MGExp

MGB & GT Forum

Lost power at 100 mph.

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

Topic List Forum List Newer Older Start New Topic
Lost power at 100 mph.
#1
Jeff in Iowa Avatar
Jeff in Iowa Jeffrey J
Iowa City, IA, USA   USA
Sign in to contact
1970 MG MGB
1970 stock MGB, high compression 1968 motor. I had it out and decided to see if it would do 100, it did but lost power at that point. I limped it home, no power at all low rpm, a little at high rpm, it's in the garage.
The points look okay at a glance and the fuel filter has fuel in it. Also, it has a Schlemmer distributor. Did I cook a spark plug? Did the distributor mechanical advance go awry? Did the distributor twist itself?
I'll have a chance to look at it this coming weekend but am hoping for some brain fodder between now and then.

Reply
Was this post helpful or interesting?
Yes No Thank
. Become a Supporting Member to hide this ad & support a small business
Re: Lost power at 100 mph.
#2
Jeff in Iowa Avatar
Jeff in Iowa Jeffrey J
Iowa City, IA, USA   USA
Sign in to contact
1970 MG MGB
It did this earlier at 80 mph but shook it off after a couple of minutes. That was two months and many miles ago. It's been a strong engine, I don't think this is something major.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#3
The Dave Avatar
The Dave Dave Cameron
Dracut, MA, USA   USA
Sign in to contact
If I had to venture a guess it would be that the distributor slipped and the timing is off, I've had it happen under load (though not 100mph of load!). Easy to check, easy to fix. Hope I'm right and it's not something spendy!

Reply
Was this post helpful or interesting?
Yes No Thank
. Become a Supporting Member to hide this ad & support a small business
Re: Lost power at 100 mph.
#4
favedave Avatar
favedave David Church
Saint Joseph, MO, USA   USA
Sign in to contact
1967 MG MGB GT "Marilyn"
1967 MG MGB GT "Marilyn"
1995 Ford Probe "The Probe"
1995 Ford Probe
Yeah, most likely the Dizzy clamp is not tight enough and it slipped out of time.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#5
mgbanthony Avatar
mgbanthony Platinum Member Anthony Henderson
Eastern Thousand Islands, ON, Canada   CAN
Sign in to contact
1962 MG MGB
1974 MG MGB
Distributor moved, or possibly the point cam got hot enough to take a bit off it. Double check point gap to .015.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#6
gooser Avatar
gooser Drake Myers
Danville Va, USA   USA
Sign in to contact
do a compression check. may have lost a head gasket.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#7
mayberry97 Avatar
mayberry97 Jon Z
Mayberry, NC, USA   USA
Sign in to contact
With a babe like that in your profile picture with fingers on a shifter, 100MPH is understandable.


79 MGB with Weber 32/36
It's yellow and his name is Brian.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#8
mac townsend
Fairfield, CA, USA   USA
Sign in to contact
1973 MG MGB MkIII
common to run out of power at 100 mph or so, the top speed in a B is limited by available power not by rpm. But it sounds like something broke.


1973 Pale Primrose Roadster. A nice 10-footer!
SUs, Datsun 5-speed

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#9
NOHOME P P
O, ON, Canada   CAN
Sign in to contact
1967 MG MGB GT "Maggie (GT From Hell)"
Broke a spring in the distributor advance?

Did it backfire when you let off? Might have a vacuum leak.

The answer is to always start with a compression test then move to ignition and then intake. Compression gauge, timing light and vacuum gauge for tools.

Pete

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#10
Jeff in Iowa Avatar
Jeff in Iowa Jeffrey J
Iowa City, IA, USA   USA
Sign in to contact
1970 MG MGB
A stuck valve is what it were.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#11
Speedracer Avatar
Speedracer Platinum AdvertiserAdvertiser Hap Waldrop
Taylors, SC, USA   USA
Sign in to contact
1967 MG MGB Racecar "The Biscuit"
In reply to # 2815110 by Jeff in Iowa A stuck valve is what it were.

I guessing a exhaust valve, does it have bronze guides?


Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com


Member Services:
Acme Speed Shop www.mgexp.com
MG/ Triumph Performance Street Engines - Cylinder Head Porting for street performance and race - DIY Engine Rebuild Kits With Free Tech Advice - VTO alloy wheels for British Sports Cars, and others
Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#12
joron Avatar
joron Silver Member Michel Joron
Roxboro, QC, Canada   CAN
Sign in to contact
1977 MG MGB "Victoria"
2018 Jaguar XE "Meghan"
What did you do to unstruck the valve ?


1977 MGB Red 'Victoria' and 2018 Jaguar XE 'Meghan'
All Classics:
Married, 2 children, MG MGB May to December, Jaguar the rest of the year.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#13
Jeff in Iowa Avatar
Jeff in Iowa Jeffrey J
Iowa City, IA, USA   USA
Sign in to contact
1970 MG MGB
The head's off now, a slight tap and the valve clicked back into place. I'll take that one out tomorrow and see if there's anything awry, (emery cloth). It's the front valve #3 cylinder, is that exhaust?

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#14
Rod H. Avatar
Rod H.
Amity, OR, USA   USA
Sign in to contact
1964 MG MGB
1968 MG MGB GT
Yes, that one would be an exhaust valve.
In reply to # 2815372 by Jeff in Iowa The head's off now, a slight tap and the valve clicked back into place. I'll take that one out tomorrow and see if there's anything awry, (emery cloth). It's the front valve #3 cylinder, is that exhaust?


Friends talking around a fire is the history of mankind.

Reply
Was this post helpful or interesting?
Yes No Thank
Re: Lost power at 100 mph.
#15
jrhickmn Avatar
jrhickmn Gold Member James Hickman
Sevierville, TN, USA   USA
Sign in to contact
1958 MG Magnette ZB "Maggie Rose"
1963 MG Magnette MkIV "Iris"
1966 Jaguar 3.8S
1968 MG MGC GT "Maisy"    & more
hmm. i would surmise that was heat related at that speed...but it does suggest that its time to rebuild the head. i hate to spend money...though i will admit that its easier this time since its your money and not mine, but i'd be real tempted to replace those exhaust valve seats and guides while i had it apart...if you don't have hardened seats for unleaded, then this is your chance.


Jim H. is a grease-stained wretch, too.

Reply
Was this post helpful or interesting?
Yes No Thank
Topic List Forum List Newer Older Start New Topic

To reply or ask your own question:

or

Registration is FREE and takes less than a minute

Forums  

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





Join The Club
Sign in to ask questions, share photos, and access all website features
Forum List
MGB & GT MG Midget Buy, Sell & Trade Vendor Market MG Performance MG Engine Swaps Factory Correct MGA MGC MG Magnette 1100 & 1300 T-Series & Prewar EV Conversion Modern MG MG Motorsports Events & Meetup MG 2024 Katy Texas Other Vehicles Off Topic Clubs Archived
Your Cars
1978 MG MGB
Jim H.   Gold Member USA
1978 MGB
Text Size
Larger Smaller
Reset Save