383 engine low vacuum

[Ferd]

I’m having two issues with my 1969 roadrunner that may be related. I purchased the car last September. The engine was totally rebuilt by a professional. What I am experiencing is the engine always takes roughly five seconds of cranking to fire whether cold or hot. When driving if you shift into second gear with a light foot on the gas, the engine will lightly sneeze up the carburetor. I have set the timing using a mark at 36 degrees and power timed the engine to that setting at 3000 rpm with the vacuum advance plugged. I jetted the Edelbrock 650 carb to the old 1969 factory specs. It has a factory distributor with a Petronix conversion. The voltage to the distributor and coil are great. A vacuum gauge reading at 750rpm idle is only 5 inches. However, the idle is steady and once started the engine never stalls. The plugs look great with a light tan. I thought maybe a vacuum leak, but all intake gaskets are fresh and intake bolts are tight. I replaced the vacuum advance, and the vacuum hoses are good. I pulled a valve cover and tried a crude measurement of valve lift from the top of a valve retainer to a head surface and discovered only .300 lift . So I’m puzzled. There is absolutely no lifter or valve train noise. I’m debating on pulling the intake and lifter cover for a visual inspection of the camshaft and lifters, but I have a feeling based on the lack of valve train noise I’ll find nothing. Your constructive feedback is appreciated!

 

[STXCUDA]

What is your base timing at? What port on the edelbrock did you get that vacuum measurement from? Edelbrocks only have manifold vacuum at the front center 3/8 pcv port and the 3/16 port on the front driver's side. You may also consider buying a new vacuum gauge, yours may be defective. Based on everything else sounds like it's a healthy engine just need to figure what tune it wants.

 

[Ferd]

What is your base timing at? What port on the edelbrock did you get that vacuum measurement from? Edelbrocks only have manifold vacuum at the front center 3/8 pcv port and the 3/16 port on the front driver's side. You may also consider buying a new vacuum gauge, yours may be defective. Based on everything else sounds like it's a healthy engine just need to figure what tune it wants.

I went out and looked at my vacuum advance hose. It was hooked up to the incorrect “timed vacuum “ port. I changed over to the drivers side full time vacuum port. The car started up immediately. Thank you for your excellent advice. I then hooked up my vacuum gauge (which works fine) and determined 6 inches of vacuum at 750 rpm idle. Putting a timing light on with the vacuum advance plugged, the advance at 750rpm idle is roughly 15 degrees btdc. When I increase the throttle to 3,000 rpm the total advance of 36 degrees btdc is reached. I believe there is an aftermarket camshaft in the engine causing the low vacuum at idle. I have replaced the metering rod springs to the lightest ones from a kit (blue springs). Car is running strong. Will spin the tires on a firm second gear shift. Thanks again!

 

[r_charger]

I went out and looked at my vacuum advance hose. It was hooked up to the incorrect “timed vacuum “ port. I changed over to the drivers side full time vacuum port. The car started up immediately. Thank you for your excellent advice. I then hooked up my vacuum gauge (which works fine) and determined 6 inches of vacuum at 750 rpm idle. Putting a timing light on with the vacuum advance plugged, the advance at 750rpm idle is roughly 15 degrees btdc. When I increase the throttle to 3,000 rpm the total advance of 36 degrees btdc is reached. I believe there is an aftermarket camshaft in the engine causing the low vacuum at idle. I have replaced the metering rod springs to the lightest ones from a kit (blue springs). Car is running strong. Will spin the tires on a firm second gear shift. Thanks again!

I believe you're right about it being the cam. When I bought my RR back in 04, I tried everything I knew but could not get idle vaccum of more than 5. The cam and intake were terribly mismatched to the gears in the rear. I pulled the engine out and built it myself so I'd know what was in it.

 

[Ferd]

I believe you're right about it being the cam. When I bought my RR back in 04, I tried everything I knew but could not get idle vaccum of more than 5. The cam and intake were terribly mismatched to the gears in the rear. I pulled the engine out and built it myself so I'd know what was in it.

