250 Inline 6 is SLOW

[Matt Man]

First off I know this is a low power engine but this truck is just ridiculously slow now and seems to be getting worse. My 67 GMC C15 has a reman 250 inline 6 with about 7,000 miles on it and is paired with a 3ott and a dana 44 rear end with a 3.54 gear and 26.5 tall tires.

So the issue that I am having is that the truck will barely do 65mph on the highway and the truck has always had a issue with not wanting to rev over 3000rpm. I have verified top dead center and timing, the timing is set at 6 degrees. Spark plugs look good and has been converted to hei ignition. Engine runs smooth and has good throttle response. Another thing is that this truck uses more fuel than my 81 k10 with 35" tires and 4.11 gears! The carburetor has no adjustments on it besides the idle speed.

Like I said, I know this engine is not a powerhouse but I think there should be a little more performance out of this engine. Any tips or ideas?

[geezer#99]

6 degrees initial!
At your elevation you need likely 14-16.

[jayoldschool]

3.08s and taller tires will help top speed. Not acceleration, though.

[geezer#99]

Originally Posted by jayoldschool

3.08s and taller tires will help top speed. Not acceleration, though.

That’ll limit his top speed to 45 mph.
It’ll be a struggle to even get that high.

[jayoldschool]

Originally Posted by geezer#99

That’ll limit his top speed to 45 mph.
It’ll be a struggle to even get that high.

Stock 65 230/threeOTT/3.08/235 75 15 truck will do 65 all day long with no complaints...

[geezer#99]

Originally Posted by jayoldschool

Stock 65 230/threeOTT/3.08/235 75 15 truck will do 65 all day long with no complaints...

Of course you can.
But his won’t.
Due to one difference.
Elevation.
Makes a huge difference when your stock timed motor runs rich. No power.
You need way more timing.

Do a test.
Close your choke 1/2 way and see how your truck responds.

[jocko]

Originally Posted by Matt Man

First off I know this is a low power engine but this truck is just ridiculously slow now and seems to be getting worse. My 67 GMC C15 has a reman 250 inline 6 with about 7,000 miles on it and is paired with a 3ott and a dana 44 rear end with a 3.54 gear and 26.5 tall tires.

So the issue that I am having is that the truck will barely do 65mph on the highway and the truck has always had a issue with not wanting to rev over 3000rpm. I have verified top dead center and timing, the timing is set at 6 degrees. Spark plugs look good and has been converted to hei ignition. Engine runs smooth and has good throttle response. Another thing is that this truck uses more fuel than my 81 k10 with 35" tires and 4.11 gears! The carburetor has no adjustments on it besides the idle speed.

Like I said, I know this engine is not a powerhouse but I think there should be a little more performance out of this engine. Any tips or ideas?

I hate to ask, but are you certain on the rear end type and gearing? Don't believe either is stock for a 67 C1500. 3.54 is usually associated with Dana 60 behind a big block. I'm sure you know what you have, but just wanted to confirm.

Originally Posted by geezer#99

That’ll limit his top speed to 45 mph.
It’ll be a struggle to even get that high.

Do not understand this statement. If it is a 3.54 now and is swapped to a 3.08, highway speeds will increase for a given rpm compared to the 3.54 setup.

[68 P.O.S.]

Originally Posted by geezer#99

6 degrees initial!
At your elevation you need likely 14-16.

Agreed, 6 degrees is way too low

[Matt Man]

Originally Posted by geezer#99

6 degrees initial!
At your elevation you need likely 14-16.

Wow the timing needs to be that high? Isn't the stock timing 4 degrees?

[57taskforce]

Originally Posted by Matt Man

Wow the timing needs to be that high? Isn't the stock timing 4 degrees?

Yea I agree with the rest I’m at about 12* I’m close to 6,000 ft.

[68Gold/white]

Originally Posted by geezer#99

6 degrees initial!
At your elevation you need likely 14-16.

Put it at 14, drive it, see if it pings under a load. If it does not go to 16, see what happens.

