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77 MGB engine trouble WHAT THE HECK is happening???

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timatron Avatar
timatron Timothy McKee
virginia, USA   USA
1974 MG MGB "MGBoop"
1977 MG MGB
Boy am I stumped. starting this MGB after sitting for several years being restored by a fellow in Maryland. Fixed many wiring issues. Installed a 74 engine I got out of another MG. I have finally got the engine running after replacing the fuel pump, electronic distributor and coil. Problem is dead cylinders 3 and 4 most of the time. Originally thought bad ignition wires since exhaust pipe for 3 & 4 was cold. The electronic Distributor was replaced with a 74 MGB points type since I had one under the bench. Engine started but ran poorly replaced Points, rotor, distributor cap, wires and plugs, Ran better but still vibrated bad. Pulled plug wires 3 & 4 with no change to engine vibration. Checked compression and all cylinders were at 103 to 105 PSI. That would indicate no problem with valves or piston rings (I think). I can rev the engine and it smoothes out a little and then goes back to rough idle. The header pipe for number 3&4 was warm after high reving. Adjusted Valves to .015 and observed no sticking of valves and appear to move the same amount open and closed. Pulled intake and exhaust to make sure critters hadn't crawled up and blocked anything. all clear. Pulled top off Weber carb and it was clean and everything appears to be functioning properly. I am stumped at what to try next, what am I missing? I don't know any history other than it came out of a 74 but the oil was clean and the antifreeze was green with no corrosion. Engine has no sludge build up and the engine was painted nicely. May have been overhauled. HELP!!!

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trmgb Avatar
trmgb todd martinez
littleton, co, USA   USA
Did you check the water level in the rad



72 engine, hif carbs, black od, rest of car stock

dohc281 Avatar
dohc281 Ira Eckstein
Laurel Springs, New Jersey, USA   USA
It sounds like cylinders 3&4 might have stuck valves. I know you say they are fine but I would double check that. I had a rebuilt head gum up the valves after only being run for a few minutes before bad gas gummed them up.


In reply to # 3608474 by timatron Boy am I stumped. starting this MGB after sitting for several years being restored by a fellow in Maryland. Fixed many wiring issues. Installed a 74 engine I got out of another MG. I have finally got the engine running after replacing the fuel pump, electronic distributor and coil. Problem is dead cylinders 3 and 4 most of the time. Originally thought bad ignition wires since exhaust pipe for 3 & 4 was cold. The electronic Distributor was replaced with a 74 MGB points type since I had one under the bench. Engine started but ran poorly replaced Points, rotor, distributor cap, wires and plugs, Ran better but still vibrated bad. Pulled plug wires 3 & 4 with no change to engine vibration. Checked compression and all cylinders were at 103 to 105 PSI. That would indicate no problem with valves or piston rings (I think). I can rev the engine and it smoothes out a little and then goes back to rough idle. The header pipe for number 3&4 was warm after high reving. Adjusted Valves to .015 and observed no sticking of valves and appear to move the same amount open and closed. Pulled intake and exhaust to make sure critters hadn't crawled up and blocked anything. all clear. Pulled top off Weber carb and it was clean and everything appears to be functioning properly. I am stumped at what to try next, what am I missing? I don't know any history other than it came out of a 74 but the oil was clean and the antifreeze was green with no corrosion. Engine has no sludge build up and the engine was painted nicely. May have been overhauled. HELP!!!



If you hit your pony over the nose at the outset of your acquaintance, he may not love you, but he will take a deep interest in your movements ever afterwards.



Rudyard Kipling

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ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Your compression is miserably low across the board and 3 and 4 have flat flat cam lobes.
Tuning isn't going. To solve it



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

Andrys Posthuma Avatar
Delft, Z-H, Netherlands   NLD
1935 MG N-Type Magnette
1969 Jaguar E-Type 2+2 "The E"
1976 MG MGB GT "The B"
Do you have a double SU carb? Maybe shift your attention to fuel delivery to the rear carburettor, or the rear carburettor having a stuck float needle. Does the situation improve when pulling out the choke*?

*I know, "mixture enrichment device".

Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
Did you verify spark for 3 & 4 and do not have 3 & 4 wires reversed?

Agree with Rob that you have low compression but it should run.

Compression test would indicate that the valves aren't sticking.



