Keihin Carb overhaul. Jetting suggestions requested

[London_Phil]

HI All

As the thread title says, Im going to have to pull the carbs due to poor low throttle running.

The carbs were bought and fitted a while ago, to replace some worn Mikunis, and I just put them in and ran them , I'm not even sure if they were for a Steamer,  so I've no idea what the current set up is, except to say its been running pretty well for a while .

Now however, even when ridden for a few miles, the 1st quarter throttle response is very poor, and tickover is laboured.
When cold, will only run with choke fully on, and the bike revs wildly.
Bike has Venom exhausts with the Type Approved baffles and oem airbox and filter.
My riding is quite sedate, with mostly commuter traffic, and A road bimbling.
Not comfortable revving over 5K, upsets my feels, as Im used to big twins...

Looking for some suggestions on jetting


I have a ratty spare set of carbs from a late steamer with 104 mains and 38 pilots, which I assume is orig spec for UK.
Looking for best running setup, not ultimate power.
Wondered if a Dynojet kit would be any use.

Appreciate that everyone has different opinions, but its not my area of expertise, and even though I used the search function, it seems the Keihins are less common
Any thoughts?

Regards

Phil

[GavD]

Phil,

When I was getting mine up and running again, I found my Keihins had 38/98's fitted. When I had to ditch the original cans due to them rotting out, I bought some venoms and slung the carbs on to see how it ran. It was not good lower down the rev range, it didn't rev cleanly at all. (The bike wasn't road legal at this point so most of my assessment was carried out in the garage with the odd little blast up and down the road for confirmation).

Anyway, after reading Mustangs posts on jetting, I opted for a 40/105 set up to start with, and found it spot on. It revs well all through the rev range, and, if I'm in that sort of mood, I can sit in sixth and just roll the throttle on and off down to about 35mph before i feel the need to drop a gear. I am running a standard airbox setup.

From cold it would need 3/4 choke, Since then I have bought a brand new standard airbox as the old one had a number of cracks in it, and now it only needs 1/2 choke.

I'm getting 180 miles until the fuel light comes on, then about another 30 until I have to switch to reserve, so I reckon I could get 240 miles out of a tank which is about 45mpg.

Edit; I just remembered that I also shimmed up the jet needle, as per mustangs suggestion using a small brass washer, as the keihins don't have the option of moving a circlip up/down a notch. think this is aimed more at the midrange, I did this later on, but I don't remember feeling a big difference with it.

[ssevy]

I have rebuilt the Keihins on my '99 Legend, which is basically the same engine we have in the Steamer. Following the suggestion of someone who had rebuilt a bunch of Keihins and factoring in the TORs I have on this bike, I used 42 mains, but would definitely go down a size to 40 if I were you, as my plugs are black and sticky. I left the stock 98's in there, but did switch out the stock needles for some Thruxton needles, which have a faster taper.
If you've mucked about with the Mikunis, you'll be delighted with the Keihins, as they don't require 50 different sizes of O-rings when you rebuild them!

[London_Phil]

Well, carbs are off, and although pilot jets look okayish, the pilot screws are pretty dirty.
So Currently I seem to be running 38 pilots and 98 mains
Pilot screws are in 1.5 turns.
Amazingly, other than a bit grumpy when left for a few weeks, its run ok, imho

think its probably time for a clean and rejet....

[GavD]

Phil,

I'm running the pilot screws at 2 1/4 to 2 1/2 turns out from memory.  That's with the 40 pilot jets.

[P3aK]

 

I also found mine had 38/98's when I bought it.

Went up to 40's pilot which made a big change for cold start and low revs. I initially had problems with 105 mains when I switched but that was sorted once I did the valve adjustments.

And yes 0.7-1mm needle shims need to go in (at least when the valves are proper).

2-2.5 turns on screws.

That's with 3-1 Venom exhaust and K&N filter for me.

[London_Phil]

Thanks Guys.
Have ordered 105 mains, 40 pilots and some shims for the needles.
Currently cleaning the muck out whilst waiting on the parts.
Will check float heights, have left the float valves as they look in good condition.
Will start with set up as generally agreed on with 2 1/4 turns on the pilot screws, and we'll see how it goes.

Regards

Phil

[London_Phil]

Well I finally got it out today, and I'm now running 40 on idle jet, with screws 21/4 out, and 105 mains, with a 0.5 shim under needle.
Balanced carbs with Morgan Carbtune.
Its  definitely got improved starting, as mentioned by a lot of you, and comes off throttle much more quickly.
Nice and clean pull through the rev range, but i don't tend to rev much past 5k, as I don't tend to push it hard.
Its a difficult comparison, because I know that even with 38/98 setup, but with clean carbs, it ran ok.
Its certainly a softer sounding bike, and the exhaust has a more waffly sound, so its probably now somewhat richer.
Idles quite stable at around 1200.
Might get a Co tester from the Ebays, to check the actual values, but its working so Ill probably be  leaving alone.
I did the cutting of the airbox, as per Sin, but I was not able to successfully glue the U-Channel, so ended up using a strip of tape from Screwfix.
Have a spare airbox, and will modify that one at my leisure.
All in all, she's back to her lovely self.
I may need a FI bike if I stay in the London area, at least that's what I'm trying my luck with at home. Have to keep the "classic" Steamer, of course..

Thanks for all the suggestions and advice, as always.

Regards

Phil

[ssevy]

If you have to err on the side of too lean or too rich, I'd always pick too rich for engine longevity. Too lean can cause a lot of damage long term. Glad it worked out for you.

[Trimac]

Hi Phil
I'm in plymouth I need to order 40 and 105 jets
And I o rings
Where did you buy youres
As I've had problems finding them here

Kinds regards
Mike

[London_Phil]

http://www.ebay.co.uk/itm/200917872573

http://www.ebay.co.uk/itm/Genuine-Keihin-CVK-PE-PZ-Carburetor-Main-Jet-short-8mm-99101-393-70-190-/142135568143

http://www.triumphparts.gbr.cc/product.php/2033257/

Think that should cover it.

Regards

Phil

[Trimac]

Thanks Phil
Looks good
regards Mike

[Trimac]

Sorry
forgot to ask about size of brass needle washes
regards Mike

[London_Phil]

I used one of these on each needle mount.

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=322268777623

My feeling, based on how mine is running, is that .5 is fine, unless you like grabbing big handfuls of throttle whilst loaded.
As mentioned before, mine definitely a richer ride now.

I'd send you a few for the price of a stamp, but I've mislaid the spares !.
TBH for 2.25 its negligible.

regards

Phil

[Trimac]

Thanks
you have saved me a lot of time
So one washer per needle
Did you put any thing on the threads of the jets
When replaced them.
Regards Mike