350/th350 question

[86gmc23]

It is a Holley carb (#80508), thanks for your help. I really appreciate it, I found a diagram for carb and where the distributor vacuum advance is plugged in, the diagram says its Timed Vacuum.

Can anyone tell me if I should replace my oil pump? Like I said before, start up is around 20-25psi, then drops down to 10-15psi. Engine was cold during this test.

[bd]

...Can anyone tell me if I should replace my oil pump? Like I said before, start up is around 20-25psi, then drops down to 10-15psi. Engine was cold during this test.

Approximately how many miles on the engine?  Check the rod and main bearing clearances.  Installing a new oil pump on a worn out engine may not gain much.

[86gmc23]

From what I was told engine was recently rebuilt so not real sure on miles.  I just installed it on my truck, just got it able to drive down the road last weekend.  That's when I found the bad shifting, fixed that earlier today and now it shifts great.  When engine tested between 140-150psi on compression I took the guys word for the recent rebuild. What would you recommend I do, definitely don't want to burn anything up

[bd]

Ten PSI at idle won't damage your engine, but it is unusually low for a stock configuration; 25 - 30 PSI is more common.  The guideline for high performance engines is 10 PSI/1000 RPM.  Still, 10 PSI at 700 RPM doesn't leave much margin.

If you're confident the hand-held test gauge you used is accurate and the oil pressure is indeed ~10 PSI warm, remove the oil pan and one-at-a-time remove the rod caps to inspect the bearing surfaces and measure bearing clearances using green Plastigage.  Do the same with the main bearings.  Procedure and specifications should be in the 1986 Service Manual, small block Section 6A5.  Smear some engine assembly lube on the bearings as you reassemble.  Clean then lubricate rod and main cap bolt threads with 30 wt motor oil and torque all fasteners to factory specs.  Diligence and cleanliness is important during this process, so take your time and be careful.  Also, crankshaft bearing journals are very soft, so take precautions to avoid 'dinging' them.  You're trying to determine whether bearings were replaced during the 'rebuild' and whether the correct size bearings are installed.  But, be prepared - depending on what you encounter, this job can escalate.

Plan on replacing the oil pump with a Melling standard pressure/volume pump and screen, but upgrade to a hardened drive shaft.  Melling also markets a screen installer that makes screen installation a snap, but the tool is pricey if only used once.

[rich weyand]

The oil pressure indicates engine wear.  The more worn the engine, the lower the pressure will be because the bearings are passing more oil.  The aviation people monitor at-operating-temp idle oil pressure on their engines over the engine lifetime to monitor engine wear.  Sounds like some of the engine may have been rebuilt, but perhaps the main or rod or cam bearings, or some combination of them, were not replaced.

Remove the vacuum advance off the "Timed Spark Vacuum Source" and tee it off the PCV line.  Timed vacuum is emissions nonsense, to clean up idle emissions because that is one of the things tested.  Back in the day, vacuum advance always ran off manifold vacuum; timed vacuum came in in the 1968 model year when they added AIR pumps and all the other nonsense.  Running off timed vacuum idles the engine way retarded, and will cause off-idle hesitation, until the advance catches up with manifold vacuum, after the throttle plate is opened and the two vacuum sources become the same.  Running off manifold vacuum means the engine is always 'ready to go' as far as ignition timing.

Why do I call the emissions stuff nonsense?  Because it makes engines run like garbage, and 90% of the emissions benefit is obtained from simply running a PCV valve.  The rest is a lot of hassle and performance hit for not much emissions reduction gains.

Here's where your ports are, I think.  Just move the vac advance line around the corner and tee it off the PCV line.

[86gmc23]

Thanks a lot for everyone's help,  will get into it this weekend

[86gmc23]

Well the oil dipstick that was in engine when I got it is not the right one so I was reading full engine oil when there was only about a quart and a half in engine. Found that out when I did an oil change, so know my oil psi is good, glad I didn't run engine to much with that little oil in it.

Rerouted my vacuum lines and next thing today ill be tuning the carb. Hopefully ill be driving it to work on Monday, cant wait.

Thanks again for your help

[bd]

Well the oil dipstick that was in engine when I got it is not the right one so I was reading full engine oil when there was only about a quart and a half in engine. Found that out when I did an oil change, so now my oil psi is good, glad I didn't run engine to much with that little oil in it.

The simple things will getcha every time.  lol

[86gmc23]

Yes they will lol. Having another issue I'm hoping someone can shed some light on for me.
I have my Holley 750 idle air mixture tuned and idle screw set. Runs nice and smooth at idle pulling around d 16in of vacuum, only problem is when I give it WOT it instantly dies. I'm somewhat familiar with my carb but don't have about of experience, can anyone point me to an area to check out.  Power valve?

[rich weyand]

Not to harp on a point, but did you move the vacuum advance hose to manifold vacuum?  Poor off-idle transition is one of the results of having it on timed vacuum.  Also, what is your base timing?

When an engine dies due to carburetion, it is almost always too lean.  A very rich mixture will still burn, but you don't have to go too far lean for the fire to go out.  When you open the throttle plate, and all the air rushes in, the accelerator pump on the carb should inject a long squirt of gasoline to keep it from going lean until the gas flow can catch up.  You can test the accelerator pump easily.  With the air cleaner off, and the engine off, open the throttle all the way with your hand while looking down the primaries.  You should be able to see and hear a several second stream of gasoline into both primaries.  Poor operation of the accelerator pump will cause your symptom.  It's usually the gasket/o-ring on the piston, but it can also be clogged accelerator pump metering jets.

[86gmc23]

Base timing is set at 14°. Yes I did move distributor vacuum. I'll try that when I get off work. I remember seeing a lot of fuel squirting in when I hit throttle, but I'll get a good visual today.

[rich weyand]

It could also be clogged secondary jets or a really bad jetting causing lean mixture, but I would not think the engine would die instantly.  That sounds like a transition issue, hence accelerator pump.

14* base timing is good.  You can go higher, to 17-20, and get more power, but that is enough that it shouldn't be the problem.

Does it do this warm as well as cold?  A properly tuned and functioning cold engine will do that anyway.  That's why some carbs have lockouts on the secondaries until the choke turns off.

[86gmc23]

I figured I would keep timing on the lower end until I get kinks out, then maybe advance somewhere around 36 total timing.

Yes it does it hot or cold.  When I test the accelerator pump later today i should be looking for a nice solid stream of fuel shoting into both primaries for several seconds right

Right now my electric choke is not hooked up and I have choke held open with zip tie. Not sure if that has any effect of this but thought I would put it out there

[rich weyand]

Yes it does it hot or cold.  When I test the accelerator pump later today i should be looking for a nice solid stream of fuel shooting into both primaries for several seconds right

Yep, exactly.

[bd]

Here's a little more to think about and do...

Holley 4160 pump shot should be no more than 1 - 1.5 seconds in duration with rapid throttle opening.  Wiring the choke open will have no effect on a warm engine, but a cold engine will be effectively lean.  Verify the float setting to the base of the float bowl sights and accelerator pump adjustment (0.015" lash at WOT).  Make sure the secondaries are not opening prematurely.  What happens if you hover the carburetor with a shop towel and gloved hand (protection from backfire) to enrichen fuel delivery when you rapidly open the throttle?  Are the symptoms noticeably decreased or alleviated?

Does your engine have a fully mechanical distributor - is the centrifugal advance working correctly - does the timing still advance with increasing RPM when the vacuum can is unplugged?