installed reman carb - now won't run

[BillyBob]

OK, so I 'violated' one of my own Rules - never make more than one change at a time! Recently bought a '77 - all OE as best I can tell. Wasn't running well (flat spot, hesitation, rough idle at cold start, etc...). Ordered a reman Quadrajet. While waiting for reman ('old' term: rebuilt) carb, replaced most vacuum hoses and the 'white' and 'green' ported vacuum switches (one on intake manifold, other on thermostat housing - both broke when removing old vac hoses). Now, all back together, including installation of reman carb. Cranks well - as it always did. Now, hard starting and, once started, engine either won't stay running (loses RPMs) or, RPMs 'run away' - immediately go to 3000, at which time I shut-down engine. Not good. While I believe all vac hoses are connected correctly, I cannot be 100% certain. While running new vac hoses, I found the ported vac switch hose to the heat riser vac motor had a plastic tube insert that was 'blocked' - making the heat riser inop. I removed the vac motor, checked its function (checked OK) and verified heat riser movement - it, too, was functional. All reconnected.

Ideas on where to start?

Is there a vacuum hose diagram?

...any and all suggestions and help is appreciated!

[Gunslinger]

I've never seen an assembly line rebuilt carburetor worth a damn...too many variations in jetting, choke setup, etc. Assembly line rebuilders generally clean the carburetors and install some new gaskets, etc., and give them a baseline adjustment and box them. I have always had my own carbs rebuilt by someone who knew them. When I worked in an auto shop, the failure rate of assembly line rebuilt carbs was horrendous.

As much as I think Quadrajets are as good a street carb as you can find, you should either have that carb rebuilt by someone who knows what they're doing or replace it with something else. I would install an Edelbrock Performer EPS intake with an Edelbrock 1406 electric choke carb or an Edelbrock Performer intake with a Demon carburetor...I'm not a big fan of Holley.

If your state exempts cars over 25 years old from emissions testing, I would scrap all the emissions equipment...it will run better and somewhat more cool, as well as clean up the underhood appearance.

As far as finding a vacuum line chart, check some Camaro or Corvette websites or suppliers. Many will have have vacuum charts of at least a generic variety to look at.

[WayneC]

Ideas on where to start?

Is there a vacuum hose diagram?

...any and all suggestions and help is appreciated!

It does sound like a vacuum issue, or that the choke valve is flapping loose or a butterfly valve is

sticking or rubbing on the carb base gasket.

I'd suggest you start by re-installing the old carburetor (if you have it) to see if it acts the same

as the reman'd one.

IIRC, there is a temp-operated vacuum "tree" valve sticking out of the

thermostat housing that has 2 or 3 emissions hoses on it, and the correct match of hoses to

ports is undoubtedly critical.

Your best bet on a vacuum diagram would probably be to get hold of a shop

manual for a Corvette or Chevy of that vintage.

Or try this webpage: http://tinyurl.com/2bps5j

Or find a stock Chevy of that vintage to look at.

Or maybe one of these books:

http://tinyurl.com/2h2xbq

http://tinyurl.com/25le6a

http://tinyurl.com/ypkynk

http://tinyurl.com/284g9j

Edited July 8, 2007 by WayneC

[BillyBob]

You guys are awesome! Thanks - let's hope one of these links/suggestions finds the answer. Yes, I've been cautioned about reman/rebuilt parts - especially starters, generators and alternators!

[BillyBob]

July 9 status:

1) found carb to distributor vac advance & carb to fuel vapor canister vac hoses reversed -> corrected - no appreciable difference

2) reinstalled OE carb -> no appreciable difference

3) while replacing OE vac hoses with 'new' service hose, a port on ported vac switch (OE#3731385, white) in the intake manifold broke - this was replaced with a service part

4) what the hay, while replacing one ported vac switch, I elected to also replace ported vac switch on t'stat housing (OE#3031386, green) with a service part

--> FYI: 3) & 4) were done prior to installation of reman carb

5) as received, car had 'plug' in vac hose to heat riser vac motor; in re-doing vac hoses and prior to initial start-up with reman carb, this 'plug' was removed

6) replacement vac hose to heat riser motor was plugged to simulate 'as received' condition -> no difference

Here's what I've got today; with coaxing, car will start on about 4th crank (yes, choke is functional and appears to be functioning properly). Will stay running only if I pump gas - much like as it performed with reman carb - so, I think I'm about back to where I was after installation of reman carb, only now with new vac hoses, (2) new ported vacuum switches, and 'as received' carb. Motor will, on occasion, over-rev temporarily and then revs drop off quickly and motor stalls. During accelerator pumping process, there will be occasional back-firing thru the carb (yes, fire extinguisher is near-by!).

I suppose this is my question - could timing possibly have slipped during all this activity? Is the L48 'prone' to have timing slip? Distributor appears tightly secured.

I'm thinking of this as my next step - remove #1 plug, get #1 cylinder as close to TDC as I can and check rotor position. FYI: firing order has been determined correct.

Seem like a logical next step?

Anyone got a better one?

Edited July 10, 2007 by BillyBob

[Gunslinger]

There's the possibility your distributor is installed a cog off...not likely but it is possible. You may also have a loose timing chain. Check the timing...see if the timing marks jumps around or holds steady. If it jumps around you need a new timing chain.

What kind of vacuum does your engine hold when running? The vacuum gauge can be very helpful in disgnosing problems. A healthy engine should hold 17-21 inches on the gauge. If it's less or jumping around it could be several issues. Check for vacuum leaks...that can cause many kinds of troubles. Not all vacuum leaks are obvious...it could also happen inside the brake booster. You've already done some of this...pull all vacuum lines one at a time and see if the engine runs differently (it should speed up). The engine should run differently when you pull a hose...if it doesn't, you've found a source of a vacuum leak. It appears someone before you suspected vacuum leak problems or there wouldn't be plugged lines.

