Idle strangeness

htown16 · Dec 11, 2012

Another sort out issue on my 72 rebuild. I can't seem to get the idle dialed in. When I ride the bike for a while it seems to idle slower and slower finally to the point that I have to turn the screws up to keep it running. It may start out at around 1300 and then drop to 800 and start stalling. This is completely backwards to the other Brit bikes I have that idle slow til warmed up. I would be thinking advance sticking but it has a Pazon.

concours · Dec 11, 2012

Tell us about your carbs... Amals? How many miles? Slides renewed?

Hobot · Dec 11, 2012

Air leaks and advance not retarding all make idle speed up as you know, so I'd check battery first then pilot jet next. I"ve had water in bowels do similar.

mike996 · Dec 11, 2012

My commando would increase Idle speed - turned out that vibration was causing the large idle speed adjuster screws on one carb to rotate and raise the slide. Went up to 2500 RPM! Worth checking - it shows that an "upgrade" often isn't! Standard idle speed screws have no such problems!

htown16 · Dec 11, 2012

Amal 932's rebuilt with new needle jets and stay-up floats but original slides which seemed reasonably tite. Pilots were clear and engine responds when turning them in and out.

pete.v · Dec 11, 2012

Heat? Maybe when things get heated up, the air leaks in the carb flanges seal up. Cam chain a little tight? Primary? Anything beyond that I choose not to suggest.

Anyways, how's that battery and charging system?

BillT · Dec 11, 2012

You say its a recent rebuild. Did you re-torque the head and check the valve adjustment?

Do you have a carb stick? what's the vacuum at the manifold nipples at idle? Low vacuum would indicate leaking manifold-to-head or manifold-to-carb

Carb stick (vacuum gauge for synching up to four carbs)

DogT · Dec 11, 2012

If it's any consolation, mine idles slower as it warms up too, but only from about 1200 down to 900 or so. I thought it was normal.

Dave
69S

Matt Spencer · Dec 11, 2012

The plastic junction box on a 2 - 1 throttle cable doesnt help matters any . also the top cable has twice the load so friction is more
than twin cable twistgrip set up .Usually needed for fast throttle responce (up & DOWN ) ancient brass junction boxes are a vast improvement .
If you get a really modern one ( post war

) , Theyre CROMED on the outside . :shock:

J.A.W. · Dec 12, 2012

Jeeze Matt, when I saw you`d responded to this thread I wondered if you thought it was about you..L.O.L.
But seriously, a 2-1 junction box cable is nothing, my H2 cables are 4-1 [ 3 to carb & 1 to oil-pump metering-quadrant], & never give any hassles...I doubt that is a likely cause..

lrutt · Dec 12, 2012

I had that problem on carbs that only had 4500 miles on them. You'd think they would be tight and all is good. I put in a set of hard anodized new slides and problems all went away.

I'd suggest getting a new pair from Burlen. Sure solved my problems.

J.A.W. · Dec 12, 2012

Hobot..."water in bowels"? Tried a coffee enema?, that might get you off idle quicker..L.O.L.

pommie john · Dec 12, 2012

I take it that you have set the idle mixtures with the engine good and hot? Over rich idle mixtures could have the symptoms you describe.

Hobot · Dec 12, 2012

Oh crap I meant bowls, ugh, brain static, multi skull smacked. Ideal float level is when best hot slow idle is 1.5 pilot air screw turns out, but even if half a turn more or less its a non issue for one kick warm starts and slower idle than safe for cam and battery charging lights off. I discovered a trick to get super low idle, with each contact break set in a spread retard/advanced from optimal inital timing. Trixie Combat has registered 400 rpm till out of gas or battery died. Btw Commandos don't heat up just sitting with idle fuel flow, even in still hot sunshine, nay they cool down till first hint of throttle to say easy off a down hill slope some, then EGT, CHT instantly start to climb to normals. Oil take a min. or more to show. Blow by pollution and cam/lifter wear need about 1000 rpm to avoid, charging should break even with lights on around 1800

J.A.W. · Dec 12, 2012

Ok hobot, I thought maybe you were thinking of a water-skiing get off, [sans wet suit], that can sure shock load a few kilos of uncomfortable water ballast...

htown16 · Dec 19, 2012

Finally got a nice day yesterday and went for a long ride. I'm now at about 250 miles on the break in. Idle was consistent most of the ride until I stopped for gas after about an hour. When I started it back up it dropped off about 3-400 rpm. This is similar to what has happened befor.
I'm wondering if this is some sort of heat related issue due to everything still breaking in.
Plugs look slightly rich and I checked for airleaks at the manifold by spaying with starting fluid while it was running. No apparent pick up in rpm.
Head bolts were retorqued and valves adjusted after the first heat cycle. I was planning a redo at 500 miles. I synchronized the carbs initially with the dril bit method. When I have the tank off to do the retorque, I will hook up my manometer and check them again.
Alternator and battery are new. Voltmeter reads 13 with the bike off and jumps to 15 when it is running. When I set the timing after I got the motor running the advance seemed to be working properly.

htown16 · Dec 19, 2012

I am noticing when I shut down that the rh carb is wet on the outside with gas. This is only after a long ride. I am considering the possibility that it is getting hot enough to boil over and cause a partial vapor lock. Air temps are in the high 70's. It currently has the paper intake manifold washers part # 06-2700. I have seen the 06-3458 washers listed which are phrenolic. Worth switching ?

BillT · Dec 20, 2012

htown16 said:
I am noticing when I shut down that the rh carb is wet on the outside with gas. This is only after a long ride. I am considering the possibility that it is getting hot enough to boil over and cause a partial vapor lock. Air temps are in the high 70's. It currently has the paper intake manifold washers part # 06-2700. I have seen the 06-3458 washers listed which are phrenolic. Worth switching ?

The phenolic gaskets do a better job of heat insulation than paper gaskets, and those 063458 are what you should have.

How are your fuel lines routed? Mine originally were in an 'H' layout, with the crossover in front of the fuel bowls. I changed this over to a pair of 170 deg banjos with the crossover going straight from inlet to inlet, keeping the fuel line well away from the cylinders. I believe this setup is how the MkIIIs came, and should be readily available.

htown16 · Dec 20, 2012

Going to order some of the phenolic gaskets. I had a peek in Santa's bag and he has a pair of the new Premier carbs in there for this bike (actually ordered them from Jerry). I went through this same process on my BSA and screwed around rebuilding the original carb for about a year. I never could get it to run quite right. I put one of the new Premier carbs on it and it was like a new bike. One thing I hate is a bike that dies at a stop lite.

htown16 · Dec 23, 2012

Okay, found the problem after I pulled the carbs off and got them apart. When I assembled the carbs the first time the needle on the lh carb had slipped out of the clip. When I started it up it was running pretty rough and really rich on that side. I got that fixed and the bike ran pretty well except the idle was all over the place. With the lh carb apart I can see that slide had a raised edge along the side of the cutout for the idle set screw. This was undoubtly causing the slide to bind and apparently at different temps it was worse. I feel pretty lucky it didn't sieze. I am assuming when the needle came out it must of cocked the slide somehow and raised the edge.
Just out of curiosity, I took a little sand paper and knocked the edge off and the slide dropped in fine. I could probably get by with these carbs but I'm still worried about damage to the slide and body. So I'm looking forward to getting the new Premeires here and getting back out riding again. At least having the tank off gives me the excuse to retorque the head bolts and check the valves again.

Idle strangeness

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