From: sosmi@xxxxxxxxxxx
To: mailing-list@xxxxxxxxxxxxxxxx
Subject: Re: IML: 68 No-start when hot
Date: Tue, 11 Sep 2007 19:52:14 +0000
Since there has been a lot of very good input, I would only add, spark, and fuel. The two easy tests. It is possible the accelerator pump is in-active. The 68 should have a Holley carb, it's possible the cold start is good because the choke is closed, and vacuum during cranking will draw enough fuel to start, once the engine is hot it may be too lean to start. Then when the engine cools enough the choke may close enough to start. A simple check to see if the accelerator pump is working would be in order. And as others have stated, check for spark, however I have the impression the engine will start, only with delay. If that's the case,it's most probably a carb/fuel issue. A very low fuel pressure, or blocked fuel filter may also be a problem. You also didn't say if the car was driveable .Ya'll have a nice day, Dave.
-------------- Original message --------------
From: "Geoff Fors" <wb6nvh@xxxxxxxx>
> It sounds like the fuel is boiling and/or vapor-locking in the carburetor
> bowls. I presume that once the engine is running, it doesn't stall when
> hot, it just won't re-start when hot? That points to a fuel system problem.
> I also presume this is running a Carter AFB series carburetor.
>
> Sounds like the heat riser valve is frozen in position, and its thermal
> clock spring may be rusted away or missing as well. The spring keeps it
> closed until the engine heats up, then it holds it open after that. The
> exhaust force will also push it open when the engine is brought above idle.
> It's possible to wire the heat riser open, especially if you live in a warm
> climate. However, the heat riser assists in rapid warm-up
and reduces the
> time that excess gasoline enters the combustion chambers during warm-up.
> You may have a valve which is rusted closed, even though it appears to be
> open from an exterior view. That would certainly account for this problem.
> Sometimes the paddle moves on its shaft. The only way to tell is to drop
> the exhaust pipe from the manifold and look, rather a pain.
>
> It's not unheard of for the heat riser paddle to come off the shaft and fall
> down the pipe, jamming at some point, plugging that side. I haven't seen
> this on Imperials but on 1960's Buicks it will totally plug the exhaust
> system!
>
> The gasoline sold today tends to vaporize far more quickly than 1968
> gasoline and thus older cars without fuel injection can vapor-lock easily.
> This is a perennial subject in other make car clubs at the moment.
>
> The paint always cooks off on the heat r
iser path section of the intake
> manifold after awhile. That's normal. It shouldn't come off the rest of
> the manifold, however!
>
> If we were to take a survey, I think you will find that there are quite a
> few of us running stock '67-'68 setups with working heat riser valves and
> original carbs and gaskets, and we don't have this hot-start issue. Thus I
> think something is wrong. It wouldn't hurt to make sure the choke is fully
> pulled-off when the engine is warmed up. The setup worked fine originally,
> so if it isn't doing so now, something is not right.
>
> I don't see any point in changing the intake manifold. The heat comes from
> the riser tube, not the manifold design. If you really want to eliminate
> the heat, use a gasket that blocks the riser tube holes, and you can retain
> the stock manifold. And, it wouldn't hurt to use a thicker gasket under the
> carb. B
u t, it should run fine as a stock setup, and not get too hot.
>
> Geoff
> Monterey CA
>
>
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