Iffy idle, low vacuum problems....

[KJMason]

Sorry for the long post but I like being as thorough as I can be.

What I've got: 1970 Q-jet (7040562) with manual choke on a Pont 455.  According to the number the carb is a California model off of a 400-455.  GM HEI ignition with Accel Supercoil.  Heads are aluminum Edelbrock Performer RPM which should give an estimated 10.5 to 11.2:1 CR (depending on the head gasket used which I don't know).  Intake is Edelbrock Performer (yes, just plane Performer, not Perf RPM).  Cam is Edelbrock Performer Plus (advertised 278*/288*, @.5 204*/214*, LSA 110*, ICL 105*).  I don't know the timing as installed.  Comp Cams 1.65:1 roller tip rockers giving .462/.486 lift.  Hedman headers, 3"dual exhaust,  TH400 and 3.08 rear end.  PO claimed the engine had been dynoed at 400hp and maybe it is but then he also tried to tell me it was numbers matching (this was right up till I pointed out the block code right there for the world to see...lol).  I'm thinking with the cam and intake it's probably more like 350hp.  BTW, this engine runs fine on 92 octane.  I did have a mild part throttle ping that I fixed by simply limiting my vacuum advance.

So, here are the problems: 

Original setup from the PO: 14* initial (no vac advance) and mechanical advance of 18* (in by 2800RPM) for a total of only 32*.  Vacuum advance was disconnected (probably because of the part-throttle ping).  With this setup idle @750RPM gives only 11-12" of vacuum which seems low for this fairly mild cam.  Engine starts easily but idle is somewhat uneven (+- 50RPM) and RPM drops by 100-150 when put into gear.  Tuning idle mixture has little effect on the idle quality or vacuum, they seem to act more like on/off switches.  When I turn them in a bit the engine quit.  When I turn them out the engine basically runs the same and vacuum stays at 11-12" between two turns and five turns out.  Beyond four or five turns it runs rough.  Whatever mixture setting I use I have nozzle drip.  I've tried lowering RPM below 750 to see if that would stop the drip and, while it runs, it's fairly unstable and wants to stall. I've checked everything (several times) for vacuum leaks and can't find any.  After playing with it for a day this was the best I could do with no vacuum advance at idle.

I brought the initial timing up to see what the engine liked at idle which was 32* so switched to manifold vacuum with the vacuum advance can mechanically limited to 14* max and initial timing set to 18*.  Vacuum advance rate was set to bring in the full 14* vacuum advance by 9" so it keeps a solid 32* even when going in/out of gear.  Engine still starts easily (just a slight touch of cranking hesitation), idle is a bit smoother and varies a bit less (+- 30RPM) and vacuum is up to about 14" @ 750RPM but I still have nozzle drip and RPM still drops too much when put into gear especially with A/C on.  I had to raise the RPM in Park to 850 to get a 700-750rpm idle in gear which gives me quite a lurch when going into drive.

I monitored the timing when putting it into gear thinking the vacuum advance might be dropping out but it stays right at 32* (probably because vacuum doesn't drop below 9" which is full in on my vacuum can).  Initial acceleration is good with no hesitation to indicate my timing is retarding (or at least my centrifugal advance comes in quickly enough to compensate) plus my power brakes are liking the additional vacuum so manifold vacuum seems to work best for this motor.  Mixture screws are about two turns out but act pretty much the same as before.  I did notice that with these settings unplugging a manifold port idle RPM goes up about 100 so I'm thinking it's rich but that's more of a reflection of the nozzle drip than a too-rich idle circuit.

So, here are my questions: 

The carb has been rebuilt previously and I don't know what's in it so I plan on pulling it apart after the holidays but before I go through that, is there anything I'm missing?

When I do pull the carb I'll do a full cleaning and I'm guessing that I'll need to at least open up the idle tubes, restrictions, and idle air bleeds.  I'll most likely just rebuild the whole thing with Cliff's rebuild kit and at first was thinking recipe #1 due to the mild cam but then with the high CR, 1.65 rockers, higher lift and vacuum I'm thinking #2 might be better.  Thoughts?

Last, and thinking long term, I don't think the build is really a balanced collection of parts.  It runs well (ok, with the exception of the idle issues) but seems to be missing a bit of the "umph" that you'd expect from a 455.  The thing that really jumps out at me is that the cam and intake don't match the high compression, RPM heads, headers, and exhausts.  I'm pretty much stuck with the Performer intake as I run an original (and functional) shaker and don't want to give it up for a few more HP but that shouldn't affect the idle and vacuum.  Other than that I'd like to get a cam that would take more advantage of the rest of the build.  I'm not a racer so I'll be keeping the RPM reasonable (5500 max, another reason I'm not particularly worried about the intake) and looking for good street torque but don't want to trade away my vacuum as I'm not really interested in jumping through hoops putting in a boost pump or hydraulic brake assist.  Any recommendations are appreciated.

