- Thread starter SVcummins
- Start date
Steve@Advance
Well-known Member
Fluctuating can indicate worn valve guides.
Is there some problem you are having with it?
Any other symptoms?
Is there some problem you are having with it?
Any other symptoms?
Just tuing it up a little . Had the carburetor rebuilt it was all out of shape it runs ten times better now but it could still use a little more tuning . Plan on replacing pligs and wires I really want to pull the distributor and see if it wore out . It backfires and farts a long
used red MN
Well-known Member
- Location
- Coon Rapids, MN
Is there some drivability issues that you are trying to diagnose? I was listening to some pod casts by a guy from NJ that calls himself ..The Hot Rod Farmer.. one of his early careers was a ..drivability diagnostic..tech at a shop. Later he was a drag race tuner and also a technical writer for Hot Rod magazine. In one of his tutorials he was explaining using a vacuum gauge to check for an exhaust restriction. He said the vacuum should increase from what is seen at idle as you steadily raise the engine rpm. I have a 97 3800 Buick that does not run 100 percent especially in higher ambient temps. I tried it and found that when I raise it off
idle it may increase the vacuum slightly. One you increase the rpm say over 2000 the vacuum actually decreases slight by a couple inches or so but holds relatively steady. I have a 99 3800 that runs okay and tried it on that one same result. In your case if you bring the engine up from idle with no load and you start at 20 in. then drop to 15 and keep going down I would say you have an restricted exhaust. What I see your gauge showing is a normal response. Could be a slight concern seeing the fluctuation. Not sure what that might indicate, maybe a valve train issue.
idle it may increase the vacuum slightly. One you increase the rpm say over 2000 the vacuum actually decreases slight by a couple inches or so but holds relatively steady. I have a 99 3800 that runs okay and tried it on that one same result. In your case if you bring the engine up from idle with no load and you start at 20 in. then drop to 15 and keep going down I would say you have an restricted exhaust. What I see your gauge showing is a normal response. Could be a slight concern seeing the fluctuation. Not sure what that might indicate, maybe a valve train issue.
Spark timing could be retarded , or it could be running lean. Also need to check for slack in timing chain.
Easiest way to do that is line the crank pulley up with zero. Take the distributor cap off. Turn the engine backwards till the rotor just moves. Count the degrees on the timing indicator and you have the amount of slack in the timing chain.
Easiest way to do that is line the crank pulley up with zero. Take the distributor cap off. Turn the engine backwards till the rotor just moves. Count the degrees on the timing indicator and you have the amount of slack in the timing chain.
used red MN
Well-known Member
- Location
- Coon Rapids, MN
Most of those had a Rochester 2 bbl on them. Does the guy that did the rebuild really know his stuff? Does the accelerator pump start squirting right off idle with very slight throttle movement? Having the linkage set right and in the proper stroke hole is very important for these carbs to get good ..tip in.. coming off idle.
rustred
Well-known Member
is that original engine and how many miles on the engine? those engines will have the 4 bolt mains and double roller timing chain, which lasts a lot better than the light duty engines single gear chain. and it should be the orange engine. plus dont forget the compression check when changing plugs. let me see if i can remember the timing , i think i was 8 degree's advance at idle. it would have the unipoints also gap was .018 or 30 degree's dwell and adjusted with the 1/8" allen wrench tool when running if you have the tack dwell meter. back then we used a scope on them. and yes the guides can be worn also.
plus check your fuel pump for flow and pressure.
plus check your fuel pump for flow and pressure.
Steve@Advance
Well-known Member
The gauge is sticking or you have it on a restricted vacuum source.
Hey SV, just a thought but some of my C65’s have a vacuum tank as an option to give you another boosted brake application or two with a non running engine. Likely gauge in cab is showing reserve vacuum and not engine vacuum. Like I said just a thought without seeing the truck, Mike
1946 BN IHC
Member
I would like to chime in with a question on Chev engine also.
Our chevy 350 in a p 30 van body modification to factory R.V. creca., 1983 has an electronic distributer; with maybe 10 to 12 mpg >>Thinking of converting it back to points distributer [ new off shelf designed for 1973 350 Chev] and re fitting the carb with leaner jets. All for hoping to reach 20 mpg. Would one of you mind writing me directly so we may discuss this via e mail.
Thank you.
Wm.
Our chevy 350 in a p 30 van body modification to factory R.V. creca., 1983 has an electronic distributer; with maybe 10 to 12 mpg >>Thinking of converting it back to points distributer [ new off shelf designed for 1973 350 Chev] and re fitting the carb with leaner jets. All for hoping to reach 20 mpg. Would one of you mind writing me directly so we may discuss this via e mail.
Thank you.
Wm.
caterpillar guy
Well-known Member
Best results I ever had with a chevy was to put a tongue in it and drag with the tractor made great wagons and trailers from GM. I stll get old chevys or any old truck to do that with.
20" is a good number. Is that ported vacuum, manifold vacuum or venturi vacuum?
Not moving tells me your gauge is sticking.
'72 350 should be running about 8 degrees timing.
