carb mixture H6 - exhaust

[NutmegCT]

Randall: "But I'm still thinking it's a leak inside the carb, allowing fuel to be sucked around the jet rather than through it. Since the amount of fuel sucked through the leak is more-or-less constant, it has more effect at low idle and little or no effect when the throttle is open.

What do the plugs look like, after you've let it die at low idle ?"

Randall - thanks for the comments and suggestions. Interesting thought about a leak inside a carb. Any thoughts on where/how to check on that?

The plugs are definitely black and sooty now. Before I did the timing and carb work (including needle/jet replacement) the plugs were actually quite clean - slight white ash.

Might add that the top of the jets is about 1/16-1/8 inch below the bridge when fully up. Even using a hammer on a crescent wrench, getting that jet seal nut screwed up tight is pretty much impossible. It's now about half of what it was in the earlier picture, but certainly not the "nearly invisible" it apparently should be. However, shouldn't that slight amount be "moot" if I use the mixture adjusting nuts to raise/lower the jets? Even as lean as they're adjusted now, the (slight) soot continues.

Edit: Would this evidently rich situation, and slightly low jet relate in some way to the "dying idle" problem?

Thanks.
Tom

 

[NickMorgan]

Tom,
I am just back from two weeks' holiday on the Western Isles, so I may have missed something just quickly reading through this thread. Are you sure those jets are centred properly? If you have subsequently tightened up the main nut securing them to the body, they may have tightened up. If the pistons and needles are not dropping as they should, then the car will be running rich. If you then leave the car idling after a run the needles may work themselves down and lean out the mixture, reduce the idle speed, etc. You will also find it impossible to set the mixture correctly if the jets are not centred.
Nick

 

[NutmegCT]

Nick - welcome home! Were you out near Arran? Skye?

I'm 99.44% sure the jets are well centered. Lift and release the pistons while I centered the jets, then tightened that large nut while the pistons dropped with a nice click on the bridge. Lowered jets, then repeated and still hear the click.

The only thing that's not quite right is the top of the jets still have about 1/16 inch to go before being level with the bridge, even with the mixture nuts full up. Very likely due to the fact that the tightening nut (cork seal nut) is as high as I can get it - even using a hammer and tapping on an open end wrench - and won't go any higher. Like the cork seal won't compress enough.

The first picture below is from Art, showing the proper seal compression. The second is mine, showing the "lack" of proper compression. Interestingly (!) the distance between the bridge and the jet top equals the width of the visible seal.

But I'd think that this small distance wouldn't be that material to mixture - as the mixture adjusting nuts should more than make up for the difference. Edit: And I'd also think this slight difference wouldn't be related to my "dying idle speed" when below 1000rpm. The only time the jets should be flush with the bridge would be when the mixture nuts are fully up, right? And you begin adjustment with the jets down two (or 2.5) full turns, which would lower then top of the jets well more than the small amount they're showing at the bridge.

Thanks.
Tom

 

[Deleted member 451]

<div class="ubbcode-block"><div class="ubbcode-header">Quote:] After installing the new throttle vertical and horizontal linkage, plus new SM needles and .100 jets, I'm trying to re-set the mixture.[/QUOTE]
Just a thought. Next time you have the car running and warm pull up on the gas peddle with your toe.If the idle drops you're fighting a linkage problem.
I learned the hard way to disconnect the linkage before doing any carb adjutments. /bcforum/images/%%GRAEMLIN_URL%%/crazy.gif

 

[TR3driver]

Interesting thought about a leak inside a carb. Any thoughts on where/how to check on that?

I can't think of a good check offhand, other than disassembling the carb and making sure everything is in the right place and in good condition.

Even using a hammer on a crescent wrench, getting that jet seal nut screwed up tight is pretty much impossible.

*cringe*
Uh, it's not obvious, but it should not be the cork that limits how far the nut goes up. The nut also holds the jet bearing (95 in the attached diagram from Moss) against the upper jet bearing (91), and the upper bearing in turn against it's sealing washer (90) and the carb body.

However, shouldn't that slight amount be "moot" if I use the mixture adjusting nuts to raise/lower the jets?

