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#704525 - 08/10/17 2:57 pm DBD Clubman w/1038 Carb  
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I know this is a well worn subject, and I have done many searches to try and solve the puzzle but with no joy. I have a 1961 DBD Clubman (big valve head) and have installed an AMAL 1038 carb that is fully converted for four stroke use. The bike has a standard exhaust pipe and muffler, I am riding on the street at sea level and I am running with a "minimalist" gauze type air filter. The magneto has been rebuilt and the spark is healthy and blue at all times.

I have gotten the jetting sorted to the point where the bike runs well most of the time (idle, off-idle, steady part-throttle and uphill acceleration are all okay), except at wide open throttle ... or perhaps as soon as the needle leaves the needle jet. At that point, the RPM burbles and bobbles around 4500 to 5000 RPM, not 8-stroking and not surging, just hitting and missing until I close the throttle adequately to get things cooking again. At part throttle and load, the engine will pull nicely above 7000 RPM, so I don't think that this is a mechanical issue.

My current carb settings are: 35 pilot jet, 4-stroke 107 needle jet, 4-stroke (2-notch) needle in the richest position, 3.5 slide (a well-fitting, brass one), and a 4-stroke spray nozzle. I have tried main jet sizes from 330 to 430 ... all with the same result.

Is it possible that the standard 4-stroke needle is not long enough for the bigger 1000 series carburetors?

Thanks for any light that you might shed.

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#704527 - 08/10/17 3:16 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Originally Posted by Belfast Bikes
Is it possible that the standard 4-stroke needle is not long enough for the bigger 1000 series carburetors?


For 1000 series:
Throttle needle (paired with needle jet 622/122): 622/124
Throttle needle (paired with needle jet 622/0789): 1034/063

I haven't had my 1036 apart for quite a while so I don't remember what the differences in needles and jets are, but from the above it certainly seems possible you have a mixed set in yours.

#704530 - 08/10/17 3:40 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Thank you, MM;

Forgive me if this is a redundant message, there seems to be a glitch in the reply system:

I believe I have the firstly mentioned pair: a cross-drilled needle jet with a 2-notch (above the clip grooves) needle. Also, I am using the longer main jet/needle jet holder. Is the second pair that you listed meant as a 4-stroke set-up specifically for the 1000 series carbs?

#704532 - 08/10/17 3:49 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Originally Posted by Belfast Bikes
Is the second pair that you listed meant as a 4-stroke set-up specifically for the 1000 series carbs?
I copied that information verbatim from an AMAL parts list for 1000-series. The list doesn't specify 2 or 4 stroke for those two items. However, I now see that further down the list it say the 622/122 is "preferred for 4-cycle engines," while the other is preferred for 2-cycle.

#704536 - 08/10/17 4:06 pm Re: DBD Clubman w/1038 Carb [Re: Magnetoman]  
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Thank you, MM;

Yes, it looks like AMAL had a 2-stroke needle for the 1000 series, but no 4-stroke needle. Hmmm.

#704538 - 08/10/17 4:19 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Originally Posted by Belfast Bikes
Yes, it looks like Amal had a 2-stroke needle for the 1000 series, but no 4-stroke needle. Hmmm.
I think you misread my original post. The 622/124 is the 4-stroke needle and the 1034/063 is the 2-stroke. I don't have the information in front of me now but, assuming the same was done as for the smaller carburetors, they're probably distinguished from each other by the number of lines around them.

#704540 - 08/10/17 5:02 pm Re: DBD Clubman w/1038 Carb [Re: Magnetoman]  
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From what I can glean, the 622/124 is the only "standard" 4-stroke needle. There is a 2-stroke needle specific to the 622 and 900 groups and then one 2-stroke needle specific to the 1000 series carburetor. That begs the question, why no 4-stroke needle specific to the 1000 series?

What did Velocette do for models fitted with the 1000 series carb? Maybe John Healy will weigh-in on this?

