Losing power/slipping belts?

[iamseth]

hello, new to this site, here's my first post:

I have a 72 chevy 400 big block. Lately I feel i'm losing power when going up hills, or that the trans is not shifting. So when that happens the RPM's get higher and I hear this screeching noise. I've been told that it's a belt slipping, but I can't seem to find it.

Can anyone point me in the right direction??

thank you

[swamp rat]

is it an automatic or standard tranny? I'm assuming automatic.

If you can make the screeching noise while stationary and reving the engine than yea probably the belts.

Loosing power on uphills and the rpm goes up: (Assumption that its an automatic)

You should be able to know if its not downshifting, if its not than the kickdown cable is not adjusted properly or there is an internal problem in the tranny.

check tranny fluid if an automatic, does it smell burnt? look black or dark brown? of it does than you may have a tranny problem.

[ChevLoRay]

to the board from the Heart of Arkansas!!

Belts can squeal because they're old, or it could be you have a bad bearing in the alternator that is putting a drag on the drive belt. If it is the power steering belt, you'd have the squeal when you are turning the steering wheel at a slow speed, as in parking your ride.

But, the suggestions for the transmission above are also a good place to start.

What size tires on the rear? Taller tires (larger diameter) will affect how it performs going uphill. Are you carrying anything in the bed?

How long have you had it, anyway?

[iamseth]

thanks for the responses!

the trans (which is automatic, 3speed) fluid is red and full. Not burnt or anything. It does downshift (or at least it feels like it is), but thats when i lose the power and the screeching starts.

When in park, I can re-create the screech by reving the engine. AND when i turn the wheel hard left or right, it screeches too. However, I recently changed the power steering belt. Actually, i changed the alt belt too. Could the belt not be on tight enough? Or is the pwr steering pump going out?

[swamp rat]

Its a common problem to have a screetching belt on these trucks, i have been fighting it for a long time, new belts, new repop pullies new alt and i still have a problem with it, its frustrating to say the least. I have thought the angle of the belt going across the water pump pully and the lack of surgace contact may be the culpret, but i have not incested in a fan clutch yet to see if that helps. I have a rebuilt PS pump as well.

Do you have a 4 barrel carb? Is it possible your actually feeling the secondaries kick in too early and not the kick down on the tranny? and possibly not jetted correctly? Or possibly the fuel filter is in dire need of changing and you have a bit of restricted fuel flow? does it feel like its surging?

[iamseth]

You know, I'm pretty sure the carb needs some work. I've got a rough idle and smells rich. Got a new carb kit and float and plan to pull it apart this weekend. Hopefully that will do the trick. I will look into the fuel filter as well.

Do you think it could be both the carb and tranny? Last night I put it in reverse and gave it a little gas.... nothing. It felt like the trans was not engaging. But if I just let my foot off the brake, it reverses fine. I don't know what that means.

Any ideas?

[cdowns]

have you had a major tuneup done to address your power loss/that's the first thing i'd do

[ChevLoRay]

My personal feeling is that it isn't the transmission. Since you say you changed those belts, my bet is that you didn't get them as tight as you needed to have done. If you have a pry bar, find a good leverage point and use that to put a little more oomph on it, then tighten it down. Having three arms makes it easier, but sometimes you gotta get by with just the two that God gave you.

Carburetors that need to be rebuilt, can definately affect your power. Like the suggestion for a tune-up.....a rebuilt carb can be a plus. The fact that it runs rich (black smoke) means your plugs are getting a generous coating of carbon on them, and they will not perform very well. That will result in a poorer spark, and it will just add to the poor performance and the black smoke you are seeing.

What kind of carburetor is on your truck? I'm guessing a Q-Jet. If so, there's a very strong possibility that the plugs in the float bowl are leaking fuel into the secondaries. That would be a "normal" problem for an older Q-Jet. It is also one that is easily fixed with or without a rebuild, but you do have to pull the carb off of the engine, turn it upside down and remove the throttle plate. When you do, you'll see two "wells" that have lead-looking plugs in them. The cure is to use new plugs from NAPA that also include o-rings and spongey pieces that also help hold these plugs in place when you re-install the throttle plate and tighten it down.

[iamseth]

man, thank you for all the information! I will address all the issues this weekend and report back.

Any ideas on the reversing issues?

[iamseth]

So, it turned out i didn't have enough time to pull the carb off. Instead I replaced the spark plugs, ignition cables and fuel filter. Actually there was NO fuel filter to begin with!

Anyway, I believe the new plugs and cables have helped with the power issue but now the truck seems to be idling worse. It feels like it could stall out at any second. Could the installation of the fuel filter change the air/fuel ratio? I read in another post that when the air/fuel ratio is not right, it will idle too low.

I've never adjusted this ratio before, any tips?

thanks!

