MKIII slightly lean

At top end. Search turned up many different opinions but generally it suggested going to 260 main jets although some say too rich.

Stock engine, original airbox, peashooters, no exhaust cross-over. Stock needle/jet, relatively new, leanest needle position with good acceleration and mid-range.

Suggestions?

Ta.

Stock engine, original airbox, peashooters, no exhaust cross-over. Stock needle/jet

My MK2a runs this set up which is identical to the STD MK3 except the peashooters and had to go from 230 to 260 when I got the bike as the previous owner had fitted the peashooters but the bike would not pull at full throttle, 260's cured this. 260 may be too high but not by much as 230 is definitely too lean.

A couple of thoughts:

What are the engine symptoms when trying to run at full throttle; surging, hesitating?

A fuel flow measurement from one tap open into a measuring cup for 1 minute may be informative.
Very well could be a fuel supply demand issue.

Have you made any changes or done any work on this bike lately that might have affected the mixture ? If not and you are just getting the bike to run right, could you be seeing the effect of more volatile winter gas ?

Greg

Some fuel taps cannot flow enough to meet an engine's demand at higher speeds, either from poor design, or because of a defect. This was a fairly common problem during the 1980s with some brands of fuel tap having a rubber insert in its valve ... the inserts tended to rotate slightly and impede fuel flow. (These days I favor the BAP taps).

Could also be due to a blocked or partially blocked gas cap vent.

As Bob Z has already suggested, check the amount of fuel delivered during a one minute test. Also, try running your bike at high speed with a full tank and both taps open. If no change, pop open the gas cap (being very careful not to let gas esacape ...)

.. Gregg

JimNH said:

At top end. Search turned up many different opinions but generally it suggested going to 260 main jets although some say too rich.

Stock engine, original airbox, peashooters, no exhaust cross-over. Stock needle/jet, relatively new, leanest needle position with good acceleration and mid-range.

Suggestions?

Click to expand...

How have you tested the WOT mixture?

From an older post:

Using a Interstate tank with 5 gallons of 92 octane inside, clean screened BP fuel tap, clean 1/4" fuel line, good performing 32mm Amal concentric with removable bowl plug I got:

9 oz or 250 ml in one minute.

Ta.

Bob Z. said:

From an older post:

Using a Interstate tank with 5 gallons of 92 octane inside, clean screened BP fuel tap, clean 1/4" fuel line, good performing 32mm Amal concentric with removable bowl plug I got:

9 oz or 250 ml in one minute.

Click to expand...

250ml/minute sounds rather marginal. How much less flow when fuel in the tank is down to half a gallon?

Quick calculation: 9 ounce/minute = 4.2 gallon (US)/hour

If running 100 miles/hour that would give about 24 miles/gallon

So, 9 oz/min should be adequate fuel flow for most engines

Thanks gentlemen,

The bike was parked for 20+ years. Carburetors were cleaned including the idle passage. The symptoms I experience is that on high speed running it makes a little better power at slightly less than full throttle which I take as a lean condition. Now I don't remember it having that many years ago but at that time I had a GS1150 that I used for spirited riding and may not have noticed it. I did install new fuel taps and it's possible that they are more restrictive. A good test would be to try it with both taps open. It's too cold now and I'm just wondering if the general experience is that larger main jets are necessary as I will replace them while I'm making some other changes like sleeving the master cylinder.

If there's no consensus I'll leave it alone and try both taps open and a throttle chop if that's no help.

JimNH said:

Stock engine, original airbox, peashooters, no exhaust cross-over.

Click to expand...

850 'A' and MkIII models with the annular discharge bean can exhaust system had smaller main jets (220-230) when compared with the non-'A' peashooter equipped 850 models, so it's logical to assume that an increase in main jet size (up to 260) will be required when peashooters are fitted in place of bean cans.

Bob Z. said:

Quick calculation: 9 ounce/minute = 4.2 gallon (US)/hour

If running 100 miles/hour that would give about 24 miles/gallon

So, 9 oz/min should be adequate fuel flow for most engines

Click to expand...

Ok, my marginal isn't what you call marginal. I'd like fuel starvation to be an impossibility.

Full throttle at max revs: are you getting much more than 24 miles per US gallon?

And that flow is with a full tank.

An over advanced ignition can give the same symptoms even lean looking plugs. You could try a couple of degrees off the ignition.
Just a thought.

Dave

We's better slightly RICH than Lean .

Stuff the book .

