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Old 05-15-2019, 09:31 AM   #1
Nobighurry
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Can't hold speed on even small hills

Looking for some insight from those that know the 366. I have a 1970 C60 with the 366 (30k miles) and 4+2 465 trans. The truck is rated for 24k GVW and weighs 10.6k empty. I am using it to haul dirt and gravel but can't seem to keep any kind of speed on even the smallest of hills - empty or weighted. I have replaced / updated most everything without going internal - fuel tank and lines, filter, carb (edelbrock 1405), converted to HEI, new plugs and exhaust. I don't have any knocking or any real telltale signs of the issue. I've tried climbing in 4 low, but it is screaming and doesn't like it. It feels like it doesn't have the fuel to needed, but I am no expert. One of the first things I did was replace the worn out Holley for the Edelbrock and everything I read said to run the 600cfm. I'm wondering if it is not large enough to handle it???
It slows drastically with only 6k pounds in the bed and it should be able to handle twice that. Any ideas on what I am missing?
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Old 05-15-2019, 10:36 AM   #2
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Re: Can't hold speed on even small hills

I would start with ignition. Make sure it is advancing correctly. Make sure you have a good strong spark.

600 CFM should be good since it sounds like you aren't reving it too high.

Tuning the carb to your engine is essential for it to run properly.
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Old 05-15-2019, 01:06 PM   #3
C10 - C90 Bill
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Re: Can't hold speed on even small hills

Another possibility is the Exhaust System.

Does it have Duals or is it a Single?

The Muffler(s) could be too restricted or you could possibly have a collapsed Muffler.

I like running Straight-Thru Duals, especially on an engine like that.

I also agree on the Ignition. If a Timing Light doesn't fix the problem, try Timing it by ear. It's also possible that it jumped Time or has an incorrectly installed Timing Gear and Chain.

Also Carburetor, but not if it's doing the exact same thing with both the old and new one. But also could be a Fuel Pressure problem. Also, when you adjust the Carb, adjust it to the Highest Vacuum. Going back and forth with both set screws. And check to see if it has an In-line Gas Filter somewhere.

Keep us posted on it.
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Last edited by C10 - C90 Bill; 05-15-2019 at 01:15 PM.
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Old 05-15-2019, 01:14 PM   #4
Nobighurry
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Re: Can't hold speed on even small hills

New exhaust from manifolds back. Straight duals with flowmaster mufflers. So exhaust isn't an issue.
I haven't checked the timing lately. Might be a start.
Thanks
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Old 05-16-2019, 02:31 PM   #5
Pvmt-Pndr
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Re: Can't hold speed on even small hills

I just want to follow this. I'm having a similar problem with my 427
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Old 05-16-2019, 07:26 PM   #6
Nobighurry
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Re: Can't hold speed on even small hills

I am going to get a tuning kit for my carb. Change to larger jets and see what happens.
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Old 05-17-2019, 07:37 PM   #7
Bimus
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Re: Can't hold speed on even small hills

The 366 is a true truck engine small intake ports they build rpm slowly to build torque even listening to them idle they don't sound like a big block in a pickup .

it's been years the last time I drove one. Running the same loaded as empty really sounds right .
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Old 05-17-2019, 08:13 PM   #8
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Re: Can't hold speed on even small hills

Is this something that has happened over time and gotten worse? The 366 my wife has revs quickly. Granted it's in a truck that only weighs 4500 but it has all the same things that you've listed with the addition of an aftermarket computer.

Did you gap your plugs to. 045" when you went to HEI?

What you've looked up as far as carb size is right in line with what I have found also 600-650 cfm so it maintains the high velocity. Tuning the carb made a noticeable difference on her truck also.

What else I have found is timing should be 6-8 BTDC on the tall decks with the vacuum advanced plugged off on a warm engine but don't quote me on that.

Are you taking advantage of the vacuum advance? Or is it not hooked up?

Check the springs and weights on the distributor. It doesn't take much to throw them out of whack if you're in a hurry on the install.

If you think you don't have enough fuel then a gauge has to be installed to verify.

One thing I plan to do is a leak down test and if none of the other things work I would suggest you do one also.

Just my .02
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Old 05-18-2019, 07:21 PM   #9
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Re: Can't hold speed on even small hills

I agree with Bimus, you will find these rigs do not notice the load like a pickup. The big wheels and torque work together to keep you rolling.
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Old 05-18-2019, 07:29 PM   #10
Nobighurry
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Re: Can't hold speed on even small hills

Didn't get a chance to change jets yet but did run 3 yards of mulch today. Didn't notice it was there but still struggling on the hills. Maybe tomorrow.
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