Jump to content

Recommended Posts

On 3/29/2021 at 10:18 AM, Charlie D said:

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.

No, the stuck piston increases the depression over the jet  and that easily defeats the smaller fuel annulus area. Its not obviuos but that how the pressures work. Peter

Link to post
Share on other sites
On 3/29/2021 at 1:03 PM, Hamish said:

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 ?

Thanks Hamish for the tip. You are right and the rear carb is just as you describe. So now I am looking for a repair for this fault. Is there any part that needs polishing ?

Thanks Richard & B

Link to post
Share on other sites

I had this problem with both my Strombergs.

Worse with them because the top is held on by FOUR screws. I found that I had to tweak them in various sequances to get the piston do move freely, and even when I found the perfect position I could not screw them down as much as I would have liked because the piston would bind again.

Charlie.

Link to post
Share on other sites

Make sure the mating surfaces are flat, & clean i.e. no dirt or burrs anywhere to tilt the dome. Test the "drop" as you tighten the screws, that should help identify where the problem is.

Bob.

Link to post
Share on other sites
2 hours ago, Richardtr3a said:

Thanks Hamish for the tip. You are right and the rear carb is just as you describe. So now I am looking for a repair for this fault. Is there any part that needs polishing ?

Thanks Richard & B

 

1 hour ago, Lebro said:

Make sure the mating surfaces are flat, & clean i.e. no dirt or burrs anywhere to tilt the dome. Test the "drop" as you tighten the screws, that should help identify where the problem is.

Bob.

As Bob says. 
but if the carbs were stripped at the same time. Did the same piston set up end up in the original carb body. I think over decades of use they become matched and if they were swapped over you may struggle to re set.

it’s only a theory that I have picked up on here.

Edited by Hamish
Link to post
Share on other sites

I had this problem with my carbs, one piston would drop freely the other would drop freely only when the screws were loose but started to catch when the screws were tightened.  Eventually I tracked it down to the pistons having been swapped at some time before I acquired them.  They now both drop at the same speed - provided I have the jets properly centralised :unsure:.

Rgds Ian

Link to post
Share on other sites

Thanks for the advice. I have taken the dashpot domes off the carbs and run the base edge over a very fine and very flat rubbing down paper. The surface now  looks bright  and even all round but the tabs  that take the screws must be a bit higher because they are not quite polished the same. So I reassembled it all and the piston now falls with the  metallic click that I have been searching for for years. I tried the same clean-up on the front carb and it is better but not as good as the rear one.

I have now been able to tune the two carbs and it is now possible to set  a weak mixture which has not happened for a long time. The plugs still look a bit black so I am thinking of changing the needles from SM to something a bit weaker. Has anyone done this with any success. ??

Thanks Richard & B

Link to post
Share on other sites

The jets are centralised and the pistons are dropping well but not perfect. There was some sign of dampness under the jet so I stripped it out and was looking at fitting a new large cork gasket. I have had two soaking in oil on the shelf for about two years. I offered it up and there was no way it was going to fit into the cupped washer, So it is back in the oil with his friend. I fitted the jet assemble again and dropped the top copper washer on the ground. It was getting cold and I could not see it anywhere. I had a replacement in the store ( plastic bag ). It is the thicker one of the two choices. I fitted it all back and after a few minutes running ,and backing into the garage, the jet was dry to the touch.  I am hoping that tomorrow it will be the same, but not so cold.

Richard & B

Link to post
Share on other sites

The jets are now dry after a few miles to the post office and back.  So I am making great progress. Now I need to get the upper bush higher so that it is nearer to the bridge level, as advised here. How can i manage that ? The plugs are very black with the jet only two flats down from fully wound up. It is running OK but I feel that I must have some wrong part here. Are there any options .

Thanks Richard & B

Link to post
Share on other sites
Posted (edited)

I have cleaned and adjusted both the carbs. I removed the spring which is located above the adjusting nut on both  and I think this allows the jet to sit higher. I adjusted the nut to fully upwards and then down two flats only. After a ten mile run the plugs are showing a grey colour, which is the first time they have not been black for years. I am very please with this result. It starts and runs well and the last 1/2 mile at 3k revs went very well.

 Once I get the mixture correct I plan to use a tippex pen to mark the adjusting nut so that I can monitor any movement. If the nut does move the springs will have to be replaced and the jets will not go so high,so this may mean a weaker needle. has anyone used another needle instead of SM

When lifting the piston with a screwdriver the front carb shows hardly any alteration but the rear carb stalls  the engine immediately. Why are they reacting so differently

Any tuning advice would be very helpful

Richard & B

 

Edited by Richardtr3a
remove typing space errors
Link to post
Share on other sites

Richard, 

      If you raise the piston slightly (there is a rod you can push up on the side of each carb that does this and its part of the tuning) then ideally you should see little change for a while followed by a slowing down, this suggests the mixture is right. If on raising the piston the engine starts to stall I think that suggests that carb is weak. If it speeds up its rich.

     I'm not sure changing the needles will fix this for you. Most of the needles recommended for these engines have a very similar idle profile. They differ in areas away from the idle. It sounds like you have got this in the right position now, you just need to understand why those springs mean the jet sits lower. 

Neil

Edited by NCS_TR3A
Link to post
Share on other sites
36 minutes ago, Richardtr3a said:

When lifting the piston with a screwdriver the front carb shows hardly any alteration but the rear carb stalls  the engine immediately. Why are they reacting so differently

You probably haven't got the airflows balanced Richard. Sounds as though at idle you only have airflow through the rear carb, so when you lift that piston the mixture goes very lean.

Link to post
Share on other sites

Thanks for the advice so far. I now have the carbs balanced better, My trial experiment of running without the adjuster spring failed and the adjusters move  too  easily on their own without the spring. So the springs are being replaced. The car is running well and the choke only works on the front carb. After a few cold starts there is some  leaking from the jet. This is hard to understand because I have changed  the cork seals to Teflon  and a Buna R main seal. It does seem to be under the jet. It was dry for a bit but I think that the choke must be causing some damage. The sharp edges of the holes on the jet are chamfered. I am planning to collect a new  throttle spindle from Burlen at the end of next week and will see if I can get some advice. I am passing through Salisbury on the way to Wellington in Devon/Somerset

After some engine running, and attempted tuning , the engine just stopped and would not start. I have a dipstick for the fuel tank and it was showing 1/2 gallon and the pump was full. It took me too long the remember that I had fitted  a new tank and the supply pipe was pushed in about an inch as advised. My spare can was in the boot and the engine fired up immediately. It is starting well , plugs are grey, but it leaks. There is a member of our local group who is a professional and I will be asking him for a home visit soon.

So I am almost there. Thanks to lockdown I have no deadlines.

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.