Commando was 220 to 270 with sfa difference , because of the lousy half assed excuse for an ignition coil . I believe .
At 100 + sustained the entire insides of the exhaust system was WHITE , as were the plugs etc. Though they wernt
melting the edge of the center electrode ( which is indicative of over advance ) Points Ign. 31 BTDC timing on 750
2S cam and Mk IIs . 8.7 : 1 Comp .

If there were real petrol , youd run the center insulator yellowish and outer Chocolate , for power / cooling / tourque .
if it took 500 main jets . The Bonevel took 270s in the monoblocks . 1 5/32 though the valves wernt ( quite ) as big as JoMoCo ones ,
the exhaust was .

Jetting depends oon altitude , Gas ( that's why they went to low comp on the 850s - international touring melted the pistons on 3rd world
60 octane watered down back street currie muncher petrol )
your right hand & rideing technique
( Ex CV carb Honda Boys have no inkling of ' Milking ' the throttle for optimized performance . They think WFO is fastest . )
and your EXHAUST SYSTEM . A free Breathing Exhaust is going to be a few up on the mains in comparison to a quiteish one ,
and several / no relation - to a silent one that breaks & chokes all the pulses .

old farts used to vary the tailpipe length on straight pipes to vary the powerband . hot Roders put crayon or paint stripe down the pipes
to find the ' pulse length ' to know where to cut it . Doing this ( stripe ) would tell you if its OFF at least . Which'd affect jetting .

Silly plastic Air Box silencing ears , etc etc & so on ALL influence it . So up to 280 ( 200 to 280 ) COULD be considered normal ,
from a choked wet rag set up to a overpiped gung ho one .

Keep a eye on the centre electrode for indication off correct advance .

Be interested if someones put a Mk II on the DYNO & varied the Full Advance bit at peak HP Revs , and its influence there over what range .

seeing you asked . :lol: :wink:

Jeeze Loouise, reading these posts make me chuckle a bit. These engines have like what, 50 HP or so? Most of our lawn mowers(or snow blowers) are working harder than our Nortons.
Jim, our bikes are similar, though I still retain the original balance pipe in the exhaust I have forgone the original airbox for a K&N unit ( the kind that will stop sparrows at least, the type that hobot speaks so highly of). The bike came with 230 mains and is now, and for the past two decades been running well with 240's. Unless I missed it in one of your posts you never mentioned your current jet size.
Interestingly enough I have felt that top end power has been down a bit and confirmed to me at least with the same test you did with closing the throttle a bit and gaining 'pull'. Classic too lean confirmation.
Ethanol has a bit more oxygen in it than 20 or 30 year old unleaded so a bit of leanness can be accounted for there. This next spring I'm going to start with 260's work down to 250's and see what's best.
Winter in NH is a bitch, all we can do is think up things and try them out in the spring.

Biscuit said:

Jeeze Loouise, reading these posts make me chuckle a bit. These engines have like what, 50 HP or so? Most of our lawn mowers(or snow blowers) are working harder than our Nortons.
Jim, our bikes are similar, though I still retain the original balance pipe in the exhaust I have forgone the original airbox for a K&N unit ( the kind that will stop sparrows at least, the type that hobot speaks so highly of). The bike came with 230 mains and is now, and for the past two decades been running well with 240's. Unless I missed it in one of your posts you never mentioned your current jet size.
Interestingly enough I have felt that top end power has been down a bit and confirmed to me at least with the same test you did with closing the throttle a bit and gaining 'pull'. Classic too lean confirmation.
Ethanol has a bit more oxygen in it than 20 or 30 year old unleaded so a bit of leanness can be accounted for there. This next spring I'm going to start with 260's work down to 250's and see what's best.
Winter in NH is a bitch, all we can do is think up things and try them out in the spring.

Click to expand...

Eureka! That’s it! Twenty plus years ago there was no alcohol in the fuel. Being 10% ethanol means there’s less energy in the fuel therefore the jet size will need to be increased. Thanks for kicking my brain into gear.

LAB also make a good point that the less restrictive exhaust comes into play.

My timing is just under 28 degrees of advance with a strobe (about 27.5 and I figured close enough).

260 main jets transformed my mk111, stock air box, pea shooters, balance pipe.

michael levato said:

260 main jets transformed my mk111, stock air box, pea shooters, balance pipe.

Click to expand...

Thanks for that; I was vacillating between 250s and 260s. I'll start with 260s.