Inconsistent rpms

[Basin_Runner355]

I have a built Ls motor. Carbed with the msd 6014 box. Motor had 2 inch primary to 4 inch collector.

My motor was turing 5300 dead push on the trailer with big headers. I put my new Norman clay headers on 1 3/4 to 3 inch collector dint change anything Else Gained almost 200 rpms. On timing curve ls2 ho on the box
Switch to the custom timing curve the builder had. Lost rpms no big deal.
Put it back on ls2 ho same setting it just turn 5500 on. Is it’s turing 5300. Where could theses inconsistencies be coming from 200 rpm on the big end is a lot. I have a afr gauge when it turned 5500 on the trailer afr was at 12 even. It’s floating around 12-12.8. But I think I have a bigger inconsistent than .8 afr to make that big of rpm swings.

 

[Slidin Gator]

Basin,

You lost me in the weeds.

LS2HO timing on trailer = 5,500

What's the difference when it turns 5,300? Is it a different timing selection on your MSD or are you saying it varies 200 RPM with no discernable differences?

 

[Basin_Runner355]

Correct change noting and it’s varying 200 rpms I just moved my ground to my battery I had it on the engine stand.

 

[Slidin Gator]

Correct change noting and it’s varying 200 rpms I just moved my ground to my battery I had it on the engine stand.

So yah, first thought is dropping a cylinder on ignition or intake leak. If popping or spitting lean (lean, get it) towards leak. You got carb system, make sure you don't have vacuum issues there (replace all vacuum plugs etc.).

1-2 cans of ether to check intake tightness, then a good inspection and dielectric on the ignition, I'd probably swap in new plugs just to save the head scratching.

Are you running a coil on plug or single coil setup?

 

[Basin_Runner355]

It’s not leaning out holds 11.8-12.0 afr

I already checked for intake leaks sprayed some carb cleaner around everything while it was idling no change in rpms.

I’ll check vacuum caps they all less than year old and boat sleeps in a shop don’t sit in the sun.

I did move my ground for the msd box to the battery had it on the engine stand dint try it again yet and I have what I believe is coil on plug all the coils are on top the valve covers right over each plug. I pulled all the plugs 6 all look to be a good burn. 2 were clean 4&6 cylinders. Check compression on 2 4 6 8 and tney all consistent 185-190 lbs compression.

 

[Slidin Gator]

You got coil on plug. 4&6 Clean ones ain't sparkin all the time is my new guess. Check wiring to coils. I'd put new plugs and swap some coils around. Run again, see if new looking plugs chase the coils from 4&6.

 

[Basin_Runner355]

I’ll mess with it this weekend and see what I find. I’m leaning towards ignition my self. Only thing it can be. Got consistent fuel and consistent compression. Gota be fire

 

[Basin_Runner355]

If I use my digital timing light that reads rpm off the plug wire if it’s dropping a cylinder it should read zero correct?

 

[Slidin Gator]

As long as you are between coil and plug yes. No flash no spark.

 

[John Fenner]

4 and 6 are clean, wet with fuel I suppose, likely when you swapped out the headers could've cracked the insulators of the plugs, it doesn't take much to do so, swap out plugs, timing light will tell you if coil(s) are firing but if insulator is cracked it will jump to ground vs to electrodes and no fire.
An infrared thermometer to shoot header temps is a good tool to have.

 

[Basin_Runner355]

Yea I have a temp gun as well guess I’ll be busting out all the toys this weekend

 

[hdsadey]

This will let you know if the coil is firing. I doubt the MSD box is the issue unless there is some corrosion between the coil harness and the MSD harness. Could have a coil breaking down under high rpms. Should be able to find ohm specs for the coils and test with a multimeter.

 

[Basin_Runner355]

Found my problem I had one not firing. For sure other headers were around 150 degrees one was cold. But I found my issue when I pulled the coil. To swap it to see if if the problem followed it to a different cylinder. They had paint on the bosses so it wasn’t getting a good ground. Sanded them down put them back on and it’s firing. But I have two cylinders running way hotter than all the rest cylinders 4&6. The one that wasn’t firing was 7 I might have mixed up the plugs when I took them off the first time

 

[Basin_Runner355]

.

 

[Slidin Gator]

This will let you know if the coil is firing.

I buy 1 about every time I see one, stuff them in every tool box, damn things are handy. But I have had them tell me there was spark, but swap in new plugs and it fixed it anyway, so basically it tells you the ignition system is working, but like John said, I can be ham fisted with them things.

But I have two cylinders running way hotter than all the rest cylinders 4&6.

Glad to hear you found a simple problem! What is this based on, header temp? Bring the engine to full operating temp before drawing too many conclusions based off header temp. The go to advice is warm engine up, shut down and swap in new plugs while warm. Crank engine and go immediately to high power, hold it for a bit and then bring it down and shut off quick. Check/compare plugs, that will give you the best indication of fuel distribution and any lean cylinders.

4&6 is interesting, are you running the belt drive on this setup? Because STBD side middle cylinder runs the hottest on my air-cooled direct drive. Use IR temp gun to shoot the spark plugs and heads right around the plug to compare too.

Diagram to make things easy.

 

[Basin_Runner355]

Took the boat out yesterday and ran like a beast and never missed a lick. Also my my first time Running it with my AFR meter hooked up. That was interesting. But consistent stayed in 11.8-12.6 range. May be leaving some power on the table. Now that I got my inconsistencies lined out I Gota decide if I put wanna put my other headers back on.

Stay with the Norman clay 1 3/4 primary to 3 inch collector. Or go back to my alumitech 2 inch primary to 4 inch collector. Can someone shed some light on that Norman clay said that 3 in will flow enough until you get in the 700 hp range and I’m a ways from that at 540.

 

[Deano]

Stay with the Norman clay 1 3/4 primary to 3 inch collector. Or go back to my 2 inch primary to 4 inch collector. Can someone shed some light on that Norman clay said that 3 in will flow enough until you get in the 700 hp range and I’m a ways from that at 540.

You evidently don't realize that Norman Clay is THE Header Guy, not just a header guy.
Dave @ Water Thunder had Norman build his headers for years (most likely still does), and he is the only one I remember ever seeing Dave recommend where headers were concerned. Given that Dave is THE LS Guy, your question almost answers itself.
Generally, you won't need four inches unless it's a big block and turning a lot of RPMs. You would need to wind a 355 up like a tabby cat on crack before it would move that much air. Hence, I'd see no reason to doubt Norman's statement for a second.

 

[Basin_Runner355]

Deano. I was planing on keeping the 3 inch just wanted to see ppl thoughts cause over here they swear up and down bigger is better. But not always the case. And yea when it comes to ls or anything really if water thunder makes a statement i normally don’t think twice about and it becomes fact in my book.

Browsing some old fourms I did see where he said he wouldn’t run anything less than a 830 cfm on a 347 ci ls which got me thinking back on if the 650 I’m running now is hurting me.

 

[Basin_Runner355]

Im not running a 355 any more that was on my old boat Upgraded to a built 408

 

[John Fenner]

The 650 is fine, 408x5500 = 2,244,000÷ 3456=649.3055555556.
I would work on getting the AFR closer to 14, plug the power valve off, and start playing with jet sizes. All the while checking plug color as well.