170 rough acceleration, unless choked.

Hi folks,

My '63 Ranchero has a stock 170, 1100 L-O-M carb, 4-speed manual trans, pertronix I ignition, new plugs gapped to 0.040", new rotor, new cap, recently replaced accelerator pump diaphragm. I live at about 200 ft elevation if thats pertinent.

The problem is after she warms up, I can open the choke all the way (push the knob all the way in) and it will idle fine at about 850 rpms (is this too high?). But from a stoplight, it will accelerate but roughly - almost stuttery like its missing. However, if I begin to close the choke just a little, say I pull the handle about 1/8-1/4 of the way out which just barely moves the butterfly, it kicks the idle up a tad (to about 1200ish), and then there is smooth acceleration, no sign of trouble. If I push the choke in just a bit, the idle speed decreases a little and the roughness starts to appear but not as bad. The roughness only appears on acceleration, if I just pick a speed and cruise along - no trouble.

The pertonix ignition is fairly new, but the roughness problem was present before the new ignition. After the ignition, the engine does run smoother than before but the roughness on going fromt stand-still up to cruising speed feels about the same.

I don't know if its related but another issue is if I'm cruising along with the choke all the way open, and I take my foot off the gas to decelerate, the exhaust sounds like a motorcycle: "Brap-brap-brap-brap" for lack of a better explanation. But, with the choke-knob in the slightly pulled out position, it makes no such noise on deceleration.

I'm actually fine with driving with the choke out just a tad, but I figure I shouldn't NEED to, y'know?

Any thoughts on diagnoses or diagnosing this problem?

Thanks in advance.

Adam

If you yoke it down with the choke, however slightly it might be, you are leaning it out. There is an A/F mix problem.

Hmm... Someone will have to explain that to me. I was thinking that pulling the choke knob out a little would make the mixture richer. Here's my (apparently wrong, and not the first time for sure) reasoning: the butterfly closes a bit, which restricts the space for incoming air. The cylinders still want to draw the same amount of air which has to move through a smaller opening which makes the incoming air have to travel faster. Faster moving air exerts less pressure, less pressure means it would suck more fuel.

If choking it a bit creates a lean(er) condition, can I adjust the float down a little? Would that lean it out a bit?

Now that you say that, it does help explain why the old plugs had some carbon build-up. I was thinking it was because I had been making it too rich with the choke, but that would explain that was running too rich all the time.

Thanks.

Adam

You're right. Busted my over hasty trigger finger. It richens it. What is still true is that it needs adjustment of the A/F mix.

8) try raising the float level a bit, about 1/64", and make sure the accelerator pump is working properly.

Accelerator pump is working properly, I get a good stream of gas shooting in when I press the throttle linkage.

I'll adjust the float level and see how that goes.

Adam

While I'm at it, what is the factory spec float level and how do you measure it? I'm assuming you pull the top of the carb off, and then flip it upside down so the needle (and float) are seated, and then you measure from where the gasket sits to where?

Adam

before you take the carb apart, try re-setting the idle mix screw - prolly end up backing it out maybe a half-turn is my guess.

Lud, you were right kinda...he was correcting a lean condition by richening the mixture.

I would check your idle vacuum and spray down the carb base gasket with starter fluid. If you hear the idle pick up, then you have a vacuum leak at the base. You should be getting at least 15inHG.

Also, double check your timing. I wonder if you aren't running a little retarded based on the exhaust note.

Recheck your idle mix.

Try doing some hard acceleration with the choke engaged. If it doesn't have problems, I doubt your fuel float is the issue.

Otherwise, your carb may need a fresh rebuild as one of the ports (probably the power circuit) is clogged.

With the new Ethanol mixed fuels, it is vitally important for these old cars that don't get driven regularly, to have fuel stabilizer. Ethanol has a REALLY BAD habit of falling out of suspension in gas and collecting water which then creates a nice gel in fuel systems and is particularly bad in carbs which don't use a lot of fuel pressure to move fuel. It also causes havoc on 2 stroke outboard marine engines that aren't used often. I had this happen with my Holley. All of a sudden one day, the idle circuit got clogged and I could only keep the car running by keeping the car rev'd above 1600RPM which kept it in the cruise circuit, though a little lean with the lack of the idle circuit adding.

Your idle is a tad high. Should be around 600RPM for a manual transmission (unless the Falcon is different). But that is something I have a hard time rectifying with the other symptoms. Usually high idle is caused by advanced timing and/or high idle mix. That's why I don't think your idle mix is the problem.

Try this test when you get a chance. Get the car over 2000 RPM, warmed up, and disengage the choke. Keep it over 2000 RPM and try accelerating hard. If you are still having problems, my guess is the carb needs to be rebuilt because that would usually indicate a clogged power circuit.

Got a spare 1100? swap it and see if the problem persists.

Beofre doing anything else, first check your fuel filters. On such an old car you could have alot of junk in the tank and if it gets stirred up it could plug the tank filter or the carb filter.

Check the carb filter by removing it and blowing through it in both dirrections. You should feel very little restiction.

