So, I have a weird issue that I can't work out with the new engine. It's a carbbed 350 with a 670 street avenger, thumpr cam, and procomp 3003a heads.

It's got hydraulic lifters, roller tip rockers, and when I adjust them like non-hydraulic's, with 30 thousandths lash, it runs perfectly smooth, but has terrible performance. Best I can get at the track is a 15.22. Now, if I adjust the rockers pre-load style, (half turn past zero lash) it gets all of it's power back but under 2000ish rpms it runs like garbage. Just trying to cruise in 4th at 35mph (lightest throttle pressure possible) the whole car shutters back and forth.

Any one heard of a problem like this before?

 

if it has too big a cam then you are lugging the motor at part throttle, could be fuel mixture, could be timing issues....but you are hammering those lifters when you don't set the lash properly. That isn't a solid cam, set the lash to spec and figure out whatever other issues you have. Putting all that lash into the motor is making the cam act smaller so of course it'll run better at low rpms

 

Thanks for the info. Half turn past zero lash is proper for hydraulic lifters right?

Timing is my current focus. The guy who set my initial timing mark is used to his 70's camaro (back when chevy had the 8degrees off crank) so I'm not 100% sure his mark is correct. I have to run at LEAST 16 advanced off his mark just to run without sputtering. I tried locating TDC myself yesterday, thought I really had it (exactly 180 degrees rotation after the intake rocker finished moving) but the mark I made there was way lower than the old mark, as in, I would have to retard way past his mark to get to mine. I'm really at a loss.

My next step is to buy the tools and follow the procedure in this link to find TDC: http://67-72chevytrucks.com/vboard/s...d.php?t=290498

 

why don't you have a balancer mark and timing tab on the timing cover? The balancer mark and timing tab should line up to be rather accurate for TDC and the timing marks should line up. You didn't mention how big your cam is, but 16 initial isn't out of the ball park if it is a larger than stock cam. For larger cams, 24 initial 36 total isn't uncommon. Your timing should be done with the vacuum can disconnected as well just in case you didnt know that.

 

I do have a timing tab, but by balancer mark, do you mean the notch that was cut into the balancer when I got the engine? I was never sure if that was actually representing TDC or not.

I'll call the guy who put together the short block for me and get exact numbers on my cam. I know it's the base thumpr, but there are a few different profiles for that one.

 

yes take a yellow or white paint marker and fill in that notch so you can see it better when using a timing light

 

Here is the cam

Thumpr Hydraulic Flat Tappet Cam Adv/Dur 279/297 RPM Range 2000 to 5800 Intake/Exhaust Lift .479/.465

I notice it says that it's RPM range is 2k - 5.8k. How weird that I'm running like garbage under about 2k. Is there a correlation?

 

your carb is that vac. secondary ?


like stated above get your timing figured out. that makes one of the biggest differences.

 

If you have hydraulic lifters, then why would you adjust them like non-hydraulic lifters?

Roller tip rockers don't have anything to do with rocker adjustment! They just have rollers on the tips - that's all.

So, you need to adjust them like they are hydraulic lifters (since, uh ... they ARE), and then deal with your under 2000rpms issue elsewhere.

 

Yes.

I'm learning all of this on my own and on the fly. Now that I know the proper way to adjust rockers with hydraulic lifters, that's how they are going to stay adjusted from now on.

Question on that, is it enough to simply crank the engine past each rocker closing and adjusting it there, or do I HAVE to do the method where I go 180 degrees rotation after exhaust then intake close, then adjust them both?

 

As long as the end result is the same, you can use any method you like.

 

Try checking your cranking compression. for the duration you have on your cam I think (not an expert) you should be around 160 -180. also check your vacuum at idle you should have at least around 14`15 inches. If cranking compression and vacuum are lower the than 150 and 11inches respectively, the cam might be to big.

this should be done after you have adjusted your rockers properly.

 

I just took my dizzy cap off for the first time since we put the engine together and noticed that the tip by the contact on the rotor is broken.

Take a look...

http://i99.photobucket.com/albums/l306/Zerocyde/012.jpg
http://i99.photobucket.com/albums/l306/Zerocyde/014.jpg
http://i99.photobucket.com/albums/l306/Zerocyde/015.jpg
http://i99.photobucket.com/albums/l306/Zerocyde/016.jpg

Could my problems have to do with this? The shuttering back and forth at low rpms, like trying to maintain a constant cruising speed, etc...

 

looks like its cutting to the cap as well. thats defiantly a problem

 

Yea, all the contacts on the inside of the cap have that black mark on them. I'm heading down to the parts store now.

 

That was the problem, it runs smooth as **** now. Well almost. I got a 4.5 power valve in from when I had 10 psi vacuum. Now, with the new rotor, I got 15 psi and need to put my 7.5 back in.