TR4/4A Sticking Accelerator or SU Jets: TR4A

[KVH]

My RPMs stay high after each shift, and at stoplights, UNLESS I quickly jolt the gas pedal with a kick. The RPMs stay well above 1300 unless I go through that exercise.

I've tried greasing and oiling the accelerator linkage, and all of my return springs look good. I'm puzzled because it seems "linkage related."

Would one of you suspect I actually need to centralize the jets in the carbs instead of working on the linkage? Is that more likely the culprit?

(I hate to get into the carbs because the mixture right now seems perfect. I suppose I could count flats)

Thanks for all the help.

 

[TR3driver]

What carbs do you have?

I had a similar problem with my 3A with SU H6 carbs, which I eventually traced to worn carb bodies. The front carb in particular was worn enough that the throttle plate would actually drag on the bore, and prevent the throttle from closing all the way unless it was "slammed" shut (by kicking the pedal). The solution was to rebush the carbs.

But also look for anything that could make the linkage bind just as it comes to idle. On the 3A, the linkage pivots use a "ball" joint, and my balls were so badly worn that they looked more like footballs. Due to some linkage adjustments I made, they would bind just as the throttles closed.

Poor needle centering shouldn't cause a high idle, unless your mixture is also rather rich. Easy test is to turn the engine off, and lift the pistons to the top with your finger, then let them fall and listen. If they land with a 'click', the centering is close enough. But I think maybe this applies only to SU carbs.

 

[KVH]

Thanks, Randall. I'll check those items. Mine are HS6 like yours.

This car has a heat shield, and it's a bit cluttered around the linkage, but I'll check again.

I'll also look at those bushes and see if I can draw any conclusions.

 

[tomgt6]

Someone wrote about polishing the jets as well on another questions and this could be an issue maybe.

 

[TR3driver]

Mine are HS6 like yours.

Er, just for clarity, there are some substantial differences between H6 and HS6 carbs. The H6 (stock for TR3 through early TR4) carbs are an earlier and more primitive design than the HS6 used on late TR4A. I'm actually planning to upgrade to HS6, partly because they supposedly flow better but also because they are much less prone to leak, and the smaller size fits better with my paper element filters.

Not certain offhand, but I don't think polishing the jets would help on HS6 carbs, as they don't slide through a seal the way they do on H6. The fuel is delivered directly to the bottom of the jet by a hose from the float bowl, rather than the Mickey Mouse arrangement found on the earlier carbs. And anyway, that would apply only to the mixture staying rich, not a high idle.

 

[Mkutz]

I had a similar problem on H6 carbs for the TR3. My problem was that the butterfly plate was not closing far enough, causing a high idle. What I did was install a new return spring, AND increase the tension on the spring that sits on the shaft next to the throttle body and which is clamped in place. All i needed to do was loosen the clamp, turn the spring about 1/2 turn, tighten it, and problem solved.

 

[tdskip]

Any luck?

 

[KVH]

I just got it fixed! Runs great now.

I loosened the two clamps on the throttle shafts and rotated each clamp and the "lost motion lever" to eliminate any possibility that when at rest they were impeding the throttle and butterfly from closing.

It's hard to explain, but there is some play in the linkage, and I moved those levers to be sure the pins were not interferring with the movement of the throttle in the closed position.

I also discarded the heat shield. I find it hard to believe that really adds much. It sure does get in the way of a lot.
I suppose I could put it back on if convinced otherwise.

But the car runs great. I may take it out tomorrow.