Engine surging at low speed

[peejay4A]

If balancing the carbs improved things I don't think it's a linkage problem. If the symptoms are a little transmission shunt when on partial throttle, such as trickling along in traffic then you're looking at a fuelling, air leak or timing problem.

[boxofbits]

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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I would agree with Stuart on that. The engine mounts are an outsider but a remote possibility.

 

The throttle linkage is an awful set up on these cars and I'm surprised they didn't delete it and go cable on the revised 4a, like the MGB which had cable from 1962 on. I suffer with a similar issue on my own 4a when on rough road, as most now are, and trying to move slowly the problem can exacerbate itself without trying too hard.

 

I set mine up with a bit more slack than originally intended for this reason as the roads are now much bumpier.

 

Kevin

[stuart]

What cam have you got as I have come across low speed issues with so called "Fast road cams"

Stuart.

[boxofbits]

In my case you could be right Stuart as I opted for a Triumphtune TT1105, which is the Sprint cam. It's probably a Kent or Piper cam I should think?

 

It ticks over a little bit on the lumpy side, but I've never had it dyno'd. I believe I used a multi-hole sprocket on the Triumph so supposedly its within a 1/4 tooth which is okay but not great.

 

On the 2.0 Litre Pinto Hot Rod engine supplied by Toovey in Birmingham, and the Cooper S 1275 from Midland Engine Developments they both came with vernier style sprockets which I thought was much more accurate and easier to set up, so I would assume this is the choice of most professional engine builders. It seems pointless using dial guages and then being 1/4 tooth out!

 

Kevin

[Rob19]

I've just installed a Newman TRI4/280/405 PH1 - fortuitous as I found three followers breaking up with consequent damage to cam lobes. Anyway that's all academic since the problem has been evident for 6+ years

The new cam has smoothed out the idle and may have helped a little with the surging but hasn't eliminated it.

 

Pete that's been my thinking and the improvement following the change back to a slightly richer needle might suggest more investigation required in that area. Whether with a UEGO or dyno is the next question.

 

Rob

[Tom Fremont]

Hi Rob,

 

Silly question - have you played with the mixture settings ( sorry if I missed it above )? " Snatch" is symptomatic of rich running at low revs, at least with Webers. If you haven't, you might try leaning them out by a good amount and see if it changes anything. If so, you're down to refining the needle selection. I marvel at those who get these right across the range with non-original camshafts. Seems like a lot to ask of a single tapered pin.

 

Cheers,

Tom

[Peter Cobbold]

Hi Rob,

 

Seems like a lot to ask of a single tapered pin.

 

Cheers,

Tom

Tom,

There's much more going on around that needle than is obvious. It can deliver different mixtures at the same point along it, depending upon how much pulsation gets past the butterfly. More at IWE.

Peter

[Peter Cobbold]

I've just installed a Newman TRI4/280/405 PH1 - fortuitous as I found three followers breaking up with consequent damage to cam lobes. Anyway that's all academic since the problem has been evident for 6+ years

The new cam has smoothed out the idle and may have helped a little with the surging but hasn't eliminated it.

 

Pete that's been my thinking and the improvement following the change back to a slightly richer needle might suggest more investigation required in that area. Whether with a UEGO or dyno is the next question.

 

Rob

A UEGO and a 2metre inspection camera to see piston lift would be my choice. You can then tune the cruise mixture on the road and let the inherent mixture spread of the SU do wot mixture.

Peter

[boxofbits]

A UEGO and a 2metre inspection camera to see piston lift would be my choice. You can then tune the cruise mixture on the road and let the inherent mixture spread of the SU do wot mixture.

Peter

The design feature of the SU that incorporated hydraulic damping oil was something of a failure really, and allowed a somewhat variable performance of an otherwise brilliant carburettor. I wonder if an additional variable rate spring would have been a better idea, or a sealed oil chamber. But I suppose the suction of the oil also alllows the piston to drop at a controlled rate.

 

The oil was found to periodically disperse or was often at the owners discretion as to what grade of oil, or indeed any oil at all, was replenished, leading to thousands of underperforming engines.

 

Kevin

Edited June 3, 2017 by boxofbits

[Peter Cobbold]

The design feature of the SU that incorporated hydraulic damping oil was something of a failure really, and allowed a somewhat variable performance of an otherwise brilliant carburettor. I wonder if an additional variable rate spring would have been a better idea, or a sealed oil chamber. But I suppose the suction of the oil also alllows the piston to drop at a controlled rate.

 

The oil was found to periodically disperse or was often at the owners discretion as to what grade of oil, or indeed any oil at all, was replenished, leading to thousands of underperforming engines.

