Cutting out at higher RPMs after big cam swap

[ZMAR]

I was stuck on putting a .515 lift cam in my 350. I put the bigger springs that Lunati suggested but when it gets to higher RPMs at the top of every gear it starts really cutting out. Not a huge motor head but kinda know my way around a little. I also just have center dump ram horns without any exhaust on right now. Could it be lack of back pressure? Also 600 cfm edelbrock could be lack of fuel i dont know. Any help i would really appreciate

[jborders]

does it ever have a loud pop when it cuts out? usually does if it needs more fuel. Also could if your rpm range is a little higher now could be ignition related.

[ZMAR]

Yes it does pop. It's so loud with open manifolds though it's kinda hard to tell the difference. It has a hei billet MSD but just a delco cap, regular wires but high current plugs
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[ZMAR]

The motor was out of a street stock dirt track car so he said he couldn't put ALL the goodies on it. It ran like a champ until I had to go put a big a$$ cam in it!
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[jborders]

its probably a lean pop. Not good. could be a misfire also but i would try a bigger carb or bigger jets for the one you have.

[ZMAR]

Originally Posted by jborders

its probably a lean pop. Not good. could be a misfire also but i would try a bigger carb or bigger jets for the one you have.

Ok I'll see what I can do about it. Thanks
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[geezer#99]

WEak springs or a flat cam will make your motor do that!

[GASoline71]

A cam is also where bigger is not always better. If you have a stock converter and that small carb... it's gonna be a mess driving on the street. What manifold do you have?

Gary

[ZMAR]

Originally Posted by geezer#99

WEak springs or a flat cam will make your motor do that!

Cam and springs are brand new tho
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[ZMAR]

Originally Posted by GASoline71

A cam is also where bigger is not always better. If you have a stock converter and that small carb... it's gonna be a mess driving on the street. What manifold do you have?

Gary

Just old ram horns. My dad told me to leave it alone but I was the one bein hard headed. I know it's probably to big but I'm sure I can get it to running good. Just looking for a direction to start looking
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[prairewolf]

only trying to be helpful but a firebird I had had a similar problem ... it ran fine to 90 Mph or so and then would stutter,stumble etc. ... ended up being a pinhole leak in gas line. ...on that car there was a short piece of hose half way up frame connectiing front and back lines..... such a small hole I do not even think it leaked on ground but would suck air at higher rpms.

[ZMAR]

Originally Posted by prairewolf

only trying to be helpful but a firebird I had had a similar problem ... it ran fine to 90 Mph or so and then would stutter,stumble etc. ... ended up being a pinhole leak in gas line. ...on that car there was a short piece of hose half way up frame connectiing front and back lines..... such a small hole I do not even think it leaked on ground but would suck air at higher rpms.

Come to think of it I remember the mechanical fuel pump was leaking when I first put the motor in. It quit leaking and I never thought twice about it. I'll put a new one on and see what happens. Thank you
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[Zeke's Garage]

Might be worthwhile to put a fuel pressure gauge on it as well, just to make sure you're not losing pressure at higher flow. Either way it does sound like it's leaning out, or you're not getting any distributor advance.

Is this just while you're revving it in the driveway, or is it under load while driving? You mentioned it just has open manifolds, so It's hard to be sure.


Shawn

[geezer#99]

Did you break the cam in with the new springs or did you use some old break in weaker springs?
If you have a lot of seat pressure with strong springs you can wipe a lobe real quick!

[Sillyoldman]

Take your distributor to a machine shop and get it curved.

[ZMAR]

Originally Posted by Zeke's Garage

Might be worthwhile to put a fuel pressure gauge on it as well, just to make sure you're not losing pressure at higher flow. Either way it does sound like it's leaning out, or you're not getting any distributor advance.

Is this just while you're revving it in the driveway, or is it under load while driving? You mentioned it just has open manifolds, so It's hard to be sure.


Shawn

It sounds great in the driveway idles great and everything. It's only under a load. I will get exhaust on this weekend and see what happens.
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[ZMAR]

Originally Posted by geezer#99

Did you break the cam in with the new springs or did you use some old break in weaker springs?
If you have a lot of seat pressure with strong springs you can wipe a lobe real quick!

I left the old springs in for almost 2 weeks driving it almost everyday. I put the new springs in because I thought it was floating the valves. Didn't change a damn thing
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[msgdsrf]

I'd try changing secondary jets. Not hard to do and inexpensive.

[geezer#99]

What method did you use to adjust your valves?

[s_nova69]

I agree to look at fuel supply first. Make sure the secondaries can fully open. Fatten up the secondaries w larger jets. If the pop happens at the very top range ie above 4500 it is trying to tell u fuel. The spark curve is probably working correctly but it would be good to check. Borrow or buy an advance light and set it to the total advance amount you think it should have and while shinning the light turn up the rpm to above 3500 ish and see if it is at zero. Then stArt turning it down until it moves above zero. It should have around 32-37 degrees total. This will help u figure out if it has enough advance.

Exhaust is really an essential piece and should be on before trying to diagnose a tuning problem

Good luck
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[GASoline71]

Originally Posted by ZMAR

Just old ram horns. My dad told me to leave it alone but I was the one bein hard headed. I know it's probably to big but I'm sure I can get it to running good. Just looking for a direction to start looking
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What intake manifold?

Gary

[davepl]

It sounds like valve float to me, but if you had it with the weak springs and it didn't change with the stronger springs, it's likely not that.

Did you check coil bind at max lift? I had that once and destroyed the cam (wiped a lobe) and it backfired like that, but I do not recall if it was only at higher RPM... I actually don't think so.

If its fuel, I would expect it to do it only under full load. If you can get it up to the upper RPM range under partial load, and it still does it (or doesn't) then you'd be a step closer to knowing whether or not it was a lean issue.

[ZMAR]

Originally Posted by GASoline71

What intake manifold?

Gary

Oh im sorry its a edelbrock performer

[GASoline71]

your intake is for off idle through about 5,500 RPM's. A big cam is gonna like more upper RPM and really start making power close to 3,000 to 4,000 RPM's and peak around 6,000.

You really should step up to a set of headers as well.

When you swap to a big cam... it affects a lot of things. Do you have a stall converter?

Gary

[jborders]

you can have a lean issue anywhere from off idle to only 5500 rpm and up. Just depends on how bad the restriction is. I was getting a lean pop once every time i slightly stepped on the gas with a slight load. wouldn't do it if it was just revved up. it was a very clogged filter. then another time with fuel injection it only happened above 5500 rpms. that time it was a weak fuel pump. it could be valve float but i doubt it with new springs. These old trucks are bad about getting big chunks of stuff in the lines if your running the stock tank still. Keep that in mind.