[geezer#99]

Yes!
That high.
I run 14 at sea level.
When I was young I grew up at 3500 feet elevation. Learned quickly that the first thing was bump the timing up at least 10 degrees from stock. The early firing like that compensated for the rich mixture from the carb due to elevation. Same fuel with less air makes for richness
Try 14.
Listen for pinging.
Test in high gear at 30 mph. Squeeze the throttle down. If it likes 14 with no pinging and starts good hot then try 16.

[RustyBucket]

Do you see your mechanical advance working... not sure how much total advance a 250 likes. I've had a couple of Chevys with 250's, '69 Nova & '71 Van and they performed adequately. Gas mileage wasn't good though, maybe 'cause I was pushing 'em harder.

[RustyBucket]

Originally Posted by RustyBucket

Do you see your mechanical advance working... not sure how much total advance a 250 likes.

Do you see the timing advance as you rev the engine with the timing light hooked up. Have you twisted the distributor shaft w/ the cap off? If it's not advancing it won't make much power. Probably need 30+ deg. of total timing to run right.

[i82much]

check your plugs. high elevation means you need leaner carb settings. on a holley you would be running smaller main jets and maybe a smaller idle feed restriction.

if you clean your plugs and it runs a lot better at first, you will need to figure out some way to lean out the carb. what kind of carb is it?

[geezer#99]

His motor doesn’t have enough jam to pull it up to 60 now. Swap to 3.08 gears will make it worse.
My old 67 gmc had a 3.54 Dana in it.

[i82much]

Something is wrong with his engine and swapping gears is way overkill. He needs to figure out why his engine isn't making any power.

As for whether taller gears will raise the top speed, that depends. If you've got weak valve springs and the engine will only pull to 3k before giving up, then with taller gears you'll go faster at 3000 RPM. If you've got a bad tune and the engine isn't making enough torque to pull past 3K, swapping gears might allow you to get up to 3500 RPM or so and go a little faster.

Does it give up at 3000 rpm in first or second gear? Or only in third?

[jocko]

Thx Geezer - was that stock for your 67? Good to know.
I agree that with a weak engine to start with - and a swap to 3.08 on top of that - it will take noticeably longer to get to highway speed, but when it does get "there" it will be at a higher speed than it would have been at with the 3.54 at the same rpm.
Do agree that this is in engine issue, not a gearing issue.

[geezer#99]

Sure you will be at a higher speed but you’ll need to keep the pedal down to maintain it.
Just like driving into a 50 mph headwind.
More throttle opening drops the vacuum and wastes more fuel.
My 3.54 I assumed was stock. Forgot to mention it was a 930 Canuck truck.

[weim55]

I don’t usually answer these drivability questions as the information is usually too vague and the possibilities run into 100+ Without being able to put eyes and ears on the truck. You spell some of your situation now so I will throw the main places to over check right off the bat.

Truck has no power above 3000 RPM: You mentioned it had an HEI distributor. A very common problem with these conversions is an inadequate power wire to supply them or The original points system wire is still in place with resistance and is only supplying six volts. Check the voltage of the power wire at the distributor with the key on, it needs full battery voltage. Even a full battery voltage is present it’s not uncommon to still have inadequate voltage under load through the stock wiring and ignition switch. A quick way to check the issue is to make up 10 or 12 gauge wire that runs the short distance between the distributor and the positive battery terminal. Drive the truck and see if it changes the issue.

Truck has no power above 3000 RPM and it is getting worse : exhaust system restriction. I’ve seen two things cause this : The muffler on the truck is old as the hills and has had many a mouse nest or the baffles are rusted out and falling apart causing a restriction. Second it has a garbage Chinese muffler that just simply falls apart on its own causing a restriction. And of course a pipe could be linked or bent. Quick check: Remove the head pipe from the exhaust manifold And bailing wire it up so it won’t drag and test drive the truck for evaluation.

Truck has no power above 3000 RPM at highway speed and it’s getting worse: you mentioned the carburetor only has adjustment for idle speed, what carburetor is on this truck, is it the correct one barrel for the era ? Is it a two barrel Rochester from the 80s, I won’t even get into just how bad that is and it’s a throwaway if your truck has one. Get an original 250 intake and exhaust manifold 67-70 With the correct carburetor and start from there. If it is not original there could be a whole plethora have issues to point out and check out the simplest with the limited information : Are you certain the carburetor opens properly to full throttle? Be certain the choke is completely open at operating temperatur.

