69 C10 l6 lack of power with 4bbl, offy intake, exh manifolds

[Option13]

Hi Everyone,

I hope someone can help me with my C10. Just recently installed a 4bbl Holley Carb 390, Offy Intake, Exhaust Manifold and it feels like I lost a tremendous amount of power. Aside from the gas pedal being extremely hard (need to figure out a better way to link it) when on the road I need to MASH the gas to accelerate.

My rochester single barrel gave me more power than this current set up. After reading many reviews, people said this should provide more power than the single barrel and smooth everything out.

Have an HEI distributor as well. Truck sounds like its running well...just lack of power.

Any help would be greatly appreciated.

Joe

[geezer#99]

What’s your timing like?

[bill3rail]

What did it run like prior to installing the 4 barrel?

What was the reasoning behind installing a 4 barrel?

Was everything brand new or used?

Should we assume you installed it with new gaskets?

Did you check for intake leaks?

Are all vacuum lines hooked up and all unused vacuum ports plugged?

Bill

['68OrangeSunshine]

Are you getting full WOT?
Has your Holley had a big backfire lately? If the power valve is shot they run poorly.

[firedemon]

how large is the carb, stock cam ?

[Steeveedee]

You have a massive mismatch of components. The four barrel will give you great top end if you have enough cam and compression, etc. That looks like enough carburetor for a 400 cubic engine- too much for a bone stock 6 cylinder. High force on the throttle is just a matter of getting the linkage right, and maybe a lighter return spring.

[Option13]

This was a similar build and highly recommended from 12 bolt and clifford performance who specialize in straight 6s. They recommended the holley 390 since it "wakes" up the engine.

['68OrangeSunshine]

If it's the Holley R8007, 4160 390 CFM, it's good for a 292.
I ran that carb for 25 years on my 2nd 292. When I rebuilt the L6, I used my Holley for the break-in period [500 miles] but after that, it had trouble holding idle. We suspected a warped base plate and it was cheaper to just convert to an Edelbrock 1404, 500 CFM.
I got my money's worth out of the Holley though.
My 2nd engine was bored .030 over, but bone stock otherwise. I was able to run it for 250,000 during 25 years. My 3rd engine was also .030 over, but with a Crane 260H cam and 1.84'' Intake valves. [1.6'' is stock.]
I was able to re use my Offy intake and Clifford headers from the 2nd engine on the 3rd.

[Option13]

What did it run like prior to installing the 4 barrel?
Lot of carb pops with the single barrel before and after having it rebuilt and tuned.

What was the reasoning behind installing a 4 barrel?
12 bolt and clifford performance highly recommend a smaller 4 barrel (holley 390) and say it really helps improved the performance even with all stock internals.

Was everything brand new or used?
Brand new

Should we assume you installed it with new gaskets?
New crush gaskets

Did you check for intake leaks?
Yes, but wouldn't be bad to double check

Are all vacuum lines hooked up and all unused vacuum ports plugged?
As far as I know, but will double check as well

[Option13]

Had a shop tell me its "good." I am unfamiliar with timing so that is why I took it to a shop to have it checked.

['68OrangeSunshine]

So is the accelerator pedal/throttle linkage binding? Looks like a cable at the throttle. Does the cable come from the firewall?
I have a '68 C/10 Stepside with an OEM 292 L6, SM 465, 3.73:1 12 Bolt Corp Rear w/ Eaton posi. I'm currently running an Edelbrock 1404 on an Offy intake and Clifford headers. I now have HEI.
As stated in an above post, I used to have a Holley 390. Originally my '68 cab had a gas pedal that pushed on an arm that pulled up outside the firewall, as you pushed the other side down inside. This was connected to a long link arm and a bellcrank that opened and closed the Monojet. The Offy kit included some more bellcranks, pivot balls and stuff, so we constructed some kind of "Rube Goldberg" affair to operate the Holley's throttle. Never worked well. Would not reach wide-open-throttle [WOT]. And it would fail at the worst times. After 24 years, I replaced it with a '72 style gas pedal inside the cab and a cable-style throttle from the firewall. Much smoother action, no kinking or binding, no possibility of a tiny hairpin vibrating off and losing my carb connection. Impervious to carb icing.

