Technical Rough idle

[BarryH]

Evening all - At last today have had a chance to re-visit the garage to try and resolve the rough idle on my 1966 500F. The list of items replaced thusfar are as follows. A service kit was bought for the 26IMB 10 Carburettor and was cleaned as much as possible and any debris removed with an airline. The fuel mixture has been checked with a Colortune and is showing a nice Bunsen blue colour ( currently carb mixture screw is 1 1/2 turns out). I have set the static timing using a test lamp which lights at 10 deg BTDC using the timing gauge.

As per the attached video the roughness seems to smooth out when the revs are lifted but when idling the engine rocks and sounds like it is missing. Hoping for any divine inspiration, given the list of parts all ready changed I am running low on ideas . Could it be a worn carburettor or idle jet issue. The fuel flow is constant though I do get the occasional bubbles appearing in the fuel pipe

More than happy for any suggestions or areas to re-check if I had hair I would be pulling it out by now . The carburettor is stamped 'Made in Italy' is it possible this is the original carburettor

Ignition Coil
Distributor Cap
Rotor arm
Spark Plugs
Spark Plug Leads
Contact Points
Condenser now changed to a Swifttune
Fuel pump
Fuel tank sender

Many Thanks again

Barry

 

[ZaphodB]

See if you have any HT leak by taking a look at the engine bay in the dark whilst it's running.

 

[the hobbler]

See if you have any HT leak by taking a look at the engine bay in the dark whilst it's running.

Have you checked that the base of the carb AND the spacer between the carb and the cylinder-head are flat. If they are not, air will bleed in and affect the running. Warping of of both of these parts is not uncommon---usually brought on by people over-tightening the carb's securing nuts.

 

[BarryH]

See if you have any HT leak by taking a look at the engine bay in the dark whilst it's running.

Hi Thanks - yes I did peer into the engine bay with the lights off and couldnt detect any arcing between the HT leads

 

[BarryH]

Have you checked that the base of the carb AND the spacer between the carb and the cylinder-head are flat. If they are not, air will bleed in and affect the running. Warping of of both of these parts is not uncommon---usually brought on by people over-tightening the carb's securing nuts.

Hi thanks - i did run a straight edge across the base of the carb when off the car and used some wet and dry to be certain of a flat service, also a bead of sealant between mating services. Also sprayed carb cleaner around base when running to detect any change in engine note to point towards a leak - all appears to be ok

 

[ZaphodB]

Air filter?
Fuel filter?
Idle speed too low?
Distributor drive slop ?
and distributor radial play ?
Can you check dynamic timing with a strobe light ?

 

[Toshi 975]

Valve clearances / tappets?

 

[BarryH]

Air filter?
Fuel filter?
Idle speed too low?
Distributor drive slop ?
and distributor radial play ?
Can you check dynamic timing with a strobe light ?

Hi Thanks -
Air filter - New
Fuel Filter - only filter is on the fuel tank pickup. Have removed fuel pipe at carb end and turned enginevover and the fuel flow is good. Filter in carb is clean when checked
Idle speed - sitting at 960 rpm
Distributor - have checked throw out weights and springs but that is one to check further
Timing light - beyond checking static as previously mentioned if i connect my strobe light to the generator output I dont get the light to function until i raise the revs above 1200 rpm so maybe not sufficient power output but at this speed the timing marks are advancing correctly @ 12 deg if that makes sense

 

[BarryH]

Valve clearances / tappets?

Hi Thanks - I did check the tappets to " the rule of 5' maybe i should revisit this again.

If the tappets were incorrect would this cause rough running only at idle?

 

[ZaphodB]

Distributor - have checked throw out weights and springs but that is one to check further

Slop in the dizzy gearing will have an effect on spark timing, also excess radial play of the lobe/shaft at the top will affect dwell angle

 

[Goldnrust]

If you're really scraping the barrel for ideas then check the points gap, then turn the crank 360 degrees and check the points gap on the other lobe of the distributer. My original engine had been bumped in transport and had bet the distributer shaft slighting giving me a very uneven points gap that I'm sure would have caused some strange spark issues had I not spotted it.

