Misfire when cold

Post by **mgcjag** » Sat May 30, 2015 4:57 pm

Hi All...S2 4.2....having a problem with a misfire/splutter when accelerating from about 1500rpm..have changed all the ignition components, and checked carb fuel levels and fuel filter.....on 3 trips now the car misses untill its hot, about 15miles then runs fine.....any thoughts...sticking valve untill engine is hot?...what else can you think of that can cause a misfire when cold but stops when hot.....Steve

Moeregaard · Post by **Moeregaard** » Sat May 30, 2015 5:31 pm

Have you tried engaging the choke when this happens? My first thought tells me that things are going lean, and this will have a pronounced effect when the engine's still cold. If you're running SU carbs, the stock needle will be stamped "UM".

Post by **mgcjag** » Sat May 30, 2015 5:46 pm

Hi Mark...yes i have SUs, using UM needled.....does running
lean have more inpact before engine is fully hot, I have to drive about 15 miles before the misfire stops...worth a try with the choke, any other ideas....Steve

Post by **mgcjag** » Sat May 30, 2015 6:41 pm

Hi All.....your thoughts please......Im using engine oil, 20/50 in the dashpots, however just checking the manual says SAE20, is the 20/50 too thick when cold and possibly causeing my misfier till it hots up......searched on dashpot oil here and see that engine oil was specified.. Thanks Steve

iani · Post by **iani** » Sat May 30, 2015 8:14 pm

mgcjag wrote:Hi All.....your thoughts please......Im using engine oil, 20/50 in the dashpots, however just checking the manual says SAE20, is the 20/50 too thick when cold and possibly causeing my misfier till it hots up......searched on dashpot oil here and see that engine oil was specified.. Thanks Steve

I've never had an issue with using engine oil in carbs in summer months, I do think it a little thick in Winter however. These days I tend to use su carb oil, in the absence of that, some 3in1 is fine.

Ian

Moeregaard · Post by **Moeregaard** » Sat May 30, 2015 9:00 pm

mgcjag wrote:Hi Mark...yes i have SUs, using UM needled.....does running
lean have more inpact before engine is fully hot, I have to drive about 15 miles before the misfire stops...worth a try with the choke, any other ideas....Steve

Living in southern California, I was usually able to start my E-Types on summer mornings without using the choke. Still, it was a little bitchy until everything was up to temperature, with some spitting back out of the carbs and minor misfiring until things warmed up. A richer mixture is required when an engine is still cold; leaner when things are up to temp.

A word about choke usage: The "choke" on SU carbs adds fuel, rather than restricting airflow as on other carbs. I've seen people allow their cars to warm up in the drive for several minutes, with the choke fully on. In addition to wasting fuel, this practice contributes to fuel dilution of the engine oil and increases engine wear. Better to start driving slowly and progressively get off the choke as early as possible.

I've seen dashpots filled to the brim with everything from automatic transmission fluid to motorcycle fork oil, but all that's really needed are a few drops of SAE 20 or 30 motor oil. Castrol 20-50 works fine as well, if you can't get the stuff sold by the usual suspects as dashpot oil. All the oil does is dampen the movement of the carb pistons.

jag68 · Post by **jag68** » Mon Jun 01, 2015 5:10 am

Actually the oil in the damper is an enrichment device that acts as the equivalent of an accelerator pump in a non constant depression (CD) carb, say like a Weber. When you open the butterfly valve and expose the bridge and needle area to the vacuum in the manifold, and thereby increase air flow through the throat of the carb the piston rises due to the fact that the vacuum in the throat has increased (the upper chamber is connected to the throat through holes drilled in the rear on the portion of the piston that holds the jet). As the piston rises it increases the area of the throat, slowing the incoming air down, and keeping the vacuum (depression) over the bridge constant (Bernoulli's principle). The rising piston exposes a thinner needle section in the top of the jet which permits more fuel to enter the throat at the same vacuum to feed the needy engine. The design of the needle, as opposed to increasing vacuum as in a non CD carb regulates the amount of fuel entering. The problem is that this is not fast enough to provide the immediate need the engine has for fuel for instant acceleration. If you could delay the rise of the piston momentarily there would be a significant increase in the vacuum over the bridge instantly sucking more fuel into the throat, tapering off as the piston rises and the vacuum decreases. Well that's what the damper does. So SU's Stromberg's etc are actually only "Mostly Constant Depression" carbs.

It's obvious that the heavier the oil you use, the slower the piston rise, the greater the vacuum over the bridge, and the richer the immediate mixture is. The spring in the carb vacuum chamber mostly does the same thing.

Your problem with the car "gasping" when it is cold is almost certainly too lean a mixture. The damper, by design, has little to do with the extra enrichment required by a cold engine.

Terry Sturgeon

abowie · Post by **abowie** » Mon Jun 01, 2015 5:40 am

My money's on it being a lean mixture. Try the choke trick or try enriching the mixtures say half a turn on each carby and see what happens.
Thicker oil in the dashpot will actually make the mixture richer in the instant you open the throttle rather than leaner so that's probably not it.

Post by **mgcjag** » Mon Jun 01, 2015 4:27 pm

Thanks Guys, will wait for the weather to brighten up in a few days and richen the mixture. If i am running lean wouldnt i still get a misfire/stumbling on acceleration when hot?.... .Steve

Post by **mgcjag** » Wed Jun 03, 2015 3:19 pm

Hi All.....checked the spark plug colour all look ok....gave 1/2 turn to richen mixture on all carbs.......started car and went for a run....after 4mins choke fully in and started to accelerate,,,,again at about 2000rpm started to miss/stumble...increased choke but no different.......kept driving for 5miles, fully hot and then all ok....any more ideas thanks Steve

Post by **Heuer** » Wed Jun 03, 2015 3:48 pm

Could be electrics. Have you tried a different coil, rotor arm, diff cap, plug caps?

Post by **mgcjag** » Wed Jun 03, 2015 5:31 pm

Hi David...yes had a misfire and the points were corroded so fitted a pertronix, still missing so started replaceing bit by bit...plugs,leads,coil.dist cap,red rotor,.pertrinix, remade wireing ends......just cant understand why its ok after about 5miles, but now not sure if the parts ive replaced could be faulty.....next stop possible distributor.....Steve

dal2.0litrefrogeye · Post by **dal2.0litrefrogeye** » Sun Oct 17, 2021 2:34 pm

Hi did you ever resolve this . Having the exact same symptoms just about to post a put some oil in dash pots

Post by **mgcjag** » Sun Oct 17, 2021 4:12 pm

Hi Darren......yes turned out to be one of the connectors to the coil.......wasnt crimped tight onto the wire.....just happened that i tugged on the wire and it came out leaving the spade on the coil........i reckon that as it got hot and expanded slightly i got a better connection......re crimped and soldered and all ok.....Steve

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