mixture and exhaust note

[jvandyke]

Since I don't really feel like working, I'll report an observation.
Car seemed to be running great on Sat. I noticed the choke was out a tad.
Before leaving for a joy ride Sunday, I reached under the carbs and turned each jet adjuster about 2 flats richer (I think). Sunday's run was fine. Only noticeable difference was it seemed a little quieter. In fact, Sunday's run was amazing, about 60 miles, the last 25 was interstate (which I normally avoid but I was running late). She cruised at 65/70 (according to her speedo which is off I believe) perfectly the whole run and seemed willing to go all day. Would a slight mixture adjustment be noticeable in exhaust note? I know part of the tuning procedure is to listen to the exhaust during mixture so I guess so.

 

[DrEntropy]

With a bit of practice/experience the exhaust "note" can be a good indicator of mixture. A ColorTune or two are a better way to fine tune 'em, tho. Plug cuts work as well.
And as always, a tad "fat" is better than too lean. :wink:

 

[SilentUnicorn]

When you get it right is is truly Music.

m

 

[Sarastro]

The mixture does affect the exhaust sound, but it's really not a very good basis for tuning carburetors--just not sensitive enough to the changes. The lift-the-piston method or a colortune is better.

 

[jvandyke]

I tried the standard procedure in the spring and just couldn't tell a difference in rpm "lifting the pin". As I recall, I went 6-8 flats either way (from the 12 flat starting point) on both carbs and just couldn't detect a difference (at least not the slight rise and hold I read about). Plugs looked good, car ran well, went all summer. Was probably a bit lean all summer than?

 

[davester]

How are your throttle shaft bushings? I found that when I was getting a significant vacuum leak around the bushings the mixture adjustment was pretty unresponsive and inconsistent so that the "lifting the pin" test didn't do much. To test this, just spray some carb cleaner around each end of the throttle shaft where it exits the carb body. If the engine speed changes significantly (because the fluid will momentarily block the vacuum leak) then you have a very worn shaft and/or bushing.

 

[jvandyke]

No bushings on HS2s as I understand it, the shafts ride right on the carb bodies themselves. When I tore them down they "looked" unworn but the above test did produce some rpm variation. I sealed the outside of each shaft with a dab of RTV ( where they exit the carb body, can't do much about the inside though). Good you mentioned that, I should retest and perhaps tear down and renew those shafts over the winter.

 

[BuggerAll]

fluid doesn't seal the leak but gets sucked in and burned, which is way engine speed increases, it's extra fuel to a lean setup.

 

[Sarastro]

I've never worked on HS2s, but I imagine that the shaft/body joint can wear just like the H1s. So, leakage around the shafts is a possibility, for sure. Fixing that might require some machining.

If all is well, you should be able to tune them by the lift-the-piston method, pretty clearly. It is especially easy if you start lean--the lean conditios is somewhat easier to see than the rich. Take a look at the article on this in the Wiki.

 

[lesingepsycho]

I agree and I can attest that, when all is right, tuning by ear, lifting the piston, and exhaust note are all very clearly identified and executed. The problem is that everything with the carb has to be just right (and it rarely is). I once spent a great deal of time rebuilding a pair of SUs to perfection and once they were on the car, tuning them was just like what you read in the manual. If the tuning isn't very distinct and obvious, there is likely a problem elsewhere.

JACK

 

[jvandyke]

I think Hap told me the brass throttle shaft will wear before the carb body so usually new (regular sized) shafts) are all that is needed. I will order some up. The carbs deserve at least ONE new part each. I reused everything.