Popping through the Carb

[jrork]

Hi Everyone,
We just took the boat for it's first time out and stumbled onto something.
It fires right up and idles great. As soon as I lean into it (2000 or greater), it starts popping through the carb.
Plenty of power but the popping is frustrating.
I did a quick retard on the dist. and it might of helped a bit but sure didn't clear it up.
I'm curious about my advance curve. My dial back timing light shows,
Idle=8BTD
2000=20BTD
3000=24BTD
It doesn't seem to be smooth at all but a sort of herky jerky curve.
Does this sound right?
The dist. is a Mallory Dual Point with a MSD6 box.
One thing I don't see is a ballast resistor anywhere.
The condenser is mounted on the backside of th distributer.
It's a 454/Panther and this is our first run with it. All in all it was a success but embarrasing as hell.
Any ideas and all?
I'll do a compression test first chance I get to see if I have a valve problem but I want to look at the more obvious things first.
Thanks everyone.....John

[Taylor LP]

Advance curve seems to be close enough.
Don't think the MSD needs a ballast resistor, the distributor is only a trigger for the box. MSD website can give you info on this.
My guess is the popping is due to a fuel delivery problem or a lean condition. Check for vacuum leaks, low float level, clogged fuel filter or bad fuel pump.
Other possibilities might be bad plug wires or wrong firing order.

[cave]

jrork I had the opposite problem. After I gave the engine fuel then let off, my motor would pop then it started poping with fire out the rear end of the pipe. It turned out that my Exaust gasket on the side that was poping had a piece missing. It must of blown out. That was a simple fix. Check your gaskets too. Also check your powervalve if you have one. Alot of locals told me that that was my problem too.

[jrork]

Thanks TaylorLP,
I've got a 780cfm pulled apart and soaking in some carb cleaner. I'm just waiting on some parts from Jegs (quick change vac sec) and I'll throw that on. It currently has a 650 double pump and I didn't feel good about that as the secondary pump doesn't act like I think it should.
If I lean on it and get into the secondarys without the enrichment of the secondary pump, would that cause enough of a lean condition that it would pop through the carb? I would have expected it to have a serious bog but not a pop. Does that still sound possible? If I slowly bring the revs up it doesn't seem to pop and runs okay(not great) but if I jump on it, it pops instantly.
Thank again for the reply....John

[oknozelman]

I think same as Taylor, if it is lean poping dont stand on it! Check sparkplugs to see.

[oknozelman]

I think same as Taylor, if it is lean poping dont stand on it! Check sparkplugs to see.

[jrork]

Thanks again. The plugs look okay, though we spent most of the time with the rpms below 1800 as any more would make it pop. I guessing that the plugs might not be a good indicator as I not really getting very far out of the idle curcuit.
What cha think?

[comin' unscrewed]

Any chance an exhaust valve isn't opening for some reason? I've had an exhaust lobe go flat resulting in popping through carb. (The intake charge gets burned has no place to go until the intake valve opens again.)

[jrork]

Thanks Cave,
I don't believe that this particular carb has a powervalve but I'll double check. The exhaust gaskets seem to be okay but I'll go over those as well.

[jrork]

Unscrewed,
I suppose the exhaust valve might not be opening but wouldn't the engine miss all the time? As it set's now, it idles sweet, and will run up to 2000rpm nicely if I accelerate slowly.
Still sound familiar? It be a great way of talking myself in to a new cam.

[Taylor LP]

jrork:
If I lean on it and get into the secondarys without the enrichment of the secondary pump, would that cause enough of a lean condition that it would pop through the carb? More likley to happen like that on a mech. secondary carb where the secondaries open as soon as you stomp it. 454 will like the 780 better anyway.

[Hotcrusader76]

Tell us more about your motor; cam, carb, intake, exhaust, ignition, etc.
~Ty

[jrork]

I don't know that much about the motor since this is a fairly new purchase and this was it maiden voyage.
It's a "fairly fresh" 454 running the stock intake manifold, panther waterlog exhaust exiting the boat through the transom. The distributor is a Mallory dual point with no vacumn advance. The coil appears to be stock. The wires are some no name 7.5 silicone units that "appear" to be in good shape though this weekend I will do a resistance test with my ohm meter.
The carb that is currently on the boat is a 650cfm Holley with mechanical secondarys and two accelerator pumps. Since this carb looks pretty ratty and needs to be gone through, I've decided to go with a Holley 780cfm that I've got on the shelf.
I've also added a MSD 6 box to the mix that I had also on the shelf. I've gapped the plugs at .50 as recommended by MSD.
The popping through the carb was there previous to the addition of the MSD.
The timing is set at 8btd at idle and advances to 20btd at 2000 rpm and then 24btd at 3000 rpm.
I was always taught that "if it pops through the carb, it's an ignition problem. If it pops through the exhaust, it's a fuel system problem".
Do you think that if the 650 is that nasty it will cause this problem? I guess I'll find out real quick as I hope all the parts for the 780 will be here today.
I will add that the boat had sat for 2+ years before we bought it and the gas had turned bad. Weve cleaned out the gas tanks, changed all the fuel lines and filters, and put in new gas just to get it to run.
The motor is not running hot and seems to hover around 160 degees.
Thanks everyone for all the help.....John

[Jim Hall]

I had the same thng happen to me a couple of weekends ago, although not with the same regularity as you. I adjusted the floats a couple of turns and it went away.

