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Originally Posted by DavidP
Originally Posted by Allan G
Oif used 200 mains, I beleive the rest are the same.
.
My '71 had 200 mains when I bought it. Those proved to be too large, stock air filters, but different exhaust.
I ran VM 32's for most of the time I had the bike. Gas mileage dropped from 45 to 35 MPG, and I got very tired of rejetting to try to get the mileage up. The VM is a large instrument, as mentioned it's difficult to mount and find air filters. Unless you can find a 4-way fuel line adapter you must turn on both petcocks and have no reserve (90 degree aftermarket petcocks are required for clearance.) I guess they can work, but I cant recommend them. I was really looking for some Mk2 Amals when I bought the Miks.
Later I found a set of new Concentric bodies and reverted to them. The bike was running quite well when I sold it.

Interesting David, how we've followed the same path for such a long time (except for your abandoning your A65!). Although I must admit, my devotion to my A65 is waning a bit since I've acquired my 2007 Bonneville Black, a marvelous machine, and now my main ride. I recently did a "hone and ring" job on the A65, leaving the piston clearance at .006-.0065". Now the engine is noisier and vibrates more than before. Indications are I may have to bore it to +.060, and this time I should probably check out the lower end. And a re-bore will necessitate crank balancing again. Unfortunately, the mechanic who has done my last three A65 lowers has died. All in all, I'm not sure I have the gumption to go all through this once again.

But I can ride the A65 a while longer, saving it for "special events" (shows, swap meets, etc.) and an occasional romp around the 'hood, and I can see how it runs with the Amals.


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Originally Posted by DMadigan
Bacon gives for the Lightning 30mm carb main jet:
'68 190
'69/'70 180
'71/'72 200

The Thunderbolt 28mm carb main jet:
'68 230
'69/'72 230

The '68 Hornet and '69/'70 Firebird had the same mains as the Lightning but the '71 Firebird Scrambler have 220 mains.
Possibly the '71/'72 Lighting had larger mains to reduce warranty claims due to more WOT riding.

Thanks, and this agrees with Alan's and Phil's direct experience. I think I'm good to go.

This polishing is just with fingers and rags. I could probably do more with Dremel tools, but three evenings of polishing is enough - not necessarily museum quality, but respectable. Time to put them together.
[Linked Image from i.postimg.cc]


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The only thing I would worry about with shiny carbs is its ability to disipate heat... though they don't look "polished" so will likely be ok.

Like David P, with pre-oif silencers I found anything bigger than a 180 to be too big, didn't have much detriment on the running but it ran its fastest at WOT with the 180. Since then fuels have changed slightly and to be honest so have my bikes somewhat and im finding bigger main jets suit my bikes running. Though with non standard cams I wouldn't take that as gospal for anyone elses bike.

Dave M could be right about the warranty claims, I have yet to try anything different. I think the 69 on Firebirds with the 1 3/8" pipes make much higher BHP and torque at a higher RPM??


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Mark Z Offline OP
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You were right NickL in stating that the Mikunis "look crap".
[Linked Image from i.postimg.cc]
I haven't started the bike yet. The right carburetor is overflowing. The shutoff needles are new, so it probably just needs cleaning. Flushing it with the drain plug out didn't fix it, so I'll have to drop the float bowl.


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I got the right side carburetor to stop overflowing tonight and ready for startup tomorrow. I cleaned the shutoff needle and seat with WD-40, although there was no visible dirt there. Then I noticed a bit of dirt inside the hollow fuel banjo bolt - I almost missed that, and that could have been the source of the problem. Anyway, it's fixed now.

On a side note, a quarter, or better yet, a washer just a bit thicker than a quarter, fits nicely into the slot in the plastic float bowl drain plug, and IME is the only tool that should be used to loosen and tighten the plug. Just mentioning this because I've seen too many drain plugs chewed up from using channel locks on them.


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Hi Mark, .When it comes to setting idle mix, blank off the inlet balance pipe, so the RHS doesnt mess with the LHS ,and visa versa, makes dialling the idle mix much easier, reconnect when done.