I was so concerned about the low vacuum that I decided to pull the intake to visually inspect the camshaft lobes because I suspected a bad cam lobe. To my surprise the camshaft lobes looked great, so I just reinstalled everything and changed the plugs. I attempted a crude measurement of the cam lobes. The lift at the lifters is roughly.300 which means .450 at the valves. I didn’t attempt using a degree wheel to determine duration so I don’t know what cam it is but it has a red paint dot just in front of the last cam journal. I was successful in eliminating popping back through the edelbrock carburetor at light loads by installing heavier step up springs for the metering rods. This allows a richer mixture. I’m still dealing with the carburetor staying on fast idle after warm up.

 

[Ferd]

I believe you're right about it being the cam. When I bought my RR back in 04, I tried everything I knew but could not get idle vaccum of more than 5. The cam and intake were terribly mismatched to the gears in the rear. I pulled the engine out and built it myself so I'd know what was in it.

I was so concerned about the low vacuum that I decided to pull the intake to visually inspect the camshaft lobes because I suspected a bad cam lobe. To my surprise the camshaft lobes looked great, so I just reinstalled everything and changed the plugs. I attempted a crude measurement of the cam lobes. The lift at the lifters is roughly.300 which means .450 at the valves. I didn’t attempt using a degree wheel to determine duration so I don’t know what cam it is but it has a red paint dot just in front of the last cam journal. I was successful in eliminating popping back through the edelbrock carburetor at light loads by installing heavier step up springs for the metering rods. This allows a richer mixture. I’m still dealing with the carburetor staying on fast idle after warm up.

 

[r_charger]

It's all about the components matching each other. I'm hoping to finish things with mine this year or next. I built a stroker 440 the summer of 09 then was diagnosed with lung cancer. That really took a back seat to getting healthy as I'm going to get physically and being able to retire a couple of uears ago. By the grace of God am I still here and healthy. Bought a set of Trickflow power port 240s to put on it. Still left, get a good set of rocker arms and shafts. I wish you luck on figuring it out. I know a fairly stock engine at idle should pull 14-17" vacuum.

 

[SomeCarGuy]

.450 lift would be like the stock cam. That’s extremely low vacuum. I’d be looking for a leak if it’s running strong I’d suspect the gauge.

 

[Ferd]

.450 lift would be like the stock cam. That’s extremely low vacuum. I’d be looking for a leak if it’s running strong I’d suspect the gauge.

I believe there could be a problem with my vacuum gauge. I don’t have a second one to compare readings. The engine runs great. My only remaining issue is hard starting when warmed up, and the fast idle seems to remain when warm. I think the problem is related. When you start the car cold, it starts immediately and idles at about 750rpm. It stumbles a little going through the gears until warmed up. Then it runs great, but often stays at fast idle or 1,500 rpm. So the choke/fast idle mechanism is operating just the opposite of what I would expect. I think the hard starting when warm, is because the fast idle cam is holding the throttle open when warmed up. However, when I park the car warm and pull the air cleaner I don’t see the fast idle screw resting on the fast idle cam. I set the choke with the engine cold by turning the choke coil clockwise until the choke blade starts to close, and left about 3/8 inches as the opening. At this setting, I’m about four notches lean on the choke coil markings. I tried bending the fast idle linkage to make it longer to see if that would eliminate fast idle when warmed up, but it didn’t work.

 

[Ferd]

I believe there could be a problem with my vacuum gauge. I don’t have a second one to compare readings. The engine runs great. My only remaining issue is hard starting when warmed up, and the fast idle seems to remain when warm. I think the problem is related. When you start the car cold, it starts immediately and idles at about 750rpm. It stumbles a little going through the gears until warmed up. Then it runs great, but often stays at fast idle or 1,500 rpm. So the choke/fast idle mechanism is operating just the opposite of what I would expect. I think the hard starting when warm, is because the fast idle cam is holding the throttle open when warmed up. However, when I park the car warm and pull the air cleaner I don’t see the fast idle screw resting on the fast idle cam. I set the choke with the engine cold by turning the choke coil clockwise until the choke blade starts to close, and left about 3/8 inches as the opening. At this setting, I’m about four notches lean on the choke coil markings. I tried bending the fast idle linkage to make it longer to see if that would eliminate fast idle when warmed up, but it didn’t work.

Here is an update. The fast idle cam is not contacting the fast idle screw at all because I over adjusted the linkage. So the engine is running 1,400 rpm from the regular idle screw. I will back out the regular idle screw a turn and see the results. Possibly the regular idle screw being set so far open, is the reason I have to crank the starter a long time when the engine is warm. I will eventually adjust the choke and correct the fast idle cam.