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Carb Backfire...now gas overflows into intake!

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Old 06-14-2018, 04:02 PM
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doorgunner
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Default Carb Backfire...now gas overflows into intake!

Holley 600 cm carb has been great until backfire happened after replacing dist. coil.

The cylinders must have been loaded with fuel from cranking with the bad coil. I installed the new coil......BIG BACKFIRE.

I removed the air filter and cranked the engine over a few seconds.....fuel started pouring into the carb throat from the nozzles.

I removed the front and rear bowl plugs to check the fuel levels......gas poured out of the sight holes.

The float adjustment screws on top of the bowls have no effect.....the fuel overflows/gushes out of the sight holes and nozzles when cranking.

IS IT POSSIBLE THE FLOAT ARMS ARE NOW BENT SO THAT THE ADJUSTI G SCREWS ARE NOT CONTACTING THE FLOAT ARMS AND NEED TO BE RE BENT?
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Old 06-14-2018, 04:22 PM
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The backfire created a whole lot of pressure that had to go somewhere. Check for ruptured power valve, blown gaskets, etc. Power valve bit the dust on me about a year ago when I had the timing way off and made it backfire.
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Old 06-14-2018, 04:24 PM
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as stated above you have to remove both bowls and see what damage occurred, wonder if the floats were crushed.
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Old 06-14-2018, 04:43 PM
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Originally Posted by ronarndt View Post
The backfire created a whole lot of pressure that had to go somewhere. Check for ruptured power valve, blown gaskets, etc. Power valve bit the dust on me about a year ago when I had the timing way off and made it backfire.
Originally Posted by MelWff View Post
as stated above you have to remove both bowls and see what damage occurred, wonder if the floats were crushed.

The power valve and bowl gaskets are still good. The floats are made from solid material and can't collapse.

Im thinking because the float adjusting screws have no effect that the extreme pressure jammed the float ARMS against the needles causing the ARMS to be BENT to the point that the adjusting screws will not close the needles in either bowl.

Does this sound POSSIBLE to you?
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Old 06-14-2018, 05:40 PM
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Which 600 Holley? The DP or the vac secondary spreadbore?
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Old 06-14-2018, 05:43 PM
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Are you draining the bowl before you try and adjust the floats?
Long shot but wondering if your oring on needle and seat isnt fubared?

Last edited by cuisinartvette; 06-14-2018 at 05:44 PM.
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Old 06-14-2018, 05:56 PM
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The needle & seats may shot, those o-rings can disintegrate and that little viton tip can get screwed up. I don't know about bending float arms but you might've boogered up that tip cranking down hard. They are cheap. I'd replace them after I dried the car out and cleaned the plugs.

One note about those floats. I made a driveway gasoline fountain when I my Holley 4160 over the winter. The floats that were in it were yellowish and did not have a spring on the float arm - and no way to put one in. Now there are two new black Holley floats in there with the proper springs. Holley said that was a product bulletin a long time ago.



Good luck
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Old 06-14-2018, 06:10 PM
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Originally Posted by maj75 View Post
Which 600 Holley? The DP or the vac secondary spreadbore?
Originally Posted by cuisinartvette View Post
Are you draining the bowl before you try and adjust the floats?
Long shot but wondering if your oring on needle and seat isnt fubared?
Vac secondary/yep....drained bowls/needle tips and o-rings look new/will post pics when I get home tomorrow night

Thanks.....any more ideas are welcomed.
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Old 06-14-2018, 06:19 PM
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Originally Posted by pigfarmer View Post
The needle & seats may shot, those o-rings can disintegrate and that little viton tip can get screwed up. I don't know about bending float arms but you might've boogered up that tip cranking down hard. They are cheap. I'd replace them after I dried the car out and cleaned the plugs.

One note about those floats. I made a driveway gasoline fountain when I my Holley 4160 over the winter. The floats that were in it were yellowish and did not have a spring on the float arm - and no way to put one in. Now there are two new black Holley floats in there with the proper springs. Holley said that was a product bulletin a long time ago.



