carb mixture H6 - exhaust

[NutmegCT]

After installing the new throttle vertical and horizontal linkage, plus new SM needles and .100 jets, I'm trying to re-set the mixture.

Throttles balanced. Fast idle screw full out (not touching cam). Mixture nuts screwed full in, then backed off (enriched) two full turns. Engine starts immediately at crankover. Idle at 800 warm. Slight whitish "smoke" from exhaust. Idle very lumpy - "blmp blmp blmp etc." - sorry for the highly technical sound description ....

Lift piston 1/8", speed increases slightly, turn in one flat to lean. Lift piston, speed still increases slightly. Repeat ad infinitum.

I can actually get the mixture screws all the way up (full lean) and still the engine runs with that "lumpy" sound and feel. Lift the piston, still increases rpm.

Owners manual, Haynes, and Standard manual refer to "clear exhaust" or "black exhaust". Also say to lean mixture if idle increases when piston is raised. I've got neither. Manuals say mixture is set when exhaust is clear and idle slightly raises *then returns* when piston is slightly raised.

What does the whitish/greyish exhaust tell me? What am I missing in the mixture adjustment?

Thanks.
Tom

 

[DrEntropy]

"Whitish/greyish" ain't fuel. Oil or coolant. Unburned fuel gives a sooty or dark exhaust ejecta. The pistons need not be raised as far as 1/8".

My suggestion is to get yer mits on a ColourTune or two and use those to judge your mixture.

I wouldn't get too concerned over the "slight" smoking yet, either.

 

[TR3driver]

White generally indicates steam, which would be more or less normal for a cold engine. But you shouldn't see that with a warm engine, unless perhaps a blown headgasket or similar is allowing water where it shouldn't be.

Given how your carbs are acting, I think it's more likely you're seeing soot ... so I'd concentrate on the carbs first. If it will even idle with the mixture nuts all the way up, then there is something very wrong at the carbs.

Are you sure the jets are rising with the nuts ? Float level correct ? Needles installed so the shoulder is even with the bottom of the piston ?

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).

Another possibility would be the upper jet seal leaking, or the seal between the upper gland & carb body leaking.

Or, you may have been sold the wrong (or defective) jets &or needles.

Oh yeah, it's also important that the big nut above the mixture nut be tight. Not entirely obvious that it's what clamps the jet housing to the carb body, and pinches that copper washer that has to not leak. And after you tighten it, be sure to recheck the jet centering.

 

[martx-5]

On the TR3 carbs, there's a little piston lifting pin that raises the piston the proper amount when checking the mixture. I don't know if you're using that or not, but see the attachment. I crudely circled the lifting pin. /bcforum/images/%%GRAEMLIN_URL%%/smile.gif

Whether it will have a different affect then other methods, I don;t really know, but that's always what I used.

 

[NutmegCT]

DoctorE - the Colourtune must be a great tool. Maybe I can find someone near me who is in a lending mood.

Randall - if I really goose the throttle, there is indeed black soot from the pipe. I had thought the reference was to continual exhaust, not just "goosed" exhaust. Also noticed the carb between the butterfly and the manifold gets quite cool (cold!) as I do the adjustments. I assume that's the normal reduced (venturi) pressure lowering the air temp. I'll follow your suggestions tomorrow when I get back from work.

Art - yes, that pin is how I lift the piston. Only enough to raise the piston about 1/8 inch (altho' I hear that's too much). Interesting that the TR3 owner's manual ("Practical hints ...") says 1/8 inch, the Haynes says 1/32 inch, and the Standard manual also says 1/32 inch. The higher I lift the piston, the more the engine speed changes, so I'm figuring the *smallest* lift of the piston is best.

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

thanks guys.
Tom

 

[DrEntropy]

Tom! Another thought: To disconnect the choke cables and insure the jets are fully "home" as you try to dial it in. One of Randall's suggestions triggers a recollection of some "odd" cable binding. It'll hold the jets from moving with the adjusting nuts. Just a thought.

 

[MGTF1250Dave]

Aloha Tom,

The SU manual also list the "lift" amount as 1/32" or .8 mm, a very small distance.

If you notice in the photo above, very little of the cork seal is visible between the carburetor body and the aluminum sealing ring. This seal needs to be compressed until about only 1/32" of it is visible. This allows the the jet to be able to rise to the top of the upper jet bearing when the mixture adjustment nut is fully tightened. Often the cork seal needs to be soaked in oil to soften it before assembly so that it can be compressed.

