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Discussion Starter · #1 ·
So whilst doing a service on the rod & hunting for an exhaust leak I discovered the plugs on 7 & 8 cylinders are fouling "black slightly rich"
All other cylinders look spot on!

Has anyone else ever had this issue ?
Im running a dart shp airgap manifold 2" spacer & holley 670 Street avenger carb with 68/70 jets.

Thanks
 

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Rollers are not without failure modes but they are usually pretty brutal and noticeable.

I also would remove the spacer having one is not a guaranty of improved performance, some engines like ‘em probably most don't care to just don’t. Your driving habits impact this as well if the engine spends most of its time at WOT the extra plenum volume can be helpful up on the top end. If this is a street driver where the induction system is not taxed toward its max capacity nearly all the time then adding a spacer which add plenum volume only serves to drop flow velocity which in turn drops fuel out of suspension then it runs usually toward the lowest cylinders. Since most engines are tipped a bit to the rear manifolds are usually cast to level this up but you need to check and see how you’re installation sits.

Airgaps are rather sensitive to where you live. The southwest and deep south not too much of a problem. If you live where the weather is cool to cold especially if damp these manifolds can be a headache.

Trying to drive a racer on the street is always a PIA especially without a stick gear box that makes it easy to put the trans in a gear that winds the engine up to where it climbs out of these low rev bad habits of these type engine set ups.

Bogie
 

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Discussion Starter · #11 ·
Well rain stops play ... I drove the rod to work with the view of pulling the spacer & cleaning rear plugs ,however it looks like the k&n 14 x 3 filter overhangs the hei distributor...
I may have a filter spacer at home but uncertain. If.the spacer is ported would this reduce the chance of the fuel dropping out of suspension??
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Just out of curiosity what is your cam specs and rest of your engine build? I have a Trick flow/Edelbrock RPM air gap intake and I am having some werid fuel distribution issues and my worst two cylinders are the two front cylinders that will foul out in very short time and it gets lesser as the cylinders going back and no amount of carb change and calibration and even using different carbs is getting rid of the problem.

All things have been checked on mine such as the ignition system along with valve train and other stuff and mines a new build. Mine starts off super foul rich the two front cylinders and then slightly lesser on numbers 2 and 4 then lesser yet on 5 and 6 and the leanest on 7 and 8. By going through my firing order when it fires on number one it will be pig rich and then on number eight it will fire way leaner. Mine is so weird that in believe it or not I had it out the other day in 20 degree weather with the wind chill at near zero and for some abnormal reason it ran the best it has ever been since I had started to trouble shoot the problem.

Most likely because a carb will run leaner in the cold versus in summer months when it will run slightly richer but how much of a difference I don't know. I have another posts elsewhere that you might want to read up on and it might help shed some light on your issue and maybe you have a similar problem like I have but I don't know as I have not gotten my dialed down yet but it comes down to the intake not liking my camshaft specs for some reason.

I am from Ohio so it gets really cold up here but I used an air gap intake before with a bigger sized cam and all year around and never had any problems. I tried different spacers and it had a really abnormal effect and something is going on with my intake and the signal its getting and the air fuel ratio is some how going all over the place up and down depending on throttle position and certain driving factors. I am getting ready to replace my intake with a non air gap and see how it does. I have always used a four hole spacer with a dual plane and it always worked excellent on any of my builds from mild to wild without any ill effects. Good luck on your quest and hope you find out what the problem is.
 

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Not saying this is your probem but it is very similar to what I am experiencing and my cam is very similar to this guys shaft he had and my spark plug readings are just way abnormal and the firing order shows the problem its having going from super fat rich to leaner to fat rich to leaner on each firing cycle on mine. I have ran many different cam and intake combos over the years with a Holley 670 street avenger and other carbs and never encountered this sort of abnormal problem till now.

On my current engine I have a Holley 600 vacuum secondary just like your street avenger with all the same features as yours and I have calibrated mine and its just not running right period and the problem does it with any carb I stick on it. Something about those air gap intakes that the cut down plenum can just cause chaos with some camshafts depending on certain factors. They seem to not like certain combos and is more then just temperature factors coming into play from my research. If yours is the dual plane air gap intake. I previously used an Edelbrock Victor Jr intake on my previous build which was a 377 cubic inch Dart shp build and it ran excellent and fine but once I changed and put my slightly milder cam on and with the air gap dual plane intake I have had problems since day one and no carb adjustment on four different carbs have changed the problem and I have tuned carbs enough to know when its not a carb problem and my plugs are showing an abnormal pattern of an air fuel ratio going from cylinder to cylinder in the firing order and other factors.

