Technical Revisiting exhaust backfire on decel issue

Getting zapped by tugging on a sparkplug wire/boot is not an unusual event; that's why they make insulated pliers specifically for the job.

 

I was using insulated pliers and it was still lighting me up like a Christmas tree...none of the other wires were doing it.

 

Dang!

 

Yeah super fun experiment. Like I said I couldn't visually find anything wrong with that boot either. Hoping maybe it was arcing right to the head and I never saw or heard it.

 

If #1 was grounding, it would explain low temperature on #1.

A little dielectric grease inside plug boots makes them a little easier to remove from plug.

If boot not cracked, be sure plug is not cracked, but the problem is most likely the boot.

 

Just for kicks and giggles what kind of plugs are these, brand I mean. Lippy

 

Vacuum leak will cause hi resistance ,air don't burn.

 

I'd be tempted to replace the whole wire, not just the boot. Have you got a spare you can rob from something else, just to experiment with?

 

Plugs are AC Delco and yes I should have a spare wire somewhere I can swap on for it

 

You know what's funny....I was thinking more about the low temp thing last night and I may be making a mountain out of a mole hill on that. The driver side in general runs cooler than the passenger side. At idle yesterday my temps were around as follows with the IR gun....1:180, 3:240, 5:245, 7:195. So 1 is the lowest but really not that far off from 7.

I'm going to make a different test pipe tomorrow that captures the full exhaust flow rather than a slip in design and get some O2 readings. Despite how the truck seemed to feel best with 63 jets last season I think I'm super rich. With less than 1,000 miles on the engine there's already a high amount of carbon on the intake valves and some on the pistons as per the bore scope yesterday. And despite having my garage door open with a fan going and a back window also open the truck was so rich at part throttle in there yesterday that my eyes were burning.
Today is a long day of work and then kids activities but tomorrow I have half the day to make the new test pipe and gather some data. I've also read through the post on here about wide band tuning a carb'd engine and want to go through it again today while I'm at work. Lots of good info in there.

 

I've used this to help clean carbon deposits.
https://www.chevronlubricants.com/e...omplete-fuel-system-cleaner.html?gclsrc=3p.ds

It seems to work, but I don't have access to the boroscope.

When I use it, I'm on the road, where I can burn through a tank of fuel in five or six hours.

 

Thanks for the link. Once I get my a/f ratio close I'm going to run some GM Top End Cleaner through it I think.

 

Ok finally maybe getting somewhere. I had 60 jets in the center carb and after making a better test pipe I went for a ride. Suuuper rich. At part throttle cruise my A/F was down in the high 10's, low 11's! On acceleration it would move up to about 12's. I had a set of 58s here that I swapped in and my part throttle cruis was more into mid 11's. Those are the smallest I have on hand so I'm trying to get some 53's- 57's to mess around with tomorrow hopefully. That would explain the rich looking plugs and hopefully my backfire...has to be tons of unburnt fuel in the pipes.

The one question I have is on float adjustment. I know Mike over at Mike's Carburetors says to set to factory and leave it but I know other guys tweak it for tuning purposes. What are the effects on part throttle with lowering and raising float height and how will it effect idle or WOT? I'm assuming once I put my end carbs back on I just need to be sure the bowl doesn't run dry at WOT.

Here's a pic of my new test pipe.

 

OK, another grasping at straws suggestion. I know you said you switched distributors to no avail, but I'm wondering if you used the same distributor cap or not. Also, I think you said the backfire was on the passenger side but #1 cyl is on the drivers side. Could a spark be jumping from #8 or #2 since #1 is between them? Maybe I'm remembering what you said incorrectly.

 

Cap and rotor were new with the new distributor and it was #1 that seemed to be missing on the driver side.

 

I was just able to grab some 53 and 55 jets from the local carb shop....going to start with 55s tomorrow. Have to head to work shortly. Shop also gave me another piston for the power valve. He still thinks lower than 58 is too lean. Said with 14-15hg of vacuum the power valve may be hanging open when it shouldn't. If the lower jets don't seem to help he said start cutting spring coils off the power piston and see what happens. Is there any way to really confirm is the valve is stuck open or not when it's together?

 

Calling @carbking for technical assistance, please

 

Pretty sure I called his shop the other day and he basically told me if someone brought him tripower on a 350 he'd tell them to just junk it because it was meant for bigger engines and he had no other suggestions for me. I could have called the wrong place but I believe the number was the same.

Sitting here at work I'm leaning towards the power valve being the problem maybe but I'll look into both. The truck seems to pull well under hard but not WOT acceleration. But idle circuit is rich just off idle and cruise is super rich. Last fall I felt some surging at part throttle cruise as well. Under hard acceleration the power valve would be open normally and that could be why it felt good...needed all that fuel with the jets I had put in. But then it could be stuck open at cruise and idle making it wildly rich. That would explain the 10mpg I'm only getting as well.

