Topic: ok carb guys, riddle me this

[samm_j2]

so I am going through some carbs off a 78 cb750 and I am noticing that when the pilot screws are seated (lightly of course), they all stick up into the carb body at different heights. as far as I cam tell there is nothing else in there with like an old washer or o-ring. my thought is that if I am suppose to be 1 turn out or maybe two then they are still at different heights? Am I over thinking this? this can't be normal can it!?

[Pielz]

Sanity check: are they all the same length when they're taken out and laid side by side?

[samm_j2]

haha yes, absolutely checked that, same length!

[Pielz]

Oh jeez well I'm out of ideas! Lol

[kerryb]

Think about the prescribed measurement, "one turn out" instead of 1mm  from the seam.  If the carb bodies aren't drilled exactly the same depth, the air screws will each still be open the same amount with respect to each other.

Edit:  the amount of space for air is determined by the distance between two tapered surfaces, and not the length of the taper.

[BenelliSEI]

+1 to KerryB. Just follow the instructions.

[ckahleer]

If you take the washers, o-rings, and springs off, are they still difference heights.

[samm_j2]

yes, if I remove washer, spring, and o-ring there is still a major difference in height.

KerryB I understand what you are saying and for the most part agree. I was just wondering if this was "normal" or if any of you guys with lightyears more experience than me had seen this and if I need to account for it in any way.

[kerryb]

yes, if I remove washer, spring, and o-ring there is still a major difference in height.

KerryB I understand what you are saying and for the most part agree. I was just wondering if this was "normal" or if any of you guys with lightyears more experience than me had seen this and if I need to account for it in any way.

Ok, I'm guessing here, but, the variance probably comes from the allowable variance (size tolerance) of the castings (carb bodies) combined with the jigs and fixtures used to locate and drill the passages in the carb bodies.  A "thin" carb body combined with a "thick" drilling fixture, may result in a mixture screw port being drilled deeper in one carb than another, resulting in the needle poking further into the bore on one carb vs another.
Personally I've never noticed or thought to question it!...but inquiring minds see this and think "how come those are different lengths" ...good eye samm_j2

[scottly]

That is exactly why the idle screws should only be "lightly" seated. Someone had over-tightened the ones that stick further into the carb throat, enlarging the hole. This will throw off any standard "turns out" spec, as with a larger hole there will be more flow at the same setting.

[samm_j2]

thanks Kerryb

and  scotty I think you are correct. after comparing these carbs to another set it seems like someone probably over tightened these pilot screw. on the other set of carbs all the pilot screws sit the same height.

thanks for helping out everyone!

[HondaMan]

Yep, this is a common issue, especially with the round-top 750 carbs. When I get carbs for rebuild and take them apart, the air screws' number of turns inward to seat can vary as much as 1 full turn on any given set of carbs (often the outer 2 are the worst) from someone turning the screws too hard. The screws and the seats get a real polished look to them when this happens, and the setting needs to be 'shortened up' a little bit to get the same results. In the round-top carbs, this often ends up being 3/4 turn out instead of 1 turn out to get a proper idle mix.

[samm_j2]

thanks HondaMan!