 

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Discussion Starter · #14 ·
Just out of curiosity what is your cam specs and rest of your engine build? I have a Trick flow/Edelbrock RPM air gap intake and I am having some werid fuel distribution issues and my worst two cylinders are the two front cylinders that will foul out in very short time and it gets lesser as the cylinders going back and no amount of carb change and calibration and even using different carbs is getting rid of the problem.

All things have been checked on mine such as the ignition system along with valve train and other stuff and mines a new build. Mine starts off super foul rich the two front cylinders and then slightly lesser on numbers 2 and 4 then lesser yet on 5 and 6 and the leanest on 7 and 8. By going through my firing order when it fires on number one it will be pig rich and then on number eight it will fire way leaner. Mine is so weird that in believe it or not I had it out the other day in 20 degree weather with the wind chill at near zero and for some abnormal reason it ran the best it has ever been since I had started to trouble shoot the problem. I have another posts elsewhere that you might want to read up on and it might help shed some light on your issue and maybe you have a similar problem like I have but I don't know as I have not gotten my dialed down yet but it comes down to the intake not liking my camshaft specs for some reason.

I am from Ohio so it gets really cold up here but I used an air gap intake before with a bigger sized cam and all year around and never had any problems. I tried different spacers and it had a really abnormal effect and something is going on with my intake and the signal its getting and the air fuel ratio is some how going all over the place up and down depending on throttle position and certain driving factors. I am getting ready to replace my intake with a non air gap and see how it does. I have always used a four hole spacer with a dual plane and it always worked excellent on any of my builds from mild to wild without any ill effects. Good luck on your quest and hope you find out what the problem is.
Hey there . I live in uk , so temps range between 5c & 30c in the summer so never as extreme as you get I the states.
My engine specs are as follows :
Sbc gen1
+40 bore flat top pistons
Edelbrock e Street heads
9.5.1 compression
Howards roller cam conversion
280/288 lift .525/.530
Comp cams ultra gold 1.5 roller rockers
Dart shp dual plane Air gap manifold
Holley 670 Street avenger carb with 68/70 jets
2" 4 port carb spacer
Performance hei dizzy
1/34 medium lengh headers in to dual 2.5 pipes
Guessing about 400hp so not a crazy spec motor.
 

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Here is my post of my problem which is the weirdest thing I have ever dealt with in over twenty years.

 

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My engine is not very crazy as well and if you read my post you can see my specs there and I sure hope you can find out what your issue is. I am taking my air gap intake off and I have used a four hole spacer with it on a previous build and it worked just fine and with no problems. It seems to me at times I wonder depending on the build if the cut down plenum is causing a negative effect of causing the fuel to come out of suspension because of the sharp wall that is cut down in certain situations and stuff and just the way the design of the runners and the air factor going beneath the plenum causes erratic air fue ratios in certain situations. I know on a dual plane intake the cylinders will vary some but what I am going through and seems like what your going through is just out of the normal. If all your stuff checks out.

I had checked everything on the valve train and the cam which is a billet roller shaft and nothing radical and the whole ignition system and even compression checks and other things and it boils down to the intake just not liking the cam I have and working with it and has the same identical problem on a 377 and a 350 build and no amount of adjustment or changing the calibration on my carb will get rid of the issue. Using a four hole spacer makes it worse and using an open spacer makes it better and helps my engine run better and the problem is a lot less there but still a problem none the less. If you check the one link I posted above I have nearly 75 percent of an identical problem like the guy had on his 383 with using a spacer etc.

I have ready others having issues with the air gap intake causing such fuel distribution issues that after swapping intakes to a non air gap style and running a four hole spacer or an open spacer it ran fine so go figure. I will not use an air gap intake ever again on my build after I can get some good luck and time to get mine swapped out and it fixes the issue.
 

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Discussion Starter · #18 ·
Hi all , so been busy tinkering so not had much chance to post.
So I've now fitted fresh plugs & gone up a heat range to ny12yc which edelbrock recomend to use with their heads & also removed the 2 inch carb spacer to rule these factors out.
Performance wise it feels a little flatter but still pull strong. I took it for a good blast to the local rod meet this weekend then pulled the plugs to inspect. Still a little sooty, but look to be running a bit cleaner " however they look to burn darker as you go backward from 1+2 - 7&8 ! ?
Ill need to put more miles them to confirm. Whilst out On this run I noticed a part throttle intermittent miss i could almost describe it as a flutter . when cruising at 1200-2500 rpm. Very quiet & un pronounced but their
At this point I checked the leads & discovered the no: 8 plug boot was dry & split through. Which I presumed was causing this , so I've now shelled out in a new set of msd leads I fitted today making sure to rout as cleanly as possible separating the new leads so that they are not sat on top of one another.
Took for another tube & said issue is still present , however if I put my foot flat to the floor the car takes off like a rocket cleanly all the way to 5k+
Really frustrated as money is tight. Any further thoughts on this would be appreciated.
 
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