Firing order 1-8-4-3-6-5-7-2
Point gap around .018 giving you the 30 degree dwell angle.
Set the dwell first, then set timing. Place an index mark at the base of the dist base.
If the mark does not move, neither does the dist in the future.
Reset your dwell back to 30 degrees and the timing will fall back in.
As long as your playing, Hold the timing light on the timing marks.
Does it bounce around at idle? Worn timing chain.
Rev engine about 1800
Do the timing marks bounce?
Worn dist shaft bushings.
[email protected]
Looking for higher mileage? GM Lean drop carb adj works well.
Not moving tells me your gauge is sticking.
'72 350 should be running about 8 degrees timing.
Firing order 1-8-4-3-6-5-7-2
Point gap around .018 giving you the 30 degree dwell angle.
Set the dwell first, then set timing. Place an index mark at the base of the dist base.
If the mark does not move, neither does the dist in the future.
Reset your dwell back to 30 degrees and the timing will fall back in.
As long as your playing, Hold the timing light on the timing marks.
Does it bounce around at idle? Worn timing chain.
Rev engine about 1800
Do the timing marks bounce?
Worn dist shaft bushings.
[email protected]
Looking for higher mileage? GM Lean drop carb adj works well.
Backfiring and farting sounds like lean mixture or intermittent ignition. Also intermittent valve sticking.vacum gauge holding steady at idle says valves are fine at idle. If the trouble is when you accelerate I would suspect accelerator pump.shut off engine , look down carb, open throttle, you should see two streams of gas squirting down carb throat. This should start as soon as you start to move the throttle. Some of those accelerator pumps are leather. There is a discharge ball that may be stuck or missing. Connect timing light set timing at idle with vacum hose disconnected from advance and plugged. Then increase throttle slowly and watch timing marks. Timing should advance (centrifugal advance). Then hold throttle about 2000 rpm and connect vacum advance hose. Timing should advance further(vacum advance). Don't set initial timing too high on heavy trucks.
Backfiring under load can be several things including bad spark plug cables or even stuck mechanical advance. It is important to remember that as cylinder pressures increase, so does the the voltage required to create a spark. Having 20" vacuum at idle is a good thing, that means the engine is relatively healthy.
You are just gonna have to troubleshoot.
For the guy trying to gain mileage by changing carburetor and going back to points, that makes zero sense. I would put an aftermarket EFI with computer controlled timing if trying to make gains. There are kits out there, or can get the parts off a newer engine and put microsquirt on it. You would then have the opportunity to really tune it.
You are just gonna have to troubleshoot.
For the guy trying to gain mileage by changing carburetor and going back to points, that makes zero sense. I would put an aftermarket EFI with computer controlled timing if trying to make gains. There are kits out there, or can get the parts off a newer engine and put microsquirt on it. You would then have the opportunity to really tune it.
Indiana Ken
Well-known Member
With no load on the engine this is normal. If the vacuum dropped as the throttle was opened it would indicate a plugged exhaust system.
To have the manifold vacuum drop you have to open the throttle with the engine under load. For example, if you were traveling 50 MPH on a straight/level road the vacuum reading could be say 10". To maintain 50 MPH on a hill you would need to open the throttle which would cause the vacuum to drop. A drop in manifold vacuum indicates the increased load on the engine.
rustred
Well-known Member
yes , along with the wires and plugs. plus i remember years ago the module used to go bad in them and your dead in the water or a bad one would cause missing and sputtering. the old points never will let you down. plus you had to make sure that you put dielectric grease under them. and when ei ignition came out ot was ahrd on the rotors also. it would burn the top. then they came out with a better white rotor in stead of the black one.
timcasbolt
Well-known Member
doubt switching from HEI to a points distributor would have much effect, but if you do make sure you have the proper incoming voltage to the hot side of the coil. An HEI would have full voltage whereas a points system would likely have some sort of resistor, either with a ballast resistor or a steel wire coming from the ignition switch. Sticking a points distributor in a vehicle that came with HEI would cause the points to burn up very quickly.
Robert (ID)
Member
Check under the cab behind the step.. they liked to hide me there...
Hdonly
Member
- Location
- Florida Panhandle
You may have trouble fitting an HEI distributor in there. I put one in my 1964 Chevy pickup when I swapped a six out for a v8 305. Had to take a big hammer to the firewall.
Hdonly
Member
- Location
- Florida Panhandle
I would too. Nothing wrong with points.
Destroked 450
Well-known Member
- Location
- Harned, Ky
Easy way to check the gauge is stomp on the gas pedal, the vacuum should momentarily drop to near zero and then come back up, if it doesn't the gauge is connected to a tank with a check valve to keep vacuum from bleeding back.
Something that wasn't been mentioned is the vacuum advance on the distributor, if the internal diaphragm goes bad it won't give the proper timing curve resulting in a retarded timing condition.
Jim.ME
Well-known Member
- Location
- central ME
Do some looking under the cab. behind the running board shields. Many of those had a vacuum tank and a frame mounted brake booster, split systems had 2 boosters. The gauge should be reading brake system vacuum.
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