The only issue would be leaking at the joint between the upper bearing and the body.

Edit: Would this evidently rich situation, and slightly low jet relate in some way to the "dying idle" problem?

My theory (and it's only a theory) is that the leak is causing the rich idle. The slower the idle, the richer it becomes. The richer it becomes, the slower the idle.

The other place it might leak and cause similar problems is at the upper jet seal (92 in the diagram). This seal may have gotten damaged when you changed the jet, and be leaking fuel into the space between the jet & the upper bearing, which also comes out into the carb throat.

Hmm, I guess one way to check would be to remove the dashpot and float lid, then carefully pour in a bit more fuel, until the level come almost to the top of the jet. If you can see fuel around the outside of the jet opening, there is probably a leak. But I don't know if you could see it between the jet & the upper bearing, as they fit very closely.

Maybe you could very carefully plug the jet somehow, like maybe an oversize round wooden toothpick ? Then you could raise the fuel level and watch for it to run down the bridge.

Anyone else got any ideas ?

 

[NutmegCT]

Randall - thanks for your persistence. Just to clarify a bit - I replaced the jet seals, bearings, washers, etc. with new several months ago. That was my first attempt to get the idle below 1500 and make the choke linkage operable (those bent jets and needles were TIGHT). Didn't change the idle problem, but did get the choke operable. But the mixture seemed fine - no soot, no overheating, smooth idle (at 1500 anyway ...), clean plugs.

In the last week or two I've (1) reset timing and points, (2) replaced bent needles and jets with new, (3) replaced all throttle linkage (except the horizontal at the firewall), (4) rebalanced and reset carbs, and (5) set idle to 800 (but it won't stay that high so I had to up it to 1100). Also cleaned the sooty plugs every time I worked on the engine, but the sooty plugs (and sooty exhaust on wide open throttle) returned. So I'm a tad reluctant to tear the carbs apart again.

I suppose I could keep cranking those mixture nuts upward to "full lean" and see what happens.

Only other thing I haven't done is check valve tappet clearance, which I read should be .010 cold.

Edit: note also that both my carbs have the same "thick and not easily compressed" big cork seals showing as in the picture. I'm thinking those seals (which were new when I put them in) just are too hard to fully compress to that 1/32 inch in Art's picture. I really think that's the reason the jets don't come fully up to match the height of the piston chamber bridge.

Onward through the fog!
Tom

 

[TR3driver]

Just to clarify a bit - I replaced the jet seals, bearings, washers, etc. with new several months ago.

Sorry, I missed that. No chance you got the washers out of place, or the new ones were wrong ?

Only other thing I haven't done is check valve tappet clearance, which I read should be .010 cold.

That's right, assuming you have the stock cam. Good thing to check, since valve adjustment can definitely affect idle mixture and low rpm performance.

Hmmm, you don't suppose a previous owner has put a 'performance' cam in there, do you ? Some of them require substantially more valve lash, and still don't like to idle slow. I ran one for awhile that took something like .018" and tended to die below 1000 rpm. Pulled really well above 3200, though, wish I still had it.

Edit: note also that both my carbs have the same "thick and not easily compressed" big cork seals showing as in the picture. I'm thinking those seals (which were new when I put them in) just are too hard to fully compress to that 1/32 inch in Art's picture. I really think that's the reason the jets don't come fully up to match the height of the piston chamber bridge.

Could be. Easy way to find out would be to assemble without the cork, and see how high the ring winds up. As I mentioned, if the cork is holding the nut down, that means the upper sealing washer isn't compressed, and the upper bearing isn't clamped; both of which could cause problems.

The corks I got last time seemed really big too, but they crushed down easily. Cork just isn't very hard, and that nut can exert a lot of force.

