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Unbalanced carbs and red hot manifold!


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I have been trying to recommission my 1976 USA TR6 imported to here in Belgium since a respray and several mechanical improvements. I am being frustrated in trying to get the carbs balanced. The Stronbergs were replaced several years ago with SUs (nothing unusual in that) and before the work done on the car they were working fine.

I started by checking the points and timing. I set the dynamic timing to 4 ATDC at idle and checked that with the vacuum tube removed they fell back(?) to 12 BTDC which is what various experts seems to recommend. So far so good. I then moved on to the carbs and this is where it all went wrong. Firstly, I checked the level of the jets with dashpots removed and the top of the jet level with the surface. I marked the adjusting nut and unscrewed both carbs by two full turns. I used full choke and the car starts. However, when it comes to balancing I am going bananas. I get the synchroniser up to the same point on each carb so they are sucking in the same amount at idle. Then when I try to accelerate there is a hesitation and the occasional backfire out of the rear carb. Also I noticed the rear half of the exhaust manifold heating up to red hot!

Reading some online forums have left me very confused. Some say it is due to too lean a mixture whilst others reckon it is due to being too rich. Can it be either or?

Another thing I have noticed is that while the front piston in the carb moves freely up and down when applying the gas the rear one is more reluctant. Its as if it is getting stuck but I've checked and cleaned the dashpot and it moves freely enough. I've checked the plugs and they all seem ok - perhaps the rear are showing signs of a leaner mixture but not drastically so.

I would really welcome some advice to point me in the right direction.

My next step, I guess, is to remove the rear carb and check that the butterfly is working properly. Could it be as simple as the type of oil in the dashpot? Although both have the same oil.

Thanks in advance - and if any of you happen to know a nearby (I live near Ghent) mechanic I'd be delighted to hear from you!

   

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Steve,

Red hot manifold <<<< Timing retarded far too much.  The fuel is still burning in the exhaust!    This retard may account for your other problems.

I suggest checking that TDC is correctly indicated by the mark on the pulley.     The pulley has two functions, the second being to act as a damper for torsional vibration in the crank.   The outer part was bonded by a rubber ring to the hub, but age and the hostile environment under the bonnet may have caused the outer ring to slip around and show TDC where it is not.     If you can locate TDC by the Piston Stop Method (QV) then you can know if your pulley is correct or not.

John

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Lift the piston fully in both carbs and let drop - there should be an audible 'donk' as the piston hits the carb bridge.  If not the piston is sticking and needs attention be it bent needle, needle centring, dirt or perhaps a missing carb suction chamber spring.

Search the youtube for balancing SU carbs - lots of methods on there for MG, Triumph and Mini.

Peter W

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Manifolds will glow red. 

If you have ever run in a new camshaft by holding at 2000rpm for 20 minutes you will definitely see them glow.

The odd thing here is that not all are looking the same.

Without stating the obvious have you got the plug leads with the correct fitting order?

Is the fuel in the float chambers at the same level?

Do both dashpots contain oil.

 

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On the problem carb make sure the diaphragm is in good shape (no holes or tears) and if so, seated properly with the locating tab in the recess in the carburetor body.  

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Timing your engine to 4 degrees ATDC I assume that you still have a vacuum retard unit fitted to your distributor. This runs on a vacuum port on the underside of the stromberg flange. SUs normally have the vacuum port on the top of the flange to operate a vacuum advance unit.

If you have connected the retard unit to the port on the top of the SU your retard unit will be retarding the ignition when the engine requires advance. Try disconnecting the vacuum unit, plug the port on the carb and set your timing to say 10 deg BTDC and see how it runs. If this works OK  either get the distributor set up with a vacuum advance unit or get the carb flange drilled on the underside for the retard unit and use 4 deg ATDC

George 

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17 hours ago, john.r.davies said:

Steve,

Red hot manifold <<<< Timing retarded far too much.  The fuel is still burning in the exhaust!    This retard may account for your other problems.

I suggest checking that TDC is correctly indicated by the mark on the pulley.     The pulley has two functions, the second being to act as a damper for torsional vibration in the crank.   The outer part was bonded by a rubber ring to the hub, but age and the hostile environment under the bonnet may have caused the outer ring to slip around and show TDC where it is not.     If you can locate TDC by the Piston Stop Method (QV) then you can know if your pulley is correct or not.

John

My question is why that would appear to effect only the rear half of the exhaust manifold? Surely, if the engine was retarded too much then both halves of the manifold would get hot? Also I did use the static as well as the dynamic timing and they were both in agreement. Can retardation affect only part of the timing?

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16 hours ago, BlueTR3A-5EKT said:

Lift the piston fully in both carbs and let drop - there should be an audible 'donk' as the piston hits the carb bridge.  If not the piston is sticking and needs attention be it bent needle, needle centring, dirt or perhaps a missing carb suction chamber spring.

Search the youtube for balancing SU carbs - lots of methods on there for MG, Triumph and Mini.

Peter W

Was wondering if that could be the problem. Will check with youtube to see about checking these out. Might also explain why the piston in the rear carb does not move up and down with the same ease or to the same extent as the front one when under acceleration. Thanks for the advice.

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16 hours ago, Andy Moltu said:

Manifolds will glow red. 

If you have ever run in a new camshaft by holding at 2000rpm for 20 minutes you will definitely see them glow.

The odd thing here is that not all are looking the same.

Without stating the obvious have you got the plug leads with the correct fitting order?

Is the fuel in the float chambers at the same level?

Do both dashpots contain oil.

 

The first thing this morning I went out to check the plugs leads! As you say, pretty obvious but something that does not spring to mind! Result- all in the right order.

