Keihin Carbs

[Roy gardner]

I have been digging around in the search facility of this esteemed forum for info on Keihin carbs. I see a bunch of stuff regarding FCRs but I have an engine with 39mm CR Specials fitted.  It is a 1000 LeMans Mk IV which I acquired. When it came to me it had foam pods, which I detest, so when I fitted it into my Convert I fitted a standard LM air box & filter. It also had large diameter aftermarket headers & open megaphone exhaust with crossover under the gearbox. It did go quite well. I now run standard headers & Cali 1100 mufflers, and standard Convert cam.
It starts & idles well, pulls solidly through the rev range if using 1/3 to 1/2 throttle, but if the throttle is opened with enthusiasm, it bogs, then rolling off the throttle gives it some pick up and it takes off again.

Anybody else using CR Specials who could give me some ideas for jet numbers?

Current  settings;
 Float height;14 mm as recommended by several internet sites.
Main Jet; 135
Idle,(slow running) jet;58, screws open 2 turns off seat
Slide; 20 (Smallest cut out available)
Needle; 5179, 2nd notch from top.
I lifted the needle one notch, to 2 recently, made a small improvement, 2 notches, to 3, was too far.
 

[mtiberio]

lifting needle was the wrong direction (unless you mean moving the clip to a higher notch, in which case you are lowering the needle). You want to lean out the needle and/or main jet.

[Roy gardner]

Thanks Mike, definitely lifted the needles, from full bottom to 1 ring up. 2 up was too far.
The way it bogs with the slides wide open, then surges as they are eased down again tells me it is too lean, getting a gutsfull of air, not enough fuel, then the slides dropping reduces the air to match the fuel that has been drawn in. Thoughts????

[Tony F]

I see from the picture in the tyre thread that the carbs have Sudco stickers on them so they probably came from Sudco as a kit. You could try asking Sudco for jetting recommendations for your engine spec which probably differs from what the carbs were originally spec'd for. On my SP I have flat slide mikunis which were originally supplied by Sudco in a kit for and airhead BMW and they were helpful in sorting out the change.

It does sound like a needle/needle jet issue. 5179 seems like a mid range needle according to the (extensive) Sudco catalogue. Did it run cleanly right through the range in the spec it was when you bought it? What ignition system are you running?

[Roy gardner]

Thanks Tony. I didnt realise they offer that service. I have looked at their site, looks positive. Ciao

[guzzimatic]

What do the plugs look like? From your description you took a hotrodded motor and choked it down,stock air box,smaller headers,Convert cam,so you have way too much carb for a sorta mild motor...I would guess the plugs would be black,carbon fouled... I am not sure if a LeMans IV motor would ever run right with this combination of mismatched parts...I would say a pair of 30 or 32mm Dell'ortos would match the intake and exhaust restrictions,kinda like a CX100.

[bodine99]

Thanks Mike, definitely lifted the needles, from full bottom to 1 ring up. 2 up was too far.
The way it bogs with the slides wide open, then surges as they are eased down again tells me it is too lean, getting a gutsfull of air, not enough fuel, then the slides dropping reduces the air to match the fuel that has been drawn in. Thoughts????

Its rich

[Roy gardner]

What do the plugs look like? From your description you took a hotrodded motor and choked it down,stock air box,smaller headers,Convert cam,so you have way too much carb for a sorta mild motor...I would guess the plugs would be black,carbon fouled... I am not sure if a LeMans IV motor would ever run right with this combination of mismatched parts...I would say a pair of 30 or 32mm Dell'ortos would match the intake and exhaust restrictions,kinda like a CX100.

Thanks. I was a bit concerned about the "soft" cam, but it's quite punchy in the mid range & still runs out to 100MPH+ comfortably if I ease the throttle open. The air box is a stock LM one, not the horrible old Convert one.
I have contemplated fitting a pair of 36 Dellortos I have earmarked for the Round Barrel motor but I am sure I can get the Keihins to work properly with a bit more fiddling. I think 30 or 32's would be worse with those monstrous valves.
It is hard to read plugs with the fuel we have here these days. They are always black & sooty, even 98 octane in my standard V7 Sport.
I had the carbs apart recently to check the float heights then adjusted the idle screws. It is leaner at idle now, reluctant to idle until its had 30 or 40 seconds of holding the throttle just open, whereas previously it started & idled immediately from cold with throttle dead shut.   
I think that if the carbs were too big, the airflow through them at idle would be so slow that it wouldnt pick up fuel???

[Roy gardner]

Its rich

Thanks, so are you suggesting that its getting a gustful of fuel & not enough air then easing the throttles back is slowing the fuel supply down to match the air flow?
Should I go for a bigger slide cutaway or smaller main jets?

[Roy gardner]

So, I communicated with Sudco, nice fellow called Chad replied with this;

"That is just a big carburetor. So if you get on it quickly at a lower RPMs there is lack of vacuum signal. Basically, the motor isn't sucking hard enough to draw the fuel out of the bowls. You can try going richer on the slow jet and needle to help compensate for that but it won't 100% complete go away. You just have to be a little slower on the throttle till the RPMs are up."

I asked if that is why the standard 40mm Dellortos have accelerator pumps,

He replied; "Exactly, accelerator pumps are there to compensate for that lack of vacuum signal."

So I guess thats why others use Keihin FCR's accelerator pumps?

Guess I'm flogging a dead horse. Need to look for a pair of standard 40mm  LM1000 Dellortos. I do like the light throttle feel, but I know the Dellortos can be made better   

[Dukedesmo]

Not Guzzi related but I have 41mm FCR carbs on my Monster and whilst they run great, it will bog down if the throttle is cracked open too quickly, especially if in neutral - when in gear it generally responds OK but it also has a light flywheel and so is more responsive than a (my) Guzzi.


