Fettling a Primus #4 Inner Cap

Interesting problem. We need to collect all the different symptoms and try and apply some sort of logic to them to see if we come up with an underlying cause.

Here's another thought I'll throw into the ring. With outer caps that appear identical, but perform differently maybe we should be looking at differences in finishing?

Kerophile brought up the subject of pressure - you've got the stream of vapour coming up from the jet under pressure, mixing with air and then being guided and distributed through the gap between the inner and outer caps to exit at the holes in the outer cap. I might also assume that the burning of the flamelets and the heat produced external to the outer cap might also produce a "pull" pressure from the outside of the outer cap.

So here you have a complex set of pressure conditions. You've got the initial stream of fuel and air mixed vapour entering the system under pressure. Ideally within the burner system I'm assuming it is designed to create an level of back pressure through flow restriction such that the gap between inner and out caps remains full of the vapour. This will also be a delicate balance as too much back pressure and there will be a vapour build up right back to the bottom of the burner tube resulting in release and potential flare-up outside the burner system (or possibly internal - does this create the underburn scenario?). And then you've got the pull pressure from the burning flamelets.

So, back to finishing. If you've got an outer cap with a lot of burrs around the holes on the inside of the cap, or alternatively rusting and/or pitting on either the outer surface of the inner cap and/or the inner surface of the outer cap, does this create a bunch of vapour eddies internal to the system that disrupts the flow of the air/fuel mix and therefore affects performance? Does this change as the system heats up to full temperature and pressure resulting in the symptoms described in the previous post?

 

I got a rule that i had all my life. When fixing something just do one thing only try it then move on to the next never two things at once or you say witch one did the trick.

 

Hi Graham, Here is a link to earlier discussions about burner caps:

https://classiccampstoves.com/posts/52575

You have certainly identified many of the variables affecting silent burner performance. However, I suspect that silent burner design is as much an art as a science.
If you look at any Primus catalogue you will see that each type of stove equipped with a silent burner seemed to have its own dedicated outer cap design. The same inner design was sometimes shared between models, but is is rare to see any attempt at a "Universal" outer cap.
The lack of part nos. on the inner and outer caps further complicates matters.

Welcome to Burner Cap Hell.

Best Regards,
Kerophile.

 

For no good scientific reason, I put my outer burner cap problems down to different metals in the caps.

As I said, no good scientific reasoning involved ..

" .... ...... fascinating"

 

Hi, folks.

Does anyone have the dimensions for an innercap on a Optimus No:1 S?

Best regards, Vidar.

 

Hi Vidar, The Optimus No.1 S was originally intended to have a roarer burner. It is likely that it was retro-fitted with a No.5-size silent burner at some later date.

Check out this extract from an earlier post I made on silent burner cap dimensions:



Here as promised are the dimensions I took of some silent burner caps;

Primus No.4 stove;
Inner Cap = 33.5mmOD, but lip at bottom extends to about 35mm diam. Centre hole diam.= 16.3mm, Height = 14.5mm
Outer Cap = 38mm OD, but about 40mm on lower lip. Height at max, the top is domed, = 24mm. 5 Rows of holes.


Primus No.5 stove;
Inner Cap = 39.5mmOD, 42mmOD on bottom lip. Centre hole diam. = 16.4mm. Max Height = 16mm
Outer Cap = 44.6mm OD, 47mm on bottom lip. Max. Height = 26mm. 4 Rows of holes.


Primus No.6 or No.25 (# 4142)
Inner Cap = 44mmOD, 45mmOD on bottom lip. Centre hole diam. = 16.4mm. Max Height = 20mm.
Outer Cap = 49mmOD, 52mmOD on bottom lip. Max Ht = 26mm. 5 or 6 Rows of holes.

If you need help in finding a No.5 inner cap, PM me.

Best Regards,
Kerophile

 

Hi, George.

You say that it probobly is a No.5 burner. I have a no.5 i my collection, and I belive it to be smaler than the no.5 burner. I will do some messuring tomorrow and doubblecheck.

Regards, Vidar.

 

Hi again.

You are absolutely right about the dimensions on the burner being similar to a no:5 burner. But on the burner sitting on my Optimus there are two holes on the burner center tube. Shall there be holes on the inner cap then? Like in this picture I found in another post on the forum:



Best regards, Vidar.

 

Hi Vidar, i believe that the burner with holes in the centre spigot should use a inner burner cap with the series of holes around its top.

This was the subject of a Swedish? Patent, claimed to stop, or self-correct, the dreaded "under-burn" which sometimes occurs on silent burners.

Under-burn is when the flame ignites just above the jet, rather than at the apertures of the outer silent burner cap. When this happens you get a loud roaring noise and rapid temperature rise in the outer burner cap, to the extent that it will glow red and sometimes melt!

In fact an inner cap with top apertures will still work on a burner without holes in the centre spigot. Similarly, a burner with holes in the centre spigot can operate with an inner cap with no top holes. I guess the Patent under-burn action will not take place but the burner should still work.

Best Regards,
Kerophile.