I think I had my Holley oriented 180* to yours, with the throttle arm closer to the cab. I had to saw off the Ford automatic kickdown arm to clear the Offy, but I don't drive fords and it's not an automatic. When I swapped to the Edelbrock AFB clone, the throttle was in the front. No problem with the switch over.
I had also put a 50CC accelerator pump on my 390, but that was not as necessary as a rebuild of the whole engine. It did cure the dreaded Holley bog.
The 4160 Carb family have a Power Valve inside the metering block. Mine was rated at 8.5"" Hg. If the optimum [non-decellerating] manifold vacuum is 17 inches of Mercury, then you want the power valve to actuate at one-half that or 8.5''. When you have your foot into the throttle that deep during the power phase, [accellerating] it opens up increasing the charge. These power valves can also be wiped out by a backfire, rendering them useless.
[Holley says they fixed this problem, but that was after I left their world. Edelbrocks use springs and metering rods and are impervious to backfires.]
If you blew out the power valve, you'll have to remove and replace it. It could also be the wrong-rated power valve.
Since you have about the same set-up as me, I recommend the 8.5" Power Valve:
Holley 125-85, www.Jegs.com p/n:510-125-85 ....$10.24 .
Hope this helps.

[Caddylackn]

You have the HEI connected to a full 12 volts and not the stock resistance wire that drops it to 8 volts? HEI needs a min. of 12 volts.

Also could be a vacuum leak. With engine at idle, try spraying little shots of starting fluid, wd40, or propane around the gasket places on the intake. If idle goes up, then there is where the vacuum leak is.

[Option13]

Thank you for this. Purchased the Holley power valve and now am looking into the rest of it. I am going to have someone step on the throttle so I can see if its binding and check to make sure it's able to hit WOT. Will also look into the 72 style pedal.

I appreciate the feedback!

Originally Posted by '68OrangeSunshine

So is the accelerator pedal/throttle linkage binding? Looks like a cable at the throttle. Does the cable come from the firewall?
I have a '68 C/10 Stepside with an OEM 292 L6, SM 465, 3.73:1 12 Bolt Corp Rear w/ Eaton posi. I'm currently running an Edelbrock 1404 on an Offy intake and Clifford headers. I now have HEI.
As stated in an above post, I used to have a Holley 390. Originally my '68 cab had a gas pedal that pushed on an arm that pulled up outside the firewall, as you pushed the other side down inside. This was connected to a long link arm and a bellcrank that opened and closed the Monojet. The Offy kit included some more bellcranks, pivot balls and stuff, so we constructed some kind of "Rube Goldberg" affair to operate the Holley's throttle. Never worked well. Would not reach wide-open-throttle [WOT]. And it would fail at the worst times. After 24 years, I replaced it with a '72 style gas pedal inside the cab and a cable-style throttle from the firewall. Much smoother action, no kinking or binding, no possibility of a tiny hairpin vibrating off and losing my carb connection. Impervious to carb icing.

I think I had my Holley oriented 180* to yours, with the throttle arm closer to the cab. I had to saw off the Ford automatic kickdown arm to clear the Offy, but I don't drive fords and it's not an automatic. When I swapped to the Edelbrock AFB clone, the throttle was in the front. No problem with the switch over.
I had also put a 50CC accelerator pump on my 390, but that was not as necessary as a rebuild of the whole engine. It did cure the dreaded Holley bog.
The 4160 Carb family have a Power Valve inside the metering block. Mine was rated at 8.5"" Hg. If the optimum [non-decellerating] manifold vacuum is 17 inches of Mercury, then you want the power valve to actuate at one-half that or 8.5''. When you have your foot into the throttle that deep during the power phase, [accellerating] it opens up increasing the charge. These power valves can also be wiped out by a backfire, rendering them useless.
[Holley says they fixed this problem, but that was after I left their world. Edelbrocks use springs and metering rods and are impervious to backfires.]
If you blew out the power valve, you'll have to remove and replace it. It could also be the wrong-rated power valve.
Since you have about the same set-up as me, I recommend the 8.5" Power Valve:
Holley 125-85, www.Jegs.com p/n:510-125-85 ....$10.24 .
Hope this helps.

