Technical Revisiting exhaust backfire on decel issue

Lol I'm almost ready to convert to fuel injection at this point but can't get over wanting "the look". Not sure if this helps at all but the number on the bowl portion of my center carb is 7041125. Again these were built by "that guy down in Florida" so not sure what may have been changed internally besides opening the idle tubes.

 

7041125 is a late production 1971 307 Chevy small truck, it was not used on the larger engines.

I do not know "that guy down in Florida" and would make no comment if I did, either way. It MIGHT be worthwhile to find out what modifications, if any, that he made so you have a baseline; and also ask his thoughts on your problem.

As far as "the look" as you call it:

To me, dual quads says "I am serious", tripower, not so much.

And even with a radical cam, I could tune 2 genuine Carter 9500s carbs on your build a lot quicker than the time you have spent typing out your issues; let alone the time in trying to solve them.

Jon

 

I get it but I'm into this setup pretty deep now so if I can get it to run good (maybe not great) I'd sure like to. What would be the differences of a carb designed for a 350? Just the parts of the cluster or all of the other passages drilled into the body of the carb as well?

Spent half of today cleaning and regasketing the end carbs and plan to do the same to the center carb tomorrow. Floats on all 3 carbs were set around 1/4" off. I need to clean all the leaked fuel off as well.

Then I'll try to run an independent line to my center carb and see if I can tweak it a little more with just blockoffs on the ends. Then I'll bolt them on one at a time with no fuel and check where I'm at. Then add fuel to those. Just need to find the right barbed fitting for the carb so I can temporarily run a rubber line from my regulator to it. Should have done it in the first place and saved all this cleaning.

 

Hey Jon....are these the same cutouts you were referring to in the gaskets in the picture? This is what the local guy told me would possibly allow more air and lean out my cruise afr. To me it just looks like it would create a vacuum leak. Is this in the "dead spot" you referred to earlier?

Actually with it being above the throttle blades I suppose it's not a vacuum leak?

 

Those are the cutouts, check the link I posted for more information; and you are correct, NOT a vacuum leak.

Jon

 

@carbking , please see prior post.

 

I read through both links thank you....you're certainly a wealth of knowledge. I put new gaskets in all my carbs and cleaned up all the fuel stains. I have just the center carb mounted now and a makeshift stand alone fuel supply to it rather than the rail so no more messes.

I just went for 2 rides with 2 different sets of jets in it. I could adjust idle afr to about 14 with both and it idled ok given the cam....no threats of stalling.
On moderate to slightly aggressive acceleration with 62 jets I saw numbers in the high 11s and 12s on accelerating and cruising at 1800 RPM in third barely on the throttle I was around an 11.5 afr. Dropped down to 60 jets and repeated the test....moderate to aggressive throttle I saw high 12s to 13s. And on the same cruise speed and gear afr was around 12.2 so better. HOWEVER on just that drop in jet size I develop an instantaneous bog from cruise RPM. It coughs for a second and picks right up. Any idea why? Tomorrow I want to try 58s in it to see what the afr is during acceleration and cruise since I have them on hand. But from previous testing I know the bog will be even worse. I have cut some coils off of my power piston so maybe it's opening too slow now? Or is this a function of the carb originally being sized for a 307? Again @carbking thanks for the help so far.

 

If it's coughing, it's too lean, somewhere. Is the accelerator pump working?

 

Yes it sprays instantly. Right now it bogs for a second then takes off. Again with the smaller jets it'll probably be longer.

 

Since the end carbs are off right now I may pirate the plug for one of the power valve holes and swap it into my center carb and try it at cruise speed only to see what I get tomorrow. Maybe the piston isn't staying up under vacuum and the power valve is always open?

 

Any reason why you're trying for a 14:1 idle AFR? My carbed engines have always run best at 12.5-13:1 at idle.

 

Strictly because I can right now. It seems to idle the same at 14 as it does at 13. My big tuning issue is the rich cruise numbers. Right now I'm at 12 and I'd like to see at least mid 13s

 

I have another question about tuning the 2G.... can I assume none of this is related to having to possibly drill holes in the throttle blades? I know my idle is definitely up into the transfer slots however my mixture (which I understand is just volume) screws do make a difference in idle. If I run them too far in the engine will stall. I assumed this meant I was good and no need for air holes in the blades....but could this be putting me at some weird throttle position at cruise where I'm picking up fuel from 2 circuits when I shouldn't be? At the 1800 RPMs I tested cruise afr's at yesterday I highly doubt it but I'd rather ask and have a good explanation. Thanks guys. As soon as it's not too early to start with open pipes here today I'm going to attempt to take the power valve out of the mix and if that isn't the issue I'll replace it and go down another 2 jet sizes to see if it leans out my cruise. Then I may have to chase the bog it creates though.

 

When you say cruise, I assume something like a steady 60 to 70 mph for ten miles or more. That's what I call cruise. That's where you are trying to improve your mpg, I think?

 

I haven't driven it that far...just on the local farm roads....4th or 5th gear at 50 or 60 mph. I don't think duration at that speed should matter once it's warmed up. Should be load and throttle position only. Mileage has been bad since day 1 and plugs show rich.

 

Remember that at slow cruise, sometimes up to somewhere between 2000-3000 rpm, the main jets aren't yet in play. Have you been looking at your IFR size and idle air bleeds? If you're only cruising at 50 or 55, chances are good you're not even into the main jets yet. I apologize if you've covered this earlier (or someone else has).

