TR4/4A TR4A mixture problem - need advice please

[Adrio]

As per Randall's suggestion I measured the needles and at the shoulder they are 0.0975" But they are stamped TW which should be 0.099"

I also checked the compression again and it might be my gauge is giving a low reading because my TR3 is only 115 and it is running great and the head on that was rebuilt 3000 miles ago and my notes say that it was 155 (with a friends gauge before I bought mine).

Do you think I should buy a few lean needles and start experimenting or should I keep trying to find "a problem"

Still at a loss on this one.

 

[KVH]

You say the jets are "full up" lean. Two more ideas: 1) Are you positive that all of the choke linkage components of each carb (the levers, etc.) are correctly assembled, including the springs and rods beneath the carbs--with the springs applying pressure and pushing the jets upward at all times? You assembled them off the car and know they work? You "wound" the springs up so that constant pressure is applied?; and b) Can you disconnect the choke and see how the car runs?

After all you've said, I doubt it's the float needles.

 

[Adrio]

yes, for sure the choke is fully off. I can confirm that the jets are all the way up and that the choke comes off and the springs push the jets back home.

I had an other thought on this. The carbs are constant depression type. That, to me, means the air velocity across the jets should remain the same (exept for a brief period on acceleration when the oil damps the upward motion of the dashpot). So in the steady state the thing that determins the area for air flow is the weight of the dashpot and the force of the dashpot spring. Is this assumption correct. If so then the spring is starting to look like a possible problem, in that if the springs I have are too "strong" the effect would be to have a choke on because the area of the venturi would be too small all through the range.

This leads to a question. I know I should have the red springs in there, but there is no sign of colour on mine. Does anyone have the physical characteristics of these 'red' springs. That is, the number of turns and the wire diameter? Or know of a place I can find that information. One book I have gives a spring length with a given mass applied on top. Do I have to fashion a jig to measure this?

 

[KVH]

I have a 4A, and my springs are not red. They are steel grey, weak and floppy. I'll try to measure them this evening. That would be rare for someone to put the wrong springs in there. I keep thinking it's something more straight forward.

By the way, the springs I was referring to as applying upward pressure are the jet return springs, Moss Part 372-910.

I'll look at my car and get some more ideas this evening.

 

[Adrio]

Thanks for offering to check tonight. In my case the carbs did not come from the TR4A or from the engine I used. They were random carbs collected from some unknown application. that is why I wonder if they may have the wrong springs for this engine. These carbs may have been from any engine enven a larger volume one. All the other parts on the carb I replaced when I rebuilt them but those dashpot springs I did not.

As to the Moss Part 372-910 springs, I did check last night that they were in correctly and putting pressure on the jets.

I too keep thinking it is something more straight forward. It could also be more then one thing and that is throwing us off. One day/week/month/year I will get to the bottom of this one.

 

[KVH]

I'd sure like to see some digital pictures of the carbs and set up, linkages, etc. Can you post some? The Knowledge Base tells how, and you can just open a free page with mypicstudio.com. Then post using the syntax: [image]url of post [/image].

 

[piman]

Hello Adrio,

there is quite a large variation in spring pressures available so it is possible that they are too strong, just remove them and see what happens to the mixture? As I said earlier, different needles will make no difference to the idle so the first step is to get an acceptable idle mixture. If that is possible without springs that looks to be on the right track?
Unfortunately I'm in the middle of a house move and can't lay my hands on my S.U. manual which gives the spring weights.
The HS float chambers are not designed to be adjusted, unlike the H types. However too hogh a float cahmber level can be adjusted by putting a washer(s) under the needle valve.

Alec

 

[Adrio]

I have a picture of the right side of the engine on my web site.

I don't know if the detail is enough for you to see what you are looking for. I will take a picture tonight and add it.

I like the idea of removing the springs and seeing if it lets me set the mixture well. Also I have the Haynes SU book and it has the spring weight as 4.5 oz at a length of 2.635 inches. I am going to try and rig something to measure that tonight. I can also try to put the TR3 springs in there (the book says they are also the 4.5 oz springs).

Lots to try. I do appreciate very much all the help you folks are giving me on this.

 

[Adrio]

Also, now that I look at the picture I notice that the picture was taken back when I had one of the air filters upside down. That is no longer the case and has not been for two years. That was a short term mistake I made.

 

[KVH]

Yes, I'd need more pics. I note your choke cable goes to the front carb? On both my 4 and 4A the cable goes to the rear carb. Not sure that's relevant.

 

[NutmegCT]

Just out of curiosity (as I have little history with the various SU carbs) -

How would changing, and/or variance in, the internal jet spring change the ability to set the mixture? Seems that if the adjusting screw at the bottom of the jet assembly is screwed up as far as possible, the jet would be up as far as possible.

Thanks.
Tom

 

[PlaidMan]

Adrio, don't know if it makes any difference but the front carbs Daspot Cover is not facing the air filter like the rear is. Maybe some one could pipe in on this.

 

[PlaidMan]

Dashpot Cover that is!

 

[TR3driver]

HS6 carbs don't have a spring inside the jet assembly as the H6 carbs do. Instead, we are talking about the spring inside the dashpot.
The spring applies downward force on the piston, which decreases the venturi opening and hence increases the speed of the air flowing through the venturi. This in turn increases the Bernoulli effect that sucks fuel through the gap between the metering needle and the jet.