['68OrangeSunshine]

On the Jegs website, when you hit''description'' one of the options is a Holley film clip on Power Valves, which details a test for checking out the PV's status.
Also, they have put in a new checkball in the carb body that supposedly protects the PV from minor backfires. ''New'' since 1992. I bought my Holley 390 in '78, so I was plagued by bad power valves.
If your original PV checks out OK, it's good insurance to keep a spare in the glovebox -- next to a spare HEI module.
As Caddylackn says you need a full 12 vdc feed to the HEI coil. The old yellow 20 wire that went to the points coil will drop to 7 - 9 volts as it warms up. This keeps point from burning up. Not necessary for HEI.

['68OrangeSunshine]

Originally Posted by Option13

Had a shop tell me its "good." I am unfamiliar with timing so that is why I took it to a shop to have it checked.

Buy a timing light. I hear the Harbor Freight unit is OK. Timing check is easy.
Clip the pickup on plugwire #1, and the power leads on the Pos and Neg terminals on the battery. Run the engine up to operating temp. Set at idle ~750 RPM. Disconnect the vacuum hose from the carb and plug the end with a golf tee. Now point the light at the Harmonic Balancer. It should flash everytime #1 fires. The timing marks will show where the ignition timing is set at. I like 12*. Stock setting was 8*, but gasoline isn't what it was in 1968. Rotating the distributor will avance or retard the timing.
A good Vacuum Gauge should not set you back much more than $20 at your local autoparts store. Tweaking the fuel/air screws, each one in turn for the highest manifold vacuum, will find your best fuel/air mixture.
Investing in tune-up tools will cost a lot less than shop time.

[jfnar]

I rigged up a linkage for my carb without any cables. You can see most of it in the picture. It took some time to get all the friction out of the system. I can get full throttle with this linkage but haven't needed it yet. I did find that I had friction where the plastic foot pedal rubs on the steel throttle lever inside the cab. A little grease helped but did not fix it. A new pedal was even worse than the old one. I rigged up a pedal that I could attach to the lever without attaching it to the floor. It works much better but doesn't look very good. I have more work to do there. The small 4 bbl should work fine on the 6 cyl because the 2 primary barrels are smaller than the single barrel that was on the engine originally. All the references say it's the best carb for the application. Stay with it and you'll get it worked out.

[Caddylackn]

Buy a good timing light, one that shows idle speed and degrees advance. Such as this: Innova 3568 Digital Timing Light. Only $80 on amazon. It lets you know when your distributor timing advance kicks in and if it is working correctly. You need to make sure that your centrifugal advance doesn't kick in until you are at least 200 rpm over idle speed or you will have drive-ability and inaccurate timing issues. With the idle speed display, you can accurately adjust idle mixture, and set fast and slow idle speeds like a pro. You can also measure total advance for setting up custom advance curves for gas mileage or power.

[RichardJ]

I have to question why you guys are running the primary side of the carb inboard. Inline engines typically suffer from unequal fuel distribution from a single center carburetor. Because of the large variation in intake runner length, the end cylinders can run lean and the center cylinders on the rich side.

In the 60s Chrysler played around with various intake manifolds on their slant six. Even with their lowest power engines they placed the carb farther away from the engine in an attempt to reduce the wide variations in intake runner lengths.
For racing they went to extremes with manifolds that had 16"-17" equal length runners. The aftermarket still produces a similar manifold.

