Monobloc Pilot Circuit

[dpaddock]

OK, all you Monobloc tweakers and tuners - here's one for you!

     A10 SR with 389 Monobloc 1-1/16 inch choke, #3 slide, 270 main jet, .106 needle jet, needle in #3 position, #20 pilot jet; engine is lumpy and 8-stroking in pilot region (up to 1/8 open), runs fine all other throttle openings. Does not respond at all to adjusting the Pilot Air Adjustment Screw. In fact, the engine runs almost perfectly with the Screw removed entirely! And it idles well with and without the Screw.

     This is a new setup for me; a standard SR head which has been rebuilt with new valves and bronze guides, and a Monobloc from another source which has been completely rebuilt (new float needle and seating, air and fuel passages blown with compressed air and wire-poked, new gaskets/washers including the paper washer between the body and the jet block). New K & N RU 0810 air filter. Clearance between slide and body is .005 to .006 inch. Float level on-pip using transparent cover.

     I am probably going to ignore remarks about timing, tire pressures and the like, and I am not going to lose any riding time even though my Monobloc is (gasp!) is missing its Screw. But what are the thoughts of Our Gang here?

David
     

[chaterlea25]

Hi David,
It sounds like the pilot jet is not fully seated, remove the little blanking cap and tighten the jet with a thin flat bladed screwdriver, then replace the fibre washer and blanking cap!!
I take it there is no fuel leaking out the pilot air screw hole with the screw removed ?
This would indicate too high a fuel level!!
HTH
Cheers
John O R

[bsa-bill]

Hi David
is this a new carb, the reason I ask is I have a new carb on my flash and it has the same problem.
Screwing the pilot jet screw in will make the engine falter but screwing it out will make it run better ( but not perfect ) and continued screwing it out just makes no difference.
I have read other mails about this and wonder if it is the pilot circuit that needs cleared ( air not getting through) some talk of putting a very fine drill through ?

Will be interesting to read others comments

All the best - Bill

[Richard]

Just something to check, make sure that the inlet tract from the carb to the head is smooth, for example there is no step between the carb and the gasket and the drip tray and gasket to the head, this will give poor running as most gaskets are purchased for A10 as well as the drip tray but the S/R has a larger inlet tract.
Worth checking unless you know you sorted this out as it was put together.
might help
Richard

[Triton Thrasher]

Bear in mind that #3 is quite a rich slide.

[tombeau]

Mine was running very lumpy at just open throttle setting. I fitted a no. 4 slide and that has helped lots. Its now possible to actually drive the bike slowly. I'm still playing with my carb though.
I think I'm running a 290,position 2, slide 4, 20, 106 and  at the moment, though I don't have my notes with me just now.
Do you have spare sizes you can play around with them?

It gets expensive and tedious to buy them, fit them, try them, remove them and put them in a drawer.
Cheers,
Iain

[dpaddock]

It is NOT a new carb. In fact it shows some pitting in the bottom of the float chamber and the bottom of the jet block, presumably because it was left with water standing in it at some point. This raises the question of possible pitting at the pilot jet seat, doesn't it?

I'll try a No. 4 slide, too.

I'll keep you posted. Thanks for your inputs.

David

[tombeau]

Remember, you can get a 3 1/2 slide too.
I moved from a 3 1/2 to a 4. But I had started out with a 3.
A10s seem to be like children though, every one is different.
Cheers,
Iain

[beezalex]

If I recall, the pilot jet should be about .018" diameter.  You can check this with a jet gauge or some guitar strings.  You'd be surprised how many main and idle jets I've found that someone has drilled and not marked.  It will drive you INSANE...(ask me how I know).

[ebsbury]

SRs normally use a 1 5/32" bore carburetter, whereas yours has 1 1/16" bore and I notice you say the bike is running well on a 270 Main Jet compared with a normal SR Main Jet in the region of 410. I wonder if the smaller carburetter is creating an increased air velocity through the venturi which is pulling an over-supply of fuel from the pilot jet? That would also explain why a seemingly too small Main Jet is working well. In that case a 15 Pilot Jet might be the way to go, and perhaps a smaller cutaway on the throttle slide.

[ebsbury]

 "perhaps a smaller cutaway on the throttle slide."


Actually I meant a larger cutaway!!

[dpaddock]

Thank you all for your inputs.

After several tweakings, I have the bike running much better. At this point, I modified the slide to a #4 cutaway, and the needle to No. 2 position.

Again, thanks!

David