Rebuilt engine lacking power

[MJN]

I rebuilt the original 350 in my '74 K20 and got it running this spring and it runs great but going down the highway at 65/70 mph it seems to need a lot of throttle (at least 1/2 throttle) to stay up to speed and usually need to floor it to get up hills. Even when I floor it to accelerate up a hill there doesn't seem to be any difference, I will lose speed or stay constant speed up hills. So far I've got around 750 miles on it. No mis-fires, exhaust doesn't smell rich, no soot. I haven't pulled plugs or changed fuel filter yet, doesn't seem to be starving for fuel.
It got bored .030" over and I put flat top pistons in it with a Melling MTC-1 cam (Intake Duration at 050 inch Lift: 204
Exhaust Duration at 050 inch Lift: 214
Duration at 050 inch Lift: 204 int./214 exh.
Advertised Intake Duration: 278
Advertised Exhaust Duration: 288
Advertised Duration: 278 int./288 exh.) Otherwise its a stock rebuild, factory intake and exhaust manifolds, 3998993 casting 1.94 heads. I had the quadrajet professionally rebuilt. I have a Mallory HEI distributor and set the timing at 10-degrees BTDC (yes I had the vacuum advance unhooked and all vacuum ports plugged). I've been running 93 octane, no ethanol gas in it. Not sure where to start since it runs so well otherwise. I can light up the tires and it has great pickup and acceleration but seems to just fall on its face going down the road.

The truck itself is a '74 K-20, 4.10 gears, 4-speed manual (SM465) and I have 37" tires (actually measure 35") on it.

[geezer#99]

You’re likely only turning 2400 rpm at 65 mph.
Your motor would like to be happier and run up closer to 3000.
You could try more timing.
Try it at 14 and see if that helps.

You could do an easy rpm test.
Try the same hills at 45-50 mph in 3rd gear. You should be near 3000 rpm.
See how the power feels then.

[cadillac_al]

Single exhaust? It really sounds like lack of fuel either pump or carb. When it is running weak, what happens if you floor it and kick in the secondaries? If it stays flat its probably the pump if it takes off and rips then the power piston probably needs attention.

[Dead Parrot]

Check the RPM your mechanical advance starts coming in and is full in. Might be too late for your running down the road RPM range.

When you stomp it, do you hear the distinctive Q-Jet roar when the secondaries open? If not, possible the re-builder missed something and you only have the front two working.

If still equipped, make sure the exhaust riser valve isn't stuck shut.

[MJN]

Dual exhaust. The pump is a stock replacement AC Delco mechanical pump, brand new. If I'm in second or third and floor it, it will take off and accelerate, no hesitation or delay. I can floor it to accelerate on to the highway no problem. If I'm cruising on the highway doing 65/70 mph I need to stay well over half throttle or more just to maintain speed and if I come to a hill I can floor it and it will not accelerate, just maintains that speed then or slowly loses speed. I haven't contacted the place that rebuilt the carb yet to see if maybe its a carb issue.

[MJN]

No exhaust riser on this one. I haven't changed anything in the distributor, pulled it from the old engine and dropped it in this one, same springs. The old engine had a much lumpier cam and would only pull about 9 inches of vacuum and seemed to go down the road with no problems. This engine pulls 14/15 inches of vacuum. I'll check the secondaries, mechanically they're working from what I can tell working the throttle by hand. I'll see what the rebuilder has to say too.

[geezer#99]

Easy to check secondary air door. Tap it with your finger. It should pop open. If it hardly moves then it needs adjusting.
Warm your truck up completely so the choke is wide open and take a pic of the choke side of the carb for us. Possibly your choke lockout is hanging up.

[cadillac_al]

It still shouldn't have to get into the secondaries to run 65/70. I just wanted to know if the fuel pump could feed it. It's been years since I had my K20 with 4.10's and 35's on a highway but it ran just about the same as my K10 with 308's and 30's. They will roll 70 with the primaries about half open. Hit the secondaries at 70 and they turn on and get to unsafe speeds pretty quick. I have a tach but I forget the rpm's right now. I hate to push it over 3k rpm's for extended periods. I think that's around 70-75.

