Dellortos - strange rising revs?

[jono]

Not my Elan but my 2.0 1976 Alfa Spider - twin cam on DHLA 40's so very comparable.

All is well however when I pull the car up on a steep hill, idling, with nose pointing up (such as a junction) after a few seconds the revs inexplicably rise very markedly up to 3.5k rpm and continue. It's quite alarming

Any thoughts on what might be the issue here?

It has twin servos and both appear sound and in any event it does not appear to be affected by applying the brakes (my thoughts were vaccuum leak, but it only happens on a hill)

...over to the masses!

[JonB]

A guess...

Check the float levels. Could be fuel getting into the jets by virtue of the angle the carbs are at? Like I say, a guess only. Strikes me that if this were happening, the engine would run too rich and conk out..

Second guess. Something is causing your throttle cable to get tighter. Maybe test by introducing play?

Guess over..

Logically, either the engine is getting more fuel, or more air, or the timing is altering (advancing, I'd guess). Eliminate each one until you have your culprit.

Someone with more knowledge than me will be along soon, I'm sure!

[jono]

None of those I'm afraid. I've fitted a 123 electronic dizzy and the timings spot on (no vacuum advance)

...and the Alfa has a rod and rose joint throttle linkage

[JonB]

What about the float levels?

[jono]

The float heights are perfect, the motor runs faultlessly save for rising revs when pointing up a steep incline in idle

[mbell]

My first thoughts on this were also float levels. I am not familiar with Dellortos, but on webbers at the recommended fuel level of 25mm there is about 1mm between the fuel level and the passage to the carbs throats. So a steep hill could potentially cause the fuel level to be above the passage and supply extra fuel, increasing the revs quite a lot.

I'd suspect the same could happen on Dellortos. I'd pull the main jets and see how far the fuel level is from the passage to the throat and if angling the carb might be enough to rise it above the passage.

[jono]

I see what your saying however this occurs with no throttle input and therefore the butterflies are closed so I would expect that any extra fuel would just cause the engine to bog or stall without the addition of more air.

[vxah]

3.5k! your not going to do that rpm with a few degrees of timing or fuel seepage? What about this throttle linkage then, your nose up so could a soggy engine mount allow rearward engine movement opening the throttle a little? Or is there no way the linkage could do that?

[69S4]

I've had rising revs at idle happen a number of times on various vehicles and its usually been an inlet 'manifold' air leak. It happened on the Elan when the linkage between the two Strombergs was slipping and one carb wasn't always shutting down. But all of those would happen no matter what orientation the car / bike was in. If the nose has to be upwards to bring it on my first thoughts were, like others, around float levels but you say no.

If the ignition doesn't have moving parts subject to gravity induced slippage then next I'd be looking for what can move and open the throttle just a fraction. You might try adding an extra closing spring for a trip or two to see if that makes a difference.

[jono]

I thought exactly that in bed last night (engine moving forward and causing linkage to move). So much so I checked it this morning but the mounts are very firm and would not cause that degree of movement even with a extreme nose up attitude.

As recognised above 3.5k is a lot of revs at idle, so much so that I knocked it off (near the bottom or Wrynose Pass in Little Langdale on Saturday) as it sounded as if it was going into melt down!

It's a real puzzle, i just can't fathom it!!

[JohnCh]

Are you able to duplicate this by raising the front of the car with a floor jack? If so, it might be worth doing that then moving around various parts of the engine to rule out things like throttle linkage and intake leaks. You may also be able to better understand how the angle affects the revs which could help with the diagnosis (i.e. do the revs start to increase with angle, or is there an inflection point where they suddenly jump?)

-John

[jono]

Good suggestion John, this had crossed my mind.

I have a two post lift - will give it a try.

Cheers

Jon