Juddery engine at speed with throttle

[hanvyj]

Hi,

I've got an intermittent problem (which I hate because you never know when its fixed). I took about an hours trip on the motorway recently and it surfaced about 50 minutes in. Parked up then it was doing it for the first 30 min of the way back then got better...

The engine was juddering/jerking - felt like it was loosing power on and off causing the car to buck a little. It was varying in intensity from making a slight vibration to it struggling up hills (I had my eye on the hard shoulder expecting it to die).

It only occurred at high speed (well, 45+) and only when the throttle was over about half way down - so hills were trouble. With the throttle open downhill (where the engine can't have been doing all that much work) it was the same as the throttle open uphill with the engine at its max. I could only get it to do it in fourth.

I have had it before at lower speed (and in lower gears), I thought it was the timing being too advanced, so I retarded it a little and it went away - but its quite intermittent so I don't think it that necessarily fixed it.

Any ideas? I have a HIF38 carb and inlet to fit at some point but I've been putting off cutting the current manifold as I don't have a grinder - now might be the time to take the plunge and fit it but I was wondering if anyone else has had similar behavior and might know what it is.

Thanks

[lambrettalad]

if its electrical it's a part breaking down,I'd start by looking at :the points,condenser, cap and rotor arm(condenser favourite)>What colour are your plugs?How powerful is the spark?

other wise fuel starvation ?clean out carb.Do you have an inline filter fitted?

start with the basics then get moreand more worried

[JOWETTJAVELIN]

Check the engine mounts and steady bar.

Look at the CB points which will give a clue as to the health of the condensor.

[MarkyB]

Did you try giving it a little choke to see if that improved matters?

[bmcecosse]

Probably fuel starvation -clean the pump filter.

[hanvyj]

I thought it might have been a fuel problem too, I'll try give the pump a clean. Steady bar seems very solid, I'll check the mounts though I don't think the engine vibrating itself is the issue.

I didn't try giving it a bit of choke, I'll see what happens next time it happens - would that mean it is too lean?

It has an electronic ignition, relatively new - so unless its faulty I doubt its that. No inline fuel filter fitted. I'll give cleaning the carb a go before fitting the new one.

Thanks everyone, I'll give everything a go and report back in a while if/when it happens again!

[bmcecosse]

If it's fuel starvation - pulling the choke won't help. Clean the filter.....

[hanvyj]

Well, I took apart the pump.

Its some aftermarket electronic one so I don't trust it much, also I couldn't find any filters in the pump at all - everything was clean and serviceable though - the valves moved freely.

It fits at a different angle to the SU pumps, so there is a rubber pipe from the (copper?) fuel pipe to the pump which looks a bit tired so I might try replacing those.

That said, I'm tempted to just shell out and get an SU pump I can trust.

[bmcecosse]

Oh dear - the rubber pipe may be collapsing, or the non standard pump may not be able to pump sufficiently - they are generally designed for flooded suction - which the high position beside the battery does not provide......

[Piston broke]

Have you tested the compression of all cylinders?? My engine is lumpy at certain speeds and revs. It likes to burn out its exhaust valves. Tappets are correct so I don't know why this is (?!) but it judders and I THINK it may be the uneven pressures its running. Perhaps yours is the same??

[bmcecosse]

Set the exhausts to 15 thou..and check/reset every 3000 miles.

[Neil MG]

Definitely check valve clearance, points gap, plug condition and all the standard service checks that are often missed!

Also, if the jet tube on the carb has been replaced there is a rubber o-ring at the bottom of the float chamber end. If the old one was not removed it can partially or intermittently block fuel flow, which would give the symptoms you describe. It is tricky to see and to get out so it's not an uncommon fault!

Fuel pump problems would more likely give much more on/off symptoms. Otherwise rough running is likely tuning (too lean mixture), air flow restriction, timing or possibly a breakdown somewhere in the HT system (arcing/too high resistance).

Another potential problem that might give similar symptoms and normally occurs after cars have stood for a long time is sticking balance weights in the distributor, but I have not come across that (starting to happen) on cars in regular use.