H6

[Richardtr3a]

I have been cleaning up my carbs and they are now running very rich. Needle in correct place, mixture control at high adjustment against spring. The piston is quite hard to push up with a screwdriver with the engine off. I think this may be the problem. If the damper is not fitted the piston moves OK. It was all OK until I cleaned everything and put it back together.

Any help would be very useful.

Thanks Richard & B

[John McCormack]

6 minutes ago, Richardtr3a said:

I have been cleaning up my carbs and they are now running very rich. Needle in correct place, mixture control at high adjustment against spring. The piston is quite hard to push up with a screwdriver with the engine off. I think this may be the problem. If the damper is not fitted the piston moves OK. It was all OK until I cleaned everything and put it back together.

Any help would be very useful.

Thanks Richard & B

Have you centred the jet? If not, probably best to look it up in the manual and do it by the book.

[Richardtr3a]

I spent a long time yesterday checking needle height, centering  jet etc. all as per the manual. All correct washers are fitted and oil is in the piston until the damper is 1/4” from closed but they are both hard to lift 

thanks

[Lebro]

Has the same damper gone back into the right carb, did you dismantle the damper, & perhaps put it back together incorrectly, have you got oil in it which is too thick ?

Bob.

[Charlie D]

If the piston is hard to push up, would this not result in the mixture being too lean?

My logic (quite possibly wrong) is that with the needle down there is a smaller gap for petrol to get out of the jet, resulting in a weaker mixture.

If the needle was sticking in the up position for too long then it would result in a rich mixture.

 

Charlie.

[Richardtr3a]

1 hour ago, Lebro said:

Has the same damper gone back into the right carb, did you dismantle the damper, & perhaps put it back together incorrectly, have you got oil in it which is too thick ?

Bob.

Yes the same dampers are in correctly and the oil is SU oil co. product from Burlen. But if the piston is slow to lift will it also be slow to close and therefore giving a rich mixture. It was OK before I cleaned it and I should have left the whole thing alone. 

Richard

Edited March 29, 2021 by Richardtr3a
spelling

[Charlie D]

37 minutes ago, Richardtr3a said:

But if the piston is slow to lift will it also be slow to close and therefore giving a rich mixture.

Richard,

From the Burlen site:

"A one-way valve is incorporated in the damper plunger (11) and this is fitted in an oil-fitted reservoir in the hollow piston rod (12). The piston damper assembly will restrict the rate by which the piston lifts, but will allow it to fall freely on throttle closure. The primary purpose of the piston damper is to provide the enrichment necessary for a satisfactory 'pick-up' during rapid opening of the throttle."

 

This also indicates that I am wrong thinking that the needle being down makes the mixture weaker.

Baffles me a bit...

37 minutes ago, Richardtr3a said:

It was OK before I cleaned it and I should have left the whole thing alone. 

How many times have I said that to myself...

Charlie

Edited March 29, 2021 by Charlie D

[Hamish]

Try just loosening the screws on the dash pot domes. To let the pistons find their own sweet spot. Do the pistons tighten up again when the dash pots retightened ?

[Richardtr3a]

Back on it after lunch if it is not raining. Thanks 

[RobH]

1 hour ago, Charlie D said:

This also indicates that I am wrong thinking that the needle being down makes the mixture weaker.

Baffles me a bit...

You were right first time Charlie- but only for steady-throttle conditions.

The SU carb is designed to maintain a constant depression or partial vacuum across the bridge above the jet. It does that by raising and lowering the piston in response to variations in air flow caused by opening or closing the throttle butterfly. That in turn adjusts the position of the needle to provide the correct amount of fuel - the further out it is the more fuel that is supplied. The needle is contoured so that under constant depression conditions it is always providing the right amount of fuel for the air flow. 

Under acceleration it is necessary to give the engine a pulse of richer mixture.  When the throttle is suddenly opened the velocity of airflow across the bridge increases which in turn increases the vacuum above the jet. More fuel is sucked past the needle than is correct  for that amount of airflow and the mixture becomes a lot richer.  The vacuum is transferred to the space above the piston, which then starts to raise to open the gap (venturi) again and drop the vacuum back to its constant value. Without the damper the rise would be too fast to provide sufficient richening but with it the piston slowly rises and the richening decreases until a new balance is reached between the downward force of the spring plus weight of the piston and the upward pull of the vacuum. At that point the constant depression is re-established and the new increased fuel flow is again correct for the volume of air flow.

 

[Charlie D]

Thank you for that Rob.

I am now de-baffled.

Charlie

[Richardtr3a]

I have cleaned up both carbs inside and out. The two pistons fall fast and with a metal click when there is no oil in the suction chamber.  I am attaching a picture of one of my spark  plug which is very black after 17 miles.Today I have adjusted the jet to the highest point on both carbs and have not adjusted downwards at all. This is not according to the tuning guide  instructions from Burlen. Tomorrow I will try a short trip and when up to temperature I will see if it is running weak which it should be.