The floats and the float lever control the level of fuel in the bowl and also the jet. The float lifts the lever when the bowl is filled and closes needle value stopping fuel flow. As the float drop, the needle drops allowing a refill. If the adjustment is to high it will tend to make it very difficult to lean out the mixture and in the extreme fuel can flow freely into the carburetor. If the level is low you will have to tend to richen the mixture beyond "normal". The proper setting of the float lever is an easy static test and depends upon the type of float bowl cover you have (push on fuel connections or bolt on connections). I would recommend that you check the float lever adjust.

 

[NutmegCT]

Dave - Take a look. The cork seal on both my carbs is about 1/8 inch visible.

Good heavens - 1/32". How on earth do you folks know these things?

Thanks - will tighten those seals up tomorrow, then replace the jet return springs and try another mixture adjustment.

Tom

 

[MGTF1250Dave]

Aloha Tom,

Don't forget to check the float adjustments also.

Here is a link that describes the adjustment:

https://www.burlen.co.uk/faqDetail.aspx?faqID=15

 

[Merlin63Tr4]

I don't know if it makes any difference, but you seem to be missing the jet lever return spring ! /bcforum/images/%%GRAEMLIN_URL%%/frown.gif

 

[TR3driver]

Also, the nut looks to be set very rich ... makes me wonder if perhaps the threads are damaged so it's binding instead of seating when you screw it all the way up.

Can't speak for others, but I've owned a succession of TR3s, starting with the TR3A I talked my Dad into buying (so I could drive it sometimes) way back in 1974. No Bentley, no Haynes, no Internet ... just kept trying things until something seemed to work. Dad expected me to keep it running even when he was driving it. I'll never forget spending a Christmas vacation repairing the engine after it swallowed a valve, laying in the snow in the side yard.

 

[NutmegCT]

Dave - thanks for the float adjustment link. Looks like I have to wait 'til I've got it opened up to know which type of float I have (and which measurement to use).

Merlin - yes - I took those springs off so I could get the wrench around the nuts when I adjust them.

Randall - the picture shows the nuts at the "full up then backed off two full turns" point. From all I can find, that's the initial setting to use when you begin mixture adjustment.

If I'm mis-reading or mis-interpreting something, please let me know.

OK - off to check the floats and tighten those jet sealing rings.

Tom

 

[Simon TR4a]

You might try using 8 flats down as the initial adjustment, 12 flats (which is 2 full turns of a hex nut) sounds like a lot to me. Final adjustment is usually 8-10 flats, and a small adjustment makes a big difference.
How does the car drive when you get it off idle?
Are the needles installed the correct distance into the piston of the carburettor? (Sorry, Randall already asked that.)
How much smoke do you have in the exhaust, there should be very little and it should clear up quickly, there is something else wrong. Perhaps worth looking at the plugs to see if there are any obvious problems, don't worry much about the colour at this point as it will be wrong because of all the starting and stopping and cold running as you try to tune the carbs!
Good Luck
Simon.

 

[TR3driver]

Dave - thanks for the float adjustment link. Looks like I have to wait 'til I've got it opened up to know which type of float I have (and which measurement to use).

All H6 TR3 carbs have what Burlen calls T2 floats, and use a 7/16" float adjustment. (I'm pretty sure all H4 TR2 carbs do, too.)

Merlin - yes - I took those springs off so I could get the wrench around the nuts when I adjust them.

FWIW, the wrench included in the SU tool kit is designed to fit under the spring. But removing the spring is no problem, as long as you remember to push the jet up firmly in contact with the nut after every adjustment. The mixture nut should be free enough that you can turn it with your fingers (if the engine is cold or you're willing to burn them a little bit).

Randall - the picture shows the nuts at the "full up then backed off two full turns" point.

I pulled out a couple of housings from the parts bin just to check. With the nut full up, there is only one (incomplete) thread still exposed. So at two turns down, should be only 3 threads exposed. It's a bit hard to tell, but your photo looks more like 4 or 5 threads exposed to me. Compare to the photo Art posted, which looks about right.

Still, it doesn't matter so much where you start, it's where you end that is important. And you are saying the engine still runs rich with both nuts as far up as they will go, right ?

From all I can find, that's the initial setting to use when you begin mixture adjustment.

Actually, both Bentley and Haynes say 15 flats (2.5 turns), but obviously that's not the problem here.

Since I learned to do this without benefit of a book, I've always used a slightly different approach. I leave the nuts down about 1 turn when I assemble the carb, then disconnect the springs and pull the jets down about 1/4" by hand to start the engine the first time. Push the jets up after it starts, and keep adjusting them as the engine warms up. When the thermostat opens, I screw the nut down to touch the jet, and fine-tune from there.

BTW, the "lift the piston" test is very sensitive, once everything is just right. Even one flat of the nut makes a noticeable difference.

One other thing to think about : if the valve (rocker) adjustment is way out (especially too tight), it will screw up the mixture adjustment. Setting the valves should be one of the first steps in doing a tune-up.

