I have a 1970 with a NOM 350, M-20 4spd, and 3.08 rear. The engine has Edelbrock aluminum heads, a Torker intake manifold, and a Holley double pumper carburetor. I do not know what the cam is. I have set the timing and advance per Lars papers. The engine starts easily and idles ok, but it idles rough, like it has a wicked cam in it. The idle manifold vacuum is 9-10" hg. The idle mixture screws act as they should, and I can easily adjust the idle mixture and idle speed.

If I push the rpm's up to about 1500, it runs rough, and if you drive it at this rpm, it jumps and jerks around, and is not fun to drive at all. If you run the rpm up above 2500, it seems to smooth out and wants to go like hell!!!!

My friend suggests that this behavior is because of the single plane intake manifold and what ever cam the engine has in it. I have looked at the Torker intake manifold specs., and it is certainly a higher rpm manifold.

Having been through the entire ignition system, I am confident it is not at fault. The carburetor seems fine, although I have not rebuilt it. So, I am guessing that this engine was built for high rpm, 1/4 mile use, and not for a grocery getter. As it stands right now, it is not very street friendly.

I am an older guy with a lot of experience with stock engines, but I am not sure what to expect out of this built up engine combo. My friend suggests I switch to a dual plane intake manifold as a first step and see how much that improves things. Any insight or suggestions you guys might have would sure be welcome. I want to make this car more street friendly.

 

Runs rough like a miss? From your description (If you run the rpm up above 2500, it seems to smooth out and wants to go) I'd guess the cam is getting into the power range it's designed for. I have a 350 = single plane intake, Holley, and GM's 3927140 cam and it smooths out around 3000 RPM. Below that, it's a rough running PITA.

 

I have a polished Power Plus dual plane manifold that will fit your car and will ship it to you for 85 bucks if you want to try it.

 

I would not say rough like a miss. At idle, it sounds very powerful, but has a lot of lope to it. I am certain it is not missing. I am pretty sure your are correct, that above 2500, the cam/intake combo is really starting to work as it should. At lower RPM's, it chuggles and bucks, like it is way too lean or the timing is way to advanced, but I am certain they are correct. It makes it a pita to drive as you say.

 

Thanks. Like I said, I don't have a lot of experience with after market performance manifolds, etc. I am thinking of going with an Edelbrock 2101 as a first try.

 

Jegs is listing them now for $129 bucks with all the stuff to install them - very cheap in my opinion!
http://www.jegs.com/i/Edelbrock/350/2101/10002/-1

 

Thanks for the link. I had actually looked at Jegs a couple of nights ago. There are a lot of used ones on Ebay and CL, but I would prefer one that has not been ported or messed with in any way.

 

I wonder if the engine was built for higher rpms and by installing a low rpm intake, you'll have just a bunch of mismatched parts that won't work well together Also single plane intakes work well with 4/5/6 speed sticks with the shorter runners, dual planes like the performer you're considering has longer runners to increase torque which is beneficial with automatics.

 

I am concerned about this, that is one reason I posted here to get some input from those with more experience in this area.

 

What power valve are you running? I would think a change from 3.08 to 3.55's would make it more drivable.

Dave.

 

Dave, I don't know what power valve is in there, but the idea of checking it had already crossed my mind. I agree, a rear ratio change would make it more driveable, but I want to stay with the 3.08. I like the M-20/3.08 combination.

 

Well, you know where the timing is set now. Just loosen the dist. clamp a bit, start the car (after already warming it), then slightly turn the dist. one direction then the other to see if the idle improves [or not]. I suspect that the cam needs different timing at idle to smooth it out some. If that doesn't help it, you either have too radical a setup for everyday driving, some valve problems, or some bad plugs/wires/ignition.

 

New plugs, wires, etc. It has a Mallory electronic ignition module in place of the points. The higher I run the timing, the smoother the idle, but I set it all by Lars papers, and don't want to go too advanced. You may be right on, that the setup is a bit too radical for the street. I was thinking someone else might chime in with a similar setup.

 

Lars advocates having all mechanical "in" by 3000 rpm...then leaving the idle timing where it falls. In your situation, since idle quality is better at higher timing, I would think that you need to set the timing at idle first, then work to get the full mechanical set as Lars recommends. By the way, is your vacuum advance can working? ...or is it a "low-vacuum" advance can? At your low vacuum level, you may not be getting any advance at idle with the can you have.

 

I don't know if my setup is necessarily like yours but I can give real experience regarding ignition timing. I have read Lars papers and it is correct and great info, however radical cams spin to the beat of their own drum. Sometimes you have to listen to the engine and do what she wants not what she should have according to paper knowledge.

My base timing is 26 degrees with a total of 42 degrees. I know, sounds all wrong and shouldn't run but she purrs like a kitten.

If I try to follow the book and time it at 12-14 base it barely idles and shakes terribly. Throttling the carb at idle is sluggish with this timing. Bump it up to 26 and the throttle repsonse is crisp the way it should be.

My distributor is an MSD Pro billet, no vac adv and I did change to springs to bring the timing in a little earlier. Compression is 9.5:1 cam is not that radical but the lobe center is being 107. Sounds like you may have a similar rough idle cam that really does want more timing. Running 93 octane in mine produces no detonation or pinging and starting is no problem hot or cold. The trans is a TH350 with a stock converter and 3.08 rear gears.

As was suggested earlier it would be worth a try to advance your base timing and just try it, worst comes to worse you can put it back where it was.

Should you end up replacing the intake manifold with a performer I would suggest buying the EPS (#2701) since it is set up for a square bore carb like your double pumper. You could also use the Performer RPM intake (#7101) it does fit under a stock hood with the right air cleaner. I have the Performer RPM Air Gap with a drop base 14" air cleaner and a 3" K&N element, it's close but the hood closes.

 

Right now, my initial timing is about 18 degrees, with all the mechanical advance in by 3000 rpm. The vacuum advance can is the one thing I have not changed. It works, but it is a unit that is not fully in at the low idle vacuum I have, so I am not getting the beneifet of full vacuum advance at idle. I need to switch to another vacuum can no question.

 

I agree with all you say. I will advance the initial timing a bit more and see how it behaves. As long as it does not ping or kick back when starting, it should not hurt anything. I have looked at the EPS 2701, but was going to go with the 2101 because it will accept a square bore or a spread bore. The car has a Holley square bore on it now, but I like a Quadrajet, and thought I may go that route at some point.

 

A Torker is kind of the worst of both worlds; a low-rise single plane. I agree you can probably make some timing adjustments, but ultimately the Torker is going to have to go.

If you're willing to invest some time you can get something approaching a cam card for the cam by measuring the lift and then degreeing it in the car...it would tell you approximately what you have, and it would be apart enough at that point for a cam swap

 

double pumper holley is probably not helping you any either

 

I figured kinda one thing at a time