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Originally Posted by Mark Z
On a side note, a quarter, or better yet, a washer just a bit thicker than a quarter, fits nicely into the slot in the plastic float bowl drain plug, and IME is the only tool that should be used to loosen and tighten the plug. Just mentioning this because I've seen too many drain plugs chewed up from using channel locks on them.

i think its sized for a half crown.


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Originally Posted by gavin eisler
Hi Mark, .When it comes to setting idle mix, blank off the inlet balance pipe, so the RHS doesnt mess with the LHS ,and visa versa, makes dialling the idle mix much easier, reconnect when done.

Bigger fish to fry right now - they won't idle, I mean, at all. They have all the signs of clogged idle passage, have to tickle and give it throttle to start every time, even warm, mixture screws set to 3/4 turns out, runs great as long as I stay on the throttle, if I let go of the throttle for even a second and then rev it, it almost dies and then catches - any longer than that and it dies, period.

I tried cleaning out the (fixed) pilot jets with my torch tip cleaner, to no avail. Maybe swarf behind the pilot jet, how the heck do you get that out of there? I guess I should try a tiny drill bit before I remove and soak them in Chem Dip. Anyone know the drill size? I think I figured out once it's .016-.017" diam., I could bring my calipers to the store?


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on a Mikuni you can take out the pilot jet and see daylight through the immense hole that remains. easy to clean, no mixing chamber for mung to clog up . . . just sayin

try turning the air screws 1-1/2 turns out and see whether that changes anything.


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Drill size #78, essential tool.


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The welch plug and the 2x passages often get blocked meaning that no matter how clean the pilot jet is… it still won’t run right. Get some carb cleaner spray, remove the pilot air screw and insert the straw from the carb cleaner. Look down the threat of the carb and with the throttle open or slides removed. Spray!!! You should see 2 good streams squirt from the holes. Until it’s does that the idle circuit won’t work properly.

The one engine side of the slide is the pilot circuit. The one under the slide allows transition between the pilot half of the carb and the main Venturi. Which is why I think sometimes a bike will idle ok but then cut out when you open the slide (providing those carb settings worked fine previously)


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Originally Posted by kevin
on a mikuni you can take out the pilot jet and see daylight through the immense hole that remains. easy to clean, no mixing chamber for mung to clog up . . . just sayin.
Now now Kevin, I'm painfully aware of what I'm giving up here.
Originally Posted by kevin
try turning the air screws 1-1/2 turns out and see whether that changes anything.
Ran "best" (if you can call it that) with the air screws at 3/4 turns out. Another indication of a clogged pilot circuit.

Allan, thanks for a thorough description of the pilot circuit. I'm off to the store for a #78 drill bit - already have some spray carb cleaner (and a face shield!).


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Originally Posted by Mark Z
Originally Posted by kevin
try turning the air screws 1-1/2 turns out and see whether that changes anything.
Ran "best" (if you can call it that) with the air screws at 3/4 turns out. Another indication of a clogged pilot circuit.

Allan, thanks for a thorough description of the pilot circuit. I'm off to the store for a #78 drill bit - already have some spray carb cleaner (and a face shield!).

if it runs best with only half the air going in the past the airscrew, then i imagine youre right that the pilot system isnt passing enough fuel. you may find it difficult to buy a 78 at any ordinary source-- hardware and auto parts stores will have only much larger bits, around 1 or 2 millinmeters. you may find that hobby stores that cater to model builders or electric train people will have something on the shelf. failing that, ebay sells them.

https://www.ebay.com/itm/312994898373?hash=item48dff321c5:g:4FsAAOSwrrleRubT&amdata=enc%3AAQAHAAAA4Hq8lOrRKzoRPTAKrJBkUroEVSZQoexBgUYVUKTx4Vk2Kn5F7fB2S1hnt5Eds8eRCh8qVpa1PlPR0sx6WVCPh%2By%2FGLjy0ecN14CHoBH2c65Wp3j3n32jiHbBZMlYGSx80lAEPWmgL9kIK5YVJsGhqHOYQSoWZzFdhobYATtHq192tZIfOUU39fe%2FOkp%2BkgT8U1EV%2BQDOv21ogCvz1PwhKBvmPcfhTybEOqD0JdwqCKXSXpv%2BA4wUiJIw4unP%2F4s27omnGuAwdlg%2BJEhMwF2EDQT2HIWIDfBKJD6%2BMwub3DTW%7Ctkp%3ABk9SR_7uy5_qYA

but im not against amals. im in the process of trying to get my 441 to run as we speak

