Idleproblems on my Cadillac 500 stroker

[MonteCarlo]

Hi. I am new at this forum but I give it a try.

Have a Cadillac 500 stroker 540 CID with rollercam .600 lift 237 duration 112 LSA. 10:6 compression. Ported heads bigger valves edelbrock intake etc.

Quadrajet 750 17059216 rebuilt in Sweden from a good workshop.

Ignition base 16 max 35   10 degrees vacuum advance.

The engine seems to pull approx 10 hg.

My problem is that I have to give it huge amount of bypass air to get it idle. I have done that througt a controlled vacuumleak (1/4 inch) together wit 0.135 bypass air. With the mix screws out about 6 turns it working but it not feel right because of that big extra hole. The butterflyes open about 1 turn, think it is pretty much ok for the transition slot. Idle between 900-1000

Specs:
Idle tube: 0.04
Restriction: 0.064
Upper air: 0.071
Lower air: 0.071
Holes in baseplate: 0.96
Idle bypass: 0.135

I think it should idle without the extra bypass througt the extra nozzle. I read in another thread where a guy had simular problems that the airbleeds was to big and that it was better to move the upper airbleed up to the horn?
happy to get any suggestions. Ready to give up

Erik Olsson Stockholm Sweden

[77cruiser]

That's what I did move the idle air bleed to the air horn. Drill from the bottom the hole is already started. 6-32 brass set screws 1/8 inch long. You'll plug the original bleeds.
This is how I did mine, not my pics though.

[MonteCarlo]

That's what I did move the idle air bleed to the air horn. Drill from the bottom the hole is already started. 6-32 brass set screws 1/8 inch long. You'll plug the original bleeds.
This is how I did mine, not my pics though.

Thanks, i think I try that. Was this a big inch engine to?

[77cruiser]

421 SBC idles with about 12 " vac.

[Cliff Ruggles]

That vacuum reading seems low to me for the CID, compression and cam specs.

Anyhow, did you try tipping in the choke flap or gently placing your hand over the choke housing to richen it up some at idle?  If the engine responds well to this it may simply need more idle fuel instead of a bunch of idle bypass air, and may like a little of both.

With .096" holes under the mixture screws you're probably at or close to maxed out on idle fuel delivery with the screws backed out 6 turns (metric threads).

I'd do the test mentioned above before blocking off the upper idle bleeds and moving them to the airhorn as it's always best to determine if the engine actually needs more fuel, or more bypass air before drastic changes to the original set-up.......Cliff

[77cruiser]

One of the nice things about doing that mod, if if you miss the calibration a bit you can just change the air bleed to get it where you want.

[MonteCarlo]

That vacuum reading seems low to me for the CID, compression and cam specs.

Anyhow, did you try tipping in the choke flap or gently placing your hand over the choke housing to richen it up some at idle?  If the engine responds well to this it may simply need more idle fuel instead of a bunch of idle bypass air, and may like a little of both.

With .096" holes under the mixture screws you're probably at or close to maxed out on idle fuel delivery with the screws backed out 6 turns (metric threads).

I'd do the test mentioned above before blocking off the upper idle bleeds and moving them to the airhorn as it's always best to determine if the engine actually needs more fuel, or more bypass air before drastic changes to the original set-up.......Cliff

Thanks for your tips. I have not tried tipping the choke. Should do that. I think the vacuum is a bit low to. I have checked cylinderpressure and camtiming and Find nothing wrong. Maybe get some more vacuum if I get the idle right? Your suggestions seems logic now when I think about it. I was lost in the wrong loop  . Another thing, do you ship spareparts to Sweden or is it to far away?

Thanks

Erik

[MonteCarlo]

One of the nice things about doing that mod, if if you miss the calibration a bit you can just change the air bleed to get it where you want.

You have a point there. Maybe try it later after trying Cliffs suggestions.

Thanks
Erik

[Cliff Ruggles]

One has to keep in mind that this is 540cid, so it's going to require a significant amount of idle bypass air right to start with simply to feed the required cfm to sustain a "normal" idle speed.

I do a LOT of carburetors for big block Chevy engines, and many of them are over 500cid.  The factory used HUGE idle bypass air on the carbs used for the 454 engines in the 1970's due to lower compression and "weak" idle signal.

We've found that many are completely happy with generous amounts of bypass air to keep the throttle angle low and not expose too much transfer slot.  This fact applies as long as idle fuel delivery to the mixture screws is adequate. 

When you don't have enough fuel to the mixture screws you never find out if the engine really wants/needs more bypass air because it refuses to idle well at low throttle angles and the shear size of the engine has it pulling hard enough on the carb that it quickly/easily pulls from the boosters (nozzle drip) to sustain itself.

