Slight 'pinking' when hot.

I have an 1974 850 MK2A that has a slight problem you gus may be able to help with.
It is setup with new amal mk1 carbs and a mk3 boyer (both new).
When I start the bike it needs the choke and takes a while, maybe two minutes, before the choke can come off.
The bike then seems to pull strong, for a while.

The problem is, when good and warm, and especially 2 up and loaded, it starts to 'pink' slightly.
I was pulling up a duel carrigeway (slight uphill) 2 up this weekend and seemed to hit a wall (proverbial one!) at about 90 mph.
There was more on the throttle but if I twisted more it just pinked worse, so I backed off.
I have Champion ny7 plugs in at the moment.
I also tend to put the higher (97) octane fuel in and also chuck abit of octane boost/valve lead additive every now and again.

Any advice would be greatfully received.
Stu

Back off the timing a couple of degrees for now, the marks in the primary cover can be out by 3/4 degrees both ways. As you are hitting a brick wall at 90 then either the main jets are too small or the needles are too low and need the clip moving down 1 or 2 notches, in which the timing can be readvanced once you have checked the marks are true..

Amal carburettors have to be tuned according to the procedure in most workshop manuals. It's just not enough for the seller to say it's correctly jetted.

Most of your symptoms point to lean running. Check float level.

Hi Stu,

My '72 combat is doing the same if not worse. Setting the timing back to 26BTDC helped and now trying some Lucas Oil Products (fits the British theme) octane booster. It has original Amals that have been sleeved by Mike Gaylord. I'm thinking about going up to 240 main jets from current stock 230. Pretty sure I'm a bit lean as the pipes are blued near the exhausts ports and the machine is getting 55 to 60 miles per gallon fuel economy.

I will be joining the NCNOC weekend in the Sierras with a ride over Sonora Pass above 9,000 feet. Think I'll wait until after than to change jets. Amazing how any grade of gas will work at that elevation.

Do you know what mainjets came with the carbs? It would be interesting to hear what they are. They are the easiest thing to check without pulling the tank, ect.. and everything you say points to them.I can't see suppliers fiddling with a special recipe for each application, so what most people are going to get is a ballpark starting point.

The carbs were supplied with either 230 or 240 jets. I remember looking and coming to the conclusion he had supplied me with jets for a 750, not an 850.
I replaced them with bigger jets, 260, I think. (Old age is a terrible thing, I can't remember!)
The carbs run with a K&N filter.
The right exaust is blue at the head for maybe 3" then chrome, the left is straw/blue at the top, so the right is slightly hotter, but not excessive.
I set the timing (boyer) at about 30dbtdc static.
I failed to set with the strobe.....when I got close to the 5000rpm the signal seemed to break up.

I will check the jet size tonight and also do a plug snap at full bore and see what it says.

One question: In tech. specs on this site it states 'needle position : top'.....does this mean with the needle higher (richer) or lower (leaner).

But the conclusion seems to be either to advanced or too lean. I will investigate both.
Thanks for the advice.
Stu

Stu
Interested in how you set the timing manually with a Boyer? Didn't think you could with any accuracy cos it's not as if you can lock it in full advance and check the spark as you would with points. Boyer advance curves vary hugely but if you say you're losing the signal at 5000 try 3500-4000 which will be around 26 and 28 deg for an analogue. (my old digital Boyer was 29 and 30 at these rpms).
If I'm honest I've never been completely happy with the needle terminology used other than middle!! Top I've always assumed is clip in top groove for weakest mixture. Can't help with an 850 but it does sound main jet weak if it stops accelerating at full throttle.

Hi Keith
This is what Boyer told me for my alalogue MK3 system:-

'Dear Sir, Strobing at the crankshaft, the ignition will be 7 Degrees short of maximum advance at 3000 rpm.
At 4000 rpm the ignition will be 2 degrees from the maximum advance timing.'

That would mean at 3000 it should be 24 deg btdc, and at 4000 29 deg btdc

To set up static you set at 31 deg btdc and line the screw in the rotor pickup with a hole in the stator plate, as per Boyer instructions. Just how accurate this is is anybodies guess.
I will try again with a stobe, and a mate to hold the bike still!
Stu.

Ok Got you, the initial get it running set up. On mine with the alternator rotor on 31 and the pickup magnets inline with the Norton script it was way retarded. Mind you if you got it strobed at 24 and 3000 that's more than good enough unless you have a Boyer that overadvances at 4k+ and they sometimes do. Try it on the wheels with someone holding/sitting on the bike and revving it, doesn't move as much! Good luck.

.....when I got close to the 5000rpm the signal seemed to break up.

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If you are losing the flash you could be losing the spark and or, you are using a poor quality strobe that will flash after the spark. These strobes are fine at low rpm but at high rpm the delay adds up and is significant.

Best of luck,
Cash

bigstu said:

I set the timing (boyer) at about 30dbtdc static.
I failed to set with the strobe.....when I got close to the 5000rpm the signal seemed to break up.




Stu

Click to expand...

Hi Stu,

Regarding the flash breaking up at high revs, are you using the bike's battery to power your strobe light? It'll do that on mine everytime. Gotta hook it up to the car or another bike, (not running).

Bob

illf8ed said:

My '72 combat is doing the same if not worse. Setting the timing back to 26BTDC helped

Click to expand...

I'm wondering if Combat owners are actually aware that the factory issued a service release that recommend ignition timing on Combats should be set to 26 deg. BTDC F/A with 250 main jets if lower than standard 97 RON octane (= around 92 pump octane in the US) fuel was used instead of the standard 100 RON (around 96 US pump octane)?

http://www.btinternet.com/~madmole/Refe ... ONPON.html

Keith1069 said:

If I'm honest I've never been completely happy with the needle terminology used other than middle!! Top I've always assumed is clip in top groove for weakest mixture.

Click to expand...

Yes, the needle grooves are numbered from top (weakest) to bottom (richest).

Yes, the needle grooves are numbered from top (weakest) to bottom (richest).

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Thanks LAB but if I'd read what I'd written or stopped to think I'd have realised that's not what I meant!! It's when positions 1,2 &3 are mentioned that I've never been happy with, so is 1 Top and 3 bottom groove? (2 is self explanatory for me!). It's academic but would be nice to clear that up.

Ah yes, I see what you mean, and let's hope that everyone who describes the grooves by number understands that the groove giving the weakest setting is generally regarded as being number 1.
http://www.jetdial.com/i/clip_positions.gif

Here is a quote from the Amal tuning notes: " if the mixture is still too rich with the clip in groove number one (nearest the top), the needle and needle jet probably needs replacing due to wear."