Re: IML: surging at highway speed

[David Whitney <hazegreen66@xxxxxxxxx>]

Hi, Robin --

 

I agree with what Dave says below and want to throw in my two cents.  Please check the spring gauge on your metering rods.  It's possible that what you are experiencing is the step up from one range to the next and then the step down.  Is your Imperial modified?  Especially if you are in a lower RPM range than stock on the freeway, i.e. you have an overdrive installed, you are susceptible to this problem.  IIRC, the '72 had three-stage metering rods.  Even if your vacuum is steady (no leaks), the gradual increase and decrease of load on the engine due to the freeway's not being entirely flat could easily move you around on the vacuum gauge, causing that step up and step down.  I recommend hooking up a vacuum gauge and seeing what the reading is when you experience this symptom on the freeway.  Then change metering rod springs and see if the symptom happens at a different vacuum reading.  Then you know this is perfectly normal and can incorporate this fact into your regular freeway routine.

 

How's your exhaust manifold temperature?  Do they glow?  Do they have heat stress indications?  Suspect lean running and/or retarded timing.  After that you are looking at a run on the dyno to look at carburetion and mixture distribution problems, and as a last resort slipping timing at high RPMs under load.

 

Happy motoring,

 

David

sosmi@xxxxxxxxxxx wrote:

Surging can be described several ways, gentle slow surges are usually fuel issues, (lean) sudden bumps, or cut-outs are more likely to be electrical, ignition, low voltage etc.Since the carbs of yesteryear, were jetted for better fuel, re-jetting or smaller metering rods are due.Low fuel pressure, restricted fuel filter, restiction in the tank pick-up, or possible air leak on the suction side of the fuel line are also possible causes. An ignition control unit usually gives, a go-no go kind of failure.One thing you could try, to verify a lean issue, try closing the choke valve half way, and road test again. This might richen the carb enough to reduce the symptoms.That might head you in the right direction.Since all carter carbs have the jets on the bottom of the float chamber, any trash in the carb will give problems.Ya'll have a nice day, Dave.

 

-------------- Original message --------------
From: "Robin Giesbrecht" <robings@xxxxxxxxxxx>

> Any ideas on a surging that is slight but getting worse at highway speeds on
> a 72 Imperial. I thought about a possible vacuum leak which I have to check
> out with a gauge but what about coil or electronic control unit? Any
> experience there? Everything else is great starts, idles, power you really
> can't say if there is any difference when you step on it.
>
>

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