wandering idle....thoughts?

[JamesH]

so, on my 1980 droptop TR7, once the engine is warm, the (aftermarket) choke is off, the idle will bog down to about 700 then bounce up to 1500....and then repeat and repeat, ad nauseum. the car has the original 4 with the set of ZS carbs. i had somebody suggest that the bypass valve needed to be replaced for leaking, but i though i would ask y'all before ordering a set. your thoughts?

[busheytrader]

Albeit on a UK 7, l had similar issues. It turned out to be one of the rubber breather hoses connected to the carbs. The hose had a crack on the underside which opened up once warm, but when cold it closed up.

Adam

[JamesH]

hmm. i will check the hoses, but this doesn't feel like something that is temperature variable--it really feels like is always there once the choke is off. it doesn't seem to get better and worse--it is just there. i think it is interesting that it doesn't do it at all until the choke is off and then it is just consistently there--whether the car has been running for 10 minutes or has driven 45 miles.

[HDRider]

I have had a couple of TR7's do this. It was caused by air leakage at the rubber carb mounts. The easy fix is to just add paper gaskets on both sides of the mounts.

Originally the mounts have a thin rubber protrusion on the mounting faces that shrinks with age. Of course you could replace the mounts but others have had issues with the replacement ones. I have been running old mounts with added gaskets for years without any issues.

Edward Hamer
Petaluma CA

[FI Spyder]

Air leak after the carbs (hoses, mounts etc,) runs it lean which is why it is less noticeable with choke on (which runs it rich).

[JamesH]

'Spyder, thats what i thought...i know it dropping and surging is probably the carbs trying to figure it out....and an air leak somewhere is the most likely reason for that....i just needed y'all "experts" to tell me where. i have a couple of the manifold gaskets on the way here from one of the usual suspects. thanks.

[DNK]

Best way to tell is propane, carb cleaner, and I think Brake cleaner

[john 215]

Hi,

I agree with my learned friend Don, have a good spray around with WD40, brake cleaner etc, using a straw on the can spray one area at a time little by little, when the idle alters, normally will rise there is the source of your leak. Will save a lot of guess work and cash !

The world is full of carb experts ( in reality they know nothing ) who love to fiddle with there carbs on a Sunday morning, so possibly just req. setting up and balancing.

Cheers John

[HDRider]

Actually I think that you will find that the surging is caused by the timing retard device kicking in and out as the manifold vacuum drops near the stall speed.

Edward Hamer
Petaluma CA

[Robin Powell]

Have a look at the throttle disks in the SUs and see whether there is a sping controlled air bleed on the disk as I had one that didn't always seat properly and cause a variable tickover. I changed them to plain disks which cured the problem.

[JamesH]

wow, Robin, i know exactly what you are talking about...i soldered the pop-valve holes closed in the SU HIF carbs in my '72 BGT, but unfortunately, my wedge is a US model (makes sense, since i live in the US) so my car has ZS carbs. i wish i had SU, at least since i have alot more experience with them from my collection of MG, but, oh well.