The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > General Truck Forums > Engine & Drivetrain

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 11-04-2013, 11:20 AM   #26
geezer#99
Registered User
 
geezer#99's Avatar
 
Join Date: Aug 2009
Location: Bowser
Posts: 13,495
Re: 72 chevy 350 running rough under load!

You're not an expert Donut but you're thinking like me. I suspected the normal lock up o/d no rear gear scenario too. Good at cruise but till you get there a real dog. My 84 with turbo 350C and 3.42 shifts into lockup real quick and runs the same till I get up past 30mph. I shift mine manually. Put it in second at the lights and shift to 3rd when I get up to 25-30mph.
As for shifting manually it won't hurt it. Some people hook a switch into the lockup circuit to delay it but usually forget to flip the switch.
FWIW your hydraulic lock up if it's working full time will drop your idle rpm from 750 rpm right to zero when you put it in gear.
Don't worry about seeing over 50 degrees at times. Some factory set ups had close to 55 degrees total.
Your dyno guy or you or anybody can't set your a/f ratio with the mixture screws. You do that with jetting and/or metering changes. Your mixture screws are for idle mixture.
The other major problem you got right now is your local weather. Waste of time tuning until it gets back above 15*C.
If your motor tolerates 12*, starts and runs and doesn't ping then try shifting manually until spring. Right now you're banging your head on the wall.
And my apoligies for coming across as such a hard....s!
geezer#99 is online now   Reply With Quote
Old 11-04-2013, 05:06 PM   #27
donut
Registered User
 
donut's Avatar
 
Join Date: Aug 2009
Location: S.C.
Posts: 1,593
Re: 72 chevy 350 running rough under load!

Couldn't remember exactly how the lock-up worked (only turbo350c I've had was in the mid '90's) But everybody was looking at the engine and missing the elephant in the room.

You mentioned the different curves and got me thinking. If mine was acting like the OP's, drop a gear and go. I had similar issues with an engine when I lost 2nd gear in a 3 speed.

Didn't even consider the air temp. Usually when it got cold enough to need gloves, the tools got put away. Course I have been down south for a couple years now...
__________________
'86 C-30 dually, 454/tbh400
'73 K-20 350/350/205 (sold )

I'm kinda like duct tape- no real purpose, but handy to have around.
donut is offline   Reply With Quote
Old 11-22-2013, 08:05 PM   #28
martyminnesma
Registered User
 
Join Date: Jun 2011
Location: calgary alberta
Posts: 28
Re: 72 chevy 350 running rough under load!

ok so i did some more testing and diagnosing and I think I've gotten closer to discovering the main problem...

I took the vacuum off the carb and plugged the port. turned the initial timing up to twenty and then took it for a drive. I was also careful not to rev it too high as i didn't want the total timing to come up too high. the purpose of this was to see if the motor would miss under load and low rpm even with the timing higher. I originally thought it was missing because under load the vacuum advance would disappear and without the engine reving high the timing would be around 8 degrees. well even with it set up at 20 degrees the problem persists, if not even worse. so I checked what my gut has been telling me the whole time.

I rigged up my air compressor line to connect to the spark plug port. so far i've just tested two cylinders, number 1 and number 7. I've often thought 7 and 8 were problem cylinders and the front cylinders were fine, based on the coloring and carbon built up (i think i wrote on that earlier). however, both 1 and 7 had bad leak down problems. they would leak at any pressure I threw at it 30 lbs, 100 lbs. I can hear the air coming up into the valve cover through the lifter galley, but i of course dont know if its leaking past the head gasket or the piston rings. I took the rocker arms off and found top dead center when i tested both cylinders, that way i know the valves were closed. I'm going to find a proper leak down tester and get the numbers and then talk to the engine rebuilder guys here in town. question for you though!...

How can I have a bad leak past the pistons or head gasket and still come up with good numbers on a compression test?
I was getting about 70-90-120-150 on every cylinder when doing the compression check.
martyminnesma is offline   Reply With Quote
Old 11-23-2013, 12:39 PM   #29
68gmsee
Active Member
 
68gmsee's Avatar
 
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,947
Re: 72 chevy 350 running rough under load!