[swamp rat]

Originally Posted by iamseth

So, it turned out i didn't have enough time to pull the carb off. Instead I replaced the spark plugs, ignition cables and fuel filter. Actually there was NO fuel filter to begin with!

Anyway, I believe the new plugs and cables have helped with the power issue but now the truck seems to be idling worse. It feels like it could stall out at any second. Could the installation of the fuel filter change the air/fuel ratio? I read in another post that when the air/fuel ratio is not right, it will idle too low.

I've never adjusted this ratio before, any tips?

thanks!

The fuel filter addition has nothing to do with the air fuel ratio.. It won't hurt to unhook the hose from the fuel pump and have someone crank the engine a few seconds to make sure its pumping fuel, or even hook up a temporary hose into a containor and crank it.

Set your idle speed to about 700rpm, helps to have a tach but when i was a kid i would just set it by ear.

You have not answered if you have a 4bbl or a 2bbl so i will try to be a little generic here:

Get a screwdriver and a pencel and note pad, find the air fuel screw(s) and turn it in to the lightly seated position, record how many turns out it was at. The average starting point for the air fuel mix screws is 2 to 2 1/2 turns out from the lightly seated position. So screw it in with a light feel untill it seats, don't use force while doing this as the screw and seat are soft metal and you can deform it, then back the screw out 2 1/2 turns. Unless i missed it I still see now answer as to if you have a 2 bbl or 4bbl carb, if you have a 2bbl you have 1 air fuel mixture screw, if you have a 4 bbl you have 2 mixture screws.

When your done try firing up the truck, it should fire up,If when you first fire it up it runs really rough then try both screws at 2 turns out instead of 2 1/2, now with it running get your screwdriver and experiment with the screws, start with only 1 screw on a 4bbl, turn it out about 1/4 turn at a time and pause for a minute and listen to how it runs, your listening and trying to find the point where it starts to run rougher, then turn it back in untill it smooths out, then repeat on the other screw if a 4bbl.

Next get a timing light and pull the distributor vacume and check the timing, should be around 8 degrees, adjust if nessasary and tighten the distributer clamp and hook up the vacume hose, now do some final checks on the carb settings.

I have sometimes found that when adjusting air fuel settings sometimes it can be fine tuned better by having a friend sit in the cab, shift it in gear on an automatic and hold the break when your checking the air fuel, something about having a little load on the engine sometimes helps..

[iamseth]

thank you! This is going to help a lot. It is a rochester 4bbl. I'm going to tackle this tomorrow am. I take it the 2 screws are on the front, bottom of the carb? I've included a picture to make sure i'm turning the right screws!

thank you

[swamp rat]

Yea thats the correct screws. Heres an exploded breakdown for ya to look at too, its kinda rough but its free

http://webspace.webring.com/people/s...ssess/R4ex.htm

[iamseth]

So it turns out that the rough idle was coming from improper spark plug gap! The service manual says to gap it at 0.035". Then after a little research, everyone one is saying to gap them at 0.045-0.50". Still doesn't idle perfect, but WAY better, and since this is my only way of transportation right now....the carb will have to wait for a weekend.

thanks for everyones tips, i will post an update once I get the carb overhauled.

[iamseth]

1 month has gone by, and i'm essentially in the same boat as before. After i changed the plugs (gaped at 0.45), the cables and added a fuel filter everything was running great. Then about 2 weeks ago, I noticed the rough idle slowly coming back(which sounds to me like a mis-fire). Then the power loss. Which goes like this:

1st gear is no problem, I can hit the gas and burn some rubber.

2nd gear is fine until i'm going about 45-50mph. Which is where it would usually drop into third gear. Instead, the RPMs rev real high, belts start screeching and the truck just goes nowhere.

Finally, after flooring it- belts screeching and all -it shifts to 3rd. Although, everytime i give it a little gas to get up a hill or something, it does the high RPM thing again..... this is VERY annoying.

please, insight, tips, a solution... anything.

truck specs: '72, 400 big blk, auto trans, quad jet carb.

thanks!

[allengator]

I think your belts slip because the RPMS are shooting up...
Just like if you had it in park and hit the gas!

Sounds like your trans is slipping...

[BB72CHEVKT]

Sounds to me like the tranny is starting to go out.

[swamp rat]

While you at it pull the rear plugs on both sides and see if there oil fouled, the oil seals in the heads are not the best. May be why its running rough.

If your tranny isn't shifting then you may need it serviced/adjusted.

[iamseth]

actually when i pulled the original plugs, about a month ago, there was some oil on the rear drivers side plug. I'm having someone run a diagnostic on the trans right now. I'll check the new plugs tonight, i'm thinking this is the culprit for the rough idle as well.

thanks for everyone's input!