[Linked Image from i.imgur.com]


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Right you are Kevin, the smallest bit they had at the hardware store was a #60. I don't think I mentioned this earlier, but my torch tip cleaners are smooth for about 1 centimeter at the end, and there are no burrs or sharp edges, so I'll continue with those (the right one, that is, which is .017"), and try the carb cleaner flush.

But first I have to get them off (which is a bit more involved than the spigot-mounted Mikunis), and I have to go back to work tomorrow.


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If you cannot find a 0.016" (#78) drill (McMaster has them) bit you can use music wire from a guitar store. A little handier since they can reach the bottom of the slide floor. Drils are only 7/8" unless you get an extended length 1-1/2".

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Using adhesive bond the #78 drill into a WD40 flexible "straw".
#78 is 0.016" so if you use a 0.017" tip cleaner you will have an oversize pilot jet.
A 17 thou hole compared with a 16 thou hole has 13% larger cross sectional area so if you use a 17 thou tip cleaner you will f**k up your [pilot jet.
When using the #78 drill then twist the drill bit so that you drill all of the crap out of the hole.
The crap is tenacious and needs actually drilling out.
Think of it as a solid wall that you need to drill through.
Just my two cents worth of course.

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Tridentman, I got the carbs off the bike tonight and explored further with the torch tip cleaners. You are right, the .017" cleaner does not fit into the jet. The next smaller size is .014", which does fit. But I'll start checking out local hobby shops for that #78 bit.

What still troubles me is that long passage from the pickup hole (where the removable pilot jet was on earlier carbs) to the main jet. There's no way to get in there with any sort of tool, so I think soaking and compressed air may be required.


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There is a "Welch " plug in the roof of the float chamber, if you have a spare welch plug, you can remove the old one by drilling a wee hole in the middle , screw in a self tapper and pull the plug, beneath it you will have access to the mung pit/ pilot mix chamber . I have cheaped out before and refitted the drilled plug with a dab of epoxy, now have spares, Burlen sell them, should come in the rebuild kits. In the roof of the mix chamber you can see the bottom of the venturi delivery jets, at the side and rear you have access to air and fuel drillings. Since I got some #78 drills I no longer bother opening this up, somethings are best left unseen.

Last edited by gavin eisler; 09/19/22 10:34 pm.

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Originally Posted by Tridentman
Using adhesive bond the #78 drill into a WD40 flexible "straw".
#78 is 0.016" so if you use a 0.017" tip cleaner you will have an oversize pilot jet.
A 17 thou hole compared with a 16 thou hole has 13% larger cross sectional area so if you use a 17 thou tip cleaner you will f**k up your [pilot jet.
When using the #78 drill then twist the drill bit so that you drill all of the crap out of the hole.
The crap is tenacious and needs actually drilling out.
Think of it as a solid wall that you need to drill through.
Just my two cents worth of course.

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You should be able to get #78 drill bits from any good engineering supply store.
That is where I get mine-- I think I got 10 for about $12---but that was several years ago.
If you want to really hit the problem on the head then drill out the plug opposite the pilot jet and tap it 2BA.
Then take any old pilot jet, cut off the end and use the part with the screw head to blank off the port.
Then you can take out the plug every time you have a problem and really get it clean.
If you dont want to do that then use the #78 drill and use an aerosol of carb cleaner to flush out the ports with some force.
Do it several times and eventually you get it clean.
Best of luck!

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Ok, so I soaked the carb bodies in Berryman's, triple rinsed them in water, and blew them dry with compressed air.

Now, on one carb, I can get carb cleaner to squirt out the two tiny holes in the venturi by pumping it in into the air mixture screw hole. I can also get carb cleaner to squirt out those two tiny holes by pumping it into the fuel pickup hole in the ceiling of the float chamber.

