Premiers

Hello all,
ive fitted Amal Premiers on my mk11a and I'm having problems. On first fitting they were a big improvement over my old ones, but now they keep missing and backfiring below 2000 revs. Above this the motor runs fine. I've cleaned the pilot jets out but it keeps coming back whenever it feels like it.
Ive cleaned the fuel pipes and there is no sign of any dirt in them or the carbs. This is the third time I've cleaned the carb by compressed air and guitar string, plus its only the left side carb, which is strange as they are fed from the same fuel line.
When you release the throttle it ticks over perfectly, now I would have thought that if the pilot jet was blocked then it wouldn't idle.
If you blip the throttle fast it revs freely with no missing, but if you gradually increase the throttle it will miss and bang, even cutting the engine out.
The pilot jet blanking screw on these carbs is only for sealing the jet and does not affect any mixing or does it? I screw it all the way in, ( the pilot jet blanking screw that is, not the air screw adjuster ).

The other thing with the left one is, its running lean according to the plugs when pulled after 5 minutes at 4K revs on a run ( this is the normal range for me as it is 80mph), the right side is perfect. If I raise the needle one notch would this richen the mixture.
The carbs are in perfect balance.
Its very annoying as I like to keep the motor running about 1500 revs when I come to a stop so as to avoid it stalling, as they do when you least expect it, and as I`m only 11.5 stone its a circus act to kick over without being on the centre stand.

It has new Boyer ignition, good sparks. New battery. New valves, guides and springs ( done by Mick Hemmings), new rings and hone.
Any help appreciated as I`m taking the carbs off again this weekend for another look.
Thanks
chris r

Chris, I chased a carb problem just like yours. I only had a single Mikuni though. I was chuffed when I found the pilot jet 3/4 blocked... And a tad peeved when cleaning it made no difference.

Long story short, my carb problem was a broken wire on the Boyer backplate! I have since learnt this is a comon problem, I would therefore urge you to check it out carefully.

Fast Eddie, I thought about that, but this problem was there when I had the old boyer and I've treated it to the new micro digital system. I've changed all the original mk4 bits to the new one.
I will check it out anyway, just in case lightning strikes twice. Which part exactly was at fault, the solder joint on the plate?
Thanks
chris r

Yes Chris, broken soldered joint on mine. You'll have to be unlucky if its happened again! But maybe still worth a look...

I too had a similar problem ... I ran a dedicated ground from head back to battery and installed a new mark 4 boyer .... has been great these last few years .... so your problem sounds electrical from my experience ... please post up your fix when you get to it , thanks
Craig

Thanks but why would it only misfire below 2k and not miss a beat above this and at tickover ?
Also if I put full choke on when it misfires this usually stops it or stalls it , Plus when I start it up from hot it still needs full choke to start. This is probably my lack of weight.
From cold I flood the carbs , turn it over once then ignition on and kick, it starts first time.

blacklav said:

Thanks but why would it only misfire below 2k and not miss a beat above this and at tickover ?
Also if I put full choke on when it misfires this usually stops it or stalls it , Plus when I start it up from hot it still needs full choke to start. This is probably my lack of weight.
From cold I flood the carbs , turn it over once then ignition on and kick, it starts first time.

Click to expand...

Specific revs = specific vibrations = finding a specific weak spot / natural harmonics in electrical connections somewhere.

I've no answer for the choke effecting electrics though !

Thanks, I will post an update on sunday.
chris r

Did those new carbs have all the flat spots milled from Amal? I had to grind those flats on the new Amal premiers in both places like the originals. If you missed that or if they aren't ground down enough then one carb will tighten up but the other one won't Then they will hit each other in the middle between them and keep it from seating tight and it will lean out and backfire. Just something to look for and thought I would mention it.

Misfire at same rpm in different gears usually indicates an electrical problem. Misfire at same throttle position in different gears usually indicates carb problem. Put a piece of masking tape next to the throttle and mark 1/4,1/2,3/4 and WOT. See if the problem is consistent at one position. Different parts of the carb need to be addressed for each position. Basically idle to 1/4 = idle jet and slide cut away, 1/4 to 1/2= slide cutaway and needle jet size and clip position, 1/2 to 3/4 =needle jet size and 3/4 to WOT =main jet size.

