360, idles great stumbles uptop

[zano]

Howdy, i have a 1976 d100 swap with a 79 360 bored .030 over just swapped from 2 barrel to a 4 barrel intake off of a mid 80's? 360 have a 670 holley street avenger with long tube headers. The issue I'm having is it idles great and runs mostly okay up to about 3000 rpm the start spitting back through the carburetor and it doesnt seem to matter where I set the timing and the vacume gauge seems happy and it starts nice right now. Debating on if I need to rebuild the carburetor not sure if its getting to lean uptop.i got the carb second hand I did set the carb back to factory settings and didn't seem to help much, its pretty clean for the most part inside. So not sure what the plan of action should be next

 

[Oldmanmopar]

Disconnect the vacuum advance and see what happens. If its mopar electronic ign. you may want to switch the box out or check the ground on the box itself,

 

[zano]

I have deleted all the factory mopar it is running on an hei distributor now. Even after switching it runs the exactly same basically

 

[Murray]

Is vacuum advance good? Low float level?

 

[zano]

Float level seemed okay it was just coming out of the sight holes, which is where it says to set it. I can set it to advance a bit more if need be but it was the same for 3 factory distributors /vac advance. My thought is could it be gaskets failing in the carb or something? Maybe letting air in or something

 

[George Jets]

'79 360 flat tappet cam engine.

Is that the original cam and lifters yet?

Original heads... kind of sounds like valve float. Original weak, lost tension valve springs. Cams start to wear rounding the peaks off not fully opening the valves at rpm.

Take a heat temp gun and check header exhaust temp at the head exhaust ports. Should be 400° or so.

If running cooler, probably needs more fuel.


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[zano]

It may or may not be original it was rebuilt at somepoint. That could be a possibility though. The cam definitely has some miles on it but it looked okay when I changed the intake, but could definitely be weak springs or wore lobe, I just find it weird that it always ran really good with the 2 barrel and could go through the entire Rev range. And it only happend after putting the 4 barrel on

 

[George Jets]

Try another known good working 4 barrel (from a friend).

Process of Elimination

Carbs sit around and clog up real easy.


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[zano]

Yeah I have an edelbrock 650 I could throw on for testing definitely worth giving it a shot

 

[George Jets]

Or take your carb and put on a known good running engine and see if that engine acts up.

That's what I use my "Test Run Engine" for.

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[zano]

Can do both

 

[middleagecrisis]

Any update?

 

[zano]

Haven't had a chance to do it yet, picked up a car this weekend to toodle around in. Got some gaskets and a powervalve to pull the carb apart probably do that tomorrow. Needed a small break after all the work on it lol

 

[AJ/FormS]

I just find it weird that it always ran really good with the 2 barrel and could go through the entire Rev range. And it only happend after putting the 4 barrel on

and runs mostly okay up to about 3000 rpm the start spitting back through the carburetor

Here are some of my guesses;

1) maybe it's just a Secondary tip-in bog, cuz the secondaries are coming in way too fast. Try disconnecting the Secondaries and wiring them closed.
) a muffler is severely restricted. or
3) somebody left a rag in the intake, or
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4) check your EGR system. With headers, it should be defeated.
5) I woulda said cam-timing, but you said it ran fine on the old 2bbl. but I would still check it cuz it's so easy if yur not looking for exact cam timing. I would just check to see where split overlap is occurring.
6) if it had started at a lower rpm, like 2000/2500, I woulda said to check yur rotor-phasing as the VA starts dropping out.
7) if you woulda said seems to be related to the throttle-opening at lower revs, I woulda said, check the pick up connections.
8 if you had said the timing light goes crazy as the revs pick up, I woulda said, reverse the pick-up polarity.

Good luck

 

[zano]

Howdy, so had some time today, rebuilt the carb, put a spare power valve in it, and put 68 primary jets with 76 secondary as it has a 65primary and 68 secondary, found some gouges in the metering plate but nothing that should effect it. Ran it again and it ran the same.

6) if it had started at a lower rpm, like 2000/2500, I woulda said to check yur rotor-phasing as the VA starts dropping out.

Might be onto something with that i guess it starts right around 2000 rpm. The egr system is deleted, has virtually new exhaust on it so that shouldnt be an issue. Will be posting a video on YouTube

 

[67Dart273]

If it is rhythmic, might be a cam going flat

 

[George Jets]

Time for a compression check. Burnt Valve or Stuck Valve, or bent push rod not opening valve far enough.

