Confusing Carburation

[Brucie64]

My Spitfire Scrambler uses a 389/37 Monobloc carb originally with a 105 needle jet, 270 main jet, 30 pilot jet and the needle positioned at level 3. The bike ran well with these settings and started first time "most" times so I wasn't worried but it did always have very sootty plugs and used lots of fuel.

I then read somewhere that my bike (an S/R motor) should be using at least a 400 main jet as standard so I decided stupidly to tinker!. I purchased a replacement 105 needle jet, new needle and a 400 main jet and started to dismantle and clean out the carb. However once I re-assembled it with the new bits the bike now runs like a dog. 

It now is very difficult to start, especially when the motor is warm. Once I finally get it started and open the throttle more than 40% the motor struggles to pick up and feels like it is choking itself. Once you close the throttle a smidge the motor sparks into life. The plugs are still quite sooty but not as bad as before I serviced the carb.

After adjusting the pilot air screw out (now 2.5 turns out) and throttle screw the situation is a lot better regarding throttle use as I can now ride the bike but still can't wind the throttle open for fear of it stuttering and choking again. I have brought the needle down to level 4 to try and lean up the mixture a bit which also seems to have helped.

Question though?....I thought anything less than 1/8th throttle was controlled by the pilot jet and all mid range was controlled by the needle jet so how come fiddling with the pilot adjustment has helped my cause and changing the needle and needle jet for exactly the same size items has caused so many problems (incidently putting the old ones back didn't fix the issue for me!).

Also does anyone know what can be causing this stuttering/choking mid range issue?

[dpaddock]

See "Monobloc Pilot Circuit' posting 28 Sep.

David

[Triton Thrasher]

A bike that runs well on a 270 main jet cannot possibly run right on a 400 main jet, except on small throttle openings. 


I really am tempted to be sarcastic here, but that wouldn't help.


If you must play about with the carb, get new spark plugs plugs and follow the well-publicised (it's even in workshop manuals) Amal tuning procedure.

[Brucie64]

I know Triton Thrasher...........if it's not broken don't fix it...will I ever learn!

I shall re-read the Pilot Circuit posting as you suggest David .

Regards
Bruce

[Triton Thrasher]

Some people increase the main jet by two sizes at a time until it's obviously too rich at full throttle, then reduce by one size at a time until it runs well at full throttle.

That involves having a big handful of jets though.

[ebsbury]

Your carburetter began life as a 1 1/8" Monobloc on a 1958-9 Super Rocket.

The 1957-59 Spitfire Scrambler uses 376/89, an 1 1/16" carb with 400 Main Jet, 25 Pilot Jet, 106 Needle Jet and 3 1/2 Slide. Standard needle position is 2.

The 1960-63 Spitfire Scramblers use 389/47, an 1 5/32" carb with 290 Main Jet, 25 Pilot Jet, 106 Needle Jet and 3  Slide. Standard needle position is 4.

No A10 ever used a 105 needle jet as a standard fitting and the new needle jet is almost certainly why the engine is struggling. The difference between a 105 and 106 needle jet is 0.001" so an old or worn or deliberately modified 105 needle jet could easily have been performing like a 106 or beyond. A worn needle jet will make your carburation extremely rich from around 1/4 to 3/4 throttle.

[dpaddock]

A footnote:
      The tuning sequence for the Monobloc (and other Amals) is seamless, i.e., the adjustment ranges overlap from the one to its adjacent range. Thus, the pilot air will certainly be affected by the cutaway as the carb flow transitions from pilot to cutaway. Don't be fooled into thinking that only one adjustment will do it.

     How worn is the slide? Use a feeler gage. Any more wear than .006" or so should tell you to stop right there and get a fresh slide and, perhaps, a rebore w/ sleeve. Only then can you get proper results from your adjustments.

David

   

[Brucie64]

Hi David,

Thank you for your pearls of wisdom concerning the throttle slide, much appreciated. I shall try to measure this at the weekend with my gauges. The odd thing is that when I bought the bike 8 months or so ago there didn't seem to be anything wrong with the carb settings as the bike revved freely throughout the range (albeit running quite rich....hence sooty plugs).

I therefore think it is something I have done/adjusted that has caused the problem.

To reply to Ebsbury's posting... the original needle jet (when I bought the bike) was a 105. I subsequently replaced this with a new 105 jet and a new needle so hopefully I can discount any worn or modified needle jet issues in this case.

I think the idea that a 400 main jet is causing these issues must be red herring also as I put back in the original (again when I bought the bike) 270 main jet and the problems were still there.

[ebsbury]

I think my explanation was poor.