The goal - Reliable summer driver interspersed with mechanical tinkering...
Motto - "Driving fifty in the twisties..."
On Mods - It's your damn car - Do what you want. Haters gonna hate...
On SUVs - Drive your B like a soccer mom is texting her friends about how she wants to kill you...
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HardlySlept Avatar
HardlySlept Chris Gatti
New Berlin, New York, USA   USA
1980 MG MGB "Blythe"
Compression is low, but even tells me it's either a tired engine or your testing procedure didn't let the engine 'breathe' during the test. Firing order should be double checked. How about engine vacuum? Any chance there's a vacuum leak?

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rony12 Ron Roberts
Avoca, PA, USA   USA
I just had this same issue w/a 53 TD last month. After doing everything you have already done AND a valve job with all new valves,seats and guides I still had the exact same thing. On the TD it was cyl's 1 & 2 but same comp. issue,etc. It finally turned out to be the rear carb . I know you said it has a Weber on it but in the past I have had several issues with Webers similar to this problem. And I like the Weber !
To verify my running issue I borrowed a set of carbs and put them on temporarily . It worked. I just rebuilt the originals and problem solved. Customer said the car never ran this good since he bought it 7 years ago.
Good Luck

ClayJ Avatar
ClayJ Silver Member Clay Johnston
Mt. Olive, MS, USA   USA
1972 MG MGB
Folks read the original post, the car has a single Weber. Definitely needs to verify firing on 3 & 4.

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NOHOME P P
O, Ontario, Canada   CAN
1967 MG MGB GT "Maggie (GT From Hell)"
Put your timing light on #3 or #4 plug lead and see if it is missing a beat. It will be obvious.

Do a vacuum test. That cheap gauge will tell you a lot about the core state of the engine. Do a google search if in doubt about how to use.

Your compression numbers are at the bottom of the barrel for a running engine. Could be a good time to pour a can of Seafoam in the engine and cross your fingers. Not a strong believer, but seen it work so why not?

Less common but very useful is a leak- down tester. It will TELL you where the compression has gone.

The cam lobe thought is a valid one. Dial indicator will sort that out in a hurry.

Sinewave Avatar
Sinewave T. Keith Vezina
Kenner, LA, USA   USA
In reply to # 3608687 by NOHOME Put your timing light on #3 or #4 plug lead and see if it is missing a beat. It will be obvious.

Do a vacuum test. That cheap gauge will tell you a lot about the core state of the engine. Do a google search if in doubt about how to use.

Your compression numbers are at the bottom of the barrel for a running engine. Could be a good time to pour a can of Seafoam in the engine and cross your fingers. Not a strong believer, but seen it work so why not?

Less common but very useful is a leak- down tester. It will TELL you where the compression has gone.

The cam lobe thought is a valid one. Dial indicator will sort that out in a hurry.

X2 - All of the above tests are quick, easy and cheap



T. Keith Vezina
British Motoring Club New Orleans
1976 MGB, 1976 MGB Trailer & 1967 MGB MK. I

timatron Avatar
timatron Timothy McKee
virginia, USA   USA
1974 MG MGB "MGBoop"
1977 MG MGB
1. yes, coolant level is good
2. Valves appear to be moving properly but I cannot tell if the are fully seated without pulling head however I thought the compression test showing all cylinders equal would rule that out.
3. Compression is low but cyl 1 & 2 run, why would 3 & 4 only be affected, (unless as you say flat cam lobes) wouldn't you notice that visually by rocker arm movement? 3 & 4 visually move similar to the others. I don't have a dial indicator to check for sure.
4. I do not have twin SU's, It is a Weber two barrel. Just pulled plugs on 3 & 4 and rotated crank to open intake valve and blew compressed Air through to ensure no blockage. Repeated for exhaust and then repeated for each valve on each cylinder. No blockages.
5. Checked and double checked firing order and proper placement of ignition wires.
6. Followed testing procedure on YouTube MG Guru (pull plugs rotate 10 revolutions etc.). Firing order double checked. Verified good intake gasket and used hi temp sealer on intake flanges.

HardlySlept Avatar
HardlySlept Chris Gatti
New Berlin, New York, USA   USA
1980 MG MGB "Blythe"
Were you able to check for a vacuum leak? How about the timing light check on ALL cylinders?

GlennMGB Avatar
GlennMGB Glenn G
Fort Worth, Texas, USA   USA
1967 MG MGB GT "Rose"
What do the spark plugs look like? If 3 and 4 are not wet with fuel, those cylinders are not getting fuel, indicating an overhaul of the Weber. If they are wet with fuel, the problem is spark related.

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
Please check firing order yet again -- should be 1-3-4-2, counterclockwise...

Dick



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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