Can use some guidance on final QJet/Combo setup

Discussion in 'The Venerable Q-Jet' started by 79BlueShark, Aug 5, 2019.

  1. 79BlueShark

    79BlueShark Well-Known Member

    I purchased a 73 q-jet for my combo from quadrajet power last year to replace the stock 67 qjet for my 455. The 67 ran ok , but never could get past the drivability problem with the choke when cold. Divorced choke with crossover passages blocked.

    The new carb is giving me fits on getting the idle to be consistent oover the past two summers. In particular I could not get the idle speed down as the idle screw is all the way out. I did finally settled on ported vacuum, initial timing set to 17 deg with 14 deg bushing in the MSD distr. Vacuum Adv will add about 15 deg total. Carb was fitted with an electric choke at build time and that works great. Issue with this setup was the car would just run too hot driving around and never come down past 200 with the A/C is on.

    This year I tried switching back to manifold vacuum, and that caused the idle to be way to high without backing the initial timing way back down. So now I am set at 9 deg initial running the 23 deg adv bushing. The idle is so slow at this point with the vac adv disable it is very difficult to adjust the mixture screws. Eventually I was able to get to 16 in.-Hg. Reattached vac adv and set the idle to 1000 rpm. TA catalog specified cam will idle around that speed. In gear it will drop to about 750-800 and another 50-100 with AC turned on . This seems to start well and runs a lot cooler, 185-190, about where it was with the 67 carb. Doesn't see to drop that much either with the AC on.

    Now to the issue I seem to have when the carb is setup either way. First start up of the day warms on choke then settles to 950-1000 rpm. Drive to get the temp up, stop and put the car in park and the idle is now 1200 steady. I cant seem to get it to kick down to 1000 messing with the pedal. Shutdown the car and start it right back up it will return right back to 1000 rpm. Now drove it to the local cruise night and let the car set for about an 1/2 hour or so. Little difficult to start hot with the lower initial timing setting but it starts with slight pressure on the gas pedal. Now it will only idle at 650 RPM, I try a few pumps on the pedal without avail to bring it up. So put it in gear and head out. Seems very sluggish and barely stays running. Once on the road the RPMs are up it runs fine. Stop at the next light its back to the 750-800 idle in gear. Get it home put in park and we are back at the 1200 rpm level. Again shut the car off and it immediately starts and idles low again.

    I am at the point of just not sure how or what to look at to get this idling a more consistent speed. I have tried running the heaviest springs in the distro to verify the timing returns and it does. I check the car and intake gasket for vacuum leaks, replaced all vac line/plugs. I really don't want to abandon the newer carb, but I am now thinking of just trying to make one of the electric divorce choke kits work for the 67 carb as it would idle consistently. I am sure the carb is good the car run and pulls hard with it, more so than the 67 carb but I can't stand the constant worry that it wont start or stall when hot when I go places.

    Any suggestions on other items to check would be appreciated, I just seems if I add timing I run out of idle speed adjustment, if I use ported I am back to a run hot situation. I would hate to give up a carb I spent good money on.


    Setup info

    455 .030 over
    9.5 to 1
    TA Stage 1 SE aluminum heads (no porting)
    TA 290-94H
    MSD Ready to Run Distributor
    Crane Adv Can set to come in at 5 in -Hg and limited to (15 degrees crank timing)
    MSD 6AL-2
    Stock 67 exhaust manifolds
  2. Cliff R

    Cliff R Well-Known Member

    Your tuning methods are incorrect.

    Set the total timing first (mechanical) at full load for best power. This can be done with a dyno, or even better a drag strip for best ET and MPH. The "seat of the pants" deal doesn't cut it for me, but if that's all you have go for it.

    Once you find the right total timing for the fuel you are going to run adjust the distributor curve to give you the best initial timing. For most well thought out set-ups this is usually around 10-14 degrees. Some really low compression set-ups with hefty cams may enjoy a tad more, some high compression set-ups with smaller cams a bit less. You are shooting for the initial timing that provides instant restarts and does NOT "buck" the starter when the engine is fully heat soaked on a restart.

    So now you have base timing and total timing dialed in and can play with the advance curve. I like to see a curve that starts right off idle and slow and smooth, all in around 2800-3200rpm's. Very few, and I mean very few engines will like, want, or need these super-quick timing curves folks are putting in them. That seldom if ever works all that well, and in almost all cases some of the timing comes in at idle, even when cranking, and makes tuning difficult if not near impossible.

    I've fixed more "carb problems" with cars brought here for custom tuning by throwing that cheap-ars offshore weights and springs in the trash and putting stock parts back on them that I have messing with the carb. I've tossed out a good many Petronix units as well and went back to points, but that's a story for another thread. Bottom line here, IF the timing curve isn't right the carb will NEVER work well and the engine will let you know it.

