Edelbrock carb flat spot

[jmeydenbauer]

I've got a '77 OMC 235 HP (Ford 351) with an Edlebrock 1409 carb that is stumbling just off of idle. I've followed the tuning procedures listed in the carb manual and just can't get this stumble cleared up, it happens ~ 900-1,200 RPM and runs fine everywhere else. I've tried every combo of .098 and .101 primary jet with metering rods 1447, 1444, 1445, and 1451. I've also tried the orange, blue, and silver step-up springs. I've checked my fuel pressure and it's at 6.5 PSI at idle. The carb is only ~ 1 year old and has always had the problem but I'm just now able to work on it because of other problems that the motor had which are now fixed. I don't have any vaccume leaks that I could find and I've got 16" of vaccume at idle. I've already taken the carb apart and cleaned it out with carb cleaner and compressed air. The carb was still very clean and it didn't make any difference. The ignition has been upgraded to the Pertronix II along with a new coil, plugs, wires, and cap. The timing has been set to 10 deg. BTDC as per the manual. The carb sits on a 1" 4-hole carb spacer with vaccume port<br /><br />The best settings that I've been able to come up with so far are:<br /><br />Primary Main Jet: .101<br />Metering Rod: 1444<br />Secondary Main Jet: .101<br />Accelerator pump: Middle hole<br />Step-Up Spring: Orange<br />A/F screw: 2 ¼ turns out<br /><br />The stumble is a lean condition at that RPM as a shot of staring fluid will cure the stumble and RPM's increase. If I try the shot of starting fluid at an idle, the engine dies (as expected). I am also able to move the point of the stumble up a few hundred RPM's if I adjust the A/F screw out richer than needed at idle.<br /><br />Again, the motor starts great, idles great, and revs great, it just stumbles when slowly increasing the RPM's through that 900-1,200 RPM range. Any ideas?

 

[WSUDERMAN]

Re: Edelbrock carb flat spot

The 1409 has 3 fuel control systems Idle, Primary & Secondary.<br />Sounds like you have a problem with the transition from the Idle System to the Primary Main System. I would guess that one of the passageways for air or fuel is plugged or blocked. Maybe the idle jet/tube got pinched.<br /><br />Couple of questions…<br />Does the stumble recover if you leave the throttle alone or only once you increase the throttle?<br />How is the timing advance curve? Maybe it’s advancing to quickly?<br />What does your vacuum gauge do during acceleration just before and during the bog?<br />Have you e-mailed Edelbrock tech support and what did they say?<br /><br /> http://www.edelbrock.com/automotive/eps_sect1.html

 

[jmeydenbauer]

Re: Edelbrock carb flat spot

78, you're correct ... it is the transition from Idle to Primary. That's the same document I used for my tuning.<br /><br />The stumble never recovers, if held at that RPM, it continues to have a lean surge as long as it's held there. Vaccume is solid at 16" at idle and then drops a few inches and bounces once it hits the flat spot. I've watched the timing and it looks to be advancing normally.<br /><br />I have emailed Edelbrock tech support and they are having me change to a larger nozzle and start my jetting over from the stock settings. Thankfully these things are so easy to work on that it shouldn't take too long. Hopefully that will do it and I'll post my results if it fixes it.

 

[John_S]

Re: Edelbrock carb flat spot

You have probably already tried it, going to the rich accelerator pump setting. It is the stock setting for merc's sbc weber equiped engines.

 

[jmeydenbauer]

Re: Edelbrock carb flat spot

yeah, I tried all of the accelerator pump holes. The problem is a stumble while the throttle is cracked open and held there so the accelerator pump isn't in use. Edelbrock had me change the nozzles but as far as I can tell, fuel only comes out of those when the accelerator pump is being used so that was a waste of $30.<br /><br />I will say that while Edelbrock hasn't been able to sove the problem yet, they are very quick to respond which is nice.

 

[Richard Petersen]

Re: Edelbrock carb flat spot

When you get a problem like that it is USEUALLY a blocked air or fuel or vacuum passage in that power circuit. High pressure may blow it or not into somewhere else. I hated to do those jobs. You are on your own.

 

[Richard Petersen]

Re: Edelbrock carb flat spot

Get a different carb. Borrow or beg one. A junker may be easier to live with.-------------Send it back to Eddy for a good one.

