Carburetor/intake question?

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10drum

10drum

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Greetings,

Getting this 72 interstate with combat engine back on the road. I rebuilt the carbs, and the bike runs out strong, it has good compression. I struggled getting it to idle consistent, which my experience usually points to worn sleeves and/or bad pilot jet(s). I found a set of new 932s at vintage triumph supply for $325 w/free shipping (not bad). I installed them and still have a similar problem. The left side just wasn't right. I could turn the pilot screw all the way in, and it would idle pretty good. So, this would indicate it's sucking air from somewhere? I installed a cNw starter, and installed the k&n filter that Matt sells to replace the stock filter.

I'm going to replace the insulators between the spacers and the head, and I'm going to make sure the the faces of the spacers are true as well. I may pull the head, just to have a look. I don't think the problem is in the head, but before I do, is there something I'm missing?

Thanks for all replies,

Phil
 
You seem to have a handle on it. I have had intake manifold leaks as you describe. Check for cracked manifold too. Do a compression test before yanking the head.
 
Carefully check the balance hose between the two manifolds for leaks/cracks.

You can check for leakage at the spacers.head/manifold without pulling anything apart by shooting some WD40 at the joints. If there is a leak, you will hear the engine RPM change (drop) as the WD 40 is pulled in. You can use a variety of substances for this but WD 40 is cheap/easy and who doesn't already have some on hand? :)

It is very common to have warped mating surfaces of the carb/manifold flanges due to over-tightening the fittings. But if they aren't actually leaking, why pull them apart? ;) You can check/true them the next time you have them off for whatever reason. IOW, I would check them/true them up if/when they are off but I wouldn't pull them off just to do that if they are not leaking.
 
10drum said:
Greetings,

Getting this 72 interstate with combat engine back on the road. I rebuilt the carbs, and the bike runs out strong, it has good compression. I struggled getting it to idle consistent, which my experience usually points to worn sleeves and/or bad pilot jet(s). I found a set of new 932s at vintage triumph supply for $325 w/free shipping (not bad). I installed them and still have a similar problem. The left side just wasn't right. I could turn the pilot screw all the way in, and it would idle pretty good. So, this would indicate it's sucking air from somewhere? I installed a cNw starter, and installed the k&n filter that Matt sells to replace the stock filter.

I'm going to replace the insulators between the spacers and the head, and I'm going to make sure the the faces of the spacers are true as well. I may pull the head, just to have a look. I don't think the problem is in the head, but before I do, is there something I'm missing?

Thanks for all replies,

Phil
Click to expand...
The ignition curve has a significant effect on idle characteristics, as well as oil temp. points/coil or EI?...which one?
 
Dave, It has a Boyer. Ignition curve? Please humor me, how does oil temp have an effect on idle characteristics?
 
10drum said:
Dave, It has a Boyer. Ignition curve? Please humor me, how does oil temp have an effect on idle characteristics?
Click to expand...

1. When the oil is cold the engine will idle slower based on a set throttle setting
OK idle attempted at 900RPM (450 on the chart) oil cold advance is 2-3 deg (4-5 deg on chart)
hot oil alone will let engine spin a fair amount faster ESPECIALLY if the advance (as shown) is progressive.
Between the two It is very difficult to make it act like points/coil where the curve is ideally flat to about 1250 before the advance starts to respond.
These simplistic analog advance curves including the RITA short coming is very easy to overlook. So turning the idle screw (or heating the oil) also affects symultaniously the net advance, cold =slow hot = race a bit higher with no other sdjustment....
Carburetor/intake question?
 
Last edited:
Do the "usual suspects" thing... look first for a vacuum leak OR a clogged idle jet. That's the most likely cause based on the symptoms. Of course it COULD be something else but eliminate the easy, and most common stuff first.
 
Dave, thanks for the info, and here I always thought it had something to do with fuel atomization changes due to temp.
 
I cleaned and surfaced the left manifold , and sanded the insulator. It's better. I'm glad I installed the starter, makes the dial-in much easier. I should have gaskets tmr, so I'll adjust the valves on weds. Thanks for the replies.
 
10drum said:
Greetings,

Getting this 72 interstate with combat engine back on the road. I rebuilt the carbs, and the bike runs out strong, it has good compression. I struggled getting it to idle consistent, which my experience usually points to worn sleeves and/or bad pilot jet(s). I found a set of new 932s at vintage triumph supply for $325 w/free shipping (not bad). I installed them and still have a similar problem. The left side just wasn't right. I could turn the pilot screw all the way in, and it would idle pretty good. So, this would indicate it's sucking air from somewhere? I installed a cNw starter, and installed the k&n filter that Matt sells to replace the stock filter.

I'm going to replace the insulators between the spacers and the head, and I'm going to make sure the the faces of the spacers are true as well. I may pull the head, just to have a look. I don't think the problem is in the head, but before I do, is there something I'm missing?

Thanks for all replies,

Phil
Click to expand...
curious - were they new "premiers" @ the $325 price?
 
A piece of fine sandpaper on a piece of glass or a true machined surface (table saw?) makes a fine way to true up flanged surfaces like carb manifolds.
 
They are not premiers. I can't justify the extra cost for carbs that may last 30K miles, on a bike that may only get ridden 3K miles.

I also took the new carbs off and treated them as if the were used. Cleaned all ports with carb cleaner, blew them out with compressed air, and set the float height. There's a good chance these carbs, @ $162.50ea, are some of the ill fated bad castings? I was tempted to go with the Mukunis setup that Matt sells for $400, but I had already bought the air cleaner for the Amals, and I have not had good luck with mukunis, or keihens on anything but Japanese bikes.
 
Dave,

We all throw money at these bikes. This is not my first visit into vintage British motorcycles. This is my first Norton. Idle consistent, means, that you can warm the bike, adjust the idle to where it is right on, ride it, then it doesn't fall back, or falls too far past, the previous set idle rpm. There are some boneheads, that this doesn't bother, they are they same ones who say, "British bikes are suppose to leak". Every bike I build, I attempt to use the carbs that came with the POS I purchased. Usually, the idle won't be consistent. Sometimes you know the carbs are bad before installing them, like some gorilla used something that ruins the pilot jet. Most times the cause of inconsistent idle is due to worn slides or sleeves. Back in the day, you could re-sleeve the carb, and install new slides. I guess you could re-sleeve them today, but at $162.50 for a carb, why? These aren't like a BMW Bing, that sometimes cost in excess of $500, if you want to talk about throwing money at a bike, buy a R75/5.

I do appreciate all of the replies, I think I have it on the run.
 
Thanks for the details. The idle trend you describe IS normally assigned due to mechanical wear.
FWIW The concentric carb reconditioning is where the body is bored a few thou (after straightening), then the slide is sleeved then sized to the new bore.
At reported $75 CDN each, that last much longer than original zinc body/slide, it may still have some appeal.

My combat that I rebuilt in about 1988 did not leak until after 2000. I do not think that aluminum cases and iron barrels with large difference of expansion coefficient will stay oil tight indefinitely, unless never run...
 
As for throwing money at stuff...

Well, you know at some point I just want to go ride. I have lot's of stuff that needs work and sadly I never throw anything out without trying to fix it. This is why I am able to fix a lot of old stuff, but at some point the law of diminishing returns kills the pleasure in it.
 
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