PHF Conundrum

M

mtnduc

Junior member
74 SF2
I just went through my carbs again due to ridability issue.  It would idle & run great for about 1.5miles then die like it was getting no fuel, after sitting for 45mins run for another 1.5 miles and repeat.
Disassembled carbs and cleaned all orifices, jets, etc....  Found the 3 small holes on the bottom of the intake trumpet were clogged and probably causing some sort of pressure f'up in the carbs which may have lead to fuel starvation.

I'm at 5000ft above sea level with one size over pistons & short velocity stacks with the following jets :
Pilot 50
Main 140
Needle K2

While I was at I replaced my voltage regulator with a NOS from Wolfgang and redid all my grounds.

Now it starts and idles at about 30-40 mph.
Turning the idle screw all the way in has no effect.
Any suggestions????

TIA
Brian
 
mtnduc said:
74 SF2
I just went through my carbs again due to ridability issue.  It would idle & run great for about 1.5miles then die like it was getting no fuel, after sitting for 45mins run for another 1.5 miles and repeat.
Disassembled carbs and cleaned all orifices, jets, etc....  Found the 3 small holes on the bottom of the intake trumpet were clogged and probably causing some sort of pressure f'up in the carbs which may have lead to fuel starvation.

I'm at 5000ft above sea level with one size over pistons & short velocity stacks with the following jets :
Pilot 50
Main 140
Needle K2

While I was at I replaced my voltage regulator with a NOS from Wolfgang and redid all my grounds.

Now it starts and idles at about 30-40 mph.
Turning the idle screw all the way in has no effect.
Any suggestions????

TIA
Brian
Click to expand...

Simply means the idle circuit is blocked...

Doesn't explain why the engine cuts out after a while.  I'd start at the tank vent and petcock filters and go downwards from there, sounds more like fuel starvation.

What does "idles at about 30-40mph" mean?  Riding at 30mph with a closed throttle?

piet
 
"Now it starts and idles at about 30-40 mph".!!!
Nor sure what you mean by that, 3-4 thousand rpm?
If you found blocked airways to the idle and diffuser systems from the inlet side, you are definitely best to have the carbs cleaned ultrasonically, the idle system has very small apertures. If you mean that screwing in the mixture screws (the small ones forward on the carb, as distinct from the slide stops that hold the slides up for idle speed) has no effect then it does sound like a fuelling problem and they are generally set at between one and two turns out with the correct pilot jets. They do have their orings on them?
Are you sure that the float level is set up correctly and that the float bowl breathers are open? Does fuel run freely through and drain into the floatbowls? Put a container under the carbs and remove the floatbowls or drains and open one or both fuel taps (my SFs run fine on one fuel tap) for longer than you have experienced it runs for, do it with the fuel cap closed, if flow slows down open the fuel cap. Carb problems do not generally kill both cylinders at once unless it is fuel starvation to both sides. Your altitude is not extreme and should not have major effect on your jetting. I find that 45 pilots do the job for SF1-3 but it should run ok with 50s. Your 140 mains are smaller than I feel safe with and I use 145 but that is something you have to work out preferably on a dyno as it only affects 3/4 throttle and up anyway, it has no effect on idle at all (unless of course the mains are blocked).
Can you run at full throttle through the gears while it is going?
Check for air leaks, that your manifolds are not cracked and the o rings are in good nick, the balance pipe between the manifolds is not leaking.
Are you running with air filter? Try removing them they should make no difference.
You are sure that timing is correct and that it advances? Tappets are set correctly?
It does sound odd that it takes 45min before it wants to start again, and that it runs great up to the point where it dies. It sounds a bit like a heat related problem or electrics. The only elect item that should affect both sides at once is battery power to the coils.
Have to try to isolate if it is a fuel or ignition or mechanical problem.
 
Piet -- It used to run fine but then after about 1.5 mikes it went Bah ... Bah ... Bah and died under throttle.
It didn't matter if the gas filler cap was open or not.  I've also pulled the petcocks and in tank filters and fuel flows fine.
I've got no tach or speedo but it currently idles very, very fast.... like too fast to even think about letting the clutch out. My estimation is a 30-40mph idle.

Tippie -- I've got no tach or speedo but it currently idles very, very fast.... like too fast to even think about letting the clutch out. My estimation is a 30-40mph idle.
Yes, orings are installed.
I will pull the bowl drains and check tomorrow.
Right now I have no plans to even try and ride it again...it is unsafe at the current idle speed.

It seems like fuel to me but I replaced all electric all connections to rule that out.

It may be time for a professional to dig into it.
 
Do you have free play at the throttle and does it snap shut when you let it go,check for a detached cable connection at the throttle,eg not fully seated in its notch. Or the choke isn't fully coming closed.Check the choke cables as well
 
Vince -- the slides do indeed slap closed when the throttle is released.
I have manual chokes on each carb so no cables.

I did replace the slides and from the measurements I took they appeared to Bo identical to the ones I removed.  I'm wondering now if they are slightly different at the notch/ angle on the rear/air inlet side.
 
Brian, the new slides have the bottom shape changed around the area where they are seating on the slide stop screws. This newer type of slides will not seat properly down in older carb bodies and needs a little rework before it can be used on older carb bodies. Will try to post some pictures  today evening when back home from work.
 
That is the problem.
New slides need to be relieved otherwise the slide sticks way too open and would cause your symptoms.
Take a new slide and an old slide and hold them upside down.
You will easily spot what we are talking about.
 
Gravel -- That would be fantastic.
I compared overall heights, and the angle of the slope but I obviously have missed what you are describing.
It does seem to me that your diagnosis matches with where my head told me the problem lies.

I do recall when I purchased them that Dell Orto only makes one slide and that you have to trim it for each application but it looked to me like I had a easy swap.

I should have known better, nothing is ever that easy !!

Thanks for all the support gents! (goodjob)

Brian
 
Here you go, Brian:

SlideDiff.jpg


All you got to do on the new slides is to grind the inside of the two noses evenly, until the distance between them becomes 29mm. Best hopes that this will solve your problem.

Regards,
Stefan
 
Tippie said:
Andy, coil and condensers are individual per side.
Click to expand...

I had a similar problem on my 650 Triumph in the States a couple of years back: would run fine for a while and then die away and not restart for an hour or so. Initially thought it was fuel related (needle jets; petcock filters or whatever..) fuelling seemed fine as was compression. eventually found that when hot I had an atmospheric spark but not one under compression. Turned out the +ive feed wire to the coils was perished (ie the wires oxidised black); voltage down and resistance up: caused the coils to overheat to the point where avalanche breakdown effect set in after a while and left me at the side of the road on more than one occasion until I selected a nice spot in N.Cal north of Crescent City to breakdown and investigate the situation more thoroughly with a multimeter... Wouldn't think the setup would be much different on a Lav 750 twin: just sayin...
 
Bloke who rebuilt the spark system in my 82 Husky had a rig that generated power and had a hot air gun directed at it to test how his work was heat affected.Set this gear to 3 or 4 places around OZ for all to fail but his work.Thats Better Bikes in Adelaide, he did most of the extra lighting work for the 24hr offroad trail that happens there.Most of that on bikes that usually wouldn't light anything.
 
Exactly right Andy. Power feed to the coils (plural) is from B+ via many connectors on an SF and it will kill both sides. There can be resistence at fusebox, ignition switch, several passes through the terminal block etc. That is a different issue to one faulty coil.
 
OK, I think we're saying the same thing then: the coils aren't necessarily faulty per se but the problem is transferred there and they then go out with overheating issues..
 
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