TR6 low speed stall

[charleyf]

I am working on a 1974 TR6 that has been completely rebuilt. The PO put new HS6 SU carbs on the car. We have had a devil of a time trying to get the car to run smoothly. We are now finally down to only having a low speed stall between 1000 and 1500 RPM and especially when turning 90 degree corners right or left. In this low RPM band the car will stall or occasionally spit or backfire. If one totally lets off the accelerator in the turn and coasts around the corner it is easier to get it to go forward.
A couple of us have been working on this car for a few weeks with this problem. We have tried lowering the float and that made it worse. So we have raised the float with two gaskets and that was marginally better, but the same things persist. We have considered putting yet another ( a third) gasket under the float bowl lid to further raise the float / fuel level. We have as well tried to advance the timing and that did nothing.
The car has the stock distributor with the vacuum retard WHICH HAS BEEN DISCONNECTED. We are convinced that it is something to do with the carbs but cannot determine what to do to change it. We have also tried heavier oil in the dampers with no success.
So I am asking if anybody out there has a remedy for this low speed stall? I would add that once beyond 1500 RPM it improves and then at 2000 it feels very responsive then at 2500 the car will push you back in the driver's seat. The new cam is stock.
A friend with a TR3 had a similar low speed turn stall and was only able to get rid of it by rebuilding the carbs. But these are new carbs so that should not be the problem here.
Any ideas or recommendations????
Charley

 

[mrv8q]

Fuel pump: manual or electric?

 

[poolboy]

I once had an occasional "miss" when making a turn. Turned out to be one of the distributor's internal wires touching something it shouldn't.

 

[charleyf]

Fuel pump: manual or electric?

A new manual fuel pump..
Charley

 

[charleyf]

I once had an occasional "miss" when making a turn. Turned out to be one of the distributor's internal wires touching something it shouldn't.

It has the same stall going forward when between 1000 and 1500 RPM in anything but first gear. And not in first because it jumps through that band so very fast.
Charley

 

[poolboy]

See if the carbs air valves are moving freely and in sync when you rev up the engine.

 

[titanic]

Charley-You might try removing the carb air valve after the engine is at operating temp and check the fuel level in the jet. Some people that have used SU hs6 carbs on a TR6 have had flooding problems. Joe Curto has recommended using float bowls from a Spitfire )#2163) that are about 1/4" lower.
Berry

 

[charleyf]

Berry,
you us the term "air valve" are you referring to the piston and needle. The only way to see the fuel level in the jet on a SU is to remove the domed suction chamber, piston and the needle -- that I know of.
Charley

 

[titanic]

Charley-Sorry, same parts, different terminology.
Berry

 

[Brinkerhoff]

You aren't really raising the float level much by using another gasket. You need to take the float lid assy. off and flip it upside down to check the measurement against the manual spec. I think off the top of my head its only 1/8" at rest when upside down and the needle valve is closed.

 

[charleyf]

The floats are not adjustable. So the only way to raise the floats appears to be to use the gaskets. Likewise to lower the fuel level use a gasket under the needle valve. We started with one float lid gasket, then went to two and then three. Three made it run worse two improved it over a single gasket. BUT we still have the stall between 1000 and 1500 RPM.
Charley

 

[Brinkerhoff]

You can check the functioning fuel level by removing the dashpots and pistons and looking down into the jet. If set properly the fuel level should be just below the bridge. By blowing down into the jet with your mouth , fuel should bounce down and up , overflowing into the carb a bit.

 

[arcom]

I don't know if a maladjusted float is your problem, but the float level is adjustable, at least for HS4 carbs. I know this from years ago when I was in my MGB phase. Moss has an excellent video:

https://www.youtube.com/watch?v=Z6Le_HkrkKQ

 

[charleyf]

I as well do not know if the float level is the problem. I can not figure out if the problem is to much fuel, to little fuel or possibly even a timing problem. We have moved the flat up and down, the needles up and down and the jets up and down. I suspect that there is a combination of the three above items that will work. But we have not found it yet. Kind of like shooting in the dark.
Charley

 

[Brinkerhoff]

One thing for sure , an SU carb is not complicated in the slightest and will give you good practice to figure it out. Follow any factory service manual , the diagnostics are the same for any British Car.

 

[poolboy]

Poor carburetors...always getting blamed for everything.
https://sucarb.co.uk/technical-hs-type-carburetter-description-adjustments

 

[Brinkerhoff]

Every one of these cars I work on with a running problem gets the valve clearances checked ( so the engine can pump air properly) and a new set of ignition points installed before I'd start messing with the carbs. Especially if the owner tells me it has just been done ! Its a waste of time otherwise.

 

[glemon]

Might be interesting to pull a couple plugs and see if they indicate rich, lean, or about right. Would agree about checking ignition/electrics as well. Also make sure the pistons and needles go up and down smoothly with no friction or catches.

 

[charleyf]

We have checked the valves. Replaced the condenser and points. Today called Joe Curto and ask for help. He is sending us a new set of needles that will give us a better ( richer) low RPM end.

We have adjusted the carbs and checked the spark plugs many times. BUT I believe the carb adjustments are at idle and the spark plugs show what is happening at higher speeds. So neither of those things tell us what is happening at 1000 to 1500 RPM. We will try the new needles a soon as we get them. Then one more thing to look at is to try to use a colortune and see what is happening at that 1000 to 1500 RPM area. But then again this would be at a no load situation, which does not replicate when the problem occurs.
So we are still guessing!!!
Charley

 

[NutmegCT]

Just a guess, but often when you make a turn, you let up on the gas pedal.

Does the "stall" occur if you don't let up on the pedal during a turn?