750 SF Poor carburation

[Vince]

Q Are you not jumping over 300mm logs? My 250 Husky ran a 37mm Kehan, and wacking that fully open, wheelying over obstacles would have it falling on its face. The 450s ran 41mm carbs. There were pages and pages of modification tuning out this stuff from Horizontal blades on either side of the Carbs to Rubber O rings on the pump arm to keep solid contact, to drill leak jets and dozens more. My favourite was triple taper carb needle kits, a red needle and a blue with 5 clip positions, plus you get a page of dino found settings calibrated to ambient temperature and altitude, very handy if you're riding in the high desert of Cali or Colorado at 10,000ft. The temperature guide had a feeble, THAT SHOULD SAY FEELABLE difference between Summer and winter temperatures.

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[iis_iis]

.....just to add to the stampede to the keyboard and resulting tsunami of pointers, check the back tyre pressure.... oh wait that may not be it... if suspect open road test, and any engine misoperation, at specific revs and engine load, lean or rich, slow operate the choke, for better or worse condition under seat feel. Bit clunky though quickly indicates any fueling issues that are problematic. HTH j. ***Most important, use brand new plugs, ( a box of ten if you have to ) no matter what they look like, cheap removal of unknown factor. An engine will feel under powered or pinking with ignition "advance" nee "retard" issues as a guide. Lads know why i say this, a wry smile here.

 

[chrisk]

.....just to add to the stampede to the keyboard and resulting tsunami of pointers, check the back tyre pressure.... oh wait that may not be it... if suspect open road test, and any engine misoperation, at specific revs and engine load, lean or rich, slow operate the choke, for better or worse condition under seat feel. Bit clunky though quickly indicates any fueling issues that are problematic. HTH j. ***Most important, use brand new plugs, ( a box of ten if you have to ) no matter what they look like, cheap removal of unknown factor. An engine will feel under powered or pinking with ignition "advance" nee "retard" issues as a guide. Lads know why i say this, a wry smile here.

You should have checked the back tyre pressure before anything else

 

[piranha-bro2]

Swhy I mentioned big dirt bikes needing them Vince. My point is, people carry on about pumps not working on Dells on Laverdas and imagine they can actually tell the diff - and want them operating perfectly 'because the factory fitted them and so they must be essential'.

 

[motoddrob]

No pumps on the SFC carbs. Maybe that's saying something

 

[Tippie]

No pumps on the SFC carbs. Maybe that's saying something

Saying that they spend their life on the main jet

 

[motoddrob]

That's the plan!!!

 

[Paul Marx]

Saying that they spend their life on the main jet

Saying that filling up is lost seconds in an endurance race.
Paul

 

[piranha-bro2]

Saying that filling up is lost seconds in an endurance race.
Paul

?

 

[Paul Marx]

?

If you gain a lap or three before filling up, you've gained seconds or minutes.
Paul

 

[minigunz]

Small throttle opening so nowt to do with main jets!
Usual suggestions for PHF carb I think can be read across to VHB.
Are pilot air and fuel passageways truly clear of crud? How did you check?
Correct float level is critical.
Atomiser jet and needle wear and benefit from replacement.
Choke plunger rubbers get hard with age and don't seal well. Are the plungers fully seated with choke off?
Do the pilot mixture screws have o-ring in place?Does adjusting the screws affect engine running? - it should.

I worked for a Mikuni Factory carb company, and his / these suggestions are spot on, Because the stumble is the Pilot circuit up to 1/4 slide position. then the Needle in the atomizer runs up to 3/4 slide position, that requires clean atomizer to spray the fuel with the in coming air.
At 3/4 throttle up the main jet takes priority to meter the air and fuel out of the atomizer. the needle no longer has priority.

so just follow up on these 3 stages are used to work the circuit on any carburetor.
Pilot 0-1/4 throttle (check the flow)
needle 1/4-3/4 throttle (clean the atomizer)
Main jet 3/4 to wide open
Base line the air screw at 1-1/2 turns out from lightly bottomed out. this is very important as the air screw
meters the pilot jet mixture only, 1/4 throttle, after that the screw has no more purpose. (this is not a COX airplane)
If you follow this you can track down the stumble issue.
too bad you're not here in California I could finesse the issue and make it right,
Carb people are a dying breed,
Jeff 909-374-6715. pacific time usa,

 

[piranha-bro2]

You forgot to mention slide cutaway ... which directly affects response off closed throttle. On my FCRs I had a stutter off a closed throttle but there is no slide cutaway on FCRs per se. I was able to ALMOST completely cure the issue by playing with the slow air jet (not found on Dell'Ortos). To determine whether it would help I simply screwed it as far out as it would go and the effect was immediately positive, virtually curing the issue.

