Small SU problem

[Deleted member 451]

After I rebuilt the SUs on the TR3 I have a small problem.
Every time I apply the breaks the idle drops. When the engine is warm this is not a big problem but when the car is not fully warmed up it will die if I'm not fast with throttle or choke. I'm thinking I may not have the floats set right and the fuel is running back into the float bowl.
Any Ideas?

 

[piman]

Hello Bill,

what is the idle speed adjusted to?
The other thing that could be the cause is that the jet(s) is not centred and the piston(s) is not dropping as you brake.

Alec

 

[Andrew Mace]

Does this literally happen when braking, or does it happen as soon as you lift off the accelerator? I'm wondering if there might be something funky such as a soft or broken motor mount that might be contributing to the problem?

 

[Deleted member 451]

Happens only when braking with the clutch in. idles at 750.
No sticky slides. I was thinkg that the float boals being in front of the carbs that when braking the fuel would be pushed forward causing it to be low in the jet.

 

[jsneddon]

Have you tried pulling the float lids off when it dies and checking the levels?

 

[piman]

Hello Bill,

I don't think that your float theory is valid. You say the slides aren't sticky, but can you hear them stop when you lift and drop them?, the other possible thing is that they are set too weak?

Alec

 

[willeng]

Bill,

Is there a brake booster fitted? If so firstly check the operation of it to make sure the diaphram is ok. To check this rest your foot on the brake pedal with "light" pressure--engine off then start the engine with your foot still on the brake pedal. If the booster is ok you will feel the pedal go towards the floor a little when the vacuum from the engine reacts with the booster.

This being ok with no vacuum leaks the problem you will find to be the idle mixture, when the engine gets warm things open up--jets etc with heat of course & the problem will correct itself a little.
Just wind the Su's down or up one flat at a time-keeping note of adjustments made & see if it will correct the problem or if it made it better or worse. If it corrects the problem when cold but plays up when hot you will have to change the needles to suit.

Remember you shouldn't be driving with a cold engine anyway?

Hope this helps.

 

[piman]

Hello Willeng,

just a small point, the idle setting of varieties of S.U.needles are as near as makes no difference, identical so new needles won't make any difference.
What I have remembered is that some later S.U's had a small check valve in the throttle disc which could cause this sort of problem if they are sticking, that and the horrible wax temperature compensating jet. Neither of which should be on a TR3 but you never know what may have been fitted at some time.

"Remember you shouldn't be driving with a cold engine anyway"

hmm, it's the best way I know of to warm an engine up. An idling engine doesn't really warm up and creates a lot of condensation in so doing. Just don't drive it hard.

Alec

 

[willeng]

Piman,

It also depends on how far--in--out the needles are set in the slides & various different characteristics can be found by adjusting things with different needles. You can make a rich needle leaner or a lean needle richer with this method.

Your right idling will not warm engines to good but holding say 1300-1500rpm or so will in no time. Again horses for courses, most race engines have excessive skirt clearance with forged pistons when cold so it's better to warm the sucker up first. Driving around is the only way to warm up the driveline though.

willeng

 

[piman]

Hello Willeng,

I'm sorry but I disagree with moving the needles in the piston. The needle should be set with the shoulder flush with the bottom of the piston. The jet is what should be moved to adjust the mixture. As I said, at idle the mixture will be the same whatever needle is fitted without moving the jet.

Alec