Engine Performance suggestions

[kblancha82]

Hey Guys,

I am looking for some advice/suggestions on what to do about the performance of my 73 Mach 1. It has a 351C - 4V, 4spd toploader and 9" rear with 3.50 gears. The engine has pertronix ignitor/coil, a 600cfm holley, stock exhaust manifolds, 2 1/4" pipes, h-pipe, and flowmaster 50's. I am having acceleration issues, it just doesn't seem to get off the line as fast as I would think it should. I am guessing 75% of my problems are carb related (it runs better when the temp is colder and I get a lot of lag/bog when I put the throttle to the floor) but I am having trouble picking a new carb...

Now for a little background...

When I first got the car about 10 years ago I had a bearing spin on me about 2 months after getting it. I brought it to a mechanic who was going to have the engine rebuilt by a local machine shop. The machine shop ended up being a joke, the heads were rebuilt 3-4 times due to popping lifters. I had to threaten lawsuit to get them to finish the job, so the relationship was pretty sour. When I asked for the specs that they built the motor to I was told they put a "race" cam in it and it should be plenty fast so I shouldn't worry about it but they wouldn't give me any details as to what the cam was, if they modified the block or heads etc.

So now that I am a little bit older and a little bit bolder, I have more experience with engine internals ( far from an expert) I am trying to get some more performance out of the engine, but since I don't have any idea what I have currently I am not sure what to do.

I would rather not go for a complete tear down to measure the bore and heads as the internals seem to be working alright (I know I should, but not looking to do a complete rebuild right now). I am guessing pulling the cam out to see exactly what is in there is a must so I can pick a decent carb to match.

Any advice??

Much appreciated.

Kyle

 

[marks73]

Carb seems awfull small. Borrow a 780 and see if that don't bring it to life. You can measure your cam lift at the springs for a really rough idea.

 

[OpiemusRex]

Carb seems awfull small. Borrow a 780 and see if that don't bring it to life. You can measure your cam lift at the springs for a really rough idea.

I agree, my 306 is sucking the 600 cfm Edelbrock dry right now, my builder is going to try a 750 cfm today. We are working out the final bugs before he will turn it loose to me.

 

[jims72]

Hey Guys,

I am looking for some advice/suggestions on what to do about the performance of my 73 Mach 1. It has a 351C - 4V, 4spd toploader and 9" rear with 3.50 gears. The engine has pertronix ignitor/coil, a 600cfm holley, stock exhaust manifolds, 2 1/4" pipes, h-pipe, and flowmaster 50's. I am having acceleration issues, it just doesn't seem to get off the line as fast as I would think it should. I am guessing 75% of my problems are carb related (it runs better when the temp is colder and I get a lot of lag/bog when I put the throttle to the floor) but I am having trouble picking a new carb...

Now for a little background...

When I first got the car about 10 years ago I had a bearing spin on me about 2 months after getting it. I brought it to a mechanic who was going to have the engine rebuilt by a local machine shop. The machine shop ended up being a joke, the heads were rebuilt 3-4 times due to popping lifters. I had to threaten lawsuit to get them to finish the job, so the relationship was pretty sour. When I asked for the specs that they built the motor to I was told they put a "race" cam in it and it should be plenty fast so I shouldn't worry about it but they wouldn't give me any details as to what the cam was, if they modified the block or heads etc.

So now that I am a little bit older and a little bit bolder, I have more experience with engine internals ( far from an expert) I am trying to get some more performance out of the engine, but since I don't have any idea what I have currently I am not sure what to do.

I would rather not go for a complete tear down to measure the bore and heads as the internals seem to be working alright (I know I should, but not looking to do a complete rebuild right now). I am guessing pulling the cam out to see exactly what is in there is a must so I can pick a decent carb to match.

Any advice??

Much appreciated.

Kyle

Check the accelerator pump on the carb?

 

[goodnigh]

Hey Guys,

I am looking for some advice/suggestions on what to do about the performance of my 73 Mach 1. It has a 351C - 4V, 4spd toploader and 9" rear with 3.50 gears. The engine has pertronix ignitor/coil, a 600cfm holley, stock exhaust manifolds, 2 1/4" pipes, h-pipe, and flowmaster 50's. I am having acceleration issues, it just doesn't seem to get off the line as fast as I would think it should. I am guessing 75% of my problems are carb related (it runs better when the temp is colder and I get a lot of lag/bog when I put the throttle to the floor) but I am having trouble picking a new carb...

Any advice??

Much appreciated.