Basically the opposite of why the mixture goes lean if you lift the pistons slightly.

That front carb does look odd ... I'm pretty sure the dashpot covers can only be installed one way as the tabs are not evently spaced. Which implies to me that the front carb lacks the vent tube for the top of the damper. Might be some problem there.

To answer an earlier question that I just saw; I'm not exactly sure how a float can fail to work when it gets hot. My theory is that the fuel density falls, which means the float must be farther submerged to generate enough force to shut off the float valve. At some point the float is fully submerged, meaning it can't generate any more force and the valve stays open. I couldn't find a chart for gasoline density vs temperature; but I did find an article that says it drops about 1% for every 10C increase in temperature.

Just a SWAG, but I know for a fact that the Thermoquad on my motorhome would work just fine as long as underhood temperatures remained below some threshold. When the carb got good and hot (meaning the fuel was hot too), fuel would start running all over the place, including into the carb throats. After stopping for an hour or two (hood open), it would work OK again. This happened with 3 different pairs of plastic floats (Thermoquads have two per carb) until I finally managed to source some brass floats, which cured the problem forever.

No, I don't know of any source for brass floats for a HS6 /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

 

[NutmegCT]

Randall - if that vent were clogged, what would result?

Is there a damper vent on all the H and HS carbs?

Tom

 

[TR3driver]

There should be a vent on all H and HS carbs, though some of the early ones don't have the tube but instead a hole through the cap or the side of the dome.

I'm not certain exactly what would happen if the vent were blocked or missing; but I'm sure it would screw up the mixture. If the engine was stopped when you installed the cap, and it sealed tightly, then I think it would have the effect of making the mixture a little richer at idle, and a lot richer at larger throttle openings. As the piston rose, it would have to compress the air inside the damper, which would push down on the piston. However probably, after driving for awhile, the compressed air would leak out and then you'd have a vacuum to lift the piston when the throttle closed.

 

[Adrio]

Randall,

I took some pictures of the carbs and set up. Here they are (the loose screws are because I put the pots back on just for the picture):

The question about the vent and the missing part on the top of the pot. You are right the pots only go on one way and for some reason the front and rear carbs have that little slopped part at different positions. There is no vent hole drilled through that like there is on the TR3 carbs, but as you said the plastic caps have the vent hole.

Note the last picture. That is the rear carb and it has an extra vacuum hole and a large one that I capped off. This is what makes me think this carb came from some other care with an other type of engine.

Tonight I measured the jets and as best I can tell they are 0.100" for sure they are not the 0.125". I also tried to measure the springs and they seem to be softer just a bit then the book says they should be. I also did the drop time test on the dashpots and both came in exactly in the middle of the acceptable range or about 6 seconds (sometimes 5.5 some times 6.5 over several tries).

In desperation I tried lowering the needles even though the theory says this will not work. It was late and I got some dirty looks from the neighbors (my Stebro exhaust is a bit loud) so I gave up for the night. SO the saga continues.

 

[KVH]

It's still hard to tell from the pics, but:

a) where are your two throttle springs? Removed for the pictures? You'll see them in the Moss Catalogue. I don't see them here. They are material;

b) what is that black hose draped over the intake manifold? That doesn't look like it belongs there;

c) do you have a circuitous steel line running from one fuel bowl to the other? Mine is a straight shot forward of the air cleaners;

d) do you have the choke screws adjusted 1/16th from the little choke cams?;

e) that choke cable--before you tighten the little cinch screw, you have to apply a little force to lift the linkage and take up most (not all) of the slack.

Also, CHECK AND CLEAN YOUR PCV VALVE! A bad PCV valve can cause poor fuel economy and a richer running fuel system.

Here's a quote from an article:

"The flow/no-flow nature of the PCV valve requires that the metering slots be somewhat larger than needed for idle conditions, so that there is sufficient flow under most operating conditions. This increased flow at idle causes the intake manifold pressure to increase, leading to a richer idle mixture and increased emissions and lower fuel economy, as well as potential idle stability issues."

Sounds relevant.

Good luck.

 

[Adrio]

TR4A mixture problem - need advice please -UPDATE

It seems that Randall was right about the plastic floats. I could not find anything wrong with them but out of desperation I changed them. The interesting thing was that the new floats gave me a fuel level about 1/4" lower then the old ones. Also the old ones were very brown/tan and the new ones are clean white. I now wonder if the old ones were heavier due to 40 years of accumulated "gunk" or stuff they had absorbed. The result is that now I can get the mixture set just fine and both the colourtune and a plug cut check confirm that.

For the first time since I have had this car it runs great and I don't smell or see the exhaust. I sure feels good.

Thanks again to all you folks for you help on this one.

 

[blhazzard]

Ever get this resolved... ?

Just some comments on my current project car which is now running.
I am having trouble getting it to idle lower than 1500rpm.

>>I was wondering about the 130 myself
I have been told this should be a minimum of 150-160.

Did you install a stock cam ?
Non-stock (e.g. Hot) cams are supposed to sound like a dragster/harley at idle...

Who did your crank/cam/timing chain alignment on the rebuild.
It is quasi easy to check with valve cover off and spark plugs out.
It is easy to be off by just one tooth (or half tooth by flipping gear over) on cam gear.
Just finished a top side rebuild and really took my time to make sure that was correct per original alignment and shop manuals.
Not sure what one tooth off would sound like... guessing not pretty...