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High Idle

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scott_74

scott_74

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Hi everyone.

I have a 1974 Midget with the factory 1275 engine. I recently installed a brand new set of SU carbs. However, they couldn't be tuned properly due to a leak between the intake manifold and cylinder head. Turns out one of the ears on the exhaust manifold (which shares a gasket with the intake manifold) had actually broken off. I had the ear sodered back on and made sure everything was level and smooth. Put on a new gasket and reinstalled the manifolds and carbs.

However, the car now idles very high. I don't have any experience with these carbs so any help is appreciated with this. I followed directions I had found online and I'm able to get the carbs balanced with eachother. However, with the idle adjustment screws all the way out, it idles around 1500-2000 RPMs (significantly lower with the clutch in). If I back out the jet adjusting nuts anymore, the car runs very poorly...it misses and backfires.

I've looked everywhere for a vacuum leak and can't find anything. I've been told to check the timing, but I didn't have this problem prior to the new carb installation. I'm kind at wit's end with this.

Any ideas? Any thoughts on the matter is much appreciated.

Thanks!


Scott
 
Back fires through the tail pipe I assume?

Did you plug the vacuum advance at the manifold to be sure you do not have a vacuum leak at the dizzy?

Does the car still have the emissions equipment?

If not, what vacuum lines do you have connected?
 
It is for sure an air leak.
 
Has to be an air /bcforum/images/%%GRAEMLIN_URL%%/iagree.gif leak somewhere.
 
Have you loosened the interconnecting linkage and made sure the throttle plates are all the way closed? Checked for proper clearance between the lever and heat shield? Looked for a frayed or kinked throttle cable?
Jeff
 
Ok, fire it up. Spray carb cleaner all around the intake gaskets then around the throttle rod stems, both sides, both carbs.

Any increase in idle speed is indicative of an air leak at that spot.

You can also peak inside the throttle body with the dash pot pistons held up and see if the butterflys are fully closed. They should be when engine is off.
 
My bet would be that the weld-up on the manifold is the problem, was the flange surface machined (filed) completely flat? You will probably find the vacuun leak on the underside of the manifold at the weld.
Bill
 
I know I saw some Permitex prodect that was for exhaust and intake gaskets. something copper I think.
 
Thanks everyone for responding. Much appreciated.

The throttle plates appear to be all the way closed, so I don't think it's the throttle linkage.

The manifold was not surface machined after the weld. It was filed down and tested against a flat surface to ensure everything was flat. I now realize a more precise, machined file would have been ideal.

I did the test with the carb cleaner and there was a very slight increase in idle when I sprayed the welded ear of the manifold. But it wasn't apparent enough for me to start dismantling everything for a third time.

My father called up a friend of his who has a "Leak Chek" system where visible, harmless vapor is fed into the car. Surprisingly, nothing smoked out from the manifold connection. (And nothing from the dizzy.) A small leak from an intake manifold hose was found and fixed....but a significant amount of leakage was coming from the base of the car...near the engine block. Is there any kind of a breather down there? Any ideas what this could be from?

Tomorrow we'll jack the car up to get a good look from underneath. Also, from the sound of it, it seems I have an exhaust leak as well. I'll look into that as well once the car is up.

Thanks all...and I'll keep you posted.
 
Funnily enough one of my cars idled fast the other day when I started it up. It had been running fine the day before, but now was idling at 1500rpm and the accelerator was real stiff. I hadn't done anything to it, other than give the interior a clean out, but how would that cause such a problem? So what was wrong? Simple: when I replaced the floor mats the drivers one had cleared the brake and clutch, but was sat on top of the accelerator. Fortunately I figured it out only a few yards down the road, but it didn't stop me feeling like an idiot!
 
The ending isn't too different for my story, Pete. Just took me longer to figure it out.

Turns out there was a slight misadjustment on the new front carb. The butterfly disc was attached in a way, that when it was 'closed', there was still a very slight gap. So I readjusted the disc so it closes completely. The idle went down.

I couldn't see the gap when it was attached to the manifold. But when I swapped in the old carbs for the heck of it, the idle went down; and after inspecting the new ones, found the glitch.



Thanks to all who replied!

Scott
 
I'd like to add a comment similar to Jeff's.

Since you're not familiar with these carbs there's and easily made mistake regarding the interconnect linkages. As you certainly read when researching tuning, the linkages have to be disconnected so each carb works independently. That includes the choke linkage, both must be free/loose. You also have to make sure that the fast idle adjustment screw (on one of the carbs and actuated by the choke linkage) is backed off and not touching its cam.

If the fast idle screw isn't backed off, it's very easy to have a car whose idle stop screws are completely backed out... but you can't lower the idle speed. Make sure your linkages are disconnected and that the fast idle screw isn't touching its cam, then try setting the idle speed again.
 
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