The tank filter is much harder to check. You can use a compressed air line to blow back into the tank to dislonge any debree on it. Other then that you would most likely have to remove the tank to remove the filter from the tank. Do this last.

Or attach an inline pressure gauge between the carb filter (or pump filter depending on the year), and check the pressure. IIRC, it should be around 7psi.

Thanks for the good ideas guys -

Where do you measure vacuum on a L-O-M carb? It seems like, at idle, I shouldn't be getting any vacuum to the distributor so where do I "plug" the gauge in?

Timing is spot on, if my balancer is correct, I need to verify TDC - I will do it this weekend.

I have sprayed starter fluid around the carb while she's idling and the only point where I notice any change is near the back of the carb, near the choke shaft - but I think the gases were being sucked up past a bad gasket at the base of my air cleaner. Or is the choke shaft a likely place for an air leak? I need to get a can of starter fluid with a straw so I can be more precise with my aim. I'll do that this weekend too.

Thanks again,

Adam

Adam,

I think you may have found part of the problem (if not the whole thing).

It will be tough to measure vacuum with LOM 1100 unless you have a base plate with a vacuum port. But the spray down showed a vacuum leak, which is all the vacuum test would have done.

Just gotta do like me...keep a spare carb in the garage to swap out.

You got a spare one I can put on my shelf - or bolt on my engine? J/K. I'll post something in the wanted section.

I haven't got a vac port on my base plate, so no luck there. I need to pinpoint the exact location of the leak - beacuse my suspicion is that it is just sneaking up into the air cleaner and down into the carb - which shouldn't cause any trouble. I'll let you know.

If there was a leak at the choke shaft - would that cause a lean condition? I didn't think a leak above the venturi would lean it out...but I'm learning something from you guys everyday as I'm lurking.

Regards,

Adam

If there was a leak at the choke shaft - would that cause a lean condition?

Click to expand...

The choke shaft is above where the fuel enters the carb throat.

The other thing you can do to check for vac leak is to squirt WD 40 here and there. When it plugs the hole, the idle will level out. Use the straw.

You can also plug the vac port on your base plate and measure the vacuum somewhere else. The vac tube at the distributor is not the place. In my Mustang there is a 3 way nipple off the side of the log with several vac ports for auto tranny modulator, power brakes and something else I don't have. That is where I check my vacuum level. The carb port varies more sharply due to the changing throttle position and the air velocity in the throat.

8) and as i have pointed out in the past, dont forget the gasket between the log and the carb adapter. too many of us forget that gasket when we are looking for problems.

ludwig":12t3c6iy said:

If there was a leak at the choke shaft - would that cause a lean condition?

Click to expand...

The choke shaft is above where the fuel enters the carb throat.

The other thing you can do to check for vac leak is to squirt WD 40 here and there. When it plugs the hole, the idle will level out. Use the straw.

You can also plug the vac port on your base plate and measure the vacuum somewhere else. The vac tube at the distributor is not the place. In my Mustang there is a 3 way nipple off the side of the log with several vac ports for auto tranny modulator, power brakes and something else I don't have. That is where I check my vacuum level. The carb port varies more sharply due to the changing throttle position and the air velocity in the throat.

Click to expand...

Yup, and as you suspect a slight leak around the 'choke' plate shaft (not throttle plate shaft) should not be an issue. The two small pin-shafts that retain the air breather mount are also a loose fit.

This pic has an arrow pointing to the vacuum access on the log Ludwig mentioned, and is usually the best place to measure/set timing by vacuum...mine just has square head plug in that location (manual tranny & no power brakes).

Adam, I wrote a reply earlier this afternoon, but evidently it did not go through. Back a year or so ago I installed a 'rebuilt' autolite 1100 carb (that came off of a 63' Fairlane) on my 66' - 200cid engine (with manual trans.). When I first installed it I started with the fuel mixture screw turned out 1 and 1/2 turns (which is a normal starting point for many carbs), and the only way I could keep the engine idling was to do exactly what you are doing. I had to adjust the idle to at least 850-900 rpm, and also keep the choke partly closed for more gas. What I found out, after checking the spec's on my carb (which like I said was off a 63' Fairlane) is that the fuel mixture screw is suppose to be turned out approximately 3 and 1/2 to 4 turns instead of 1 and 1/2 turns. It made all the difference in the world with my carb (and engine) and has been running fine ever since. This may not be your problem, but I thought I would throw this out there since it appears we both have 'stock' Autolite 1100 carbs from 1963; both with manual transmission! Good luck! Jim

I'll see...I have so much stuff sitting on the shelf in my garage, I'll see what's lying around. I know I have a few 2100s, a rare Holley 2V carb (forget the number), and I think an old YF and an RF. Not sure if I got rid of all my 1100s or not. I know I sold the Pony Carb 1100, and got rid of 3 when I sold the Offy set up.

Jim -

That is interesting. Right now I'm idling right around 850-900, and my idle mixture screw is at about 1 1/2 to 1 3/4 turns out, just like you.

Were you able to get it to idle down to 600 with the mixture screw open to 3 1/2?

Hmmmm....Well, there's something else to try. It would be nice if that was all it took.

Thanks,

Adam