Placing the primary bores outboard, doubles the distance from the #3 & #4 cylinders without changing the distance to the end cylinders. This is still a long way from ideal, but it is a change in the right direction.
This should also make it easier to get to the primary mixture screws and the Holley float level window.

You'll be running on the primaries 95% of the time, so distribution when all 4 barrels are open is hardly worth considering with that manifold.

[RichardJ]

Option13, your picture above with the one bbl still shows the points distributor. Did you change to the HEI, the same time you installed the 4bbl?

I would highly recommend you return to the old distributor with the same timing until you get the 4 bbl problem sorted out. The HEI timing curve may not be as suited to your engine as the old points distributor curve.

My greatest fear is that you listened to the internet cowboys that insist that the sky will fall if you don't connect the HEI to Manifold Vacuum.

[Option13]

Well I did not realize to even check that. I can confirm I have the stock yellow wire rigged to it. Will run a new, higher gauge wire and confirm its 12 volts to the distributor. Thank you!

Originally Posted by Caddylackn

You have the HEI connected to a full 12 volts and not the stock resistance wire that drops it to 8 volts? HEI needs a min. of 12 volts.

Also could be a vacuum leak. With engine at idle, try spraying little shots of starting fluid, wd40, or propane around the gasket places on the intake. If idle goes up, then there is where the vacuum leak is.

['68OrangeSunshine]

Originally Posted by Option13

Well I did not realize to even check that. I can confirm I have the stock yellow wire rigged to it. Will run a new, higher gauge wire and confirm its 12 volts to the distributor. Thank you!

I recommend a 12 ga line. Some hotrodders say only a 10 ga wire will do, but I don't think it's necessary. Choose a color not used elsewhere, or one that will be prominent. I have Orange on one truck and Red on another. The feed end is plugged into any open spade on Ign Unfused on the fuse block. It goes out the firewall and can join the rest of the harness on the firewall going over the hump and clip into the metal tabs on the valve cover as it runs to the BATT+ tab on the HEI coil cap.
I have still preserved the original wiring [Orange/purple/white resistance line and the Yellow 20s that split off it] so I could go back to the points distributor if I wanted. I taped off and folded back the OEM points wires. When my Pertronix II burned out, I went back to points for a year, before I sourced a 292-specific HEI.
The Yellow 20 connection on the Starter is no longer necessary.
On my 292, I got a ''low boy'' cap for Model Year '75-'77 HEI w/ Remote Coil for Chevy C/10. I had to make a bracket to hang my coil since the OEM between-the-lifter-cover-bolts-type was not available. [Could not find my filed L6 HEI pix.]
I made sure I got a Vacuum can for that vehicle, which approximated my '68 C/10.

[AussieinNC]

Way too much carb...all a six will ever need is a good 350 cfm 2 barrell....

HEI needs 12 volts to run effectively...points use 7-9 volts ...so you need a rewire as discussed by others...

What engine did the HEI come off? It will need a recurve to make it work with this vintage six.

What timing do you have in it?

['68OrangeSunshine]

Option13: PM sent.
68O/S

['68OrangeSunshine]

Originally Posted by AussieinNC

Way too much carb...all a six will ever need is a good 350 cfm 2 barrell....

HEI needs 12 volts to run effectively...points use 7-9 volts ...so you need a rewire as discussed by others...

What engine did the HEI come off? It will need a recurve to make it work with this vintage six.

What timing do you have in it?

The Clifford website has a carb CFM/Displacement chart. They recommend 390 CFM for a 292. I ran that combo for about 25 years. Now I run a 500 CFM Edelbrock on my 292 L6 [but I built this one with some goodies.]
Jack Clifford was an aerospace trained engineer who saw the untapped potential inherent in the underdeveloped stock inline engines of the big 3. The major automakers were building underperformance into their drivetrains to save an uneducated public from itself.
He proved that with the right components, ''Six equals Eight.''
I've had a pair of Clifford Research headers on a 292 in my truck since 1978.

[BCOWANWHEELS]

Hows compression..............