Jump to content

SU carbs running too rich


Recommended Posts

Hello,

 

I am trying to adjust the SU carbs in my TR3A. To make the story short: (see below for a longer explanation)

 

At idle, with the jet at its uppermost level, mixture is too rich with about 9 % CO at the exhaust. Spark plugs are sooted, as well. I see two possible causes: a too low jet or a too high needle.

 

When the jet is fully home, it remains about 0.5 mm (0.020") below the bridge. I could bring it higher, but some of the jet bushes or washers should be machined.

 

The needle shoulder should be flush with the piston surface. However, the piston rod that really holds the needle is a little recessed (see photo).

 

- Must the needle be flush with the piston bottom or with the rod bottom?

 

- Should I lower the needle in both carbs? How much?

 

- Any other hint to weaken the mixture?

 

Thanks for your help.

 

Jesus

 

------------------

 

Some more details:

 

The SU carbs have been reconditioned (seals, needles, bushes) and adjusted (needle flush with piston rod face, jets centred, idle flow balanced, throttle opening synchronised, correct float hinge level, enough damper oil, etc). The jets are the original that came with the used carbs, but seem to be in nice conditions with no important wear (and they are the correct 0.100" for the SM needle).

 

However, it is impossible to lean out the mixture at idle. With the jet at its uppermost level (adjusting nut fully home) not only the engine keeps idling (shouldn't it stall?) but mixture is too rich with about 9 % CO at the exhaust.

 

The ignition timing has not been fully adjusted (I guess that it could benefit from some advancing) and the valve clearance has been set as per the manual. The engine has 86 mm liners, a TR4A cylinder head and new original camshaft.

Link to post
Share on other sites

Jesus

 

The most common cause of your problem is the needle valves in the float chambers leaking or the cut off levels set at the wrong height.

 

You have the needles set correctly and the correct position for the jet is 0.038" or 1 mm below the jet bridge. This is where they were set when the car is new and it gives almost exactly the right mixture provided the float chambers aren't flooding.

 

Take out the Dashpot look to see if fuel is spilling over the top of the main jet. Pump the fuel pump and watch to see if fuel is escaping and flooding. Make sure there is no petrol inside the float.

 

It could be dirt getting through the fuel lines, the needle valves are always a real nuisance, hence my suggestion that they be replaced by the Moss ones which have cured difficult cars for us.

 

Also take the lid off the float chamber turn it upside dow and make sure that the forked lever that presses on the float is exactly 7/16" off the lid flange.

 

If you follow these instructions, you'll be an SU expert!

 

Ash

Link to post
Share on other sites

Thanks, Ash for your quick response.

 

I will check if there is some flooding with the engine stopped. Nevertheless, I adjusted the float lid height as you explain, and also checked the needle valves shut off by blowing air. I also "boiled" the floats for detecting any pinhole.

 

 

Jesús

 

 

Link to post
Share on other sites

Thanks, Ash for your quick response.

 

I will check if there is some flooding with the engine stopped. Nevertheless, I adjusted the float lid height as you explain, and also checked the needle valves shut off by blowing air. I also "boiled" the floats for detecting any pinhole.

 

 

Jesús

 

 

Jesus

 

I'm sure you have! We have too, but they can be a complete B----d! Make sure the main jets are 0.038" down and work back from that setting, because it is correct. We've just had a pair of 2" HD8s flood with three different new needles, so then we bought the ball valves from Moss and immediately it stopped.

 

Ash

Link to post
Share on other sites

I have checked the needle valves in the float chamber by priming the fuel and they seem to work properly. The fuel level in the jet pit remains about 7 mm below the edge -- difficult to appreciate accurately, but there are definitely no spills. However, I will order from Moss two ball valves (I assume we are talking about the so-called Grose jets, that also share some reputation for leakages, indeed), and two new jets, just in case.

 

In order to weaken the mixture I plan now to lower the needles by about 0.5 mm and try to adjust again the CO in the exhaust at idle.

 

Is there any negative consequence with this action?

 

Jesús

 

 

Link to post
Share on other sites

Jesus

 

The position that you have your needles in is correct. The CORRECT position for the jets is 35-38 thou down from the jet bridge measured with a vernier micrometer, so if they aren't giving the correct mixture, something else is wrong and you must find it.

 

I'm not sure of the value of an exhaust gas analyser on an old engine because modern fuel has ethanol in it which needs a higher compression to burn properly. In my experience it is better to set the jets as I suggested and find the cause of the richness.

 

I do have an exhaust gas analyser but am not as confident with it as just setting SUs as I've suggested. I've done a huge number over more than forty years and it's right every time with any car if everything else is right.

