My cam is too big for my motor (Comp Cam 292H-10), having a hell of a time
getting it to idle properly (more engine specs linked in my signature).

The best I can do is ~ 4 in./hg. with carb adjustments, cam phasing (dowel offset bushings) and 0.005" pre-load. I have set the intake centerline back
to MFG specs and have been driving it like this for weeks.

I tried to open the carb throttle blades with the idle screw, but going too far
takes the transfer slot out of the equation. I've even opened the secondary
blades with the adjustment stopper...but no luck.

Timing is 20 degrees base @ 900 RPM, 37 degrees full advance. Moving base
timing lower and adding manifold vacuum at the distributor doesn't help either.

At this time, I don't think there's anything else I can try, but I'm open to suggestions.

I guess I'm going to need a vacuum canister to save this engine at ilde and
restore my brakes.

Are there any links to how a canister functions? Found tons of threads on
what they are, what they do - but not how they work. Google returns the
same stuff, and a bunch of links for vacuum cleaners

 

The vacuum can isn't going to increase your idle vacuum. All it does is store the highest reading it sees until you hit the brakes and then it dumps to the brake booster. Basically all it amounts to is a sealed can with a one-way check valve. If your only seeing 4 inches then all it's going to store is 4 inches. You'll see more on decel from a fairly decent RPM, but it's going to be gone the first time you hit the brakes.

I would start looking for leaks somewhere, and I would begin the process with a leakdown test to make certain the rings are seated. However, if you have no leaks (either outside or inside in the lifter valley) and your dead set on running that cam, then you can always look at a fluid assist for the brakes. Works by t-ing off the power steering pump to provide power assist instead of vacuum. Not sure which exact cars/trucks came with it, but I'm pretty sure the turbo diesels do (the chevy versions anyway). I'm sure someone will chime in here on that part #.

Good luck man.
Dave C.

 

So the vacuum canister is essentially a remote brake booster (same concept)?
I realize it's not going to give me more vacuum, I meant I need it to get my
brakes to work properly.

The brakes are weak below ~2500 RPM. Even from decel, it's not very satisfying.

What I might end up doing is installing higher flow mufflers (which I had before).

Trouble with that is the police.
I might have to hire David Vizard to design a zero loss system which runs quiet!

I'm getting the feeling, the pressure is so high in the exhaust that it's
not allowing enough to flow out. Sort of feels like EGR is stuck open at
idle.

As for the cam, I'm torn between keeping it. The engine really picks up,
and the power is amazing. The kicker is, it's more a strip setup now.
With 3" x 24" extension pipes off of the collectors, the engine is happy.

I've also made some supporting mods which are going ot be tough to undo.

Thanks for the heads-up on the canister!

 

Why not try tapping your exhaust at the collector and running a line to a pressure gauge? It'll at least tell you if your truely up against a positive pressure problem of if it's somewhere else. Another thing to remember is that if your only seeing 4 inches at idle, your carb is probably not seeing enough signal to properly meter at idle. Oh, and with that much cam and a single plane, I would also consider a higher stall than 2800. More like 4000 to get the thing to swing where you need it. Just an observation...

Dave C.

 

Yes, the stall is a big problem and I've been thinking about going to a 3500.

As for the exhaust pressure, I know it has issues with the current mufflers.

I used to have these (Purple Hornies) coming off the collector exiting to the
side:
http://gmthunder.com/tino/exhaustside.jpg
http://gmthunder.com/tino/exhaustside2.jpg

Now I have what seem to be stock mufflers turned down at the axle:

http://gmthunder.com/tino/stockmufflers.jpg

If I take the mufflers off, or install collector extensions the engine idles much
better.

I haven't measured the difference, but it wouldn't take much to unclamp
the mufflers and try (I know...clamps!)

I'm planning for a complete revision of the exhast which will be a 2.5"
pipe, dual system with h-pipe to twin straight through mufflers for the street.
It's just a matter of funds right now.

http://gmthunder.com/tino/exhaustcalcs.jpg

It's nice to have a second set of eyes checking out the setup.

Good points.

 

About the carb metering issue:

Have you known flow plates to help with this problem?

I'm just looking for other ways to get around this problem since I can't get
around the exhaust issue at the moment.

I was hoping there were some tuning tricks, or minor modifications to get
some more vacuum of out the idle.

I'll look into the vac. can to help the brakes.

Thanks for your time.

