New 351w Fired up Today but 7 and 8 not firing **Solved**

[Ol'Blue]

Happy 4th everyone! My plan worked out to fire up the new 351w on the 4th of July however its definitely not firing on all cylinders. Lazer temp on 1-6 all read in the mid 300 degrees but 7 and 8 closer to 200. I checked my firing order and spark plug wires several times all checks out.

Im thinking it might be spark plug gap. I have a new Pertronix distributor and coil. I read that .050 gap was recommended, but I think that's too much?

Its too hot right now to reduce the gap and try again so thought throw this out there for any other suggestions while I wait for it to cool down.

Besides this issue everything else went well.

 

[DirtDonk]

Some have run that large of a gap, but for a first time out and looking for a reliable combustion, I would go down to 45 and see if anything improves.
Also, seven and eight might be having an induction issue. What about the brake booster and PCV valve? Where do you have those plumbed?
Is it EFI or carburetor?

 

[Ol'Blue]

It’s seems to be something with the timing. It ran smooth on initial start up but when I set the timing at 10* at idle that’s when it started running rough. I turned the distributor clockwise and it smoothed out at idle and driving but now I’m reading 25* advance. Maybe I installed the distributor wrong?

 

[Ol'Blue]

Some have run that large of a gap, but for a first time out and looking for a reliable combustion, I would go down to 45 and see if anything improves.
Also, seven and eight might be having an induction issue. What about the brake booster and PCV valve? Where do you have those plumbed?
Is it EFI or carburetor?

Hey Paul, I already rushed to gap 40 and that didn’t change anything. I’m thinking timing. No brake booster but I do have a new PCV. Everything new except quadrajet which ran fine on the 302.

 

[DirtDonk]

Anything is possible, but it doesn’t sound like it.
Ignition timing is ignition timing. If you’re reading a particular degree on the pointer and damper, assuming it’s correct, changing the distributor “location“ isn’t going to change the fact that you’re still going to have to adjust the timing to read a given value on the marks.

But that leads to another question(s). What damper are you using?
And following that up, what front accessory drive, what water pump, what timing pointer, and all of that stuff related to ignition timing, are you using?
Remember how often we’ve talked about the timing marks being incorrect on our engines.

Did you verify TDC with the marks and the crank when you were putting the engine together?

 

[DirtDonk]

The PCV valve is connected to the carburetor, correct? And not to just an individual runner on the back of the intake manifold?
I see the valve in the cover, but I can’t see exactly where the hose picks up the vacuum.

 

[Ol'Blue]

Damper is Proproducts 80006. Water pump is Flowkooler from your shop, timing pointer I copied my 302 (pictured blue block), actual pointer (pictured at 10* advance) distributor is Pertonix D131700. I had the long block built but did install everything else. The PCV is connected to the front of the carb.

https://professional-products.com/p...y-small-block-ford-28-4-oz-in-harmonic-damper

https://pertronixbrands.com/product...gnitor-ii-technology-vacuum-advance-black-cap

 

[DirtDonk]

Having only two cylinders act different, especially when they are next to each other, does not sound like a timing issue in general.
Sounds like a fuel distribution issue, a spark issue on those individual cylinders, or something along those lines.
Check your plug wires to make sure that seven and eight are in their correct spot, check your plug wires to make sure they are not crossing next to each other for too much of their distance, and check that none of them are touching the headers or otherwise burned or chafed.
Those are a few things you can do right now.

I would still want to verify TDC and the pointer however. You can get it pretty close by bringing it up to zero and making sure that the piston in number one cylinder is at the very top of its stroke.
Sticking some wire in the spark plug hole or something like that to feel the top of the piston.
Better yet is a piston stop, but it shouldn’t be specifically necessary.

What firing order are you using? Standard 351, or early 302?
If standard 302, then seven and eight fire one after the other.

 

[Ol'Blue]

Checked all plug wires a few times and separated some, none are touching and made sure they were not twisted etc.

