Cant drive 55 Literaly

[66CHEV]

dont know if this will trigger any new ideas from anyone or not but the fuel mileage has dropped from 15-16 to around 8. does have sort of a fuel smell when you shut the engine off. does this mean i am pumping more fuel than the engine is burning and maybe flooding the thing or what?

[66Submarine]

The float might be stuck, or the fuel pump might be building too much pressure or something. You don't have any way to take a video, do you?

[luvbowties]

Originally Posted by 66CHEV

dont know if this will trigger any new ideas from anyone or not but the fuel mileage has dropped from 15-16 to around 8. does have sort of a fuel smell when you shut the engine off. does this mean i am pumping more fuel than the engine is burning and maybe flooding the thing or what?

BINGO on the drop in gas mileage! There IS such a thing as TOO LITTLE back-pressure on an engine's exhaust system. A heat riser stuck open all the time is not good. That will really affect the performance of an engine, esp. a 6-cylinder, giving too little back press. plus too little heat to intake. (Read about guys installing headers, on a 6, without any connection to the intake. This results in too little heat to intake/carb, and manifests itself much greater when cold than after it FINALLY warms up.) Your engine will not reach operating temp. as soon as it was designed to; neither will the carb reach cruising mode soon enough; this means the carb will run in the much richer, power mode for much longer; meaning the mpg will drop significantly.

In the end: FREE up the thermostatically-controlled butterfly! And a 6-cyl. seems to be even more susceptible to this condition than a much larger V-8. And, poor power can easily result.

**An example of too little back pressure that I readily recall: I bought a 1977 Buick LeSabre with a small--301--V-8 from an auction. It ran and drove fine. That is, until I got about 10 miles from home, where when I hit a bump the exhaust pipe came loose from the single outlet crossover pipe--someone had "rigged" it by poorly-welding the 2 pipes together. That car did not want to run, and probably would not reach the speed limit. Afraid something else, deeper, was wrong, I called the lot and had it towed the rest of the way. My mechanic assured me, after listening to the engine idle and for any noise when we tried to rev it, that all it needed was more back pressure. Sure enough, when he welded the pipes properly, it ran great again with normal power.**

Hope this helps. I feel confident that your problem is at least worsened by that stuck heat riser. When operating properly, it will vary progressively in degree of opening, from fully-closed to fully open--and probably you can see it fluctuate/bounce a little even after it is fully heated up, as you rev it sitting still.

Sam

[66Submarine]

A couple points

NONE of my trucks have working heat risers; they are all open all the time or completely omitted when I run exhaust. I can assure you all my trucks will handily exceed 55MPH, lol. My fathers truck doesn't even have manifold heat at all. (350 with vortec heads) None of my trucks get single digit MPG, either. And the inline sixes have zero manifold heat problems IME; the exhaust manifold is bolted to the intake, so why should they?

Power circuits in carbs are actuated by vacuum or mechanical throttle position, not heat.

I also do not run a choke. When I start something up on a real cold day, I have to work the accelerator pump to richen it up when starting off; after I get rolling I just drive. Normally once you exceed a fast idle the balkiness isn't even that noticeable.

And at any rate, the heat riser should be open at operating temp, and not making backpressure.

[luvbowties]

Originally Posted by 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??

After re-reading this entire thread, I realized I had made wrong assumptions about your symptoms. And, it sounds to me MORE like your heat riser is stuck in a nearly closed position than a nearly open one. I also recall that the cars so adversely-affected by low back pressure were those that were computerized, like late 70's and early 80's--ones with cat. convertors.

I also recall that our 6-cyl. trucks of the 60's would run just fine dumping directly out the exhaust manifold, tho' the law wouldn't allow it AND it was a fire hazard. I still believe our trucks perform much better--with better MPG--esp. when COLD if the heat riser is operating correctly, as it takes less time to warm up and less time needing the extra fuel required during warm up.

So, my 2nd opinions--after disregarding my 1st--would be still to free up the heat riser. Adjust the choke properly to where you won't have to "pump the gas" so much to keep it going when cold. Get a good set of FRESH plugs THAT ARE CORRECT FOR YOUR ENGINE. None of this will be wasted $--if present plugs are good (and I am skeptical), you can save them for next tune up. Ensure your accelerator/linkage is opening the carb all the way--takes 2 people(unless you're awfully fast).

Describe how the engine acts when it is struggling to run faster at the near-45 mph range--popping, bucking, smooth and just flat out, skipping, making a whooshing sound thru the exhaust, etc... Tell us what your plugs look like IMMEDIATELY after you've just tried and failed to run 45 mph. Be best to show us a picture of those plugs at that time! Also, show us 2 pictures of your heat riser, one with engine 'overnight-cold' and 2nd with engine fully warmed up after a relatively hard run. We need to see its position relative to your manifold.

Surely all of us together can "cure" your truck. May even all get a good laugh when it is finally determined how simple the fix was. Perhaps every one of us wishes he could have your truck for just one day!

Sam

[cg285]

leave all settings to factory specs. take a vacuum reading at idle, take another one at around 2k (holding steady) compare readings and report

[awbrown]

Did anyone mention to check the distributor shaft for slop??? that would make setting the points impossible...