Your EGR valve could be bad. On a '78 Corvette I had, I eliminated the EGR and installed a block-off plate on the intake, and removed the rest of the smog controls, also replacing the thermostat housing with the vacuum switch with a standard housing. The car ran better and also a bit cooler after that.

Does that Quadrajet have an anti-stall solenoid mounted on it? Where earlier cars used anti-stall dashpots GM had moved to electric or vacuum solenoids to keep the engine from stalling when there's a sudden throttle drop...your solenoid could be bad or missing. It could also be a result of a vacuum problem, so don't condemn it right away.

The occasional backfiring through the carb could be simply the result of too much fuel in the bowl after aggressive pumping the pedal.

[BillyBob]

Would like vacuum reading - can't keep car running long enough to check. Same with timing check - car won't idle on its own. Yes, car has anti-diesel/run-on solenoid mounted on carb. It's electrically operated and is functional ('bench test').

Vacuum hose re-do and reman carb installation were done to resolve poor idle when cold and occasional bog/flat-spot when pressing accelerator. I do (just about) anything to have those 'problems' back!

[Gunslinger]

The L48 is no more prone to jumping time than any other engine. It does sound like your timing chain may have jumped. How many miles on your engine? Small block Chevy engines are quite reliable in that regard but depending on mileage and previous care (or lack thereof) they can be prone to timing chain problems...not really common but it does happen. It makes you wonder why other manufacturers didn't simply use timing gears as Studebaker did.

I think you should log onto some Corvette sites and ask some of the same questions about your problems. Try <www.corvetteforum.com> and click the forum link to C3 Corvettes...they're the same vintage as your '77 and scan the topics and ask some questions. It's a pretty heavily used forum so someone should have some answers for you.

[WayneC]

Would like vacuum reading - can't keep car running long enough to check. Same with timing check - car won't idle on its own. Yes, car has anti-diesel/run-on solenoid mounted on carb. It's electrically operated and is functional ('bench test').

Vacuum hose re-do and reman carb installation were done to resolve poor idle when cold and occasional bog/flat-spot when pressing accelerator. I do (just about) anything to have those 'problems' back!

Seems to me you've eliminated the carburetor as the problem. Still sounds like a vacuum issue.

If you haven't monkeyed with the distributor internally, or the intake manifold, or played with timing or spark plug wires (eg, you haven't done ANYTHING with the ignition since before you changed the carb), and it's secured tightly and the vacuum can on the distributor holds a vacuum applied by mouth, I would not consider it suspect (although it can't hurt to make sure it's aligned with #1 cylinder wire at TDC on the compression stroke). I would consider those new vacuum valves as suspect, though. Assuming you've convinced yourself that the vacuum hoses are connected as the diagrams show as proper, and there are not any "extra" nipples on the new valves, and since re-installing the old carb didn't fix/change the problem, it would seem prudent to backtrack on the other changes you made (like the vacuum valves). I might even be tempted to block off the holes the vacuum valves are in and simply run a ported-vacuum hose to the distributor for a test run. Also, you did not make any other alterations elsewhere, like a new heat riser valve, or other exhaust changes, etc, did you?

[Gunslinger]

Also, you did not make any other alterations elsewhere, like a new heat riser valve, or other exhaust changes, etc, did you?

That brings up another possibility...check the exhaust system thoroughly. If you have a collapsed or otherwise completely restricted muffler(s) or exhaust pipe the car would run very poorly or not at all. This is pretty uncommon, but I have seen cars have this happen, even if the parts look good from the outside.

You don't have any neighborhood kids known for stuffing potatos up tailpipes do you?

[IndyJimW]

The first thing I noticed is the engine is over-revving by itself. The only thing that could cause that is the carb suppling too much fuel. I would check that out first, would explain hard to start and backfiring also.

Jim Wood

Fleet Care

[ernier]

I'm curious, did you solve your problem???

Ernie R2 R5388

quote name='BillyBob' date='Jul 7 2007, 08:19 PM' post='2979']

OK, so I 'violated' one of my own Rules - never make more than one change at a time! Recently bought a '77 - all OE as best I can tell. Wasn't running well (flat spot, hesitation, rough idle at cold start, etc...). Ordered a reman Quadrajet. While waiting for reman ('old' term: rebuilt) carb, replaced most vacuum hoses and the 'white' and 'green' ported vacuum switches (one on intake manifold, other on thermostat housing - both broke when removing old vac hoses). Now, all back together, including installation of reman carb. Cranks well - as it always did. Now, hard starting and, once started, engine either won't stay running (loses RPMs) or, RPMs 'run away' - immediately go to 3000, at which time I shut-down engine. Not good. While I believe all vac hoses are connected correctly, I cannot be 100% certain. While running new vac hoses, I found the ported vac switch hose to the heat riser vac motor had a plastic tube insert that was 'blocked' - making the heat riser inop. I removed the vac motor, checked its function (checked OK) and verified heat riser movement - it, too, was functional. All reconnected.

Ideas on where to start?

Is there a vacuum hose diagram?

...any and all suggestions and help is appreciated!

[BillyBob]

Yes, "fixed" today! Thanks for asking! OK, so I don't proclaim to be an expert and now have a Garage Trophy, a carb gasket. Yes, it was installed front-to-back, or - as some would say, bassackwards. I am now much more intimate with the details of my L48 motor at a much earlier and quicker pace than I'd ever imagined. Thanks to all, Bill