Firing order is 1-3-7-2-6-5-4-8 and verified with my comp cams 35-512-8

It really runs smooth when I advance the timing and rough when I bring it back to 10* at idle.

 

[Bajabrewer]

Are you disconnecting & plugging the vacuum advance when setting the initial timing? Also make sure the 7 & 8 plug wires are seated in the cap all the way & on the plugs all the way.

You'll get this!

 

[Ol'Blue]

Dang, I didnt disconnect the vacuum and plug.

I checked TDC on #1 several times and somethings definitely up. The HB has two sets of timing marks and is at around 45* on one and 10-15* short of 0 on the other set of timing marks when at TDC. I plugged the vacuum and set the timing close to that area and seem to run better but very hot.

Also, it sounds like I have some valve noise or something around cylinders 7 and 8. Maybe the builder didnt set them right.

And low on power but thats probably expected with the issues im having.

 

[bmc69]

Dang, I didnt disconnect the vacuum and plug.

I checked TDC on #1 several times and somethings definitely up. The HB has two sets of timing marks and is at around 45* on one and 10-15* short of 0 on the other set of timing marks when at TDC. I plugged the vacuum and set the timing close to that area and seem to run better but very hot.

Also, it sounds like I have some valve noise or something around cylinders 7 and 8. Maybe the builder didnt set them right.

And low on power but thats probably expected with the issues im having.

New cam and lifters?..flat tappet?

 

[Ol'Blue]

New cam with hydraulic roller lifters and roller rockers. Motor is a 1995 roller motor.

 

[Broncobowsher]

I would do a compression check. Might have a couple low holes. I would guess a valve too tight and hanging open. Just that one bank, 5-6-7-8 will tell you.

I know you said you double checked the firing order. I would still take all the plug wire off and start over again. I was sure I had it right for about 2 months. Fought and fought it. I had it wrong.

You could also swap those plug wires to a different location (move the whole wire). Try to rule out a bad wire or two.

 

[Broncobowsher]

New cam with hydraulic roller lifters and roller rockers. Motor is a 1995 roller motor.

Stock valvetrain? rebuilt heads? Lifter preload is adjusted by replacing the pushrods with longer/shorter ones.

 

[Brush Hog]

Maybe carb issue? I had valve noise, rough idle, ran hot. Finally realized I was running the carb way too lean. Once I gave it more fuel all those went away. Prior to adjusting the carb I kept adjusting the timing and would get improvement with it way advanced.

 

[73azbronco]

Ditto compression check before going any further, cheap, except lost time. Do all the cylinders.

Carb wont effect just two cylinders.

Unless wires not seated in cap, wires or routing wont make a difference.

 

[DirtDonk]

I agree that normally the carburetor won’t affect just two cylinders. But in this case, seven and eight are next to each other, and both are fairly notorious in the old days for not getting the same combustion mixture as the other cylinders. Manifold designed partially, but perhaps it was more a function of the old 302 firing order. I don’t know, I just remember lots of talk about seven and eight being a problem.

So to the previous point, brought up by Brush Hog about it being too lean, if it’s borderline to lean (or even too rich perhaps?) the two cylinders could be burning even less efficiently than the others.

I don’t know any of this for a fact. Just know that it feels plausible. One possible scenario…

 

[Brush Hog]

What cam was installed? I remember reading somewhere that if the cam installed had a 302 firing order that could cause problems.

 

[DirtDonk]

I’ll have to go back and reread, but I believe the OP mentioned a comp cams part number with the 351 firing order. Unless I’m thinking of another thread.
Bill, with the state of cam manufacturing these days, it wouldn’t hurt to verify with fingers over the spark, plug holes.
However, all that said, they mention it running extremely well under some conditions. Getting four cylinders out of firing order would lead to all sorts of other running issues. so I would think that the firing order is correct at this point.
Trust, but verify!