I have a rebuilt 408 that has an edelbrock performer intake and a 670 cfm holley truck avenger carb. It wasn't running that great but i adjusted the acc. pump and now it runs really good. The only problem is it kind of sputters at higher rpms, usualy right before i need to shift. What would make it sputter only at high RPMs? it isn't even too terrible but it is noticible and i know it shouldn't be like that.

Thanks,
TheBlueMule

 

Before spending any money, try running it without the air filter housing on. If the sputter goes away, you have too small a filter housing, and it is restricting your airflow at the higher RPM.

I was going ape sh&t trying to tune a carb last week which was sputtering at high RPM, took the air cleaner housing off, and it ran PERFECT. Put on a bigger housing and filter and it still runs perfect.

 

Well the aircleaner didn't make a difference. Do you think it could be an adjustment on the acc. pump on my holley? I will try and take a look at that tomorrow.

Thanks,
TheBlueMule

 

What rpm are we talking about here? What ignition are you using and do you know the total advance?

 

Well I don't think this post will be much help but its all I can tell you.

I put a tach on my truck but it seems to be wayyyy off. Cause it says my truck will rev to 8k RPMs. But on this tach it happens around 5500-6000+ RPMs when it sputters.

The ignition is all stock. stock distributor, wires (new but stock), and ignition box.

And I do not know the total advance.


I dont know if you can get much out of that but if you can help me I would greatly appreciate it.

Thanks guys,
TheBlueMule

 

Well it sounds like the tach is off. You might float the valves at 6,000, most 400s wont turn that fast. Is this like a wide open run?

 

It does it whenever you drive it at higher RPMs ( 5500+ but i think my tach is off), however, it seems if you floor it or drive it harder, its not as bad of a sputter. It seems to happen at even higher RPMs compared to when it happens when you are just driving regularly and proper.

Thanks,
TheBlueMule

 

The 400 has such a massive crankshaft and long stroke, all you are accomplishing at such an engine speed is shortening your engine life. You are not gaining anything from running such high RPM unless the engine is HIGHLY modified. The torque is falling off so far at that speed that you will accelerate harder by shifting at a lower RPM.

That said, if you insist on doing this, besides recommending that you wear kevlar, I would start by checking the ignition curve. There also could be several things causing it to lean out at such high revs.

What do the plugs look like?

 

I am not driving the truck like an *** all the time. I was just trying to see if the sputter went away at all after giving it a little more gas.

The engine was a 351m that I bored .030 over and installed a new 400 crank and 400+.030 pistons from Tim Meyer. It also has a hyd roller cam, hyd rocker arms and lifters, dual timing chain, new valves springs and retainers, HV oil pump, headers, edelbrock intake, and a 670 CFM holley truck avenger carb.

I realize the torque falls off at higher RPMs but its not like I am horsing it when it sputters. It only occurs about when I need to shift.

I havent had time between work, school, and football to realy work on it yet. I am hoping to check the plugs tonight.

Thanks guys
TheBlueMule

 

his power drops at 5500 rpm, he has enogh mods to safetly rev his engine till there, i remember i did a dyno sim of his built, so it shouldnt sputter till 6k when your valves float, it could be a lean condition or a bent dizzy shaft as utfball said...

 

You might check your tach and see if it has a selector switch for number of cylinders. High RPM miss is ofter in the ignition. I'd also use a timing light to check initial timing, vacuum timing (pull the line off and note the timing drop), and watch the timing tab while reving to see if it starts jumping around.

 

this may sound stupid but how old is your battery and alternator?

we couldn't figure out a miss on some racecars and we tried everything in the carburetor, ignition system and could not figure it out. turned out we were a couple of volts low and it just couldn't keep up to the rpm's.

 

Well the truck fires up imediatly every time i start it. However, the battery looks old school. Its fairly clean, but it looks like it could be the battery that came in the truck. I had some starter problems though because my starter got too hot from the headers and they cooked it. The volt meter doesn't work though so I can't see the volts at high RPMs.

I don't know how the distributor could have been moved but I am going to put a timing lite on it on Sunday and if that makes no difference, i will look into the battery/alt. It might be that because like I said, I have had a few issues with starters and shorting out when I go to start it. I will take a look.

Thanks very much guys,
TheBlueMule

 

I would look in two directions;

Leaning out, judgement call only way to be sure is to install bigger jets.

To much advance, it sounds like when this happens both the mechanical and vacuum advance are doing there thing, pull the vacuum line off the carb see if it gets better.

 

I haven't had much time at all lately to really look into the problem. Work and school have been kicking my but. I am planning on moving to an aftermarket distributor, ignition box, and coil. I also want to change my carb to bigger jets. I don't mean to sound stupid but if I want to go one size bigger jets, what size do I need? I tried to find the jet sizes for a 670 CFM Truck Avenger Holley, but I cant seem to find what they are. So, if someone could tell me what size and where to get them I would greatly appreciate it.

Thankyou guys very very much. You guys are awesome.
TheBlueMule