# stumbling and backfiring through carb



## Mr_Roboto (Apr 19, 2013)

OK, fired the engine up. I have a brand new fuel pump on as the old one decided it needed to spew into the crank case. I have a 350 with a 2bbl. Right now I don't have vac advance hooked up, and I need to replace the lines. I plan on doing that before going ahead and possibly installing a new HEI dizzy, plugs and wires before hand to eliminate those as a variable. I also have the carb kit here that I'll replace if and when I get there, but figure I should do things 1 at a time so I don't make a ton of changes and break it even worse. I think that the accelerator pump or needle seat seat or a related circuit are having issues as when in steady state it runs fine. It's only bogging/backfiring in the tranisitions and it's worse off idle. Thoughts/opinions before I start yanking the carb and/or ignition off?


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## gjones (Oct 8, 2012)

*Antiquated and fusty...*

Still running points? If so, change them & condenser and set dwell. Vacuum leaks? Bad vacuum hoses? Plugged fuel filter/ screen in carb? Carb. bolts tight? Rebuild the carb. Did you change the oil after it dumped gas into the crankcase? HOOK UP the vacuum advance . Got fluid in the radiator? How long has it sat? Like they say, need more info. Check those points, first off, then go down the list.


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## PontiacJim (Dec 29, 2012)

Very hard to give advice on just looks here. By the looks, I'd go with a carb rebuild. Make sure you clean it real well, set the float level, adjust the choke, etc.. Stumbling can be easily had if the accelerator pump in the carb is shot, you are opening up the carb for more air and the additional gas required is not being squirted in -thus stumble, but runs great at idle. Make sure all vacuum lines are good and no open vacuum holes left open to suck air. Make sure the choke is fully open when its warmed up or this will cause a problem. Then of course it could be an ignition problem if points -burned, mis-gapped, bad dwell, and bad cap,rotor,wires,plugs, etc.. New fuel filter? PCV valve if it has one? All basic type stuff you can do yourself and if you still have problems, it may be time to take it to a garage or buddy who works on older cars after you have done your thing.


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## gjones (Oct 8, 2012)

*Jes' a lil work, is all...*

He did say his vacuum advance wasn't even connected. The fact that he got it started is a good sign, though.


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## Mr_Roboto (Apr 19, 2013)

Yeah, I'm still running points the PO put a new condenser and points in the dizzy, looks like new cap/rotor too but ancient wires and maybe a new coil. Dwell is a good question, I haven't got a meter is there an easy way to check/set it otherwise? I have a new HEI here that I would love to replace the points with, just wanted to be sure I should do it before the carb. 

Based on what I've read I shouldn't even have vacuum advance in this engine until it's warmed up due to the emissions gear which is why I didn't sweat it. Mechanical is definitely functional though.

Oil definitely got changed. I have seen some tins that didn't quite make it due to explosion. Forget that idea. Not only that, but I value the bearings this engine's got and the dip stick was 2" over the full level so hydrolocking was a concern. 

Overall it runs awesome except the transitions. Starts fairly easy, idles really nicely overall a very quiet engine. That's why I was leaning towards accelerator pump. Should I just do a stock rebuild or should I make any changes to this 2GC before I install it?

Lastly, what port on the 2GC should I be plugging my vacuum line into? There are a few, and I know that they can have varying functions. I also know the vacuum digrams on this aren't that redable and have a solenoid. It looks as if there are 3 or 4 connections for small vacuum hoses available on the carb and intake. I have a 71 which means no EGR, and I have no air conditioning. What should these go to if anything?


