The Stovebolt Site Forums The Shop Area Engine Shop need help with flat spot

OK so I've got a newly rebuilt 58 235 that I'm trying to fine tune. Dual rochester 1 bbl on offy intake. Used the uni-syn to get them close to in sync. Here's the problem. There is a huge dead spot coming off idle to accelerate. I'm pulling 20Hg of vacuum at idle. I've tried advance and retard everywhere on the dizzy. Motor only wants to run when very advanced. Retarding it makes it worse. Sometimes if I blip the throttle real fast it backfires thru the carbs. It seemes to run better with the linkage disconnected to one carb or the other. Oh and I rebuilt the carbs myself (didn't think there was really much to it)The base of the carbs are also dripping with water which I attribute to the venturi affect making them cold at the base It's also humid and rainy outside today. So any thoughts on what's happening?

First try richining up the idle jets. Next raise the float levels a little. If that doesn't do it you might try larger main jets.

forgot to mention that when trying to start it will catch and die everytime. I have to hold the pedal to the floor to get it to stay lit

Is the intake manifold heated?

Might the float level adjustment be off?

That flat spot makes me think it's lean at that point. I'd try a quick test of putting my hand,or hands over the carb to restrict air intake to richen things up just to see what the result was.

It' hard to comment on that Offy manifold but I do know sometimes it's possible to get great volume but a lose of needed velocity in the air/fuel delivery. Which is to say, the fuel mixture is there but at some point is not moving to the cylinders as fast as it is needed.
It's something I ran into when I modified a motor.
Dan

the intake is not heated however this same setup came on the truck when I bought it. It ran decent with the old tired 53 motor that was in it . No starting or flat spot but I guess I'm comparing apples to oranges. I plan to put heat to the manifold in the future. It's got Fenton duals on it. There is a pipe plug in them so I guess I have to plumb something to the base of the intake. Any posts on how to do that? by the way the gas in the bowls is about 1/2 to 3/4 of in inch down from the top of the bowl when I take the top of the carbs off.

Sounds like a flooding condition. After you shut it down look down the carb floats. There should be no fuel running in. The fuel pressure may be too high. If so you need a pressure regulator.

you mean pull the top of the carb off after I shut it down and see where the float level is? and if more is running in.I'll check that but I don't see how fuel could be pumping in with the engine off

The carbs may be icing due to humidity causing some of your problems. I would set the floats pretty high, and the idle jets rich, then install a fuel regulator set to 2 1/5 lbs so the floats will hold once the engine is turned off.

How are the carbs jetted? stock?
Hows it run at highway speed?
Hows it run full throttle?
Hows the tail pipe smell or look?
Hows the tail pipe look when you start it? smoke? clear?
Hows the fuel usage?

Get the intake heated ASAP, even in your climate you need it.
I would bet most of your problem will go away once you plumb the heat.

Joe

Joe

I rebuilt the carbs myself (didn't think there was really much to it

You did something wrong to the power valve.

reply to JoeH, The truck is not on the road yet, Just a motor in a bare chassis so I can't answer most of your questions.I guess before I pull them apart ( the carbs)I will try running it with the the choke partially on to see if I have a lean condition. What could I have done wrong with the power valve? and does anybody know how to plumb for carb heat to the manifold with Fentons and offy

The frist time I rebuilt my carb I took my time and made sure I did everything the way it showed in the books , it did the same thing Yours is doing now took it apart again and paid more attention to the power valve took some valve lapping compound and nice and easy turned the power valve back and forth by hand till the seat was nice and shiny cleaned everthing real good and when i reinstalled the power shaft made sure that it went up and down easy on the spring after that the truck ran fine no more hesitation on coming off a idle.Put timing back on ball to.

Pete

No! Just look down the throats with the carbs complete & the air cleaners off. You are right, there shouldn't be gas running in after it is shut off but there is a residual pressure even when shut off. If the pressure is too high or you have dirt in the needle & seat it will continue to drip in to the throat. Float level is measured by the distance from the top of the air horn with the gasket installed to the bottom of the float with the air horn held upside down. Like shown on the instrucktion sheet. The level in the bowl after you remove the top is immaterial.

thanks Pete Guess I'll be tearing into the carbs again.

" . . . and does anybody know how to plumb for carb heat to the manifold with Fentons and offy . . . "

This manifold heating kit is the easiest to install and it does the job with minimal plumbing and no fear of water/coolant leaks.

