I've been playing with the jetting on my quad, went from the stock 77 jets to 78's and stayed with the stock rods which are 48's. At wide open throttle it's great (actually far better then I ever expected) but at cruise it's pretty non responsive and not very smooth especially between 1500-2000rpm, over that it seems to clear up a bit, on heavy throttle will clear up and run perfect. So it seems that my cruise/part throttle mixture is a bit on the rich side, so I need to go to a bigger rod. Now my stock metering area 77/48 combination has an area of .002847, my current setup is 78/48 .002969. Now my question is should I go to a 49 or 50 rod? 49 will put me at .002893, 50 will be .002815. The secondary side has CE rods with a G Hanger.

thanks
Pat Kunz

 

 

I would think you'd want a smaller rod to let more fuel in, unless you're running rich at part throttle.

 

It acts rich at part throttle, sluggish part throttle response. It should have gone richer at part throttle with the bigger jet, since the metering area will now be larger at all times. All the 74 and older q jets have the same tip diameter so the primary rod size at wide open throttle will not matter, the size will only affect the cruise/idle mixture. Evidently my 78 jet was big enough to throw off the cruise mixture enough to cause my problem, and unfortunately there isn't a primary rod that will get me back to exactly the stock area it would have had with the stock rod/jet area. A 49 rod will put me at slightly more area then stock (slightly richer then stock) but closer then current, and 50 will put me slightly leaner then stock. So I'm trying to decide which way to go. If I was setting this car up for race use only I'd leave it as is, Full throttle performance is great!, it's just not real smooth at a low rpm cruise or slight throttle.


Pat Kunz

 

Go ahead and make the rod change. You may also look at using a power piston spring that is one step leaner (weaker). This will allow manifold vacuum to hold the rods down in the jets a little bit more.

Nice to see someone tinkering with this kind of stuff. Go slow, keep notes.

PS. I just thought of something else. You can also use the APT to fine tune the primary mixture.

BigBlockk

Later.....

 

Thanks for the reply BigBlockk,

No APT available until 75. Just trying to decide which rod to go with the 49 or 50.

Been keeping notes through this entire process along with the dates they were done. I find it amazing exactly how sensitive these things are, just by going up one jet size and nothing else I got quite a bit more power out of my combination but at the same time loss some of my formerly great driveability.


Pat Kunz

 

Pat -
You can still adjust the power piston height on your carb - just like on an APT carb, and this should get you the level of fine tuning you need if you don't want to swap out rods.

But before you do that, make darned sure your timing curve is set up right and that your vacuum advance is functional. Taget timing numbers are:
36 degrees total at 2500 rpm
16-18 initial
16-degree vacuum advance hooked up to manifold vacuum.

Once you've verified that, take a look at your power piston: With the top off the carb and the gasket removed, push the power piston down until it bottoms out. At this point, the power piston brass sleeve (the part that the rod hangers are pressed into) should be about .020" above the upper lip of the plastic retaining collar. Obviously, the higher it is, the richer it will run at cruise. To adjust, pop the throttle plate off the carb float bowl. You'll see a little spring steel tang sticking up between, and slightly forward of, the primary throttle bores. This tang is adjustable with a screw located under a stamped steel cover in the front of the throttle plate. The adjuster screw is usually seized solid and nonfunctional, so you can adjust the power piston height by simply bending the tang.

 

Lars, glad you chimed in. It's not that I don't want to swap out rods (actually was planning on it), just if I should go with the 49's or 50's. with the 49's I'll be a tad richer then my original setting of 48/77, but leaner then my current setting of 48/78, or go with 50/78 which would be a tad leaner then my stock setting. Hoping your expereince can help me out here. As I said before it runs great on hard throttle, much better with the 78 jets then with the stock 77's, just need to get my cruise mixture back to where it was (or close to) with the 48/77 setting. No problems before the jet change (I try to change one factor at a time so there isn't too many variables). I'll check the power piston height when I get back in there. I doubt anything happened with the ignition since cruise was fine before the jet change and only changed afterwards, unless something happened to my vacuum advance? I'll check on that. Any thoughts on which rod to go with 49or 50?

Thanks
Pat Kunz