Maymyvetteliveforevr

75 Vette TH400 Auto. 55,000 miles.

Problem

-Engine doesn't seem to want to idle, presently at 600/800 rpm in drive 850/1050 rpm in neutral.
-Engine surges 200 rpm at idle.
-Very high smell of gas, (my clothing and I, smells of gas after a 20-minute drive)
-Poor pick up, (maybe due to 308 rear end ratio).

Engine rebuilt at 20,000 miles by me.

350, L48
10:1 flat top pistons
¾ race cam (nick name 35 years ago) don’t know exact specs.
Hydraulic lifters
Double valve springs
3 angle valve job.
305 hp to the rear wheels.
2.5” dual exhaust, no cats, crossover.
Blackjack alumicoat headers.
Rochester Quadrajet, rebuilt by Lars 500 miles ago (over 5 years ago). 75 Vette Carb.
New ignition wires and plugs 500 miles ago.
New pickup coil, cap, ignition coil, electronic module, 500 miles ago.

Engine specs at present.

Timing 12 degrees tdc. 28 degrees total @ 2800 rpm
Highest vacuum, 12 psi @ 900 rpm (connected to front, bottom, right port of Rochester Quadrajet)
Both fuel/air mixture screws set at 5 ¼ turns out.
When I place my hand over primary (front two barrels) the engine surges 500 rpm higher.
While I’m checking timing, idle ranges from 800 rpm to 1000.

TimAT

IF you cover the carb with your hand and the RPM increases, it's a good bet you have a vacuum leak somewhere.

'75

It's lean for some reason or other, that's why the bad smell. I would start eliminating vacuum connections, to try to narrow it down. You could also use spray carb cleaner to try to locate the leak. Just don't use brake cleaner, becomes very toxic when the engine burns it.

Maymyvetteliveforevr

Thanks for the reply TimAT and '75.

According to the link below using the hand-choke method is either a lean or vacuum leak issue, Since it smells rich, I'll go with the vacuum leak.

What would be a good vacuum pressure for my spec engine?

"3. Handy Test Pattern
You can determine whether an engine is running rich or lean at idle without resorting to gauges or fancy devices. One method involves removing a hose that connects directly to the manifold vacuum (the engine must be at operating temperature). If the engine speeds up, the carburetor is too rich. If it stalls or stumbles, it’s too lean.
Another way to achieve the same results is to hand-choke the engine. Place the palm of your hand over the carburetor. If the engine speeds up, it’s too lean or, worse, could have a vacuum leak. By the way, two places that are seldom checked when it comes to vacuum leaks on street/strip cars are the PCV hose and the power brake booster hose. Check them if you’ve encountered an impossible-to-find leak."

http://www.hotrod.com/articles/mscp-...tem-tech-tips/

'75

A vacuum leak makes a lean condition since air is being drawn into the engine without going through the carb where it would be enriched with fuel. The advise to check the pcv and booster is sound and easy to do, basically unplug them and block the ports to see it the problem changes, same with the small vacuum hoses on the manifold for trans etc.

Maymyvetteliveforevr

Update:

Vacuum

Sprayed a full can of carb cleaner around
Front and sides of intake
All vacuum hoses & lines on intake
All fittings, egr valve etc.
Base of carb as well as each section.
Around throttle plate.
Pinched each vacuum hose coming off of intake.
Around pcv valve hose.

Conclusion

I found one minor vacuum leak going to the headlight vacuum tank so I removed the hose and plugged it at the intake.
The vacuum pressure has gone up one to around 13.5 psi but no change in the engine stalling or surging.

The car idles higher when I raise the timing from 12 tdc to 14 and more if I set it at 16.
Fuel air mixture is still at around 5 ¼ turn out for highest vacuum @13.5 at 950 rpm.

I covered the primary flaps and the car idles 500 rpm higher (same as before),

I also did a test and while covering the primary (front barrels of the carb), fully closed both fuel air mixture screws and the engine was still running at 1100 rpm.

Am I to assume there's still a major vacuum leak? If so how can I find it as I tried the spray?

REELAV8R

Looking at your specs (305 HP at the rear wheels on a 350) and what you got for timing it looks to me like the timing is too retarded and the idle mixture screws are too far out.
5 1/2 turns is a lot for those unless this is a 79 carb or something similar with the fine thread idle adjustment screws.

12* of timing for what looks to be a performance cam is fairly retarded as well. I would try something in the teens. Not knowing the cam specs makes it a bit of a guess, the engines response should help tell you what it wants though.

My 350 with a 270/270 108 LSA cam is set at 19* initial timing. I had to limit the mechanical advance to get total advance at 32*. Yours may need something similar.

Also if your using vacuum advance and your vacuum can can't respond to your low vacuum then it may need to be changed. A vacuum can that cuts in and out with the RPM changes is one that has too high of a vacuum requirement to pull all the advance in with the available idle vacuum. It should fully advance at idle and stay there, not cycle in and out.

MelWff

are you aware that you adjust the idle speed and mixture screws with an automatic in Drive?
are you using a vacuum gauge or tachometer to set the idle mixture screws?
the 5 1/4 turns out does not sound correct, way too rich.