 

Ash

Link to post
Share on other sites

Hi, in your picture the needle in the piston must be flush with with the piston face. if it isn't, it will be like running with choke [ie lowered jets] all the time, which accounts for your high CO reading.

 

Andy

Edited by 67_gt6
Link to post
Share on other sites

You may wish to try the viton tipped valves in the float chamber lids.

 

In my experience they are better than the Grose Valves.

 

Tony

Link to post
Share on other sites

You dont say how old the needles and jets are, if they are badly worn the standard measurements do not apply!

Link to post
Share on other sites

You may wish to try the viton tipped valves in the float chamber lids.

 

In my experience they are better than the Grose Valves.

 

Tony

 

 

Not our experience. We've been fitting viton to replace the old metal tipped ones and it's been terribly hit of miss and finally the other day on a special 4.9 Litre car with R-R's special HD8s, we just couldn't stop the thing from flooding, it was a nightmare because the customer drives thousands of miles across Europe and back all the time and he's very fussy. In desperation we fitted the Grose Valve and immediately and decisively it stopped flooding and we were able to get a decent tick over.

 

Ash

Link to post
Share on other sites

Not our experience. We've been fitting viton to replace the old metal tipped ones and it's been terribly hit of miss and finally the other day on a special 4.9 Litre car with R-R's special HD8s, we just couldn't stop the thing from flooding, it was a nightmare because the customer drives thousands of miles across Europe and back all the time and he's very fussy. In desperation we fitted the Grose Valve and immediately and decisively it stopped flooding and we were able to get a decent tick over.

 

Ash

 

 

I wish I could agree Ash, on my trip to Spa last year, a Grose valve stuck open and flooded ..... I had a strong smell of petrol for a good few miles before pulling in and found the petrol pouring from the float chamber. With a plastic float fitted there is also no way to adjust the levels in the float chamber?...unless you fit extra washers under the valve?

 

john

Link to post
Share on other sites

Johnny

 

Do you think that there could have been a possibility of dirt causing your problem?

 

We've only used a pair of the Gorse Valves so far but they worked immediately where the Burlen ones rarely do. It's often a case of knowing they are flooding and hoping it stops as it often does. With the Bentley I described we'd tried two new sets and one old one and each intermittently dribbled. It's a problem that's driven me up the wall for years.

 

Altering the thickness of washer under the needle jet to get float level correct is what you do with Solex and they flood for a pass time too.

 

I'm going to fit the Grose Valves to my TR3A, so I'll apologise if they give trouble, I promise ;)

 

Ash

Link to post
Share on other sites

Johnny

 

Do you think that there could have been a possibility of dirt causing your problem?

 

Ash

 

 

Yes that's quite possible as it was OK after lifting the float chamber lid and inverting, although there is a very large filter/regulator fitted as well as a pre-pump (Facet) filter as well. I still occasionally get flooding on very tight hairpin bends and roundabouts (high g) :)

I suspect the float levels ar a bit too high, but with no float adjustment I've considered fitting 'old style' adjustable floats?

 

john

Link to post
Share on other sites

A little off track perhaps, but still interesting for this thread, I think:

 

In this month's Dutch Triumph Club magazine there's an article about float chamber levels too. The writer of the article wonders if the correct adjustment of the valves position and float chambers is affected by the position of the carbs. Most engines are tilted (English, spelling?) backwards. And when you hold a spirit level between the carbs, it's clear that the float chambers aren't level too. His conclusion: since the rear float chamber sits a little lower, it will be filled earlier than the front one! In his opinion, this will affect the adjustment of the float valves.

 

Any opinions?

 

Menno

Link to post
Share on other sites

A little off track perhaps, but still interesting for this thread, I think:

 

In this month's Dutch Triumph Club magazine there's an article about float chamber levels too. The writer of the article wonders if the correct adjustment of the valves position and float chambers is affected by the position of the carbs. Most engines are tilted (English, spelling?) backwards. And when you hold a spirit level between the carbs, it's clear that the float chambers aren't level too. His conclusion: since the rear float chamber sits a little lower, it will be filled earlier than the front one! In his opinion, this will affect the adjustment of the float valves.

 

Any opinions?

 

Menno

 

 

The small difference in the carbs' height above the fuel pump is immaterial because a typical 3psi fuel pressure (about one fifth atmospheric) will lift petrol over 2 meters. So a cm or two difference in height between carbs due to engine tilt makes no difference.

