Technical 500D wont run off choke

[Franko500]

Hi all
its been a long time since i have been on here.
My 1962 D is finally on the road. It took nearly a year to get it registered in the UK due to Covid and then i have been busy rebuilding my garage/workshop so my D has been safely tucked away.
Its a standard 500D engine with no modifications.
Since i have had it running it starts perfectly every time regardless if it is hot or cold.
I have been out for a number of runs to check it for teething problems and was all set to go to a Car show this Sunday.
We went for a drive this evening for about 5 miles and then i noticed it kept trying to stall when i came to a stop and had to keep my foot on the throttle to keep it going. So we turned round and went home.
Here's the problem. It still starts straight away (hot or cold). It will run fine when on throttle with no back firing etc.
If I leave the choke on a bit it will continue to run but I sense it is getting choked up.
As soon as I turn the choke off it want to stall.

What i have checked so far:
Valve clearances
Points gap, rotor arm, cap, leads, plugs (all new)
Condenser is an old one mainly due to the brand new one was causing running issues when revved.
Timing
I did notice that the distributor base doesn't always return properly when the points are open, but this is most likely resistance on the points arm and shouldn't be an issue when running as would cause it to be slightly advanced. With the points closed the base is free and returns properly.
Its a brand new fuel pump & pipe work. Filters are clean
Removed carb.
Carb is clean of any debris.
Carb face and head face are flat, new gaskets and spacer
Checked and cleaned all airways & jets. Choke device functioning ok. Mixture screw is new.
Refitted carb.

Restarted engine, started straight away. Runs fine.
As engine warms up, I slowly close the choke.
As soon as I close the choke fully (last 3mm on the lever) the engine stalls. Even if I have increased the tick over speed to compensate for the choke being off.

My thoughts here are it must either be a fault in the carb or potentially an issue with the timing when the revs drop below a certain speed.
I don't have a fully understanding of how this choke works on these carbs so looking for some advice.
I am rather stumped on what to check next and cant understand what has suddenly changed to cause this issue.

Any ideas please?

 

[the hobbler]

Hi all
its been a long time since i have been on here.
My 1962 D is finally on the road. It took nearly a year to get it registered in the UK due to Covid and then i have been busy rebuilding my garage/workshop so my D has been safely tucked away.
Its a standard 500D engine with no modifications.
Since i have had it running it starts perfectly every time regardless if it is hot or cold.
I have been out for a number of runs to check it for teething problems and was all set to go to a Car show this Sunday.
We went for a drive this evening for about 5 miles and then i noticed it kept trying to stall when i came to a stop and had to keep my foot on the throttle to keep it going. So we turned round and went home.
Here's the problem. It still starts straight away (hot or cold). It will run fine when on throttle with no back firing etc.
If I leave the choke on a bit it will continue to run but I sense it is getting choked up.
As soon as I turn the choke off it want to stall.

What i have checked so far:
Valve clearances
Points gap, rotor arm, cap, leads, plugs (all new)
Condenser is an old one mainly due to the brand new one was causing running issues when revved.
Timing
I did notice that the distributor base doesn't always return properly when the points are open, but this is most likely resistance on the points arm and shouldn't be an issue when running as would cause it to be slightly advanced. With the points closed the base is free and returns properly.
Its a brand new fuel pump & pipe work. Filters are clean
Removed carb.
Carb is clean of any debris.
Carb face and head face are flat, new gaskets and spacer
Checked and cleaned all airways & jets. Choke device functioning ok. Mixture screw is new.
Refitted carb.

Restarted engine, started straight away. Runs fine.
As engine warms up, I slowly close the choke.
As soon as I close the choke fully (last 3mm on the lever) the engine stalls. Even if I have increased the tick over speed to compensate for the choke being off.

My thoughts here are it must either be a fault in the carb or potentially an issue with the timing when the revs drop below a certain speed.
I don't have a fully understanding of how this choke works on these carbs so looking for some advice.
I am rather stumped on what to check next and cant understand what has suddenly changed to cause this issue.

Any ideas please?

On all the IMB carbs used on the various versions of the 500 and 126, the "choke" is in fact a "fuel enrichment device"---there is no 'choke' butterfly. What you're describing sounds like a lack of fuel when the 'choke' is shut and stops delivering. Have you checked the float height? With the top of the carb (off) held vertically, there should be 7mm between the gasket and the edge of the float. The easiest way to check this is with a drill bit of the correct size. I know that you have checked the carb for 'debris' but have you been able to do this with an airline---there could be something stuck in the 'idle' jet passage causing the engine to die as soon as it is required to run on the 'idle' jet (aka, slow running jet)

 

[Franko500]

On all the IMB carbs used on the various versions of the 500 and 126, the "choke" is in fact a "fuel enrichment device"---there is no 'choke' butterfly. What you're describing sounds like a lack of fuel when the 'choke' is shut and stops delivering. Have you checked the float height? With the top of the carb (off) held vertically, there should be 7mm between the gasket and the edge of the float. The easiest way to check this is with a drill bit of the correct size. I know that you have checked the carb for 'debris' but have you been able to do this with an airline---there could be something stuck in the 'idle' jet passage causing the engine to die as soon as it is required to run on the 'idle' jet (aka, slow running jet)

Hi Tom
good to here that you are still on the site providing your wonderful advice.
Yes I checked the float level and the float itself. Its one of the brass type.
It also has a new float valve.
I was just reading up about the IMB carb operation and it is leading to me also thinking there must be an issue with the idle jet/passage as once the choke is turned off it should default to this jet for tick over.
It is absolutely fine when using the throttle so cant be fuel starvation due to pump, filters or float.
Looks like the carb is coming off again tomorrow for a full strip, clean and blow through with the airline

 

[Bigvtwin996]

I think you may need a new carb... well New to you...
It just so happens........ I think I know where there is one....

