Q-Jet Problem?

I bought a rebuilt QJet from a reputable rebuilder/modifier for my 1969 Buick GS400. I just got around to installing it last weekend (had a few other things to address first on the car) as a replacement for the Edelbrock that was on the car. Ran fine with the Eddy, but I wanted to go back to a QJet. I ended up buying a 800 CFM w/electric choke modified for my engine (bored .030 over with a small cam).

Swapped the Q-Jet in and I cannot get it to idle (not or cold), and it runs rough at cruising speed. I've now been fussing with it for the past few weekends with no success. Here's what I've done/checked:

There are no vacuum leaks. All ports are either attached to something (PCV and vacuum advance) or are plugged. I tested for leaks. Solid as a rock. Pulls about 16 lb of vacuum at idle with the QJet.

Acceleration is OK once it's moving.

Won't idle. I have tried to adjust the idle mixture screws and they have no effect but for turning them all the way in which will cause the car to stall. Once I start to turn them out I don't see any difference. I checked with vacuum gauge and a tach.

The Eddy was run w/o vacuum advance so my first thought was that perhaps I have the hot idle speed screw too high and it's running off the primaries. I thought that maybe advancing the idle timing using the vacuum advance would help so that I could lower the hot idle screw. I checked the timing and then even connected the vacuum advance to manifold vacuum. It really didn't make any discernible difference. I also tried the ported vacuum just for fun, but as expected, it made no difference.

Once it's warmed up, if I turn the key to start it, it will start, but then will die. Won't stay running. However, if I hit the throttle to open secondaries, it will then idle at about 1200-1300 RPM until I turn it off or drive it. Once I turn it off, it will start back up but won't idle again unless I repeat the big throttle hit to open the secondaries. When it is idling at 1200-1300 RPM like this I can back off the idle speed screw so that it doesn't even engage the throttle linkage without any impact. In other words, it will just keep running like that. However, if I drive it like that (which is not a good solution) it runs rough at cruising speed (surging, etc.). If I crank up the idle speed it will idle at 1200-1300 RPM, but that's not a solution. The mechanical advance is already kicking in and that not really a viable idle speed.

Since the car was running pretty nicely off the Eddy, I cant imagine I have some other problem, since the only thing I changed was the carb (and I did install a new mechanical fuel pump as well).

Is there something else I can do/try on my own or do I need to send it back to the seller for adjustment/replacement?

 

Turn the idle mixture screws all the way in and then out 1.5

Then turn idle speed all the way out, then back in till 2.5 turns then pump it twice and try it.

If it cranks, hook vac gauge up adjust each idle mix screw in or out 1/8 turn equally at a time to get highest vac reading, then adjust idle speed to desired rpm

 

Thanks. I'll try that again. I've pretty much tried all combos including what you've noted, but there's no harm in trying again.

 

Was someone who sells/posts a lot here, so I don't want to be a jerk and have anyone think his product is inferior given the high likelihood that the issue is mine and is not based on his rebuilding skills.

 

What intake manifold? Any chance the Q-jet (the base gasket) is uncovering the manifold casting, leading to air leaks or exhaust contamination of the air/fuel mix?

Wrong base gasket?

I would consider removing the idle mixture screws, and use a rubber-tipped blow gun to blast the idle fuel/air passages. Aerosol carb spray can do the same thing if you don't have access to compressed air.

HOW can you connect the vacuum advance to manifold vacuum and there's "no discernible difference"? Failed vacuum canister? What is your initial timing (@ what rpm), total timing (@ what rpm), and amount of vacuum advance?

 

Shurkey, thanks and good questions. The intake was on the car when I got it and looks to be something like an Eddy or TA manifold. Probably not Eddy since there are no discernible markings on it. It's a dual plane unit that can accommodate both a spread bore and a square bore carb. So, I unbolted the Eddy and bolted the Qjet on (using the thick gasket normally used for QJets). Tested for leaks around the carb and there are none (sprayed carb cleaner around the base with no difference in engine idle). So, I don't think it's leaking.

