No power at low RPM

JEFFWGRAY

MEMBER
1972 CHEVROLET CHEVETTE
5.7L
V8
2WD
AUTOMATIC
35,000 MILES

1972 Chevelle with 1975 Monte Carlo 350 block. Stock rebuild except for RV camshaft, 1.6 rocker arms and hei distributor. 1976 nova th350 transmission rebuilt with shift kit. Rear axle is 8.2 with 2.73 highway gear also stock rebuilt. Rear tire will not spin from complete stop and WOT. Rear tire will bark when it shifts to second. Good power through mid range rpm. As rpm get around five to six grand engine starts to stumble. I have adjusted on the stock Q-jet multiple times with vacuum gauge and by ear with no improvement. Hei vacuum is connected to a tube on passenger side of Q-jet where I get a better cold start and hot idle. When I connect the hei vacuum to a tube just above PVC on front of carburetor idle fluctuates a little and cold start is difficult but low rpm power will spin the tire. I have been up and down the spectrum on timing from eight to ten degrees to as high as twenty because manual for Chevrolet hei introduction in late 1970's has timing at 22 degrees.

Friday, November 24th, 2017 AT 10:00 AM

1 Reply

STEVE W.

MECHANIC
15,240 POSTS

2:73 gearing is going to ruin low end power with a TH350 because the engine is not going to get into an area it can make real torque. But it should do pretty good on the highway.
If you want to get more low end power you need to drop the gearing, 3:42 or so would give you low end but will also ruin the highway ability. If you want the better parts of both, switch the rear to 3:42 or even 3:73 and put in a 700R4 to get the overdrive gear to make up some for the taller rear gearing.

5,000 is about as hard as you want to turn a stock 350, they tend to start shedding parts not long after that. RV cam will make that worse as it is designed for low rpm power not high revs.

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Friday, November 24th, 2017 AT 12:11 PM