sb 350 troubleshooting?

[PEG_DRAG'R]

Hey Everyone, new here...I have some motor drama...lol...'78 chevy sb 350 ,starts right up,idles fast but smooth, when i put the truck in gear, the motor almost chokes out...new carb,cap,plugs,wires,oil,im stumped...smooths right out in park or nuetral...trans fluid is full, and I've checked all the vac. lines.....anyone got any ideas....

 

[70Chevy]

Check for a vacuum leak.... spray carb cleaner around and listen for a change in engine speed.

 

[PEG_DRAG'R]

tried that last night with WD-40...didnt find anything that changed ...

 

[69GSCAL]

Did the truck run/ operate as intended before, or is this a new to you truck that you are trying to get into operating condition?

If it did run/ drive before, what has changed since?

When you say it idles fast but smooth in Park or Neutral, at what RPM is it idling? Is this with the choke engaged or disingaged?

Quickly, a new carb requiring set up could be the issue. Check manufacturer's recomendation for initial set up then tune.

Timing, especially too much mechanical advance too early, could also be the issue. Pull the cap and rotor, put in a set of stock, heavy mechanical springs and weights (if they aren't already in there), then set initial timing according to the manufacturer's recomended setting. 5-8 BTDC should be a good start for a stock truck.

Once you have verified Carb (choke, idle, fast idle) and timing (initial, mechanical, unplug vacuum for now, dwell if you still have points) you can start some real troubleshooting. Likely, this is a tuning issue.

Vacuum leak is easy to look for as suggested. Keep the air filter on when doing it and use the straw attched to the carb cleaner. Otherwise, it's easy to mist the carb cleaner too close to the air horn of the carb and get a false positive.

 

[ddawg16]

You have a vacuum leak.

In park, if you can not get the idle down to 600 RPM....you have a leak......this is assuming that you have already set the timing.....

Old backyard trick....hook up the vacuum gauge....adjust timing for max vac....while keeping the RPM's as close to 600 as possible.

You should not get more than about 100 RPM difference when you put it in gear....

BTW....Q-Jet carb? If so...did you epoxy the bottom where the rear jets are installed? Do you have the thin metal plate installed? The right gaskets? the Ported vac ran correctly? If I remember correctly...HEI runs off the ported vac of the carb....

Take a pair of needle nose pliers and pinch every hose to see if it drops the idle.

 

[StaggeringGoat]

Check for a vacuum leak.... spray carb cleaner around and listen for a change in engine speed.

...and melt everything plastic or rubber in the process...BAD IDEA!

 

[FRONK 1969]

ok 2 ideas

I have had the same truck LOVED it.

COIL ?

Plugged fuel filter in the carb if it is a Q -jet?

Best of Luck

 

[DKerns]

Ditto on the vac. leak. Check the base gasket real well. Especially if it is a square bore/spread bore mismatch!! I prefer to use propane or a spray bottle with water in it to check for vac. leaks. Carb cleaner is a fire hazard and can puddle on a hot intake!!!! Not a good thing!! And as stated above, make sure you do not have the wrong vac. signal to the dizzy!!

 

[Gary S]

Can you describe what you mean by "chokes out"? If it is truly choking out, you need to get the choke adjusted right. If the engine is simply idling too low and dying, the vacuum leak and timing issues are most likely places to look.

 

[Wadd2]

...and melt everything plastic or rubber in the process...BAD IDEA!

Carb. cleaner will NOT melt everything plastic or rubber! Spraying carb. cleaner to check for vacuum leaks has been used for ever and of all the times I've done it, I've never once melted anything plastic or rubber, especially with the newer formulas of carb. cleaner, they're not as nasty as some of the older stuff. I suppose if you soaked it in there it might but not just a light spray of it.

