bwinterstine

I'm rebuilding my '75 Vette (see separate thread in General: Small Garage Restoration); I'll make a long story short for this problem I'm having.

I installed a 1970 355 that I had purchased from another member. The engine was rebuilt less than 1000 miles ago and seems to run strong; it came without an engine and it has a Erson TQ-20H cam (mild cam from what I read). I installed my original '75 quadrajet on the engine. The carb appears to be original and not rebuilt; the idle mixture plugs are still present.

- The engine is set at 8 degrees BTC timing.
- All vacuum ports are closed except for distributor advance and PVC which are connected.
- Vacuum ports on head behind carb are closed off.
- Fuel delivery from tank to carb is new or cleaned and in good condition. New fuel pump, new fuel lines, new fuel filter. Tank and pickup clean.


Problem: Hard starting and poor/bad idle characteristics

Here's what I see:

-Engine will run smooth at higher RPMs but will not easily idle. In order to idle, the choke needs to be closed with about 1/4 opening. Fully open and the engine will stall.

-Any attempt to get the engine to idle at less than 800 RPM and it will stall. But, once the car is warmed up, it will run at 800 RPM with the partially closed choke for ever.

-At 800 RPM, there is about 13-14 inches of vacuum. At 1000 RPM there is about 20 inches of vacuum.

-I checked the PVC hose ... no leaks ... there doesn't appear to be any vacuum leaks.

Question: It appears at idle the care is running lean; so, if I remove the idle mixture plugs do you think I'll be able to adjust the idle mixture enough?

bwinterstine

CORRECTION: The carburetor is not an original vette carb. The number is 7059216 and according to what I can find, it's a '79 Camaro carburetor.

I also chatted w/ the original owner of my engine (thanks JNB5101) and he was running a '68 396 quadrajet and it was running well. His 396 carb's primary metering area is 0.00237 and my Camaro carb's area is larger at 0.00281. Whould my larger meter make my car run lean at idle? The secondary's are different also but I'm not too concerned about that right now.

I guess I need some expert quadrajet advice on my carb setup.

7T1vette

Send an e-mail to Lars Grimsrud at V8FastCars@msn.com requesting advise on proper setup of this carb. It would be helpful for him to know if the carb you are using has been modified from its original build, or not.

If it has not been modified and you feel confident at working on the carb, he may be able to advise you of changes that will solve your problem. Or, if he is willing, you might send it to him for rebuilding it for your needs. His fees are very reasonable.

bwinterstine

I may have to "bug" Lars if I can't find the problem. I'm still looking for advice and suggestions to try and find the problem.

I did check the secondaries to make sure that they are fully closed and not sucking in air. They are closed.

My next step is to pull the carb and remove the plugs for the idle mixture screws. I need to see if they can be adjusted.

lars

The first thing you need to do is fix your timing. 8 degrees initial with a mild cam isn't by far enough. You need to get the initial timing up in the 18-degree range and limit the total to 36. Make sure you have a vacuum advance control unit appropriate for the level of vacuum you're pulling, and that it's hooked up to manifold vacuum. This should produce a combined actual timing at idle of about 30 degrees. Anything less than that and you're going to idle like crap.

Secondly, if that carb is all original with the limiter caps still installed, it's way too lean for your application. You need to get the float level raised to about .300, check the IFR tube size and possibly enlarge them a tad, and get the limiter caps off so you can set your idle mixture correctly - the lower engine vacuum with your cam won't pull enough fuel through your stock-set idle system.

Lars

bwinterstine

Thanks Lars, I may still need to "bug" you more. Today is a wash because it's just too darn cold again and snow here in South Jersey. Tomorrow's plan is to move the vacuum line off of the ported supply to the manifold vacuum and to set the initial timing to 18 BTDC. I'll check the total advance also and report.

I'll post an update once the timing is set correctly.

bwinterstine

I decided to try again today. I moved the vacuum advance off of the carb port and to the manifold port. I warmed up the engine and set the timing w/o the vacuum line attached to 18 degrees. It idles much better...not perfect but probably good enough for now.

I don't have the choke warm air attached so once the engine is warm, i need to hold open the choke w/ a (large) screw driver. Anyhow, I was able to get the engine to idle at about 700 RPM which is a big improvement. Vacuum is still a little low, 13 inches, at 700 RPM but that's to be expected w/ the cam the car has.

I'm going to install an electric choke and then try to tweak it but I think it's good enough for now. I probably still have many months before the body goes back on.

I may still have to open up the idle per Lars suggestions at some point but I think the carb is good enough for now and until I get a chance to drive the car.

