• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGB MGB tachometer erratic, engine cutting out....

Share this page

B

borjis

Senior Member
Offline
It does this when it's cold from a first start.

Tachometer seems to be cutting out (never gets to zero before bouncing back up) and the engine does to.

The same time this began also I noticed it would not start by the first few seconds, some times it takes two or 3 tries before starting.

Once the car is fully warmed up it hardly ever does it again.

I've checked:
coil connections, no wires are touching each other externally.
cap, rotor, points, plugs all look good.

I have a gold lucas sport coil....possible it's going south?

thoughts?
 
One of my previous MGB's displayed almost this exact problem.

In the wiring under the dash on the drivers side is a 4-way connector with 4 white wires. One of these is the tach, one is ignition, one is fuel pump, not sure what the other is, maybe just hot. The rubber 4-way connector had a small split - just enough to not hold good pressure on the little metal connector inside. Occasionally - going over a bump, my knee brushing it, even going downhill sometimes was just enough to allow the wires to momentarily loose good connection. The tach would drop toward 0, the ignition would die, but almost instantly, apparently the wires make connection and everything returned to normal.

Took me forever to figure that out. Replaced the 4-way connector with one from my parts stash - fixed in about 2 minutes for $0.

Might not be your problem, but check it out.

Bob
 
Thanks for the Suggesion Bob.

Looks like that connector is just fine.

I've also tried swapping out the spark plugs, ignition coil and the distributor rotor.

Still has the problem.

I'm stumped on this one.
 
Maybe it's time to check all ground connections. Give them a good cleaning. What about the engine ground strap?
 
I had a '59 Sprite that did this until I found and fixed air leaks in the throttle linkage going into the carbs. The easiest way to diagnose air leaks is to get the car started and idling, then carefully spray a little starting fluid or brake cleaner around the surfaces where the carbs bolt onto the intake manifold - any place where air can get in after the gas is introduced to the carbs. In my case, the tachometer was only showing me what the engine was doing, it wasn't a problem with the tach electronics.

Tim
 
Jacmo, that's what other locals are telling me...investigate those ground points.

Car is in the winter storage now, but I'll try that come spring and report back on this thread.

Thanks for the words Tim. I'm fairly positive it's not that since I'm using a fairly new Weber down draft...that and the erratic tack and cutting out seem electrical.
 
Look to the points again. Some #600 wet-or-dry run thru 'em when closed.

"Schmaze" build-up on the contacts will do what you describe.
 
OK, so it's been getting warmer these past few days so I decided to get this problem tackled, and tackled I did!

What I've learned from this experience is if the car is having very odd behavior its quite possible multiple issues are causing it and that was the case.

1. my distributor was a little loose. not super loose, but loose enough to make it rotate on a nudge.

2. my points...were also loose. they were not fully opening, just opening a hairline, that was the root of the problem.

as a result of #1 my timing was way off, thats why the car had such a hard time starting.

the car running really bad and tachometer erratic was caused by #2 and just reared itself last august.

so I got out the timing light, re-adjusted the points to a gap of .025 and now the car starts on the first crank and idle's fantastically at 900 rpm. It revs smoothly and no erratic tach now.

thats all folks.
 
Good to hear it's running well.

Just FYI:

The points gap on those is 0.015"

The spark plug gap is 0.025"
 
:iagree: The points gap was always a matchbook task in the old days if ya got caught out without your feeler gauges... 0.015" or "close enuff fer gummint work!" :jester:

And be aware not to over-tighten the dizzy clamp. If ya do, the housing will crack and MORE issues will ensue.
 
ah ok thanks guys.

I'll put the points to .015 then

thanks for the tip on the dizzy clamp.
with a slight force it would turn,(the distributor)
I turned the dizzy clamp nut 3 revolutions to tighten
it. not super tight, but it easily turned.
 
Back
Top