Mysterious (to me) power drop

[RichardL]

Jeff,

The concentric is the same one (rebuilt, of course) that was on the bike when I bought it in 1973. I was figuring to just use the tube for static setting of the float level. It would be great to be able to see it while riding, but I think that's physically very difficult, if not impossible. One gag would be to rig a camera and call out the mph as you go. Regardigng checking the amount of gas in the bowl, that would be easy enough at idle, but following a power loss is more tricky than I want to deal with.

Richard L.

[RichardL]

G'day Richard.
I wish you had pulled the lead on the left to compare. I know you have to increase the idle speed to run the one plug test but to hear it at proper idle (1/2 the speed it was) would help.
As a side, running a concentric on an alloy head it's hard to get anywhere near the recommended fuel height in the bowl. 0.170 to 0.240" below top of bowl. With the downdraft angle I could only get 0.260", any higher it would flood.
So in saying all that, yours must be on the lean side if it doesn't flood. At 70mph cruise on a flat road you wouldn't be on the main jet. Lift the needle!
Cheers

Muskrat,

I can't actually work on the bike today, but I might redo the running-on-one test at lower idle when I get the time.

Regarding concentric bowl capacity, has anyone ever seen a custom siamese bowl setup? I think it would work and look wild. Anyone know if Amal castings can be brazed or soldered? I plan to run a test (on scrap) tomorrow.

Definitely will be lifting the needle.

Richard L.

[JulianS]

If you think that the float bowl is starving of fuel the attached US service sheet from 1969 may be of interest.

[RichardL]

Thanks, Julian. I've seen this before and forgot about it. I want to do this. It makes perfect sense, considering the fuel has to wend its way past the flats on the needle. I think the best I'll be able to do, tooling wise, is a Dremel tool.

Richard L.

[muskrat]

Gee, I vote JulianS as the new librarian.
Yes I did that to the bowls when running methanol (12mpg on an A7), till I found some methanol bowls with a much larger hole and a tri sided needle.
Cheers

[RichardL]

I think this picture might say something about fuel starvation versus nipping up at 70 MPH. This was taken with me sitting on the bike and off the stand. I could swear I checked fuel level in  the bowl with the bowl off. If I did, I did a ***-poor job of it.

The fitting is just a 3/8" copper tubing to hose-barb connector, using the male thread at the copper end (without the female compressing nut, of course) to go into the bottom of the bowl, then just sliding my 3/8" hose over the female thread that attaches the barb, skipping the barb. The thread at the bottom of the bowl is not exact, but close enough to go in a few turns before the mismatch makes it snug. Not even fully finger tight, so there won't be any damage. Thread sealer makes up for the washer on this temporary rig.

Richard L.

[worntorn]

Oh dear, makes one wonder how it ran at all.
Is the temp. tube sealed completely on the carb?
If it weeps even a tiny bit at the connection you will get a false low reading for the float bowl fuel level.
I just went thru the same test with the SR
Monobloc and thought the level was way low. The tube connection at pilot jet was weeping ever so slightly. Pushing the tube on further made a seal and brought the fuel level in the tube right up to the pip on the float chamber cover.

Glen

[RichardL]

Here's a picture after using the tickler to flood the float bowl. This level is holding stable, so no line leaks, methinks. Anyway, there's been no drpping.

It would be fun to rig a camera to shoot the fuel kevel while riding, but the weather's not good for that right now and I plsn to correct this in time for better weather.

Richsrd L.

P.S. Not on the bike for this picture.

[worntorn]

Amal says the ideal fuel level for the mk1 Concentric is.  .21" below the top of float bowl with a +- of .040", so anywhere from .17" to .25".
Your tested fuel height looks to be about an inch below the top of bowl, does that sound correct?
This would mean that the fuel level would need to be almost at nothing in order for the float needle to allow full flow into the bowl. Also, the extreme low fuel level should cause general lean running at lower speeds as well.

Glen

[RichardL]

Here is a try at dimensioning the actual level below the low edge of the float bowl joint. Disregard the number of decimal places. The dimension on the hose itself is my way of verifying that I've scaled the picture approximately correctly in AutoCAD.

Richard L.

[worntorn]

For a tilted carb it's good to set the prescribed fuel level at the centre of the bowl. With that in mind, your fuel level is about 5/8" below the top of bowl, so about 3/8" lower than the lowest figure in the Amal range.
Definitely looking like the root of the problem.



Glen

[muskrat]

I agree with Glen. Measure from the blanked off idle screw bit ( about where the top arrow is in the picture. As I said I doubt if you will get it to 0.25" but it's a lot higher than you have. Once you get it as high as you can without flooding you'll need to adjust ALL your settings to get the best performance. If it's been running like that I'd say your current settings would need to be very rich to make up for the low fuel level. Do you have a "stay up float" to be able to adjust the height?
Cheers

[JulianS]

Worth reading what Amal say about rebuilding the Concentric, fuel level and possible problem with brass float needle.



http://amalcarb.co.uk/rebuilding-mark-1-concentric-carburetter

[RichardL]

Good morning and thanks, everyone.

All,

I have a Viton float needle and stay-up float. I have previously read (maybe, just "perused") the Amal article Julian referenced. (Thanks for the reminder, Julian). Also, done the same with this one,  https://www.princeton.edu/ssp/65-cub-data/library/amalbritbike.pdf  , but it is a lot to absorb. In the middle of this article the author says that he prefers to adjust the needle seat rather than bending the fork of the stay-up float. Considering how far off my level is, maybe I need a combination of both. Obviously, I'll know better when I open it up.

In that same article there is no mention of the measurement point based on the angle of the carb, that is, centerline at the joint?, center of the idle-screen extension?, etc. It does say to be 0.17"-0.24 below the joint, so I am thinking centerline of the joint might be the right compromise, but I live in mortal fear of going against advice from Muskrat. (What say ye, M?)

Once the float level is corrected, there is still the issue of whether, or not, the main needle should be raised a notch. Seems like it may not have been an issue, based on the float level problem.

Finally, is it possible that all of this explains why my idle screw, right now, only requires 3/4-turn out instead of about 1-1/2, as the tuning guides give as a starting point?

Finally, finally, I hope you all recall that I have long confessed my carburetor/carburetion ignorance, but I'm learning.

Thanks for reading my lengthy posts on this. Truly appreciate your thoughts.

Richard L. 

[worntorn]

My neighbour , Jim Bushman, put out this guide.
It's easy to follow and gets the job done.

http://www.jba.bc.ca/Bushmans%20Carb%20Tuning.html