Pontiac Street Performance
Home | Profile | Register | Active Topics | Members | Search | FAQ
Username:
Password:
Save Password

 All Forums
 Pontiac Performance
 Tempest / Lemans / GTOs
 Help Tuning a Rebuilt 350
 New Topic  Reply to Topic
 Printer Friendly
Author  Topic Next Topic  

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 20 Apr 2017 :  10:17:42 AM  Show Profile  Reply with Quote
I need advice on tuning a freshly rebuilt 350 from my 1970 LeMans.

With the vacuum advance disconnected:
- At 6 DBTDC (17" vacuum) has poor performance and little to no ping
- At required 9 DBTDC (17.5" Vacuum) has mediocre acceleration with a slight ping and still runs at near 200 degrees temp.
- At 10.5 DBTDC (18" vacuum), runs hotter (215 Temp) with more ping
- At 12 DBTDC (18.5" vacuum), runs even hotter with even more ping.

When the Vacuum advance is reconnected, the performance drops and the pinging is much greater.

I have checked for other vacuum leaks and found none. The Quadrajet was freshly rebuilt but originally tuned for a 400.

1969 XS Block 350 with the std 9779067 grind cam. Rebuilt Quadrajet from a Firebird 400 and an HEI ignition.

I have struggled with this for weeks and am at the end of my rope. Please help.

George

Blued and Painted
Chief PONTIAC

USA
3292 Posts

Posted - 20 Apr 2017 :  10:48:11 AM  Show Profile  Reply with Quote
The hei wont work well if connected to the factory resistor or restisor wire. The hei needs a dedicated power source with full battery voltage.
Whats the casting number on the cylinder heads at the center exhaust outlet?



Bull Nose Formula, 461, TH400/ 3.08 8.5 / R44TS.
Go to Top of Page

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 20 Apr 2017 :  10:50:43 AM  Show Profile  Reply with Quote
The heads are a number 47. Good catch on the HEI. I'll find a dedicated source.

George
Go to Top of Page

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 20 Apr 2017 :  12:22:58 PM  Show Profile  Reply with Quote
HEI power change to direct battery power made no difference. Should the vacuum advance hose go to the metered post on the Rochester or the straight vacuum?

George
Go to Top of Page

Admin
Ye Olde Webmaster

829 Posts

Posted - 20 Apr 2017 :  3:00:43 PM  Show Profile  Reply with Quote
George--
The HEI works differently than a typical points distributor. To be specific, the weights operate the same in both, but the HEI is designed to work off of metered vacuum off the Q-jet, whereas, the points distributor was set up, along with the old timing specs, to run off of manifold vacuum.

At idle, you say your manifold vacuum is around 17. The pulse from the carb outlet at idle is zero. Consequently, most HEI run engines want 12-16 degrees of initial advance. Adding the zero vacuum at idle you have only 12-16 degrees. If you keep this concept in mind, you should be able to dial in your timing for good performance.

Vacuum advance increases during part throttle with the Q-jet. Under full throttle vacuum advance falls off while mechanical advance increases with rpm.

So if your mechanical advance is set right, you should see a total mechanical advance of about 34-36 degrees. Most HEIs I've worked with come close to giving 20 degrees advance. Point distributors typically provide more mechanical advance because the initial is set at a lower advance.

HEI's require a full 12 volts to operate. Direct power is needed--no resistor.

The advance curve is another area that can be fine tuned. Most street car engines should have a curve that plateaus around 3000 rpm. Some HEIs I've tested continue to provide mechanical advance as the rpm increases. They should be restricted or limited to around 3000 rpm. The spring tension plays a large part in creating a quick curve. However, there's a fine line between using low tension springs that allow the weights to fly out fast. Typically those light springs don't have enough tension to pull the weights in to close when the engine slows and the car comes to a stop. This results in an inconsistent idle--one that may fluctuate.

I suggest tuning your HEI based on the highest vacuum reading you get from your intake manifold at idle. ( Hook up your vac gauge to the manifold. ) Initial could be between 12-16 degrees. Then hook up your vacuum line from the metered outlet on the Q-jet to the vac canister. Minor adjustments may be needed to the carb to improve idle quality. Make only small adjustments and note what you do. Keep tweaking as needed.

Hang in there--you'll get it.


"Dedicated to keeping the classic Pontiac engine alive."

Go to Top of Page

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 20 Apr 2017 :  4:27:34 PM  Show Profile  Reply with Quote
Thanks for the input. Lots to think about. I have three vacuum ports on the quadrajet. I believe the upper left one is metered, so I'll start there. Should I then adjust timing from the get go with the Vaccum attached using a higher rev power tuning?