Read the spark plugs, are they all burning the same, or is there one or two odd balls?


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[zano]

so what I'm hearing is time for an ls swap lol, i have nothing invested really into the drive train of this thing, so really on the fence if it would be worth touching it since it is an la engine and price to performance isn't great and I'm not super concerned about purity. its a shame cause it ran good, but at the end of the day is it do i just swap it to something else, could just sell the carb. and headers and get the swap. time to ponder anyways. truck need to come back for next year i miss driving it lol.

 

[middleagecrisis]

What I'm reading is, you need to do a compression check, which is zero cost. This is something you should have done on Sept. 26th or sooner. If you don't have a compression gauge, you can rent one at the local auto part chain stores, however, everyone on this board should own one. There's an old saying, "Don't throw the baby out with the bathwater". If it's got even, strong compression (~150 psi) across all cylinders, then it's a Fuel or air issue and you'll have gotten rid of a good engine under a bad assumption.

 

[zano]

Sorry for not saying before, I had just swapped this engine in and had done a compression check prior to swapping was good then, but you are right it is worth doing it again never know I guess maybe the gauge was wrong. I am leaning towards maybe a bad batch of plugs, they are new but they were part of the list of things I changed before this happens, so maybe one is cracked or something, have about 8 other projects on the go lol

 

[middleagecrisis]

Sorry for not saying before, I had just swapped this engine in and had done a compression check prior to swapping was good then, but you are right it is worth doing it again never know I guess maybe the gauge was wrong. I am leaning towards maybe a bad batch of plugs, they are new but they were part of the list of things I changed before this happens, so maybe one is cracked or something, have about 8 other projects on the go lol

I get it, I see undone projects everywhere I go in my property and my shop. You just do them as you get time and prioritize. I'll bet it ends up being something simple that you'll stumble across. My latest frustrating project is my wife's T-Bucket we bought and trying to figure out how to wire the gauges (not wired up at purchase) that all the factory marks have worn off, you'd be surprised at the number of options on a gauge with four terminals, lol. I probably have 8-10 hours into this project alone. I did get lucky and stumble across these older gauges on ebay that had pics of the back of the gauge with the factory marks on them, SCORE! I'll be trying that tonight. On my Duster I chased the exact same problem (mid-high rpm misfire) and it ended up being a partially burned ballast resistor. I just lucky because I removed it for something else I was doing and saw the coil in the back had burn marks. Replaced and misfire gone!

 

[zano]

I get it, I see undone projects everywhere I go in my property and my shop. You just do them as you get time and prioritize. I'll bet it ends up being something simple that you'll stumble across. My latest frustrating project is my wife's T-Bucket we bought and trying to figure out how to wire the gauges (not wired up at purchase) that all the factory marks have worn off, you'd be surprised at the number of options on a gauge with four terminals, lol. I probably have 8-10 hours into this project alone. I did get lucky and stumble across these older gauges on ebay that had pics of the back of the gauge with the factory marks on them, SCORE! I'll be trying that tonight. On my Duster I chased the exact same problem (mid-high rpm misfire) and it ended up being a partially burned ballast resistor. I just lucky because I removed it for something else I was doing and saw the coil in the back had burn marks. Replaced and misfire gone!

dang thats pretty lucky haha, the time has finally come im going to be pulling the heads tomorrow, just a couple bolts away form having the engine and trans pulled going to do a rebuild on it same time since it leaks like the Exxon Valdez, going to do a full inspection of everything. and have a decent set of 340 j head to put on it, and going to be putting a cam in it but not sure what to go with so will be making another thread likely. and now i have even more of a dilema, do i put this engine in my 4 speed 4x4 truck after all this and debating on doing body swaps. but bare minimum i should be able to find the issue if its mechanical

 

[B3422w5]

Went through similar sounding issue last summer, Idled fine, ran fine until it got up in the rpm’s
Changed everything, ended up being plug wire.

 

[CRUZE 418]

Rehash, it ran fine until he installed the 4-barrel.

 

[zano]

well decided to go get another comp test before pulling the heads left side in order was cyl 2 120psi cyl 4 120psi cyl 6 125psi cyl 8 was 138psi
right side cyl 1 110psi cyl 3 90 psi cyl 5 120psi cyl 7 was 132psi

so something is definetly going on im guessing a valve is hanging or weak spring maybe possibly even burned valve/ valve seat. it weird that it doesnt smoke at all doesnt overheat doesnt burn any coolant going to tear it down more to find out likely and runs fine otherwise