Carb trouble?

[D-Rat]

Some questions for the v8 experts....

This is a 400 that was running great up until about a month ago. There's about 12k on the engine since it was rebuilt, balanced, blueprinted, etc. The engine runs like a dream, but seems to have recently developed a carb issue. The Holley was rebuilt at the time the engine was.

Symptoms:

Runs great until up to full temp, then acts like its running rich and "loading up". Will occasionally backfire out the exhaust. Driving on the freeway, I'm feeling some intermittent power loss as if someone was lightly tapping the brakes.

When I posted this in the engine forum, I was told to check my timing, which I haven't yet been able to do since I seem to have lost my timing light. I do ask how the timing could change that much when it hasn't been touched. It has an HEI distributor, and the wires are in excellent condition with about 12k miles on them.

Fuel flow is good, and the filter is clean. I haven't been able to detect any vacuum leaks on the lines and fittings, and the hoses are new - brake booster, vacuum advance for the dist, etc.

When I pull the air cleaner when the engine is warm, the butterflies are open, indicating the choke seems to be working as it should. It has an electric choke.

Carb appears to be a 730CFM Holley, but I'm not dead sure, as I've had some difficulty cross referencing all the stamped numbers on the thing.

I'm ready to buy another carb - looking at a new 600CFM Edelbrock, but want to rule anything else out first.

[Shyguy]

My truck had similar symptoms. I looked at the distributor cap and rotor and found a hole burned in the rotor. I replaced the cap and rotor and it runs ok now.

Just some thing to check.

[Wrenchbender Ret]

Holleys are bad about the power valve blowing out. If it is running rich take off the primary fuel bowl & metering block & check or replace the power valve.

[D-Rat]

Originally Posted by Wrenchbender Ret

Holleys are bad about the power valve blowing out. If it is running rich take off the primary fuel bowl & metering block & check or replace the power valve.

How do I tell it's bad? Since I'm having trouble ID'ing the carb, I'd be concerned about getting the right parts. The casting#s are inconsistent, and the only one I've been able to xref on any of a number of googled sites points to this thing being a carb from a Ford 428 Cobra Jet engine, but who knows. For all I know it's a hodgepodge of several Holley carbs.

Also, are there any good links to some exploded diagrams that I can print out? Sadly, I know a lot more about EFI than i do carbs. I'm not afraid to tear into it though, provided I can see what is what. I'm a little more familiar with the AFBs (like Edelbrocks) because I had one on an old Buick I used to tinker on.

I'll check the cap and rotor as well. I have HEI, but have no idea what it came off of....any tips on part identification there too to get the right replacement parts? Seems if the rotor was bad it wouldn't run good cold either though.

[Wrenchbender Ret]

Stamped on the vertical plate on the air horn there should be a List-xxxx number. That will tell what it came off of. You could buy a kit that would have the exploded view & parts but they are pretty expensive for a holley & you probably don't need to much.
If it is running rich & hard to start after setting for a little while you probably have a bad power valve or needle & seat. You can check the float levels on a holley by removeing the brass plug on the side of the fuel bowls. The gas level should be right at the bottom of the hole.
If you can't find the info on the net e-mail me & i'll send a copy in the snail mail.
georgec2434@yahoo.com

[D-Rat]

Originally Posted by Wrenchbender Ret

Stamped on the vertical plate on the air horn there should be a List-xxxx number. That will tell what it came off of. You could buy a kit that would have the exploded view & parts but they are pretty expensive for a holley & you probably don't need to much.
If it is running rich & hard to start after setting for a little while you probably have a bad power valve or needle & seat. You can check the float levels on a holley by removeing the brass plug on the side of the fuel bowls. The gas level should be right at the bottom of the hole.
If you can't find the info on the net e-mail me & i'll send a copy in the snail mail.
georgec2434@yahoo.com

Thanks for the carb info.

Thing is, it starts up easy and runs great until it's fully up to temp. Then the symptoms start. That's what has me baffled and thinking choke over timing and other carb parts, but it's hard to say.

