I have a 69 350/350HP 4sp - pretty much stock with some miles on it. Idles OK but is rough on acceleration and is missing 1/3 of it's power. Above 4,000 RPM it pops and backfires. Changed plugs, points, wires - usual tuneup stuff. Checked firing order - it's good. Check timing - it's good/conservative 4 BTDC. Check compression in all the cylinders - all are 140-160. Swapped the carb with an extra I have and still get the same thing. Pulled valve covers and there's no obviously broken springs. I was thinking maybe valves - but the compression seems reasonable - so now maybe timing chain??? I'd appreciate any small block wisdom out there before tearing down the top end.

Pete

 

First of all welcome to the forum. You are sure to find lots of valuable information from the members here.
Your problem could be a multitude of things. It's hard to know without being there, but it definately sounds ignition related. I would be cautious about tearing the engine down at this stage. I doubt that the timing chain is your culprit, but there is a very simple check for a loose timing chain. Pull the distributor cap and observe the rotor position. Take a breaker bar and a appropriate socket and put it on the crankshaft damper pulley. If you have a degree wheel put it on the damper pulley else you can just chalk mark the timing mark position at the appropriate time.

Now - slowly turn the crankshaft pulley in a clockwise direction. Watch the distributor and observe that the rotor is moving. Stop turning. Now - mark the damper pulley position with the chalk or observe the degree wheel. Very carefully turn the crankshaft in the other direction and VERY carefully observe the rotor in the distributor. The instant it begins to move STOP turning and mark the crankshaft position again. Measure the number of degrees of rotation of the crankshaft. If there is a lot of slop in the chain then you will have moved the crankshaft ten or fifteen degrees (or more) before taking the slop out of the chain after the reversal before the camshaft began to turn. Get the picture?? If all is well and there is no slack in the timing chain then you will see about three to five degrees of "reverse motion" before the distributor begins to turn.

If you are not sure how many degrees it turned during the procedure there is a simple way to calculate that based on the spacing between the chalk marks. Take a string and wrap it around the crankshaft damper where you made the chalk marks to measure the circumference of the damper. Let's say it was 18 inches. If there is one inch between the chalk marks then divide 1 by 18 and multiply the result by 360 (the number of degrees in a circle). In this case the answer is 20 degrees and it is time to replace the gears.

Have you checked your distributor closely?

You should read this thread if you haven't already.

http://forums.corvetteforum.com/show....php?t=1430339

 

Well - I followed the procedure - the crude procedure as I don't have a timing wheel. I measure the slack and caculated the # of degrees based on the length of my breaker bar. Came out to less that 10 degrees - so based on that it sounds like it's OK. I was wondering if maybe the timing chain jumped. Haven't had that happen on a V8 - only on OHC 4 cyl - but maybe???

I'll go in and read the thread you referenced

Thx

 

that almost sounds to me like a fuel deprivation situation.......how do you know you are getting enough fuel? both in terms of pressure and quanitity?

 

I would start with checking the cam by taking the valve covers off and have someone bump the motor over, unhook the distributer so the motor doesnt fire, and check all the rocker arms. Make sure they are all moving and that you dont have 1 or 2 that are not going up and down as much as the others. If there is 1 not moving, or just moving a very small amount, the cam is going flat and will kill performance. Goog Luck.

 

You need to first fix your timing. 4 degrees BTDC is way off. You need to set up for 36 total, which should put you somewhere in the mid-teens for initial timing.

Once you have the timing set correctly, tear that carb down and take a look at float level and jetting. Make sure you're running .375" on the float, make sure the float is a new NitroFill float, and verify jetting to spec. Set your secondary windup spring to 3/4 turn and install a new filter.

Assuming you have compression on all cyluinders, this setup will get your carrunning well.
Lars

 

The timing is 4 BTDC at 750 with the vacuum disconnected - I think that's right - or at least that's what my book says. I'll check the total advance also. Swapped out the carb - same issue as before. Compression on all cylinders looks OK 14-160.

 

that may be what the book says, but Lars is giving you advice for performance tuning. if you want performance you need to alter the stock timing

 

pbennett,

Take Lars advice and follow his directions to a tee. I, and others will attest to Lars' knowledge on setting up our cars to run properly. Total timing is 36* between 2500 - 3000 rpm with the vac line disconnected. After that is dialed in, follow Lars procedures on the carb tune up/diagnostics to get it running properly.

 

Thanks, appreciate the help!