Last edited by Belfast Bikes; 08/10/17 5:04 pm.
#704548 - 08/10/17 7:32 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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There is not really a definite 2 stroke or 4 stroke setup.
Usually 2 strokes have the angled spray tube but not always.
I'm thinking maybe the needle jet/needle is too lean and not letting the main jet flow its full potential. A big problem is that AMAL never made enough different needles but you can fit Mikuni needles and their choice is vast


1955 BSA B31 400cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350cc
1967 Greeves 360cc Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
Modified Nu-Trak GM500cc sprint/LSR bike "Deofol"
2003 Aprilia RSV Mille "Lo Stregone"
#704549 - 08/10/17 7:46 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Hmmm...I never tried by DBD with the MkI 1038 -I went straight from GP to 36mm Mikuni, though I did have all the bits. Looking at the current AMAL spares site, here's what they list for the 1000 series:

http://amalcarb.co.uk/mk-i-Concentric-series/1000-series/needles.html

If things improve when you close the throttle, that's usually a hint at richness.

I did collect details of what others were using in various carb setups for both DBD and Thruxton, and many seemed to settle on the 108 needle jet for sea level and a 106 at 3000 above for the MkI Concentric. Oddly, many didn't share their main jet settings. One things I did find, was that what others think is a good setup was well wide of the mark on both the BSA and Velo when I swapped both to Mikunis (definitely the thinkers choice, frankly!) - I suspect that for many near enough is good enough.

The 1036 on the Thruxton was setup with a 320 main jet and a 106 nozzle.

Suggestion: put a 250 main jet in it and just see what happens. If it's lean, you won't cook the engine for a try-out, but it just might be illuminating. Let us know!

Last edited by Kerry W; 08/10/17 7:47 pm.

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#704552 - 08/10/17 8:16 pm Re: DBD Clubman w/1038 Carb [Re: Andy Higham]  
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Thank you, Andy;

Looking at the size of the needle jet (.107") vs. the size of the main jet, I'd have to say that once the needle clears the needle jet orifice, that should not be a problem. I do like out of the box ideas, keep 'em coming.

#704553 - 08/10/17 8:20 pm Re: DBD Clubman w/1038 Carb [Re: Kerry W]  
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Thank you, Kerry;

You might be right that for these museum pieces, once they are running down the road, that's good 'nuf. Once I get a bug in my shorts, though, I won't rest easy until I've gotten it out.

#704555 - 08/10/17 8:28 pm Re: DBD Clubman w/1038 Carb [Re: Andy Higham]  
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Andy;

I completely glossed over the important bit ... which Mikuni needles will fit, and how do you gauge the appropriate sizes?

#704557 - 08/10/17 8:38 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Mikuni needles are spec'd by taper (single or double), length and diameter. There is a table HERE


1955 BSA B31 400cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350cc
1967 Greeves 360cc Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
Modified Nu-Trak GM500cc sprint/LSR bike "Deofol"
2003 Aprilia RSV Mille "Lo Stregone"
#704559 - 08/10/17 8:45 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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If you mark an AMAL needle with a pencil or fibre tip pen at 10mm intervals and measure at the marks, you can make a direct comparison between AMAL and Mikuni needles


1955 BSA B31 400cc "Stargazer"
1962 Greeves 200cc "Blue Meanie"
1962/67 Greeves 350cc
1967 Greeves 360cc Challenger
1984 Suzuki GSX1100 EFE "Sorcerers Apprentice"
Modified Nu-Trak GM500cc sprint/LSR bike "Deofol"
2003 Aprilia RSV Mille "Lo Stregone"
#704560 - 08/10/17 8:53 pm Re: DBD Clubman w/1038 Carb [Re: Andy Higham]  
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Andy;

That is a great tip ... thanks.

#704562 - 08/10/17 9:11 pm Re: DBD Clubman w/1038 Carb [Re: Andy Higham]  
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Originally Posted by Andy Higham
you can fit Mikuni needles and their choice is vast
I've never tried to fit a Mikuni needle in an AMAL so can't comment on that, but I found their choice is only half-vast. It's true that their catalog lists a gigantic selection of needles and needle jets in tiny increments but, unfortunately, many (most?) of them haven't been made for years.