I live in a high elevation (5,000-7,000) . Some of the comments mentioned are correct that more initial timing is necessary at higher elevation. I have found most 250s are more sensitive than V8s to more initial timing. 12degrees seems to be the sweet spot and going more noses over with reduced performance and detonation. That aside, because your issue is performance/power above 3000 RPM the important thing to check is their centrifugal advance in the distributor. Check to make sure that it’s functioning properly and smoothly. Set the initial timing with the vacuum advance disconnected.

Digging deeper there still could be a dozen places to throw darts and of course there is a likely hood of a compound problem here I could be a little bit more difficult to diagnose.

Steve weim55 Colorado

[RichardJ]

You are talking about "The Peach build http://67-72chevytrucks.com/vboard/s...d.php?t=733903" in your signature

You bought the truck three years ago and mentioned problems with the title transfer.
A '67 wouldn't originally have the front fender clearance lights or a monojet carb.
The truck looked pretty nice in the pictures when you bought it, yet you've done a considerable amount of work on it since that time. Considering that, you probably should recheck much of the information you were given at the time of purchase.

A compression check to verify the 7k rebuilt engine.
3.54 gears should be verified.
How accurate is the speedometer?
What are you using for a tach in your comments about 3000rpm and is it accurate?

>>carburetor has no adjustments on it besides the idle speed.<< I'm sure that isn't true.
From your build thread, >> tuned the Carb was running VERY rich before<<. What did you do at that time three years ago?

[Matt Man]

Originally Posted by RichardJ

You are talking about "The Peach build http://67-72chevytrucks.com/vboard/s...d.php?t=733903" in your signature

You bought the truck three years ago and mentioned problems with the title transfer.
A '67 wouldn't originally have the front fender clearance lights or a monojet carb.
The truck looked pretty nice in the pictures when you bought it, yet you've done a considerable amount of work on it since that time. Considering that, you probably should recheck much of the information you were given at the time of purchase.

The truck is a 67 just had 68 fenders on it when I got it. I have since installed aftermarket 67 fenders without the marker like it should be. The engine was replaced at a shop with documentation that came from the seller including receipts.

A compression check to verify the 7k rebuilt engine.
Don't have a compression gauge might be able to borrow one.
3.54 gears should be verified.
I counted the ring and pinion when I changed the gear oil and the rpm checks out via some online calculators.
How accurate is the speedometer?
Checked with my phone, the speedo reads about 3 mph slow.
What are you using for a tach in your comments about 3000rpm and is it accurate?
I am using a sunpro parts store tach and my timing light reads the rpm and is pretty accurate.

>>carburetor has no adjustments on it besides the idle speed.<< I'm sure that isn't true.
I cannot find any adjustments on the carb besides the idle speed?
From your build thread, >> tuned the Carb was running VERY rich before<<. What did you do at that time three years ago?
The choke was stuck making the engine run very rich.

I increased the timing to 14 degrees and it made no difference in the upper rpm still has a hard time at 3000+ rpm. I am going to go check out the vacuum advance.

Here is a picture of the engine slightly:

[Steeveedee]

Have you verified that you have wide open throttle when the gas pedal is on the floor?

[RichardJ]

Great response. Thank you.

I'm not a fan of the Monojet, mainly because there are about 2 thousand versions of the same carb.

1st image shows idle mixture screw on bottom front, below float bowl, next to that plug.

2nd shows small slotted Idle Speed screw.

3rd shows idle solenoid in place of idle speed screw. Turn to adjust. Screw on left of image is High Speed Idle screw.

The Monojet was first used on the inline Chevy in '68. The previous Rochester B on the Chevy six, remained pretty much unchanged from about 1950 until 1967

[geezer#99]

If extra timing didn’t help then a couple other things could be happening.
Your valves may be set wrong and they’re being held open a little.
Or your cam is installed advanced from stock which limits higher rpm.