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Missing at speed?

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Darrel

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998 engine. Accelerates smoothly to 50/60 miles an hour and cruises there with no issues - as long as the road is flat. As soon as you hit a hill and place more load on the engine it starts to "misfire". I place it in inverted commas as I am not sure this is what it is doing - but that's how it feels. The following has been checked/adjusted/replaced - valve clearances, float, dashpot oil, points and condensor replaced with magnetronic, dizzy cap, rotor, spark plugs, carb mixture, timing (not sure if I got this right, hence previous post).

The problem occured before all these changes and persists. Someone has suggested that the petrol/gas tank may not be breathing adequately? Can anyone help?
 
Does it "misfire" if you try to accelerate quickly in top gear from (say) 60mph to 70 mph, on the flat?
I'm trying to ascertain if it's the load or the incline that's giving the problem.
 
While your commenting on the test... does the misfire feel like the car has hit a wall and won't go faster? Does the car jerk forward and backward slightly when you reach the point of misfire?
 
Thanks for your comments to date. I did the test this weekend - accelerates fine on a flat road. Tried 40 to 70, 50 to 70, 60 to 70 - she was flawless. But climbs a hill...

I think the best description of the problem - is like some thing grabbing hold of the car from behind and pulling and letting go, pulling letting go... As soon as the hill in conquered it smooths out again.
 
Pulling and letting go and only under load is what I was looking for.

This may not be a misfire. Before you start replacing ignition parts look over the fuel system as it may be the beginnings of fuel starvation. Make sure the pickup screen in the tank is not covered in muck. Look for inline filters from the tank to the pump and from the pump to the carb(s). Replace any filters you find. Also check for debris in the bottom of the float bowl(s) and clean out the float valves to make sure their seats are clean.

I haven't seen a fuel pump fail to delivery the right amount of gas but I suppose it's possible. Do you have the mechanical pump, an electric SU, or an aftermarket electric (like a Facet pump)?
 
It is an electric fuel pump - not sure if it is SU or other. Is there an ideal/best make to install? It was my next set of check to go through the fuel system. Thanks for the tips on where to look and what to do!
 
I'd start by looking for the filters before thinking about a new pump. If there are plugged filters anywhere, a new pump won't fix a delivery problem.

The original SU is a great pump with an undeserved reputation. They are rebuildable using a manual and kits that can be ordered from the "usual sources". If you're not interested in the original pump, the common replacement is the metal, rectangular Facet pump in the low pressure range (up to about 3 PSI). These are available from most auto parts stores (sometimes sold under the Facet name, sometimes labeled Purolator). Depending on the source I've seen the price range from $25 to $50 for the Facet pump.

The link below is for a picture of my car's Facet pump. My pump has been repainted silver. Out of the box these pumps are yellow chromate colored.
https://home.mindspring.com/~dklawson/minipics/facet05.jpg
 
If it happens only going up hills, check the float positions. Not getting proper amount of fuel on grades, but is OK going level and down. Move the float slightly and retest until you are good to go all the time. May not take much. I'm going on the hill up only time! Assuming no less than a 1/4 tank of fuel.
 
I had a look under the car yesterday. It is a Facet pump - 4 psi. Looks and sounds ok and giving a good stream of fuel at the engine.
Will fiddle with the float on the weekend.
 
Any update for us?? How has it been running??
 
Errmm... The problem seems to have disappeared! About 2 weeks after posting this I took her for a drive again. Sailed up and down and along with not a hint of an issue!

This is not to say it won't come back so all suggestions are archived to follow if the problem "reappears".
 
Isn't that the way it goes?? Like a toothache to the dentist, ask about it and it disappears. Good to hear you are up and running! :smile:
 
I like a car that fixes itself. Those are keepers.
 
I had a Toyota that fixed itself. Then it had a relapse when I was going through a mountain pass in the middle of the night on snow covered two lane road without lights. Lost power, bucked and jerked, coughed and died as I slid sideways into a snow bank. The poor old girl had a fuel filter plugged full of rust and scale. Fortunately I happened to have a length of old fuel hose in the boot and bypassed the filter... no hose clamps... I had to wire the hose onto the metal fuel pipes.

My point is, be prepared for the problem to come back unless it was associated with something transient... like a bad tank of gas.
 
Ok. So spoke too soon :frown: However, there is an interesting development - I was driving at night and each time I experienced the drag/hesitation the headlights and dash lights became dim. When the hestation went away the headlights and dash lights became bright again. Interestingly too, the issue occurred constantly - not just uphill. I think it was just more noticeable previously under load - as you need all that the 998 can give you up hills at times :smile:

Checked a few elementary things when I got home - like fan/v-belt on the alternator (yes I have replaced the gennie with an alternator), checked and cleaned up the earth points. They all seemed nice and tight and cleaned.

Any other suggestions? Could it be a voltage regulator issue?
 
I would not expect it to be voltage regulator related... but it could be.

I suggest that you remove the belt and try spinning the alternator and the water pump by hand. You may find that one of the two has bearings that are beginning to fail.
 
The belt may not necessarily squeak. I can't say with certainty that the belt would slip or even that the bearings are your problem. I'm only suggesting it as something to check.

Many years ago I owned a Honda Accord. It seemed to be having some pulsing when driven. I couldn't find the cause. While driving to my father-in-law's house the car practically did a nose stand while cruising at about 35 MPH. It was as if I'd punched the brake pedal and then suddenly released it. It did this three times in quick succession and then the car started moving again with pronounced bucking and surging. I limped the last mile to his house. When I checked under the hood I found the water pump had completely seized. When I fit the replacement water pump all surging and bucking was gone. Had it not been for the water pump bearings finally failing I wouldn't have thought to look at it. The car was otherwise running fine... no belt squeaks, no charging problems, and no overheating.
 
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