Jump to content

Recommended Posts

Sorry Guys but I have had to start this again as I am getting no where. Previously we didn't come to any conclusion that helped the issue, so i kicked it into touch and went on to do other jobs on the car like rebuilding the water pump. That's easy!

But the same fault still exists and I have spent months trying to sort it: The HS6 carbs on our 2l TR7 are still impossible to tune. I cannot achieve any tick over below 1500rpm. 

Just to reiterate:

1 Original fault tick over a little fast 1000rpm. Putting pressure on front carb throttle spindle reduced tick over OK

2.Butterflies and throttle spindles replaced.

3 Rubber mounts removed cleaned and inspected for cracking, they are fine and even if they were not, there's none in the country.

4 All emission hoses inspected, removed and ends blocked off.

5.Both throttle screws backed right off.

6. Push pistons down and hold down and the revs decrease to a suitable tick over level. The pistons lifting is causing the rise in revs.

7. Block off carb throat on rear carb and revs drop. Block carb throat on front cab and engine stalls immediately.

8.During the warm up period using choke and tick over performance is normal. When hot and choke in then tick over is far too high.

9. The pistons will drop with a clunk when lifted with a screwdriver, so the jets are not off center.

10 The piston springs are undamaged, and look the same and SAE 20 oil is in the dash pot, up to the top of the tube.

The question is why with the throttle screws back completely off does the engine even run and why if I manually push the pistons down do the revs drop to an acceptable level. What is causing the pistons to rise and let more air into the carb. Is there a clue somewhere there?

I have managed to get it somewhere near right over the months, but slowly the revs will drop and the car stalls. Or if tweaking the tick over before it stalls, up a little higher, then the engine will slowly speed up till its back at 1500rpm completely on its own.

Before I completely social distance myself from this heap of.......has anyone ever come across this problem and more importantly solved it?

 

 

 

 

 

 

 

Link to post
Share on other sites

Sounds like an air leak develops as it warms up, check the inlet manifold for cracks. Do you have the original temperature compensating jets? what about the butterflies have you deleted the poppet valves?

Stuart.

Edited by stuart
Link to post
Share on other sites

Hi Stewart

Having had the manifold off there was nothing untoward I could see. Would that explain why holding down the pistons the tick over is OK?

The carbs are fitted what I believe at called Waxstats. There were replacement fuel feed and jet outers in the Burlen repair kit, but as I hadn't a clue how to fit them I left the Waxstats in place.The new butterflies are equipped with poppet vales, I saw no reason on a new butterflies to remove and solder these up.

I pulled the servo pipe and blocked off the outlet to see if the non return valve on the manifold was at fault. Needless to say it made no difference.

 

Link to post
Share on other sites

Just to add, check the breather pipes, cam cover and oil filler cap for air leaks.

On to the carbs - The throat maybe too worn for the butterfly to seat correctly. Are the butterfly's centred correctly in the throat. Maybe the spindle holes are too worn and are still letting in air.

The centring of the butterfly's should be done when they are full shut and the throttles set screws are either removed or turned right back out of contact. 

Dave

Link to post
Share on other sites

Hi Dave,

The breather pipes are disconnected and all SU emission ports blocked off including servo outlet on inlet manifold. Both rocker box and filler cap gaskets were changed during this battle anyway.

I believe that the butterflies are centered in the ports, I did have them checked by a TR guru at the time.The spindles holes being worn is a possibility, but it is more usual for the brass spindles to wear in the harder casting.

However my next step, and thanks to you guys there is always a next step, is to purchase a can of spray carb cleaner and go carefully around all mating surfaces on the inlet system including spindles. Also I will check to the SU book the float settings. I believe that this is a fairly rudimentary task using a 4mm drill shank if I remember.

 

Link to post
Share on other sites

Hi Stewart,

I have a new pair of Jet Assemblies from Burlen as shown in the pictures attached, however I am not sure whether this is all I need to scrap the Waxstats and fit these. I think there is a bracket needed? 

Still not 100% sure how they fix into the base of the carb. As far as I understand the Waxstat arrangement is to compensate for temperature keeping the mixture constant. And what is the "Penny mod" I keep seeing?

You may be interested in the next findings........

DSC_0054.JPG

Dsc_0053Jets.jpg

Link to post
Share on other sites

 Hello Dave,

I grabbed a can of carb cleaner and went round all the inlet joints including spindles with absolutely no change to the nut revving 1800rpm tick over. 

Then I pulled the floats to inspect the 4mm clearance. We'll as you can see the front float was set at about 12mm. Very high indeed with a big fat washer under the old fuel cut off valve. I changed the cut off valves for new ones and set them both as close as I could get to 4mm, not an easy task with fully plastic floats, The car ran, albeit at 1500rpm and no slower, but fuel dribbled from the front carb  jet hole at the base of the carb body when the car was stopped. Setting the float a little higher (which means less fuel in the bowl and therefore lower in the main jet body) solved this problem.

Now does this point to crappy Waxstats and if so why does she not respond to lowering of the tick over speed?