I hope it's OK to post a link, but here's a good article on the 2G:
https://www.motortrend.com/how-to/ctrp-1101-rochester-2g-carburetors/

 

Air bleeds are fixed unless I were to drill them out. What's the IFR? I'm assuming fuel related? I know the idle fuel tube have been slightly opened on this. I have some stock ones I plan to try swapping in.
And my carb understanding isn't the greatest so I wasn't sure where the 2G transitions over to the main jets.

 

So I have a little time to mess with it again now. I just pulled the carb off and based on where idle was set yesterday I'm actually not into the transfer slots...about a half turn or so away. This would only matter off the line though right? The bog my smaller jets created was from cruise RPM.

 

The IFR is the idle feed restriction. The article I linked discusses that.

 

At this point, are you still getting the backfire?

 

Just took it out with no power valve and still same afr at cruise of about 12.2. I'll check that link again about the IFR.

Yes still getting backfire when I let the throttle snap shut. I'm taking all of my AFR readings from the other side of the engine though.

 

May have just had a breakthrough. I have what I was told by my local carb shop is a set of stock size IFR's. I just swapped them in and it threw my idle super lean....up near 20. I knew the other set had been drilled by the original builder. If they were opened up too much that could be causing my rich cruise if I'm still on that circuit correct?

I'm thinking I need to go pick up some pin bits and slowly work this new set up in size to get to a good idle and cruise AFR now?

 

Keep in mind the opened up IFR tubes are meant to compensate for a small amount of air leakage from your dumper carbs butterflies.

 

I thought about this fact. I know they say you should tune with the ends blocked off but maybe I need to put my end carbs back on without fuel and see what I'm at?

 

I would keep going as you're doing, but don't get too hung up on achieving optimum A/F ratios at idle...because what you have now (with the blocked off outer carbs) is not what you plan to run in the long run. Don't get too lost in the rabbit hole.

 

Ok do I put the drilled out IFR tubes back in and went for a ride after setting my idle again. Checked my AFR at idle, part throttle, and cruise. Then I bolted just the front carb on instead of the blockoff as it's much easier to install than my rear because of some things in the way. Front also has throttle blades that don't seal as well as the rear. It definitely threw the idle AFR off...by over a point. However everything right off of idle was back to the same readings. I didn't mess with the idle screws at all but probably he'd enough to get it back I'm the acceptable range still.

So at this point is it safe to assume the IFR tubes have been opened up too much? I couldn't find pin drills locally and we're actually getting ready to take the kids away for spring break so I don't have any more time for test and tune now. I can order pin drills and have then waiting for me when I'm back in a week. If my thinking is correct the IFRs are the same essentially as the low speed jets on a motorcycle carb right? Once I get into the main jets on this setup which is later than I thought my acceleration AFRs are pretty good in the 13.5 range. Hopefully putting the new set of IFRs in and going 1 step at a time up until I reach a good cruise AFR works. I think my rich condition was a combo of this and the floats being set too high. Readjusting the floats brought my AFR up from the 10.5-11 range to my current low 12s already. I'm assuming if I get right around 14 that'd be ideal? Possibly even higher if acceleration numbers stay good?

Now the real question is if I lean it out will this help my backfire too?!

 

A set of pin drills is always a good idea if you're going to be messing with carburetors, but remember that you can't put material back when you drill! About .002 at a time is all you ever want to do, then retest.

 

I'm going to literally go 1 but at a time and retest cruise AFR. I'll also grab another set of factor IFRs in case I go too far. Am I correct that this should finally help my cruise mixture? Like I said my acceleration ratios are pretty happy so I'm hoping my jets then are close to optimal.

@Aaron65 thanks for pointing me at the IFRs again. Too bad I have to wait over a week now to try it

 

You’re a really nice guy,,,,and I know you have been struggling with this for months.

However,,,,you will never get this system to run correctly !

The 3x2 Chevy manifold is a horrible design with severe flow characteristics.
The runners will never be correct for any air fuel ratio,,,,it just doesn’t work .
When you get the idle ratio close,,,,the other rpm ranges will be out,,,,,it has always been that way .
I had a very nice looking one on a 350 about 40 years ago,,,,never could get it correct.
I thought it was just me,,,,,but over 40 years,,,,,,that has always been the story I have heard from everyone that tried one on their engine.

Now,,,yes,,,,some 6 pack systems work well,,,,but they were completely designed for that use . And they also used dedicated carbs for the application .
The flow characteristics were optimized for all around use,,,,and they worked very well in daily driving,,,,and high rpm hot rod use .

This Chevy intake is a quick and dirty copy to make it look cool,,,,,and it does .
But , it doesn’t deliver on performance value,,,,sorry .

From your earlier post,,,,don’t go F I,,,,,,a carb is still excellent in my book !
Like carb king eluded to earlier ,,,,,2 x4 is still the best looking setup ,,,,and can be tuned to perform very well !

Please forgive me for my comments,,,,but I’ve seen you struggling with this for months .
I really feel bad for your troubles,,,,, I truly do .

You have a beautiful truck,,,it’s very impressive,,,it deserves to run flawlessly .
Good luck !

Tommy

 

Thanks for the comments on the truck Tommy...I spent many years on it between customer work.
I've heard from many people at this point that the 3x2 will never run right. Yet other people swear theirs do....however I understand everyone's idea of right differs. At least for this season I'd like to get the idle/cruise as good as possible. For the driving I do back and forth to shows and on weekend drives I don't need the top end to run like gangbusters. If playing with the IFR's gets me into a better cruise AFR for this year I can drive it a couple thousand miles and decide if I want to try a different induction setup next winter.