I am calling in at Burlen in Salisbury for a new main throttle spindle kit at the end of the month.  It may be time for new needles and jets.  What are  else would be sensible to replace ? The springs are not red and there is no washer under the spring. The needles are SM and it was running well last year. There are no leaks at present after I was sent the correct gasket for the float lid by a forum user who has experience and answers.

Thanks for the help so far. 

Richard & B

[iain]

Perhaps  it’s time to check  the choke lever mechanism on the front carb....notorious for being very stiff, lubricate with some lithium grease and “viola” it all moves freely and doesn’t cause rich running.

Iain

[Ralph Whitaker]

13 hours ago, Richardtr3a said:

I have cleaned up both carbs inside and out. The two pistons fall fast and with a metal click when there is no oil in the suction chamber.  

I would say then that your problem lies elsewhere, perhaps as has been suggested sticking choke mechanismn. The pistons on my carbs also take quite a bit of pressure to lift with oil and dampers in place, they then drop fairly quickly in a steady controlled sort of way, which is how they should be.

I don`t know what viscosity the correct SU oil is, but have seen reccomendations for everything from 3 in 1, auto transmission fluid, and engine oil, but these only vary the speed with which the pistons lift, the eventual resting place of the pistons at a steady throttle opening being controlled by the balance between the spring and the inlet vacuum. Weak springs would cause rich running, as would worn jets and needles.

I have all new parts in my carbs, springs, needles, jets, and although I have only just started using the car my first 100 miles seems to have worked out at 17 miles per gallon, and also have sooty plugs. I have turned the jets up 2 flats, and the car now hesitates until warm, so eager to see what the next 100 miles gives me.

Ralph

[Richardtr3a]

I am checking the jet height when fully adjusted upwards. It is 4mm below the bridge which may be a bit low. How can it be improved. Anyway a road test today and see how it goes when up to temperature.

Thanks Richard & B.

[Drewmotty]

There’s something wrong there Rich. The jet should come up near flush when screwed right in. Are you sure that it’s assembled correctly and that the choke is fully seating?

[NCS_TR3A]

2 hours ago, Drewmotty said:

There’s something wrong there Rich. The jet should come up near flush when screwed right in. Are you sure that it’s assembled correctly and that the choke is fully seating?

Agree, mine are nearly flush when fully adjusted up. I think that will be your problem and certainly explain why rich. 

Neil

[Lebro]

I would go so far as to say that the top of the jets should protrude slightly above the saddle when wound fully up. 

Bob

[Richardtr3a]

This is becoming more hopeful. We need an owner who gets high mileage and easy starting. If we can find someone we could ask them to measure the height on their jets.  I have spoken to Burlen who had no answer.

If my jet is too low by as much as 4.0mm,  then I am thinking of ways to solve the problem. It would be possible to alter the jet bearing, but I am not keen on home made answers when there are owners out there who do not have the problem.

So who is the owner of a high mpg TR3A who could help ???

Thanks Richard & B

[Lebro]

Top of jet will be approx 1 to 2mm down, but after undoing the adjuster by several turns. There must be something wrong in your assembly

Bob

[Ian Vincent]

My TR3a has been fitted with HS6 carbs and is not that economical but when I remove the plugs after a run they are the correct colour. I set them using two colourtunes and a carbalancer. on the front carb the jet is 0.88mm below the bridge. The rear one is 0.66mm down. 

I don’t know if that helps.

Rgds Ian 

[Richardtr3a]

Thanks Ian. I am now working on which parts to replace . My carbs were overhauled by a "specialist" about 3 years ago. Do you know who supplied your jet bearing kit. It seems as if I need a shorter jet bearing or a longer jet. Are there different kits for H6 carbs.

It looks like a call to Burlen again. They are the only supplier ??

Thanks Richard & B

[NCS_TR3A]

Richard, 

  Do you need any measurements of parts. My carbs are presently off and I intend to strip the bottom again and see if I can fit vitron seals with them leaking (which they did first time) but it seems that everyone else has been successful with this so I should get it to work.

Neil

[Richardtr3a]

Thanks very much for the offer. This morning I was lucky. The sun was out here is Sussex and that is always a good omen for work on the Tr.  I investigated the jet levels further and found that they were binding on old cork seals which were brittle. Even after removal the jet was a tight fit  on the bench and the temptation was to use some wire wool.. This must be a mistake , so I will now order new jets and needles. SM should be OK ?

Strange that both jets would stick at the same time. I thought that the springs should pull the jet up to the top. So I am waiting for  new seals and will hope that the car will be in good shape in a couple of weeks.

Thanks for all the answers here on the Forum.

Richard & B

[NCS_TR3A]

Getting the jets to return has always been a challenge for me. Even with new springs everything has to be just right for the choke to return fully. That's was one of the reasons I was trying the vitron, less friction than the cork. You did leave the cork in fresh engine oil overnight before fitting? That's a must. 

Neil