Quote:
Originally Posted by martyminnesma View Post
......How can I have a bad leak past the pistons or head gasket and still come up with good numbers on a compression test? I was getting about 70-90-120-150 on every cylinder when doing the compression check.
A compression test is more or less a quick way to determine ring/valve condition but is not as accurate at pinpointing where the problem is as a leak down test.
At this time, if the engine is still running rough and you have eliminated ignition, timing and/or vacuum leaks, then it's time to think about rebuilding.

If your engine is high mileage it may be rings, valves or both may be worn. Usually a head gasket leak can be detected by two adjacent cylinders having low compression compared to the rest. With a leak down test you can listen for air escaping between two cylinders. However, a head gasket leak may be harder to detect if it only acts up with engine running.
__________________
68 GMC 250/3 speed Saginaw p/b p/s
69 Chevy 350/350 currently in pieces still lookin for a cab
06 Trailblazer
I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...
68gmsee is offline   Reply With Quote
Old 11-23-2013, 06:28 PM   #30
donut
Registered User
 
donut's Avatar
 
Join Date: Aug 2009
Location: S.C.
Posts: 1,593
Re: 72 chevy 350 running rough under load!

-I'm assuming that the carb float level is correct or a tad low?
-That ignition timing is correct?
-The advance springs and weights you used were plotted and not just thrown in because "the internet said so".
-All 8 cylinders are firing? (for that matter, why keep the dizzy that was set up for propane instead of replacing it with a new one?)
-break in was properly done.
Set the carb back to stock, iirc your about 2 stages leaner now.

Take truck to engine shop, ask them why would those two cylinders are acting like that. One of the techs should be able to diagnose the problem (compression, leak down ...) and decide which direction to go.

This way, you both can decide whose problem it is. (if it's not an engine problem, they can probably test it cheaper than you can.)
__________________
'86 C-30 dually, 454/tbh400
'73 K-20 350/350/205 (sold )

I'm kinda like duct tape- no real purpose, but handy to have around.
donut is offline   Reply With Quote
Old 11-23-2013, 09:30 PM   #31
martyminnesma
Registered User
 
Join Date: Jun 2011
Location: calgary alberta
Posts: 28
Re: 72 chevy 350 running rough under load!

Quote:
Originally Posted by donut View Post
-I'm assuming that the carb float level is correct or a tad low?
-That ignition timing is correct?
-The advance springs and weights you used were plotted and not just thrown in because "the internet said so".
-All 8 cylinders are firing? (for that matter, why keep the dizzy that was set up for propane instead of replacing it with a new one?)
-break in was properly done.
Set the carb back to stock, iirc your about 2 stages leaner now.

Take truck to engine shop, ask them why would those two cylinders are acting like that. One of the techs should be able to diagnose the problem (compression, leak down ...) and decide which direction to go.

This way, you both can decide whose problem it is. (if it's not an engine problem, they can probably test it cheaper than you can.)

ok I'll give as simply as a breakdown as i can to tell you everything i've tested/redone

- engine ground
- full voltage at distributor
- new high output msd distributor
- new spark plugs
- new spark plug cables
- heat protection boots on cables
- new holly carb, professionally dyno tuned by a guy who im told is the best in western canada
- redid the intake manifold gaskets, the originals did not have any leaks
- re-torqued the heads
- tried removing the vacuum advance and bumping initial timing to 20 degrees to see if that would help with the off the line failures. it was worse if anything
- tried running sea foam through the carb and in the engine oil. seemed to change the running a little bit, but not necessarily for the better, and it was a short lived change
- did a vacuum test (at the dyno shop) checked out fine
- fuel pressure test (also at the dyno) also fine
- air/fuel ratio checked out fine after the new carb
- re-did all the valve lash stuff. 3/4 turn with hydraulic lifters
- checked to make sure the timing mark on the harmonic balancer did in fact match up with TDC on the marking plate - it was bang on
- did a leak down test. all cylinders were around the 15-20% mark, which i was told is fine for a cold motor.
- compression test, all cylinders fired approximately 70-90-120-150... so im all good there.

other things of note, i've never had any oil show up in the radiator, no rad fluid in the oil except the one time my heads came a little loose, but i fixed that. i've never had any metal shavings come out with the oil. the break in process was done exactly as they told me to. I had the proper oil with the zinc and everything, ran it for 20-30 minutes varying the rpm from 1500-2500 every few minutes.

any thing else I should check on?
martyminnesma is offline   Reply With Quote
Old 11-25-2013, 03:20 PM   #32
martyminnesma
Registered User
 
Join Date: Jun 2011
Location: calgary alberta
Posts: 28
Re: 72 chevy 350 running rough under load!

