With spring approaching, I think I need to correct my engine combo from last cam/head swap. Here is what I have on the motor:

350, stock bottom end.
TH400 tranny, with stock convertor (1800??) and 3.08 rear.
Crane XRi conversion kit.
Q-jet by Lars.
Sportsman II heads (200 runner, 2.02/1.6 valve, 64 cc, 1.6 stamped rockers). CR is estimated at 9.2-9.5.
Hooker competetion headers, with MAD chambered true duals, no cat, no muffler, 2.5" dia.
CompCam XS256S cam (218/224 duration, 0.465/0.477 with 1.5 and 0.496/0.509 with 1.6 rockers).

Car runs like crap between 2,000 - 3,500 rpm. Pulls hard after 3,500 but flattens out after 4,500. Car sees around 1,000 miles/yr on the street.

Some say the heads are too big. Some say cam is too small. I am confused. Looking for help.

I have to keep:
1. the redline at 5,500 rpm
2. stock convertor
3. stock rear

What should I change? H E L P P L E A S E

 

The set up should be fine. Have you checked timing? What do you have as far as timing goes?

 

the combo seems fine to me as well, how have you got the cam dialed in? advanced at all, semms to me thatthe trouble may lie eslewhere that the combo, but just out of curiosity, what is the overlap/ lobe separation angle?

 

The heads aren't too big. I'm using the same heads on my base '72 350 and the stock cam. Mine pulls hard to 5.5k rpm with no soggy or flat spots from idle to redline.

It's not in the cam either. That cam should have a robust and responsive off-idle through the midrange and pull nicely to 5k rpm.

You got something going on in your tuneup. Just because Lars tuned the Q-jet doesn't mean all is well today. You could have something creating a blockage or some friction in the primary metering rods or some trash in the metering circuit.

Your ignition could alse be falling down with a sticking advance.

My guess is that you'll find the problem in the carbs primary metering circuit or a vacuum leak.

 

Cam was installed without advance or retard.
LSA is 110 deg.
Timing is set at 12 initial with 35 total all in by 2500.

14 in of vaccum.
idles great. sounds great. but gutless....

 

Gerry72, I thought the same thing about the carb. So at the end of the summer, I installed a second Q-jet (15 minutes after Lars tuned it) to rule out the possibility of bad carb.

I have not swapped the dist. to rule that one out. I am going to borrow an HEI unit to be 100%.

One way or the other I have to resolve this before summer. Car is not as pleasurable to drive as it used to be with stock heads, stock mufflers and a smaller cam.

 

And I would check the linkage for full throttle and make sure the choke isn't locking out the secondaries.

 

Getting full throttle and removed choke assembly completely.

 

Are you running out of fuel? I had that problem with my Q-Jet. Or maybe your fuel line is crimped, or maybe your fuel pump sucks? Bring it to 4,500 slowly in second gear then hit it. There should be more available fuel that way.

 

That could be it. Runs lean under 3400 rpm and rich over it. Here is the dyno with A/F

I have 76 jets and secondary rods are CH. What would be a better combo based on the graph?

 

At what rpm's did the dyno run start? If it started at 2,500 the accelerator pump may not be working. It also appears that the primary jets are too big. Maybe go down 2 steps.

 

Not sure what rpm the run started. But judging by the hp curve, I am assuming it started at 3,250.

I think I should try another carb before I swap the cam/head.

 

Let's also discuss how these heads could be too big a runner for my set up.

Given my redline of 5,500, what is the best size? 165? 170? 190?

Is there a "guide" to selecting a proper runner?

 

I looked at your spec. again, and the problem as I see it is the runners are too big given the cam and compression ratio. To use those heads, I think upping the cam to maybe 230/238 @ .050, .510 lift and bumping the compression to 10.5-11:1 would wake the engine up. That beign said, 170 cc runners would pick up port velocity, the torque and h.p. will rise also. But, the 3.08 rear gears are killing any real acceleration.

 

Fix that mixture first. Try a A/F gauge ($150)?? Sorry but i can't tell which line is A/F on that graph?? Darker line looks like what ur describing and yes that rich mix could steal ur low & midrange pwr.

Ur idle vac is lower than mine while u have higher compression and smaller cam??? U may have vac leaks too.

Yea, fix that carb and look for vac leaks until idle vac gets over 15" at least.

I would not spend another dime on heads until u fix the carb tuning issues.

cardo0

 

big G, I am not planning on changing the CR. But am entertaining cam swap (as a last resort).

cardo0, getting A/F corrected is #1 on the list. What jet / rods do you guys recommend?

As far as vaccum goes, I checked at the dip stick and at carb, both were same without any drop. Doesn't that mean there's no leak? Also, the needle was stable...very stable.

 

Correction: my vaccum is 18" at the carb and 0 at the dip stick.

 

DING DING DING!!

We have a winner!

Cam too small

Heads too big

Too little CR

You also left out an important part of your combo, the fact that you are using a Performer intake, which is also not really matched to those heads. Aamir, why haven't you bought a Holley yet???

 

I have a potential buyer for heads. Gonna sell them and put Performer with 170 / 64 heads.

Cam is the last thing I will change. I don't think it's too small. A lot of guys are running XE268H here with around 250 rwhp. My cam is pretty close to that.

I still think I can make the Q-jet work. After all it was working fine before.

 

Beg, borrow, or steal another carb to see if it makes a difference, that way you can elimintate the carb from the equation. It does sound to me like there is more going on besides the mis-matched components.

Not with heads that large, and the XE268 has 224/230 duration,only the lifts are similar. Those heads need more cam to make the power you are after. What cam did Wayne use with those heads, a Holley carb, and a Performer RPM intake to make 300rwhp?