Kyle

I have the same setup as you in a '71 Grande and use a 770 Holley.

mike

 

[kblancha82]

Thanks for the suggestions guys. I guess I am in the market for a new carb. I knew my carb was in rough shape and had planned to tear it down and rebuild it but I guess it makes more sense to grab a bigger carb.

 

[woodsnake]

Is this a recent change in performance? Lots of ignition issues are mis diagnosed as carburetor problems. The 600 should actually give you very solid bottom end performance. Tough call on pulling the cam out, just to check it out. You could just plan a new cam upgrade, and then get a new carb while you are at it. I like the comp cams kits, as they have everything you need in one box. Timing chain, etc....

 

[Don65Stang]

Welcome.

I think you'll find the solution in a bigger carb. I have almost the same setup but I have 3.25 t-loc gears. I run a Street Avenger 670. Goodnigh (above post) has same setup with 2.75 gears he run a Street Avenger 770. I probably should have a 770 on mine as well.

Get one from Holley Performance on ebay and your smiles will return when you mash the pedal.

 

[c9zx]

The carb is too small and likely mal-adjusted but, it sounds like there may be other problems as well. Even if the carb is not calibrated/adjusted well the torque curve should not be soft at the bottom especially with the 3.50:1 gear. While a larger carb is probably in order eventually, a larger carb will just make the bottom softer until you sort out the other issues. Was it soft at the bottom before it was "rebuilt" several years ago? What was happening too the lifters? What is the idle speed and manifold vacuum at idle? Before spending money I'd do some diagnosis. Check the timing. If it is correct do a compression check and if you can do a leak down check. (Autozone and others have a tool loan program if you don't have the tools) Let us know what you find and we can be of more help. Chuck

 

[Don65Stang]

^^^^^

Listen to Chuck no matter what everyone else says.

 

[marks73]

^^^^^

Listen to Chuck no matter what everyone else says.

HEY! I resemble that remark.

 

[Wolverine]

There is no reason for that car to be soft at either end...provocative remark withheld.

Trying to be a good boy for Santa.

 

[cazsper]

Not a pro when it comes to the Cleveland, but isn't a low bottom end from a 4v Cleveland normal? Everyone I have "spoken to" on this forum tells me the 4v Cleveland doesn't really make any power till at least 3500 RPM. Unless you went with a stroker. Or an aftermarket dual plane intake.

 

[73StangCoupe]

There is no reason for that car to be soft at either end...provocative remark withheld.

Trying to be a good boy for Santa.

Come on Wolverine... no holding back. Let us hear what you have to say. I kind of feel the same way about my car. It doesn't seem to have ooomphf that you would think it should have. I have almost the same set up, only differences are a holley 650, hooker headers and 2.91 gears. What do you think??

 

[Don65Stang]

Not a pro when it comes to the Cleveland, but isn't a low bottom end from a 4v Cleveland normal? Everyone I have "spoken to" on this forum tells me the 4v Cleveland doesn't really make any power till at least 3500 RPM. Unless you went with a stroker. Or an aftermarket dual plane intake.

I dunno, mine seems to scream from about 2000 onward. M-code, 670 street avenger, 3.25 gears, 4 speed.

 

[kblancha82]

On the 3rd attempt for the machine shop to get the heads right I had them taken to a different machine shop to be "checked" prior to being installed on the motor. They found the reason the was because they were using brass valve guides that didn't have the appropriate clearance and they were swelling up and pinching off the valves causing the lifters to crack. Once everything was cleaned up it had plenty of bottom end power. I am guessing the decrease in power has been gradual as I didn't even notice until a few weeks ago I tried to stretch it out at a stoplight at the bottom of an interstate and some guy in a dodge stratus was keeping up with me.

I'll try to check timing, compression and vacuum this weekend and get back with details.


Also, I can't believe its taken me this long to find a forum dedicated to the 71-73! Nice to be among friends rather than listening to everyone talk about how ugly my car is.

 

[caspianwendell]

What ignition system do you have?

 

[will e]

If it seems to have decreased then, yes, your best bet is to check out the basics. Like you said, timing, vacuum, compression. You might just have a weak spark or ? Try to get it more back to normal before you swap out carbs and such.

It's not impossible to figure out your cam without tearing into your engine. You would need a solid lifter and some way to measure it's travel. You can also pull off the water pump and timing cover, timing gears and I think you should be able to see what is stamped onto the cam itself. (someone will correct me if I am wrong).

 

[kblancha82]

What ignition system do you have?

Pertronix Flamethrower coil and Petronix Ignitor, Stock distributor.