 

[NutmegCT]

Per the Burlen site:


"T1 (1 7/8” OD) and T2 (2 1/4" OD) Float chambers. With the float chamber lid inverted and the hinge lever resting on the float needle, it should be possible to just slide a 7/16” diameter between the radius of the hinge lever and the face of the float chamber lid. See diagram.

T4 (3” OD). A 5/8” test bar should be used.

Early HS carburetters with brass floats. A 5/16” test bar should be used."

The OD of my float chamber is 2 1/4". But the float is brass (not plastic).

So should the test measurement be 5/16 or 7/16? Both currently measure 7/16". Fork is free to move up and down; brass valve (?) in the "cage" is free.

Tom

 

[TR3driver]

Tom, the photo you posted is of an original H6 carb, not HS. Thus it uses the 7/16" measurement.

Late TR4/A did use HS6 carbs, which can be retrofitted with some minor effort to the TR3/A (have to redo the choke & throttle linkage). But they are an improved design, distinguished by an external tube running from the bottom of the float bowl to the bottom of the jet (thus eliminating all those pesky seals in the H6), so you don't have them.

 

[NutmegCT]

Thanks Randall. Just wanted to be sure.

My float looks excellent, so I reconnected the whole shootin' match, started from scratch on the jet adjustment. Seems the jets are adjusted (very slight rpm increase when piston is barely lifted), and tried lowering idle to 800.

Set any lower idle and the rpm actually decreases slowly 'til the engine dies. Goose the throttle and a *small* amount of soot spews from the exhaust. Drives well, plenty power, no miss or ping under load. No backfire or dieseling.

Jet nuts are about 5 flats from full up. I think I could lean them all the way and the engine still runs ok (as long as idle is set at least 1000rpm.

Hmmmm ...

Tom

PS - after my test drive (no problems), I got home and found the idle was *much* smoother than before the drive. Still at 1000-1200. Again tried lowering to 800 but the idle then slowly dissolves down to nothing. Weird.

 

[NutmegCT]

Just a stab in the dark -

When I opened the float bowls, a small "squirt" of fuel came out, like there was a bit of pressure inside. The float was indeed "floating", the spring set correctly, the steel ball free in its cage. Same for both bowls.

Trying to figure out why the idle slowly drops to zero when set below 1000 - could it be a weak fuel pump?

The engine has only one pump - the original with glass bowl. Bowl is clear and I can see fuel in it. Is there a way to know if the pump is sending enough fuel? I've heard it just "squirts" and should have low pressure. Wondered if maybe the gradual drop to zero rpm is because - at a low idle speed - the pump isn't sending enough fuel.

Would a "test" be waiting for the engine to die at low idle, then open up the float bowls and see if there's fuel inside?

Grasping at straws again ... but otherwise, TeeYah sure runs great on the road, with hardly any exhaust soot when the throttle is goosed.

Tom

 

[piman]

Hello Tom,

the number of turns or flats from full up is only relevant if the needle is correctly installed in the piston, with the shoulder flush with the bottom of the piston. Two full turns or twelve flats should have you near enough correct for idle mixture. If it runs better at a significant deviation from that something else is incorrect. You really need to remove the piston\dashpot assembly to get the initial full up setting with the jet top level with the jet tube (Which should be flush with the bridge). Then screw down the 12 flats. When tuning the carburettor mixture, from time to time blip the throttle to get the pistons to settle where they should, sometimes they gat a little 'lost'.
By the way all H carburettors used brass floats so 7\16" is your float setting.

Alec

 

[TR3driver]

Is there a way to know if the pump is sending enough fuel?

Several ways to test this. Best, IMO, is to temporarily tee in a pressure gauge at the front carb and measure the pressure when you see the problem. If you can find a vacuum test gauge (very worthwhile tool to own, IMO), they usually will also read fuel pressure in the range of interest.

I've heard it just "squirts" and should have low pressure.

True enough, although it won't seem so low if a line comes apart with the engine running. The pump only delivers fuel on every other crank rotation, so there is a very strong pulsation in the flow.

Wondered if maybe the gradual drop to zero rpm is because - at a low idle speed - the pump isn't sending enough fuel.

Seems unlikely to me, but couldn't hurt to test.

Would a "test" be waiting for the engine to die at low idle, then open up the float bowls and see if there's fuel inside?

If, as before, you find pressure still in the lines, that would pretty well prove the problem wasn't a lack of pressure.

If I've got this right, you can tell that it's running rich even at the higher idle; it's just not so rich that it doesn't run well at higher rpm (and throttle). Sounds more like too much fuel rather than too little, to me. Might be more interesting to let the engine die at low idle, then remove a dashpot and look at the fuel level in the jet. 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 ?