The Stovebolt Site Forums The Shop Area Engine Shop choke not reving engine

i have a newly rebuilt 261 and a rebuilt rochester carb, i have it running good except that when cold and i pull out the choke it will rev up a lil then it starts to load up and miss, i know that it will load up if the choke is kept on but i push the choke in a lil and the rev comes down, i would like it to be like my old 235 that would rev up nice when choke is on and slightly push in the choke to drive...

something is not right and is bugging me, also there is to much vacumn on my vac advance at idle, i had to lossing the vac line a tad that way i created a leak so that the vac advance would not b ingaging at idle...

please help
jeff

Did the gasket between the carburetor and the insulator have the notch orienated to the notch in the insulator and the hole in the base of the carburetor?

the carb insulator did have this notch and the gasket has the notch cut out there aswell, they are lined up, this carb came off a 235 and it is now on my new rebuilt 261.

Jeff,

I think I would pull the carburetor and check the float level setting. I think the rebuilders don't go for quality, but for quantity.

How did you determine there is too much vacuum on the vacuum advance at idle...with a timing light. I think vacuum will be an arbitrary figure. Depending on where it is picked off the carburetor will influence when the advance unit sees it, but it can't be adjusted per se. Your vacuum advance unit might be for a different application and that could change how much advance it give the distributor.

Stuart

the advance came off my 235, cause when i picked the old 261 from the donor truck the advance did not work, so i used the one from the 235, and i do not have a vac guage i made new lines the same size as the old ones for the gas and vac and when its at idle the distributor will advance, i assume that this is not correct, is my problem from mix match parts, the carb and advance from a 235?

the float level, shold i check that, the engine is loading up, i have the mixture screw 2 1/2 turns out..

jeff

Jeff,

Although this isn't cast in stone, a common adjustment for the idle screw is somewhere in the realm of 1 1/2 turns out or less. Maybe your way over-rich at idle and this is part of the problem.

I think your components are compatible, and line size if fine. If I understand the vacuum thing correctly, your vacuum advance unit should be advanced at idle...yours is! Your line should attach to the carburetor near the idle mixture scew...this is referred to as ported vacuum. At idle, this will give you maximum vacuum and only start to drop as the throttle plate opens. I am curious why you think you have a timing or advance issue.

You should have timed your rig with the vacuum advance disconnected and plugged at the carburetor. Did you time the engine to the mark on the flywheel with a timing light? There are two different unrelated problems being discussed here.

Stuart

Ported vacuum SHOULD NOT be present at idle. If it is, you've got the throttle plate adjusted too far open. Try reducing the throttle plate angle, and drill a small hole in the throttle blade if necessary to bleed enough air to let the engine idle properly. The vacuum doesn't need to be on the vacuum advance until the throttle goes to just above idle in most cases, to reduce the tendency to bog and stumble when transitioning from idle to part throttle.

There should be a way to adjust how much fast idle you get when the choke is closed, either with a screw, or by bending the fast idle linkage. Make sure the fast idle cam isn't worn, or sloppy to the point it doesn't open the throttle when the choke is pulled.

The idle mixture screw is adjusted by turning it clockwise (with the engine fully warmed up) until the engine stumbles or misses, then opening it up until the manifold vacuum maxes out. Any further richening to prevent a tip-in stumble should be done 1/8 turn or so at a time until the best compromise between good idle vacuum and throttle response is achieved.
Jerry

There is a link from the choke to a fast idle cam, I would guess that is not adjusted properly. When you push the choke off the idle screw stays on the fast idle cam when working right. It will drop back to a regular idle speed when you touch the throttle.

Sheesh...there goes that cigar again. Just so I understand all this stuff, the ported vacuum hole in the carburetor is above the throttle plate and sees zero vacuum until the throttle is partially opened. When the throttle plate is opened more, vacuum drops and the advance unit springs back...is this correct!

Sorry for the bum info Jeff...I'll try harder.

