Anybody seen these before?

Screens over the mains in the carbs. Looks to be a little restrictive and plan on leaving them out. Any thoughts????

Never ever. Leave those screens out. Someone's idea I think.

Very clever idea - great if you are out in the country, and the fuel is full of rust and gunge.

Where do I get some ??
Could have used some a few years back...

Until they block up, and you are back to square one with opening carb and cleaning.
A good filter in the fuel line is probably a better idea.
But needs to be non-restrictive. Not all are.

They were a pretty popular accessory about 40 years ago.

A good fuel line filter is a better idea. Jim

They look manufactured and not thrown together. We're these factory or add-ons?

They were manufactured and sold through aftermarket sources. Jim

Those look like a scary little item. I think I will continue to go bareback with my mainjets.

Glen

Thanks for the info. Going to keep them off.

Scott

just out of curiosity...

what possible harm would there be if there were used as intended?

surely gas would flow through the screen and into the jet just fine, no?

and if the screen did collect some crud from the gas in the bowl, well then so what, mission accomplished?

You have screen filters on top of the pet cocks inside the tank, inline filters going to the carbs and nylon filters on the banjos feeding the carbs so if one of those filters were to catch some dirt you likely have problems up the line. Any dirt that far could have fowled the needle valve off the float.

they prevent your float bowl from getting pregnant

After seeing what was left of my friend's engine after a small fine screen similar to this clogged and extreme lean burn occurred, I have been getting rid of such items.
The replacement is a large surface area, see thru, in line filter. It would take a lot to clog it and the debris is visible.

Glen

I was told these were used to mitigate foaming due to vibration at high rpm. It apparently causes the fuel to adhere to the jets.

I used them in my race Commando but cannot attest to their effectiveness.

worntorn said:

After seeing what was left of my friend's engine after a small fine screen similar to this clogged and extreme lean burn occurred, I have been getting rid of such items.
The replacement is a large surface area, see thru, in line filter. It would take a lot to clog it and the debris is visible.

Glen

Click to expand...

+1.. absolutely the right way to go

I got some hand crafted MKII's from Jerry Burak a few years ago. Jerry was still riding his bicycle many miles to work @ his place out Mullholland in the LA area, the last we talked. Got to be in his early 70's now. Jerry provided much of the R&D when Amal cam out with the MKII's those years ago. Well, those carbs had the screens that are shown in your photos. They have an Amal part number. I called Jerry to see what the story was. He purposely included the screens in his carbs that would be going on a tuned engine. Had years of evidence that showed a more consistent power application on road race applications. It also changed the main jet tuning. He also had some well formed opinions regarding velocity stacks. Seems there are more situations than not where there was no increase in power on a dyno with any selection of velocity stacks. Once mounted in a chassis and exposed to buffeting air, the velocity stack becomes more important. Seemed it was about as important to get a smooth airflow into the carbs as it was to get the optimum fuel air mixture when it came to lap times. I don't think that as much could be gained on a engine tuned for the street. Jerry did say that he sold a lot of the screens to LA shops that tuned Mikuni VM carbs.
PS: I do not have any comparison data on those carbs. They are in the bin for projects that I plan to work on during my first year of retirement.

Yamaha SR500 carbs have 'em, I've seen 3 and they were all the same.

I wouldn't use anything which might alter the rate of flow of fuel through the main jet. What do you do when tuning, make a judgement about the pieces of crap adhering to the screen ? In racing an aluminium tank and fuel out of drums is the way to go - no filters or restricted breathers on the tank or carbs.

grandpaul said:

Yamaha SR500 carbs have 'em, I've seen 3 and they were all the same.

Click to expand...

I am sure many bikes have these strainers over their main jet, I know I have seen them before also, but I doubt not without calculating the need for proper fuel flow. That is to say, 260 main with a screen vs 220 without. I guess you could adjust you mains by using different mesh screens.........not.

You have a strainer at the petcock and one on each banjo, I mean really, someone must have had a serious issue or lacked the filtering up stream. There must have been a reason or read something in a book somewhere or maybe their other bike was a sr600 and maintained continuity.

So its a screen, so it must be meant to screen - right? I don't think so.

Tell me who has had a main jet plugged. When you think about it, there are plenty of minor circuits that can get so easily plugged and these screens over the main jets do nothing for them.

The screen opening to jet opening is so biased that there's way more cross section of screen than jet so jetting adjustments (w & w/o screens) is a non starter; and I certainly never noticed a difference. And in all my races I never accumulated debris on the screens.

I am pretty sure this is to mitigate fuel surge (as rotorwrinch mentioned) & starvation by increasing surface tension and fuel availability at the jet. Maybe this is an old dirt bike application where things get tossed around a bit in the float bowl.

Even Mikunis have funny little dishes about the main jet with holes in them; probably attempting to address the same problem but in a somewhat different way.

Ok Dance's pique'd me with the most reasonable reason suggested, ie: involving assisting fuel flow function rather than filter function. Turns out its a unique feature of the AMAL 2 stroke carbs, which were adapted to 4 strokes in the late 60's,with press in pilot jets but not with the screens installed. According to John Healy on page 9. Two tokes have less inhale [low vacuum] force so must assists flow, but I don't know the principle its based on yet, even with the briefing Dance's made on it so far. http://www.tioc.org/amalbritbike.pdf

Hm this nozzle site implies its a disfuser screen, which lowers turbulance which can trip up flow entering a jet orifice.
http://www.kinsler.com/page--constant-flow--14.html


hm, even some popular cars have fine screens in fuel pick ups.
http://www.totallyamaha.net/forums/prin ... hp?t=78285