 

[rusty500]

Since it had been running fine and then started all of a sudden, it does sound suspiciously like a clogged idle jet. Also, is it possible something has gotten in between carb base/gasket/spacer/gasket/manifold to cause an air leak?

 

[rusty500]

Or wide open idle mix screw?

 

[the hobbler]

Or wide open idle mix screw?

Sean:-- Upon reading your initial request for help more carefully, would I be correct in thinking that the car hasn't been run for about a year? If this is the case, there is a chance that the old fuel in the carb has vapourised away leaving a varnish, mucky residue in the carb. When you have the carb off, strip it and see if you can get somebody to put in their 'bubble-bath' (ultra-sound) bath for about a week. You have 2 places in your area who might be able to help----Thanet Carburettors (07909-794746) and, Faversham Classics (01795-539163). If that fails, I do have a 24IMB which is surplus to requirements which you could have, for a very modest sum.

 

[Toshi 975]

Sean:-- Upon reading your initial request for help more carefully, would I be correct in thinking that the car hasn't been run for about a year? If this is the case, there is a chance that the old fuel in the carb has vapourised away leaving a varnish, mucky residue in the carb. When you have the carb off, strip it and see if you can get somebody to put in their 'bubble-bath' (ultra-sound) bath for about a week. You have 2 places in your area who might be able to help----Thanet Carburettors (07909-794746) and, Faversham Classics (01795-539163). If that fails, I do have a 24IMB which is surplus to requirements which you could have, for a very modest sum.

Tom if you have got a 24IMB carb that is the restricted carb that was fitted to the 500R. The 500D would have originally been fitted with a Weber 26IMB 4. There were 4 variations of the 26IMB carb fitted to the standard 500 range , IMB 1, 4, 6 & 10 and they all had the same basic specification. The main differences were structural with the carbs being beefed up a little although the last version , the 26IMB10, incorporates a CO limiter for idling. I have worked on all of the versions.

 

[the hobbler]

Tom if you have got a 24IMB carb that is the restricted carb that was fitted to the 500R. The 500D would have originally been fitted with a Weber 26IMB 4. There were 4 variations of the 26IMB carb fitted to the standard 500 range , IMB 1, 4, 6 & 10 and they all had the same basic specification. The main differences were structural with the carbs being beefed up a little although the last version , the 26IMB10, incorporates a CO limiter for idling. I have worked on all of the versions.
View attachment 408102View attachment 408103

Thank you for that snippet of information David---I shall make a note of it in my "Little red book" (and yes, it really does exist!). I was aware that Fiat put a smaller carb on the 500R to slow it down--initially it was quicker than the 126, which was the new car that the factory was trying to promote--I wasn't quite sure which carb they fitted to achieve that; now i know..

 

[lazylobster]

Thank you for that snippet of information David---I shall make a note of it in my "Little red book" (and yes, it really does exist!). I was aware that Fiat put a smaller carb on the 500R to slow it down--initially it was quicker than the 126, which was the new car that the factory was trying to promote--I wasn't quite sure which carb they fitted to achieve that; now i know..

You are quite correct David, my 500R had a 24 carb, since replaced with the go faster 28 for that extra 0.25BHP and imporved 0 - 60 (KMH) time of 4 mins 59 seconds

 

[Franko500]

I fitted a 26 IMB 10 to my D engine.
Yes Tom the car had been living in the garage for about a year although had been started on a regular basis and fuel had been replaced.
Since obtaining my registration I had covered about 100 miles running locally to sort out any teething problems.
One thing i can honestly say was I had no issues with the engine in all that time.
When we started off for our trip out it was idling fine. So the problem started about 10 minutes into the journey after going over a few speed bumps and going up a steep hill.
The good news is the following morning from when i posted this message I went out at 7am and took off the carb, took it apart.
I couldn't see anything wrong or any dirt. (The petrol was clean).
I completely stripped it down and gave everything a good blow out with the airline.
Re-assembled with new gaskets and it started straight away with no problems.
Another road test and 30 miles later all good.
Today he successfully went to his first car show with no problems.

Thanks for your advice as always

 

[Bigvtwin996]

Tom if you have got a 24IMB carb that is the restricted carb that was fitted to the 500R. The 500D would have originally been fitted with a Weber 26IMB 4. There were 4 variations of the 26IMB carb fitted to the standard 500 range , IMB 1, 4, 6 & 10 and they all had the same basic specification. The main differences were structural with the carbs being beefed up a little although the last version , the 26IMB10, incorporates a CO limiter for idling. I have worked on all of the versions.

there was also the IMB 2 & 3 they are ultra rare and used in the N Sport