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## BearGFR (Aug 25, 2008)

Make sure any vaccum ports on the carb that you aren't using, or have temporarily disconnected, are cappd off/plugged. If the carb hasn't been "into" and has been sitting for a long time, at the very least it should be torn down and have every passage in it cleaned out. Any problems in these areas can definitely show up as "issues" in transitions.
You can set 'dwell' also by setting the point gap with the rubbing block on the highest point of the distributor cam, but it's a little harder on GM distributors than using a meter, and less accurate.
As to which port to use, there are really only two kinds: ported and non-ported. A non-ported source will have vacuum present even at idle. A ported source won't start to "show" vacuum until the throttle is cracked open past idle a little. Whether to use a ported or a non-ported source for vacuum advance is one of those questions that sort of tends to turn into a religious war --- the real answer is to try them both and then use whichever one your engine seems to like the best.

Bear


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## Mr_Roboto (Apr 19, 2013)

Sounds good to me, I think I'm going to do the HEI first, shake it down to make sure nothing is any worse then do the carb. I'm just trying to figure out the order I want to do this in, and how to minimize introducing several variables at one time which I'm leery of.

Besides that, I'll try the ported vs unported vacuum. I'd hate to start a holy war on the forum, I'll leave it to trial and error to determine the best option.


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## Mr_Roboto (Apr 19, 2013)

OK, so installed the HEI tonight. Hooked it up sorta kludgy (straight to the alternator post) but I was wanting to see how it was going to change anything like the carb. As it turns out I still have issues, so I'm planning on rebuilding the carb. I don't think the accelerator pump is trashed, it looked like it was shooting fine. 



Found this in the fuel bowl. Doubt that's a good thing.



Gotta remember to put the check ball back in for the accel pump. This pic is so I do. 



Booster assembly, screws are there so I know what to put it back in with.



Nitrophyl float. I've been told to replace this. Should I get one ordered? I am thinking yes.



Dunno why I took this. It has an accelerator pump in it though. 



This rod goes into that hole.



This screw is longer than the others. I don't why, but hopefully it will go into the right place now.



New HEI dizzy insatlled. Need to take the hood liner out, the PO took it out with a lidless air cleaner and a backfire I suspect. Need to do a lot of cleaning too, but one thing at a time.


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## gjones (Oct 8, 2012)

*XXX (dirty) carb*

Don't let that hood insulation material go down the intake holes. You should also let the pieces of the carb sit in a container of cleaning solution for several hours to loosen the junk up.


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## Rukee (Feb 8, 2007)

I always let them sit overnight at least.


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## BearGFR (Aug 25, 2008)

Wow... if all that gunk inside your carb isn't the cause of your problems, then I'm a ballerina.. 

After you let it soak and clean, go find yourself some pipe cleaners and/or some Q-tips and clean out every passage you can find.

Bear


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## Mr_Roboto (Apr 19, 2013)

I'm using Berryman dip, it says no longer than 4 hours for coated aluminum parts. Is it safe to do over night?

I'm also ordering a new float, going to do a nitrophyl again. I've heard that they can start absorbing gas and sink.

Yeah, I think the power valve was jammed with garbage the passage definitely looks like it too. I'll probably soak and blow it out. Will probably flush out with carb cleaner since I'm wary of there being a seal of some kind in it.

I stuck a sock in it, literally. No need to worry about junk in the intake. Figured that wouldn't be fun.

Debating on converting to Holley jets, but will probably leave it be. How are the tunes on these stock overall?


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## BearGFR (Aug 25, 2008)

If you really want to step up in performance, then consider swapping to a 4bbl intake and carb. You'll get the best results in your present situation from going to the factory cast iron manifold with a QJet.

If you're staying with your present setup, then I'd get it cleaned up and running right first before thinking about swapping any parts. What's significant about jets is whether they're the correct size to provide the proper air/fuel mixture - brand doesn't matter.

Bear


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## Mr_Roboto (Apr 19, 2013)

Yeah, right now my big goal is getting everything running right then think about mods. I'll have about $50 into this carb including the can of dip, so that's not horrible IMO. I need to cut the original accelerator pump off the mounting bracket as well which I haven't done yet. 