Also, on the ignition side, is it worth temporarily adding an extra earth strap to the engine, to rule a poor earth out?

 

[BarryH]

Thanks all once again for your input it is so beneficial to bounce ideas off you all. I have had another look this morning and I had already looked for any evidence of arcing as suggested by ZaphodB and there isnt anything obvious but I did get a zap off of the LT lead at the coil negative terminal. The swiftune condenser ( new) connects at this point and the coil has been replaced previously. It does feel fuel or ignition related
I plan to recheck the tappets once again as Toshi suggested as a sanity check and also rig up a fuel feed directly into the carb to rule out petrol feed from tank and fuel pump even though the fuel pump was replaced. Also have sprayed damp start around the carb base again and no evidence of change in idle. Barry

 

[ZaphodB]

but I did get a zap off of the LT lead at the coil negative terminal.

That's back EMF as the primary coil is switched by the contacts. it will hurt lol

As an aside, it's not gonna help getting a belt off that terminal but make sure the lucas connectors are all snugly biting onto their associated lucas blade, (or are they screw down terminals on this?)

 

[BarryH]

If you're really scraping the barrel for ideas then check the points gap, then turn the crank 360 degrees and check the points gap on the other lobe of the distributer. My original engine had been bumped in transport and had bet the distributer shaft slighting giving me a very uneven points gap that I'm sure would have caused some strange spark issues had I not spotted it.

Also, on the ignition side, is it worth temporarily adding an extra earth strap to the engine, to rule a poor earth out?

Hi Thanks - will check the points again, also good shout on the earth strap. I had fitted a new one bypassing the rear engine mount but worth doubling up as a temporary measure. Barry

 

[BarryH]

That's back EMF as the primary coil is switched by the contacts. it will hurt lol

As an aside, it's not gonna help getting a belt off that terminal but make sure the lucas connectors are all snugly biting onto their associated lucas blade, (or are they screw down terminals on this?)

Hi Thanks - That would explain it then . The coil are screw type terminals so all tightened down

 

[the hobbler]

Hi Thanks - That would explain it then . The coil are screw type terminals so all tightened down

With regard to checking the timing with a 'strobe-light'---it is better when you have a battery as remote from the engine as the 500s is (and the 126s) to use a spare battery at the rear of the car to power the timing-light.----the pick-up lead onto No1 ignition lead and the earth to some-where on the engine BUT AWAY FROM REVOLVING PARTS!

 

[BarryH]

Valve clearances / tappets?

Hi Thanks - just a further update, I have re-checked the tappets today all good. Also i have rechecked the static timing and points

If you're really scraping the barrel for ideas then check the points gap, then turn the crank 360 degrees and check the points gap on the other lobe of the distributer. My original engine had been bumped in transport and had bet the distributer shaft slighting giving me a very uneven points gap that I'm sure would have caused some strange spark issues had I not spotted it.

Also, on the ignition side, is it worth temporarily adding an extra earth strap to the engine, to rule a poor earth out?

Hi Thanks - checked points gap on both cams and all good, I have ordered an additional earth strap

 

[BarryH]

Hi Thanks - just a further update, I have re-checked the tappets today all good. Also i have rechecked the static timing and points

Hi Thanks - checked points gap on both cams and all good, I have ordered an additional earth strap

Valve clearances / tappets?

Once I have eliminated the fuel pump action and fuel tank feed I would like to eliminate the carburettor, as previously said I did use a service kit but cannot be 100% of any wear to the internals.I can see that the available 26 imb 10 replacements from the recognized Fiat 500 suppliers are likely to be made in Spain under license but I would prefer to keep the made in Italy option, would anyone know where i could get mine serviced or offer a source for a replacement. Many Thanks