Idle Creeping Up

[whitenviro]

A couple years ago my car idled at a very nice 800-900 rpm when warm and manual choke off. Last year, without doing anything that would change it, it idled at around 1,100. High, but not too bad. I checked for vacuum leaks everywhere but didn't come up with anything except cracked hoses back at the fuel tank vapor return. Changing them didn't lower the idle, but eliminated some of the gasoline odors. I also checked the choke cable and pedal linkage and they seem free. This year, it seems to idle at around 1,200 rpm, which has me a bit concerned. I haven't touched the timing or ZS carb settings, so keep thinking there must be something simple. Any ideas?

[troubador]

I found the adjustment on the choke cams seem to wind their way forward.
Re adjust and ensure they are locked in place securely.

[john 215]

Hi,

Not sure about US spec Strombergs but seen similar issues with carbs over here, IF they are fitted with ' poppet valves ' in the throttle disc the spring looses some tension so allowing it to opened fractionally on idle, hence the speed rises. Fix is simple, solid throttle discs or if you are brave.... the BL repair was silver solder them closed, it was all about cost !

Alternately you may just need a tune up.

Cheers John

[Dukmans]

Hi John,
Did you check the oil level in the carbs? In my Spitfire it sorted out the problem.

[FI Spyder]

ZS carbs have a rubber seal in the pistons. These can develop cracks (starting with hard to see micro cracks) . I think these can cause idle to edge higher like any other vacuum leak.

[whitenviro]

Good thinking! That's one thing I did not check. The last time I had the lid off, the rear carb diaphragm was looking a bit distorted. It may have failed by now. I have a spare, so I think I'll give them a look.

[whitenviro]

I'm still trying to figure this high idle out before I make any changes to the carb settings or timing. The car runs so well I hate to mess with it too much. I checked the diaphragms in the carbs and both look good. I looked over the rubber carb mount and don't see any cracks ( a couple years ago I had it out and coated the entire outside edges of it with silicone, so I don't think that is the problem). I still suspect a vacuum leak from somewhere.

Today while dinking around I discovered that the rear carb seems to be doing all the work. If I cover the intake to the rear carb while at idle the engine stumbles immediately. Same if I lift up the carb piston. Not so with the front carb. I can cover it completely or play with the piston and nothing changes. A couple times it actually seemed that the RPMs increased slightly when I covered the front intake. The front carb does gulp some air when I press the throttle.

This car has ZS carbs, but the FASD has been removed. There is a manual choke only on the front carb, and it does make a difference when starting from cold. So I know the carb is not just sitting there dead; its doing something. I just don't know what! And still don't know how it is all related to my high idle. Any thoughts?

[Hasbeen]

Twin carbs when much above idle depend on the linkage between them to control the balance. This linkage is not precise enough for balance at idle, where a much smaller difference in opening will cause a problem, & they depend on the throttle stop screws for balance.

I find my SUs need adjusting of throttle stops to maintain good suction balance at idle, on a regular basis, perhaps annually, or a few thousand kilometres.

I have foam sports air cleaners, so these get a wash & oil, & I then do the hose in ear carb balance job, without touching the linkage between the carbs. This is usually no more than a quarter of a turn on either of the throttle stop screw.

If it requires more than that I start looking for other reasons, such as a vacuum leak, a high float level in one or both carbs, a dirty/tired spark plug, [not found again since changing to iridium plugs], or valve clearances.

I did once find the front carb doing nothing at idle. I spent quite some time looking for any cause, but found none. It was fine after adjustment. I still wonder is someone was playing a trick on me by screwing the stop out .

If you don't want to upset your nice running, try adjusting your throttle stops with the linkages connected, you probably have enough slack in the system to do this.

Hasbeen