 

So you're saying it consistently backfires on the passenger side but the bad wire you just found was on the driver's side? (I'd be tempted to change all the old plug wires.) One of my patented ways to check plug wires is to start it up on a dark night with the lights off and look under the hood.
Okay. I can see the bad plug wire affecting the idle. But I don't see the backfire on one side pointing to the carbs as the problem. The 3 x 2 carb intake does muddy that conclusion a little bit...... But I still lean towards it not being carb related. Maybe look deeper at possible ignition issues.
But on the other hand, the "popping @ the throttle snapping shut" deal makes me think neither carbs or ignition. Which circles back to everything else....... cam, exhaust, carbs again, etc. etc.

 

Sorry I must have confused you at some point. My backfire is on the driver side and it seemed like my misfire may have been from #1 which is on that side. Passenger side has always sounded ok. Gets a little backlash/afterfire on decel but more of a normal sound not shotgun loud.
My wideband is in the passenger side for now so my air/fuel shouldn't be effected if I do still have a slight misfire. And that healthy sounding passenger side is showing way rich. If the power valve is stuck the 5and7 cylinders firing right after each other that rich are really loading up the driver side header with potential unburnt fuel.

 

LMAO 10mpg your doing GOOD ! I am gettin 6 !

 

Lol....on a 350hp that runs off 2bbl most of the time?

 

So I fought with this thing all day again and didn't make much progress. I dropped the jets down to 55's just to see what it did and the engine needs that fuel for acceleration. My A/F ratios at moderate acceleration went up to the 15-16 range and the engine stumbled all over itself. After some testing I'm back on 60 jets currently and under that same moderate acceleration I'm closer to 13 which is at least in the ballpark. The issue I'm having however is cruising A/F. In 5th gear at about 50mph the throttle is barely open and my A/F is down in the upper 11's to mid 12's! I tried removing some coils from the power piston spring....2 at a time and it made little or no change. I've removed 5-6 coils from it at this point. I pulled the power valve itself and it appears to be opening and closing no problem. I also tried applying vacuum to the port in the top of the carb on the inside to see if the power piston is pulling in....tough to get a good seal for testing purposes but it was pulling up.

I went completely back to basics right at the end of the day today and my float level may have been way off....however not sure if it was like that this whole time or if I accidentally bent it while cutting coils off the power piston. The height was at just over 1/2"...maybe 5/8". I believe the correct spec is closer to 3/4". Float drop was also lower than what I believe is the correct spec of 1.5". I buttoned it all back up and started the truck. Idle A/F was right in the 14.5 range however I didn't have time to test drive it. If the float level was that far off would that cause the overly rich cruising mixture? If not I'm just about at a loss for other adjustments. Air bleeds are all clear, jets seem to have little effect on the cruise mix, etc. I'm debating trying to put a plug in place of the power valve just for a quick test drive to remove that whole assembly from the mix and see what my mixture looks like.
Anyone else have any suggestions as to why I can't get the cruise mixture to lean out?

 

I can't remember, are you running without the air cleaner currently?

If the air cleaner is impeding air, it would show rich. It seems we should be doing this around town testing without the air cleaner, to eliminate that as a possible problem.

 

Air cleaner was off today. I just got off the phone with another car guy all the way up in Canada who has this same engine but is running a 4bbl. He has the same vacuum as me essentially and I think I've ruled out mechanical/vacuum leaks now.

I really wish I knew if I accidently threw the float level off today or of its been set almost 1/4" off this whole time. Seems to me that would certainly cause an uncontrollable rich condition...however I think it would show up at idle too wouldn't it?

 

But does he have the same back firing issue?

 

He had some before all of his tuning. He's also running a full exhaust

 

I might be able to help you understand better the carburetors:

(1) Rochester supplied fuel valves for the 2-G carburetors from 0.086 to 0.113 inches
(2) If one looks at STOCK tripower configuration, Rochester generally used the 0.086 valves in the end carbs, and 0.101 in the center carb. Changing the fuel valve orifice can dramatically alter the fuel level, even if the float is correctly set.
(3) Rochester 2-G carbs, IF USING THE FACTORY ORIFICES AND THE FACTORY FLOATS, will have no issues with 6~6.5 psi fuel pressure, no regulator needed. Before I learned tripower was not overly efficient on a V-8 and still had one, I used 7~7.5 psi with absolutely zero issues (Pontiac 389).
(4) Float settings vary significantly on various 2-G carbs, to properly set the float, one needs to know the original identification number of the carburetor.
(5) Floats should ALWAYS (and I detest that word) be permanently set at the factory specified setting.
(6) Float setting MAY be tweaked for TESTING PURPOSES! Changing the fuel level by 1/16 inch WITH CORRECT FUEL ORIFICE and correct float will change the calibration by approximately 1 calibration size (useful for testing before buying additional jets). Once the testing is complete, change if jets, if necessary, and reset the float to correct factory setting. Why do I mention correct floats? Floats with different displaceable volumes are sometimes substituted trying to adjust the maximum fuel volume of the carburetor bowl(s).

