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any ideas?

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TasunkaWitko View Drop Down
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    Posted: 25 March 2007 at 09:40

got an interesting situation here....

we recently resurrected my 1973 chevy half-ton, which had been languishing with several problems that come with simple old age. the old girl is back up and running now, but it's still got a problem i haven't quite figured out.

even after warming up, the engine (350 4bbl) will have a funny idle. it will speed up a little, then slow down, then speed up a little and slow down, like a gradually-downward spiral, until it finally dies. it will start right up again, no problem, but will do the same thing unless you keep a foot or a brick on the gas pedal.

is the idle simply set too low, or is there perhaps some other problem? the carb could use a rebuild, but i am not convinced that is the immediate problem. i thought it might be the timing for a while, but i simply don't know.

this pickup is an oldie, but a pretty-goodie. if it has a catastrophic problem, it is definitely not worth sinking a grand or even much more than a couple of hundred into it, but i would like to keep her running as well as i can for as long as i can.

TasunkaWitko - Chinook, Montana

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Post Options Post Options   Thanks (0) Thanks(0)   Quote teebogr81 Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2007 at 04:03

Tas,

Check for a vacuum leak.  Maybe even just replace all the vacuum hoses. Tubing is cheap and they crack pretty bad on older vehicles.

Tee

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Post Options Post Options   Thanks (0) Thanks(0)   Quote TasunkaWitko Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2007 at 04:47
sounds good! i'll give it a try over the weekend.

thanks!
TasunkaWitko - Chinook, Montana

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Post Options Post Options   Thanks (0) Thanks(0)   Quote soggyshooter Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2007 at 08:38
as it idles down, is it running rich?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TasunkaWitko Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2007 at 08:58
soggy -

i don't think so, but i don't know for sure. my dad was the last one to set the carb, and he tends to set them pretty lean. on top of that, it's one of those old, cold-blooded chevys that need a brick on the gas pedal to warm up on winter mornings.

having said that, it's one of those holley double-pumps, and if one of them is leaking or something into the throat, perhaps it is spurting, then dying out, spurting, then dying out etc.

i'll hopefully be able to have a close look at it this weekend, and will see what i can see.
TasunkaWitko - Chinook, Montana

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Post Options Post Options   Thanks (0) Thanks(0)   Quote saddlesore Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2007 at 09:10

Check fuel pump. Might be just on the edge, clogged lined/filter. If it's fuel, it sounds as though it is not getting enough. Slows down until enough can be pumped then speeds back up. Finally it just can't keep up.

Don't ever rule out electrical though. Coil?

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Tikkabuck Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2007 at 09:55

   All good ideas and I'd do them all,then I'd take that Holley off and sell it on e-bay and get ya an Edelbrock. If it's an old style Holley they have seats I think it's called, and if you have a back fire ya might as well pull it and replace them,they fixed that on the newer ones. GM's weren't made to run on Ford parts. 

  To me it sounds like vacume or fuel,change your filter,I use inlines so I can see if I'm getting sluge from the old tank. You say it's been down for awhile,how old is the gas?

 Check all the vacume line's you'll surprised at the differance it will make,even if they don't look worned or cracked they will be brittle and can suck air,just nice new tite ones will make a huge differance.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Guests Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2007 at 14:56

Had the same carb on my boat(351 cui).  Rebuild it once trying to track a problem that turned out to be electrical.  Check the ignition advance weights in the distributor, they can rust or those little springs rust and flop in and out of orbit.  Takes 5 minutes to check if they are free and springs working.

Oh, check all that other stuff that folks are suggesting.  Aren't engines fun.  Present project isn't guns or even making a fishing rod...rebuilding a 1965 Evinrude.  At least everything is light weight on boat engines (aluminum)

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Post Options Post Options   Thanks (0) Thanks(0)   Quote CB900F Quote  Post ReplyReply Direct Link To This Post Posted: 28 March 2007 at 15:14

Taz;