Good luck
Hmmmmm.....the carb is less than two years old But I don't remember seeing the spring on either float. The needle tips are I was easy on screwring down the threaded needle tubes.
Can I buy a repair kit at OReilley/etc. ?
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Old 06-14-2018, 07:37 PM
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Originally Posted by doorgunner View Post
Hmmmmm.....the carb is less than two years old But I don't remember seeing the spring on either float. The needle tips are I was easy on screwring down the threaded needle tubes.
Can I buy a repair kit at OReilley/etc. ?
If the carb is that new I’d be surprised if you had that spring problem. Mine is an ‘80s vintage. That said, yes you can buy the little springs on eBay for a couple of bucks but the problem wasn’t they were missing it was that they weren’t there in the first place and there was no way to put them on. The replacement float arms had small notches that allow you to put the springs in.

I had two needle/seat problems. One was a disintegrated secondary before I rebuilt it and the other was just two days ago. That’s why I responded to this thread. I must not have tightened the set screw enough and it backed out and man, flooding can happen quick. It sounds like you have a needle/seat problem and maybe, maybe a float problem but why both at once? Sounds weird. The nut is the adjustment and the screw locks it.

I saw some mention of a power valve. No help here with a car that isn’t running but Holley told me that when you are setting the fuel/air mix at idle if you can screw those two adjustment screws all the way in and the car still runs then the power valve is blown. Something to check later.

When my was really flooded it was hard to tell what was going on. I pulled the needle/seats, removed the sight plugs, disconnected the fuel line, cranked it over to clear the cylinders, cleaned the plugs and put a fan on it. Let it dry out real good.
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Old 06-15-2018, 07:36 AM
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With bowls off, you can eyeball up whether they close needle valves or not. And set float level. I also blow in the fuel inlet. Now turn bowl upside-down and blow. That checks needle sealing. Pull floats and throw them in a bowl of gasoline. They float or sink? Which bowls, side or center pivot? I have a bunch of side-hanger bowls-carbs I am not using.

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Old 06-18-2018, 11:34 PM
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Originally Posted by doorgunner View Post
Holley 600 cm carb has been great until backfire happened after replacing dist. coil.

The cylinders must have been loaded with fuel from cranking with the bad coil. I installed the new coil......BIG BACKFIRE.
A long time ago in a galaxy far far away I had the same thing happen to me when I replaced my distributor in a camaro. It was a quadrajet and had the same problem you are experiencing.

My distributor was 180 degrees off. I think you should check that before you go any further. Disconnect your coil and pull your #1 plug. Put your finger on the plug hole and a buddy tap the key until you feel the air push out of the cylinder.

Now pull the cap off the distributor and see if the rotor is firing the # one plug. If it isn't pull the distributor and turn the shaft 180 and put it back in. Then plug the coil back in and see if you can start it.

With any luck it is just a timing issue and all is fine with the carb.
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Old 06-19-2018, 08:29 AM
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Originally Posted by 80-Vette View Post
My distributor was 180 degrees off. I think you should check that before you go any further.
This ‘distributor out’ drove me absolutely nuts - start Googling that and you’ll see what I mean. From what the PO wrote he said it was all good until he replaced the coil. Should be able to put a meter on the coil and see if it’s good or bad. Unless he had the plug wires off it does sound like the problem is in the carb. He wrote that he could see fuel dripping onto the throttle plates on the primary & secondaries

As for the 180* out thing I say careful. At one point last year I had yanked all my plug wires and did fussy little cleanup stuff and put it all back and BOOM! Exactly what you described. So I got out the service manual, followed the ‘distributor install engine disturbed’ procedure and the rotor is pointed right where it should be. BOOM! Won’t run. So, just as you said I pulled the hold down clamp and ‘walked’ the distributor around in a half circle so now the rotor is pointed at where the #6 wire ordinarily goes. Put the cap on, ran the plug wire from the #6 spot down to the real #1 plug and put the rest on in the correct firing order from there. Fired right up.

If I took a closer look before I yanked the wires I would have seen that. Doh! Googling “distributor 180* out” was the source of a big useless headache for me – you’ll see people raging at one another about all sorts of completely unhelpful stuff.

