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Ian Vincent

Problem adjusting HS6 Carb on TR3a

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My TR3a has HS6 carbs on a TR4a manifold.  It has been running in that configuration ever since I finished rebuilding it in 2014.

In July this year I participated in the Liége-Brescia-Liége Rally and the car ran pretty much faultlessly for the entire trip but when I got home blew the head gasket - literally on the doorstep.

I have replaced the head gasket and all seems well in that department but ever since then I have had a problem with adjusting the carbs - or to be more specific, the front one.

When I check the plugs for the rear two cylinders, they are the requisite sandy brown colour but the front ones are greyish white indicating they are running lean.

This has been going on for some time and in the course of my investigations I have removed the offending carb, dismantled it and blown through the jet with an air line.  Re-centralised the jet and made sure that the piston is moving freely, (I have also checked with the engine running and the air filters removed and if I blip the throttle both pistons rise in unison).  I have topped up the dashpot with oil, checked and adjusted the float level and made sure that the valve in the float chamber is working properly.  I have also removed all the fuel lines to the front carb and blown them through with an air line.  And still the front two cylinders are running lean.

Yesterday, I even dug out my colourtune plugs (I don't use them much) and fitted one to  the front cylinder and one to the rear (no.4) cylinder.  No. 4 was blue turning orange - which is correct for my car - it leans out further up the rev range, but no. 1 was pale blue and no matter how much I dropped the jet it didn't change the colour.  I ended up with it over 2 mm below the bridge and it was still running lean.

So it looks as if I will have to remove the front carb again to find the problem but can anyone give me any clues on where to look please? Surely it has to be a fuel blockage somewhere but where?

As an afterthought, I have checked for leaks at the inlet manifold but I can't detect any and to be honest to have an air leak that weakened the mixture when I am driving along at 70 mph it is going to have to be a pretty big hole.  Like I said has anyone got any ideas?

Rgds Ian

Edited by Ian Vincent

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Some problem with the float/needle valve resulting in too low a fuel level in the float chamber? 

 

 

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Two things...why did the head gasket blow ?

they ONLY blow if there is a mechanical fitting problem...low liner heights somewhere completely around the liner circumference, uneven liner heights particularly in liners 2 and 3, or a low or tilted liner on the hottest part of the block at the back of number 4.

Had the engine been torqued to 105 lb run for at least a couple of hundred miles and then retorqued back up to 105 lb ?

Depending upon your answer to these the engine then will remain reliable or will again cause you angst.

Onto the carbs, what actual level did the jet get down to on number one ? you say over 2mm (80 thou) that's not appreciably much lower than I'd expect it to be (in the range of 70 thou), what level is the jet set at on the rearmost carb ?

Mick Richards

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Hi Ian,

 could you have an air leak on the inlet manifold or not bolted down fully

 

Have you got a fancy exhaust manifold with a thin manifold plate. The standard clamps have trouble with a thick inlet manifold and a thin exhaust manifold.

 

Roger

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Hi Mick,

The head gasket blew because in the past the engine had a big valve head with the shroud around the inlet valves removed.  The liners had been shaped to match the head.  On one of hte cylinders, the fire ring was only just clamped between the head and the liner (less than 1/2mm).  I hadn't noticed it when I had previously replaced the head and that thin area had burnt through.  The replacement gasket is a mega expansive one from Racetorations that is shaped for big valve engines and as a result the fire ring is properly clamped.

The rear carb is running perfectly at 0.86mm which is a tad less than the 35 thou figure that I have always used as the starting point for setting up the carbs.  How that compares with 12 flats I don't know because I don't know the thread form.  I didn't measure the actual figure that I got down to without making any difference to the colourtune image because it was way more than I have previously adjusted the carbs to but when I lifted the piston and looked it was well over 2mm.

Hi Roger,

I can't see anywhere where there is an air leak and as I said in my original post it would need to be a heck of a leak to let in the amount of air needed to make the front two cylinders run lean.  Yes I do have a Phoenix exhaust manifold and the manifold plate is thinner than the inlet manifold so I have welded 3mm packers on it at the appropriate places so that it can be properly clamped.  I suppose I could go and buy a can of "Start ya bastard" so that I can completely close that line of thought.