Maymyvetteliveforevr

REELAV8R

I agree that 5 1/4 turns is too much but if I turn it in anymore, I guess less vacuum pressure and the car stalls in neutral from 900/1000 rpm starting point.
It's a 75 Vette carb,
I advanced the timing to 17 degree and it runs slightly better but still stalls while in drive. My total advance is 28 degrees at 2800 rpm.
Yes I have vacuum to my advance/distributor.
I'm not sure what you meant in your statement "A vacuum can that cuts in and out with the RPM changes is one that has too high of a vacuum requirement to pull all the advance in with the available idle vacuum. It should fully advance at idle and stay there, not cycle in and out."

MelWff
I wasn't aware that "you adjust the idle speed and mixture screws with an automatic in Drive", but either way, I'm unable to do so as it won't idle in drive.
I started off using a vacuum gauge, then tried the tach method via a timing light. Either method seems to obtain close to the same results.
I agree that 5 1/4 turns is way to rich but when I turn it in even 1/2 turn I loose vacuum as well as rpms which is what I'm trying to avoid. I believe my gas mileage is less than 10 mpg.

REELAV8R

Let me see if I can explain this better.

You have an engine that is developing 12"of vacuum at idle in park.

A vacuum can has specs that state it's requirements for advancement of the timing.

Let's say for example that your vacuum can starts advancing at 10" of vacuum but does not fully advance the timing until 15" of vacuum. Just as an example.

So what is happening is your timing is 12* initial that you have set plus whatever the vacuum advance is giving it at idle vacuum. If the total movement of the vacuum advance can is 17* then maybe at this point (12" of vacuum) it's giving it 6* in addition to your initial of 12* for a total of 18* advance at idle. Engine is fine with that and will idle.

Now you drop it into gear and the RPM drops. The vacuum goes down to say 8" instead of the 12" you had at idle out of gear. If your vacuum can doesn't even start to advance until 10" then you just lost all of the vacuum advance because you put it into gear and your timing drops to the initial setting only 12*. This is all assuming you are using manifold vacuum and not ported vacuum.

For a cam with lots of overlap/long duration/ narrow LSA low timing lead is a problem and it will not want to continue to run.

Performance cams have their own set of problems, one of which is low cylinder pressures at idle RPM's. Due to the low pressure the fuel burns slower. Slower burning fuel needs to ignited earlier to extract the most power from the current RPM. This is why lots of timing may be needed to get it to idle correctly.

Another factor is the stall of the torque converter. With a big overlap/long duration/narrow lsa cam the torque converter will drag the idle way down and maybe even kill the engine if it is still stock stall or does not have a high enough stall. With your cam it sounds like you should have in the neighborhood of a 2600 to 3000 stall torque converter for it to idle in gear comfortably.

Lars has papers on various vacuum cans and their associated specs for the HEI.

His papers may help you quite a lot for setting up what you got.

Let us know how it goes.

What heads are you running? I would venture that you can bump that to at least 32* to 34*.

Maymyvetteliveforevr

REELAV8R

Thank you for a more in-depth explanation as I'll need to read it over a few more times to get a better understanding. I'll try to locate Lars papers and learn more about timing.

I'm running the stock L48 heads, during the rebuild, I went with double valve springs and a 3 angle valve job only.


EDIT;

In the meantime, I'm hoping I can find the source of the vacuum leak and hopefully boost my pressure up one or two more psi.

CanadaGrant

Did you plug or check the vacuum line to the brake booster? Sometimes the plastic fitting at the booster or the booster itself can be leaking.

lars

Let me check the setup specs I did for your carb when I get home to the workshop this evening. If you're only pulling 12 inches of vacuum, the carb will run lean at idle since it won't pull fuel through the idle circuit. Try backing the idle screws out to 6 or 7 turns. The lean idle will cause a surge, and when you put it in Drive, vacuum gets worse and the carb will lean out even more.

You need to run a lot more than 12 degrees of initial timing with a "3/4 cam." Get your initial timing up around 18-ish, and make sure you have vacuum advance hooked up to manifold vacuum so you pull in an additional 12 degrees of timing at idle (actual timing at idle with vacuum advance hooked up should be around 30-ish). With your vacuum, you need to be running a very "soft" vacuum advance - if your vacuum advance is not correctly matched to your engine vacuum, it will cause idle problems and idle surging. This could be your problem as much as any carb issue.

Your rear end ratio with an automatic and a big cam will make the car a real dog from a standing stop...

Lars

REELAV8R

You can always take every vacuum source off except the vac advance and plug the ports to see if you get an increase in vacuum. If not, you have no leak or one not significant enough to effect your vacuum levels.

If there is a difference then start putting one item at a time back on and see which one affects the vacuum.

Stock heads flow terrible (compared to today's standard) both intake and exhaust. To counter that the cam may be of pretty long duration with a good split intake to exhaust and given it age may also have slow lobe ramps, all of which contribute to poor vacuum.
Stock heads run better with around 36* of timing, unless your pulling timing due to detonation then of course that takes precedent.

