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P Lomas
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Discussion Starter · #1 ·
Bought this 67 Firebird with a 400. This came with an engine out of a 70 gto. The motor had some work done to it we weren't sure how much had been done so we removed it and the transmission. We installed new distributor (MTD), plugs, wires, and had the heads redone. The intake is an edelbrock performance and it has a 650 quick fuel double pumper carb, and 268h cam. It ran rich from the start, I tried to replace the #68 jets with #64 and that seemed to help a little but now it backfires when it uses the secondary. Vacume is ok at 6.5 and timing is set at 18 before. I looked on line and some are telling me that if I have to bring the time to around 32 before that might be necessary because of the performance engine. It seems extreme to me but wanted to see if anyone else has any suggestions. Leaning towards replacing the carb all together.
One other thing, the motor distributor #1 is towards the intake and not back by the firewall like most of the diagrams I am looking at. I'm seeing that this is the case on some GTO's. I'm guessing this is the case with this motor?
 

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It sounds to me that you don't know much about carb tuning and adjustment and if I am wrong I apologize, but I am just going by your post information you posted. There are many factors that can cause it to be rich. Post some more information on what your problem areas are and some answers from some of the other questions from the other replies and hopefully we can steer you in the right direction.
 

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P Lomas
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Discussion Starter · #5 ·
Rich where? Idle, tip in? cruise? WOT? everywhere?
Whats heads?
Sorry if not specific enough. of course I cannot really tell how rich it burns while driving down the road. I notice it when I'm idling. The cylinder heads were rebuilt with new valves, springs etc. I've had a couple of home garage mechanics, like myself, tell me my carburetor was built more for racing than just street use. Like I said I am wondering if changing the timing will make a difference. Its my first performance engine and I know they need to be tweaked.
 

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P Lomas
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8 Posts
Discussion Starter · #6 ·
What does 6.5 vacuum mean?

If idle vacuum that is mighty low for a 268H cam. this would indicate something is amiss from vacuum leak to miss timed cam and a few possibilities in-between.

Bogie
Yes Idle vacume. I am learning as I go for sure and getting advice from people that have more experience than I. the vacume guage reads about 14" vacume at idle.
 

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P Lomas
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8 Posts
Discussion Starter · #8 ·
It sounds to me that you don't know much about carb tuning and adjustment and if I am wrong I apologize, but I am just going by your post information you posted. There are many factors that can cause it to be rich. Post some more information on what your problem areas are and some answers from some of the other questions from the other replies and hopefully we can steer you in the right direction.
Not offended. I'm trying to figure this out for sure. The motor runs well while running exept that its running rich eanough to chase me out of my garage after a couple of minutes, but since I've changed the jets (which has helped) it now occasionally backfires when I step on the gas at around 3500 rpm. Also the plugs are black and fouled from the improper fuel burn. I'm trying to figure out how to get the best performance.
 

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First you need to differentiate whether the cam is a 268H or the XE268H?

Next if the idle is rich and the idle mixture screws have little to no effect it is then highly likely that the curb idle screw is opening too much throttle. When this happens the transition circuit comes on. This feeds off the same source as the idle screws. The general state of carb tune is to have .020 to .040 inch of the transition slot exposed as measured from the bottom of the slot to the bottom of the throttle blades. Otherwise both idle and transition will feed fuel with no control available from the idle mixture screws. As a rule the closer this is to the .020 dimension the better. This is set with the carb off the engine as it cannot be seen once it is bolted in position. This is a common problem as cams get bigger, the 268 is walking on that edge. It’s a good idea to mark the throttle position on the carb that indicates this point of opening that produces the correct dimension of the transition slot exposure so you can get back to it again without pulling the carb off.

More advance may help get the idle speed up with that cam around 12 to 16 degrees plus full time vacuum advance of about 7 to 10 degrees at idle. It needs to be remembered that most distributors are designed around having 10 to 12 degrees static base. Getting beyond 12 degrees usually requires modifying the centrifugal advance so that the total an peak RPM is 34-38 degrees keeping in mind that vacuum advance goes away as the throttle is opened so at WOT max RPM the vacuum system is not contributing any advance.

Ticking up the static advance as outlined plus the vacuum will speed the idle up. If this doesn’t work then other steps to get more air into the engine at idle besides opening the curb idle has to be found. This varies from drilling small holes into each primary throttle plate to replacing the carb with one that has 4 corner idle adjustment.

