Fullauto head on a 750

QUESTION...I built a 750 with the best of everything. The motor has a JS 1 cam and a Fullauto head. I installed BRAND NEW 32 Amals and cannot get them to idle under about 3000rpm. I have used Amals for years so I've done all the usual things. As far as I can see, there is no possibility for air leaks in my installation. However, the idle problem can only be from a big air leak. Same on both sides. I can turn down the slide screws to almost nothing and it barely affects the idle. If I didn't know better I'd think these were the dreaded MK2 amals. I then removed the carbs and installed a single 34mm Mikuni. All the problems went away as soon as it warmed up. With no tuning I could idle it down reliably to 1200 rpm. Motor performance was excellent. So it's the carbs.
Has anyone else encountered a bad set of carbs? I even double checked the dumbest possibility...that I might have reversed the air and idle screws. Of course not. While the idle screw and brass jet were out I could see through the carb, rather a big hole.

2ND...low speed performance of the fullauto head. I am expecting some small differences such as jetting changes (mostly pilot) and different idle. Should I require a slightly higher idle?

I guess the usual things included cleaning any swarf, checking passages are open fitting 19 idle sets etc.

So can I add a new one I learnt only today. From a very experienced guy who was fitting two brand new 28 mm premiers to a Lagonda car. Pre war Rapier I think with a small ohc engine.

He found the very bottom of the slide passage had been drilled a bit oversize. So the slide was actually leaking. When he pressed down and sideways on the slide the car would idle.

Believe it or not he cured the problem by resleeving two brand new carbs.

seattle##gs said:

QUESTION...I built a 750 with the best of everything. The motor has a JS 1 cam and a Fullauto head. I installed BRAND NEW 32 Amals and cannot get them to idle under about 3000rpm. I have used Amals for years so I've done all the usual things. As far as I can see, there is no possibility for air leaks in my installation. However, the idle problem can only be from a big air leak. Same on both sides. I can turn down the slide screws to almost nothing and it barely affects the idle. If I didn't know better I'd think these were the dreaded MK2 amals. I then removed the carbs and installed a single 34mm Mikuni. All the problems went away as soon as it warmed up. With no tuning I could idle it down reliably to 1200 rpm. Motor performance was excellent. So it's the carbs.
Has anyone else encountered a bad set of carbs? I even double checked the dumbest possibility...that I might have reversed the air and idle screws. Of course not. While the idle screw and brass jet were out I could see through the carb, rather a big hole.

2ND...low speed performance of the fullauto head. I am expecting some small differences such as jetting changes (mostly pilot) and different idle. Should I require a slightly higher idle?

Click to expand...

Slightly off-topic - but, given your investment, I hope you've fitted waisted 3/8" bolts to stop the crushing destruction of your new head.
Cheers

Cables to short or wrong length needles, if the cables are to short the slides won't drop down enough and the needle compare to the length of the old needles, I have been caught out when replacing my old needles for new ones, the local British bike shop gave me the wrong needles and were shorted than the old ones and couldn't get the idle down under 3K rpm.

Best of everything and Amals?

Can't help with that, but I have an old set of re-sleeved 930's and a few throttle cable sets that would probably work like they are supposed to. Of course, they are old and constructed of silly putty metal that warps if you look at them too long.

Did you use Jim's lifter blocks and BSA lifters, or have your old lifters radiused?

I'm starting to think it sure would have been easier to have my old lifters radiused. I'm not sure I can drill and tap the barrels to secure the JSM blocks, so might end up with the barrels back in machinist jail. By the way, (off topic) everything worked out with Maloney.

Needles not properly matched to the spray tubes.
3 ring needle to standard spray tube.
4 ring needle to cut away spray tube.

robs ss said:

Slightly off-topic - but, given your investment, I hope you've fitted waisted 3/8" bolts to stop the crushing destruction of your new head.
Cheers

Click to expand...

I used the waisted bolts. What do you mean by "crushing destruction of the new head"?
I will double check the needles. I know they aren't the 850 type. And the spray tube is the correct one.
The bike is not mine unfortunately. I built it a few years ago and it was running beautifully for it's owner until an inattentive LOL got him in an intersection. Fortunately her insurance paid well and he got the bike back. We pretty much replaced everything except the motor and wheels. To add insult to injury, somebody swiped his New Amals and some other Norton bits so the known good pair of carbs were gone.

I used the JS lifter blocks and BSA lifters and had a machinist I know in Seattle set them up for the set screws, done while I waited. But I know him. He's done two sets for me, excellent work. If you need his number get ahold of me. He is located about 65th and I-5.

Thanks for the offer but I have a crate full of old Amals I use for spare parts. The most valuable parts are the #2 BA float bowl and top screw.90% of the rest is useless metal. Come to think of it, it's time to do some house cleaning.

It is a 2-groove needle. 106 needle jet

seattle##gs said:

I used the waisted bolts. What do you mean by "crushing destruction of the new head"?

Click to expand...

I meant that the overly strong 3/8 bolts will cause the aluminium around these bolts the creep, effectively crushing the head in these areas over time.
Cheers

OP says no leaks, but was this tested via spraying with carb cleaner etc on all potential joints? What about a hairline crack in a manifold, disappearing when moving to single carb one into two type?
What about carb balancing via guages or manometers?