 

[TR3driver]

I just saw this, might be something to look for

"The crack is next to and propagates to the jet hole causing it to leak fuel. "

https://www.britishcarforum.com/bcforum/ubbthreads.php?ubb=showflat&Number=320336#Post320336

 

[MGTF1250Dave]

Aloha Tom,

Here is a link that has some tips about SU carb rebuilding. Although it is about HD series carbs primarily, there are many areas that address SUs in general and the H series.

https://www.jcna.com/library/tech/tech0006.html

I have been using Jim Taylor for several years and he has rebuilt at least three sets of SUs for me. Here are his observations on some after market rebuild kits:

XV. AFTER MARKET PARTS -SU CARBS

Unfortunately, the people assembling after market carb kits don't always appreciate the significance of the original specifications, or in some cases just have poor quality control. Two items come to mind:

1. On the H series carbs the upper jet bearing copper washer is supposed to be 0.016" thick. Many after market kits have this washer 0.025" or even 0.033" thick. This holds the complete jet assembly down farther away from the needle than intended and defeats about 1/2 turn of the mixture adjustment nut (ability to lean out).

2. Jet orifice sizes are 0.090", 0.100", and 0.125" in diameter. I have made a set of pin gages to check these orifice diameters. The typical smaller gage is 0.089" diameter on one end, is 2" long, and tapers uniformly to 0.090" on the big end, When the small end is inserted in the orifice the gage ideally should become snug about 1/2 way up its length. If it slides all the way through I don't use the jet. An orifice 0.0005" too large, and many are, may give you trouble in leaning out the mixture enough.

As he notes, if the copper washer above the upper jet bearing is thicker than specified, you would not be able to 1) compress the cork seal, 2) get the jet assembly even with the bridge and 3) the standard set up will tend to be rich.

Hammering on a wrench to tighten the sealing nut should not be done, it will either strip the threads in the carb body or crack something. He recommends soaking the large cork seal (do not do this with the smaller jet seals) in hot water to soften it before installing.

 

[Dave Russell]

Quote Randall:
If you pump up fuel pressure with the lever on the pump, does it hold pressure for at least several minutes ? If not, you may have a leaking float valve (or float).

Quote Tom:
Question to all: I've never yet looked at the floats. What symptoms do float problems bring about?

Quote Tom:
The float was indeed "floating", the spring set correctly, the steel ball free in its cage. Same for both bowls.
-------------------
If you are using ball type float valves, even very slight leakage will allow the fuel level to slowly rise & flood the engine at low rpm. As rpm & fuel flow increase the "slight leakage" would have less & less effect. Try some Viton tipped inlet valves for a perfect seal.

The other possibility is that the inlet seats are slightly leaking where they are screwed into the float tops.
D

 

[NutmegCT]

Thanks gentlemen. Your suggestions promise further adventures for me.

All my carb parts came from Moss; I didn't realize I'd need to measure them and certainly didn't know there were standards I'd need to compare those parts to. Live and learn I guess. Seems this is getting very detailed, and I worry about my own competency, as I don't see similar carb/mixture/timing problems posted by other guys here.

Randall - I've PMd the guy in the other thread asking for detail about his carb crack issue.

MGDave - Among other item measurements I missed, I didn't know there are different jet orifice diameters in addition to needle sizes. I just got the standard .100 that Moss sells (they offer no other in the catalog). I will eventually pull the jet assemblies apart again and check all those measurements. Also soak the big cork seal in water before re-assembling.

Dave Russell - where would I find Viton tipped inlet valves? Are those something which fits into the stock bowl lids I have?


I have to work for the next couple days, but will start my next round of investigations early next week.

Thanks for your help all.
Tom

 

[TR3driver]

All my carb parts came from Moss; I didn't realize I'd need to measure them and certainly didn't know there were standards I'd need to compare those parts to.

Well, it certainly shouldn't be that way ! But unfortunately, lack of quality control is a very real problem today. In fairness to Moss, they don't manufacture these parts, and don't charge enough to have 100% incoming inspection. They check out the first few parts they get from a particular source, and then if they seem OK, the rest of the parts are simply passed on to us.

One of the reasons I like dealing with TRF is that Charles Runyan actually owns a TR3, and uses his parts himself. They still make mistakes and sell bad parts sometimes, but it gives him a certain vested interest in trying to prevent that.

I just got the standard .100 that Moss sells (they offer no other in the catalog).