To diagnose this condition the tuner can use a manifold vacuum leak to lower the throttle angle.  We do this all the time here in the shop by removing the small hose going to the pull-off unless there is another one that is easier to access such as the front of the main casting above the pull-off source or rear of the baseplate.

Once you add some bypass air, re-adjust the idle speed and then take a gander to see if the nozzle drip has stopped and if you have more control with the mixture screws.  Bypass air is an EXCELLENT feature in a Quadrajet and allows the tuner to get the carb off the main system and reduce fuel flow from the exposed transfer slots at the same time.

I also recommend to experiment with initial timing as well when idle tuning.  Sometimes just a few additional degrees of timing will increase vacuum and signal to the idle system without having to go into the carb and open things up. 

Most engines with well chosen cam/compression for the CID will NOT require a lot of timing at idle speed so adding a lot of timing is not a cure-all for inadequate idle fuel, but it is still very important as a tool for idle tuning.  Basically we don't want to go in and open up things too much as idle fuel also becomes transition fuel as the throttle angle is increased.......Cliff

[MonteCarlo]

Hi.

A lot of good information here. This will certainly help me get along with the idle tuning. As you say it is a big engine so it was hard for me knowing how much I should open things up without overdoing it.
The idle chanels are allready in the size of recipy 3 in your book but maybe I can go a little further then if I found out that it needs more fuel.
Should give it a try next weekend. Hopefully my next message is "problem solved" or something like that.

Thanks for information.

Erik Olsson Stockholm Sweden

[77cruiser]

How far can you open your throttle before you get into nozzle drip. Maybe you can open them more than 1 turn. I know they show a lot of transition slot but they're not Holleys. Can't believe I said that word.

[Cliff Ruggles]

The tendency to get "nozzle drip" can vary dramatically from carb to carb.  This simply happens because there were quite a few different boosters used in different carb numbers, and vastly different calibrations.

Carburetors with much smaller MAB's, such as Marine units for example, can see nozzle drip at very low throttle angles.  This simply happens due to the sensitivity of the boosters and overall calibration of the carb combined with the specific parts used in it.

Fuel level is another player as a higher fuel level allows the fuel to flow easier from the bowl, all else being equal.

Fuel pressure and fuel seat diameter are also players here.  The higher the fuel pressure and/or larger the inlet seat diameter, the higher the fuel level will be at any given float setting.

The size of the float also becomes a contributor here as well.  These are a few items folks never even take into consideration when custom tuning.

It is also why I push using are parts, because we have already tested them and will supply the correct ones for what you are doing, and follow that up with specific tuning advice.

This fact is also why I often shy away spending countless hours on the phone helping folks with "free tech" when they have purchased EVERYTHING someplace else, it doesn't work well, the folks where you bought the parts are useless for advice.........then I'm being asked to crawl thru the phone and fix all the issues.

This scenario plays itself out here WAY too often for my liking so I have my calls screened so I can stay on the job and get the carbs the very patient folks have sent here for us to work on finished up.

Besides, if I took every phone call that came in here for "free technical assistance" we'd be out of business in a month, maybe less!

That's the facts of life here folks, pardon my attitude and comments, but we do this FULL TIME and for a living and really do want everyone to have a good end result with these things, just throw a bone our way once in a while so we can continue to provide this service.....many thanks.....Cliff

[MonteCarlo]

How far can you open your throttle before you get into nozzle drip. Maybe you can open them more than 1 turn. I know they show a lot of transition slot but they're not Holleys. Can't believe I said that word.

I only know it was way to open when I started. Probably I can open a little bit more without nozzledrop. I am going to work with this during the weekend.

Erik

[MonteCarlo]

I only know it was way to open when I started. Probably I can open a little bit more without nozzledrop. I am going to work with this during the weekend.

Erik

Hi again. Getting a pretty god idle now. The trick was to change a little bit here and there.

Opened up the the bypass a little more. Raised the timing a few degrees. I get a better idle when using manifold vacuum, good or bad I dont know. Opened up the idle fuel a little bit but then I think it is going a little bit rich of idle. I think I will reverse that.

Called up MaximumTorqueSpecialities in California and talked about the camshaft and low vacuum. The guy at MTS sad that 10 hg was around the expected value with this cam.

Thanks for help and all the tips.

Erik Olsson

[Cliff Ruggles]

540cid with 237 degrees @ .050" and 112LSA isn't really a big cam.  I'd expect considerably more vacuum at idle speed that what you are seeing, unless the static compression ratio is pretty "low".

I seldom use manifold vacuum to the advance but have seen a few combinations that require a lot of timing at idle for one reason or another, which is usually a bit too much cam and/or not quite enough compression.......Cliff