The all-new Front Page News Feed is now live   Read the announcement

MGB & GT Forum

Car won't start low pressure (very confused!!)

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

pinkyponk Avatar
pinkyponk Gold Member Adrian Page
Berwick, NS, Canada   CAN
In reply to # 3908455 by jacob.cob.young
In reply to # 3908453 by tvrgeek Basically, the rivet is an issue and the black plastic has too much carbon becoming conductive, both shorting your spark.

I found a weak spark will tend to show up as more scatter on the timing light. I have not thought about the physics much, but I think it would be that too low a voltage would take longer to ionize the gap so it would delay the spark some erratic amount. Timing light sees current, so until there was flash-over, no strobe. Drop out is even worse.
Thanks for your response
I'm not sure what you mean by rivet?

Some company once made rotors with a rivet in them that caused issues. Now some people think all rotors that aren't red have rivets in them. If you get spark to each cylinder the odds are your rotor is fine. Did you use a spark tester to check for spark or just the spark plug on the head?

If you can post a picture of your spark plugs. It might help.

Did you happen to take a compression reading from before you had this issue?


Home built Eaton M62 Supercharger with 7.6psi boost, 8:1 compression, custom "supercharger" cam from Schneider Cams, Mikuni HSR48 Carburetor, Chevy Cavalier 1.6 rocker arms, Maxspeeding rods with Teflon wrist pin buttons, custom aluminum cold air intake, CB Performance computerized ignition, Fidanza 9 pound flywheel, 1.44 exhaust valves in 48cc chamber head, matched manifolds, 2 1/4" exhaust system.

. Hide this ad & support a small business by becoming a Gold Supporting Member
Was this post helpful or interesting?
Yes No Thank
29desoto Avatar
29desoto Gold Member Wayne Sanders
Outside Otis, OR - in the woods!, USA   USA
1965 MG MGB V6 Conversion "Carmine"
1968 MG MGB GT "New Yeller"
1973 MG MGB GT "Eliza - FIRST PLACE - MG2013"
1984 Chevrolet Corvette "The Green Weenie"    & more
Richard, it sounds as if you have a bad brake booster. The vacuum must be running free inside it.

In reply to # 3908508 by rover100rich i just posted my experience of my car not starting,remove the hose to the brake servo and plug it to ensure that there is no possibility of air being drawn in here instead of the carbs,this was the problem with my car anyway,it would seem odd that if the car was running ok then suddenly not that the compression on all cylinders would be the cause,rich

Wayne Sanders
Rose Lodge, OR
(TD 4288)

"I don't care to belong to any club that accepts people like me"-joined Willamette Valley Club in 2011

73 MGB-GT virtually stole this one. Someone up there likes me.

68 MGB-GT the daily mule. Solid as a rock, but rattles like a can full of them!

79/65 MGB - Carmine- V-6 - T-5
This car is now very nearly completely done. Sure to find something else, but not now.........

Was this post helpful or interesting?
Yes No Thank
slice Avatar
slice Silver Member Andrew McCue
Chelsea, QC, Canada   CAN
1979 MG MGB

100 PSI compression is low even for a LC engine. Being in the UK, engines are HC and would typically be in the 160 PSI range. You have lost compression somewhere. A leak down test can show you where the leak(s) are on each cylinder. Since, the head has been installed recently, has it been re-torqued? My bet is the lost compression.

Best of Luck,

. Hide this ad & support a small business by becoming a Gold Supporting Member
Was this post helpful or interesting?
Yes No Thank
BH Davis Avatar
Grosvenordale, CT, Warren, VT, USA   USA
1973 MG MGB
Was the compression checked after the new head was installed? If so what was the reading then.

If it wasn't checked then you could have had 100 psi in the cylinders back then. If that were the case then the low compression likely has nothing to do with the current fail to start condition.

Have you checked to see if the distributor has moved? Try moving it by hand without loosening the lock clamp. If it rotate then that is most likely the problem. Reset the timing and tighten the distributor clamp to correct the problem.


Was this post helpful or interesting?
Yes No Thank
. Hide this ad & support a small business by becoming a Gold Supporting Member

To add your reply, or post your own questions

Registration is FREE and takes less than a minute!

Having trouble posting or changing forum settings?
Read the Forum Help (FAQ) or contact the webmaster