[Cas]

jr,
I just went through a similar situation, was running around the lake at about 3000 rpm for a hour or so, everything was running good. On the way back in, I got on it a little more and it started to pop out the carb, let off and it ran fine.
When I got back home to check things out, it turned out to be a flattened cam lobe, #8 exh valve. Engine is torn down now and should be going back together starting this evening.
Pull the valve covers and see if you have any excessively loose rockers, mine had a 1/4" gap between the pushrod and seat.

[Taylor LP]

Had a cam go flat in a small block (79 305) and it behaved exactly the way Cas described. It was running fine, then I cleaned the engine at the car wash and when I refired it, it backfired then started and never ran right after that. Figured it jumped time, chain or something. But in the end it was the cam. Just that quick.
Have seen the same thing with incorrect float levels.

[jrork]

Thanks guys.
Since I can slowly lean into it without getting any popping and it idles smooth, does that rule out the cam concern.

[Taylor LP]

Can you drive it thru the "sour spot" and does it give full performance after you do?
I would try the 780 first and see what happens. One rule of tuning is not to change more than one thing at a time, but since you said you had the same problem before the MSD that tends to rule that out.
One more thing.....
You said you had a stock manifold. If you are running an open (square hole) adapter, (Holley to Q-jet), make sure that even if the carb cures your problem, dont go out and stand on it for any long passes. Open adapters tend to give a weak vacuum signal and tend to run lean on those type of adapters. Pull your plugs and take a reading to be sure it's not too lean. If it is, either try going up 2 jet sizes at a time, or switch to a 4 hole adapter. Better to be rich than lean, fuel is cheaper than pistons.
And make sure you set the float level.
[ June 19, 2003, 04:04 PM: Message edited by: Taylor LP ]

[ultra tweaker]

had a similar situation in my boat but didn't figure it out till I put my 4th motor in it and put it on the dyno where it appeared to swap cylinders. after much trouble shooting found it to be a leak at the intake manifold where the head meets the block. long story short... check the intake manifold gaskets and the end gaskets they may be sucked in just enough to allow a small amount of air to be sucked in. when I had the ends of the intake machined off ran like a champ.

[Cas]

Ultra brought up another good point. You can do a simple test to see if it's an external leak by starting the engine and spraying carb cleaner, wd40 or even water on the joint between the head and intake. You can also squirt the base of the carb and the throttle shafts. If you hear a change in rpm, you can bet you have a leak.

[jrork]

Thanks guys.
As far as being able to drive through the sour spot,I was scared to get into it too much. For my one frustration run, I stood into it faily well and it popped till I had jumped up on plane and backed off of it so I could cruise for a short distance on plane. As soon as I started to fall off plane I leaned into it to maintain the plane and it started popping again.
So I guess what I'm saying is that it has it's greatest problem when I'm placing it under load.
The parts came for the 780 so I'll throw that together and give that a try.
If still not corrected, I'll look for vac leaks.
While I'm wrenching, I'll pull the valve covers and take a quick look for excessive lash indicating a flattened lobe.
While the manifold is a stock cast iron, it's not a spreadbore. So, I don't know how common square bore cast iron manifolds are but I have one.
I'm installing a quick change secondary spring can on the carb and have the assortment of springs. Does anyone have any starting recommendations for a 18' jet with a 454? I figured I would start with the stock spring and work from there but I'm just curious if others have this similar setup.
Regardless, I'll keep the group in the loop with how were doing.
Thanks again and with hope, we'll be able to help someone else in the future..........John

[HOSS]

Intake leak, possible water getting in the intake runner. Cam ground down, possible fuel loading. I`d remoev the valve covers to see if there was a broken spring. I really do think this is your problem. Removing valve covers is easier then rebuilding carbs. Good luck.

[jrork]

Good point. I'll be sure to check them out.

[jrork]

http://www.***boat.com/image_center/showphoto.php?photo=20890&password=&sort=2&thecat=521
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http://www.***boat.com/image_center/showphoto.php?photo=20570&password=&sort=2&thecat=521
Here are a few links to pictures of the boat guys, just to show you what we are dealing with. Again thanks for all the help!

[Bahner18]

POWER VALVE IF YOU HAVE A HOLLY I WENT TROUGH THE SAME THING. IF THAT IS THE PROPBLEM INSTALL A POWER VALVE SAVER

[malcolm]

I usually pull both valve covers, pull the coil wire, and turn the key. Then just look for the rocker arm that's not moving.

[jrork]

The carb that is having the trouble doesnt have power valve.The Holley we are rebuilding does,and we just installed the powervalve saver tonight. Should have the new 780 holley on tomarrow.We can't wait to see if it will help!! Thanks again for the advice.

[jrork]

Thanks everyone,
It turned out to be a bad cam and I've started a new topic.....
Thanks again.