 

Kevin

Kevin,

The oil damper was introduced post-WW2 when the pistons changed form brass to aluminium alloy. Presumably the greater inertia of the heavier brass pistons provided sufficient transient enrichment upon flooring the throttle.

Sealing the oil chamber is no good- the trapped air prevents the piston rising as it has to be compressed. Either the damper rod screw-cap must be pierced or there is an internal drillling from the top of the oil chamber to the suction chamber, ( but not both ).

The piston needs to drop fast upon closing the throttle. Really heavy oils such as EP90 or 140 might slow the descent of the piston upon closing the throttle leading to transient lean mixture. I have tried EP140 and it gave problems- will revisit when the UEGO and camera are working.

A variable rate spring would need to be intially stiff then relax. Can that be done?

I like SUs but they are much more complicated, and capable, than I thought a year ago....

 

Peter

Edited June 3, 2017 by Peter Cobbold

[LGFromage]

My suggestion is that the ignition coil is not receiving full battery voltage thereby reducing spark plug performance, especially at low speed. Check the voltage at the switch side of the coil and compare it to the battery voltage with engine switched off. If there is a difference it is either the ammeter with a dry joint or the the ignition switch contacts need cleaning. To test for a faulty ammeter check the voltage at the A1 terminal on the regulator: full battery voltage indicates that the problem lies with the ignition switch. Since the overdrive works on the same ignition switch terminal as the coil there could also be an improvement in its operation.

 

Tim

[Rob19]

An expensive and largely inconclusive morning spent on a rolling road threw up a couple of observations which might be indicative of where the problem lies, or may just be more red herrings:

1. it gets worse when the mixture is leaned off to something near a sensible fuel/air ratio;

2. the possibility of the PCV having some bearing on the problem - although disconnecting and plugging the connection to the inlet manifold didn't seem to have much effect (the diaphragm is new-ish).

 

Anybody have any thoughts as to why the lean mixture makes it worse and what symptoms might be expected if the PCV was not working correctly?

 

TIA

 

Rob.

[Peter Cobbold]

Did the RR session include checking the spark centrifugal advance curve -see posts 7 and 14.

A way over-rich mixture will burn slowly, so your leaning to a reasonable mixture may have made a faulty over-advance even worse.

Mixture and spark timing interact.

Peter

Edited June 17, 2017 by Peter Cobbold

[Rob19]

Thanks Peter. I checked the advance at the lower end (vacuum?) before going to the RR and it seemed to accord with the workshop manual figures and the centrifugal advance was checked on the RR and seemed to be in the right ball-park, I think. However the idle advance was showing something like 13 degrees on the RR when I had set it at 5 degrees because of pinking at 8. I've just checked it again at idle and got a similar figure on my strobe - 14 degrees. So there is evidently something not quite right there. Possibly a weight sticking or an incorrect spring? It would have to be something which could create the symptoms over a long period without any change in the intensity/frequency since this has been happening for a number of years.

 

I guess a clean-up of the centrifugal advance bits and maybe some new standard bob-weight springs wouldn't go amiss - unless I just send the distributor straight to DD.

 

Rob.

[Peter Cobbold]

Thanks Peter. I checked the advance at the lower end (vacuum?) before going to the RR and it seemed to accord with the workshop manual figures and the centrifugal advance was checked on the RR and seemed to be in the right ball-park, I think. However the idle advance was showing something like 13 degrees on the RR when I had set it at 5 degrees because of pinking at 8. I've just checked it again at idle and got a similar figure on my strobe - 14 degrees. So there is evidently something not quite right there. Possibly a weight sticking or an incorrect spring? It would have to be something which could create the symptoms over a long period without any change in the intensity/frequency since this has been happening for a number of years.

 

I guess a clean-up of the centrifugal advance bits and maybe some new standard bob-weight springs wouldn't go amiss - unless I just send the distributor straight to DD.

 

Rob.

Rob,

There should not be any vac adv at idle, it needs the butterfly to be opened until its edge is under the take-off that feeds the capsule. To eliminate that capsule as faulty disconnect the pipe and blank off the carb nipple. Then test run.

 

Next, try setting the static to 5 ATDC, because it seems to be advancing about 10 deg too much on the RR. Then test. If that kills the surging then phone DD to see if he can supply the correct springs and weights. Or get him to rebuild.

 

Peter

Edited June 17, 2017 by Peter Cobbold

[PYU940F]

As I said n my post 7. I suspect to much advance at low rpm. Can you borrow a good dizzy from a member near you and then at least you can rule this in or out

 

Smon