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Hot Start Problem - 73 Riv


JZRIV
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Been having hot start issues since I got car a couple years ago.  All stock 60k mile 73 Riv 455 with original Quadrajet. Starts OK after being off long enough to get gas at station but much more than that and it does not want to start without a fair amount of cranking. I suspected maybe it was flooding but wasn't sure. Then finally when I was cranking it trying to start there was an explosion in the left muffler. I assume from too much raw fuel getting dumped in somehow.

Cold starts are fine and it runs well without any other problems. The choke is working properly. I am using a 1/4" thick insulating base gasket. No indications of engine overheating. It has a new brass re-cored radiator. Timing is advanced about 4 degrees beyond spec.

I took the carb apart today to investigate cause but not seeing anything obvious. The float chamber was bone dry. This is after sitting for only 3 weeks. I expected there to be at least some dampness left. I put about 1/4" deep fuel in the float chamber to check for leaks in the metering well plugs but there are none. The float setting was right on and needle appeared to be in good condition. It had been apart before I got the car and it appears it had a rebuild kit installed.

Any suggestions are appreciated on where to go from here. Any chance it could be caused by something other than the carb?

Edited by JZRIV (see edit history)
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OK Jason: This is contrary to my ranting of scientific diagnosis. Try a fresh set of points.  Every time I get that stupid backfire in the muff and it won't start I begin with spark at the points. So far? So good!   Cheap, easy, you'll need 'em sooner than later anyway.   Mitch

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4 hours ago, carbking said:

Thanks Jon! I will try that. I tend to open the throttle as commonly done for a flooded engine as soon as it doesn't start in the first few revolutions. And that's what I did when the explosion occurred. I've tried different techniques but believe all of them included opening the throttle at least a little. Your explanation sounds exactly like what is happening, I just didn't understand the physics behind it.  

 

Mitch and John 

I am confident the basic ignition components are in good shape and tuned precisely.......Unless there would be an uncommon ignition problem that only affected hot starts. Once it finally starts when hot, it runs great. 

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A bad condenser would also cause this because it's "Breaking Down" when it gets hot. Inexpensive experiment. Some of today's ignition components, even though they may be totted as US made, still really come from China.

Just a thought.

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Jason - the physics are more or less explained if you click on the link and scroll backward to the paragraph above. Whether the excess fuel runs out by the end of the throttle shaft, or runs down into the intake manifold depends on if the throttle plates are completely closed at idle.

 

And I certainly agree with those posting the possibility of ignition issues. The new "off-shore" ignition parts, even in a name brand box, are junk. Not only ignition parts - we recently had a "carburetor" issue after a customer just purchased a new "name brand" fuel pump. It only produced 18 psi to a carburetor wanting 3~5 psi !

 

But, as you state the car runs well when started, I think your hard start under hot conditions is related to the volatility of modern fuel.

 

By not opening the throttle you are accomplishing the same result as we used to accomplish 50 years ago by opening the throttle to a wide open position; that is, reducing the density of the A/F ratio in the intake manifold. Because of the volatility of modern fuel, bleed-over on a hot engine doesn't allow the excess fuel to be pumped out with the throttles open; whereas leaving the throttles closed allows the engine to pump the excess fuel out, and the engine to start.

 

And, unlike most repairs today, it costs nothing to try ;)

 

Jon.

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I'll post back after I get the carb back on and some other things cleaned up. Interestingly last winter I re-bushed the throttle shafts. Have to wonder if prior some fuel overflow was leaking past the shaft onto intake vs going down into the intake. Now that there is no clearance around the shafts, it all would have to go into intake. Maybe why now I had explosion. Or maybe coincidence.

Also I noted in the link higher octane fuel tends to have more ethanol. I was thinking opposite. The car will run on 87/89 but last fill up I put 91 in it.

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You might want to pop the distributor cap off and put vacuum on the advance diaphragm. Then release the vacuum and make sure the points go fully retarded. If the breaker plate is dragging some you might have too much advance on the warm start because they haven't had time to creep back.

That's the one where is started great at home, drove 4 blocks to the gas station and it just grunts.

Bernie

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Hi JZ, went thru 2 sets of mufflers after putting my car back together, Car would bog, stall and then Talk about explosions! Split like Clamshells. Replaced everything. Make sure its all working properly. Advance, TVS, All vacuum lines, AIR Diverter Valve. 4Deg BTDC, But ended up being the Carb. Might look OK but these Quads will leak and pour gas into the intake. Get a new one or a GOOD rebuilder to take care of yours.

No problems since.

Edited by Tder1 (see edit history)
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Carb back together and installed. Put about 80 miles on today with 3 intentional 10-15 minute stops. I also filled with 1/2 tank of 87 octane. 

Each time it started with same amount of cranking and no drama.  I'd guess around 6-8 seconds with Throttle left completely closed. Doesn't sound like much but seems an eternity when expectation is immediate start. I resisted urge to open throttle and so far this is working. 

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6-8 does seem like a lot when you are the one holding the Key. Just for info mine will start with a half turn of the Crankshaft within 15 minutes. Much after that requires a little pedal and fires after about 3 seconds.I would think 6 seconds with no throttle is pretty good. Did you do the Carb yourself or send it out ? Just wondering. Done many in my time but always seems hit or miss when they get back on the car. I'm sure there are pros here who know but there all kinds of tricks they go thru to eliminate potential leaks and problems. Idle Screws, did you just go with the 2 1/2 turns or set them by Vacuum? I always like to know what the trends are. I had so much trouble with my backfires I turned obsessive about adjusting Quads.

Good Luck, sure you'll be fine.

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I did carb myself. Last year I re-bushed the primary throttle shaft so that's nice and tight. Other than that it appears in excellent condition inside and does not look like it has been molested by inexperienced hands.  It runs flawlessly other than the hot start issue.

 

I always set the idle mixture screws using a vacuum gauge. I adjust to max vacuum then cut back just until needle starts to drop.

 

I do agree sometimes there is no substitute for having a pro rebuild the carb and am a proponent of that. Years ago I fought with one on a 67 and finally bit bullet and got a professionally rebuilt replacement. It really woke the car up and found power I never knew it had.

 

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