You might also check the fuel level's. I've put on 2 sets of Premiers and they all needed the levels raised. Amal says .21 +/- .040 below the top of the float bowl. Here's my u-tube rig made out of some hose, a brass barb fitting and a spare drain plug.

Hi Chris
Sounds like I have had exactly the same problems!
Erratic tick over and poor firing from one cylinder, right in my case, after fitting new 30mm premier amals to my 72 commando.
I made the mistake of doing multiple jobs at one time so couldn't be certain of the cause of my problem.
After systematically, changing plugs, ht cables, coils, plug caps, battery, fitting Pazon instead of suspect Boyer, checking all wires, checking compressions, checking exhaust tightness, leaks around carb manifolds, fuel flow, etc
Still could not cure problem.
Fitted old carb on Right cylinder and things improved, so I reached the unthinkable conclusion that the new carb was faulty. So I sent it back to Burlem for them to check.
It was indeed faulty, inside the carb is a metal plug. this plug was leaking causing air to be sucked into the pilot chamber and cause the erratic running at low revs. Burlem resealed the plug and returned the carb and my commando is now running ok.
Since then, I have seen another thread on this site with someone having the same problem. They fixed it themselves by tapping the plug back into its hole.
You could probably prove it one way or another for yourself by tapping the plug with a suitable drift or applying some fuel proof sealant around the plug joint.

Pretty annoying that we should have to go through all this hassle when it is not unreasonable to expect to fit new carbs and have instant improvement.
Poor quality control and testing by Amal I think
Let us know how you get on.
Cheers
Steve

Any pics of this plug?

Hi Steve,
Interesting answer, which plug are you on about? Is it the pilot jet screwin plug on the rightside, or the pilot air adjuster screw on the left?
I will check them both tomorrow.
Many thanks
chris r

Hi Chris,

I don't mean the pilot screw or the removeable pilot plug on the inside of the carb body.

My new carbs are on my bike so I can't photo them, so here are a few photos of my old carbs.

If you remove the float bowl and look up you can see a round brass plug.

Hers is a link to a photo.

http://i1318.photobucket.com/albums/t64 ... cszmml.jpg

This brass plug is obviously used to fill the hole that has to be there to enable them to drill the very small hole that draws air into the pilot chamber.

Here is a pic of the hole from the intake side of the carb.

http://i1318.photobucket.com/albums/t64 ... 2sdswx.jpg

It was this plug that was leaking on my new carb. This leak causes an excess of air to be erratically sucked into the chamber.

I don't know how Amal sealed it , but like I said, I have seen another thread on AN where someone tapped the plug home a bit with a soft drift.

If you smeared some petrol proof sealant across the plug you could prove if this is your problem or not.

Hope the photo links work ok for you.

Cheers

Steve

Thanks for that Steve, Ive just had them apart and refitted before I saw your post. I`ll check the plug out today and post the results later, as its pouring with rain here.
Thanks again
chris r

Well I checked all the wiring for the boyer, all ok. I removed the premiers again and checked the plug Steve told me about. They look like a shiny soft alloy and a very good fit, so just to make sure I used a flat2mm dia punch to flatten around the edges, spreading the alloy onto the surrounding casting. Took it outside and started it up, ran it for 5 minutes at various revs to get it hot - it usually misses when it warms up- and up to now , no misfire. Its a bit early yet to celebrate but once the weather pics up and I put some miles on it, I will find out if its worked. But for now Steve`s info has solved it- I hope-.
If it starts missing again I will reply on this post, if I don't, then everything OK. AMAL are to get a letter concerning their quality control.
I will go for mukini`s next time or even wassels.
Many thanks to all,
chris r

candles are lit .... good for you .... hope this is end of the troubles .... it bites when new parts are sub standard
Craig

All OK.
Went to the Norton owners club AGM on Sunday, 150 mile round trip. Never missed or stuttered once.
Thanks again all
chris r

Thanks for the feedback Chris.

Steve, that was a pretty impressive diagnosis sir! That is something I would never have thought about being a likely issue. I wonder how many Premier owners are suffering this and chasing their ass looking in the wrong places?!

A very poor show from Amal.