Oh yeah, both of my trucks have almost the same cam that you are running. I call them copies of the Edelbrock Performer cam.

[akart]

I had the same symptoms on my K20 same specs as yours. I had a plug wire that was pulled out @ the distributor just a bit to cause a non connection. I did that while I was taking off a valve cover to check head #'s. Hardly missed at all, just a little vibe. Ran darn good for a 7 cyl. Fixed it and got my snort back....

[Tom]

Couple things: 93 octane is way too much octane for a stock rebuild, even more so with a bigger cam bleeding off some compression. Go back to 87.
Also, with the cam, did you change the valve springs? Even new stock springs would be better if they fit with the cam but you'd be suprised how easy it is to have valve float robbing power.

Originally Posted by cadillac_al

Oh yeah, both of my trucks have almost the same cam that you are running. I call them copies of the Edelbrock Performer cam.

Edlebrock copied the Melling cam. Its a 40+yr old grind that I can't understand how people keep using.

[MJN]

Here are pictures of the choke linkage, doesn't appear to be any issues there.
The first picture is the engine cold, on high idle. The second picture is completely warmed up, thermostat open. The secondary butterflies are free and theres no drag on them at all. I did a couple throttle snaps and they do open on their own.

[MJN]

Originally Posted by Tom

Couple things: 93 octane is way too much octane for a stock rebuild, even more so with a bigger cam bleeding off some compression. Go back to 87.
Also, with the cam, did you change the valve springs? Even new stock springs would be better if they fit with the cam but you'd be suprised how easy it is to have valve float robbing power.


Edlebrock copied the Melling cam. Its a 40+yr old grind that I can't understand how people keep using.

You don't have to understand, its the cam I chose because it fit my application and how I use it. Does it really matter how old the grind is if it works? I'm not racing it.
No, I did not change valve springs but they have been changed in the past. I'm not running 4000 rpm or even coming close to valve float.

[geezer#99]

Secondary lockout isn’t a problem.
When you say secondary butterflies do you mean the ones at the bottom of thecarb or do you mean the secondary air door?
The secondary air door shouldn’t move at all when you do a throttle snap.

[MJN]

Originally Posted by geezer#99

Secondary lockout isn’t a problem.
When you say secondary butterflies do you mean the ones at the bottom of thecarb or do you mean the secondary air door?
The secondary air door shouldn’t move at all when you do a throttle snap.

The air door, they opened slightly but I opened the throttle far enough to open the secondaries. So, more than just a snap but I wanted to see it for myself.

The carb builder suggested loosening the secondary air valve tension spring. I'll give this a shot and see if they open sooner. Still doesn't make sense if I need to give it full throttle to get up a hill though.

[geezer#99]

Be carefull with that adjustment.
Hopefully you don’t have the spring pop off the tang.
Have you got a number for your carb?
I have a book that tells you the number of turns it should be set at.

Another thought. You could check to see if your metering rods move easily. Motor off look in the vent tube and you should see the rod hanger. Poke a small pencil in and you can push the rods down and feel them pop up.

[MJN]

Carb number is 7044213. I'll check the metering rods. The builder sent decent instructions on adjusting the secondary air valves, just need to give it a try. Thinking about bumping timing to 12 or 14 BTDC and see if that helps too. Thanks for the suggestions!

[geezer#99]

My book says 1 turn on the air door.

Another thought.
Do you have a dedicated 12v power supply to your hei?

[MJN]

Engine harness is same as factory, it was originally points but when I switched to HEI I pulled the distributor wire from a newer harness and installed in mine. Same distributor and wiring as the old engine.

[Dead Parrot]

IIRC, a #8 common box nail works great for checking for movement on the primary rod piston hanger.