Everyone, Lars and JNB5101: Thanks for your advice, it really helped!

ctuinstra

Not trying to highjack a thread here, it seems as if it mostly complete. I seen where it's mentioned about moving the vacuum advance from the carb to the manifold port. Why, shouldn't they be the same (assuming the proper carb port to begin with). I ask this also because I noticed my Holly has another vacuum port that the manual says its for no advance at idle. Should this be used or just leave it on the manifold or lower carb port?

bwinterstine

Good Question. We'll probably need some more senior opinion on this but I'm GUESSING that on my car (with a stock emissions carburetor sitting on a 70 engine w/ a mild cam) the advance vacuum is a little more steady off of the intake port and is not cut off during idle. Basically, pulling the vacuum from the intake makes sure that the vacuum advance is fully engaged at idle.

Right?

jnb5101

It depends upon where the vacuum port is connected inside the carb. If it's above the venturi (butterfly), it's exposed to atmospheric pressure, if it's below, it's manifold pressure. It was GM's attempt to reduce NO emissions by retarding the timing and thereby lowering exhaust gas temps.

7T1vette

Ported vacuum outlets have NO vacuum at idle; manifold vacuum ports have FULL vacuum at idle. The effect of the vacuum advance can [at idle] on the dizzy is eliminated, if it's connected to ported vacuum.

Once the throttle begins to open, both ports have nearly the same vacuum level.

bwinterstine

Agree

bwinterstine

The car is idling OK now but it could be better and I'm hoping that the idle mixture screws will help. Not sure when I'm going to pull the carb and remove the caps so I can access them.

I was concerned that I may have too much advance now so I set it back to 16 degrees and I wanted to double check my total idle timing. My vacuum module spec is 11-12.5 distributor degrees which is 22-25 crank degrees. If I left the initial timing at 18 and the vacuum advance was providing 25 more, that is 43 degrees of idle timing ... too much (shooting for 36 total).

So, I measured the circumference of the damper (25.25"), divided that by 10 (360 degrees / 10 = 36 degrees) and marked the 36 degree timing mark 2.5 inches to right of the timing line on the damper. If I'm getting 36 degrees of total timing at idle with the vacuum advance connected, this 36-degree line should line up with TDC using a timing light. It's close, it is steady at 4 degrees ATDC. So, I'm getting a total of 32 degrees timing at idle and the amount of vacuum advance I'm getting at idle is only 16 degrees (the spec for the distributor says 18 degrees and my vacuum advance module is 22-25 degrees).

Before I add back more initial advance, I need to understand the 1112888 distributor spec I pulled from my service manual and I need to calculate cruise timing. According to the table, the centrifugal advance at 1600 RPM will be 12 degrees and at 4200 RPM it will be 16 degrees. So, say I cruise at 2600 RPM. The centrifugal advance will add in about 14 degrees. And, at cruise, my vacuum advance will also be operating at full advance, 16 degrees. So, my cruise timing would be 16 Initial + 14 Centrifugal + 16 Vacuum Advance for a total of 46 degrees. This is well short of optimal 52 degrees.

Here's my question: Is the centrifugal advance spec given in distributor or crank degrees? If it's in distributor degrees and I need to double the 14 degrees, then; I'm way over and my cruise advance would be 16 Initial + 28 Centrifugal + 16 Vacuum Advance for a total of 60 degrees.

By the way, the reason I think my service manual distributor spec is in crank degrees is because it states that the max vacuum advance is 18 degrees (very close to the 16 degrees I measured).

Anyone else lost?

jotto

What happens if you work it a little differently.....

Set your timing to 36 degrees at around 2500-3000 rpm with the vac can disconnected and plugged. This is your all in.

Now check your timing at idle, what is it?

Reconnect vac can and see where that get you. This is where you are looking for the max of 52.

bwinterstine

I'll do it today if I can beat the rain.

bwinterstine

Final timing evaluation...

I warmed up the engine and setup my tach meter and timing light. With the engine running at 2600 RPM (strong and smooth) I checked the timing with the vacuum advance still connected. The 36 degree mark on the damper was at 16 degrees BTDC. So, at cruise, this would be a total timing of 52 degrees...perfect!

This the cruise timing breakdown is: 16 Initial + 16 Vacuum advance + 20 Centrifugal.

Thanks again for all the help. I still need to pull the carb and get at the idle mixture screws to tweak the idle but it is much better then when I started this tread. I had a timing issue.

scottyp99

There's an old rule of thumb that says to always get your timing squared away, then start tweaking the carb. I think you are on the right track, good luck,

Scott

Paul L

So you set initial at 16 using intake vacuum and advance connected?

bwinterstine

No. Initial is at 16 with advance hose disconnected and plugged. Vacuum advance adds another 16 for a total of 32 at idle. Mark to shoot for is 36 degrees so I could still add 4 more degrees.

I measured the cruise timing today which is the total of initial timing, vacuum advance and the centrifugal advance. Mark to shoot for is 52 degrees and I'm spot on.

Paul L

OK, thanks.