I really had to take the Mr.Spock approach to understand this. I picked up an old GM distributor cheap just so I could dissect it because this was bugging the hell out of me. I found the only thing that gives that shaft orientation are the square and round holes on the cam that the rotor screws to. I am pretty sure the answer is to knock out the roll pin and rotate the shaft 180*. I now know about the orientation dimples and it seems straightforward. I think I’ll rebuild the donor just for the experience and then pull my real 1111493 over the winter. I should be able to put it in according to the service manual after that and then have the fun of reorienting the thing again to get the vac advance can is behind the ignition shield bracket and straighten out the tach cable a bit.

I realize that this is basic stuff to some but this was interesting to me as a hobbyist. To boldly go where several other ******** have boldly gone before ……….. and actually figure it out. I hope.

Not the best photo but you can see the dimple, the roll pin is between the gear teeth (on the other side it's directly under) and those two black sharpie marks. Not in the picture but when they are aligned like that and I put the rotor on the tab lines up with them and that dimple.

Hope I haven't over answered a question nobody was asking.

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Old 06-19-2018, 08:55 PM
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Thanks Everyone. I removed both bowls and the float arms were bent so much that the float/needle adjusters on each bowl weren't moving the needles/floats at all. I think pumping on the gas pedal too much (hoping the engine would start)before I replace the module and coil had the intake soaked with gas. Then, when I got the distributor working again the BAD backfire bent the float arms (I had a similar problem years ago when a backfire collapsed a brass float).


I straightened the float arms/turned the bowls upside down and now the float adjusters work fine. I still need to take the power valve out and check it before reassembling the carb.


So......with everyone's help......the problem is now solved!

Last edited by doorgunner; 06-19-2018 at 09:02 PM.
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Old 06-23-2018, 07:11 AM
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Originally Posted by pigfarmer View Post
This ‘distributor out’ drove me absolutely nuts - start Googling that and you’ll see what I mean. From what the PO wrote he said it was all good until he replaced the coil. Should be able to put a meter on the coil and see if it’s good or bad. Unless he had the plug wires off it does sound like the problem is in the carb. He wrote that he could see fuel dripping onto the throttle plates on the primary & secondaries

As for the 180* out thing I say careful. At one point last year I had yanked all my plug wires and did fussy little cleanup stuff and put it all back and BOOM! Exactly what you described. So I got out the service manual, followed the ‘distributor install engine disturbed’ procedure and the rotor is pointed right where it should be. BOOM! Won’t run. So, just as you said I pulled the hold down clamp and ‘walked’ the distributor around in a half circle so now the rotor is pointed at where the #6 wire ordinarily goes. Put the cap on, ran the plug wire from the #6 spot down to the real #1 plug and put the rest on in the correct firing order from there. Fired right up.

If I took a closer look before I yanked the wires I would have seen that. Doh! Googling “distributor 180* out” was the source of a big useless headache for me – you’ll see people raging at one another about all sorts of completely unhelpful stuff.

I really had to take the Mr.Spock approach to understand this. I picked up an old GM distributor cheap just so I could dissect it because this was bugging the hell out of me. I found the only thing that gives that shaft orientation are the square and round holes on the cam that the rotor screws to. I am pretty sure the answer is to knock out the roll pin and rotate the shaft 180*. I now know about the orientation dimples and it seems straightforward. I think I’ll rebuild the donor just for the experience and then pull my real 1111493 over the winter. I should be able to put it in according to the service manual after that and then have the fun of reorienting the thing again to get the vac advance can is behind the ignition shield bracket and straighten out the tach cable a bit.

I realize that this is basic stuff to some but this was interesting to me as a hobbyist. To boldly go where several other ******** have boldly gone before ……….. and actually figure it out. I hope.

Not the best photo but you can see the dimple, the roll pin is between the gear teeth (on the other side it's directly under) and those two black sharpie marks. Not in the picture but when they are aligned like that and I put the rotor on the tab lines up with them and that dimple.

Hope I haven't over answered a question nobody was asking.

The dimple is there to align with the rotor tip. There, I said it for you.


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Old 06-23-2018, 08:00 AM
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Originally Posted by stingr69 View Post
The dimple is there to align with the rotor tip. There, I said it for you.

yeah, I do go on .....
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