You've got a classic case of over-carburetion. The velocity of the air going through the venturis is so slow as the carbs transition from idle to midrange operation that there's no venturi vacuum to pull fuel through the main jet. The carbs go lean momentarily, regardless of how much vacuum there is below the throttle plates at idle. Raising the float level(s) is one possibility, but then there's a serious risk of flooding. What you really need to do is find a set of carbs for a 216, and modify the manifold stud spacing to bolt the smaller carbs onto the manifold. You will still have far more venturi area than you'll ever be able to use, but the flat spot will go away.
Jerry

I'm thinking along the lines of Hotrod lincoln. When you open both butterflys your vaccum drops off to quickly before the engine rpm's come up, the venturi effect is lost. Thats what accelerator pumps are supposed to compensate for. Be sure yours throw a good spray and are set for the longest stroke possible if yours are adjustable like old Strombergs are. Raising the fuel level in the bowl my help a little too. Pop ing back thru the carburetor is a sure sign of a lean mixture.

Did you use the instructions that came with the offy intake, regarding the idle bleed screw adjustment? The instructions that came with mine were wrong. They suggested using half the normal setting.

What I found helpful, was to set both carbs up before placing on the manifold to the factory settings. The shop manual will tell you, its something like one and a half turns out from seating, and there is also a number of turns from closed throttle for the throttle screws.

When I first set mine up, it ran, but was super duper lean. I consulted my beloved California Bill's Speed guide, and found the answer to my lean problem.

I assume your engine has been over-bored? If it has, you should get fine results with the B's. I run two carbs on my 216, its great, and looks cool.

After checking the above, I would start by static timing, getting the ball in place on #1 firing, and putting your multi meter across the points in the dizzy. With the octane selector at zero, loosen the dizzy up and lock it just where you loose continuity across the points. ( By turning the dizzy)

If you are fancy and time with a light, and the engine running, remember to disconnect the dizzy vacuum advance, and plug it while you use your light. I don't use a light because I am too lazy to disconnect the dizzy line and plug it, too lazy to go to the store to buy one, and I'm cheap.

Once you start road testing, you might find that you can't get enough advance by adjusting the dizzy using the octane selector. If thats the case, you can start by retiming the engine, with the octane selector set somewhere in the retard zone. In the end, it doesn't matter where the selector ends up, as long as the engine is happy, and there isn't coke bottle noise coming from it when you stomp on it.

If you are planning on using a choke, you will want to use both. You also will want to make sure the carbs are matched, and both floats are set exactly the same, no excessive play in the throttle shafts, when you sync don't have any linkage attached, blah, blah, blah.

Mr lang ,I'm with you on everything but the part about static timing. I have a pertronix conversion on the dizzy. No points. I will tear apart the carbs again and check everything for the 3rd time (when it gets above freezing here in Fla.) In the meantime I will try to locate at least 1 rebuilt roch B carb at my flaps. thanks

Did you have it running before the pertronix? I don't know about anything like that. Too much high tech-ery for me.

Yea, I've tried 2 different dizzy's one with points ,one without. Same results. real hard to get it to catch.Just won't be able to get any parts till the weekend. Darn regular job keeps taking up all my time.

This may be the key.

My guess is that you have the idle mixture control screws too far out and thus a VERY RICH idle. This creates "puddling" in the intake. When you open the throttle, the sudden burst of air into the cylinders sweeps all of the puddles into the engine, creating an instantaneous overrich condition rather than lean. The mixture is too rich to fire, thus the hesitation. Try readjusting the idle mixture. No more than 1 turn on each mixture screw. Set the idle RPM with the positioner screw.

Jon.

thanks carbking I'll give it a try. I did notice though the couple times when I was able to get it to start that my best idle and vacuum reading came when I was about 2 - 2.5 turns out. But yes when I pull the carbs to check ,the manifold is always wet in the floor of the manifold.I'm anxious to get another carb to confirm that my issues are carb related and not something else. When I block off one carb flange with a plate to isolate the carb and then switch them the problem still the same but then again I'm the one that rebuilt both carbs so it could be something I did or didn't do on the rebuild. These carbs worked before I (fixed ) them but they were on a different motor.

P.S. when I go to purchase a rebuilt carbs at my flaps what do I ask for? 235 060 over offy dual 1bbl and fenton exhaust on a fresh rebuilt motor. Any manual choke rochester B from the mid to late 50's ?

May be something useful here:
http://bangshift.com/gallery/Tech-Gallery-Rebuilding-a-Rochester-B-Carb-gallery.html

The rebuilt carbs from the parts house may be worse then what you have. They use lots of parts from different stock to make good usable units. What you need are two just the same, right down to the part number on each tag. Keep at yours till they run one at a time.

Joe