 

[rayjay]

Re: Edelbrock carb flat spot

Those Edelbrock carbs can be the biggest pain in the neck. Is this motor stock other than the carb ? A HP cam will create some weird carburation problems at low rpm. <br /><br />Otherwise you need a needle that is richer at the height that corresponds to the vacuum level of the problem rpm zone. I used to fool with q-jets and reading some of the hotrod info it was not uncommon to have to take a file to the metering rods to get the shape you needed. Of course there was not a range of readily available metering components for the q-jet like there is for the Edelbrock.

 

[rayjay]

Re: Edelbrock carb flat spot

I just read the Edelbrock tuning info and the springs have a real low HG value. They never mention anything about using these carbs in a constantly loaded marine type situation. The q-jets have a complex tapered needle instead of the Edelbrock's two step design.<br /><br />I wonder if you are even beyond the transfer slot when you are having the problem? You could take the boat out and starting from idle screw in the idle adjustment screw until the motor starts missing. Then shut off the ign and pull the carb and look at the position of the throttle blades in relation to the transfer slot in the bore.<br /><br />Should'a got a Holley.

 

[WSUDERMAN]

Re: Edelbrock carb flat spot

I am with cyclops on this. If you bought it new see if who ever you got it from or Edlebrock will exchange it. It should not be that much work to get it running well on a stock 351. Almost all 1409's are a bolt and go installation.<br /><br />As for it being a bad design, well thats like the ford vs chevy argument... not worth having.<br /><br />By the way if you want a holley, let me know, we are swaping a 1409 in on my brothers boat this weekend.

 

[jmeydenbauer]

Re: Edelbrock carb flat spot

Rayjay, the 1409 carbs are Marine carbs and I know that a lot of people here use them. Originally, I had planned on getting a Holley for the boat but decided to go with the Edelbrock because of all the good things that other people had to say about them and how easy they were to work on.<br /><br />I'm trying to find someone that has another 4-barrel carb that I can swap to verify that it runs crappy on their motor or put their carb on my boat to see if that fixes the problem.<br /><br />I'm not so sure about this tech guy from Edelbrock that I'm working with. He wants me to take the boat out and see if I still have the stumble with a load on the engine!? I've never heard of a stumble that goes away when you put a load on the motor, it's usually worse. Also, he had me change to a different size nozzle where the accelerator pump shoots out. First to a bigger one which made no difference and now to a smaller one. I told him that I didn't think this was going to do it since I only see that circuit being used while advancing the throttles not while the throttles are held stationary. He kind of got huffy & puffy about it telling me that that circuit is used everywhere except durring WOT. I could be wrong, I just don't see anything coming from these nozzles except while advancing the throttle.

 

[Richard Petersen]

Re: Edelbrock carb flat spot

I would hire you and fire him as a tech rep. Last week he was a Burger Slinger.

 

[LubeDude]

Re: Edelbrock carb flat spot

Originally posted by BigBird-20:<br /> He wants me to take the boat out and see if I still have the stumble with a load on the engine!?

I dont think he is as stupid as you think he is. I think you should try it on the water. You are not going to use it in that RPM range anyway, and if you do you are going to have a load on it and have a bit more throttle down. The accelerator pump will only push fuel when you move the throtle.

 

[Richard Petersen]

Re: Edelbrock carb flat spot

Were you given that carb free of charge or did it cost you some of your worked for money? Get a GOOD one. That's what you paid them for.

 

[jmeydenbauer]

Re: Edelbrock carb flat spot

Originally posted by LubeDude:<br /> I think you should try it on the water. You are not going to use it in that RPM range anyway

I am taking it out this weekend. <br /><br />I use that RPM range everytime I (un)load the boat or approach my destination(anchor/dock) or pickup a fallen wakeboarder or run through a no-wake zone. You may be right about the load though as I may use more throttle for the same RPM under load.

 

[WSUDERMAN]

Re: Edelbrock carb flat spot

So bigbird20, how did it go?<br /><br />Just an fyi, we finished my bro's swap this weekend, did the tuning in the driveway and had it dialed in... We thought <br />Took it out to Paris lake for a test yesterday and had to make a few changes. Nothing major, just the accel pump and step-up springs. He could not be more pleased with the results, runs better, starts better, and no more carbon buildup on his hull. <br /><br />Moral is I guess final tuning should always be done under load...

 

[jmeydenbauer]

Re: Edelbrock carb flat spot

Well, I took the boat out and it did alright. Not perfect but alteast the engine didn't die on me. It's still a little rough. <br /><br />784winns, I think you're right about tuning it on the water. That'll be my next stage.