It's raining today, so I'll check timing next as soon as I can get someone to lend a hand with the throttle.

[GASoline71]

Old Holleys from somewhere around 1995 and older have no power valve blowout protection. All newer Hollys have a small one way check valce in them to keep from blowing out the power valve from a carb backfire.

A simple test to see if your power valve is blown... is with the engine running, and you can turn the idle mixture screws in all the was until they seat and the motor doewsn't stumble and die... the power valve is prolly shot. But you have to make sure you have no air leaks first.

gary

[D-Rat]

Would it be that if it runs good cold?

[GASoline71]

Not sure... the power valve is vacuum operated, so it takes a certain amount of vacuum to open it. It is designed to open at a specific vacuum to assist in richening the fuel charge at high load. If you have very low manifold vacuum at idle, then the power valve could be open causing the fuel mixture to be extremely rich. Usually happens with performance camshafts with large valve overlap.

Have you pulled a couple of plugs to see if theu are wet and reek of gasoline?

HEI modules can go bad over time as well... you can pull the module and have it tested. I had a module go bad in a 1979 GMC 3/4 ton that did somewhat what you are describing.

Gary

[D-Rat]

Thanks....that gives me some stuff to check. pulling plugs around those headers on a hot engine will be fun, but I can do it cold and still see if they're fouled. Having bought the truck a few months ago, I have no idea the history on the HEI...if it was new when the motor went in 12k miles ago, or if it was a used piece.

[D-Rat]

OK, again, forgive my SBC ignorance. I'm learning. I pulled some of the plugs - not all because a couple are a pain to get to, but all the plugs indicate proper tuning - they're light brown on the tips.

I looked at the wires a lot closer as well as the vacuum lines. They're all like new.

I tested the power lead to the choke and that's getting juice.

The timing sticky is a bit beyond me. I don't think I need to pull the whole distributor out to check timing. The cap is not like anything I'm used to. If I was to pull it, it looks like there's 2 or 3 screws at the base of it, and it lifts off. Is this a standard GM HEI piece?



Also, I don't see where you make timing adjustments. Older Japanese distributors have an elongated hole at the base of the distributor on the engine block that enables you to loosen a set bolt and turn the distributor, but I don't see that here. Am I missing it?



Moving on to the timing, I hooked up a timing light only to learn there are no timing marks on the balancer. Also, in the timing sticky, it says to set at something like 32-36 degrees at 3k Rpms. How would you measure this even if I had the timing marks? The little pointer on the block only shows up to 8* advance (see pic)



Am I right to assume that if I find TDC in the #1 cylinder, I make my mark on the balancer at zero on the little pointer in the pic above?

Thanks guys. I'm going to go look for an engine tuning manual at the bookstore next. Any suggestions on one for someone new to SBCs?

Lastly, here's my unidentified Holley:

[dwcsr]

Check your float levels, make sure they are just at the bottom of the sight hole.

I got a 1" phenolic spacer to fix the same iuuse on my 350. The engine gets hot and boils the gas out of the bowles into the engine. runs crapy untill it cools down. the spacer keeps the heat out of the carb base plate.

[GASoline71]

that is and old Holley carb... looks like it must have been on a Ford at one time since it has the crossflow tube back to the rear bowl.

Your HEI is not a points style ignition with a "little window" to adjust the dwell. the 32 to 35 degrees is total timing at RPM. It's easy to check. Just have someone sit in the truck and hold it at 3,000 RPM. but you will need an adjustable timing light to bring the mark back down to the tab.

There has to be a mark on the balancer somewhere. You just need to find it. You make timing adjustments by turning the distributor clockwise or couter clockwise. There is a hold down clamp with one bolt holding the distributor in place. It is on the passenger side below the distributor on the intake manifold. There are four spring loaded screws that hold that cap on the distributor. To adjust your total timing... you do have to take the cap off to get to the springs and weights. There are kits that you can get with different weights and spring tensions to adjust your "curve".

the cap is goofy lookin' because the coil is in the top of the cap. Also you can't make a timing mark on your balancer by just bringing it up to TDC and marking it at zero on the tab. You have to find the mark on the balancer. Your engine could be anywhere on timing right now. So until it gets timed right... between that and that ancient Holley carb... for me... it's hard to diagnose without being there.