The symptoms you described of the engine struggling and running better when you close the throttle slightly are consistent with the engine running weak. At the point where it occurs you are running on the needle jet. Because a small amount of wear in the needle jet has a significant effect on the richness of the mixture between 1/4 and 3/4 throttle, excessive wear in the needle jet fitted when you bought the bike could easily cause it to be passing fuel equivalent to a 106 or even a 107 which would explain the richness you were first trying to fix. There are also non-genuine jets around that are hopelessly inaccurate. So for all those reasons your engine could have been running reasonably well on a very worn needle jet stamped with a 105, but the replacement would need to be a 106 which is the minimum size on all A10 389 carburetters, if it is to flow sufficient fuel in the mid-range.

[Brucie64]

Hi Ebsbury,

Thanks you again for your detailed response and clarification. I thought the setup musav been running rich as the plugs were very black and sooty.....I never thought that the symptoms maybe due to weakness.

I think the best thing to do to test this theory is the put the old needle and old 105 jet back in a test again. If all is well then I will invest in a 106 needle jet as you suggest.

If all is not well then I think I might have to resort to paying for the services of our local classic bike expert/mechanic here in Bucks to sort it.

[alanp]

Just a little tip which helped me sort out a GP carb. - I put a small white paint mark on the inner end of the throttle twist grip and then marked with more white paint the 1/4, 1/2 and 3/4 positions on the fixed part. When I did this I realised how you can be fooled into working on the wrong part of the carburation sequence since you think you know where the throttle position is at any time but you're probably wrong. I was surprised how wrong I was.
As already posted, there is a considerable degree of overlap through the sequence e.g. on the GP carb. the pilot jet has an diminishing effect right through to 1/2 throttle and overlaps the cut away's main effect zone and part of the needle's. I expect something similar is the case with the carb. is question.

[beezalex]

Uh...I know it's been a couple of weeks, but did you sort this out?  I just re-discovered this thread and something came to me:  Did you replace all the gaskets on the carb when you replaced the jets?  Did you perhaps put a gasket under the needle valve block?  This will make everything run crazy rich and possibly ooze fuel.

Just a thought, anyway.

[RichardL]

Well, this seems like a reasonable place to ask my carburetion questions.

My A10 is a swingarm with crankcases from '55 (the date of original registration), but several things have changed since then and I'm trying to decide if I have optimum carburetion or if there could be some benefit from a change. The carb I have now is an Amal 930, the same one that was on the bike when I bought it in '73. The bike has been running rather well, but it has occasional tickover problems. I can't say those problems are wholly due to the carb, its adjustment or its jets, but the possibility of that raises the question as to whether or not to replace it (as a winter project).

Here are the questions:

1.  Is the 930 taller than a monobloc or TT, from its intake to its top, such that the throttle cable must take a tighter bend into the top?

2. Do A65s, which, I think, all used concentrics (correct me if I'm wrong) have less up-angle on their intake manifolds such that gas sits level in the 930's float bowl. If so, I would guess that A65s get better performance from the float and are less likely to flood. I notice that gas emerges around the tickler almost immediately on pushing it, rather than taking a couple of seconds.

2a. Is the monobloc float "bowl" (chamber?) specifically designed with its centerline across the bike, rather that vertical, to deal with the up-angle of the intake manifold?

3. (This is the really tricky question.)  With 9:1 pistons, 356 cam, 67-1126 head casting, and normal intake valve diameters, which monobloc with which jets and needle would serve best, were I to go that direction? Should I go that direction?

I am anxious to hear guesses, informed answers or bold marching orders, if you prefer to put it that way. (Please bear in mind that I am not really asking for tuning instructions for the 930. That's a different topic than the concentric vs. monobloc questions.)


Richard L.
 

[trevinoz]

Richard,
            just a point regarding the cable entries, the original Monobloc did not have adjusters, just top hat ferrules.
When you fit a carby with adjusters, the bend gets a little tight.
The A65 used Monoblocs prior to 1967 as the Concentric didn't exist.
If you are considering a Monobloc, a 389 with 1 1/8" bore should do nicely on your head.
A starting point for jetting - 290 main, 30 pilot, .106 needle jet, 3 1/2 slide, needle position 3.
This is the specs for use with an air cleaner. Without air cleaner main jet is 400.
Trev.

[ebsbury]

The height of a 900 Series Concentric from it's centreline to the top of the lid (not including adjusters) is 2.47 inches. The equivalent measurement on a Monobloc is 2.97 inches so the Concentric is shorter.

Monoblocs are not happy with steep angles of downdraught. They tend to flood their pilot jets. The maximum recommended angle of lean is 4 degrees. Concentrics were designed to run at steeper angles. The central main jet ensures that the fuel level in the jet is always the same whatever the angle, and the pilot circuit is above the float chamber.