Sounds like you made the switch with the new distributor harness but have you verified 12V at the dizzy power terminal while its running? Some older system had a resistor block, others did a resistance wire. The block is easy to spot and remove, the wire can be a bit stealthy.

BTW - motor look great in the pics.

[MJN]

Originally Posted by Dead Parrot

IIRC, a #8 common box nail works great for checking for movement on the primary rod piston hanger.

Sounds like you made the switch with the new distributor harness but have you verified 12V at the dizzy power terminal while its running? Some older system had a resistor block, others did a resistance wire. The block is easy to spot and remove, the wire can be a bit stealthy.

BTW - motor look great in the pics.

When I made the switch I pulled the wire out of the fuse block/firewall connection of a spare harness I had and installed it in my engine harness to replace the resistor wire. No resistor block on this one either. I had done the same on others with no issues. Seems like if spark/ignition was the issue I would experience the engine cutting out or surging, neither of which happen.

Thanks for the compliment, it looked really nice until I buried it under brackets, harnesses and fuel lines.

[Desert1957]

Here is great explanation of ignition timing, It explains "Initial", "Centrifugal" advance and "Total" timing.

https://www.hotrod.com/articles/chev...a-great-crate/

Without knowing the "Total Advance" its impossible to know what you engine is capable of.

[andyh1956]

Have you checked the total timing @ 3000 rpm? Be SURE the mechanical advance in your distributor is working & has the curve your truck wants.
Also be SURE the power enrichment valve is free in the carb & lifts the needles smoothly.
I know you said why you chose that cam, but that is a VERY MILD cam for a big heavy truck running 4.10 gears.
Years ago when they were brand new I bought the Performer intake / cam / chain set for my K5. What a dog. Pulled the cam & went with a Chevy L46 cam & woke that puppy back up! I mean really it ran better before I went thru it with that Edlebrock cam!

[cadillac_al]

Everything sounds perfectly fine to me. At this point i would be double checking compression (valve lash issue), fuel pressure and the fuel filter. I still think it is lack of fuel. Weak fuel pressure will show up in high gear first. I wonder if the builder changed the jets or needles. It is the original carb for the truck right?

[chris989]

andy is right, check your timing at idle, and through the rpm range and make sure it is advancing.


disconnect the vacuum advance for now and plug it until you solve this problem.

what is the CC volume of your heads? what is the static compression?

also, what is the valve separation on this cam. 214 @050 288 advertised is a long ramp. Maybe it just needs a lot more timing than your distributor can deliver? I would make sure you are in the ~35 degree range at 3000.

My lunati is Advertised Duration (Int/Exh): 262/268
Duration @ .050 (Int/Exh): 220/226

[MJN]

So, I did some digging and found out the distributor (Mallory 8548201C) came with an adjustable vacuum advance. The instructions that came with the distributor did not state this or how to set it, however I found a revised version online that stated how to adjust the vacuum advance. I also adjusted the secondary air valve spring tension and bumped timing to 12-degrees BTDC. This helped and feels much more responsive now. The carb is the correct one for the truck, I did have the builder set it up a bit differently than stock though. I went with a level 2 build, https://quadrajetpower.com/our-quadrajet-build-process/ Right now I only know base timing, not total timing.

These are the spec on the cam I'm running.
Basic Operating RPM Range: Idle-4,500
Intake Duration at 050 inch Lift: 204
Exhaust Duration at 050 inch Lift: 214
Duration at 050 inch Lift: 204 int./214 exh.
Advertised Intake Duration: 278
Advertised Exhaust Duration: 288
Advertised Duration: 278 int./288 exh.
Intake Valve Lift with Factory Rocker Arm Ratio: 0.420 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio: 0.433 in.
Valve Lift with Factory Rocker Arm Ratio: 0.420 int./0.433 exh.
Lobe Separation (degrees): 112

The distributor came with different springs, per the chart how do I know which ones to choose?