#704609 - 08/11/17 11:01 am Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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You'd need to be very accurate when measuring the AMAL needle to compare with Mikuni - the Mikuni details are to the nearest 1000th of a millimeter, at very specific points along the needle. Could be a good way of seeing roughly where the AMAL needle fits in the AMAL setup though.

The suggestion above that theneedle jet size, in relation to the main jet might be an issue - very unlikely, I'd say, unless the main jet was VERY large (alcohol-sized) or the needle jet very small (which a 0.107 isn't).

Mikuni needles are very accurate and there are mountains of them on the lists, but as MM says, some are out of production. She were specific to a particular manufacturer, for a certain bike, but make the 'general' lists.

I have read that a Mikuni needle in an AMAL can be done, though the differences will be purely a refine,emt, if the AMAL needle isn't giving the fuel delivery/slope that is required. frankly, the problems prompting the start of this thread are bigger than that.

However, 'conventional wisdom', when choosing a needle for a Mikuni on a 4 stroke is to have one with a single taper. The Mikuni needle taper is defined by letters and numbers, the decode for which is easily found. Practically though, a 6DH3 (or -2; leaner), 6DP17 or 6DP1 would work. My DBD, with a 36mm Mikuni, ran very nicely on a 6DH3 (with good response and very good fuel consumption - WAAAY better then the GP).


No generalisation is wholly true, not even this one.
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#704623 - 08/11/17 1:22 pm Re: DBD Clubman w/1038 Carb [Re: Kerry W]  
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Originally Posted by Kerry W
You'd need to be very accurate when measuring the Amal needle to compare with Mikuni - the Mikuni details are to the nearest 1000th of a millimeter, at very specific points along the needle.


I don't know much about Mikunis via Amals, but I spent a lot of years in a factory doing precision metal fabrication ...

.001 MM is .000039". That's .039 mils. Nobody's really manufacturing or measuring to that precision, I don't think. If they are, I'd like to know how they do it ....

Lannis


OK, I admit it, I'm addicted to brake fluid.

But I can stop any time I want.
#704633 - 08/11/17 3:10 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Unlike the Mikuni the AMAL needle lifts clear out of the needle jet around 3/4 throttle. When it does this is dependent upon which one of the needle's three clip grooves the clip is in. With the Mikuni the tip of the needle never clears the needle jet orifice.

The bits used to tune an AMAL for two or four stroke use are different. It isn't just the slant cut spray tube.

The jetting for the 1036 used on the Velocette Thruxton used a 320 main, 30 pilot, 622/122-106 needle jet, 622/124 needle, #3 slide with the clip in position 1 (top groove).

There is more to comparing the taper of the needle required as the main diameter of the AMAL vs the Mikuni are different. This requires adjustment to the needle jet orifice if one is to use a Mikuni needle in an AMAL. Good luck with that.

Quote
If things improve when you close the throttle, that's usually a hint at richness.


Just the opposite. When you roll back the throttle the mixture momentarily gets richer: lower throttle opening equals less air while for a brief time the momentum of the fuel flow remains steady.

If the main jet is lean the rpm will hunt and not be steady and when you roll back the throttle the bike will accelerate a bit for a few seconds. Also if the throttle is turned abruptly the engine will stop running until you roll the throttle back to where it is on the needle.

Where you can put a 420 main jet in this carburetor and see no difference I would be looking elsewhere.

The Mikuni needle thing is a bit of a ruse. While the charts list a myriad of needles only 3 or 4 would be applicable to a given Mikunii model, and as with AMAL (who list 6 or 7 for the Concentric), only one or two of those would be applicable for a typical 4 stroke application. For example one of AMAL's Concentric needles was derived to meet EPA mandated exhaust requirements. Also the vast quantity of Mikuni needle jets are of the "primary" variety and basically designed to be used with two stroke engines. Mikuni developed the "bleed" needle jet for the few applications where it was used on a 4 stroke engine. This mirrors AMAL Concentric where the first applications for these carburetors were for two-stroke engines. They later had to modify the jetting to get it to perform better when the Concentric was introduced to the 4 stroke engines. So with both you end up with a list lot of bits, but only a couple that will work with a given carburetor and application.