George
Go to Top of Page

tjs44
Cochise

USA
363 Posts

Posted - 20 Apr 2017 :  4:50:55 PM  Show Profile  Reply with Quote
Any idea of exactly what your CR is?Do a hot pumping comp test to see what you pumping.If over 175 you might not be able to run pump gas.Tom
Go to Top of Page

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 20 Apr 2017 :  8:44:16 PM  Show Profile  Reply with Quote
I adjusted the HEI Distributor to the highest vacuum at idle with Vacuum advance plugged. Then I reconnected the Vacuum advance to the metered port on the quadrajet. The engine ran at higher rev, but on acceleration, you got a lot of ping. With vacuum advance still connected I adjusted the distributor to hunt for the highest vacuum, then on acceleration it pinged quite a bit. I gave up for the night thoroughly frustrated. No matter what carb I put on in the past, I still get the same lousy performance. so I know it has to be timing, but I'm stumped.

I am almost reaching the point of putting the old distributor with points back on. Any further advice?

George
Go to Top of Page

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 20 Apr 2017 :  8:46:44 PM  Show Profile  Reply with Quote
the CR should be at 9.25, but Im at 150 psi when I use the compression tester.

George
Go to Top of Page

Admin
Ye Olde Webmaster

829 Posts

Posted - 21 Apr 2017 :  05:20:09 AM  Show Profile  Reply with Quote
At the highest vac point at idle, what was the initial mechanical advance set at the distributor? How many degrees? What is the vacuum reading at the highest point?

When you have it at this point, what is the rpm at idle?

If you left the engine to idle, how high does does the coolant get, is it running hot? When shut down, how easily does the engine restart? Any kick-back at the starter?

At what rpm are you hearing the pinging? What do you notice?

Using a timing light with vacuum advance not involved, what is the amount of advance at 3000 rpm? Can you tell what it is? Do you have a timing light that will give you that info?

What grade of gas are you burning in the engine? Have you pulled any plugs to see if you are running rich or lean? Have you noticed any peculiar exhaust smell?

When the engine was using a points distributor, did any of these conditions exist or did it occur after the installing the HEI?

"Dedicated to keeping the classic Pontiac engine alive."

Go to Top of Page

cortcomp
Coyote

USA
5154 Posts

Posted - 21 Apr 2017 :  11:05:08 AM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
Just for giggles, set your timing at closer to 15 and see what it does.
Go to Top of Page

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 22 Apr 2017 :  08:51:41 AM  Show Profile  Reply with Quote
I dont have an integrated tach with the timing light, but I can scrounge up one.

- 17.5 inch at 9 DBTDC at ~800 rpm
- At idle, ~200 degrees. Easily restarts, no kickback
- 1500-2000 rpm starts to ping
- when at high rev advance is about 20-25 degrees but its off the scale
- 92 octane plus an aftermarket octane booster
- Plugs are burning somewhat lean
- Not a truly rich smell
- Not really

George
Go to Top of Page

Admin
Ye Olde Webmaster

829 Posts

Posted - 22 Apr 2017 :  09:12:53 AM  Show Profile  Reply with Quote
- 17.5 inch at 9 DBTDC at ~800 rpm--Is that your highest vacuum reading--at 9 degrees advance???

What is the vacuum at 14 degrees?

"Dedicated to keeping the classic Pontiac engine alive."

Go to Top of Page

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 22 Apr 2017 :  1:30:08 PM  Show Profile  Reply with Quote
20" with lots of ping (vacuum advance not connected)

George
Go to Top of Page

Admin
Ye Olde Webmaster

829 Posts

Posted - 23 Apr 2017 :  07:08:50 AM  Show Profile  Reply with Quote
I'm thinking your carb is not providing enough gas to your cylinders and the lean mixture is firing a millisecond before the plug is firing.

With a vacuum of 20 the cylinder should be filling to provide a clean burn--but it isn't. This points me to a too lean situation. Your 150 cylinder pressure is not causing this problem--I don't believe it's an octane issue.

"Dedicated to keeping the classic Pontiac engine alive."

Go to Top of Page

gmaxiotis
Tribal Scout

USA
9 Posts

Posted - 23 Apr 2017 :  07:55:11 AM  Show Profile  Reply with Quote
The plugs looked a bit lean. So I shouldn't scrap the HEI and go back to points as a check? I'll start from the beginning and look hard for any obvious vacuum leaks. Starting with the trans shift vacuum line. I also have a breather on the valve cover. Since I dont fun that to the air cleaner, should that now be a one way check valve?

George
Go to Top of Page

Admin
Ye Olde Webmaster

829 Posts

Posted - 23 Apr 2017 :  11:26:38 AM  Show Profile  Reply with Quote
You could "pop in" the points distributor and see if there's any difference--ping or no ping. If you try it, test the points distributor without vac connected and make sure your dwell is at 30. If it still pings at 20" of vac...you can pretty much conclude that it isn't the distributor(s).




"Dedicated to keeping the classic Pontiac engine alive."

Go to Top of Page
   Topic Next Topic  
 New Topic  Reply to Topic
 Printer Friendly
Jump To:
Pontiac Street Performance © 2006-2016 Go To Top Of Page
This page was generated in 0.39 seconds. This Site Sponsored By:
Powered By: Snitz Forums 2000 Version 3.4.05