Thanks,
Kevin

[KJMason]

Oh, I forgot to mention that my carb has the idle air bleed adjustment screw located on the top of the carb in front of the choke housing.  It's mentioned in Cliff's book but there isn't really a description of how to use it other than it allows additional air to enter the circuit and can be used to tune the idle circuit.  I suppose I could try just loosening that a bit to see if it has an effect.  Does anyone have experience and tips on how to use this adjustment?

[429bbf]

I'm thinking some idle bypass air would help out a lot. when you overhaul the carb i would add bypass air that carb may not have it.also i don't know what kind of fuel pontiacs like but i think id start with the low end of recipe 2 that works good for big cube fords .with mild cams and a good intake system.they seem to like a little more idle fuel when you step the comp.to 10.5. I'm working on an easy external adjustment for bypass air but haven't had time to finish. hth

[KJMason]

Thanks, I was thinking that might be it but that I also might be missing something else that I could try before tearing the carb down after the holidays.  Do you have any experience with the air bleed adjustment screw on top of the carb?  I'm going to play with it this weekend, shouldn't that be similar to enlarging the idle bypass air?

[Cliff Ruggles]

The cam is very poorly chosen for the CID, compression and head flow, so it will never yield optimum performance...IMHO.

The carb still should be set up exactly for the application, start with a good rebuild with good parts, and check all the items related to the idle system while you are in there, and change the jets and metering rods to work better with that engine combo.  The CA emission carbs that year are extremely lean everywhere, so some tuning is going to really help it out.

I can spec out specific tuning parts, good rebuild kit, etc, just call the shop when you get time and we'll get what you need headed your way...thanks...Cliff

[KJMason]

Will do Cliff.  I was surprised at the cam choice myself even though I know little about them.  Got any recommendations for a good replacement for street use that'll still give me good vacuum for power brakes while taking advantage of what parts are already there?  Any tips on the idle air bleed screw on top?

[Cliff Ruggles]

On that carb the screw is main air, not idle air, so leave it fully seated (richest position) for initial testing.

A very good cam for that application at 10.5 to 11 to 1 compression is the Crower 60919 cam.  I would install Rhoad's lifers on it, but it will make over 500hp/550tq with decent idle quality and strong/broad torque curve.

We built and dyno'd a 455 with 87cc E-heads (unported) and that cam, and it made 505hp, 551tq and pushed a 3900lbs Firebird to 11.50's at 118mph in full street trim......Cliff

[KJMason]

Cliff, enjoyed our conversation last week, hope your trip went well and your new Harley is as nice as you expected. 

As you suggested, I did a compression check and came up with 220lbs,  pretty much exactly what you said to expect with the small cam.  The good side is that my rings/valves/head gaskets are in great shape but the down side is that I guess that confirms that I have the small E Performer Plus cam. I don't suppose that tells you anything about what the cam timing is though, does it?

I also ended up R&R my original distributor with one from Summit as my timing had been wandering a bit and I found the lower bushing was completely shot and the distributor shaft badly galled.  Also replaced the plugs and wires.  Plugs looked to be in the right heat range but the mixture looks just a little bit rich with a bit of black soot on the base.

The new distributor has helped the idle a bit and the timing is rock solid but vacuum is still low of course. I'm still getting the timing and advance where I need it (still haven't road tested things yet) but given the continued poor idle it sure seems to me I must have a vacuum leak somewhere.  Before I replaced the distributor I tried both the carb cleaner and propane tests looking for a leak with no success and I'm thinking that the wandering timing may have masked the engine response to the cleaner/propane so I need to look again.

BTW, I did some looking into the 60919 cam/Rhoads lifters you recommended and that's what I'll eventually do as well as looking to replace my current intake and doing a drop base mod to the air cleaner but none of that'll happen for a while so I need to get the current combo working as well as I can.

Also, a last question but I've seen several places where people have claimed the stock power brakes should work down to about 12"-13" of vacuum but mine (stock booster/master/disk/drum combo) needs more like 15"  Should 12-13" really be enough for GOOD performance (seems doubtful) or is 15" more normal?  I'm considering replacing the whole assembly primarily to improve performance but also cosmetics.  I'm just really looking to prioritize my changes to get the car more drivable.

[Cliff Ruggles]

The engine is doomed with that cam in it, and it will never run correctly.  The intake valve is just closing too soon, nand not nearly enough overlap for the CID and compression ratio.  That cam is a great choice for a 400cid engine build with 8 to 1 compression, but will never yield desirable results in a 455 build over 10 to 1 compression.

I'd start with a cam swap, and things will get a LOT better from there on out......Cliff