Gary

[D-Rat]

Would that timing mark be a notch in the balancer, or is it just a surface mark?

[peleni]

D-Rat,

One other thing to check, I had similar problems and thought it was the carb, I finally found that the wire going to the points underneath the rotor had a bare spot and would ground out, just something to check.

[Wrenchbender Ret]

Your damper may have come off an engine that the timing marks were straight down in back of the water pump. If you have a short water pump you will not be able to see it. The mark on the damper is a groove the full width of the damper. If you have a long water pump aim your timing light in back of it with the vacuum advance disconnected. Straight up is o if the original marks on the cover are gone. You may be able to power time it from ther. I'd say your problem is in the carb. The HEI unit seldom needs retiming & you said it was running good.

[Jacfourteen]

If you are unsure about your carb you should pull it out and rebuild it, they are really pretty simple to do and you can get a kit with new gaskets, power valve, accel pump, and needles for like $30. Then you just pull it apart clean it and throw in the new parts, while your in there you can find out what jetting you have and check the floats. It sounds like you're running rich though because when cold the engine likes a rich mixture but when you warm up its flooding out. I'd say to pull the carb though that way you can get familiar with it.
About the timing, if your light isn't ajustable you can get a timing tape (its just a sticker with degree marks on it) then you can tell where it's advancing to. If you get the timing tape just make sure to know what diameter you balencer is so you get the right one. then just line up the zero on the tape to the mark on your dampener. Good luck and let us know what happens.

[D-Rat]

Originally Posted by Wrenchbender Ret

Your damper may have come off an engine that the timing marks were straight down in back of the water pump. If you have a short water pump you will not be able to see it. The mark on the damper is a groove the full width of the damper. If you have a long water pump aim your timing light in back of it with the vacuum advance disconnected. Straight up is o if the original marks on the cover are gone. You may be able to power time it from ther. I'd say your problem is in the carb. The HEI unit seldom needs retiming & you said it was running good.

Thanks, I'll check again Saturday when I have time to get in there and look around some more.

After the feedback I got from you and from others indicating how old that Holley is, I'm thinking I might just go with a shiny new Edelbrock carb and see how that works out.

I do believe this is a Ford carb originally. The one number I was able to cross reference pointed to it being the same carb used on the 428 Cobra Jet - 730CFM. That's probably more than my 400 needs, and I don't think my power would suffer with a 600CFM carb.

does anyone have any recommendations on particular Edelbrock part numbers? Summit has decent prices on them, but they have several listed that all appear very similar.

[Sinister]

Edelbrock #1406

[Jacfourteen]

Originally Posted by D-Rat

That's probably more than my 400 needs, and I don't think my power would suffer with a 600CFM carb.

Yeah for a street motor that is a little overkill. It would be great for a race engine where you need the cfm for the top end, but you are definitly losing low end and responsiveness. I have a 670 on my 355 and it seems to work well, I would think somewhere between 600-670 cfm would be good for you. You lose a little power in the high rpm's, but how often do you run it up over 5000 rpm? You might want to look at the different websites that sell carbs, some of them have interactive guides that ask you questions about your app and tell you what carb would best suite you. So far I like my holley, but then again I've never messed with any other brand so I don't have much to compare to.

[Wrenchbender Ret]

If that is a Cobra jet carb it may have some good value to one of the Ford guys that want to have matching numbers.

[D-Rat]

Originally Posted by Wrenchbender Ret

If that is a Cobra jet carb it may have some good value to one of the Ford guys that want to have matching numbers.

Yeah, that's what I'm thinking. Hopefully I can get the cost of a new Edelbrock (or close) out of it.

I ordered a new 1406. $287 shipped and it'll be here as early as tomorrow.

I'm assuming it's a direct bolt on replacement, and that my throttle linkage will work just fine with it.