Cant drive 55 Literaly

[66CHEV]

Running a 250 with a single barrel rochester carb, points. Did a tune up because of poor performance still having an issue. New distributor cap, points, plugs, vacuum advance. Points are set on 19, timing is set 4 over without advance when advance is plugged back in timing advances to around 12-15 over. Problem is truck will not get above about 45 mph. any thoughts? carb issue maybe??

[KJSR]

Are there any symptoms? Does it run good otherwise? Compression check?

[geezer#99]

You got no timing!
Set your base at 12 to 14.
That'll wake it up!!

[TJ's Chevy]

If it has a hard time starting when it's hot you have it advanced to much. If it's retarded to much.....not really sure what happens! Had mine advanced to far and it wouldn't start when hot. Are you getting any advance period? I had my advance hook up to the wrong side of the carb and when I put it on the right side I noticed a slight increase in performance. How does it idle, rev, restart?

[66CHEV]

not really any other symptoms, starts good both cold and hot, in fact when its hot it will start before making a complete revolution of the engine. compression is between 100 -115 all six cylinders. idle is set at around 600

[66CHEV]

hey geezer 99, not questioning you but just trying to understand. When you say base at around 12 that is with the advance off and plugged correct??. all of the books i have on the truck suggest around 4 before zero for that setting but hey i will give anything a shot right now.

[TJ's Chevy]

Sounds like it's retarded in the timing. Meaning timing is set back a little far, might try advancing it a little(turn the distributor Counter-clockwise and see if that helps.

[Sharps40]

Should pull 0 vacuum advance at idle...i.e. ported vacuum.

Are you hooked up to manifold vacuum instead? (usually a good choice for big cams that don't make much vacuum/don't make strong signal but not so much for stock set ups....) If so, you could be pullin in to much advance way too soon.

If yer on ported vacuum and showing a vacuum draw on the dizzy at idle then yer lower butterfly may be open too far at idle or yer idle too high, again, bringing on advance too soon.

[geezer#99]

Originally Posted by 66CHEV

hey geezer 99, not questioning you but just trying to understand. When you say base at around 12 that is with the advance off and plugged correct??. all of the books i have on the truck suggest around 4 before zero for that setting but hey i will give anything a shot right now.

That's correct. Advance off and plugged.
Factory setting could be 4 degrees.
Pop the dizzy cap off and twist the rotor slightly and make sure the dizzy weights are moving. THey should move out and snap back.
Maybe look into an upgrade of an HEI dizzy in the future too!
And you likely only have manifold vacuum available on your car.

[66CHEV]

I am gonna go out on an limb here and say it is ported vac. there are only two vacuum lines on the motor. One comes out of the top of the intake manifold and that is plugged to a pcv valve in the top of the valve cover. the line that is running to the vac advance is a very small line that is connected to a port at the base of the carb. there is another vac port on the carb located on the upper body that is plugged off. could i possibly be going to the wrong port.

[geezer#99]

Easy to check ported or manifold. Turn the idle down so it's barely running and put a vacuum gauge on any port. If it has vacuum it's manifold. No vacuum it's ported.
Or just listen for a whistle when you pop the cap off.

[Sharps40]

Some carbs have both ported and manifold ports.

Ye'll have to do the checks geezer suggested.

Whether or not the lower port is ported/manifold is gonna depend on how the internal channels in the carb are routed....If the interior opening is above the throttle, its ported, below the throttle its manifold....

what gets confusing is the location of the vacuum hook up on the outside may not be a visual reflection of where the port is actually drawing from on the inside.

[66CHEV]

dont know if this makes a difference or not but the lower port where i have the vac advance hooked up is a type of port that has a small metal line with a threaded end that screws into the port. there is a small screw with a spring directly above the port that adjusts the flow of air. the other one is just a smooth stub that you would slip a rubber vac hose onto.

[Sharps40]

My guess is the lower port is manifold vacuum. The screw next to it would be the idle mix screw. The upper port would be ported vacuum for the dizzy. But, there are lotsa versions of the B and Monojet so, could be wrong....only way to know is to vacuum check it.

Plug any ports not being used since vacuum leaks destroy performance.

[66Submarine]

A little extra timing should make it run a little better, but IIRC a stock tuned truck should be able to exceed 45MPH, lol. Is the throttle opening all the way? If you open the throttle up in neutral does the engine wind up like it should?

[markeb01]

Another consideration depending on the age of the components, occasionally the outer ring on the harmonic balancer can slip making the timing mark(s) completely inaccurate. Choices are replace the balancer, or simply set the timing by ear, rpm, and vacuum readings. If this is the problem, keep putting in more advance until it won't start when it's hot, then back off a couple of degrees.

Also, when the truck won't achieve speed, is the engine racing as if the clutch is slipping?

[66customcamper]

Different motor, different carb, but I had the same issue years ago, truck ran great, just would not go over 45.

Had the carb rebuilt and BINGO, FWIW.

I've had it up to 70 and didn't top out, but I don't ever want to go 70 in that truck again

[Sharps40]

"I've had it up to 70 and didn't top out, but I don't ever want to go 70 in that truck again"

I hear that! A Brick at Terminal Velocity, Oh Wadda Feeling!!!!!

[66CHEV]

right now it runs pretty smooth at idle when the engine is warm, and will even take gas pretty good. when cold if you try to give it gas it will pop and backfire through the carb a little bit till it gets warm. btw how would I time it using a vacuum gauge?

[luvbowties]

Originally Posted by 66CHEV

right now it runs pretty smooth at idle when the engine is warm, and will even take gas pretty good. when cold if you try to give it gas it will pop and backfire through the carb a little bit till it gets warm. btw how would I time it using a vacuum gauge?

Set vac to your advance properly. Then try upping the timing 2 degrees at a time and check how it runs. Keep upping it 'til it either pings(sounds like octane is too low) or 'til it kicks back when trying to start it hot; then back up the last couple of degrees. It should then be close to where it "wants" to run. Check timing at that point & recall where it is set(or mark it with white paint) for future tune-ups.
Sam

[66CHEV]

Ok did take apart and clean on carb nothing wrong there reset timing again verified harmonic balancer has not slipped still same issues

[63burban]

Starving for gas, change the rubber hoses in the fuel line for starters.
Sounds like it's running lean, try boiling out the carb and actually putting a kit in. Does the choke work ? Try the choke when you are running down the road and see if there is any difference.

[Chevy Fleetside]

Also look at the screen on the sender in the tank. It could get clogged also. Just a suggestion.. I am guessing you changed the fuel filter also.

[nomad957]

Long time lurker and I have not introduced myself yet. This has had all of the answers that I ever wanted. Great people also. I felt compelled to chime in on this one. I had the same problem when I first got my truck. I checked everything that has been brought up and still my 230 had no speed, ran great just, no speed and if I was too aggressive under load it would pop. I blamed it on just being a 230. I was replacing my exhaust and I discovered I had over looked the heat riser. The heat riser was rusted stuck in about the 1/3 open position and restricting the exhaust. If you can’t get air out can’t get more air in. Woke it up and ran great.

[66CHEV]

good point nomad and that thought crossed my mind but the heat riser in mine is rusted and stuck all the way open. however, it does make me wonder if the muffler may possibly be slightly clogged.