Maymyvetteliveforevr

CanadaGrant

I sprayed carb cleaner at every brake booster hose joint starting from the carb and also removed the fitting to the brake booster and plugged it. No change in vacuum pressure.

Lars

The carb was rebuilt at least 5 years ago and I left it in the vacuum seal that you ship it in until recently when the restoration was done, at which point I installed it.

I will set the initial timing at 18 and check where the vacuum to the advance is being pulled from this weekend and report back.

Lars, I'm confused at this statement, " vacuum advance hooked up to manifold vacuum so you pull in an additional 12 degrees of timing at idle (actual timing at idle with vacuum advance hooked up should be around 30-ish). "
Are you saying I should set the initial timing to 18 with vacuum advance disconnected and plugged (so I don't loose vacuum) and when I connect vacuum back to the distributor advance, it should be around 30-ish?

You're very correct in your statement, "Your rear end ratio with an automatic and a big cam will make the car a real dog from a standing stop..."

I'm amazed/surprised that you would keep the specs for so long. Sorry I also corrected my initial post to say carb rebuilt by Lars, as I believe it's up to us to tune them.

REELAV8R

I believe this next step"take every vacuum source off except the vac advance and plug the ports to see if you get an increase in vacuum" is a good idea.

Equipment used

I'm using the Innova 5568 timing light & OTC 5613 Vacuum/Pressure Gauge Kit.

Question:

If the fuel air mixture is 5 1/4 turns out, which I understand should causes a lean condition, then why do I smell a very strong fuel odor to the point it burns my eyes when checking for vacuum leaks etc?

Turning the screws out makes a q-jet leaner at idle..........correct?

REELAV8R

I'm not there so I can't say for sure but it's not my belief that you have a vacuum leak causing your low vacuum. If it's something you want to check to make sure though eliminating the vacuum sources as possible leaks is a sure way to do it. Unless it's a base plate leak, ie, the carb gasket is letting air in.

That Innova timing light is a great tool. I have one as well and it works really well displaying timing and RPM at the same time.

The reason for the fuel smell despite it being lean may be due to misfire. If a cylinder fails to ignite the fuel then the fuel on that stroke does not get burned it just exits the exhaust creating your eye stinging fumes.
Turning the screws out makes it richer at idle. The screws control fuel flow.

While idling if you disconnnect a vacuum component allowing air to be sucked into the manifold your idle should get worse if it's lean. If you were rich then the idle would go up.
You could also try slightly covering the carb on the primary side. This in essence "chokes" the engine forcing more fuel to be pulled into the engine just as a choke plate would do, if that improves your idle then you are lean.

Maymyvetteliveforevr

REELAV8R

I sprayed carb cleaner around the carb base plate and no increase in rpm.

I'm wondering if the plugs have fouled due to incorrect carb/distributor etc set up.

Lars

The distributor advance hose is connected to the vacuum port between the drivers side fuel air mixture screw and the throttle linkage, ( front, lower, drivers side of carb).


Update.

I advanced the timing to 19 degrees with vacuum disconnected to distributor advance and plugged. The idle went up about 150 rpm. I adjusted the idle (in neutral) back down to 900 rpm and vacuum has increased from 13.5 to 14.5 (1). I adjusted the carb once more and it now runs at highest vacuum/rpm at 5 turns out.

I placed my hand over the primary (front barrels) and the rpms only went up 100 rpm, which is a big improvement from 500 rpm before.

I placed the car in drive and it stumbles pretty hard but didn't stall. I fell this is a step in the right direction.

Questions:

Whether I connect the vacuum to the distributor advance or not, the timing remains at 19 degrees. Is this correct?

Should there be vacuum to the hose going to the distributor vacuum advance at idle as there didn't seem to be any (by feel)? Or does vacuum come into effect once the throttle increases?

7t2vette

It sounds like you have it connected to a ported vacuum source instead of a manifold vacuum source. A manifold vacuum source has vacuum at all times except WOT. Change your vacuum source.

REELAV8R

First check to see if there is or isn't vacuum on that line at idle. If not then you have it on a ported vacuum source.

If there is and it's equal to you vacuum reading on the engine then you have a vacuum can that demands a higher vacuum level than the engine can deliver at idle.

lars

That is correct. Set the timing with the hose pulled off the vacuum advance to about 18 initial. Verify that this produces total timing in the 34 - 38 range. Then hook up the vacuum advance hose to get the additional timing from the vacuum advance. This should produce timing at idle of about 30 degrees, and will give you much better manifold vacuum and idle quality. You will probably have to lower your idle speed after doing this, and this, alone, may solve your surging and odor problems.

No, that's not right. Turning the screws out will richen the mixture. Keep in mind that CO is odorless. If you're smelling a strong exhaust smell, it's highly likely that it's from HC, not CO. High HC is caused by incorrect timing and an over-lean mixture. It's not likely that you're running rich.

I can't find the specs on your carb in my files, even though I keep files for over 20 years back. Would you please e-mail your name and address (or send me the invoice sheet I sent you if you have it) so I can see how the carb is set up?

Lars
V8FastCars@msn.com