Author Topic: Pontiac 400 running poorly under load  (Read 291 times)

0 Members and 1 Guest are viewing this topic.

Offline Atticus80pacecar

  • Jr. Member
  • *
  • Posts: 14
  • Referrals: 0
Pontiac 400 running poorly under load
« on: June 14, 2020, 12:59:34 PM »
Hello all, i'm looking for some help with getting my freshly built 400 running well.

Engine: '72 400 block, bored to 413, mild cam but more aggressive than stock. '69 GTO heads, Edelbrock 750, plugs gapped to .045, total timing set around 32 advance

It idles very well and revs in neutral just fine, but has very little power on the road, and falls on its face around 3k under load, backfires and makes a "chooching" sound (backfire out the carb?). I'm learning as I go with this project so I have very little experience with the fine tuning aspect. It almost feels as if the choke is not opening, but I'm sure that's not the issue.

I adjusted valve lash with the engine running, which helped a lot, but didn't completely resolve my issues ( one or two are still clacking on the driver side so Ill need to revisit that again). Before I did this, the spark plugs had some carbon build up (enough that they needed replacement for the engine to start, but didn't look terrible). When I pulled the plugs they were dripping with gas. I'm inclined to think I resolved this issue with the valve lash adjustment, but wanted to include it in case it's a symptom of something else.

Does anyone have any advice on what my issue might be? It FEELS like its leaning out, but it could also be running too rich, I don't have a good enough ear to tell the difference between timing and fuel issues. The engine has maybe an hour and a half of run time on it (cam break in, some idle while making adjustments, and a few trips up and down the road in an effort to begin seating the rings before my cylinder walls run out of abrasion).

I'll hopefully be getting the car to my engine builder this week to help me diagnose it, but I thought I'd make a post here to see if anyone thinks these sound like symptoms of a specific issue.

I've heard the edelbrocks can be fussy with too much fuel pressure, so i've installed an inline fuel regulator and set it to 4 psi, but have not fired it up since to see if any difference was made.

Thank you in advance for any help!

Offline Angelo

  • Sr. Member
  • ***
  • Posts: 1049
  • Referrals: 0
Re: Pontiac 400 running poorly under load
« Reply #1 on: June 18, 2020, 09:19:27 PM »
Hello!

Smarter folks will chime in but based on your symptoms, it runs great at idle but not at higher RPM's makes me think that either there is an ignition issue or fuel issue. Fuel issue though I don't think it would backfire, but I could be wrong. Regardless, this is what I would check...

In neutral, is the engine slow to rev up to 3,000 and beyond or snappy responsive? If it is not snappy it's still telling you something is wrong.

Induction: Monitor fuel pressure from the pump to your carb when at higher rpms.

Ignition: Monitor the timing advance when you rev up the engine, make sure you're getting the correct advance at 3,000+. Not sure what timing curve you picked but I idle at 14 and let it max out at 34 at 3,000 without vacuum.  What are you running ignition wise, points factory distributor, HEI, MSD box with a MSD billet dist, etc..??? What kind of vacuum do you get at idle?

Other vitals: What does your volt meter read when you're driving and start to have the drop in power? Everything else good such as oil pressure? PVC valve new and connected correctly? Check for Vacuum leaks? Nothing restricting the air cleaner or butterflies in the carb/throttle body?

I had a Pertronix distributor that 2 months of owning went bad, made my engine run like a dog over 2,000 rpm, when I checked it with a timing light it was retarding the timing instead of advancing it and it was all over the place. I got a temporary new Cardone HEI unit just to do a quick test for $75 and it solved the problem. I never had a backfire though but thinking back with the timing being all erratic it was bound to happen. I now run an MSD billet 2 wire locked out distributor connected to an FiTEch EFI throttle body setup to control the timing. I can make adjustments from the FiTech management screen without getting out of the car, as well as watch the timing from the screen.

Thanks,
Angelo
« Last Edit: June 18, 2020, 09:22:10 PM by Angelo »
81' ex-Turbo Trans Am - Resto-mod

Offline Atticus80pacecar

  • Jr. Member
  • *
  • Posts: 14
  • Referrals: 0
Re: Pontiac 400 running poorly under load
« Reply #2 on: June 22, 2020, 05:11:02 PM »
Thanks for the reply!

The engine revs normally in neutral, very responsive.

Induction: I will need to get a fuel pressure gauge to confirm, but since my last post I installed a Mr. Gasket fuel pressure regulator and set it to 4psi as recommended by my engine builder, so it SHOULD be 4psi.

