OK, for a fun project I built a '70 LT-1 motor to drop into my '75 Automatic. It has an automatic transmission and power brakes. I used CompCams 12-107-3 Factory Muscle cam for the cam, stock heads, intake and carb.

I don't have the equipment nor the space to drop this into the car at my house so I took it to my local shop and they dropped it in for me and set the timing, broke it in, etc.

They called me today and said they have tried everything (had their carb guy check adjustments there, upped the stall converter in the automatic transmission, etc.) and can't get the vacuum above 8".

The quote I'm getting is that it cruses but it doesn't have enough 'pep'.

I would assume they have checked for all the standard leak type problems, as these guys are knowledgeable.

What do I do next? I'm going to pick it up as they say it's ok to cruse with...

 

Have they TDCed the engine to make sure the timing marks are correct. It sounds like the timing is retarded.

 

Could be a vacuum leak. Did they check around the base of the carb and the intake/head area?

Wondering if there could be an issue with cam timing, index off?

Did you do a compression check?

 

I'm having a similar problem. My 76 L-82 hesitates off the start sometimes and even backfires once in a while. My car pulls about 8lbs at idle. My mechanic said my CAM is too heavy. I'm not sure what CAM I have as the engine was rebuilt many years before I got it.

Someone else mentioned there might be some kind of vacuum booster we can add?

Right now the vacuum advance is disconnected since it seemed to run worse when it was hooked up. I'm guessing that's not good.

I need to spend more time looking for leaks, checking timing, etc..

 

That can be a big cam for an automatic tranny one thing that would help is lock the distributor so your at peak timing at idle.

We have seen it work on circle track engines where there is a vacume rule and the carb has to be right on as well, Peak vacume will accure at peak timing.

Test by setting your timing at 36 degrees at idle and see what happens.

Hope this hepls Carl

 

i would not assume any thing.disconnect all the vacuum lines .most mechanics that dont work on vetts a lot forget about the head lights ,and all the other vacuum operated vette only parts.

 

Thanks guys. This is a Corvette specialty shop. I had a chance to drive the car but haven't picked it up yet. It is nothing great off the line but it's got plenty of power once I get into the 1800+ rpm range, which is where the cam wants to be.

How would I tell if this was an indexing problem?

 

I have a 70 with the same issues, and have a thread running now about it. It is a NOM, built up 350 with Edelbrock aluminum heads, single plane Torker II intake, and a Holley carburetor. It is a 4spd car with an M-20 and 3.08 rear. It idles ok, but ragged, like the cam has a lot of overlap. The idle vacuum is 9-10". It is a dog to drive, because it has no torque to get moving, but once it is above 2500 rpm, it wants to fly like a rocket. It was obviously built as a 1/4 mile machine. It is not very street friendly. Most everyone I have talked with about it agree, I need to replace the single plane intake with a dual plane, and swap out the cam for a more street friendly cam with less overlap. Looking at your cam specs., yours appears to come to life at 2800 and up, I think that could be the problem.

 

If you look at the timing card for that cam at the CC website, it's the same as the 30-30 Duntov cam as used in the '64-'65 vettes and in the Z28. However, the street high perf solid lifter cam used in the '70 LT1 had 10 degrees less duration at .050 (and .020" less intake lift), but the reduced duration was taken off the opening part of the intake lobe - as a result, there is 10 degrees less overlap. Lobe separation, as a result, increases to 116.25 degrees. The good in this LT1 cam is that it has better vacuum, but diminished high rpm power. On the other hand, the cam you have has better power, but lower idle rpm vacuum.

If you want to increase the idle vacuum with this cam, you can:
1. Raise the idle rpm
2. Shorten the centrifugal advance by a few degrees, re-timing for the same total advance, which will advance the early portion of the timing curve.
3. Use a looser torque converter
4. Ensure that you have the lowest possible restriction in your exhaust system
5. Ensure that your carb is properly tuned.

 

Also something I forgot to mention, but it's of utmost importance...
Ensure that the cam is installed with a 109 or 110 degree intake centerline.

The Duntov cam, from the factory, came with 4 degrees of advance (110 degree ICL); the cam timing from the CC website cites an ICL of 114 degrees - if your mechanic set the cam timing with this, it will kill the performance you thought you would be getting.

Let us know how it works out - Cheers!

 

OK, my timing set has the ability to be adjusted 4 degrees at the crank, netting 2 degrees at the cam. If I read the suggestions here right, I likely need to pull the lower sprocket and advance it the 4 degrees, netting 2 at the cam.

Is there something I can do before I pull the whole front of the motor apart to identify if this is the real problem?

 

You really need to do it right by checking your cam and TDC first. If you really know where TDC is (a late model balancer after ~1969(?)) will have the TDC mark 10 degrees CCW from the crankshaft keyway; earlier balancers have the TDC mark inline with the keyway. Which do you have? It is also helpful to know that the keyway exactly aligns with the #1 crankshaft journal, so when it is aligned with the center of the #1 cylinder, the crank is at #1 TDC.

Checking the cam timing - although using a degree wheel is best, the following steps will get you in the ball park...

1. Remove the left hand valve cover.
2. After knowing what you have for TDC marks, set the crank to TDC-overlap. At TDC-overlap, the #3 cylinder intake valve will be closed; if it's open, rotate engine another complete turn.
3. Remove the rocker arms from #1 intake and exhaust valves; also remove the rocker from the #3 intake.
4. Place a ruler or flat across the #1 cylinder pushrods tips and overhanging #3 intake pushrod tip. If the cam is straight up (not advanced), the ruler should be about .087" above the #3 intake pushrod tip. If the cam is advanced by ~ 4 degrees, the height above the #3 pushrod tip will be ~.130"; if it is retarded from straight up, the ruler or flat will be less than .087, possibly touching the #3 pushrod tip.

Note: The above procedure avoids having to remove the intake manifold, but if it is off, place a short machinists rule across the #1 exhaust and intake lifters, resting it vertically against the pushrods. It will be easy to see which lifter is higher. If the cam is to be advanced 4 degrees, the intake lifter should be about .030 higher than the exhaust lifter. (This data comes from profiling data which I measured with an original 30-30 Duntov cam years ago.)

 

Most timing sets with advance & retard keyways are in crank degrees, so if you move from the 0 to 4 degrees advanced, the cam events will advance 4 degrees as measured on the crankshaft. In other words, if the intake valve closed (.050) at 44 degrees after BDC, advancing the cam 4 degrees will cause the intake valve to close (.050) at 40 degrees after BDC.

 

larrywalk, I'm confusing myself. If the cam is currently set for 114 degrees and I wanted to set it to the correct 110, would I advance the crank sproket or retard it?