I took the old holley of my 350 and decided to go with a Q-Jet. I searched the site and found links to the info I needed to turn the Q-Jet into a better street performer. I used 73 jets, ck sec rods, G hanger. I also drilled out the idle dicharge and idle channel restriction to the specs given in the info I found (written by Damon Nickles). But it wont idle right. I have the idle speed screw turn up as far as it can go and the mixture screws as rich as they can go and the vaccum advance hooked up to manifold and the highest idle i can get is about 1300rpm (in park). Why is it still too lean. I cant drop the rpm by over-richening the mixture; the rpms keep going up until the screws are all the way out. It also has no power below 3000rpm and about 80% of what it had before over 300rpm. The only thing I can think of is that I used the stock primary rods, but I didnt think that would affect the idle?????
I have the idle speed screw turn up as far as it can go and the mixture screws as rich as they can go and the vaccum advance hooked up to manifold and the highest idle i can get is about 1300rpm (in park). Your kinda me here. If you have the idle screw turned all the way UP, why sure, you should be idling at(or over )1300rpm. Do you mean the lowest idle you can get? I'm not trying to bust-em on you,just help me out and maybe we can get you set-up. I've built a few Q-Jets in my time and it really ain't rocket science.:grin:
OK, even though I have enlarged the idle passages, there is still no idle adjustment. I can turn the mixture screw right out of the carb and it still wont get rich enough. If I run the vaccum advance off a ported vac source, it wont even idle ( it needs the extra timing at idle to keep from stalling). The highest achievable idle rpm is about 1100 (1300 was a bit off). My point is, the carb seems to not be able to deliver enough fuel in the idle and idle transfer circiut even though I have increased its ability to do so by the highest recomended measurements. I want to know if there is something else that could be causing a restriction that I have overlooked.
Are you sure it's lean? What happens when you cover up the carb air horn with your hand? How much of the transfer slot is exposed by the throttle blade? Should be no more than .040" How much vacuum do you have at idle? Did you use the proper baseplate gasket, you need a Buick one as it's different than a chevy one?
If you loosely cover the air horn with your hand the rpms quickly go up as if it your hand is acting as a choke plate; but if you completely cover it, it stalls. There is definatley more than .040 of the transfer slot exposed because the idle screw is turn all the way in the make it idle. Last time I checked idle vac. it was about 12-13. The gasket came with the carb kit and the carb is off a chevy so I guess it's not a buick gasket. What's the difference?
I don't know if I would mess around with the idle passages on a Q-jet. Do you have a cam that requires modifications to the carb. I know you must have some precision drill bits to do it right, and if you overdo it, there is no way to return it to stock. What did you drill it out to? Doug Roes book says .095" Emissions carbs had smaller curb idle ports.
Idle discharge was drilled to 092. Jet performance recommends 089-093 for cams with 216+ duration; mine has 224. According to the article by Damon Nickles, if this mod doesnt cure idle probs then you should drill the idle channel restriction out by 4 or 5 thou. I did this but still can get good idle or idle transfer.
Regal (do you have a real name?), if your engine speeds up when you cover the airhorn, you are indeed too lean not rich. I suspect a vacuum leak and the first place I'd look is the base plate gasket. Just so you know, after 1975, Q-Jet primary metering rod length was changed and as such, aren't interchagable with pre-1975 rods.
The primary rods I left stock. Originally I suspected a vac. leak but cant find one. The carb was just put together so all gaskets are new. Doesnt a vac leak cause the rpm to increase? If so, I doubt that is the problem because 1100 rpm is the most I can get out of it (at idle). You said in one of your earlier replys that I need a Buick base gasket; could the fact that Im using the chevy one that came with the kit be my whole problem? btw my name is Liam
Liam, This could be a couple of things, here's a quick list to check... (assuming vaccum leaks, ignition, and choke are all OK) The primary rods should produce about a 30 thou metering arewa. Basically, you want your primary rods to be "30" smaller than the jet. For example, your 73 jets should be used with rods that are ideally, 43. 41-45 is an acceptable range though. And YES, you want to use your stock rods, you CANNOT use rods that were made prior to 1976 in an 86 carb. Make sure that the rods are the right size, not bent, and are able to fully extend into the jets. This seems like the real culprit... What power-piston spring did you use? With your cam choice you probably want a medium spring. It's likely that your stock spring is a little too stiff to pull the piston all the way down with the current vacuum. To determine this, use an independent, but full vaccuum source for the carb's power piston vaccum port and see if it idles. If so, check that the power piston moves up and down OK, and then try a lighter spring. When you hold your hand over the carb it idles up. OK, to my way of thinking this means it's running better when starved. That indicates that you're lean. Float level, fuel pump, and oversixed rods would be a problem. Have you made any changes to the part throttle adjustment? If so, reset it to the factory specs before any tuning. If all this is done and you're still OK, then you'll have to continue with the idle mods: Your idle discharge ports are only part of the problem on an 86 carb. There are 2 more ports, the idle channel restrictions that control idle mixture and must be drilled out or else you won't have any increase in idle mixture. Here's the link, and the part I'm referring to: http://www.73-87.com/garage/qjetidle.htm Hope that helps!
Well Liam, that is a common mistake in assuming a vacuum leak will create a higher idle. Only if you are running slightly rich will the increased air supplied by a vacuum leak cause the idle to go up. If your lean to begin with, the added air will cause a poorer air/fuel mixture resulting in a lower idle speed. Take carb cleaner and spray around to find any vacuum leaks or take a propane bottle and open the valve(DON'T LIGHT IT) and put the tip around any suspected areas. The increased fuel will cause the idle to go up when you find a vacuum leak if your lean.