My mistake on the rings.
There are 2 ring and 4 ring needles.
2 ring matches up to straight spray tube.
4 ring matches up to cut away spray tube.
May not be your issue but I have experienced the 2 ring on a cut away spray tube.
Gave many of the same symptons you're experiencing.
Brand new set of carbs set up incorrectly. It was many years ago.

The amals are bolted to rubber manifolds so they can be spigot mounted. The alu spigots are from JS. I sanded off the raised o-ring on the manifold and used the usual amal flange gasket between the two. yamabond was used.

#2 To insure there was no air leaks at the rubber junction I used a good dollop of RTV around the joint. Very messy. More silicone around the crossover tube pipes. New rubber hose. Snug fit.

#3 I surfaced the alu manifolds on glass and sandpaper and used more yamabond on that joint.

#4 Each part was examined very closely for defects. None are visible.

I assure you that there are no air leaks in this area.

If anyone has a premier on the bench can you please take out the air mixture screw and see how big a drill bit you can pass through the hole. Remove the #19 pilot jet and look through the passage way. Mine has a rather large hole connecting the two. What size drill bit fits the hole?

I did not use carb cleaner for a diagnostic device. Always so-so luck using that method. It doesn't work every time.
I set up the carbs very close to each other on the bench. They can still benefit from a carb balancer but they are very close when I bolt them on.

When I was having running issues with two original non premier amals, re-sleeving did not cure. Went to premiers but still had issues found a good clean out of passages helped a fair amount, but only after using a carb balancer sorted issues fully. Took a fair amount of cable adjusting to get proper balanced flow, despite having descent mechanical balancing using rods etc on the slide movements. Maybe i'm just hopeless without the balancing guages. I suspect my original pair are fine.

Why wouldnt you want 850 needles with 32mm ports - wouldn't the head itself perform more like a RH4 than an RH1? I would think that to idle correctly, 850 needles on a FA head with larger ports would dictate that change? Then maybe dropping them and/or different jets to get the mix right?

From Jim's head flow testing thread you can see that the standard FA head flows a lot better than the stock 750 head.

I finished a street performance port job on a 28mm- 750 head today. Here is a graph of the tests.

The blue line is before any work was done. It had stock valves and worn seats. The original guides would only allow .450 lift so that is where the line stops.

The green line is the after picture. All the work was done from just before the guide to the valve. The 28mm section of the port was left untouched. Black diamond valves were installed on three angle seats.

The red line is the fresh 32mm combat head I tested yesterday just for a comparison.




Well I did a full competition port job on a Fullauto head today. Stock valves and 30mm port area. Reworked the bowl and guide area.
The light green line is the Maney stage three. Huge everywhere.
The dark green line is the Maney stage 2 with 1.615 valve and 33 mm port ,
The blue line is the fullauto with stock 1.5 valve and 30 mm port
The yellow line is the out of the box Fullauto.
The port velocity on the modified Fullauto is almost double the Maney stage 2.

And I just found this in another thread...more relevant.

A couple years ago when I was doing the design work for the Fullauto head ports I had a lightening strike that took out my flowbench computer. I could still test flow but I couldn't do the analyzing and charting. This morning I finally got the full system up and running again. I started out with intake flow tests on three Norton heads . None of these heads have been modified in any way and all were flowed using the same valves and a spigot type manifold with the same velocity stack.

The first line is the green line that is the lowest at .250 lift. It is a NOS 750 head with 30 mm ports.

The second line is the blue line and is a low mileage combat 750 head with 32mm ports.

The top two lines are white and green and are left and right intake ports from the Fullauto head



I dont think stock needle/jet setup for a 750 would work well for an 750 engine with a FA head, 32mm carbs and a JS1 hot street cam.

@seattle##gs I have no wish to insult you, as you're a lot brighter than I am.

But what pilot jets are you running on your Premiers?

I had to swap to size 19 because I couldn't get either bikes to idle when I replaced the carbs.

Both sets I bought were supplied with size 17 originally.

It has #19 pilots

seattle##gs said:

I used the waisted bolts. What do you mean by "crushing destruction of the new head"?

Click to expand...

He means he bought an Australian made Fullauto and has had a problem with it deforming under the head bolts. One can understand not being happy about that, whatever the cause.

I have an Australian made 77mm Fullauto (#101), supplied through Mick Hemmings in the UK at a keen price. It is on a 750 short stroke race engine, so it's been put on and tightened down quite a few times since 2014, to give it a number let's say 4/5 times a year, 30 to 35 including trial fits for valve clash/timing etc.

I have not used waisted bolts as yet, though I do have some from JSM, I have used standard style stainless from Middletons, then ARP bolts from CNW, and I have not had the problems Robs ss reports.

I am not the only one who has used an Australian made head with no problems. Robs ss isn't the only one who has reported issues, but I think it is rare, however Robs ss insists all Fullauto heads should be tarred with his brush. Which is getting a little tiring.

My understanding is that different batches were cast at different foundries and machined in different shops. It may be a batch or materials related issue, it may be something else. I do not know.

I am absolutely sure that new US made heads will not suffer materials or casting issues.

I also suspect the number of Australian made heads with issues is very small, but again, I do not know.