Random Miss-Firing?

[jedo]

Hi Guys,
Quick brain pick again, I was on the Tour of Cheshire the other day and had a great day. However, had a small random intermittent missfire problem. I'd be driving along either on the motorway so 65/70mph (to the rally) and then trundling around during the rally doing 20/25mph. Then for about 30-60seconds or so, the engine just starts miss firing. I seemed to get through it by putting the clutch in and letting it try tick over about 3500rpm. As i said, then it would just stop and not do it again for a while? When I started her in the morning of the rally, she was miss-firing for about 30seconds, stalled, restarted her, no miss-firing.

So, by the time it starts happening and I've pulled over, lifted the bonnet she's either stalled or started the behave before I get chance to have a look. Electrical or mechanical? Any ideas?

Unfortunately I cannot try anything for a while as she has gone back away in storage til Easter and then hopefully she'll be out for the summer (all being well!!).

Any ideas would be great, thanks.

James

[Henk]

My first idea is: condenser.
Had such behaviour couple of times before switching to electronic ignition. Replace condenser, tune points and give it a try!

[Edin Dundee]

Or the coil.

[Dolly-Nut]

Fuel supply problem? Check the chamber level after misfiring.

[HTO SPRINT]

I had this the first few days I had my sprint - it would only start after the third try, and them misfire for 10-20 secs before revving above 3000rpm before idling....changed the condenser - voila!

[jedo]

Ok, Cheers guys. Will have a look next time i'm home and can get my head under the bonnet!!

[jedo]

Ok guys,
FInally been home to fix problem, I have now replaced fuel pump (old one sprung a leak), fuel filter, all fuel lines and all electrics. Yet it still is playing games. It starts, struggles to idle then stalls (when idling), when I drive down the road it loses power and then comes back again. When it stalled last time, wouldn't start again until after 2minutes. Then when i got it home and left it for about 3hours before moving it into the garage, took 30minutes to get it going again. I'm really starting to run out of ideas... help!!

Thanks

James

[soe8m]

Does it smoke black?

Jeroen

[jedo]

Nope

[SprintMWU773V]

Sounds like classic fuel starvation caused by crap in the float chambers/sticky needle valves. had exact same problem with my Sprint some years back. It was caused in my case by failure of a fuel tank sealant which then clogged the fuel lines. Ignition faults wouldn't explain the issue of it working then failing to work for a while. This can surely only be caused by the process of crap flowing back to the tank or momentarily moving out of wherever the crap is.

To be sure you need to be 100% confident your float chambers are clean, needle valves working correctly and that fuel is flowing. Come back when you have done that and hopefully i'll be right.

[jedo]

Cheers Mark, will have a look.

[Jon Tilson]

Having replaced the fuel pump you need to bs sure that your fuel delivery is working as you expect. You can quicly check things by disconnecting the fule supply on the pump outlet or at the carb inlet and get an assistant to crank the engine on the starter. You want to see a nice squirt of petrol for each engine rotation. If it seems like a weak effort then look for blockages and a partial vacuum in the tank. Take the fuel filler cap off and try again...if it improves then you have a blocked vent hole.

Obvioulsy go through the ignition again systematicly too. Did you try a different coil?

When dealing with ignition....only change 1 thing at a time...


Jonners

[jedo]

Good fuel supply and tried different coils...

[MIG Wielder]

Hi James, Would this be an 1850 ? In which case can I tell you about my previous 1850, MPD . We used to have exactly this problem. And it was always worse going up hill . It would be running fine and then suddenly start misfiring. So much so that Birdlip hill in Gloucestershire was taken at 20 mph. Like you I did everything. The clue was in when it happened for the first time. ....It was the year that they started taking the lead out of 4-star. Now MPD did seem to like lots of lead in its petrol or we would get sticky inlet valves, and the trouble with the 1850 (which ran Strombergs as well) was that the mixture was being just sucked in and blown back into the carb effectively disabling the 2 adjacent cylinders.
It was actually diagnosed by a strong dose of Redex in the petrol. This made things heaps better.
So it was off with the head, where I found lots of black carbon building up behind the inlet valve stems. One decoke and valve grind and it was sorted !
HTH,
Good Luck,
Tony.

[jedo]

Cheers Tony, yes its a very early 1850. Might have to try that next time i'm home and near the car. Literally, it's gone back into the garage at the moment looking sorry for itself. Will give that a go, thanks!

James