350 doesn't come alive at, and never reaches the top

[pcguy]

Anyone have a favorite trick to test for a domed pison? It occcurred to me that bending a piece of #12 wire and trying to sweep over the piston top when it's at TDC might tell you something.

[spencerforhire]

if you want to know what the pistons are, borrow an inspection camera, sometimes called a "borescope", and take a look inside through the spark plug hole. Most well equipped mechanical repair facilities have them. They can be purchased for under $200.
I really think that before you start disassembling the engine you should at least go through all the tuneup related possibilities first; carb, distributor, timing, etc.

[ROB]

If the timing mark on the balancer was off, wouldn't it simply be a matter of changing the timing??? I must be missing something....

No, If the balancer slipped, you will show "ignition" TDC, but mechanically you could be off 4-5*. That's why I suggest you make sure you are really at TDC piston wise to insure everything else lines up. And the vacuum source I was talking about, look at holley's website for info in your carb to insure where to get vacuum from.

[ROB]

Also check the source of vacuum for the distributor.

[pcguy]

Distributor vacuum is plugge/disconnected. Heading out to get a camera. If it has domed pistons along with the unidentifiable cam, I'll probably replace the engine with a 383 crate. Any recommendations?

[pcguy]

Or assuming that the pistons are a known quantiy like Wysco 10:1 as explained in the sheet by the builder, is there an aluminum head and cam combo you would recommend? If the pistons are flat, I believe the door to change is open.

[ROB]

Well having the vacuum advance blocked off is a definite problem. Before you go and dump alot of money on a crate motor, figure out the bugs with this one.

[pcguy]

Not a big fan of vacuum advance if you're not going to use the setup on the streets/highway? What's is the downside to using just the initial and centrifugal? Correct me if I' worng here, but there may be some advance at idle and some while cruising, but when you into, there's no vacuum advance... So what am I missing?

[pcguy]

Before you go and dump alot of money on a crate motor, figure out the bugs with this one.

I'd really like to find out what the bug is and if the pistons are not domed, I will likely replace the cam and heads with a combination that works. I'm not sure how to proceed with heads and cam if the pistons are alraedy domed. Can't deal with the uncertainty of the clearance issues.

Sure, I could pull the engine and bring it to a machine shop, but the cost of that project will be equal to or greater than the purchase of a 383 420HP stroker. So if I have to pull the engine, there's a crate in my future.

[ROB]

Read this:
http://www.stevesnovasite.com/forums/sh ... hp?t=11689

It will explain why vacuum advance is helpful. Yes, in a "race" car this is not necessary, but in a street car it is. This would help explain some high rpm's at idle, and why it rev's slow (in my opinion). Just give it a try. Its free.

[pcguy]

Read this:
http://www.stevesnovasite.com/forums/sh ... hp?t=11689

It will explain why vacuum advance is helpful. Yes, in a "race" car this is not necessary, but in a street car it is. This would help explain some high rpm's at idle, and why it rev's slow (in my opinion). Just give it a try. Its free.

Thanks for that, read some of Lars' stuf - he's definitely got it together.

Going through the carb right now and it may be the secondaries are not opening. I compressed the spring a bit to see if that has any effect. The Holley kit has about 7 spring selections for this carb. Need to put it all back together and take it out for a spin but need to finish a coolant leak issue first.

More to follow....

[77Astre455]

Have a 75 Vega/350/TH350, purchased a couple of months ago as an undocumented build. The seller sold through a consigned dealer and doesn't want to talk with the buyer and the guy who sold it to him provided a semi-informative/questionalbly-accurate build sheet (have a copy) - for example, the sheet says it has a posi diff, but it doesn't.

The problem is the engine is kind of a conundrum - if it really has the 280 cam in it, as the build sheet suggests, then it may be over the top for this engine setup. The iron block and iron heads (75CC based on casting number) suggest it came from a 70's corvette. The build sheet says it has Wysco domed pistons which should help offset the 75CC's. The problem is that this beast never comes alive at any RPM – here are some facts/symptoms:

- Carb is a Holley 650 with vacuum secondaries.
- Have never proven that the secondaries are functioning properly.
- Idles best at 1000rpm.
- Very lumpy cam exhaust sound – kind of like a fuelly dragster.
- Gets about 8hg vacuum at idle.
- Have set the mechanical/centrifugal advance to 33 and all in at 2700 – have never heard any knocking on 91 octane so it look like there’s
some room to go.
- Has a GM? HEI distributor and Accel 14003 coil and new MSD 8.5mm wires
- The R45TS plugs show they burn a dark brown
- Have adjusted the idle jets to max RPM and the adjusting screws seem to function as opposed to some which appear to have no effect.
- The exhaust shows heavy black soot (same on both sides) – obviously unburned fuel.
- Doesn’t seem to burn any oil – no smoke of any kind at any time.
- Compression is 165-175 all around.
- Has sport tires with 8.5” tread and 23.3” diameter.
- At 90MPH engine rpm is about 3700 – has 2.73 gears which I will be changing to an Eaton posi with 3.73’s soon. Would like to know if
you would look elsewhere first.

Most of all, this engine seems to pull, but not like the high hp to weight ratio setup which it should be. I compare this to my 09 ZR1 and they are worlds apart.