Ignition: Total timing comes in at about 3800 rpms. I haven't messed with the timing curve springs. It has a Mallory HEI distributor with vac advance. Brand new unit. I also do not have a vacuum gauge, ill need to pick one of those up to verify vacuum. Should I have my vac advance connected to ported or manifold vacuum? Its essentially a '69 GTO under the hood, no emissions, if that matters in finding an answer to that question. Ive received mixed results to this question.

other vitals: I haven't checked anything with a volt meter so I cant say what it might read. Oil pressure does seem low when its warmed up, but has good pressure when its cold. I have a 60 pound pump in the engine, but my stock dash gauge goes to 80, so I'm hoping that is the cause of my low oil pressure reading at idle. And i mean very low, like 0-10lbs. Again, i'm hoping, and assuming this is due to the gauge. Are there options for an external type gauge to test this?

PCV valve is new and routed correctly (PCV in rubber valley pan grommet, going to front port of edelbrock carb)

Air cleaner is the stock unit off my 301, so it could be restricted more so than a normal chrome air cleaner would be, but i'd think its getting enough air. 

I like the idea of being able to make adjustments without getting out of the car, but id like to figure out my issue in its current set up.

I haven't started the car up again since my last post. As a general rule, how much can I really let this thing idle right now? The cam has been broken in for 20 mins, and i've put about 30 miles on it just road testing it. My engine builder says I really shouldn't let it idle "more than i need to". Will i cause damage letting it idle while I mess with timing and checking vacuum? I'm at a point where I'm hesitant to work on it and troubleshoot for fear it'll take too much time for me to diagnose the issue and I'll end up ruining my cam or tappets.

Thanks,
Atticus

Offline Stringer

  • Member
  • **
  • Posts: 146
  • Referrals: 0
Re: Pontiac 400 running poorly under load
« Reply #3 on: June 22, 2020, 06:12:26 PM »
Your vacuum advance should always be attached to port vacuum. Manifold vacuum will drop under acceleration as the engine pull in more air. Port vacuum is constant. The initial (baseline) timing should also be done with the vacuum advance disconnected. You should also at pick up a vacuum gauge, it will help with tuning the car at idle.

With about 30 miles on the new engine I wouldn't be too concerned about letting it idle while tuning. Of course you don't want to let it idle for an hour - but 10 minutes or so shouldn't be a problem - especially since you will probably run it up and down the rev cycle while doing a tune.

Without knowing everything about your settings with the performance cam - I would start with the factory settings and go from there. Timing I believe for the 400 is about 100 @ 1000 rpm with automatic trans.

I would also make sure that both idle mixture screws are adjusted correctly. Start with the same number of turns out from fully seated. Usually at about 1-1/2 turns out. Then adjust each in and out slowly (1/8 turns each) until you reach the highest reading on the vacuum gauge. It is best if you don't adjust the idle screws until the engine is at operating temperature. A stock engine should yield about 18 lbs or so on the gauge. Yours will be a little less with the cam.

Beyond that, someone with more knowledge may need to chime in. 

Hope this helps.
« Last Edit: June 22, 2020, 06:23:11 PM by Stringer »

Offline Atticus80pacecar

  • Jr. Member
  • *
  • Posts: 14
  • Referrals: 0
Re: Pontiac 400 running poorly under load
« Reply #4 on: June 22, 2020, 06:32:57 PM »
Okay perfect, I've had the vac advance hooked up correctly then, to the ported vacuum.

When using the vacuum gauge, where would I actually hook that up to? T off the line between ported vac and vac advance? manifold vac port?

Good to hear I shouldn't be too worried about letting it idle for a short time. I've already let it sit at idle for about 40 mins total (not all at the same time, that's between 4-5 start ups. Setting timing, idle mixtures, and valve lash).

It is a manual trans car, Muncie m20. Does that make a difference for timing? (assuming so since you mentioned auto)

Offline Stringer

  • Member
  • **
  • Posts: 146
  • Referrals: 0
Re: Pontiac 400 running poorly under load
« Reply #5 on: June 22, 2020, 06:46:05 PM »
Hook the vacuum gauge to manifold vacuum. You will see the gauge go up and down dramatically when reeving the engine. Manual trans does make a difference in the timing (as you don't have the restriction of a torque converter).  I believe it is 120 @ 700 but you might want to look that up.

Offline Atticus80pacecar

  • Jr. Member
  • *
  • Posts: 14
  • Referrals: 0
Re: Pontiac 400 running poorly under load
« Reply #6 on: June 22, 2020, 06:57:26 PM »
Very good, I appreciate the info!