Will check the float chambers when I check out the needle and jet.

Oil is the same level in both dashpots. Perhaps I'll try a lighter grade of oil?

Thanks for the pointers.

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13 hours ago, harlequin said:

Timing your engine to 4 degrees ATDC I assume that you still have a vacuum retard unit fitted to your distributor. This runs on a vacuum port on the underside of the stromberg flange. SUs normally have the vacuum port on the top of the flange to operate a vacuum advance unit.

If you have connected the retard unit to the port on the top of the SU your retard unit will be retarding the ignition when the engine requires advance. Try disconnecting the vacuum unit, plug the port on the carb and set your timing to say 10 deg BTDC and see how it runs. If this works OK  either get the distributor set up with a vacuum advance unit or get the carb flange drilled on the underside for the retard unit and use 4 deg ATDC

George 

Hi George

Having set the timing at 4 ATDC at idle with vacuum connected I then disconnect the vacuum and blocked the port on the distributor and she registered 12 BTDC. If I block the the port on the carb instead should I also block the port on the distributor at the same time? Not sure whether this would cure the problem I have with one carb as the front carb seems to be operating perfectly.

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2 minutes ago, stevefmyers said:

Hi George

Having set the timing at 4 ATDC at idle with vacuum connected I then disconnect the vacuum and blocked the port on the distributor and she registered 12 BTDC. If I block the the port on the carb instead should I also block the port on the distributor at the same time? Not sure whether this would cure the problem I have with one carb as the front carb seems to be operating perfectly.

Steve

You only need to block the port on the carb, its only a small hole but it will draw unregulated air and affect your carburation. The port on the distributor vacuum unit can be left open, (this is how injected cars left the factory)

As a thought how old are the SUs? Earlier types require the jets to be centered otherwise they can stop the needle moving smoothly. 

George 

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It is worth checking that the su needles are the same spec (mine were marked sm) by removing them carefully.

It is vital that when they are put back that you do not insert them all of the way into the recess but insert them so that the shoulder of the needle is flush with the base of the holder.Failure to do this results in the needle being too high which leads to a very rich mixture.

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1 hour ago, stevefmyers said:

My question is why that would appear to effect only the rear half of the exhaust manifold? Surely, if the engine was retarded too much then both halves of the manifold would get hot? Also I did use the static as well as the dynamic timing and they were both in agreement. Can retardation affect only part of the timing?

Ah! missed that.      Only half the manifold does point to the carb on that end!

Jhn

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2 minutes ago, john.r.davies said:

Ah! missed that.      Only half the manifold does point to the carb on that end!

Jhn

Gentlemen, they say the simplest solutions are the easiest to overlook but fiddling around just now I found that the rear idler screw was fouling on the throttle linkage. I remember seeing in a video how someone doing the conversion (from Stroms to SUs) had to cut out a small piece due to this fouling problem. Not sure yet whether it will be the solution to all my problems but it does explain why there was no throttle on the rear carb - the more I turned down the idle speed the less room it had before hitting the linkage - it seems like I only had a few mm of throttle on that carb. It does beg the question however of how it was running before I put it away. In my defence I can only say the engine was tuned by a member of our Belgian Oldtimers Club who admittedly struggled to set it up. Sadly, we are no longer in touch with this chap so I am now looking for another local mechanic who likes tinkering with old cars.

I'll let you know whether this 'adjustment' resolves the issue. In the meantime, both pistons give a satisfying clonk so I reckon they are both centered ok. Firing order is fine. Fingers crossed as I put the linkage back together.

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Well done, Steve!    

But I fear it shows the impotence of remote gurus to detect such things!     But I do find personally that just discussing them, and explaining your problem makes you think better. so that you can solve it yourself.   I call it the Common Room Solution!

JOhn

Edited by john.r.davies
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24 minutes ago, john.r.davies said:

Well done, Steve!    

But I fear it shows the impotence of remote gurus to detect such things!     But I do find personally that just discussing them, and explaining your problem makes you think better. so that you can solve it yourself.   I call it the Common Room Solution!

JOhn

Thanks John. But I'm not sure I would have noticed the problem without going through all the other checks suggested by everyone. I like to think cars are logical beasts and if one takes the right approach you are bound to come across the solution. Anyway, now got both carbs working together. Will take it for a ride tomorrow to clear the old carbon build up that's probably hanging around the cylinders! Dieseling is one of those problems I 've always had with this car and its still there. Watching a video I saw someone using 'Seafoam' to clean out the carbs and cylinders. Only trouble is no-one over here has heard of that! Anyone know of an alternative? Whilst it is an import from the USA it came with no anti-run-on valve (before anyone suggests reconnecting it - it ain't there!).

10 brains are better than 1.

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1 hour ago, stevefmyers said:

Thanks John. But I'm not sure I would have noticed the problem without going through all the other checks suggested by everyone. I like to think cars are logical beasts and if one takes the right approach you are bound to come across the solution. Anyway, now got both carbs working together. Will take it for a ride tomorrow to clear the old carbon build up that's probably hanging around the cylinders! Dieseling is one of those problems I 've always had with this car and its still there. Watching a video I saw someone using 'Seafoam' to clean out the carbs and cylinders. Only trouble is no-one over here has heard of that! Anyone know of an alternative? Whilst it is an import from the USA it came with no anti-run-on valve (before anyone suggests reconnecting it - it ain't there!).

10 brains are better than 1.

Hi Steve - I've been plagued by running on since increasing the compression ratio, even on a new engine. I managed to avoid it by using cooler plugs and 99 RON petrol. I also have a anti run-on valve which does work well if the solenoid doesn't stick shut when the engine is hot.

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