Bogie
 

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P Lomas
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8 Posts
Discussion Starter · #10 ·
Check float levels.
Adjust idle mixture screws.
Report back with the results
Yes I did that and called Holly to make sure I was doing it correctly, talked to a tech there and he walked me through it. I am confident I have that part of it correct. its a double pumper quick fuel 650, float levels are at 1/2 the sight glass all the way around and mixture screws are at suggested factory settings.
 

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More for Less Racer
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Main jet has no influence on idle mixture or transition mixture, the main jets are only in play above about 2200-2400 rpm.
All you've done by changing the jets is make the WOT mixture so lean the car backfires.

To alter idle mixture you need to change idle feed restriction size and possibly idle air bleed size. Not knowing what exact carb model you have, I don't know if those are screw-in and easily replaceable, or if they are press-in.

Quick Fuel mechanical secondary carbs come with a rich idle calibration, they seem to think everyone buying a mechanical secondary carb is a racer.

Once plugs are fouled, you need to replace them....they won't burn themselves clean, they will just fail to react to any change you make to the carb, leave you chasing your tail.

Do you know the compression ratio of the engine??
Pontiac iron heads have a real problem with pump gas above about 9.25:1 compression, they are very detonation prone.
The '70 GTO engine would have been high compression, 10.75:1 I believe.
 

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Any engine with a performance cam that has any sort of lope to it and less then 18 inches of vacuum will always to a degree have a rich idle on it to a point especially the bigger and more overlap the camshaft has will cause a lot of different effects which I won't get into things here but the carb signal is reduced and thus the idle has to be a richer fuel curve in order for it to function and feed the engine.

I have a brother who has a bone stock late 90's Mercury Grand Marquis and a tiny 4.6 v8 that has seen its last days but still runs and it has tan burning plugs and is fuel injected and has all emissions stuff on it and he has an exhaust leak before his mufflers and if you stand by it long enough especially in a garage you would swear its running pig rich but it is far from it. It will make your eyes water and you breath in enough of co2 and make you choke.

Bottom line that is a normal thing and yes you can see signs of a rich exhaust by seeing either blue smoke or black smoke if its real bad but just your eyes watering and choking the way you describe does not mean it is.

Now with that out of the way here is a good idea on what to do on tuning your carb and some of this info might not be helpful towards your carb as I copied this from another post and its a lot to type over and over again but hopefully this should give you a direction on what is needed to do.

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This is how I do things.First thing is to make sure your timing is good and correct and that your timing marks and your TDC marker is correct for your balancer and timing pointer your using. The bigger the cam the more timing it will need.

Do a search for timing and it will give you some more info on that. This is after you at least get your engine up and running before tuning your carb and staring off with getting the initial timing set and make any necessary changes as needed before going in depth with your carb.

Making sure the ignition system is up to code and proper spark plugs are being used and proper gap for what the engine needs for a good starting point. Fuel pressure checked and verified that its not too much and no problems from there. When all those things check out then I start off with writing things down on my starting adjustments on my carb.

You do one circuit on the carb at a time and then after one is done you move on too the other and then get it as good as you can dial it in and at times when you get one circuit done you might have to go back to do another tweak to the other so they can all transition from each other from one situation to another with as smooth of a transition as possible with out any issues.

I start off with the idle circuit and then after the the pump shot circuit and make changes to each as needed and then after that is dialed in then the transition circuit in relation to the idle circuit. Once that is all dialed in then I proceed to the main circuit and jetting and make changes as necessary and then after that is done I proceed to the secondary opening tuning and then finally the power circuit and make adjustments to the jetting front and rear along with power valve sizing.

On your carb transfer slot exposure is very critical to the tune on the idle circuit and the idle mixture screws and also the off idle performance of the carb.


When starting off I always will close the primary butterflies all the way and then slowly turn the primary idle screw in until it starts to make contact and open up the throttle plates. I will then see how many turns it takes to get about .020 transfer slot to show. I write that spec down and then open it up more to about .040 transfer slot exposure. I then write that amount of turns down. The goal is to have the proper amount of transfer slot exposure needed for proper idle and idle mixture screw setting without it being open too much or too little.

You don't want to have it without any transfer slot showing as it will cause off idle stumbles and affect the pump shot tune. You can have as little of it just starting to show and you don't want to get past about .040 as after that point estimated of course, then the idle mixture screws will have issues or not work at all and a pig rich idle. Transfer slot length will vary also from one base plate style of one Holley compared to another. That will also affect things as well on setting it.