On the other carb, I can get carb cleaner to squirt out the two tiny holes in the venturi ONLY by pumping it into the air mixture screw hole. That is to say, I can't get the carb cleaner to squirt out anywhere by pumping it into the fuel pickup hole.

That tells me, on carb #2, the blockage is between the fuel pickup hole and the pilot jet. I guess that means the welch plug will have to come out.

I've never removed a welch plug before; I believe the process cited above was to drill a hole in it, insert a self-tapping screw, and pull. How hard are these things to get out? Also, will the hole vacated by the plug be on the feed side of the pilot jet?

Tridentman, I will also consider your idea of drilling and tapping the plug opposite the air mixture screw, but removing the welch plug sounds like less work, if it comes out relatively easy.


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Originally Posted by Mark Z
Ok, so I soaked the carb bodies in Berryman's, triple rinsed them in water, and blew them dry with compressed air.

Now, on one carb, I can get carb cleaner to squirt out the two tiny holes in the venturi by pumping it in into the air mixture screw hole. I can also get carb cleaner to squirt out those two tiny holes by pumping it into the fuel pickup hole in the ceiling of the float chamber.

On the other carb, I can get carb cleaner to squirt out the two tiny holes in the venturi ONLY by pumping it into the air mixture screw hole. That is to say, I can't get the carb cleaner to squirt out anywhere by pumping it into the fuel pickup hole.

That tells me, on carb #2, the blockage is between the fuel pickup hole and the pilot jet. I guess that means the welch plug will have to come out.

I've never removed a welch plug before; I believe the process cited above was to drill a hole in it, insert a self-tapping screw, and pull. How hard are these things to get out? Also, will the hole vacated by the plug be on the feed side of the pilot jet?

Tridentman, I will also consider your idea of drilling and tapping the plug opposite the air mixture screw, but removing the welch plug sounds like less work, if it comes out relatively easy.

Before you remove it, see if you can find someone with a sonic bath, put some of your berrymens solution in that. WD40 seems to break down fuel quite well also.

The welch pool itself cannot be blocked if both transfer ports flow when sprayed from the air bleed side. So the issue I believe still remains in the pilot jet.

Also try not rinsing the carbs out with water. Water has a higher viscosity and density than fuel has. If you want to clean out the berrymans, stick with a carb cleaner, brake cleaner or acetone which will disperse the water and evaporate.


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Allan, it sounds like you're saying that the welch pool is downstream (with respect to the fuel) from the pilot jet. And yes, that means removing it will accomplish nothing.

It also occurs to me that spraying carb cleaner into the air mixture screw hole is not a valid test for the pilot jet or the feed side of the pilot jet, unless I can see carb cleaner come out the fuel pickup hole, which it is not. (It IS on the other carb.) I believe the pilot jet itself is clear, and the problem is in the passage between the pilot jet and the fuel pickup hole in the ceiling of the float chamber.

Now, drilling out the plug opposite the air mixture screw as Tridentman suggested would provide access to the feed side of the pilot jet, but I will try some more soaking and/or flushing first.

It seems that, after soaking in the Berryman's, dropping the entire basket into a bucket of water is the only way to safely handle the parts. After that, I can continue to flush with carb cleaner or WD-40.


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have you tried compressed air?

you cant hurt anything with air pressure, and it can loosen stuff that a spray can will not.


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Welch Plug listings on Burlens page https://amalcarb.co.uk/catalogsearch/result/?q=welch+
Not sure what the correct plug is for the 900 series carbs, possibly the listing with 9 as the first digit?
I spoke to Burlen on the phone when I got spares. Sadly these are not shown on the AMAL / Burlen exploded diagram.

The passage from the fuel pick up to the mix chamber has two 90 degree bends, poke with flexible wire/ monofilament , soak in vinegar and blast with compressed air. I boiled my carb bodies in Rhubarb leave juice ( dilute oxalic acid) that seemed to clean the internals well.
Pulling the welch plug will allow access to each end of the passage.
50lbs breaking strain fishing line is flexible/ stiff enough to probe passages with some finagling.
Best tool for the job would be an ultrasonic bath .

Last edited by gavin eisler; 09/24/22 6:40 pm. Reason: splingel

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