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Can't hold 70mph

Chevman88

Chevman88

I don't know what I'm doing
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Now I know that I'm not going to be setting any land speed records, but I wouldn't have thought I would have a hard time getting to and holding 70mph, even with the tires and gearing I'm running. This is my first carb vehicle, so tuning is going to be a learning curve, but I'm trying to gauge how much time and money I want to spend on this motor when a 6.0 LS+4l80e swap is in the future. I haven't done a compression test yet, that's on the books to be done.

This will also help me gauge on how soon the swap will be occurring.

Basically I just want to know if 70mph is unreasonable with my current setup.
 
I can’t help you with your current set up. I can say that with a 88 burb weighing in at 7K lbs, I can hold 85 all day with the 6.0 and LQ4. That’s with 35s and 4.56s.
 
Is your speedometer right? You've verified via GPS?

I'm running 350, 33's, 3.42's and a 465, running the numbers you are turning ~150RPM more than I am at 70MPH....I have no problem holding 70 and then some (a lot), at least unloaded.

I don't buy into the whole big tire thing as a large factor in drag, your frontal area increase is about 4" wider and 4" taller based on exposed tire (both) over my setup, and with 8" under the differentials, I doubt I'm getting any sort of road hugging aerodynamic advantage you aren't lol.
 
Do you have a 250 straight six ?..:crazy:...
Even one of those should maintain 70 mph,but getting it there may take a few miles..unless your climbing steep mountain grades or at high altitude ,it should be able to go that fast --though I'd consider 70+ mph a bit too fast for a lifted square body with big tires,they can be unstable at that speed,especially if you had to make a sudden swerve to avoid something..

I'd start by checking the timing,it may be retarded,then make sure all cylinders are firing,and it has decent compression...a party clogged fuel filter or air filter can reduce power a lot,as can a fuel pump thats not making enough pressure and volume..
Then things like dragging brakes,wheel alignment way off,can also make the engine work so hard it cant get up to the speed you want..
So can high winds coming towards the vehicle..
 
When carbed with 4.56s, 35' tires and 700R4 had no trouble cruising 75-80
on the freeway all day long
 
Hot damn I love this forum, just posted this before I left work and came home to all these responses.

My speed was gps verified, in fact my backlight doesn't work on the speedo (need to replace bulb), so the only way for me to tell how fast I was going was gps.

My altitude is about 5k feet, crossing the mile marker is common.

I've got a timing light and compression gauge, so I'll check those today.

I've got an Accel HEI super coil on my distributor with vacuum advance, that's supposed to be hooked up to port vacuum, not manifold right?

About how much advance should this be running at idle?
 
I'd start with 12* advance at idle and see what the advance is at 3000 rpm. If you can adjust the curve great, if not then set the idle and roll on. :)
 
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Chevman88 said:
I've got an Accel HEI super coil on my distributor with vacuum advance, that's supposed to be hooked up to port vacuum, not manifold right?

About how much advance should this be running at idle?
Click to expand...

Those things are a convoluted mess of vacuum lines, thermal vacuum switches, and so on. If those are all stripped off, running the distributor off of manifold vacuum is what you do for performance. Ported vacuum is for emissions control.

How much should it be running vs. what it will like at idle? Injection runs the SBC at around 20* at idle. You are stuck with around 8* mechanical at idle IIRC. If you need to adjust it, you can always crank if up to 20* and see how good that is...even if you can't keep it there, because mechanical advance that high will have your starter fighting ignition. I always wanted to try that, if it is possible to find a 12* can, and see if it is possible to have high/good idle timing with a carb.
 
Carbed at this altitude I set my base at 12° like @K85 Octane said
Mid to upper 30s full in by 3k

There's no reason it shouldn't run that speed if its tuned right unless its got some major compression or cam lobe issues
 
Right now it's at 19° advance at idle with manifold vacuum advance. 8° without vacuum advance. Turn it down?
 
I’d probably do that. Magic on a carb engine has always been 2* +2* for every 1000’ altitude

36-38 has been a nice number full timing for me

Flat lobes will do the same, if you get to the point of checking compression and chasing it with a vac gauge
 
Ok, got my compression and vacuum gauge out, will have to check that tomorrow. Would I be better off disconnecting the vacuum advance? I've only ever timed fuel injected/tbi engines and never had that.

Also, what's the best way to check total timing in this thing?

Adding to this, smells like it's running pig rich. Going to check spark plugs when I do my compression test and likely do a tune up.
 
Stock 350 in 86 had basically 165hp at sea level, at 5000ft you have roughly 140, now allow for high mileage and wear, looking at closer to 130ish. Add 4.10's and 37" tires to the equation, holding 70mph? not likely.

Hurry up and retire that poor motor and install the 6.0.






Edit: could always add a turbo to your existing 350, would be entertaining until it blows. :thinking:
 
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While 4.10 is poor gearing for a 37" tire, it's still better than the factory 29"/3.08 combination. If properly running, it should hold highway speed no problem. Actually, it should do it in OD on flat ground, so with a TH400, something isn't right. If you're seriously WOT on the highway, it's normal to be a bit rich, but not black smoke rich. Assume the engine is good and go over spark and fuel carefully first:
-Is the choke actually opening?
-Does the gas pedal actually open the throttle all the way?
-Do the carb secondaries actually open once the choke is off? (I mean the throttle blades on the bottom, not the top flaps)
-Triple-sure the plug wires are in the right order?
-Do the mechanical advance weights actually move or are they stuck?
-Have you verified the timing mark on the balancer is actually at TDC #1?
-Yellow coils belong in the trash

Is there any surging or shaking under load or just general lack of power?
 
It sounds like timing is the culprit. Turn it down to 7 and road test it to check for improvement..........Adjust from there.

FWIW, this is coming from the far reaches of the memory bank.
 
Am I better off unplugging the vacuum advance and then timing?
 
Chevman88 said:
Right now it's at 19° advance at idle with manifold vacuum advance. 8° without vacuum advance. Turn it down?
Click to expand...

Not clear if you understand (not trying to be condescending), but that is the point of vacuum advance...it's not there on startup (the engine isn't running yet) and it's a smaller factor to overall timing under throttle because vacuum drops instantly when the throttle is cracked open. Best idle timing is much higher than most emission controlled carb setups ran, but that is due to emissions, not because less timing at idle is better. And yes, base idle is set with advance disconnected. If it's not pinging and starts ok, your base timing is probably ok, but you can always play with it, because base timing affects total timing.

Unfortunately with a carb (just because laptop tuning is a lot easier) tuning to get everything right, under all conditions, is a nightmare. Tuning for specific conditions (WOT or cruise) is fairly easy, but it's why the Q-jet still seems to reign supreme for carbed drivability, and when tuned right, are arguably the best all around carb there is...BECAUSE there is so much adjustment. The complexity that brings (read one of the numerous Q-jet books out there to see for yourself to get an idea) is nearly mind boggling.

I would use this time to bump the swap up to number one priority. :)
 
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