I've been thinking Qjet but at the same time I'm not really 100% about it. Everyone around here wants a $100 bill for the intake, and I still would need a kit for mine as well as another throttle cable and intake gaskets. I have a 1977 divorced choke unit here from a 350 truck I was thinking about using. I probably need to pick the Ruggles book up first and think about what I actually want to do with it. I also have a few other mods I want to do such as going to a clutch or E-fan setup and a larger alternator.


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## Mr_Roboto (Apr 19, 2013)

OK dipped a buncha stuff and even though it's not 100% a touch of carb cleaner and a tooth brush should get me where I need to be.













Gotta go grab my float, and will probably rebuild in the early AM Tuesday when I get off work. There are some signs of corrosion on the exterior mostly but she looks servicable. Based on the books I have found, it looks as if I got lucky with the model/era of this carb because it doesn't have a lot of emissions garbage on it I'd need to over ride.


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## gjones (Oct 8, 2012)

That right there should get you at least squared away. You really should connect the vacuum advance, though. THEN see how well she comes off idle.


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## Rukee (Feb 8, 2007)

That looks like it cleaned up nicely!! :cheers


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## Mr_Roboto (Apr 19, 2013)

I suspect you'd actually know a ton about modding these carbs having a tri power and all. Anything I should do this this while it's off? Right now my big goal is running, but that can always change in the future. These are sort of fun to rebuild, I have another 2G that it kinda makes me want to take apart, rebuild and modify. It looks to have the smog hardware though so I'm not quite as excited about doing it. 

I'd love to do a plating kit and get her lookin really nice but it's just not in the budget right now.


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## Rukee (Feb 8, 2007)

The only thing I did to my carbs was to replace the throttle base plates on the outboard carbs as mine were rusted and not sealing proper. Then I replaced the paper filters with K&N filters. Other then that they're stock. Why screw with perfection?  :cheers


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## Mr_Roboto (Apr 19, 2013)

Hrrm, well Autozone screwed up my float order so I put it together and stuck the old one back in. I think it is part of the problem as the level measured up but it has fuel out the vent andwont start unless it is full throttle and I have a screwdriver holding the choke open.


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## Rukee (Feb 8, 2007)

If it leaks fuel out the vent tube then don't run it till it's fixed, you have a fire hazard. Either the float is bad, or the needle and seat are bad, or you have too much fuel pressure coming from the pump.


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## BearGFR (Aug 25, 2008)

Rukee said:


> If it leaks fuel out the vent tube then don't run it till it's fixed, you have a fire hazard.


:::shiver::: can we please not use that word? 


Bear


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## gjones (Oct 8, 2012)

*fresh percolated petrol*

GET THAT NEW FLOAT IN IT, PRONTO!!! :willy: (Double and triple check the rest...)


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## Mr_Roboto (Apr 19, 2013)

Yeah, she's in dry dock till I get a float contributed to it. Supposedly Autozone has it in for me, for real this time. If not the order's going to Rock Auto probably. Got plenty of body stuff to dobut really want it running again.


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## Mr_Roboto (Apr 19, 2013)

New float is in. OK, when I get on the gas it still acts like it's dumping in tons of fuel, but the stalling is gone, it starts properly and it warms up well. Something that I'm not 100% sure is screwy or not is I can turn both idle screws in and it will continue to run. Is this typical? From my understanding of these units it should theoretically stall out, right? I have another 2G I can steal the needles from if need be to test out in this one. They aren't any newer though. It's way better but it's not 100% there by any means.


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## Rukee (Feb 8, 2007)

The fuel level in the bowl might be set too high and if the idle is too high those screws won't respond.


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## chuckha62 (Apr 5, 2010)

I read this thread the other day and I apologize for not going back and rereading the whole thing right now, but I recall you saying you replaced the fuel pump. Did you get an AC Delco, or did you get an Autozone (or other) aftermarket replacement? It's been my experience that the AC Delco pumps typically put out ~ 5 psi and the aftermarkets significantly more (like 7 to 8 psi).