On writing down the amount of turns what I do is see first how many turns I can go past contact on the primary side and an example is a 1/4 of a turn would be a minimum amount to where the transfer slot is just starting to show and a 1/2 a turn would be about .020 and then 3/4 of a turn would be about .040 or to where the transfer slot is starting to become rectangle and no longer square. I always make sure that I stay within that range.

If I have to go past the 3/4 of a max turn for example by say a 1/4 of a turn, then I would open up the secondary side a 1/4 of a turn instead in order to get the proper idle setting I need and too keep the transfer slot setting in a good range of what the engine needs for the best idle. I always start the secondary side at half a turn open on bigger cams like yours to start off with and most of time depending on things it can end up open up more. It just depends on the build and what it needs and the carb being used etc.

That is setting that part. Then you want to adjust your mixture screws and I use a vacuum gauge and also by how the engine responds and does in order to get the best setting starting off on the mixture screws. If its a two corner idle I put them at 1.5 turns out and if its a four corner idle then I set all four at 3/4 of a turn out. I start off with turning the driver front mixture screw in to where the vacuum is the highest and also to where the engine will start to studder and shake some and then back the mixture screw back out a little bit to get it to be smooth again and paying attention to the vacuum signal. You can't make it have the highest vacuum reading as it will lead to it being to lean at times on anything outside of a bone stock engine with a very mild camshaft.

You have to have play around to see the best setting you can get. You want to have a good adjustment range on the mixture screws and on a two corner idle you want to be able to run them around 1 to two turns out on average and on a four corner idle you want to have it be able to run around half to one turn out with being equal on all four but they are way more sensitive versus the two corner idle type. That is not etched in stone but you want close to that. You also want to check the mixture screw settings with putting it into gear and see if it is set to lean. I always set it for the leanest I can get on the engine while in park and then put it into gear and see how it does.

If its too lean and struggles when going to drive I put it back in park and adjust them a hair at a time until I can drop it into gear without any sputtering and it wanting to quit. After going back and forth and finally nailing it down for the best setting I look at my final setting to determine if I need to note if there is a possible chance I need to either make my idle circuit richer or leaner. You will learn with time on adjusting your carb if you need to do so as while setting things if its very close it should not be too rich or too lean and if you have to have your transfer slot super exposed and your butterflies way open then you most likely have to lean of a idle circuit or if your needing to drill holes without having an over big sized camshaft in which in your case you should not have to do such a thing.

If its too rich and you can' get much adjustment out of your mixture screws and the butterfly setting is still correct like I stated above and your plugs foul out in short time and they are the proper heat range and your fuel pressure is good then it might be too rich. With bigger cams like yours its very tricky to get the sweet spot right off the start as certain symptoms can throw you off and experience and knowing how your carb operates and functions is very critical on how they can be tuned and made to perform well.


The bigger the cam is and the more overlap it has and the lower the vacuum it makes then the smaller of a window you will have to get the right tune for it especially on the idle circuit as the big cams have such a small window on what it will allow for not being to lean and also not getting to be too rich and fouling out your plugs and getting bad fuel mileage and a poor idle setup.

Now all those things thrown out there after you have determined if you carb might need to have the idle circuit made either richer or leaner first by how it is operating on settings and then after that you will have to do a few test drives and see how it operates and checking plugs. Now like stated its hard to go buy some things on the plug readings as they can throw you off at times and I just do a general guide line by looking at them and giving me a picture but now the whole picture. I also go by how it drives and performs.


I could post more on that but that is a lot more to get into detail. If on your holley carb on the metering blocks you may have pressed in idle feed restrictors and they can be taken out and your blocks modified to take brass allen screws and its super easy if you take your time. You can buy a drill bit gauge set off ebay for around 15 bucks and you can get brass allen screws from mcmastercar.com in the 6/32 size and you can tap your metering blocks and use them and drill them to size in order to tune your idle circuit without buying quick fuel billet metering blocks with changeable idle feed restrictor options.

If you have the money and don't want to customize your blocks if there the non adjustable type then buying the quick fuel blocks will give you way more tuning adjustment ability and allow you to fully dial in your carb better vs the stock options.