Quote:
Originally Posted by donut View Post
Couldn't remember exactly how the lock-up worked (only turbo350c I've had was in the mid '90's) But everybody was looking at the engine and missing the elephant in the room.
.

I think you may have a strong point here. i was reading some other forums about hydraulic lock up torque converters and one thing that came up was that with a lower gear ratio set up like mine, engine lugging can be a big problem. also, I've come to realize by reading those posts that my hydraulic lock up converter is set up terribly thanks the the transmission shop that put it in. they are supposed to have a spring pressure that keeps it from locking up until a high speed, but mine locks up the moment it shifts, and it locks up every gear. i've always hated the way my transmission worked, but the shop wanted to charge me 1000 bucks to remove the lock up. I didn't know that it was actually operating incorrectly, and i had no idea it could seriously effect the way my engine runs.
I'm going to put in a new post in the transmission section about removing or adjusting the hydraulic lock up. and once i figure that out we'll see how my truck runs. if it doesnt help the running, at least it will help my transmission issues. it would be nice if it killed both problems at the same time though
martyminnesma is offline   Reply With Quote
Old 11-25-2013, 08:52 PM   #33
donut
Registered User
 
donut's Avatar
 
Join Date: Aug 2009
Location: S.C.
Posts: 1,593
Re: 72 chevy 350 running rough under load!

Don't thank me, geezer#99 hit on it before I did. Glad your exploring other options though. At least rule them out before you go back to the engine.
__________________
'86 C-30 dually, 454/tbh400
'73 K-20 350/350/205 (sold )

I'm kinda like duct tape- no real purpose, but handy to have around.
donut is offline   Reply With Quote
Old 01-21-2014, 08:11 PM   #34
martyminnesma
Registered User
 
Join Date: Jun 2011
Location: calgary alberta
Posts: 28
Re: 72 chevy 350 running rough under load!

last update here. on another thread a guy told me to check for a ball blocking a solenoid in the transmission. it was there and I removed it which also removed the locking up function of the transmission. for a couple weeks it still would fall on it's face a bit, but it's been getting better. it's basically healed itself somehow. I guess the lugging on motor had something to do with it, because it seems to run much better now.
martyminnesma is offline   Reply With Quote
Old 06-27-2017, 08:27 PM   #35
edecisions
Registered User
 
Join Date: Jun 2017
Location: Elk Grove Ca.
Posts: 1
Talking Re: 72 chevy 350 running rough under load!

Ok, after messing with a simular issue for about 6 months and never finding this solution on line figured I'd share it as it pretty much matches his issues.

Check your Choke!

Someone on my car connected the choke line to the Dizy +12vdc so neither were getting the current they needed, meaning the choke did not fully open and what happens when your choke does not open? You run rich and have a lack of O2 when hot and load up the engine when trying to take off, car sounds like a lawn mower ect....

Hope this helps someone.

Kenhttp://67-72chevytrucks.com/vboard/i.../bryan.gif.gif
edecisions is offline   Reply With Quote
Old 06-11-2022, 05:38 PM   #36
davidh
Senior Member
 
davidh's Avatar
 
Join Date: Feb 2004
Location: Ponca city now kingfisher Oklahoma
Posts: 1,221
Re: 72 chevy 350 running rough under load!

What became of this?
Having a similar issue. 350 vortec but with carb, ac delco fuel pump, one used with boats.
Seems like it started happening when I changed the condenser in the HEI because the connector clip broke. I grabbed one from Napa thinking it was better quality than other store brands. I use a quadrajet too it’s at about 7psi. I have driven this like this for several years and this issue just popped up.

It’s weird it doesn’t always do it. I went back changed cap, checked to make sure capacitor, coil were okay. Doesn’t smell rich either just hesitation at 2200 rpm, can often accelerate past the spot but not always.
Good information on this thread mine maybe as simple as bad gas.
__________________
Even the nicest dodge-chrysler is only 2years away from being parked outside of Section 8 housing.
davidh is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 09:56 AM.


Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Copyright 1997-2022 67-72chevytrucks.com