That's odd, since the TR2 used .090 jets and needles. Your catalog doesn't list 370-760 ?

But I doubt that is your problem here. Mixing wrong size needles & jets causes huge mixture problems, and you only have a small mixture problem.

BTW, although it's probably best to remove the carbs from the car, it is entirely possible to drop out the jet and bearing assembly without removing the carbs. Just remove the choke lever and undo that big nut above the mixture nut; the whole thing will drop out in your hands. BTDT

 

[MGTF1250Dave]

Aloha Tom,

You indicated you have H6 carbs which are correct for a TR3. These carbs use a .100 jet. The earlier TR2 uses a H4 carb and has a .090 jet.

The concern I was pointing out is that since you don't seem to have the jet assembly fully into the carb body, the top of the jet is matched lower on the needle than it normally would. This would allow the orifice around the needle to be larger than normal. Also, the normal float bowl level would higher in the jet. Both of these would tend to cause a rich mixture.

Although the thickness of the copper washer above the upper jet bearing may be as much as double the specified .016", that would only be 1/64" larger. From your photos it appears your jet assembly is much lower in the carb body than that (jet sealing not fully compressing cork seal and mixture adjustment nut not fully threaded onto lower jet bearing). Since you also said that you would need a hammer to further tighten the jet sealing nut (bad idea!), perhaps something other than cork seals are causing the problem. I would suggest the after removing the jet assembly, reassemble the parts without the seals. Check that the nuts can be fully threaded into the carb and onto the the lower jet bearing.

 

[NutmegCT]

4:45pm Eastern Daylight Time - FLASH - removed front jet assembly and checked all parts for proper assembly.

All parts check out as being in place correctly. I re-assembled and put the jet assembly back up in the carb. Tightened the sealing nut (99 in picture); it screwed up fully flush with the bottom of the assembly. Cork seal was just barely visible. The way it's supposed to look!

Thought I had seen a minor miracle, but then I saw the upper copper washer (90 below) lying on the bench.

With that washer removed (or forgotten ...), the nut tightens perfectly (closing the "cork gap"), and the jet bearing and jet are absolutely flush with the bridge when the mixture nut is fully up. Put the washer back, and the nut will not tighten more than in my first picture (showing the "fat" cork seal), and the jet and jet bearing are slightly below the bridge.

The jet slides up with no binding. Again, with the washer removed, the jet and jet assembly top are slightly below the bridge - the same distance as the thickness of the copper washer.

I swear I can't find anything out of place, or anything holding that jet back from being fully flush, other than the coincidental thickness of that upper copper washer.

Assuming that all parts are really supposed to be there (!), and that there is no binding or catching inside the assembly, I have to also assume that the cork seal is the culprit. I'm going to soak it in hot water for a few minutes, then put back in place and try again.

Onward.
Tom

 

[Dave Russell]

I just got the standard .100 that Moss sells (they offer no other in the catalog). I will eventually pull the jet assemblies apart again and check all those measurements. Also soak the big cork seal in water before re-assembling.

Dave Russell - where would I find Viton tipped inlet valves? Are those something which fits into the stock bowl lids I have?
Tom

All H6 carbs use .100" jets & needles.

Moss lists Viton tipped float needles & seats as #373-626. Yes, they are interchangeable.
D

 

[MGTF1250Dave]

Aloha Tom,

Does the mixture adjustment nut (item 101) thread onto the lower jet bearing (item 95) all the way to the top of the threads when the the other items are not installed?

Is there any thing in the carb body interfering with the copper washer or upper jet bearing like a burr? With the copper washer and upper jet bearing removed from the carb, will the washer easily slide over the top of the upper jet bearing and rest flush with the flat surface?

 

[NutmegCT]

So I took the afternoon off and pulled the jet assemblies apart.

Soaked the large cork seals in hot water, replaced the assemblies.

No change.

Question: the upper jet bearing has two notches in the sides. What are these notches for? Is something supposed to "mortise" inside those notches? Maybe that's what's keeping the jets slightly below the bridge?