After you get your initial idle circuit worked out for the starting point and your mixture setting is good for starting off with then you want to move on to the pump shot circuit. When tuning the pump shot circuit its a bit of a trial and error part. The thing is to determine if your pump shot is either too rich or to lean. Start off with the stock shooter and pump cam and see how it does in different situations. Now the tricky part is the idle circuit at times can add to a stumble on the pump shot and its not always a pump shot issue a hundred percent of the time if there is an off idle hesitation or lean spot. The transfer slot setting and also the idle mixture screws adjustment will play into things as well and experience will help you to adjust each spot as necessary in order to get them all to work together well.

On the pump shot I first start off with the shooter size and if you have an off idle stumble then you need to go up 3 sizes bigger in order to get a change in calibration. The pump shot tuning goes like this, the bigger the number then the quicker and faster the pump shot will happen and the smaller the number then the pump shot will happen a little lesser at first and have a longer duration of time per stroke of the throttle.

If you have a off idle stumble for example and you go up in the shooter size and its still there then you might need to change out the pump cam and go with a profile that gives a bigger shot. On the pump cam chart below you will see the different pump cam rates and the way to look at them is the more cc's it as per stroke the less of a initial shot it will have but more travel of duration of throttle opening. The lesser cc's of stroke per throttle opening then the quicker the pump shot will happen and a bigger initial shot will happen but with less throttle opening.

The way with tuning that part is you have to back and forth to fine tune to get the best shot you can without it being too much and puffing black smoke out the tailpipe and bogging it down. After that is done I move on to the main jetting.

That is kind of a bit different from one vehicle compared to another and won't get into a whole lot but I just basically start off as lean as I can get and then go back up a few sizes richer to get it as close as I can for the best mileage. I then go on too tuning the secondary side.

I start of with only making sure I have no bogs and hesitations on the primary pump shot first before even getting into wide open throttle and also opening up the secondaries on a double pumper. I make sure the pump cam and shooter size are correct for the primary side and will even disconnect the secondary side if needed to get it nailed down so the secondary side opening up won't affect things. On a double pumper you need the primary side all set in and dialed good so you can work out any bogs and hesitations on the secondary side if they occur and to get the best optimal pump shot you can.

I always start off with a middle of the ground size shooter which would be around a 31 shooter and depending on the pump cam an orange one if the vehicle is light and picks up speed super fast and the rear gearing is on the steeper side. You want to tune it just like you do the primary side but you will have to do some quick wide open throttle shots to see what you need and you want to make minor changes at a time. Go up or down about .003 of a size on a shooter size as needed in order to get a change.

You will gain experience in time telling if you need to adjust the pump shot in relation to hitting wide open throttle and having it transition smoothly with the secondary side opening up as fast as possible in relation with the pump shot. After you get that done then you can go on to the secondary jetting and the power circuit and the power valve power valve channel restrictor size as well if you were to get a set of quick fuel metering blocks to where you can change out the restricor size in the power valve channel and ultra fine tune the power circuit if you can change the power valve channel restrictors i the primary block.

I would recommend a book on Holley tuning that I posted below as it will help you understand how things work on those carbs and the more knowledge you have the more easy it is to tune things. Sorry for the long post but I just want to help you out in put you in the right direction."



This was from another post and I changed some things as it was a vacuum secondary carb but the front side tunes the same and the secondary is like doing the front but slightly different but not by much. Hope this will help some to start you off.
www.amazon.com

David Vizard's How to Super Tune and Modify Holley Carburetors (Performance How-To): Vizard, David: 9781934709658: Amazon.com: Books
David Vizard's How to Super Tune and Modify Holley Carburetors (Performance How-To) [Vizard, David] on Amazon.com. FREE shipping on qualifying offers. David Vizard's How to Super Tune and Modify Holley Carburetors (Performance How-To)
www.amazon.com
 

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Did your machinist set your valve stem height ? Many don't understand that unlike a Chevy , Pontiacs of that vintage had xa non adjustable valve train IIRC . If stem height wasn't set , you could have valves not closing completely .
 

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1949 Ford Coupe RESURRECTION
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They are a good guideline but will also lie to you sometimes. Like any other tool, they do a good job when the operator understands what they are seeing and doing with it.
Well, it beats the hell out of a bunch of guesswork... And if the owner doesn't understand the readings, he or she will never ever get very much correct in life.
 

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Well, it beats the hell out of a bunch of guesswork... And if the owner doesn't understand the readings, he or she will never ever get very much correct in life.
Lots more to tuning an engine than reading a digital screen . Some of us got along fine without them for 3/4 of a century .
Granted , we may have been a bit slower , but we got the job done & learned just the same .
 
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