> Hey folks I wonder if you can help me with a problem that has recently
>occured with my M-43.On a recent highway journey I was'nt able to get the
>truck going faster than 40mph and it kept hesitating slightly.I had just
>cleaned the carb
>the day before.Anyways it has gotten worse and I can't hit any of the high
>speeds without it backfiring and hesitating.So once again I took out the
>carb and gave it a really thorough cleaning,put a new fuel filter and hose
>in and
>poured some carb cleaner down its throat for good measure and whattaya know
>same thing.
>
Symptomatic of lean high speed mixture or internal spot overheating.
Pouring carb cleaner in it as envisaged from the above will do virtually
nothing, a thorough clean and air blast through the passages, jets and drillings
is needed. It is suspicious you infer it was OK before the carb clean
suggesting that a problem might have been introduced by faulty re-assembly.
Carb cleaner as we know it is largely acetone and will not only remove fuel tar
and varnish but will eat away accelerator and power jet diaphragms in short
order, remove these before using.
As described the symptoms sound much like weak full-throttle/high speed mixture
which will cause hesitation and backfires as parts of the engine get seriously
overheated, e.g. the plug electrodes start to glow and ignite the mixture on the
way in.
The reason could be quite obscure and includes, insufficient full power fuel
feed to the carb, inoperative power jet system (if fitted), air leak in trunking
to air cleaner as appropriate.
>So it must be the ignition right?
>
Not necessarily but similar effects could be due to, coil or capacitor
(condenser) breaking down under high-rate stress, HT feed breakdown, distr cap
breakdown or fouled plugs.
>Well I'm getting a good spark
>from the plugs,90psi compression on all cylinders,replaced the distributor
>cap to make sure and timed it manually and with a timing gun.Still shakes
>and bangs
>when I rev it too high.Its been running good all summer and then this just
>pops up out of nowhere.Any feedback is greatly appreciated. Thanx.
>
Good sparks at low speeds is helpful but not indicative of the high speed
performance or plugs under compression gas loads.
It is assumed that timing it with a gun is appropriate, it may not be. Some mil
engines are timed TDC or AFTER statically, in these cases timing with a strobe
light effectively retards the ignition as the centrifugal advance may be
intentionally operating at idle speed. If any reference data gives static
timing or describes a procedure using a simple bulb/battery set-up then this
must be followed, applying the static timing data to a running engine is totally
wrong.
Retarded timing will have the combustion spaces run very hot as the fuel is
still burning before the end of the power stroke, this may well set the plug
tips or any carbon deposits glowing and pre-igniting the mixture, excessive
carbon build-up in the cylinder head has a similar effect and may have the
engine run on roughly after the ignition is switched off by dieseling and glow
ignition of the fuel.
Another obscure phenomena causing weak high speed mixture could be carb icing
owing to a subtle (unpublicised) change in fuel formulation and although
possible on damp summer days it may be prompted by high volatility (not octane
rating) fuel and cooler temperatures. It is a common problem with current UK
fuel formulations in many older, carburated, vehicles and MVs.
Richard
(Southampton UK)
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