Need jet reccomendations

Been cleaning up the carbs and discovered why I think it might be backfiring... 220 jets. I hear that iwas supposed to come with a 230 in stock form. I think my Dad left his Rocky mountain roadtrip jets in it from his last ride at altitude because that seems lean. It has a very free flowing 2 into 1 exhaust already and I'm setting it up with a K&N filter soon. Can anyone reccomend a good jet size?

it is a 1975 850cc dual Amal 32mm

According to my manual:

Main jet - 260
Needle jet - 106

230 main was for the Combat (932), 220 for 750 (930)

Maybe things were different for the Mk III - my manual apparently is through Mk I

My needle is 106 but the main is 220 :?

Captain B said:

Been cleaning up the carbs and discovered why I think it might be backfiring... 220 jets. I hear that iwas supposed to come with a 230 in stock form. I think my Dad left his Rocky mountain roadtrip jets in it from his last ride at altitude because that seems lean. It has a very free flowing 2 into 1 exhaust already and I'm setting it up with a K&N filter soon. Can anyone reccomend a good jet size?

it is a 1975 850cc dual Amal 32mm

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My 850 had 240 mains in it when I got the bike. Standard airbox and mufflers.

The 850s fitted with the black box air filter and annular discharge silencers had smaller main jets - Mk1A, Mk11A and Mk111 all had 230 mains.

My Mk111 which uses the earlier airbox and silencers runs happily on the 260s as fitted to the Mk1s and 11s.

I've been up as high as 300s when experimenting with velocity stacks because I prefer to start rich and work down but that was a bit extreme .

Your non-standard exhaust is the unpredictable element here. The ideal would be to borrow a few different sets of jets before deciding but I would regard 260 as a minimum.

hi ben,if my memory serves me right riding at altitude weakens the mixture so i would expect larger jets,i dont think a non std exhaust will have an effect on the size of jets, but non std air filter will ,set it up as std and do some plug chops,

My mom's old Ford would backfire with the slightest exhaust leak. That's how we knew when it needed a new muffler. My 850 will backfire if the silencer clamp or the main exhaust collar nut come loose but it's just a "pop", not flames.

Main jets are set via plug chop from full throttle and examining the plugs. You're only on the mains from 3/4 throttle and up. If you can't reach 100 mph your mains are too small.

I would try 260's, 230's are way too lean for an 850 with open pipes. If it still backfires, get a cheap set of standard headpipes and peashooters off eBay. You need to rule out a sticky exhaust valve... that'll do it too.

Well. I just ordered some 260's. Got my fingers crossed

chris plant said:

hi ben,if my memory serves me right riding at altitude weakens the mixture so i would expect larger jets,i dont think a non std exhaust will have an effect on the size of jets, but non std air filter will ,set it up as std and do some plug chops,

Click to expand...

Coppied from the web....

Altitude- Again this is an issue of air density. At sea level atmospheric pressure is around 15 psi and as the altitude increased the atmospheric pressure decreases. Because less pressure is exerted on a measured volume of air as the altitude increases the air molecules are able to relax and they take up more space leaving less space for additional molecules. The higher the altitude the less air in a measured volume and therefore less oxygen present so jetting will have to be leaned to compensate.

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Ben
I would do a WOT plug chop test with your 220 main jets and check the color of the plugs.
http://www.hitchcocksmotorcycles.com/am ... nc_ht.html Here is a good read.
Hope that helps.
Regards,
CNN :mrgreen:

Hi Ben
Just done the carbs on my 1973 850Mk11a
Using Amal concentric mark 1 new carbs with a K&N filter, like an oval shape which wraps over both carbs.
I think you may find 250 mains are standard. Everyone agrees about the needles.
If you are at altitude air jets less, which is why we gasp for air. Therefore you are correct, you would have to install smaller main jets at altitude.

I have found I have settled at 260 mains, with 250 being the minimum using the K & N..
First synchronise your carbs by setting the slides at the same height (various methods, I use a ball gauge), and MOST IMPORTANT make sure when you twist the grip the slides start to move at exactly the same time. Now fit the K & N..
Run at full bore for as long as you dare, then hit the kill switch at spped with throttle open. At least 20 seconds at full bore and at say 5000 rpm min. Pop the plugs out and look at the colour, brown to dark grey, but not light grey, is the colour. Always favour rich (dark), not lean (white). Adjust your main jet size to suit full bore.
Back off to 3/4 throttle and hold for same time (less scary)...Now adjust the position of the tapered needle which is located by a circlip onto your slider. move up for richer, down for leaner.
Now set your idle, and go through synchronise again.

The important thing is run at full bore to check main jets, 3/4 bore for Needle position, 0 to 1/4 bore for idle jet.
Good luck. Stu.

Thank you everyone! About this concept of a plug chop (new to me, as I'm a Jeep guy)... I'm guessing you have to start with new plugs, but do you need new plugs each time you test?
Mine have only been run for about an hour, will they work?
How about at each of these (do you put in new plugs for each test)? : full bore to check main jets, 3/4 bore for Needle position, 0 to 1/4 bore for idle jet.

Thank you
-Ben

You can use the same plugs. Note the area you need to look at to "read" the plug is down near where the insulator meets the metal body of the plug. There should be a 1/16" wide band that indicates how the bike is running. Magnification is needed to really see it The rest of the insulator does not matter. I'm no tuning expert and I never knew about this before... but there was a recent thread that gives links with all the details. There was an article by Gordon Jennings (my hero).