I need some help troubleshooting a couple issues. I was helping a buddy of mine set his timing on his 79 Bronco, which has a 400 in it. The timing is set at 14*, which is fine. But I noticed a problem when I disconnected the vacuum advance hose from the distributor; the rpms of the motor did not change whatsoever. Normally, when you disconnect this line you will have a vacuum leak until you plug the port and the rpms of the motor will slow down. So I hooked up a vacuum gauge to the port on the carb (Edelbrock 750) and there was absolutely no vacuum registering on the gauge. But when he goosed the throttle, the vacuum gauge jumped up to about 20 or so, but when he let it idle there was no vacuum at all. In order to verify the vacuum gauge wasn't faulty, I fired up my 79 Bronco (351m) and hooked the vacuum gauge up to my carb (Edelbrock 600) and it read 16 at idle, so I know the gauge works.

Can anyone tell me why my buddy's Bronco does not have any vacuum at idle? With the cam he has, he should have plenty of vacuum. Here's the cam he has.....

Earson Cam: RPM range = 1200-4200, Advertised Duration = 280* Intake & Exhaust, Duration at .050"= 208* Intake & Exhaust, Gross Valve Lift = .448 Intake & Exhaust, Lobe Center = 111*, Advance = 4*, Valve Lash = .000" Intake & Exhaust.

The next issue he has is when he first applies the throttle from a stop, the carb "spits" (for lack of a better term) and bogs down for a split second and then picks up and runs. What is causing the carb to do this? Is this related to the lack of vacuum issue?

Thanks in advance for the help.

 

Depends on how it is setup, a "ported" source of vacuum has no vacuum at idle, it isn't supposed to, that's the idea. If the idle RPM is spooled up enough it will start to tip in. If there's a manifold source on the carb, you can connect to that, or on the manifold, and it will pull in lots of advance at idle. Ported and manifold vacuum are basically identical as far as timing is concerned at anything other than at idle. Try both and see which one runs better.

 

Check a port on the manifold and that's where his advance should also come from not the carb.
The vacume advance works reverse in that as speed increases as you open the throttle manifold vacume drops as the throttle plates open, this allows for increase velocity or flow of air. Close the throttle and more resistance hence higher vacume.
The advance works by when the engine starts it draws a vacume on the manifold and hence vacume pot on the distributor drawing against the spring inside. This retards the timing for emissions and to help the engine run smother and be more dependable. Engines start easier with more advance but if they are idling, to much advance can damage internals such as pistons, valves, spark plugs. It's not really an explosion but an exceptionally fast controlled burn, that's why an advance mechanism is so critical as the timing to complete the burn and allow the expansion to be right as close as possible to the correct piston position to TDC. And yeah I don't know what that is.
Had to learn all this when I got certified but haven't used it much in the last15 years.

 

And yes it could also be a vacume issue as timing is effected.

 

Actually at idle (no load) just about any engine will happily purr with 30° or more of vacuum advance/ignition advance and run a lot cooler.

The problem with this is actually emissions testing, the maufacturers retard timing at idle to lower certain pollutants for this reason.

Make sure the carburetor float level and fuel height is correct in the bowl, and the idle and idle mixture is adjusted correctly

 

a bog or spit from idle to part throttle can be either not enough or too much pump shot. power valve enrichment could contribute to this also.

 

Thanks to your help, I think I figured out the vacuum problem, which I don’t think was a problem at all. The reason I had vacuum at my carb port and my buddy didn’t is because when I fired up my Bronco to test the vacuum, I hadn’t been running my Bronco, so it was cold. The choke hadn’t kicked off yet, so the engine was idling at a higher rpm because of the choke and was pulling vacuum. So yesterday I fired up my Bronco and let it warm up and tested the vacuum again after the choke kicked off and I had no vacuum reading at idle either. So turns out this wasn’t a problem as we thought.

Redark1--As for the carb spitting issue, my first thought was the pump shot also. I was figuring it’s probably not getting enough fuel during the transition from the idle circuit to the cruise circuit. I’ll get with my buddy and maybe we’ll get a tune up kit for his carb and swap out the plunger. FYI, there is no power valve on his carb because it’s an Edelbrock, not a Holley.

Tedster9--I didn't think if was a float issue but you might be right. We'll check the float level first before we buy a kit to swap out the plunger.

Thanks for the advice fellas!!

 

there still should be some kind of a vacuum based fuel enrichment. usually a piston connected to the metering rods. at least that's how the old afb carbs were.

 

If there's only one carb adjustment to get right, float height is probably it. It has to be right for both performance and fuel economy, the float height determines the fuel height in the bowl and that determines the height the fuel has to rise to reach the fuel discharge in the well. Imagine if you had a bucket full of water. Punch a hole in the side about halfway up, and another near the bottom. The hole in the bottom would send a column of water farther than the one higher up, due to the pressure of a higher column of water. A carburetor fuel bowl is sort of like that, all the adjustments and specs assumes that the fuel level is correct, and this in turn affects jetting to some degree and fuel starvation at high speeds or, flooding, etc.

SERVICE HINTS
Before carburetor service can be properly performed, it must be remembered that CARBURETION IS DEPENDENT UPON COMPRESSION, IGNITION, SPARK TIMING, VALVE TIMING, ETC THE CARBURETOR SHOULD ALWAYS BE SERVICED AND ADJUSTED LAST IN AN ENGINE TUNE-UP.

 

this is from the edelbrock website and there pdf file for tuning.

At higher engine loads, such as in a heavy part-throttle acceleration, there is a need for a richer mixture. This enrichment is provided by the Metering Rod and Step-Up Function (Figure 4).
A vacuum passage (8) communicates the manifold vacuum to the underside of the Step-Up Piston (9). This vacuum tries to hold the Piston in the bottom of its bore by working against the
force of the Step-Up Spring (10).
When the manifold vacuum is high, indicating a low load such as idle, cruise, or light acceleration, it is able to overcome the force of the Step-Up Spring and hold the Step-Up Piston at the
bottom of its bore, which also positions the Metering Rod at the bottom of its travel. At this point, the Rod has a large diameter that creates a high restriction through the Jet and the fairly
lean A/F Ratio that is desirable for low load/low power operation. This portion of the Metering Rod is referred to as the “Lean Step” of the Rod.
When the manifold vacuum is low, indicating a high load such as
a heavy part-throttle (or WOT) acceleration, the Step-Up Spring
is able to force the Piston to the top of its bore and position the
Metering Rod at the top of its travel. This action is called “Power
Mode Staging”. The portion of the rod now located in the jet has
a smaller diameter, thus the restriction through the Jet is reduced
and a rich A/F Ratio is provided for high load/high power
operating conditions. This is the “Rich Step” of the Rod.

 

Thank you for that info. Understanding how the carb functions helps to diagnose problems. Hopefully we can get this "spitting" issue figured out. Thanks again...