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Engine surging at low speed


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My 4a with HS6 SUs has suffered from a surging of engine speed at low revs and low road speed since I bought it 6+ years ago. It occurs as you depress the throttle expecting a smooth increase in engine speed and road speed. This makes it difficult to trickle along in traffic without kangaroo jumps. Various attempts at finding a cure have been made over the years without success. I overhauled the carbs some years ago and have recently replaced the suction chambers and air pistons with NOS. I'm running TW needles with red piston springs. Float chamber fuel level is acceptable and setting the jets at 12 flats below the bridge gives a steady tickover at 650 - 700 rpm with an even pulse. Plug colour after a run is light grey/brown so the mixture looks to be about right. Ignition is by Pertronix Ignitor 2 and is currently set at 6 degrees at idle (8 caused some pinking under load). The piston dampers are running in light GP oil.

 

This morning I tried running without dampers and this seemed to make the condition worse. This afternoon I hit on the idea of seeing what would happen if I ran it without the springs. Interestingly without them the engine would not idle properly on either carb individually or both together, such that I could not drive the car. I was thinking of putting 8oz springs in the suction chambers but this now seems to suggest something different going on. Replacing the springs returned the idle to 'normal'.

 

Anybody have an idea as to what might be causing this?

 

TIA,

 

Rob.

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You could try a heavier damper oil. I use EP90 in mine but it has a modified head and cam. If that doesn't work try going to a heavier pair of springs. I did both of these on Eric and it's just as I like it now.

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That sounds to me more like a sticking throttle linkage than anything to do with the carbs..

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Hello Rog, yes there is oil in the damper - mentioned in my first para

Rob that's an interesting take. The surge happens as you depress the pedal so it would need to be a stored sticking suddenly releasing. This phenomenon can continue for several cycles until you de-clutch to break the cycle. The linkage was overhauled some years ago and is well lubricated and free - not that that means anything of course.

 

Sounds like yellow springs might be worth a try.

 

Thanks all

 

Rob

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do not think tinkering and tweaking is going to cure this one. There s fundamental and probably simple problem, as simple as a piece of carpet fouling the throttle pedal. If the carbs have been overhauled and the mixture looks correct then changing damper springs is just playing around. How about looking else where. If it is kangarooing at lowish RPM's I would suspect the primary advance spring in the dizzy has come of its peg causing far too much ignition advance too early. At higher RPM's this symptom would disappear

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Mine does the same at low speeds when you try to feather the throttle, particularly as I drive over bumps in the road - I believe its due to the solid rod linkage having some lost motion, and movement of the engine as the revs rise - I think soft engine mounts do not help.

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The manual says that the clearance between the throttle connect rod spigot and the bottom of the locating bracket on each carb throttle spindle should be the "thickness of a business card". If too great a clearance there is a marginally slow throttle response but if adjusted with no clearance the gentle rock of the engine at idle will move the throttle spindles and cause lumpy idling.

 

Simon

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Rog, I'm near Leominster in sunny Herefordshire.

Simon, thanks certainly worth a look.

 

The fact that the engine would not idle properly without the springs presumably has something to do with it. I imagine the pistons are not sitting close enough to the bridge at idle without springs so are enriching the mixture more than is required for the throttle opening. The needles and jets are properly centred.

 

Rob.

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and of course it is very easy to adjust to no clearance after balancing the carbs because it s a three handed job to get it correct and a two handed job to get it wrong

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Hi Rob

I was up in Leominster last week. My son has always wanted a 70's Alfa Spider and there was one at auction at Brightwell's. He got it and I have to say it is gorgeous to drive. How did the Italians get 150 BHP from 2000 cc whilst we were struggling with 105? If my 4A gives me anymore agro I may be tempted

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Removing the springs reduces the 'constant depression'. The piston and needle will be lifted more but the mixture will weaken.

Adding a stiffer than normal spring runs the needle lower in the jet but the mixture will richen.

This is because the depression over the jet wins out over fuel area in the jet.

The fuel velocity is proportional to the square root of the depression. Thus for a fixed jet area doubling the depression will increase fuel flow by 1.4 times. If the new lower needle position does not decrease the area by 1.4 times ( it wont ) then the mixure runs richer.

More at the IWE :)

 

Most engines will run fine, but just not optimally, on a wide range of mixtures so I doubt the surgeing is a carb problem, unless say a piston is sticking eg bent damper rod.That should be obvious in a drop test.

I'd check the advance curve on the strobe.

 

Peter

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Bit of an update:

EP90 in the air piston made the situation worse. Strangely 20/50 helped a bit.

Mechanically the distributor looked OK - weights free and springs secured at both ends. It has a 10 degree cam.

Advance curve - bit of a work in progress as either the strobe was playing up or it was finger trouble. Will try again tomorrow armed with the workshop manual figures.

 

I'm thinking that a dyno session might be the only way to resolve this issue.

 

Best,

 

Rob.

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Have you checked the vacuum advance diaphragm is good and holds a vacuum?

Not sure which air cleaners you have, but is the gasket between them and carb, in the correct orientation, perhaps try running with the air cleaners off.

What sort of condition are the engine mounts in?

John

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A couple of things which may or may not be relevant. Are the engine mountings in good condition and everything tight. At very low revs/ road speeds the engine has a tendency to be driven by the rear wheels also causing a rocking motion.

 

Also when the linkage was reassembled is their adequate and equal clearance in the butterfly linkage where the levers turn the shafts. If there is no clearance or unequal clearance it might produce the symptoms you describe.

 

Kevin

Edited by boxofbits
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Thanks all. Some helpful suggestions. Have run without air filters whilst doing other investigations but no different. Dizzy vacuum is ok. Engine mounts look ok but since they haven't been changed in 20 years I expect they might benefit from replacement.

I've re-installed the sm needles I was using and set up the carbs as carefully as I can and there was a significant improvement. I think this is pointing to a fuelling issue which requires a bit more investigation with either a dyno session or investing in a lambda sensor.

 

Best,

 

Rob.

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I go with Kevin as I said in #10. There must be clearance and equal clearance between butterfly linkages and the interconnect shaft levers other wise rocking of the engine at idle will cause "throttle tremor" If in doubt increase clearance over the recommended "thickness of a business card" to ensure no butterfly movement when the engine rocks

 

Simon

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I go with Kevin as I said in #10. There must be clearance and equal clearance between butterfly linkages and the interconnect shaft levers other wise rocking of the engine at idle will cause "throttle tremor" If in doubt increase clearance over the recommended "thickness of a business card" to ensure no butterfly movement when the engine rocks

 

Simon

If your engine is rocking that much there is something seriously amiss with your mounts as well as tuning. There is also usually enough slack in the rod linkage that it should absorb any engine movement without it affecting throttle position.It should never be aligned so there is no slack at all, always set it so you can snap each of the ball/socket links together without having to pull or push either way. At the carb end there is a fork link for each and if you adjust them so the arm is midway between the fork on both this gives the right free play and means they both take up at the same time.

Stuart.

 

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Hmmmmm, whilst I can see the point of the position of the pin in the throttle shaft fork needing careful set up the surge comes when the throttle is partially open, by which time the pin is firmly in contact with the lower fork prong and the slack in the linkage has been taken up.

The engine runs smoothly at idle and low rpm - there's certainly no rocking.

Peter - thanks for that correction.

 

Rob.

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