Engine revs increase by itself

[StewRat]

Hi all

Very frustrated by a nagging problem and feel I've run out of ideas, so throw myself on the mercy of the megabrain here ...

I originally had a problem with the engine taking a long time to drop revs from say 5k back to idle - thats documented at http://cal-look.no/lounge/index.php/topic,28025.msg346088.html#msg346088

The engine drops ok from high revs now, and pretty much snaps back to idle from a sharp rev of say 3k or 4k.

The problem is that a small intended increase in revs - say to 1.5k, 2k keeps increasing by itself, up to 3k or 4k and stays there. A sharp blip on the throttle and it falls back to idle ok. I thought I understood this was an air leak somewhere meaning extra air being drawn in below the butterfly mimicking the throttle being opened.

The carbs/manifolds have now been apart a few times for this and other reasons, and most recently reassembled with thick new gaskets and permatex between manifolds and heads.

But if I spray carb cleaner around the base of the manifolds at idle, there *is* bubbling, but no increase in revs. Don't the bubbles suggest air being pushed out rather than sucked in? Confirmed by the no increase in revs? If this is the air leak, what more can I do to stop it? The manifold ends have been sanded on glass, I'm using 10/13 nuts to make sure they get tightened down, I shortened head studs that were touching the manifolds. And all testing is via the linkage, so it's not the pedal/cable.

Also ...

With the air mixture screws 2.5-3 turns out the engine runs/sounds great (to me anyway) but the adjustments for normal tuning don't seem to do anything. On some barrels at least I can turn the screw in all the way with little/no difference, maybe even a small increase in revs.

Is this all really just an air leak somewhere - and if so, suggestions for how to get rid of it?

Could this be a problem within the carbs (DRLA 40s)? I can't make a causal link but before this started to happen they did get exposed to weather for a while (another story). I know they got very wet and can't rule out possibly froze. I dismantled and rebuilt them afterwards but have always had a nagging concern about them. Part of me would like to start with new carbs but I'd hate to do it and find the problem isn't fixed!

Hoping someone has been in exactly the same position - and knows the solution.

I'd be racing on Sunday if not for this - some places I could get away with it, but the staging road/queue at Crail means I need to do exactly those little throttle blips that cause the runaway revs.

Thank you for reading.

[Mats Herrlander]

Ignition?
If the ignition curve starts too close to idle rpm, you get the type of problem you describe.
If you have an idle rpm, like you have, 900 rpm, the ignition curve should start at say, 1100-1200 rpm.


(Assuming your manifold / carburetors are dense.) Ignition?
If the ignition curve starts too close to the idle rpm, you get the type of problem you describe.
Pre-ignition increases engine rpm.
If you have an idle speed, that you have, 900 rpm, the ignition curve should start at say 1100-1200 rpm.
The closer the better, of course, but there must be a distance/possibility for the engine to find the idle speed.

[andy198712]

unless the car is supercharged and making boost at idle? the bubbling will be the carb cleaner boiling off at a guess.

so you sprayed the base of the manifolds, if try the carb bases too and the carb bodies.

does sound a lot like an air leak....

do your carbs snap fully shut well? not hanging open that last little bit? tried manually closing them but pushing the throttle arms closed with a finger to test?

[nicolas]

hello,

what emulsion tubes are you running on the DRLA; there is only one that really works. I will look up which, but it is in the dellorto book or here on the site. to me the butterflies don't close and/or worn. make sure they close fully. sometimes the carb spring and shaft work against a smooth operation, or push the shaft sideways that causes it to rub and not fully close or the linkage itself has a tight spot near the idle position.
also make sure your idle jets are clean. the mixture screws need to respond.
sometimes if jets are too big, your engine will continue to rev or even pick up revs to burn the mixture even after the trottle is closed.

just trowing around some ideas to hopefully help you out. DRLA carbs are great carbs if they work 100% 

[StewRat]

Guys - thanks very much for the input.

I agree the bubbling was probably boiling carb cleaner and permatex!
And yes while I had checked all the jets several times I always had a nagging concern over the 2nd hand linkage I was using.

Anyhoo ...

I think I have solved the revving problem - I didn't have time to do it more scientifically so did several things all at the same time:

Re-flatted the manifold surfaces
Made new gaskets from the thickest material I have, and fitted them with some blue hylomar
Made new adjustable linkage arms
Fitted helper springs to the throttle arms
... and a new throttle cable which I know didn't change anything but it was such an easy job I may put a new one on every week just for the sense of achievement.

And I can't make it do anything other than snap shut and idle after any level of revving.

On reflection my best theory is that I had a combination problem. One of the linkage arms had a kink that limited its adjustment so I think that was compromising both carbs closing fully after a 2-3k rev. Partially open throttle with an air leak might have created a vicious cycle that increased the revs by itself.

I still feel the idle mixture screws are ineffective but on more reflection and reading it's likely that I have not been waiting long enough between adjustments and so may have been getting some wierd results, so I'm going to re-check seals and washers on those screws and do a more disciplined adjust and wait attempt at a tune.
However I've also come to the conclusion that as they only affect idle mixture and this isn't a street car, I may be able to live with them not being very adjustable

Thanks again

Stewart