Solex AHD 26 - Low speed jet

[camstevens]

Hi All
My Solex is deciding to play tricks on me again.....classic symptoms of a blocked low speed jet (number 70). Starts no issue on choke, but will only run at higher revs with choke in. Push the choke in at idle, splut, dead. So, just stripped and cleaned and hopfully caught the issue.

I have a question (maybe a nieve one), but I was curious as to how the low speed jet stem seals effectivley once fitted to the main body of the carb.....the low speed draws its air from the little aperature that leads to the main air intake, but fuel drawn from the 70 jet direct from bowl. But...my question is whether air can leak around the jet stem and whether it is known to cause issues. It would certainly expalin some of the behaviors I get when the engine is hot and coming down from road speeds. The lower photo shows the jet stem just as it is entering the carb body.

C

[simon.holyfield]

I've always assumed that because the annular air gap around that jet is constant, then the jetting arrangement must be able to cope with it.

I can't offer any more than that, but I agree it seems odd. These are commonly used carburettors in the automobile world though.

[camstevens]

Thanks Simon
They are a pretty simple setup, so one would think an issue could be identified easily. If you recall the paper gasket issue (blocking hole), the gasket has gone as hard as teak, so possibly passing air. Will replace

[simon.holyfield]

Thanks Simon
They are a pretty simple setup, so one would think an issue could be identified easily. If you recall the paper gasket issue (blocking hole), the gasket has gone as hard as teak, so possibly passing air. Will replace

Did I tell you I bought a new idle screw? The thread on the old one was quite worn, and I have a couple of turns of PTFE tape around the new one. Seems fine, definitely better.

[camstevens]

Hi Simon
I had a similar issue, except the aluminium was slopped out. I carefully used JB Weld to restore the thread, which seems to.gave worked out well.

[simon.holyfield]

Hi Simon
I had a similar issue, except the aluminium was slopped out. I carefully used JB Weld to restore the thread, which seems to.gave worked out well.

That's interesting - I've never used JB weld on a thread.

I'll give it a whirl.

[Chris Sealy]

Hi all. I have rebuilt many Solex carbs; mostly for classic cars; and around half a dozen for the square four.
As Simon says the pilot jet is not sealed.The annular gap allows air in, to emulsify the fuel and push it along the internal passage toward the little aperture that Cam mentioned. Here it has more air mixed in, before emerging into the throttle chamber via the pointed end of the volume control screw. Aka, idle mixture setting.
As the throttle is opened, this same mixture emerges from a small orifice, just behind the bottom edge of the throttle disc; to smooth the transition from idle to main; in order to avoid having a weak, or flat, spot.
Most of the problems with these carbs, other than blocked jets, is caused by a worn throttle shaft allowing extra air into the mix. I fix this problem by machining two grooves into the shaft and fitting heat resistant Viton O-rings.
Chris.

[camstevens]

Hi Simon
I had a similar issue, except the aluminium was slopped out. I carefully used JB Weld to restore the thread, which seems to.gave worked out well.

That's interesting - I've never used JB weld on a thread.

I'll give it a whirl.

Very light oil on the thread seems to allow the thread to act as a former. Breaks clear once set. Challenge is not to foul the mixture tube with JB....but manageable

[simon.holyfield]

Hi all
...by a worn throttle shaft allowing extra air into the mix. I fix this problem by machining two grooves into the shaft and fitting heat resistant Viton O-rings...

Mine is not that bad, so I haven't done this yet - but what size 'o' rings do you use Chris?

[Chris Sealy]

I have used both 1.5mm and 2.0mm x 5mm. 5mm being the internal diameter.
It is important to countersink the external edges of the holes in the throttle body, to avoid cutting the O-ring. Use some silicone grease, both on the shaft and in the hole.
Fit the O-ring to the threaded end first. Push it through until the O-ring can be fitted to the other end, then pull it back to centre, before refitting the throttle disc.
The depth of cut is trial and error. It must not be too tight, or the return spring will not close the throttle when in use. The width of cut should be about twice the O-ring thickness.
If you are going to try it for yourself, I suggest that you arm yourself with both sizes of 5mm O-rings; then if you cut too deep for the 1.5mm, all is not lost. You must use genuine Viton o-rings from a trusted source. Others may melt. Ask me how I know.
Chris.