As above they're big carbs that work best at a decent RPM when the flow is good and as such air speed at low RPM is slow, causing poor throttle response which is one reason that Ducati were early adopters of fuel injection for v-twin race bikes. 

[rocker59]

Guess I'm flogging a dead horse. Need to look for a pair of standard 40mm  LM1000 Dellortos. I do like the light throttle feel, but I know the Dellortos can be made better   

Why not source a couple of FCR39s for the project??  The Keihin FCRs are much better than the Dellortos can ever be.

[Turin]

I have FCR 41's m my Lemans SE and they pull from down low, even if you snap open the throttle. no bogging at all. Thing is, I'm running a racing camshaft with high compression pistons.
I'm thinking that lawnmower cam is a step in the wrong direction. A lumpier cam will cause the motor to suck in and expel more air, using more of the carbs potential.
BTW, the B10 from the Lemans 1000 wasn't very radical to begin with.

[Roy gardner]

I have FCR 41's m my Lemans SE and they pull from down low, even if you snap open the throttle. no bogging at all. Thing is, I'm running a racing camshaft with high compression pistons.
I'm thinking that lawnmower cam is a step in the wrong direction. A lumpier cam will cause the motor to suck in and expel more air, using more of the carbs potential.
BTW, the B10 from the Lemans 1000 wasn't very radical to begin with.

Yea, so about the cam...  I took out a locally ground cam that was supposed to be a copy of the factory P3 LM race cam. You could ask;how did that grind come to be on a Convert cam??
20+ years ago a local Guzzi nut rounded up 10 or so cams from amongst mates  with a promise of getting the P3 grind done at a great price. I didnt jump in because I wasnt going to disturb my V7 Sport for something it didnt need. However, I was asked by a couple of blokes to assemble their engines with these hot cams. When I put a degree wheel over them they were shocking, the variation between lobes was anything between 3 & 5 degrees for 3 and about 8 for the fourth! The numbers related to nothing particularly, it wasnt B10, P3 or V7 Sport, just random everything in between.  It transpired that the grind had been applied to a Convert cam which I obtained, yep, it was as bad as the others, the grinder operator was consistent. I ran it for a while, and actually saw 59HP on the dyno with the Keihins. Predictably, the power was all near the top of the rev range, and although the response improved from the changes made with needles on the Dyno, it was a death trap at really low revs, it would drop its guts crossing an intersection. Putting the 25-52 lawn mower Convert cam in made it much more rideable, and economical.
Before I disturb that motor, I have promised myself that I will have the hot rod cam built up & re-ground by a reliable shop to the proper B10 LM grind. You are right, it is not particularly radical, but it does give nice power, I do like riding standard LM 1000's.
And if that works well, I will get another done the same for my Round Barrel Convert motor which will inherit LM II heads & 88mm high comp pistons to match & 36mm LM carbs.

Rocker; Why not source a couple of FCR39s for the project??
Because I would need to start again, I am sure you guys would all help with jet, needle & slide info, but I am familiar with Dellortos & have stuff on the shelf to fit,as you do. 

[Rough Edge racing]

 My opinion, bigger carbs, when jetted properly. can work just fine with a bit of restraint on the twist grip.. Roll the throttle open, not snap it open like a kid with his first dirt bike...
  OP, what type exhaust and mufflers? the problem you describe does sound like reversion, a momentary over rich situation caused by the effects of intake,cam and exhaust not working together.Air drawn through the carb is expelled and then drawn back in causing an over rich air fuel mixture in a particular RPM range, generally around midrange..It's normally not a factor on stock engines..but...long exhaust length can contribute as alternations to the intake tract length..

[rocker59]

Rocker; Why not source a couple of FCR39s for the project??
Because I would need to start again, I am sure you guys would all help with jet, needle & slide info, but I am familiar with Dellortos & have stuff on the shelf to fit,as you do. 

Makes sense.

[guzzimatic]

He stated that he is using Cali 1100 mufflers and stock headers...

[Furbo]

What do the plugs look like? From your description you took a hotrodded motor and choked it down,stock air box,smaller headers,Convert cam,so you have way too much carb for a sorta mild motor...I would guess the plugs would be black,carbon fouled... I am not sure if a LeMans IV motor would ever run right with this combination of mismatched parts...I would say a pair of 30 or 32mm Dell'ortos would match the intake and exhaust restrictions,kinda like a CX100.

What he said.

You have a big valve engine with a small valve engine cam, restricted intake and exhaust - What exactly were you wanting in putting a LM 1000 engine into a 'vert??? Why not round up a CAL II/III engine instead? The B10 Cam (standard for LM 1000) is designed for large valve engines. It can work in small valvers (I stuck a B10 into a Cal II motor with 40' Dels) with more open air and exhaust. Seems you taken the heart of a 440M runner and put it in the body of a Semi Pro Bowler. 

[Roy gardner]

What he said.

You have a big valve engine with a small valve engine cam, restricted intake and exhaust - What exactly were you wanting in putting a LM 1000 engine into a 'vert??? Why not round up a CAL II/III engine instead? The B10 Cam (standard for LM 1000) is designed for large valve engines. It can work in small valvers (I stuck a B10 into a Cal II motor with 40' Dels) with more open air and exhaust.

Because my original Round Barrel Vert motor had death rattles from the pistons & I am a Guzzi owner, I had that collection of stuff laying around and it was a way to keep the bike running until I overhauled the RB. 
I agree, it's time to get the spare cam made into a B10 
And I have scored a pair of 40mm LM Carbs for sensible money locally.

[Sheepdog]

I would enrich the idle mixture screws (out) another 1 1/2 rotations.

[Muzz]

Pushrod, I know nothing at all about those carbs.  Are they just a normal carb or are they a CV type?