• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

HS4 mixture adjustment help please

Share this page

bugedd

bugedd

Jedi Knight
Country flag
Offline
I watched a few how-to videos on adjusting the mix on HS4's, but I have one question. How far do you raise the piston? In one video I saw he just rolls a flat head screwdriver over to raise it slightly to get the desired raise of 50rpm then settling back down. Is that all you raise the piston, about 1/4"? Obviously if its raised too much the car stumbles. And is there a small air filter that goes over the velocity stacks? I have large round K&N with backing plates but they are so big I can't get to the jet adjustments. Thanks for the help!
 
Yeah, of the top of my head, about 1/4 inch - just a bit
 
I think it's more like 1/16 of an inch. You're supposed to just barely move it.
 
I have H4's on my car and I found that the best result comes from raising the piston just a tiny amount will do the trick. Anything more than that will not yield a good response. I fully agree with Keith.
 
You can also use a Gunson Colortune spark plug. I found it made a nice "crutch" until I got more experience fiddling with the mixture.
 
I thought the pin on the side of the carb supposed to lift the piston the correct height
 
Great, thanks for the help everyone! Got it running close and took it around the block, amazing how smooth it runs with carbs that aren't pulling vacuum leaks from everywhere!
 
So I drove it around a bit, and double checked the piston thing....turns out I have the pins to push them up, never saw that before. Pushing them up a skosh, not quite a smidge, resulted in the idle staying flat. Thinking that was pretty close I went to Pep Boys to get parts for the Mazda. Car ran well other than just off idle when lightly accelerating it was a touch choppy, like it wasn't getting enough of something. I'm thinking a bit lean? does that sound about right?
 
bugedd said:
So I drove it around a bit, and double checked the piston thing....turns out I have the pins to push them up, never saw that before. Pushing them up a skosh, not quite a smidge, resulted in the idle staying flat. Thinking that was pretty close I went to Pep Boys to get parts for the Mazda. Car ran well other than just off idle when lightly accelerating it was a touch choppy, like it wasn't getting enough of something. I'm thinking a bit lean? does that sound about right?
Click to expand...


What kind, and how much, oil do you have in the dashpots?
 
Keith_M said:
What kind, and how much, oil do you have in the dashpots?
Click to expand...

my first thought too - check the dashpot first and then check the colour of the spark plugs.

BTW I checked the manual and, you are correct, it is a skosh not a smidge.
 
The dashpot oil is the SU oil I got from the rebuilder of the carbs, Minimania. As for the amount, just enough to where it doesn't come out the hole when the piston is fully raised.
 
bugedd said:
The dashpot oil is the SU oil I got from the rebuilder of the carbs, Minimania. As for the amount, just enough to where it doesn't come out the hole when the piston is fully raised.
Click to expand...

Hmmm.... If it were me, I'd try richening it up a flat or two and see what happens. I assume you've checked your dwell and timing?
 
Ya, ignition is looking pretty good. I'll tinker with that today and see what I can mess up :smile:
 
Back
Top