All of the above is ASSUMING you are using FACTORY END CARBURETORS, NOT Frankencarbs. Good luck (and a virtual bottle of Bayer ) if you are trying to use Frankencarb ends.

No suggestions for the backfire issue.

Jon

 

Jon,

Thanks for the info. Improper float level was definitely part of my issue. I adjusted closer to 3/4 of an inch and went for a ride. My cruising a/f went up more into the 12.5-13 range so a step in the right direction for sure. What do you feel is a suitable cruise afr for a tripower setup? Any tips to lean that cruise out a little more? I just picked up all new gaskets and will spend today and tomorrow cleaning up the mess I made on my intake and carbs with the ends blocked off. Local carb shop gave me 2 different style gaskets for between the throttle body and the bowl assembly. 1 has a different cutout that he said will allow a little more air into the mixture. I have to see if I can find a pic of the original gaskets for this carb. The number is stamped on the side of the bowl and if I look it up it correlates with a 1971 truck with a 307. That's assuming the cluster has never been changed, etc. I did find float levels for that carb for a Chevy 350 and that is where I will set my carbs now with the new gaskets. I do know that the idle tube have likely been opened up a little on the center carb. Currently my idle a/f is good in the 14 range. My understanding of the circuits in the 2G gets limited here. Doing 50mph in 5th gear I'm probably turning around 1100 RPMs and the throttle is just barely open....as in I'm just touching the pedal. Am I completely out of the idle circuit at that point or could those massaged idle tubes be adding too much fuel at that cruise point? I obtained a set of stock size idle tunes today that I could try if necessary .
I've been beyond frustrated with this project for close to a year but feel like I'm starting to make some progress. My cruise afr started in the 10s earlier this week and I'm atleast in the 12s now. Acceleration on just the center carb is still pretty good. I'm in about the 13s but it has a slight bog or hesitation now. It seems to pull better with a little more jet. I'll chase that once I have the idle and cruise where they seem "right".

 

The cutouts (or lack thereof) in the throttle body to bowl gasket do not add or subtract from the idle circuit. These cutouts were designed to help control high temperature idle and low speed issues. When I asked a Rochester engineer about these some 50 years ago, he told me these are in a "dead" vacuum area in the throttle body, thus no air (and no dirt, as concerns many individuals) enter the air stream at this point. Think of these as "pressure relief points". During idle and/or low speed conditions during hot weather, the rapid atomization of fuel can create pressure in the throttle body rather than vacuum. This pressure would artificially increase the mixture, thus creating an overrich mixture with the possibility of stall. The vents relieve the pressure.

This link to my website may explain further: https://www.thecarburetorshop.com/Ventedgaskets.htm

Here is another link on adding tripower to a V-8 engine that may (or may not) help. https://www.thecarburetorshop.com/Aftermarkettripowersetups.htm

If this was a single two barrel application on a genuine GM intake just used for cruising, a carburetor from a Chevy 400 would work MUCH better than a carburetor from a 307. This would probably lean out the cruise AFR.

Without knowing the exact identification number, I did follow your lead and checked the 1971 307 Chevy truck carburetor which was not used on a 350. This may be part of your problem.

Contrary to popular opinion: too small a carburetor runs RICH, whereas too large a carburetor will run LEAN. At some point, the too small carburetor will just fall off rather than accelerate further.

If a carburetor is too small for the application, the venturi air velocity will be greater than normal, thus creating an excess of negative pressure at the fuel wells, causing excess fuel to be drawn into the system.

As far as cruising at 1100 RPM??? The carburetor is trying to transition from the transitory (off-idle) circuit to the main circuit, and is probably bouncing back and forth. Some different rear gears making cruise at 1500~1800 might work much better.

Good luck.

EDIT: and just think how much time and aggravation you would have saved by not putting a tripower on an over-square engine. Bolt on a dual quad intake, with, for your engine, a pair of genuine Carter 9500s carbs for mostly street and a little race, or a pair of genuine Carter 4759s carbs for more race with a little street. Straight linkage, choke on both, idle on both, both open simultaneously. Set the idle RPM and go play!

Jon