1600 Twin cam missfire

[ducat97]

Guy,
I bought a Marlin Roadster one year ago to compeate in MCC events. It has a 1600 Twin Cam Fiat engine. It managed the Lands End ( Thank you for your advice over the phone ) But was reluctant to start when hot. In the recent Exeter event there was more waiting for stages so more heat leading to a missfire on tickover and very reluctant to start. Once cooler ( 10 Mins ) It starts and runs fine.
It had the problem from the day I bought the car.
I have changed to a Luminition Optronic ignition system
New H T Leads
New plugs NGK BPR6E
New coil
Piper Cross air filter ( The one on car was very small )
New fuel filter

In The Exeter I changed the plugs a number of times and they were gray / white - Looked very lean.
I am wondering if someone has re jetted the carb to match the small ( period looking ) Air filter. Now the Piper Cross has a much larger area to breath.
The carb is a weber 32 ADF 2.
Main Jets - Primary 1.10 Secondary 1.25
I have ordered a distributor cap and rotor arm as they are the only parts on ignition side I have not changed.
All comments will be gratefully received.
Martin crosby

[Guy Croft]

Fuel pump problem

GC

[ducat97]

Hi Guy,
Fuel pump seems to be working ok. Its a Facet electronic with new filter.
Have wondered about carb icing so put some Silkolene FST in but problem still same. I am now thinking I need to rebuild the Webber carb.
Am told they are trouble free but do you think a rebuild kit would be the next thing to try ? Perhaps It is drawing in air once hot ?
Regards, Martin.

[Guy Croft]

An engine needs fuel/air, good compression and spark to start up and run well.

The carb can cause hot start problems - most usually if it is flooding:
1. needle valve under too much fuel pressure or not seating
2. pump jet dripping (usually caused by blocked spill-back jet in float bowl or worn-out diaphram

You don't mention fuel pressure - I would check it with an in-line gauge. You only need 3.4-4 psi but if the carb fuel return line back to the tank is still connected (which you don't need it the pump is pushing from the tank end) you could be losing too much flow to the carb. If it's sucking the return line helps keep the fuel from boiling under lower pressure). Electric pumps should always 'push', not 'pull'.
And of course if the idle jets are blocked (easy to check) it won't deliver any fuel on cranking, although opening the throttle will make the pump jets spray fuel into the barrels and it might start - but stall again on idle. There may be an electrically operated idle fuel cut off switch on the carb that could be faulty too. To be honest you don't need it on your car.

Flooding is a problem greatly accentuated by the engine being too hot (accumulation of fuel vapour and liquid in inlet manifold can make it too rich to fire) and I certainly advise use of a 74 deg in-head stat.

Weak spark doesn't help - the system should generate a fat blue spark that can jump at least 1cm

The thing I would probably look at first though is a compression test, it may have a blown head gasket.

GC

[ducat97]

Thanks Guy,

That has given me something to chew on. I'll do a comperssion test tomorrow.

Martin.

[vcg]

Jumped camwheel tooth maybe?