Visit our Twitter feed for more great content
AHExp

The 100-Six Forum

BN4 not running well

Moss Motors
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor
AutoShrine Sponsor

dkgraber85 Avatar
dkgraber85 Darin Graber
SLC UT, USA   USA
Hello to the Forum -

I have an early BN4 that has run very well in the past. However in the last 6 months the car has become hard to start, the idle moves around and isn't steady, and I've had to start playing with the choke to keep it running. The car runs cool (160 deg).

I have checked the timing, the carbs are rebuilt, and again, the car was running very well. I have noticed on a recent tuneup that the two of the cylinders have dropped in compression. Could this be it? Plugs are clean so I think the rings are good, but head gasket maybe? Probably going to take the head off and have the valves done and put in a new gasket anyway.

Just don't know why the idea would become erratic and starting becoming hard. Any thoughts would be appreciated...

D

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
sliproc Avatar
sliproc Kevin Quistberg E
Long Beach, CA, USA   USA
Darin,

Yeah, that would do it. Which two cylinders? Are they side by side or separated by good cylinders? What's the condition of your oil and water? Is the car smoking? Unfortunately if you've lost compression you'll almost certainly have to pull the head. When you've lost compression the only way I know of to get it back involves at least some disassembly and replacement or rebuild.

dkgraber85 Avatar
dkgraber85 Darin Graber
SLC UT, USA   USA
Yep - side by side. No smoking.



Edited 1 time(s). Last edit at 2018-09-28 09:43 AM by dkgraber85.

. Hide banner ads & support this website by becoming a > Gold Supporting Member <
sliproc Avatar
sliproc Kevin Quistberg E
Long Beach, CA, USA   USA
Darin,

If it's a head gasket you're probably losing water, either through the cylinders and out the tailpipe or into the crankcase(oil turns grey) or out the side of the motor between head and block.

dkgraber85 Avatar
dkgraber85 Darin Graber
SLC UT, USA   USA
Kevin -

Thanks for the feedback. I haven't seen any of these signs. My plugs are clean and no smoking. What about between cylinders?

Leyburn, North Yorkshire, UK   GBR
Hi,

Before taking anything apart I'd check that the vacuum advance is working, that can cause both your problems as well, there are youtube videos on how to easily check, and repair if needed.

Stan


sliproc Avatar
sliproc Kevin Quistberg E
Long Beach, CA, USA   USA
Darin,

I assume you found the two cylinder compression loss by doing a compression test on the motor correct? Are you losing coolant? If you're getting water in those two cylinders those two plugs will be much cleaner than the other four. Sounds like you'll probably have to pull the head but I'm all for getting as much information as possible beforehand. I'd do a wet and dry compression test, I'd check the torque on the head bolts, and I'd drain the oil a take a real good look at it.

Stan,

While Darin may have a vacuum problem, I'm not sure how that could account for compression loss in two cylinders.

P.S. Darin I just had another thought, are you sure your low compression is a recent problem? My Healey has low compression the rear two cylinders(60psi in #5, zero in #6)and it runs just fine and has been for the 34yrs that I've owned it. Yes, I should rebuild it, I'll get better performance for sure but since it runs good and I've got two other Brit cars and a boat I'll get to it when I get to it. My point is if I hadn't done a compression test when I first got the car and a problem cropped up now I might not know that the low compression is unrelated to the new problem.

Leyburn, North Yorkshire, UK   GBR
Kevin.

You're quite right a vacuum problem won't explain low compression, but it does explain a possible cause of the problem, and fixing it is so much less costly , and time consuming. Another related problem is the low revs mechanical advance spring has become a bit long with continual stretching, a test for this is to to check for the rotor moving by twisting with no resistance from the spring immediately, this coupled with a lack of vacuum advance could mean the ignition timing has the ability to float around a little rather than be under control at low revs, ie tick over.

Stan

sliproc Avatar
sliproc Kevin Quistberg E
Long Beach, CA, USA   USA
Stan,

As I noted in my P.S. if Darin has been running around with two low compression cylinders but not losing or mixing any fluids this new problem could very well be vacuum related but unrelated to the low compression. In any event if a fix of a vacuum problem gets the car running well again as it was, Darin still has the low compression problem to deal with.

wagonaire144 Avatar
wagonaire144 bill d
mequon, WI, USA   USA
Derrin,
i have a few questions... what type of ignition are you running? If points, look at the lobes on the the dizzy open the points the same amount for every cylinder. The other question i have is did you check for a vacuum leak at the inlet-to-head gasket? if you suck a gasket in, either at the manifold-to-head, or from the carburetter-to inlet, you could find it with a wee bit of spray carburetter cleaner, aimed at the seams. If run for awhile, the lean mixture would dry out the rings or burn the valves in two cylinders fed by thesame carburetter/gasket combo's.

best of luck, tell us what you find!?
billd

. Hide banner ads & support this website 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





Join The Club

Sign in to ask questions, share photos, and access all website features

Your Cars

1956 Austin Healey 100

Text Size

Larger Smaller
Reset Save

Sponsor Links