 

Well, I don't think your going to be able to artificially increase the vacuum to the carb. I would suspect that it would run better at lower RPM's with a bit smaller carb, like a 650 vac secondary. You can adjust the high speed bleeds to pick back up your mixture on the high end if you don't get enough fuel in the holes. Also, I've never seen a motor that didn't pick up when a carb spacer was added. Even if it's only a little increase, if you've got the underhood room, go for it. Also, you might wanna double and triple check the rocker lash. Hanging the valves open will destroy vacuum in short order. Lash em down to just zero, and try the idle like that for a minute or two. Yeah, it'll make noise, but it'll give you an indication of whether the lash was too tight to begin with and give you a chance to see what the max vacuum signal your ever gonna see is. I wouldn't run it like that for long though

Dave C.

 

Already messed with the pre-load. I went from 0.010" to 0.005". That helped
a touch. I don't want to go much lower because it's a flat tappet lifter.
Maybe I can sneak it down to 0.002" and see what happens.

The spacer I have now is a 1/4" wood open spacer which is primarily used to
insulate heat from the carburetor. A four hole spacer might be a better option
in my case. Any particular types, or brands that you recommend?

Maybe a spread bore carburetor instead of a vac. 650? I'd rather stay with
a mechanical secondary. I can always swap back to the 700 CFM at the track.
The square bore seems to even the fuel distribution to all cylinders
when wide open. I've been messing around with jetting to get the plugs
looking even from front to back (76/88 jetting right now).

I think I went a little too far with this motor. Now making it streetable again
is going to take away from the track performance. Maybe your idea with the
spacer and smaller carb will allow me to enjoy the car until the exhaust system
is complete.

Good stuff Dave.

 

As mentioned before check EVERYWHERE for leaks. You should also lock out your timing(I.E) no curve and start with about 36*. There are people that will tell you how great advance is but they have never ran 240+ degrees of duration on the street with a carb before either. You are bleeding off so much cylnder prss. at low RPM that there is virtually no chance of you experiencing detonation. Also you can drill the two rear carb butterflys starting with a 1/32 drill bit and go from there. These two mods should get you up to about 8-9" vaccum and let you tweek from there (air bleeds, ETC). I've run very simular combos in the past and never seen over 11" so just realize that there are compromises to be made. Feel free to PM me if you need any help.

James

 

Vacuum leaks are not an issue. I'm only running a PCV valve and brake booster.

Those hoses have been checked for cracks, and tight fit at the carb ports.

All other vacuum ports on the carburetor are plugged and leak free...same
goes with the mating gaskets for the manifold flange.

Instead of drilling holes in the throttle blades, I have tried opening the secondary
blades by about 0.005" with the set screw adjustment.

That allows more air to flow without comprimising the idle adjustments, or
acclerator pump circuit. Do you still recommend drilling at this point?

I like your idea about timing. I'll try fixing the mechanical advance and run
a higher base timing.

 

You may still end up drilling in the end but I would make locking down the timing priority number 1. Remember you will need a degreed balancer or a dialback timing light to set everything as the base and total timing will now be the same thing. This should get you about 80% of the way there.

 

No problem, I have an advance timing gun.

I'm going to give it a try right now!

Thanks.

 

Thank you both for your suggestions, the engine is now idling well and I have
a hint of braking assist once again.

Setting the base timing to 36 was too much. I had trouble starting the engine.

Instead, I set the base to 20 again but installed very light springs on the advance weights.

The timing is pretty much "all-in" by 2500 RPM now.

With some dyno and track tuning, I'll be able to dial in the full advance timing
and check the AFR (new jetting combo 80/88).

 

What power valve do you have and does the carb have idle mixture screws on the secondaries? Your cam is not that big, you just need to tune and possibly modify the carb. The cam in my car is a solid roller with 264 and 274 duration at .050", your cam is only 244 at .050".

I would make the following changes in this sequence:

1. 2.5 power valve
2. Open the secondaries by adjusting the set screw on the under side of the carb
3. Set it up for 4 corner idling
4. Drill holes in the throttle blades

Do not lockout the ignition. Run 20 to 24 degrees at idle with 36 to 40 total and you want it all in by about 2,500 rpms.

 

Power valve is indeed a 2.5 (down from 6.5). That's about what Holley recommends for my idle manifold pressure.

Done. Helps a touch.

Not in the budget. Doesn't seem to "need" it, but it would be a nice addition one day.

Don't want to go that route. Throttle blade adjustment is working well.
The transfer slots are about 0.040" exposed with the new setting.

Done. Set it to 20 this evening. She's snapping off the line nicely. All I need
is some dyno and track testing to find the optimum full advance timing.

THe last setup liked 37 total. That's were I'll start and dial it from there.

Might be going Wednesday night to race the car.