My guess it this setup should be producing 350+ hp and if it was 350+, it should meet my goal of 11’s, but at the moment, it’s never going to make it. Have not run a dyno cause I was hoping to dial it in a bit before then. My main thought is the the cam is over the top???

Just looking for some direction – which direction would you head off in first?

First thing I'd do is check fuel pressure and inspect filter(s). Want 5-7 psi. You might have two filters. One inline and one in carb. I'm assuming it isn't cutting out/ missing in high RPM but it might only be getting barely enough fuel to run clean at RPM but not pull hard. In racing / hotrod applications we usually run an 1/2 inch fuel line from tank up to pump/carb. Sometimes 3/8s will work under 400HP but you need volume. Haven't been inside a Vega tank yet but I'm willing to bet the pickup is small too. Might have filter screen in tank reducing flow also. If your wondering about the secondaries on the carb holding you back (which I doubt) just install a sheet metal screw (or use a tyrap etc) a little past the rod for back barrels on drivers side. Leave enough room for some movement on primaries before back barrels are forced to open. Any car with a large cam will have reduced vacuum but that doesn't mean it will be holding you back. Best thing to do to eliminate a fuel psi question/concern is mount a fuel psi gauge so you can see if fuel psi is dropping at RPM. A 280 dur cam is not that big. Your 8hg vacuum sounds about right with that cam maybe a tad low. Without a stall convertor and steeper gears with bigger cams tend to make it sluggish at low RPM but will pull hard at high RPM. Your intake is ok and your pistons is not the problem. With 150-160 psi compression they are most likely flat-tops and not domes. Your timing is close enough. Ideally you want 36 total all in by 3000 RPM but some engines like more, some like less. In my experience small blocks like a little more and big blocks like a little less but there is no set rule.
RC

[77Astre455]

Not a big fan of vacuum advance if you're not going to use the setup on the streets/highway? What's is the downside to using just the initial and centrifugal? Correct me if I' worng here, but there may be some advance at idle and some while cruising, but when you into, there's no vacuum advance... So what am I missing?

At idle the vacuum is highest, part throttle a little lower, then at full throttle it drops off completely due to fact there is no differential pressure when at WOT. As in the engine and atmos psi is equal. All a vacuum advance does is give you better MPG at part throttle cruising. You can run without one on street tho. Just make sure you around 36 degrees total by 3000 RPM. Some people run vacuum advances on ported vacuum (liitle port on carb metering block) but it should really be run on direct port vacuum. If you want to run vacuum advance and engine isn't stock it will be best to install an adjustable vac advance canister. They engineer/ design stock vac adv canisters according to engine vacuum at idle then the diaphragm starts re-acting as you give it gas. When installing cams etc.. your vacuum drops and stock canisters come in different ranges. Not a one size for all.

RC

[77Astre455]

Distributor vacuum is plugge/disconnected. Heading out to get a camera. If it has domed pistons along with the unidentifiable cam, I'll probably replace the engine with a 383 crate. Any recommendations?

If vacuum advance is disconnected and you have 34 etc by 2700 the previous owner probably set it up that way. That is NP and is most likely not holding you back, except for a little worse MPG on the street. I wouldn't go spring for a 383 till you have that one running correctly. You could have the same issue with any hi-po engine if fuel psi is dropping off at RPM. But on other hand if your expecting to run well into the 11 second ET range your gonna need more than you have now. As in better flowing heads, stall convertor, more gear, bigger better tire, etc.

RC

[77Astre455]

Distributor vacuum is plugge/disconnected. Heading out to get a camera. If it has domed pistons along with the unidentifiable cam, I'll probably replace the engine with a 383 crate. Any recommendations?

If vacuum advance is disconnected and you have 34 etc by 2700 the previous owner probably set it up that way. That is NP and is most likely not holding you back, except for a little worse MPG on the street. I wouldn't go spring for a 383 till you have that one running correctly. You could have the same issue with any hi-po engine if fuel psi is dropping off at RPM. But on other hand if your expecting to run well into the 11 second ET range your gonna need more than you have now. As in better flowing heads, stall convertor, more gear, bigger better tire, etc.

RC

Another thing to check is make sure gas is not trickling inside carb at idle. You shouldn't see any gas at idle. You could possibly have trash in needle and seat for float in front carb bowl/(s). That will make it run rougher and appear to have bigger cam/ or seem too rich on jets. I doubt its jetting on secondaries because that carb you have probably has a rear metering plate instead of rear metering block with jets. (I'll look at pic again) Also check float level with side screw on carb bowls while not running. Should be right under screw hole. If trickling gas your sooty exhaust and brown plugs can be fooling you thinking your too rich. Typically when too rich... at part throttle cruising it will kind spit/sputter/miss(not run clean and smooth). I recommend one day changing carb to a 750 dual feed or a double pumper so you can tune/ change rear jets like the fronts. An 750 dual feed vac sec is cheap and you can add a rear metering block with jets but you will need a double pumper longer dual feed line. You can change vac diaphr. spring for secondaries or you can make it mechanical secondaries by using the small screw on linkage on drivers side. I doubt your engine is too lean in secondaries from what I've heard but that will hold you back too. If in doubt take carb apart and clean and blow out with air especially on secondaries with the metering plate. It will tend to mess up gasket when you pull it apart.

RC