67 400 Timing/Carb Help - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 06-08-2016, 12:28 AM Thread Starter
 
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67 400 Timing/Carb Help

I have a 1967 GTO. I had a lifter go bad and wore a cam lobe. I rebuilt the top end. I have it all back together and I have it starting up. This is my first time setting timing. I currently have it set at 8 degrees BTDC. The old book manual I have for it says stock was at 6 degrees BTDC. I have tried it there as well. I can get it running but idling a little rough. When I am in the garage and I rev the motor, it is not missing. However, when I took it out to see how it drove I noticed it would start missing at high speeds and/or under a load. I am just not sure if it is a carburetor issue or a timing issue? For a slightly more aggressive cam should the timing be further advanced? It also seems to be very jerky when I give it gas above the 3000RPM range. I noticed the jerk in 1st gear just rolling slowly as well.

Engine specs: 67 Pontiac 400 bored .040 over with a 3.75in stroke, 67cc 1969 ram air heads, 10.5-1 compression ratio, Edelbrock 650 performer carburetor, Edelbrock performer series intake, Edelbrock performer RPM camshaft, Edelbrock valve spings, Edelbrock timing chain, Comp Cams roller tip rockers. Running 93 octane fuel. The carb, cam, springs, timing chain, rockers, and push rods were all replaced with this rebuild.

I appreciate any help. Just hoping to get it back running good again.
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post #2 of 6 (permalink) Old 06-08-2016, 10:04 AM
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Ok, a few things. First, how old was your cam and what was it? Original? Second, when camshafts go flat, the metal goes in the engine. Sometimes it acts as grit and wipes out all the bearings and cylinder walls. Sometimes you get lucky and it doesn't. Did you pull the pan and check for metal in the pump and sump? Most new flat tappet cams and lifters are Chinese made, and are inferior materials and hardness. They have a high failure rate. I would personally rather run 50 year old used junkyard parts in my own engines....made in the USA and superior. Third, a 67cc-headed .040" over 400 will be right at 11:1 CR, which will require 110 octane fuel to run, at all times. If you want to run 91-93 octane pump gas, you will need to drop the compression about 2 points, by gaining 20cc in your combustion chambers. I went to 87cc heads with my own 400. Forth, you need to do some basic checks to verify the engine is ok: vacuum gauge, compression test, etc. Until you verify that the engine is mechanically sound in every cylinder, chasing timing and ignition and fuel problems is a complete waste of time. If unfamiliar with engines, get familiar with them or hire a pro. The ability to read and tune and troubleshoot engines effectively isn't something you can pick up on the internet in a day or two. It takes time and aptitude.
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post #3 of 6 (permalink) Old 06-08-2016, 06:44 PM
 
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What are your cam specs? If you got the 110 LSA with an intake closing of 27 degrees After Bottom Dead Center, you will have to use racing gas as your Dynamic Compression Ratio is extremely high. Plug in your numbers here: Wallace Racing: Dynamic Compression Ratio Calculator

My opinion - carb is a little small, I'd have gone with a 750-800 CFM. You may have to fine tune the carb and richen it up.

What kind of manifold vacuum do you have?

Have you tried a fuel pressure gauge to make sure you are not losing fuel pressure at higher RPM's?

What type distributor? Points or electronic?

Did you set the timing curve using the mechanical advance/vacuum advance can?

Could be as simple as a bad wire, bad ground (you did put the ground strap back on the head-to-firewall?), alternator problem at high RPM's, battery up to par?, or some other electrical gremlin.
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post #4 of 6 (permalink) Old 06-08-2016, 07:12 PM
 
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the Performer RPM cam is an "041" RAIV clone... 231, 240 @ .050. .470 lift with true 1.5 ratio rockers. Verdict: Pretty good thumpin cam in a 400 with not that good of vac characteristics W/O use of Rhoads lifters...have worked with a bunch of them. About to work on customers '70 406/ with mildly worked 12's, Melling 041, headers, tricked Qjet. Car will get it, decent, but needs the short tires and 3.55's out back.

In the OP's combination, it is a mismatch...find a Local with a chebby 305 that needs the small Carter clone, and then get a good Qjet, then go through it. '71-73 800 cam Buick Qjet or a '77-79 Pontiac or Buick M4MC are great choce and are affordable as cores. Cliffs Qjet book is a GREAT guide. Can be done for under $225, ncluding throttle bushings, total rebuild, if one can buy the core right.
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Many years ago I tore up a 428 with 10.5 CR and cast pistons by running it on pump gas. Never heard it ping. Ended up pounding the bearings out of it due to the detonation, and on teardown, not only were the bearings and crank toast, but I had 4 or 5 broken pistons. Very easy to bust the lands out of a cast piston by running high compression on pump gas.....and it can lead to the symptoms described by the original poster. Something to consider. A Compression and Leak-Down test will verify that the basic short block is sound.
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post #6 of 6 (permalink) Old 06-08-2016, 09:00 PM
 
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PH: "the Performer RPM cam is an "041" RAIV clone... 231, 240 @ .050. .470 lift with true 1.5 ratio rockers."

PJ: OK, found it. Cam #7157 . I saw the Performer Plus which had the 110 LSA and a 27 degree ABDC closing on the intake - not a good choice for the already 10.5 compression engine.

So that changes things a little and the Dynamic Compression now becomes 9.31, but still, in my opinion, a bit high for an iron head street engine and will require good high octane racing gas.

I found this article on Dynamic Compression ratios and am going by this:

"DCR is much lower than static CR. Most performance street and street/track motors have DCR in the range of 8-8.5:1. With typical cams, this translates into static CR in the 10.0-12.0:1 range. Higher than this, there may be detonation problems with pump gas. Engines with “small” cams will need a lower static CR to avoid detonation. Engines with “big” cams have a later IVC point and can tolerate a higher static CR. When race fuel is used, much higher DCR (and static CR) may be used because of the detonation resistance of the fuel. Of course, race motors also have much larger camshafts which is another reason they can get away with such high static CR, often in the 13-15:1 range."
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