DSC_0049.JPG

Link to post
Share on other sites
1 hour ago, steve redway said:

Hi Stewart,

I have a new pair of Jet Assemblies from Burlen as shown in the pictures attached, however I am not sure whether this is all I need to scrap the Waxstats and fit these. I think there is a bracket needed? 

Still not 100% sure how they fix into the base of the carb. As far as I understand the Waxstat arrangement is to compensate for temperature keeping the mixture constant. And what is the "Penny mod" I keep seeing?

You may be interested in the next findings........

DSC_0054.JPG

Dsc_0053Jets.jpg

Back to the beginning.

Have you rechecked that the throttle butterflies are fully seated when the screws that hold them to the spindles were tightened?  This is done with the tickover adjusting screws completely removed so your fingers can push the butterfly fully home, centred,  and seated in the venturi.  When done hold them up to the light and you should not see any light around the disc.

Have you disconnected the throttle cable completely?

To fit those jets with red plastic bases to your carbs you will need to modify or replace the link rod between them and the choke mechanism.  A simple bent rod will do it so long as it does not bend or fall off! 

The 'penny mod' is the removal of the wax capsule in the bottom of the jet and replacing it with a penny.  This stops the wax stat part from moving when the temperature changes, thus stopping the moving of the jet height in the cab body.

Cheers

Peter W

Image of kit for Spitfire which has the wrong sized jets for the TR7.

image.png.6c676b9abe589e9c3b7c193547f92705.png

Edited by BlueTR3A-5EKT
Link to post
Share on other sites

Hello Peter

The conversion kit from Burlen comes with a suitable bracket, whilst the repair kit doesn't. I may just get them to send me a couple of those and change them over. Alternatively here is the Penny mod detail: http://www.dunlopclassics.co.uk/waxstats.htm

We made a special effort to get the butterflies central within the carb throat and the throttle cable is well sloppy and putting no tension on the shafts. The choke cams are well away from the throttle back stop so nothing there.

One way or another I will sort the Waxstats out and recheck the butterflies in the throat.image.png.bc2814610943a5f85175391c511f7def.png

Link to post
Share on other sites

As Dave  and Stuart have already pointed out you most likely have an air leak past the butterfly on the front carb, presumably temperature related.  You have proved this yourself by blanking off the front inlet and causing the engine to stall.

Air flow means there is a pressure drop across the venturi, and a lowering of air pressure above the piston which is why the piston rises -  if there was no air flow there is no vacuum; the pistons cannot rise on their own.  The pistons do not let more air in - it is the butterfly that does that - the pistons regulate fuel flow. 

It is the rise in revs- and hence of airflow - which is causing the piston lift, not vice versa. The carb is just doing the job it was designed for and adjusting fuel flow to match air flow.

By pushing the piston down you are choking the fuel flow which is why the revs drop again as the mixture alters. 

There is usually a balance pipe in the manifold between the carbs so one badly adjusted carb will feed all the cylinders, which is why choking the rear carb doesn't stall the engine and proving that the rear one is probably working OK. Have you tried undoing the linkage between the carbs and adjusting things independently for idle?

 

Edited by RobH
Link to post
Share on other sites

Rob,

Thanks. I will go around yet again with the carb cleaner on the front carb, mounting rubbers, inlet manifold, etc to try and see any alteration whatsoever in tick over. I will pay particular attention to the rubber mounts. 

Looks like the Waxstats was false hope then?

Link to post
Share on other sites

The waxstat may be a factor in poor operation Steve as may the setting of the float level but for the piston to rise there must be air being drawn in through the carb over the bridge and past the butterfly. You have already  proved that by stopping the flow by closing off the carb and causing the engine to stall.  The air is coming through the carb somehow so surely it can only be getting past the butterfly. 

Link to post
Share on other sites

It is less likely that the waxstats are the cause of the problem you have. They either adjust too rick or too weak. In both cases the tick-over is likely to reduce and become uneven.

Due to the TR7 engine being cross-flow and not having a hot exhaust manifold sitting under the carbs the waxstats are less of a problem than say on a Spitfire.

73's

Dave

Link to post
Share on other sites

Hello Rob,

Just to confirm when you say "an air leak past the butterfly", you mean that for whatever reason the butterfly is not completely stopping the flow of air. Not that there is an air leak further in towards the manifold. 

If this is the case then it's off with the carbs yet again.....I just whistle now and they pop off the car and on to the bench now.

Link to post
Share on other sites

Hi Dave

I tend to agree regarding the Waxstats, I dont think they are the cause of this fault. It's not a mixture or rough running issue it's just air getting past the butterflies and pushing the revs up. As I said to Rob I'll have 'em off again and take a pictures through the throats for your comments.

Link to post
Share on other sites

Yes that is exactly the point Steve.  The piston lifts because there is air flow through the butterfly which causes a drop in pressure across the venturi. If the air leak was elsewhere that would not happen. If the butterfly was fully closed, the piston could not lift.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Restore formatting

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

Please familiarise yourself with our Terms and Conditions. By using this site, you agree to the following: Terms of Use.