1968 L-71 Timing and Tuning

[Gary Noser]

I have a 68 427 435 corvette that I am having trouble getting tuned right. It has the original tripower setup, heads (3919840), etc the only non original component is a complete msd ignition. It has been bored 60 over. It has comp. cam (cca-11-218-4) (Int./Exh.) 236/236. The pistons are TRW-L2268F60. Bore= 4.310" stroke= 3.76"
One thing with the center carb, before I got it, two holes where drilled in the throttle plates in order to accommodate a big cam. Sorry for all the info but I thought it may be helpful in getting to the bottom of the problem. The problem is I was tuning it and driving it at what the engine builder set the timing to when I got the car back. It ran ok but I was constantly adjusting the fuel mixture and idle speed. It seemed I had to keep making it richer until the mixture screws where close to fully turned out. Now it began to back fire out the exhaust after a few trips with this setting. So I leaned it back out but the backfiring continues. So I check the timing and the distributor had vibrated loose, just so it was able to move a very small amount. So it was no surprise I was having problems. I set the timing back to where the mark was I made just after I got it back from the builder. Now I checked the timing and he had it set at 12 deg. of advance @800 rpm. I now have it set at about 8 deg and it still back fires a bit. In addition it has occasionally run-on for a few seconds ever since I got it. I did install the diode that came with the MSD, so that is not it. It stopped the run-on initially after I timed but it has since started again. I really appreciate the help here; I have been reading the archives over the past few days but did not find anything that answered all my questions. I need some guidance on what to try timing the engine at. Also any tip on tuning it would be greatly appreciated. Finally if anyone could give me an estimate of the compression ratio as well.
Thanks in advance,
Josh

[Jim T.]

Re: 1968 L-71 Timing and Tuning

Gary the backfiring can rupture the power valve in a Holley. Yours might need replaced. Cost is not much, less than $10, I pick mine up in blister packs at Auto Zone and buy new fuel bowl and metering block gaskets as well.

[Gary Noser]

Re: 1968 L-71 Timing and Tuning

I took my center carb apart today and found a solid plug where the power valve should be. I removed that and put in a #65 power valve. I then reassembled and tried to start the car. I could not get it to run very well at all. At the point when it would idle and run the smoothest it would only run for a 10-20 seconds and then die. Does anyone have an average number of turns that the idle mixture screw should be set at or any clues as how to properly adjust the mixture?

[Jim T.]

Re: 1968 L-71 Timing and Tuning

Gary I have always used a vacuum guage to adjust my 4-barrel carbs, adjusting the screws for the highest vacuum reading. Points adjusted, engine timing set, and full time vacuum hooked to vacuum advance cannister.

[Frank H.]

Re: 1968 L-71 Timing and Tuning

Gary
You have the trw 2268 .240 inch dome same/correct 435 pistons plus the 60 overbore added voluum are proably at 11.25-1 or 11.5-1 ,depening on head gasket.
But you didn't mention fuel additive
I,m sure you will need to find a ratio.
I used one can of 104+ octane booster to ten gallons of gas was the max fuel/additve ratio for me and still had to shut off while releasing clutch,
I set timing while driving then came home recorded setting.

I still adjusted dist. with some brands of fuel even at 10gal/can.
As stated above if your engine backfired,it may have burned out your power valve
I ended up useing a #50 power valve,with mech secondary linkage ,but don't suggest that for you, but any holley can use the check valve protection upgrade kit from holley protect the valves port or plug the port and get the philip screw that has the check valve in it that I used needs no drilling.

http://www.powervalveshield.com

I milled my pistons and use pump gas now.

[Gary Noser]

Re: 1968 L-71 Timing and Tuning

I am thinking today I may take the carb back off and put back the power valve block off. I was reading about it and if you have big enough jets you do not need the power valve. The jets in the carb are #66. Do you think that will be ok? I did have the car running fairly well until the distributor came loose. It is recommended that timing be set at 4-5; does anyone run that little of timing in these 435 cars?