I'll try to incorporate other suggestions in to this reply too. I
found myself to be shockingly tired all day yesterday, so I didn't get
out to do a whole lot of troubleshooting.
On Oct 26, 2009, at 1:51 AM, Paul Hunt wrote:
> Could be almost anything, especially since you have recently worked
> on ignition and carbs.
I reset the timing to 15 degrees BTDT (from 19), but I ended up not
changing the carbs. They seemed to be behaving correctly (small rise
then settle when barely lifting the piston) without adjusting.
> What does the tach do for a start? If that is flicking all over the
> place then it is an ignition LT problem. If it is relatively steady
> then HT or fuel. As it happens when cruising you may be able to
> reproduce it with your head under the bonnet, if so clip a timing
> light onto the coil and plug leads and watch the flashes.
The tach is stable. I did drive to get groceries yesterday, so I was
able to drive for about a mile at 35 (in third). It only seems to be
happening when cruising at speeds greater than 45mph.
> Erratic flashing on the coil lead indicates coil or possibly
> condenser problems. Steady on the coil lead but irregular on the
> plug leads (remember the coil lead flashes four times faster than
> any one plug lead) indicates that the rotor or cap are breaking
> down, although that would be more likely under acceleration than
> cruising.
> While on No.1 lead point it at the timing marks to check that it is
> about right, and not jumping around.
Hmm. I didn't check yesterday, but the timing mark has always bounced
around. Even after replacing the timing chain, it bounces more than I
think it should. It's been "an issue" for years.
> If all that is OK then it looks like carbs. Unlikely to be fuel
> starvation as that would be worse under acceleration. Could be the
> aforementioned vacuum leak, test with propane or carb cleaner, or
> possibly float valves, a slight leak causing mixture problems under
> light fuel demand but not under heavier demand. If you have an SU
> pump turn on the ignition but don't start the engine, and listen to
> the clicks. Once the float chambers have been filled the pump
> shouldn't click more than once every 30 secs. More than this
> indicates a leaking float valve or possibly a leaking one-way valve
> in the pump inlet, although this latter is more likely to cause
> problems under high fuel demand than low. Remove the float chamber
> vent pipes to see if one or other carbs does eventually start to
> overflow, this can take several minutes, so disconnect the coil to
> prevent it overheating.
I'll hopefully be able to check for leaks tonight. I don't have the
SU pump, so I'll just have to wait for it. I did check the plugs
yesterday. The #1 plug was a bit white, but otherwise all plugs were
a nice light brown ashy color (I'll pull that #1 again and take a
closer look at it)
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