But the float chamber's fuel level sets the fuel height in the jet and their relationship is crucial. So ideally the f/c needle valve should be adjusted by inpsecting the jet with the dome removed , to set the fuel level a mm or so below the top of the jet. Here the effect of engine tilt on the carbs might make a difference because one has its f/c to the front of the jet and the other to the rear of the jet and is lower by maybe mm. However the venturi suction that pulls fuel out of the jet is a fraction of 1psi and easily enough to cope with few mm of level difference within the jet.(but both jet levels should be equal). But f/c level should never be so high that fuel overflows the jet!

Link to post
Share on other sites

Just a few observations after many year's of tuning various types of SU carbies - doubles and triples.

 

As Norman D basically said, if the needles and jets are worn, it's impossible to set the correct mixture. When assembling an SU carby it's difficult to centre the jet, so the needle typically rubs on one side and over time wears the jet to oval. This can usually be seen under a magnifying glass. The needles wear too, so the best option is to replace both items. Always replace the jet seals and cork upper seal when renewing the jets. (soak cork seals in engine oil overnight). Carefully centre the jet. As already said, the needle must be installed with the boss flush with the piston - the jet set 38 thou or 1 mm below the carby bridge. That gives the correct starting point to finely adjust the mixture via the hex nut under the jet. Renew the top hat rubber seals between the bowl and the carby with Ethanol friendly seals.

 

An easy way to set the correct float level is to use the shank of a 7/16" drill bit.

 

With the fuel bowl needle and seat, the quality of repro items made in today's smaller volumes can be suspect. When Grose Valves were first introduced they were a revelation in stopping fuel smells over the old brass needle and seat. Then came Viton needles which sealed really well. Soon after the quality of Grose Valves seemed to fall away. I've used Viton for many years now on a variety of SU cars and can't recall a leak. Then again I use two fuel filters - one down near the tank before the electric pump and another just before the front carby. A clean fuel supply is essential for any type of needle and seat to operate reliably.

 

Another important thing with SU carbies is fuel pump shut off pressure of max 2.5 lbs sq. in. Higher pressure will overcome the float needles and cause a rich mixture or flooding.

 

Consider replacing the original mesh aircleaners with something like the RamFlo's that I use with efficient foam filters. Original aircleaners pass a lot of fine dust that will choke up the piston/ damper - typically seen as a black deposit on the piston.

 

To obtain a correct idle, the butterflies must be installed on the spindles so they start from a fully closed position. Set the idle with one W clip loosened so each carby is independent, then tighten the clip.

 

SU carbies are then pretty much set and forget.

 

Regards,

 

Viv.

Link to post
Share on other sites

Thank you all for the replies with very useful information. I find it difficult to address every post individually, so here are some general comments:

 

- Fuel level in the jet pit is difficult to measure and even to visualise, but it is definitely below 6 mm. I adjusted the float hinge lever as you suggest.

 

- In its upmost position, the jet is higher than 1 mm below the bridge – and the mixture is rich

 

- Jet cork seals were treated properly with oil, and they did not leak (at least towards the outside)

 

- Needles are new.

 

- Jets are the old ones that came with the carbs, and probably they are worn. I will order new ones.

 

In the meantime I will make a test adjusting the needle position in the pistons, and will let you know.

 

Jesús

Link to post
Share on other sites

We've just had a pair of 2" HD8s flood with three different new needles, so then we bought the ball valves from Moss and immediately it stopped.

 

One of the H1F6 on The Shed started flooding last week. A real pain as they have the 'improved' concentric float chamber, so one has to remove the carb to clean out the chamber and check/replace the valve. I overhauled them with new valves (and everything) from Southern Carbs a year ago. Now I have another new viton valve to instal, but after reading this thread I have misgivings about fitting it. Duh.

 

I also have a pair of HS6 with the separate float chamber. Perhaps a swap might be a better idea, then at least I can fix the dratted things more easily.

 

Ivor

Link to post
Share on other sites

If you buy quality Viton needles from either Southern Carbs or Burlens I have never had any problem with them. The Grose ones were a problem a while ago as they had changed manufacturers but I believe this has been sorted now. I too would also check that the ignition circuit is in good condition and the timing is correct as poor spark will not help.

Stuart.

Link to post
Share on other sites

chaps look at the picture - with the needle set like that in the piston it is the same effect as having the jet turned down 2 or 3mm too far

 

andy

 

 

Andy

 

The should of the needle should be level with the face of the dashpot and, as far as it's possible tell, it is.

 

Ash

Link to post
Share on other sites

Andy

 

The should of the needle should be level with the face of the dashpot and, as far as it's possible tell, it is.

 

Ash

 

 

Ash look at the right hand side of the hole, it is 2mm short from being flush, as Jesus pointed out in his original post.

 

Andy

post-3529-0-58107800-1307723486_thumb.jpg

post-3529-0-58107800-1307723486_thumb.jpg

Edited by 67_gt6
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.