So overall I'm stumped.  My usual reasoning with these things is "what was the last thing that you changed" but in this instance, all I did was remove the carbs and put them to one side to put back on in exactly the same place.  I fitted a new inlet/exhaust manifold gasket but I didn't replace the carb gaskets.

If it helps, here are three photos showing the offending carb viewed froom the front and rear and underneath.

Rgds Ian

PS  I have checked the float level and it is right where it should be and the front float chamber had approx the same amount of fuel in it as the rear when I lifted the lids and looked.

IMG_2547.jpg

IMG_2546.jpg

IMG_2545.jpg

Edited by Ian Vincent

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I am supporting the manifold leak theory.

Bob

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That’s fine with the gasket then Ian, going onto the fix as Roger itemised I would endorse a major air leak. 
You fiitted new manifold gaskets and it’s a known fail for when refitting the manifold to foul or snag on a stud to give a leak bad enough to prevent the engine starting at all ! I would remove and refit the manifold taking particular care to make sure it beds up snug.

Before you remove the carbs start the engine and spray brake cleaner ( or start ya bastard) carefully around the front carb inlet manifold join, if the revs pick up that’s where the air leak is ( normally underneath ) crudely mask” the front carb air cleaner ( or cover with your hand) to prevent it being drawn in the front.

Mick Richards

Edit: I see Bob backs this horse as well.

Edited by Motorsport Mickey

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I concur with the air leak theory. Are you using the thicker manifold gaskets from Revington. They work.

Iain

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Have you checked the vacuum connection at the distributor, and is the distributor vacuum diaphragm leaking perhaps, as this is connected to the front manifold?

John

 

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4 hours ago, John L said:

Have you checked the vacuum connection at the distributor, and is the distributor vacuum diaphragm leaking perhaps, as this is connected to the front manifold?

John

 

I have checked both ends, yes but to be honest, I have run the engine in the past with the vacuum line disconnected and wide open and it as had minimal effect on either the tick over or the running - other than the loss of vacuum advance obviously.

Rgds Ian

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If you unscrew the dashpot lids and leave them loose when you accelerate do both lids rise up?  If not the gasket on the air cleaner side is fitted wrong or the gasket on the other side of the carb could be fitted incorrectly?  There is a cutout in the gasket on the engine side, that should be fitted correctly.

Looks like the front float chamber is a bit at an angle or is it the photo? This will upset the level of fuel in it.

John

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If you have higher compression, achieved by skimming the head, you may need to trim a little from the bottom edge of the gaskets which fit between manifold and head in order to ensure that you get leak-free joints.

Ian Cornish

Edited by ianc
leak for eak!

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33 minutes ago, ianc said:

If you have higher compression, achieved by skimming the head, you may need to trim a little from the bottom edge of the gaskets which fit between manifold and head in order to ensure that you get leak-free joints.

Ian Cornish

Thanks for the suggestion Ian, but it's not a problem with this head although it was with the previous one.  Hopefully by the end of the afternoon I will have the answer as I am about to go into the garage and start shielding the air filters so that I can do the 'Easy start' test before I remove the carb and strip it down. I already have a spare set of gaskets.

Rgds Ian

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Just an idea but I once had problems balancing twin Strombergs and it was caused by some play in the connecting rods between the carbs. Move the throttle by hand and watch the 2 carbs. Do the butterfly valves open in unison or does one start to open before the other?

Keith

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And the winner is......?

Everyone who suggested a manifold leak.  The moment I sprayed brake fluid around the inlet manifold the revs picked up.

Actually, Roger was the nearest because one of the clamps underneath the inlet manifold - front carb - hadn't picked up on the inlet manifold.  It must have slipped around a bit as I was tightening the nut so the bottom of the manifold was unclamped.  Ten minutes to slacken the nut, rotate the clamp to the right place and wedge it with a screwdriver while I re-tightened the nut and all was well.

Many thanks to all who offered help and another lesson in don't ignore the obvious.

Rgds Ian

PS Have just realised i should have given Mick credit for saying the leak would be underneath the inlet manifold

Edited by Ian Vincent

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Excellent Ian, its great when its an easy fix!

Iain

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