Stuart

Vacuum advance is mostly a fuel economy thing, and it's only needed at cruise RPM or light-throttle applications where the timing can be advanced 5 to 10 degrees over the centrifugal advance. Centrifugal is directly related to RPM, up to the point it hits its mechanical limits. As the throttle is opened above idle, intake manifold vacuum is applied to the ported vacuum hole, and the advance diaphragm is pulled in the "advance" direction against a spring. If the throttle is opened quickly, or it approaches full throttle, the vacuum on the port drops, and the vac. advance spring retards the spark a few degrees to prevent detonation during acceleration. Total timing at light-throttle cruise is a combination of initial advance, set with a timing light, centrifugal advance, related to RPM, and vacuum advance, related to engine load and manifold vacuum. It's not uncommon to see 45 degrees or more of total advance at cruise, dropping to 36 or so as a passing maneuver is made at highway speed. Most engines max out on centrifugal advance by about 2500 RPM or so.
Jerry

no problem stuart!!!!

hotrod lincoln you have the timing thing down to a science! the timing does retard a bit when i hit the throttle hard, i thought that was odd and if i just ease on the throttle the dist advances. its pretty cool how that works.

now my 261 runs sweet, just rebuilt, i rebuilt the carb myself, although it was working great with the old 235 engine, my only problem at this point is that when its cold and i choke the carb, it will start easy and start to rev then quikly start stammering and the rpm come down till i open the choke and give it some gas to bring up the idle.

this is not right i know cause when this carb was on my 235, when i started it it would rev up nice and not stammer and bog down, soo i am lost, it worked great on the wore out 235 and sucky on this new 261...

jeff

i guess that i will need to get a van guage and check this vac thing out! as far as timing the engine i just pulled #1 to tdc and set the dist to that and then i started the engine and adjusted the dist to the best i would run at idle,, this is a step van and for me to be able to put a timing light and see the ball on flywheel would be an act that i would not care to trysince when i am in the van i am looking from the back of the engine and is the only access there is.

the throttle plate i am guessing is the bottom butterfly on the carb from inside?

jeff

check out my ice cream truck you will understand.

jeff

It sounds to me the float level is too high. It,s normal for the engine to richen up with the choke out but if it,s overdoimg it the float may be setting the fuel level too high. Try setting the timing up a little. With a roch. B carb the vacuum advance line goes to na fitting on the right side near the idle screw. Like Hot Rod says there should be no advance at idle. A poor mans exhaust annilizer is if it hesatates when cold with the choke in & runs good after warmup the mixture is about right. I worked on step vans for 35 years. I know what positions you have to get your body in to work on them.

261 minus 235 equals 26 cubic inches more air through the carburetor every two turns of the engine. You've probably had to open the throttle (yes, that's the bottom butterfly) enough to expose the vacuum port to make it flow enough air to idle properly. Start by drilling a 1/8" hole beside the throttle shaft, so some air can flow through the throttle plate to keep you from having to crank the idle speed screw open so far. If that's not enough, increase the size of the hole a few thousandths at a time until the engine will idle with the throttle plate almost closed. This assures good manifold vacuum at idle, and also prevents the distributor from over-advancing.

One good way to estimate the timing is to drive up a moderate grade, at about half throttle in high gear, and see if the engine detonates. That's a noise like marbles rattling around in a tin can that goes away if you ease up on the gas. If that happens, retard the timing a couple of degrees and try again. The idea is to get the most timing advance without getting any detonation. At that point, you'll get the best performance, and gas mileage, with no engine damage from too-early timing.

In the early 50's, when my family was racing alcohol-fueled flathead Ford engines in 1934 3-window coupes, we did some experiments on timing. With the car running on a chassis dynamometer, we would advance the timing at full throttle until the maximum horsepower was developed, and record the HP reading. Then the timing would be advanced one more degree, and the power loss recorded. Back to max HP by retarding ther timing to its former number, and start retarding it and recording power loss. It was necessary to retard the timing FIVE DEGREES to drop as much power as one degree too much advance! When in doubt, don't push the limits of timing advance! It's easy to go too far!
Jerry

Hey when Hotrod Lincoln talks we all listen you are 1 bad man.

I agree with ol' 55. I've never heard of anyone doing that before but its great information.