Does this one have a conventional points/condenser system, or an HEI system?  If it's an HEI, check that the magnets in the distributor are OK.  Check any wiring around the engine for burnt insulation.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote TasunkaWitko Quote  Post ReplyReply Direct Link To This Post Posted: 29 March 2007 at 09:58
good suggestions - thanks to all for the advice. i'll "run the gauntlet" through these and report back!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TasunkaWitko Quote  Post ReplyReply Direct Link To This Post Posted: 29 March 2007 at 09:58
900f - this 350 has the old points/distributor cap etc.
TasunkaWitko - Chinook, Montana

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Post Options Post Options   Thanks (0) Thanks(0)   Quote rivet Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2009 at 13:00
Your problem is your gasket between your carburator and manifold. That rise and fall of rpm is the giveaway. The stalling/dying is prolly clogged vaccum lines, I would spend 10 bucks and buy 10 feet of them and replace all. You will solve your problem guaranteed!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote rivet Quote  Post ReplyReply Direct Link To This Post Posted: 15 May 2009 at 13:00
Oh crap. Just realized this post is years old. Sorry!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TasunkaWitko Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2009 at 02:41

hey, rivet!

actually, we haven't done anything yet - it's been parked! i bought a carb kit a month or so ago and will also get new vacuum lines - hopefully, we'll have this baby running soon. i've been putting the job off because carbs scare me! especially holleys! especially double-pumps!

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Post Options Post Options   Thanks (0) Thanks(0)   Quote rivet Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2009 at 07:04
The dead givewaway for an air leak between carb and intake is that rise and fall of RPM at idle for no reason. The engine will suck extra air through the leak, then the carb willl compensate by adding more fuel and you are on that merry-go-round.

Although all the other advice is good and you probably should do it to keep your baby running well for another 50K miles (and it should), the solution to this particular problem is the carb and the gasket.

No worries with carb kits I've done many. Rules of thumb when doing carb work-

1) Keep things clean! No bits, no trash, no sludge, no lint. All this stuff will negate any work you've done.

2) No matter if the jet, pin or part is outta the box, use "canned air" or air hose to blow them clean. Lots of little bits of stuff on them.

3) When reusing your air-horn, bowls, base...whatever, make sure you clean them up and remove the built up varnish. And if you are going to do that get it all off. If you only do it halfway, eventually some of the old varnish will come off and cause a clog.

4) Q-tips are great, but they are LINT MONSTERS.

5) Toothpicks are great , but they are SPLINTER MONSTERS.

6) Try to find an old dentist's kit and use their scrapers and wire probes. They are the best. In the Army, my LRSD team split a kit up to keep the bolts and chambers of our weapons clear of build up. All you need is one tool, but the kits have several.

7) Follow the instructions. *grin*

8) Keep your gaskets in a sandwich / ziploc bag until you are ready to use them. They fall out of the kit and lay around and get moisture or liquid on them and start to swell. Not good. Keep them dry from the get-go.

Lastly, buy a tube of GEL-GASKET or FORM-A-GASKET (one or the other...the company that makes it is, or was,  PERMATEX) Use this to make an airtight seal between the carb and the intake manifold (and between the EGR plate if you have one. Your 1973 may have it. My 1973 Mustang did. Its the Exhaust-Gas-Recirculation plate that fits between your carb and intake manifold and the vacuum tubes that lead into its nipples feed exhaust gasses back into your cylinder's intake)

Run a nice bead around the exterior perimeter of the carb as well as around each of the barrels holes( Be careful not to put too much and cause it to squeeze into the chamber as you torque down the carb!). This liquid gasket will help ensure an airtight seal. Tighten the pieces down as usual and you will have a bullet-proof seal.

No more rise and fall of RPM's my friend!!




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Post Options Post Options   Thanks (0) Thanks(0)   Quote rivet Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2009 at 07:06
Oh, and PS. The downward spiral and eventual stall, is from fuel starvation. Your float could have a hole in it, not uncommon at all. You should replace it no matter what, since you'll rarely be able to spot the hole in your float.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote TasunkaWitko Quote  Post ReplyReply Direct Link To This Post Posted: 16 May 2009 at 16:10

excellent 411, rivet - many thanks! i'm hoping to get this carb project off the grouns ASAP - we don't use the pickup much anymore but it is very handy for trips to the landfill and for hauling stuff around ~

probably won't be this weekend, but next weekend - when we get going, i'll let you know how it turns out ~

TasunkaWitko - Chinook, Montana

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