This could be enough of a difference to keep your needle and seat from stopping the fuel flow once the bowl is "full". If you can, go get the AC Delco pump, or at least one that is known to be calibrated to a lower output pressure on par with the AC. I've had and seen this happen many times over the years. I've tried pressure regulators and have had inconsistent success with those. 

I hope you get it taken care of soon!

Chuck


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## Mr_Roboto (Apr 19, 2013)

chuckha62 said:


> I read this thread the other day and I apologize for not going back and rereading the whole thing right now, but I recall you saying you replaced the fuel pump. Did you get an AC Delco, or did you get an Autozone (or other) aftermarket replacement? It's been my experience that the AC Delco pumps typically put out ~ 5 psi and the aftermarkets significantly more (like 7 to 8 psi).
> 
> This could be enough of a difference to keep your needle and seat from stopping the fuel flow once the bowl is "full". If you can, go get the AC Delco pump, or at least one that is known to be calibrated to a lower output pressure on par with the AC. I've had and seen this happen many times over the years. I've tried pressure regulators and have had inconsistent success with those.
> 
> ...


Very interesting, sounds like I need to hook a FP gauge up to this and see where I stand. I guess I need to see if my TBI one has inverted flare adapters for it as well.

Thanks for the heads up!


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## Mr_Roboto (Apr 19, 2013)

Well, it's odd. I don't think it's right by any means, but hell it sure acts like it free revs correctly. I bumped the timing by about 2 degrees, and I put some more gas in it. Due to the starting manners I'm convinced the float was a rock, so I'm glad I rebuilt it. I'm also not sure how the timing's going to hold steady after the first full tank of gas gets tossed in it. Base is 14 degrees with the vac line off, I could pretty easily see the kerosene in the tank causing the issues.


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## Mr_Roboto (Apr 19, 2013)

chuckha62 said:


> I read this thread the other day and I apologize for not going back and rereading the whole thing right now, but I recall you saying you replaced the fuel pump. Did you get an AC Delco, or did you get an Autozone (or other) aftermarket replacement? It's been my experience that the AC Delco pumps typically put out ~ 5 psi and the aftermarkets significantly more (like 7 to 8 psi).
> 
> This could be enough of a difference to keep your needle and seat from stopping the fuel flow once the bowl is "full". If you can, go get the AC Delco pump, or at least one that is known to be calibrated to a lower output pressure on par with the AC. I've had and seen this happen many times over the years. I've tried pressure regulators and have had inconsistent success with those.
> 
> ...


I hooked the gauge up tonight. At low RPM it fluctuates but when I bump it to probably 2-3K (no tach yet) it holds steady at 7.5 PSI. So I guess I need to figure out wth I am going to do. I would imagine that on an OEM style pump there's not a way to adjust, leaving me the "get another pump" or "dead head regulator" options as mentioned. Is a Qjet as sensitive to this as the 2jet is? If it will work better with the higher pressure, it's always a thought in the back of my mind. Especially since I think I can get 3-5" HG free revving this at WOT without an air filter.


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## chuckha62 (Apr 5, 2010)

The Q-jet is pretty sensitive to excess fuel pressure, in my experience. I've had better luck with Holley's tolerating higher pressures, yet I see more and more cars with regulators and fuel pressure gauges on them. If it were mine, I'd use the "get another pump" solution. Ames has pretty good prices and they are supposedly calibrated to the correct pressure.


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## Mr_Roboto (Apr 19, 2013)

I may have found "the trick" to dealing with this. I've been looking on the net a bit, and aparrently if you add more pump gaskets it causes the pressure to decrease. Has anyone ever heard of or tried this?

ED:

yeah, went out and looked at the OEM gasket. It's at least twice as thick as the new one. I think I'm going to give this a shot.


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## chuckha62 (Apr 5, 2010)

It might help. It will effectively take some stroke out of it.


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