Misfire after 3k rpm ... need some help.

[chrisf695]

I have a 400 sbc thats giving me some misfire after about 3000-3500k rpm... It will cruise at 3k just fine pretty much but when I give some throttle it just starts missing. Motor has about 50 hrs on it and it has a 750 eddy on it. Plugs are gaped at .35 using NGK V power and running around 9.3 compression. Cap, rotor and wires where new 2 years ago.

Where should I start looking?

 

[Bt Doctur]

Re: Misfire after 3k rpm ... need some help.

missing from ignition or fuel delivery?
cross-firing
look inside the cap with a magnifying glass for carbon tracks

 

[chrisf695]

Re: Misfire after 3k rpm ... need some help.

missing from ignition or fuel delivery?
cross-firing
look inside the cap with a magnifying glass for carbon tracks

Cant really tell if its ignition or fuel... havnet had this problem before ever... Ill look in the cap tomorrow, would a compression test be something to check also?

 

[NHGuy]

Re: Misfire after 3k rpm ... need some help.

Has it always done this? Or is the problem new? What Edelbrock carb, is it a marine unit? And how is it jetted? You can normally get a jet assortment for most of their carbs. The kits come with a sheet that will help you decide which way to go.
But first make sure the ignition is right, and make sure you have good fuel pressure when you are calling for higher rpms. If the fuel pressure drops when you go to wide throttle openings deal with that first. That big motor might be calling for more fuel than your pump can send.

 

[chrisf695]

Re: Misfire after 3k rpm ... need some help.

Has it always done this? Or is the problem new? What Edelbrock carb, is it a marine unit? And how is it jetted? You can normally get a jet assortment for most of their carbs. The kits come with a sheet that will help you decide which way to go.
But first make sure the ignition is right, and make sure you have good fuel pressure when you are calling for higher rpms. If the fuel pressure drops when you go to wide throttle openings deal with that first. That big motor might be calling for more fuel than your pump can send.

Its never done this up til a few days ago... But its a edelbrock 750 marine carb, no idea on its being jetted. So timing needs to be checked, and plugs, compression test. If it was fuel wouldnt this had happen earlier... has over 50 hours on it already.

 

[thumpar]

Re: Misfire after 3k rpm ... need some help.

What kind of ignition system is it?

 

[chrisf695]

Re: Misfire after 3k rpm ... need some help.

What kind of ignition system is it?

I'm running TBI IV... Im gonna change the spark plugs today and do a compression test and go from there. Ive run the new motor kinda hard this summer... but I did my oil changes every 25 hrs and its never overheated.

 

[NHGuy]

Re: Misfire after 3k rpm ... need some help.

Well since it has run well til now... Check your separator/fuel filters, pour the contents into jar and check for junk. Replace with a new separator filter. If it has junk go test drive it. Pull the carb entry filter too. If it's stone or screen just wash it, if it's paper replace it.
How do the plugs look? How are your wires, cap, rotor? How about the fuel delivery system? Has you timing moved?
It's gotta be some stupid thing.

 

[FreeBeeTony]

Re: Misfire after 3k rpm ... need some help.

I had a similar problem once. You say it only has 50hrs on it....was it rebuilt?

 

[chrisf695]

Re: Misfire after 3k rpm ... need some help.

The motor was completely rebuilt... everything new. I changed the water sep about 25 hours ago, wires look good but they were laying on a ground part of the block and all zipped tied to each other... not good. But the good news is that I replaced the plugs and now no more misfire... but the plugs I pulled out dont look that bad in my opinion... why would it not miss now? After putting the new plugs in I did a quick 2-3 miles at 3000-5000 rpm and pulled a plug, I know thats not enough to get more color but I was running out of day light. I posted the pics below. I also reset my throttle mixture on the carb back to how it came from the factory... Ill do some more fine tuning with a vacuum gauge tonight or tomorrow. But the motor seems to running good, actually better after the spark plug change before it was misfiring.