The problem is that there's no way it should be running the slow air jet that 'large' (the aftermarket adjustable slow air jet replaces an array of about 25 options, each one worth about A$10!). I have been lazy and left it like that but I'm determined to do more testing with the pilot circuit to find a better solution. By rights the needle should have no influence on throttle opening from closed, but it's clear that - on FCRs at any rate - everything on these carbs has some sort of overlap (except the main jet). I'm pretty sure my motor runs rich, plugs at anything other than highway speeds are black as, and it uses more fuel than I reckon it should.

It would be nice if we could buy EFI systems that bolted straight on, plug and play! How else do modern road bikes get away with running CRs as high as 11.5:1/ Even Royal Enfield 650 twins manage this.

 

[Ernesto]

A plug chop shows the right cylinder is running very lean

Check the isolation thingys between head and carbs.
The original ones tend to have cracks when old.
I would replace them anyway together with the o-rings.

the left is oily/rich

Oily isn*t a good sign.

Ernesto

 

[Alzero]

You forgot to mention slide cutaway ... which directly affects response off closed throttle. On my FCRs I had a stutter off a closed throttle but there is no slide cutaway on FCRs per se. I was able to ALMOST completely cure the issue by playing with the slow air jet (not found on Dell'Ortos). To determine whether it would help I simply screwed it as far out as it would go and the effect was immediately positive, virtually curing the issue.

The problem is that there's no way it should be running the slow air jet that 'large' (the aftermarket adjustable slow air jet replaces an array of about 25 options, each one worth about A$10!). I have been lazy and left it like that but I'm determined to do more testing with the pilot circuit to find a better solution. By rights the needle should have no influence on throttle opening from closed, but it's clear that - on FCRs at any rate - everything on these carbs has some sort of overlap (except the main jet). I'm pretty sure my motor runs rich, plugs at anything other than highway speeds are black as, and it uses more fuel than I reckon it should.

It would be nice if we could buy EFI systems that bolted straight on, plug and play! How else do modern road bikes get away with running CRs as high as 11.5:1/ Even Royal Enfield 650 twins manage this.

EFI is all well and good, but can you rejet them on the roadside as you climb and descend in altitude? Or make tuning adjustments for higher and lower ambient temperatures? I think not.

 

[piranha-bro2]

EFI is all well and good, but can you rejet them on the roadside as you climb and descend in altitude? Or make tuning adjustments for higher and lower ambient temperatures? I think not.

Excuse my ignoramster musings but don't modern EFI units measure all that sort of stuff, and do it about every millisecond ... so it would detect a lean or rich mixture at altitude and compensate? Or various temp or O2 sensors? My Skoda diesel starts and idles identically whether it's on a 45 degree stinker or at 2000m above sea level (we don't have any bigger montagne) in midwinter when it's well below zero.

 

[Vince]

There is a thing called running in open loop and closed loop. It gets insanely complicated very fast. When EFI arrived on Husky Dirtbikes, I test rode one in the Wadigans trails, and it was impressive at its smooth, seamless power, and then 10 seconds later, there were aftermarket mods everywhere that improved it even more.

 

[Alzero]

There is also a thing called satire…… I hoped the emoji’s would have been a tip-off.

 

[Alan]

Funny this coming up as I was talking to a mate just last night who's just back from 2weeks spent riding round the mountainous area of Northern Spain. He was on his new Honda 500 blah blah thingy and said in the higher attitudes it did suffer a bit, which did suprise me too as I thought it's we brain would have sorted things out.

 

[piranha-bro2]

There is also a thing called satire…… I hoped the emoji’s would have been a tip-off.

Damn! I completely missed that ... I did plead ignorance

 

[Vince]

Me too,emotacons were on a different line.