As for timing, I think it was OK (my timing light died today while I was using it). I had previously followed Larry's power timing steps and had it set at about 4 deg initial (at about 750RPM idle), total to about 34 or 36 degrees at about 3000 or so (not 100% sure here). The vacuum canister is working (but I do have a Crane adjustable unit on order). When I say that there is not discernible difference, I mean in the engine idle. The vacuum advance does advance the timing somewhere around 12 degrees. It's what I was futzing with when my timing light bought the farm.

On a somewhat related note, the guy I bought it from has been offering some suggestions for me to try before I send it back. He suggested that I set the idle mixture screws nearly seated or half turn out and lower APT screw 1 turn. I haven't tried that yet. Any reason why I should try that? Seems like he's suggesting that it's running lean. I am far far far from a carb or QJet expert, so I have no way to know whether the advice is good or not. I have no reason to doubt it though.

 

what condition are the rubber hoses on your fuel pump, are they colapsing? are you runing one of those clear filters you always see with an edlebrok carb, if so toss it. Its restrictive. check your carb filter at the inlet. You dont want it in backwards. Other than that the spring may be too light on the piston for the primary needles. that would hold down the needles causing too lean all the time. But if you checked everything twice more than lickley the top of the carb or airhorn needs to be removed and a spring or float level needs a tweek or two. Also does it sound considrably louder than the edlebrok? The sound of air sucking down into it. It could be the air bleeds dont need to be exposed.

 

Yea 4° initial is part of the problem for sure

 

Thanks for all of the tips. I will fuss with it some more this weekend and will let you all know how it works out. I'm sure I'll get it straightened out. I just need to have more patience with it probably.

 

Changing the timing at idle from 4 degrees of advance to 12 degrees of advance should make a huge change in idle speed and perhaps idle quality.

You say there's no discernible difference.

Something ain't right.

 

Incorrect timing and/or vacuum leaks will make a Q-Jet virtually impossible to tune. Did the rebuilder use marine-tex on the well plugs, pull the idle tubes, and install bronze bushings on the primary shaft? If not, the vacuum leaks around the primary shaft will make it run lousy at idle. If the idle tubes were not pulled, the debris trapped in the bores will not allow the fuel-air mix to reach the intake. If the well plugs were not properly sealed, the fuel leaks will also cause trouble.

Q-Jets are great carbs, but these are some of the critical issues which must be addressed in order for them to operate properly.

 

I'm curious too. I just installed a restored Q-jet that was supposed to be custom-tailored for this 455,and my issues are very similar.

 

That's why I prefer the 4MV . Without the ideal altitude compensator. You can adjust the primary needle height, and how far down it goes

 

Following.

 

Your choke and high idle step linkages are goofed a tad and sticking.
Verify that it is actually fully open or closed.
Advance the timing as instructed, but know that it is impossible to tune or get the mixture and idle speed to respond if the choke linkage hangs in the middle somewhere.
Hoping this turns out EZ, good luck.

 

IF...the choke isn't fully opening, it will still look pretty close to open and do all that's described.
The 'ticks' on the side to reference the settings can be completely useless other than just for reference.
Sometimes they fool people into thinking that they are way past the rich or lean range when we are simply talking about a wound up spring and where it ends up leaving the hash mark.
If you think it's OK, check it again. Move things with your fingers and be certain.

 

I was running an Edelbrock on my 455 car for while too. It would idle basically no matter how bad the mix was, or how off the timing was... but it got horrible gas mileage and made no power. Switching back to a Quadrajet has been a challenge, but I think it's worth it. On the Qjet there's no way my motor would run at 4* initial, with the Vacuum Advance Controller unplugged. Advance the initial to at least 12*, maybe more. It will idle at 12*, though probably poorly. Once you reconnect the (correctly specced, fully functional) VAC it will jump up to 28* or so, which will get it really idling nicely.

You can get into a circular problem where a crappy idle gives you a low vacuum condition, which gives you insufficient vacuum advance, which causes a crappy idle, which gives you a low vacuum condition, etc. It's very frustrating to tune, but worth it in the long run I think.

 

x2 on the vacuum advance/no vacuum trap - just had this happen on my 73 and my 65. To date, all my carb problems have wound up being ignition/timing. Resist the urge to start messing with the carb, and get the basics dead balls squared away.