 

[PEG_DRAG'R]

thanks for all the advise, the truck has been sitting for about 7 yrs, i found it n a farmers feild, the original owner bought it new and passed away in 06, it has 60,151 original miles and is clean as a button, took about 3 hrs to get it running, I thought the problem was the carb so i took off the stock Q-jet and replaced it with an Edlebrock, used the correct adapter plate and gasket set, and chokes out as i referred to earlier,it sounds like the last few burps a motor makes when it runs out of fuel, but the fuel is good, new line, filter,runs great in park...lol

 

[StaggeringGoat]

Carb. cleaner will NOT melt everything plastic or rubber! Spraying carb. cleaner to check for vacuum leaks has been used for ever and of all the times I've done it, I've never once melted anything plastic or rubber, especially with the newer formulas of carb. cleaner, they're not as nasty as some of the older stuff. I suppose if you soaked it in there it might but not just a light spray of it.

Oh, just because you've done it makes it all OK! Specifically, spraying carb cleaner onto old rubber vacuum lines, is a really stupid thing to do! (unless you plan on replacing them all) Even if they don't completely melt you have weakened them. Not to mention the fire aspect that somebody else talked about! There are other ways to check for a vacuum leak.

 

[bgott]

You might have a **** carburetor, yep, new, right out of the box. More than likely, however, you have a vacuum leak, as suggested. Clamp the line going to the power brake vacuum booster, people forget there is vacuum going there and will go nuts chasing their asses everywhere else around the engine compartment. I've also seen those cheesy assed adaptor plates not cover the whole mounting flange of the manifold and leave a gap, major vacuum leak.

Oh, and since I got a really good deal on one last week on CL, a smoke machine is the best way to chase vacuum leaks!

 

[bigbubba]

You did drain and clean the tank out right? Also check the fuel pump and blow the lines out.

 

[BillK]

took off the stock Q-jet and replaced it with an Edlebrock,

Thats the first problem Get the Q-jet rebuilt, there is no better carburetor for that engine.

Just my opinion,

 

[Gary S]

Thats the first problem Get the Q-jet rebuilt, there is no better carburetor for that engine.

Just my opinion,

I agree with your opinion. The best Edelbrock carb in the world will never match an original unmolested qjet.

 

[69GSCAL]

If the truck sat for 6 years your first order of business should be replacing all of those vacuum lines. As others have suggested, you may have a vacuum leak and old vacuum hoses would be very suspect.

Love Q-jets and run two on my Buicks, but no reason that Edelbrock shouldn't work for you. Call the Edelbrock hotline for help getting it set up.

Take a good look at the distributor and make sure it is in good order.

Once again, how high are the RPMs when in Park?

How much advance are you running (initial)?

 

[uniballer]

Timing? My timing was off in a 350, done the same thing, but maybe carb too?

 

[davedriveschevys]

Vacuum leak is a possibility, but one bad enough to stall the motor when you crack the throttle should be easy to find . You should be able to hear it. Id plug all the vacuum ports off on the manifold and see if idle drops then add one hose at a time to find the culprit. If idle does not drop with vac. ports plugged check carb base gasket or intake gaskets. And youll be fine using carb cleaner to check, just dont go overboard. I use non-chlorinated brake clean, it evaporates quickly.

 

[Wadd2]

Oh, just because you've done it makes it all OK! Specifically, spraying carb cleaner onto old rubber vacuum lines, is a really stupid thing to do! (unless you plan on replacing them all) Even if they don't completely melt you have weakened them. Not to mention the fire aspect that somebody else talked about! There are other ways to check for a vacuum leak.

Yep sure does. I can get in a ******* match if you'd like but whatever. 1. If the vacuum lines are that old, a smart person would replace them. 2. Carb. cleaner doesn't weaken them after a few times of doing that. I suppose you're one of these guys that think an engine has to be built in a vacuum with air tight seals so no dirt can get in too right? And anyone that doesn't do it that way is an idiot? Fact is, methods used 30 years ago still work and just because you don't believe in it, don't spread false information. The truth is that over time yes, carb. cleaner (some types) can and will weaken or ruin the plastic in rubber but not after one time.