[66CHEV]

cg285, where should i take the vacuum reading from? the small hose going to the vac advance or the large vacuum hose connected to the pcv valve?

[cg285]

anywhere it is not ported vacuum.

[awbrown]

So , have you got this truck figured out yet ???

[66CHEV]

Ok took vac read at idle bounces from 10 to 17 very fast bak forth give it gas smooths out around 15

[cg285]

ok, it should be steady at idle and around 17. bouncing could be a compression issue on one or more cyls. does it have a skip? your readings don't give me enough information so try holding it at the lowest speed that it smooths out then raise the engine speed about 2k or so (if it smooths out at 5k don't try for 7k ) the vacuum should increase or stay the same at the higher rpm. if it goes lower it is a restricted exhaust or bad cam timing (unlikely), that will verify exhaust issues. next will be why it bounces

[awbrown]

video of the vacuum gauge in use would be handy .. you can find info on youtube or in some of the older chiltons repair manifolds....

[66CHEV]

cg 285 ok when i say bounces at idle i mean very fast back and forth between 10 and 20 on the gauge. as soon as i start to give it throttle it will steady out at around 12 then the more i give it the higher it climbs will get up to around 15-17. the when i let off the throttle it goes back to the fast bounce between 10 and 20.

also checked spring tension on valve springs all are at 180 lbs. compression is at 95-100 psi on all cylinders. could this possibly be an intake gasket problem?

[awbrown]

intake gasket would be low all the time,you have an ignition issue ,move your timing light from wire to wire and see if you have a light on all of them , or you can pull one off at a time and see if it makes a difference in the way the engine runs ,sounds like more than one cylinder is not getting fire....At least from my couch that is what it looks like ,I wish it were in my driveway...

[cg285]

is it hitting on all cyls? if so then release the park brk what is the fuel flow and filter

[66CHEV]

Ok May be on to something here took ohm reading at coil spec on book say shoul be 1.4-1.6 on primary, 5000-7000 on secondary. Mine is 3.4 on primary and 12300 on secondary thoughts???

[cg285]

Forget that.
1. Did you verify exhaust integrity With your vacuum readings?
2. Did you verify good fuel flow
3. Did you verify a clear fuel filter
4. And what is the power balance. That seems to remain on unanswered question
IE is it hitting on six cylinders

[cg285]

Of course we are all assuming that at one time in your care this vehicle ran fine. If that is the case then what did you do prior to the poor performance

[luvbowties]

Originally Posted by cg285

Of course we are all assuming that at one time in your care this vehicle ran fine. If that is the case then what did you do prior to the poor performance

Hi, cq285, if you will drive up to SC, I'll drive us to Ky. along with a comp. gauge, my 7-dollar vacuum gauge, a couple of flare nut wrenches, and a 1/2" X 9/16" wrench [and my secondary tool box that has a good pair of booster cables that we keep in my wife's van--just in case my driver needs a bit of adjustment]. Hope you like pbj and pimiento cheese sandwiches! We'll plan on spending a couple of hours out there, and then head on back home.

Dunno about you, but I've never been to Ky., and this sounds like a good excuse to visit that fine state.

Oh, while there, we'll get that 6 "hittin' on all 8" just as a "pay-it-forward" good gesture. I'm itching to get my eyes and paws on it, just to enjoy making it purr.

Sam

[awbrown]

I love a good challenge , we should take bets here and offer a prize !! I have helped quite a few members fix mystery problems .. if it were the coil going away I doubt it would make 45mph , most coils I delt with would start as long as your foot was off the gas ,but the minute you add accelerator the engine would die.. .. Remember those cool pens that you hold againt the wire and it showed if it was sparking or not ??? Champion used to make them .. http://www.ebay.com/itm/Vintage-Cham...08ee75&vxp=mtr I bet we would find one or more not firing and the distributor shaft worn so that the shaft is running out of round ... just a hunch , but I would love to see a video of it running ........... if it has compression like he says it does but yet has a vacuum issue it would have to be ignition related,unless it is missing cam lobes .........one more thing , how about firing order?? 153624???

[cg285]

i'm convinced you cannot guide someone by typing on a computer. he needs to take it to a shop-where they don't get as technical and merely fix things.

[FetchMeAPepsi]

Originally Posted by cg285

i'm convinced you cannot guide someone by typing on a computer. he needs to take it to a shop-where they don't get as technical and merely fix things.

Respectfully I couldn't disagree more. With a willingness to learn and a desire to succeed ANYONE (even me, the idiot typing) can figure out these old trucks with help from others online. This guy sounds like he wants to try, that's all it takes.

I'd hate to think of where I'd be if I just took it to a shop.
broke and stupid, probably

[cg285]

Originally Posted by FetchMeAPepsi

Respectfully I couldn't disagree more. With a willingness to learn and a desire to succeed ANYONE (even me, the idiot typing) can figure out these old trucks with help from others online. This guy sounds like he wants to try, that's all it takes.

I'd hate to think of where I'd be if I just took it to a shop.
broke and stupid, probably

wait awhile until this is 10 pages long, with hundreds of off the wall theories, and he is still at the beginning

[66customcamper]

Originally Posted by 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.

This