Hi,

I have a 79 F700 truck, has a holley 4160 (1850-4) 4 barrell carb. I bought it two weeks ago. I drove it home (approx 30 miles) and it ran great. I parked it. I started it last weekend to drive somewhere (about 15 miles) and when I started it it ran kind of bad at idle. When I got to my destination it would not idle very good at all. When I shut it off It backfired louder than any backfire I've ever heard. Now, here is what I know about the truck...it has a 429-4v engine, the carb was bought some time ago (Not sure how long, but not more than probably 8 months) and it was a rebuilt carb bought directly from Holley. I don't know much about holley carb's (or any good carbs for that matter) as I'm just getting into these type of things. I'm learning these carbs have a lot to them. The truck seems like it is running rich by the smell of things. Tried to adjust the two idle/air screws on carb and actually putting them all the way in makes no difference...engine still runs. I thought it would shut off. I can get engine to die if I cover the choke valve with a rag so that should elimiate the possibility of an internal vacuum leak...used wd-40 to try to find any external vacuum leaks and found none at all. Vacuum gauge on manifold vacuum shows about 14-18psi of vacuum, the needle on the vacuum gauge jumps quickly between the two numbers...so I should have about 16psi of vacuum. Changing the screws does change the vacuum somewhat but I don't know what to do. The engine just runs rough, wants to idle at about 300rpm, but I raised it to 600 and it still runs rough and smells rich. Might check the timing...I have timing light but don't know yet what the degrees is. Am I on the right track? Any help would be greatly appreciated.

Thanks, and I'll fill out more on my profile later...I'm new

 

When it backfired it probably blew the power valve in the carb.

 

Yeah, I was told by someone that might be, I'm going to replace them...apparently there are two, a primary and secondary, but I'm told the secondary only works at high rpm, will replace them both anyhow. I've read Holly uses 6.5's in their rebuilds, I was told if it is running rich I might want to try a 5.5, but then someone said that would cause more flooding issues. I just have never gotten this far into a carb. I've rebuilt, but never swapped out different size parts for others. Then I wonder why it was running bad before the backfire as well...just don't know.

Thanks...

 

also, at idle I can see gas dripping from what I thought was the secondary jets (the two in the back) and nothing from what I thought was the primary (the ones under the choke flap)

 

If gas is dripping from the secondary boosters that means that the secondary float bowl is running over- causing the engine to run rich. First thing to do is remove the needle and seat assembly. sometimes a pice of dirt will get in there and cause this problem.

 

I assume the secondaries are the ones in the back of the carb towards the rear of the vehicle and the primaries are the ones towards the front of the vehicle under the choke flange?

Thanks.

 

You are correct.

 

Ok...tomorrow I will remove the fuel bowl adjustment screw and blow it out and put it back in and make sure the level is right. There is no idle/air adjustment screws on the rear bowl, just on the front. I think the only screw on the secondary bowl is the one you are talking about that adjusts the fuel level. I should probably replace both power valves while I'm at it, I think I can get them locally. I appreciate your help on this. One pic I seen was a picture of a screwdriver going down into the carb to adjust something like where the stud would come out for the air filter assembly...ever hear of that? I've been doing some reading and seen a lot of stuff over and over, but only seen that once. I don't know if the 4160 has that or not. You have a 4160? What's it on? Thanks again, I really appreciate it.

 

The screwdriver going into the carb would be to pull the accelerator pump squirter out of the carb. Shouldn't need to do that unless you are rebuilding it. You can check by looking into the primaries and working throttle , you should see two streams of fuel squirting into carb. Did the truck set for a long time? If so everything may be dried out and have some leaks. Definitely check the secondary float needle and seat like FastFarmer said. Does your carb have a big conglomeration of stuff on the passenger side where the throttle shafts would be. This truck should have a a governor on that side of the carb. Most of the big F-series trucks did. One of my old racing buddies used to take the vacuum it was pulling and divide by three to use that number for the power valve size. (NOTE: never heard this anywhere else) but it seems to work.
I have also had new carbs from Holley with bad floats right out of the box.

 

Sounds like the needle and seat are sticking, or the power valve. Tap on the top of the needle and seats, if there is a piece of dirt stuck in there, this can free it. To adjust the idles, if I remember right, turning them out on a Holley is richening it, and turning them in leans it out, but I could have it backwards. In any case play with the idle screws for a bit.
Hope this helps

 

I may not get to this today...but was wondering, I have the clear see through sight plugs on my bowls. Do these screw out or pop out? I don't want to break them.

 

You said that the vacuum gauge was fluctuating fast between 14 and 16 in Hg? This is a sign of a mechanical problem in the engine to me. Might be time for a compression test and some more diag, but not before you solve your overflow problem with the back bowl- likely either a stuck needle/seat or misadjusted float as the others have said. The only clear sight plugs I've ever seen unscrew.

Does it misfire at idle only or at high rpm (2500 or so) too?

I"ve never seen one actually blow a power valve- they're easy to check with a vacuum pump though. If you still get no response from the idle mix screws, you may just have the idle speed set to open the primary throttle too much causing the idle circuit to be bypassed- this can be a problem if your not running enough initial advance. Don't just assume that you have a carb problem because it backfires. Any misfire can cause a backfire from an open plug wire to a burnt valve.

 

the clear plugs on the passenger side of carb would unscrew when turned counter clockwise pull one out start it up if fuel runs out your float level is the problem.The fuel should just dribble out.If you have the flat head bolt and thin 9/16 or 5/8 nut on the top of the case of the carb crack the screw loose then turn the nut cloak wise to lower the fuel level coming out thru the site glass.When adjusting the rear its a little harder to do you have to manually open the butter fly after you adjust it to lower the fuel level in the bowl to see where you are at worst case is your secondaries wont work as well.Also the two idle air valves turn them in and back them out 1 1/2 turns to start.

 

Well, a couple of things. The engine wants to idle at 250 the way it is, I've turned it up to 300, 400, 500 and 600rpm, even up 700rpm at one point and still runs rough. High rpm's (driving) is fine, or was before I screwed with the idle air screws...that isn't a problem right now. How would I test the power valve with a vacuum gauge? The motor only has 12,000 original miles on it too by the way. I appreciate all of the input. The motor starts on the first turn of the key, and runs great at higher rpms. Before this problem started, it ran beautifully at idle too...I'm hoping I find some dirt, something simple, but need to definately work the back bowl as well.

Thanks

 

you shouldn't have any fuel dripping in from the secondarys or back barrels at an idle so its looking bad if it is.These carbs are easy to rebuild if you wanted to go thru it.