Blackjack Avion Cold Running Issue

[Ian Caldwell]

Hello all,

I still want to use my Avion during the nicer winter days. I am, however, finding that the car does not heat up enough and I get a significant flat spot with the accelerator pedal depressed by about a third. You can work around this by briefly using full throttle, but it's a bit of a pain. I have heard comments that you may be able to adjust the jetting to make the car better for cold running , or is there another way to help the engine heat up on colder days; I have already blanked off the oil cooler but thought about a deflector that reduced the air flow over the cylinders but this may be difficult to control in reality.

I've attached a picture with the jet sizes I have and any advice would be greatly appreciated.

Regards,

Ian.

[Ian Caldwell]

Hi, I should add that I now know that the main jet is not fixed and I've seen an article(link below) that suggests you change this to a 107. I'm not sure what I have but the car runs really well once it is warm enough, so I don't think I have a modern fuel issue and therefore I'm also not sure changing this will make a cold running change. Cheers, Ian.

https://shared.yellowgrey.com/2020/07/0 ... rburettor/

[notsmartcar]

If it was mine and on the side of the carb it has 18/26 cast on it the main jet (no 7 on your picture) accesable through the 12mm hex nut on the outside is removable with a screw driver (you must have a screw driver as wide as possible that will fit in the jet hole ..I would then drill out the hole to 110 that’s for normal use any way and may fix your problem as well. Ps I know you can buy jets but recent experience with a box of assorted jets from Bu***N the stamped sizes on them were incorrect . If it runs ok in the summer you may find the floats are too high if the jet is still 102.. does your avion have the oil cooler pipes in to the manifold for manifold heating /oil cooling

[Ian Caldwell]

Hi and thanks for the reply. I do have some braded hoses from the heads but I'm not sure if these are oil but I assume they are. I've included a picture for others to comment and confirm; also one of the Carb markings which look like 24-21.

I'll also remove the main jet to check it's size and look at drilling it out to try it; I have a spare, so will sacrifice this.

Cheers,

Ian.

[IanCaldwell]

Hi all,

I finally got around to looking at the main jet. I had prepared a 110 but found that the jet installed was already bigger ; less that 1.45mm, which was the next size of drill up I had, I guess about 1.2 or 1.3mm. The car runs really well once hot but do you think this is too big? My dad has obviously already worked on this and found a jet that worked for the way he has built the engine and used the car. He lived in Bath, so always a bit warmer than Glasgow.

I still think this is a problem with the car not heating up fully on cold days (sub 5oC), and based on this, I removed the exhaust wrap, so that the air into the carb is hotter and this has improved things a bit, so I think adding more winter ducting, may further improve things.

Has anyone installed or made anything that allows for a summer and winter setting for the air intake or something that reduces the air flow over the cylinders?

On another note, it's mot day today but it's been snowing here in Glasgow. The station is on the other side of the city; to cancel or go, is the next decision 8-!

Ian.

[Ken Hanna]

Ian,
with the 21-24 version of a Solex 26/35 carburettor, the original primary petrol jet will have been either a 120, or (rarely) a 125.
In order to cope with unleaded E5 fuel,it would have needed to be opened up to 127.5, so if you're using E10 fuel, 130 is probably a better bet.
Airflow over the cylinders isn't of much relevance in this regard, restoring the mixture fed to the engine closer to stoichiometric is what matters...
Ken

Hi all,

I finally got around to looking at the main jet. I had prepared a 110 but found that the jet installed was already bigger ; less that 1.45mm, which was the next size of drill up I had, I guess about 1.2 or 1.3mm. The car runs really well once hot but do you think this is too big? My dad has obviously already worked on this and found a jet that worked for the way he has built the engine and used the car. He lived in Bath, so always a bit warmer than Glasgow.

I still think this is a problem with the car not heating up fully on cold days (sub 5oC), and based on this, I removed the exhaust wrap, so that the air into the carb is hotter and this has improved things a bit, so I think adding more winter ducting, may further improve things.

Has anyone installed or made anything that allows for a summer and winter setting for the air intake or something that reduces the air flow over the cylinders?

On another note, it's mot day today but it's been snowing here in Glasgow. The station is on the other side of the city; to cancel or go, is the next decision 8-!

Ian.