Barry Johnston and Jeff Binks did a lot of special tuning both for the factory and factory race shops. This included making subtle changes to the needle jet and main jet holder. Parts were shortened and shimmed and bleed holes adjusted, or removed to meet special requirements. If you look at the position of the actual jet orifice in the two stroke vs the four strok needle jet you will see that they are located about 1/4" away from each other. This is a variable available to the serious tuner!

Quote
That begs the question, why no 4-stroke needle specific to the 1000 series?


They do: 622/124 2 ring needle.
John


#704661 - 08/11/17 8:37 pm Re: DBD Clubman w/1038 Carb [Re: John Healy]  
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Thank you, John and all others who have taken the time and interest;

Having checked Mikuni's offerings, I did find that there are no needles that would meet AMAL Concentric requirements, and that would be any longer than the 622/124. There may be something else (other than carburetion) causing the engine stumble at WOT, but my gut tells me to continue on this path for now. If any revelations occur, I'll be sure to come back and report them ... I'm not too proud, or too smart.

#704672 - 08/11/17 9:32 pm Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Originally Posted by Belfast Bikes
the bike runs well most of the time (idle, off-idle, steady part-throttle and uphill acceleration are all okay), except at wide open throttle ... or perhaps as soon as the needle leaves the needle jet. At that point, the RPM burbles and bobbles around 4500 to 5000 RPM,... just hitting and missing until I close the throttle adequately to get things cooking again. At part throttle and load, the engine will pull nicely above 7000 RPM, ...
It may be time to remember the maxim that 90% of carburetion problems are electrical. Under load is when demand on the electrical system is greatest (i.e. the voltage rises to the highest value) so the chances of internal arcing also are greatest. If there is arcing, it could be within the coil (bad) or at a crack or carbon track in the pickup (not so bad).

What gap is your plug now? Do you have a stranded Cu HT lead? What brand and number spark plug, and type of spark plug cap, are you using?

#704677 - 08/11/17 11:36 pm Re: DBD Clubman w/1038 Carb [Re: Magnetoman]  
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Thanks, MagnetoMan;

My plug gap now is .018 to.020" (side electrode angle is a little wonky at these gaps) on a B7EV plug, using a stranded CU lead and a NOS KLG hard plastic cap that does not extend to the metal hex on the plug. The magneto armature was rewound and recapped by a long time shop here in Maine ... he seems to know his work and has a good rep., though I don't think you would agree with his choice of capacitor, which is a brightspark ceramic type. The pick-up is new, but the slip ring is not (looked for carbon tracks, didn't see any).

I'm not saying it's not ignition, let's just say that I am less sanguine about the carburetion right now. This little project has proven to be a great way of spending my newly gained retirement time.

#704688 - 08/12/17 4:07 am Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Originally Posted by Belfast Bikes
I'm not saying it's not ignition, let's just say that I am less sanguine about the carburetion right now.
I'm not saying it is ignition, but in rereading the symptoms plus the information in your latest post I'm leaning less toward carburetion than before.

#704748 - 08/12/17 2:55 pm Re: DBD Clubman w/1038 Carb [Re: Magnetoman]  
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Thank you, MM;
After re-reading my own posts and all the rest, I certainly can see why you lean that way. My work plan is to experiment a bit more with jetting, then if there is still a problem I will send the magneto for testing. It's great to be able to bounce ideas around with so many folks who have experience with such non-current equipment.

#704845 - 08/13/17 2:46 am Re: DBD Clubman w/1038 Carb [Re: Belfast Bikes]  
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Not at home but for some strange reason I recall that my 1038 (on big valve DBD) has 380 Main, 108 needle jet and 3.5 slide. Have heard some use a 3.0 slide.... Not sure on needle but top pos'n rings a bell and it has a "0" above the top notches I think. Pilot jet...can't remember...but probably at least a 25?

I always get grief on from my wife at the odd (to her) stuff I remember....damndest thing as I can't remember my kids birthdays!

Not sure if that helps...

Cheers,
Dave

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