    Anyhow, once you've dialed in the initial and total add some vacuum advance. I find that on a good engine build 10 to about 14 degrees will be plenty. I almost always use ported to the advance, unless it's some engine that isn't making chit for vacuum and needs the timing ran way off the scale to "crutch" the real issue.....too much cam and not enough compression.

    If you have absolutely and for sure verified the timing is ROCK SOLID at idle, and you still see varying idle speeds then something is amiss with the carb and you need to go in and figure out what it is. Could be something choke related, or the secondary throttle plates not fully indexed and sealing each time they return (I see this a lot). I've even seen other stupid things cause issues like something hanging up on the gasket under the carb, or the linkage not having enough slack in it, etc.

    Some idle problems related to speed will also be insufficient idle fuel to the mixture screws causing nozzle drip. You have to make certain the throttle plates are closed enough at idle and good sensitivity with the mixture screws. If that deal isn't getting it then idle speed can and typically will be all over the place with different engine temperatures. A good example of this is that it doesn't want to idle well right after the choke comes off, but as the intake heats up it gets better and better, and faster and faster. Once full heat soaked we start to see some sensitivity with the mixture screws, but little to none when it's cool/cold. You should also find when setting the mixture screws that you have full control from rich to lean. This means that when you adjust them (engine fully warmed up and heat soaked) that you can slow the engine turning them in, then back them out till it's dead smooth/highest vacuum, then continue to back them out and it goes rich and slows some or even loads up a bit.

    It is also NOT a good idea to run with the crossovers blocked off. That just prolongs the agony and causes issues until the engine is fully warmed up and intake fully heat soaked. Hope this helps some.......Cliff
    Dano likes this.
  3. 79BlueShark

    79BlueShark Well-Known Member

    Not sure you actually told me anything I haven't already done 20 times with the timing and mixture adjustments. They always end up in the same place. I am convinced its with the carb as none of these issues ever manifested with the original carb. I have changed the base gasket, and even sent it back to the builder. I didn't want to give up on it since it cost me a penny or two for the carb, but I am leaning to just going back the older carb and being happy.
  4. Cliff R

    Cliff R Well-Known Member

    I don't know all the engine specs, but an engine that wants, needs, or is happy with 17 degrees initial timing plus another 15 makes me raise an eyebrow right to start with.

    I've seen countless folks over the years try tuning with a lot of timing at idle. In many cases it takes away control with idle speed even with the throttle plates nearly or completely closed. Depending on the spring tension inside the VA you can also get some fluctuation in how much is added depending on the engines vacuum production at idle speed. Most VA cans don't let the timing budge till 7-10" vacuum, and not all-in till much higher.

    So at a minimum a low tension VA should be used (or an adjustable one) to make sure ALL the timing is always in at idle speed based on the engines vacuum production, and that's in and out of gear for auto trans cars.

    A few years ago I started opening up Saturday mornings to custom tune vehicles. Most or all will have had many attempts by the owner and every resource at his disposal to get them to run correctly. I've done many hundreds at this point, some from over 1000 miles away. Just a couple of weeks ago had one in here from Canada, and a couple summers ago one in from Atlanta Ga. Most recently a really nice 1970 Chevelle SS from Northern Ohio.

    The common denominators I see with almost all of them is attempts at some point to get the mechanical advance in really early, like 1500-2500rpm's. This is difficult if not near impossible to do without having some of it in at idle speed, causing all sorts of issues with idle speed and idle mixture tuning.

    The other BIG problem or trend I see is to install camshafts on really tight LSA's, and super-quick opening/closing ramps. Comp XE on 110LSA top the list, even worse are the Thumper and Thump-Ya-Mutha stuff on 107LSA. Talk about idle like chit and stink up your wife's hair and she woln't ride in it! I often have found myself needing to run a LOT of initial timing, butt-tons of idle fuel, and a LOT of idle bypass air to even get some of these engines to think about being happy at low rpms. They still shake, rattle and roll, and sound "bad-ass", but to date not a single one of them was overly impressive anyplace or made me want to run out and replace my smooth idle cam with something that sounds more like a Pro Stock drag racing engine.

    Anyhow, there are ways to troubleshoot your issues to isolate the carb as the problem. A little to lengthy to cover here as it would take many pages. For sure I'd block off the bypass air in the carb, then add some in via removing a manifold vacuum hose and see what it likes better. I'd also loosen up the distributor and turn it in both directions to find out the exact initial timing the engine wants to be happy, then lock it down and observe that number for tuning reference.

    In closing something to keep in mind with all of this is that the more efficient you make an engine, in terms of compression, cam choice, etc, the LESS timing and fuel it will want/need to be happy. Over the years I've found most folks to the exact opposite, throwing tons of timing at them at every point, and flooding them down with gas-sucking pig-ars rich carburetors.......FWIW.......Cliff
    Last edited: Aug 9, 2019
  5. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    Ive seen weights hang up the advance in dist. They can stick on the rotor. The plastic nubs or even the bushing that holds the chinese weights.

Share This Page