Hello all, I have nozzle drip on one side or other on q-jet (17056540) and am trying to tune engine as this is a new rebuild. Idle is rough and will surge at times (rich roll) . I modified idle circuit per Cliff Ruggles book. Engine build as follows: .030 over 455 with speed pro pistons at "0" deck ,ta heads from tri-shield, performer intake and ta 288-94 cam (10.25 comp.) Thanks for any and all replies Al.
What are your idle tube and down channel restriction sizes? Idles tubes probably need to be about .038 and dcr about .055.
DCR's seem large there. You still have bypass air in place? It's not blocked in main body is it? In base plate should be .109 What size are the idle mixture screw holes? go to .090
Have you seen the nozzle drip/confirmed? Primary or secondary? Primary nozzle drip can be stopped by reducing throttle angle via more idle bypass air. Secondary drip can be caused by poor sealing of the secondary blades in the throttle body.
Ken, yes I have seen nozzle drip from primaries, do you think going a tad bit larger on bypass air would be a good idea?
If you aren't using enough initial timing, the engine will require a larger throttle opening at idle. That can trigger nozzle drip all by itself. Temporarily increase your idle timing, close the throttle as much as possible, and see if the drip stops. If it does, you need to reduce your mechanical advance so you can run more initial timing. Most cams like lots of initial timing, 15-20*.
Larry has a great point here. Proper timing has a huge effect and should be set before tuning a carb. Check this first. See if it helps close throttle. And id be surprised if the .109 bypass air baseplate holes wasn't enough. You're sure the passages in the carb body aren't shut with solder or anything? What did you set the float level at? I'd set it about 10/32".
Yes, Larry has a good point,I think my initial timing is about 14-15...maybe not enough? And my float is set at 1/4". Also I am moving next week from California 180 feet above sea level to Reno Nv. 4500 feet above sea level, will I have to re-calibrate my carb. after move?
Al, no reason to "think" you know the timing, put a light on it and know for sure. Yes, you will need to recalibrate the carburetor. You will have less engine vacuum at altitude, about 4" less at 4500' You'll probably need a different power piston spring as well as jetting changes. There is less oxygen at 4500' https://quadrajetparts.com/quadraje...ing-assortment-color-coded-springs-p-210.html
1/4 inch is a high float. Unless you are running WOT for long periods of time and are concerned about fuel starvation I think its a bit much. That high a fuel level in the bowl makes the carb more prone to drip. Usually I only put the float that high on the earlier carbs with the huge old style float. Your M4M quadrajet has a much smaller float, thus more fuel bowl capacity. 14-15 initial degrees (without vacuum advance) seems like a pretty good initial setting to me. try lowering the float also what size are your main air bleeds? rods n jets?
Hey Larry, I did put a light on car and timing at idle is at 16* so I might try 18* or so, with 34* total. Ken, spec's on carb. are as follows: float 1/4" .135 inlet, .038 idle tube, .059 dcr, idle air bleeds: upper .060, lower .080, main air bleeds: air horn .060,main body .046, bypass air.109, mixture screws .094,#73 jets, metering rods from Cliff's high perf. q-jets ( don't know size),da secondary rods and k hanger.
16* should be enough. see if you can close the throttle some. The primary side of the Q-jet is very efficient. doesn't take much to initiate fuel flow from the nozzles. How much vacuum are you making at idle in Park and in gear? What is your idle speed in and out of gear?
Larry, idle in gear is 650, in park about 900 ( factory tac don't know how accurate ) last time I checked vacuum in park it was 15"+. Trying to sort out this new engine, however it is close to 100 degrees and cruising around town with car at 200* coolant temp makes tuning uncomfortable also been packing to move next week. Thanks Al
If the float doesn't do it, I might go with more bypass air to close the throttle and reduce air passing thru the venturi. Do a little at a time...just enough to kill the drip. Your idle system is good enough to handle it.
Thanks Larry and Ken for suggestions,I was thinking float level might be a bit high,will drop it some. Also something I forgot to mention with new engine is I am running a RobbMc fuel pump with possibly more idle pressure? (have not checked with a gauge)
That should not be a problem unless you are running the bigger RobbMc pump (1100HP). The 550HP puts out 8 psi + or- 1 psi. Early Q-jets got a bad reputation with their poor float design. Those carburetors did not like more than 6 psi, but later Q-jets can take quite a bit more. My CV pump puts out 8-9 psi, and my 1972 7042240 Q jet doesn't mind it one bit.
Yes Larry i'm using the 550 pump with a 1/2 in pickup and 8an fuel line (maybe over kill but don't want to starve engine of fuel).Also do you recall what your float setting is on your q-jet?
