Sprint engine idle speed too high

[dursley92]

My Sprint engine is idling too fast, around 1000rpm, and I cannot get it lower.
Checked for the obvious usual things like air leaks etc but nothing obvious. Problem seems to be with the front carb as the flow meter shows that is drawing quite a bit more air than the rear one and even with the throttle stop screw backed right off it does not come down. I have had the carb off and the butterfly seems to be closing fully. Thought I had found it with the fast idle screw on the choke cam but that made no difference.
I am running the club aluminium mounts so obviously no leaks there but the problem developed after I fitted them so any suggestions where to go next are welcome.

[Jon Tilson]

Air leaks? Throttle spindle bushes? Too advanced?

Jonners

[tony g]

Have you slackened the linkage bolts between the carbs? If not then the other idle screw may keep the R/H carb too high.

Tony

[Mahesh]

Were you doing any work with the carbs when this happened ?

[dursley92]

An air leak was my first thought but it would need to be really bad and there is nothing obvious. Also if it was a leak why would I be seeing a much higher air flow on the carb balancer?

I have slacked off the linkages so it is not being caused by the other carb.

It has only been happening since I fitted the club alloy mounts but I don't see why that could make this happen. All the faces were clean when they went on and the "O" rings should be giving a good seal.

Just to confuse things a bit more I think the fuel level may be too high in that float chamber so I have got a pair of new stayup floats and needle valves coming from Burlen just to eliminate that factor.

[cliftyhanger]

Does pressing the linkage shut on the carb help?
Had a similar issue on that TR7 engine, in the end new spindles and butterflies fixed it, think it was a tiny bit of wear on the spindle causing the issue, almost undetectable until it was taken apart. Not an air leak either.

[dursley92]

Does pressing the linkage shut on the carb help?
Had a similar issue on that TR7 engine, in the end new spindles and butterflies fixed it, think it was a tiny bit of wear on the spindle causing the issue, almost undetectable until it was taken apart. Not an air leak either.

No, fiddling with the linkages/spindle makes no real difference and it feels like its closed solidly. When I had the carb off to check I held the body up to the light and the butterfly shows closed with no gaps. Seems to me the only answer has to be a big air leak somehow so I am just off out to take them off (again) for another hard look at.

[SprintMWU773V]

Run the engine and spray a mist of easy start around any suspect areas where there might be a leak. The revs will rise indicating where you may have a leak.

[dursley92]

OK, carbs off this afternoon and problem found - the butterfly on the front carb was not closing fully. It was one of the first things I looked at and obviously I didn't check it properly. No one to blame but this incompetent mechanic. I have reset the butterfly and can now get a sensible idle speed.
I think it must always have been wrong but maybe the old cracked rubber mounts had concealed the problem and the new alloy mounts has shown it up.

Wish I never posted this now because I feel like a plonker !!

[Mahesh]

I wish my problem was as visible as yours, (car is holding a 23 year old grudge).

Glad yours is sorted.

[tony g]

What is resetting the butterfly?

Tony

[dursley92]

What is resetting the butterfly?

Tony

I slackened off the two screws that hold the butterfly in the spindle so that it could move and centre itself properly in the carb body. Then re-tightened the screws (they have a slot to lock them) and the butterfly now closes fully with no little gaps around the sides.
The butterfly has obviously been taken out sometime in the past and was never put back properly.

[tony g]

Ah ok cheers

Tony

[dursley92]

Just a follow-up to this post. After fixing the dodgy butterfly the idle improved a lot, but the front carb still seemed to have a high fuel level in the jet. The HS6 carbs have the one piece plastic non adjustable floats so I decided to invest in a set of the StayUp floats from Burlen These have an adjustable metal pivot and also come with new needle valves and gaskets.


The new floats went in really easily and I was able to set the float level as per Burlen's instructions. After getting the carbs balanced again I popped into my local garage to put it on their emissions tester and got a reading of 7.5%. Screwing the jets up three flats on each brought it down to 4.5% which is a shade rich but nice and on the safe side.

We took the car out for a run with the TR Register group last Sunday (the hottest day of the year so far) and it ran better than it ever has. Smooth all through the range and with a nice amount of power from low revs as well. Three of our TR5/6's broke down due to the heat but the Sprint engine didn't miss a beat.

Excellent result.

[soe8m]

viewtopic.php?f=4&t=20838

Maybe of some help.

Jeroen