipe? Just a puff of blueish?
 
tony barnhill said:
DRY:

#1 1st- 149lbs 2nd- 149
#2 1st- 145lbs 2nd- 145
#3 1st- 107lbs 2nd- 107
#4 1st- 130lbs 2nd- 130

WET:
#1 1st- 155 2nd- 158
#2 1st- 155 2nd- 155
#3 1st- 126 2nd- 125
#4 1st- 143 2nd- 143

I'd redo the dry tomorrow morning while the engine's cold....if #3 is still low, you've got a problem. IMHO, 15psi difference between cylinders on either check (either dry or wet - not from dry to wet) is acceptable; anything greater warrants looking inside the engine. IMHO.

But, lemme ask a question first: when you first start it in the morning, does the wife see any smoke out of the tailpipe? Just a puff of blueish?
Click to expand...

I checked the valves. If 1 is near the radiator, and 8 is near the bulkhead, 6 was too tight. 7 and 8 were a trifle snug as was 5, so I adjusted them as well. I was adjusting them to .013 hot engine.

Afraid it didn't make a huge difference though:
#1 157 157
#2 155 154
#3 128 128
#4 143 141

The only dry test I did was the very first one. The rest had three or four squirts of SAE 30 in the cylinders.

We've never looked at the exhaust when the car is first started from cold, but we will next time. I have noticed that when I first start it with the choke on, it puts out some puffs of greyish black smoke. I always thought some of that was a normal byproduct of the choke at first, but maybe not.

Is this sounding like a failing head gasket?
 
Valves too loose or too tight wouldn't cause difference in compression settings unless they were so loose they weren't closing (& I don't think that's possible)...you've either got valve spring seal problems or ring problems...#3 is the problem area....

A couple of questions:

1) How's the head at #3? If its cracked where the guide goes could it be leaking enough to cause a drop in compression?

2) How could a bad head gasket cause a drop in compression?
 
tony barnhill said:
Valves too loose or too tight wouldn't cause difference in compression settings unless they were so loose they weren't closing (& I don't think that's possible)...you've either got valve spring seal problems or ring problems...#3 is the problem area....

A couple of questions:

1) How's the head at #3? If its cracked where the guide goes could it be leaking enough to cause a drop in compression?

2) How could a bad head gasket cause a drop in compression?
Click to expand...

Remember, this is new territory for me. Maybe it shouldn't be, but it is.

Can you show me what I'm supposed to be looking for? Nothing looks obviously wrong, but I also may not know what wrong looks like.

As far as the head gasket goes, I've been told before that low compression can be a sign of a failing gasket. I've also been told that these engines have a tendency to do that every 12-15k miles. Also, I remember seeing coolant seeping up between one of the head studs and bolts on the outside of the engine. I torqued the head and haven't seen it since. But it was odd, and seemed like a head gasket problem.

I also thought a head gasket problem could play havoc with compression, but I've not seen signs that the fluids are mixing.

If it's a busted ring, what the heck did I do to cause it? Same goes for a damaged head.
 
Tony, did you mean valve spring seaT problems? Rather than valve spring seal problems?
 
I would suggest that he has a valve not seating properly in Number 3 and 4. It just might be time to pull the head and take it to your favorate engine shop for an overhaul.

ie, the oil seals the rings mostly and then if you have bad compression the leaks are from the valves usually.

This is not an expensive operation but you will need a new head gasket.
 
Is it possible that I've burned a valve due to not always having the mixture right? That had a steep learning curve for me, and I feel like there's still alot to learn.

Also, was the 190deg thermostat a bad idea, or was it just the weak state of my radiator?

The company that rebuilt the engine should be able to fix the valve seats. They're supposed to be hardened for lead-free gas.

Thanks,
-Bill
 
Intakes hardened for no lead gas. But then most of us do not bother.

190 degree thermostat is fine in my opinion.

Could have a burnt valve but your wet compression check has already proven that something is wrong with 3 and 4 it seems to me. Pull the head and check it out.
 
The car hasn't been run since 2:30 or 3:00, so I asked SWMBO to come out and watch the tailpipe. It required the choke to start, and she said the smoke was grayish white. This wasn't ideal, cause it was under artificial light and it's not like the car sat all night. But it might be something else to think about.
 
I was thinknig possible valve seal problems - but, you could have a valve not seating properly

Would like to know what that smoke looks like in daylight.
 
Surely could be Tony. IMHO it is in the valves area for sure.

Wonder if all the oil he used for the compression check was burnt out when he started it last.

I think I would pull head, what do you think? Haveing it shop checked is cheep enough.
 
I still want to see what happens when he starts it tomorrow morning - smoke-wise - & the dry compression while engine is cold.
 
Hi there. I don't have any answers for you but maybe some things to think about that may help.

1. LOOSE valve lash will keep valves from OPENING and will alert you by clattering. TIGHT (too small a gap) will hold the valves OPEN and eventually burn. This could show up as low compression since the combusion chamber won't be sealed. (From your post, it doesn't sound that that's the case here.)

2. Try propping your carbs open a bit while taking your compression readings. If the throttle butterfly is closed and the bushings are so good they don't allow any air in, what will you compress to take your readings? This is really true with Weber carbs! If your 1/2 carb is set at a different point than your 3/4 carb, it could explain some of the pattern you are seeing. (If the compression evens out, re-tune the carbs!)

3. I like the idea of taking it for a good long drive with lots of accelleration and decelleration. It'll be a blast and, if a couple of rings had not seated properly, you could get lucky!

4. All else fails, do a leak down to see where to look next.

Good luck! Adam
 
A 20% difference in compression is not a real worry. The engine has been rebuilt and you accidently found this. Was the engine running decent before the testing of the cylinders. Tony is in the mark with #3. Have someone start it for you while watching tailpipe and see what comes out. Also, check tailpipe (cold) and see what color inside is, black (rich fuel), gray ( on the mark), white (coolant), sooty grease ( oil).
 
The instructions that came with the compression tester said to open the throttle all the way during the test, so I figured out a way to keep it open the whole time.

The engine seems to chatter like a sewing machine. I'd worry about it if it didn't make any noise.

With what I've found so far, would driving it at all be a bad idea? I know of a potential good drive area on one of the old highways, but don't want to do it if it might make things worse.

As far as the tailpipe goes, it's black inside. Seems odd considering all but one of the plug made the mixture look a little lean. But it's not a greasy black.

How do I do a leak-down test?

Due to school, it may be a couple of days before I can come back to this other than reading responses to the post. I'll check things and keep everyone posted.

Thanks to everyone for their help so far.

-Bill
 
Forgot to answer: the engine wasn't running terribly, but seemed like it could do better. Idle seemed to be the worse problem, as getting it to idle consistently seems to be beyond me. In a way, I'm glad I found this because it gives me a reason to be having trouble instead of the problem stemming from my amateur skills.
 
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