Twincam stalls frequently - what is causing this ?

[peg_pilot]

My twincam idles reasonably well, except that at random times the revs suddenly drop, which often results in a stall.
This is a Stromberg-carbed big valve.
I have had the head rebuilt by Steve Smith at Twin-Cam Sportcars.
I have installed computer-controlled electronic ignition.
I have rebuilt the Strombergs.
I have tried various idle speed settings from ~700-1000 RPM.
I have a wide band oxygen sensor that says my idle mixture is about 13:1
Often (but not always) when the revs drop at idle the oxygen sensor reads up in the 17-20:1 range. However, I can't tell if this is a cause or effect (e.g. either mixture leaning out or misfires from some other cause could give higher EGO reading).

This is really driving me crazy, nothing seems to help. Could my carb float valves be sticking at low flow rates ? Any other ideas ?

Thanks all.

[1964 S1]

I'd go for the easiest things first and what comes to mind is fuel filter. Is it in good shape, do you have one?
Or maybe a fuel pump problem? Is yours mechanical or electric?
Is there any way you're sucking some air into the fuel line at idle? Do see or smell any gas?
Does it stall while idling at traffic lights? Is it easy to restart?
Carbs would be next area of inspection, it sounds like fuel starvation but to muddy the waters, maybe you're losing spark at low rpm with that computer controlled ignition? Does it sense fuel flow?
Good Luck, Eric

[ppnelan]

computer-controlled...

Matthew

[gjz30075]

Does it start right up after it stalls? If not, should be easier to diagnos. Does the tach drop immediately like you shut off the engine? If so, I'll bet its your computer controlled ignition acting up. Could also be the usually faulty anti theft switch in the glovebox, assuming yours has one.

Greg Z
'72 Sprint

[peg_pilot]

Gentlemen,

Thank you for the suggestions.

The stalling problem predates the computer-controlled ignition, there has been no significant change in the symptoms since I installed it, alas. Lots of testing with a timing light and an O-scope has convinced me that it is working per design.

The car usually starts easily after a stall.

I use a mechanical fuel pump I purchased from one of the usual suspects in the USA (recently retired). When I first installed it, the engine would flood immediately. Turns out it puts out 7 PSI, which appears to be much too much for the Strombergs. I had to install a fuel pressure regulator which is set to about 1 PSI to get the car to run. It seems to work fine most of the time, the car is peppy and fun to drive except for the stalling. Increasing the regulator pressure to the 2 PSI range doesn't seem to improve matters. While I have had a few leaks in the past, I think I have overcome them, there is no smell of gas at present.

As to the carbs, I have replaced the throttle shafts and shaft seals, the diaphrams, and the float valves. I have blocked off the bypass valve that is supposed to reduce engine vacuum on overrun, in case it was leaking. I have cleaned the temperature compensators - I wonder if they could be popping open unexpectedly and leaning the mixture at idle ?

Argh !

[gjz30075]

As to the carbs, I have replaced the throttle shafts and shaft seals, the diaphrams, and the float valves.

Float valves: do you mean the needle and seat? Both components would need to be replaced because it sounds as if the needle and seat are worn and the pump is flooding the carb. A stock mechanical fuel pump needs no regulator. Key word here is 'stock'. Are you using something other than OEM? As an aside, I've use the later pumps that have no glass bowl and they work as well as the glass bowl type.

Greg Z

[ppnelan]

Could there be a problem with a vacuum connection to the inlet manifold, either to the brake servo or to the headlight pop-up system?

Matthew

[rgh0]

A few items I would check

Do you have the balance pipe still fitted between the 2 carbs- The absence of this can cause problems with idle.

Are your oil dampers in the strombergs properly filled with a low viscosity oil the absence of this can also cause problems with unstable metering pistons

Is there a miss fire or blow back into the inlet before it stalls - this can cause the piston in the stromberg to close due to the pressure increase and the engine then to go lean and stall

cheers
Rohan

[1964 S1]

Fuel pump. Greg's OEM comments say it all. If I were you I'd (re)start there. Fuel pressure regulator on a stock Elan??? Buy a new stock fuel pump, they're cheap.

[peg_pilot]

Hmmm, a number of interesting ideas here.

I do have the balance pipe between the Strombergs that used to hold the secondary throttle plates, the secondary throttles themselves are gone.

The dampers are full of engine oil.

My headlamps stay down for about 24 hours after I drive the car, so I don't think there is a significant leak. I don't have a brake booster.

Greg, where did you get your fuel pump ? I have purchased 2, one from Don Tingle and one from Dave Bean, they both put out 7 PSI. I still have my original glass-bowl type pump (which I believe I found put out about 1.5 PSI) but it's in pretty bad shape.

Rohan, I am not sure if the engine is misfiring before it stalls, but it seems possible. I was wondering if poor compression might allow enough oil past the piston rings to cause an occasional misfire, leading to a stall. I did a compression test and my readings were 145, 140, 140, 135 PSI. I also often see a small amount of oil leaking around the spark plugs. I have forged pistons (inherited from DPO) that run a ton of clearance in the cylinder bores (~.007") and I am fearful that these might be hurting compression...

Thanks again all for the suggestions.

[types26/36]

[quote="peg_pilot"]The dampers are full of engine oil.quote]

Isn't engine oil too thick for the dampers? from my younger days from working on S.U. carbs I believe the spec's called for a maximum grade of SAE 10 or a very light oil......we used to use ATF fluid.

[bcmc33]

I my old Cooper "S" days a can of 3 in 1 oil was always on hand for the SU carbs.

3 in 1 was the recommendation from the SU garage following my first visit there with a Riley RME in 1961.

I can only imagine that Strombergs need the same sort of viscosity.

[types26/36]

I my old Cooper "S" days a can of 3 in 1 oil

Ah ! yes you are right, I couldn't remember the name of the light oil....3 in 1 ...wasn't there another one called "Everymans Oil" as well?

[types26/36]

....and a quick search turned up....

[gjz30075]

My fuel pump came in a box of parts I received 25 years ago. Sounds like you have the right pump then if purchased from one of the usual sources.

If you have to regulate the presssure that low, your needle and seat are probably worn and leaking. Then again, I could be totally off base here!

Greg Z