Recentered the (Edit: jets, not needles), reset the mixture, same results. Slightly rich even with mixture nuts screwed up to full lean (gun the engine, slight soot at exhaust). And the idle works smooth and dandy, but only if set above 1000. Set at 800 and it gradually lowers and stalls. After my work today however, it slowly dropped all the way down to around 450rpm and kept running (but VERY lumpy and shaking engine).

While the idle was down so low I took a look at the fuel pump bowl. About a 1/4 inch of crud at the bottom, but full of fuel.

By the way, the mixture nut does thread completely up to the top of the threads, with or without the other parts installed.

Edit: Sure can't find any interference inside the carb body; buffering that is the fact that both carbs (jets, etc.) are exactly the same in behavior: slightly low jet and jet assembly at bridge, both need can be fully lean and still run the engine. The copper washer does easily slide over the top of the upper jet bearing and rest flush with the flat surface, on both assemblies.

Those new SM needles and jets sure have made a difference!

Onward.
Tom

 

[TR3driver]

Question: the upper jet bearing has two notches in the sides. What are these notches for?

They allow fuel from the float bowl to flow into the jet bearing, so it can go into the jet. Supposed to be there, with nothing in them.

While the idle was down so low I took a look at the fuel pump bowl. About a 1/4 inch of crud at the bottom, but full of fuel.

That really tells you nothing (except you should clean the bowl out). The bowl will stay full even when you're out of gas.

Overall, it beats me. Did you ever check those valves ?

Did you say you had replaced the jet bearings ? Why, and where did you find new ones ?

Given the other weirdness you saw with timing, I think my next step after checking the valve lash would be to double-check first the timing mark on the pulley (using a piston stop), and then the valve timing.

 

[NutmegCT]

Randall - thanks for the quick followup.

Before pulling the jets out today I also checked valves. One was slightly tight, but the other seven were spot on .010. Also cleaned the plugs and verified they're correctly gapped at .025.

No - didn't replace the jet bearings. They look (deceptively?) fine to me. By the way, I use the Moss provided o-ring seals in the jet assemblies, not the old cork type.

I also read through the last twenty years of service records on the car. Nothing pops out as unusual other than three starter problems within a few months (I did have to replace the starter myself back in March).

Someone earlier in this thread wondered if I have a non-stock cam. How would I check something like that? Is there an ID stamped on the cam somewhere?

Thanks.
Tom

 

[TR3driver]

No - didn't replace the jet bearings. They look (deceptively?) fine to me.

They probably are fine, it just seemed odd that (I thought) you said you replaced them. My apologies.

Someone earlier in this thread wondered if I have a non-stock cam. How would I check something like that? Is there an ID stamped on the cam somewhere?

Many do, but the one I got from Bap-Geon many years ago had no identifying marks whatsoever. About the only way to be sure is to check where the crank is when the valves actually open and close. The instructions at https://www.cranecams.com/pdf/803.pdf will give you some idea. The workshop manual also gives a method as a "final check" after setting valve timing in the absence of marks.

However, most of the components in the Crane kit are not absolutely required. You can make a piston stop by breaking the ceramic out of an old spark plug and threading the shell for a bolt. And the pointer can be bent coat-hanger wire.

Just the degree wheel is about $30 https://performanceparts.com/part.php?partID=4723, but you could even make that yourself if you want to spend some time on it.

One good thing about the degree method is that you don't have to tear the engine down (which is a particular pain on the TR3 because the front apron has to come off to get the cam out). Since you just need approximate opening and closing points, you can just mount and calibrate the degree wheel, then note the reading when the rocker first becomes tight, and when it first becomes loose. Not the most precise method, but it should be good enough to tell you if the cam timing has been altered (by a different cam or possibly a jumped tooth on a stock cam).

If you assume the cam is stock, then there is an even easier way to check it's timing. At TDC between exhaust and intake, both valves should be open by the same amount. If you set the gap for two valves wide enough that they aren't open at that point (but the gap is the same when both are fully closed); then you can use a feeler gauge to see how far the pushrods have moved. Turn the engine until you get equal readings (near TDC between exhaust and intake) and look at how close the crank is to TDC.

That's probably total gibberish, but hopefully you get the idea ?