Hey everyone! It's been a while. Hope you've all been well.

Part of the reason I've been so quiet is because my '73 Valiant daily driver has been running pretty great for the past couple years. I've hardly even had to think about it which has been GREAT! Until...

This problem began very subtly this summer, but was always more of a curiosity than a burden. But now, its suddenly starting to get cold here and the problem is becoming massive. Goes like this:

When engine is cold I don't have a lot of power. Never seemed bad enough to bother me. After a minute or two engine would pick up temp and car runs great. However, if I try to ascend a hill or pick up speed to quickly (while car is still running cool) it lunges. Like LUNGES violently, like bucking back and forth. I have to take my foot off the gas and then gently feed it fuel until it stabilizes at a speed it can maintain. The problem seems to disappear when its nicely warmed up. Tonight, however, was a pretty chilly night here in MA. Had to travel somewhere on the highway. Car took a while to get up to temp and was lunging all over the place. Couldn't comfortably get it up to highway speed until it was fully warmed up. Before that it would start lunging every time there was any sort of incline. (I should NOTE: The lunging is NOT dependent on there being an incline, its just way worse when there is one and the engines under stress. I've been able to get it to do the same lunging when traveling on flat land if I really lean into the gas pedal). The real frustrating thing here is that my car actually runs GREAT when its warm (engine/outside)... no hesitation, lots of power...

Anyone ever seen anything like this? Obviously I need to check out carb/choke situation and make sure everything's adjusted right. Worries me a little because I can't say for certain this isn't coming from the transmission. It's very hard to tell. It seems like either the transmission is jolting back and forth between gears or I'm having sudden loss of fuel pressure in bursts.

Anyone have any suggestions? Oh, and I'll save some time for us all with some specifics about my car:

1973 Plymouth Valiant - 225 Slant - Holley 1945 Carb (1989 revision, NOS from SlantSixDan) - 904 Torqueflight Automatic Transmission - pretty much everything other than carb is stock about this car. Timing is probably 5-10 before TDC. No OSAC. EGR blocked off. Stock distributor for year. Premium distributor cap and wires, Blaster coil. All electrical connections good.

Thanks guys for pointing me in the right direction!

-Matt

Symptoms point to faulty carburetor choke and/or choke pull-off. See here for info on how to check and adjust the choke pull-off; note your '73 has a non-adjustable choke thermostat which warrants replacement if it's not working correctly—see here.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Hey Dan,

Thanks! I'm on it...

One quick question about my carb that maybe you can shed some light on:

When I got this carburetor from you I made all adjustments per the Chrysler video on fine tuning the 1945 carb. But one strange quirk it's always had is that if I start the car (kick pedal to floor and start) and walk away to let it warm up, I will always come back a few minutes later to find the engine racing at intensely high RPMs. Have to physically kick it down myself or will race higher and higher seemingly forever. Can you shed any light on that behavior?

BTW, despite said quirks, this carburetor has been the best purchase I ever made for my car. When things are running right it's incredibly responsive and reliable.

Thank you!

-Matt

1974 was the first year for the № 1945 carb; your '73 originally came with a 1920. Has someone installed a 1945, or were you adapting the 1945 procedures for use on your 1920?
That's…weird. If the choke system isn't in proper order the engine speed can increase for awhile, but "higher and higher semingly forever" is bizarre.
What carburetor did you buy?

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Properly adjusted choke and fast idle mechanisms/linkages will set both the choke and fast idle cam upon the first depression of the gas pedal. The choke will close fully. Then the choke closes, the fast idle linkage pulls the fast idle cam up so the second highest step of the cam is underneath the fast idle speed screw. Then the engine is started, the vacuum choke pull should pull the choke open roughly 1/4 inch. This should also drop the fast idle linkage and the fast idle cam should be able to move a step or so down underneath the fast idle screw.

As the engine warms, the choke opens further and the fast idle cam linkage should move and allow the fast idle cam to also move, putting smaller and smaller steps under the fast idle speed screw. By the time the motor is fully warmed up, the choke will be fully open which causes the fast idle cam linkage to be fully dropped and allow the fast idle cam to be completely out from under the fast idle speed screw.

Usually your car just needs to be on a fast idle for about 30 seconds. In normal operation (in temperate climates, not cold climates) you hop in, start the car, let it idle for about ten or fifteen seconds, then drive away. Because your foot is on the throttle, the fast idle cam is free to move. However, if you just pump the gas once, start the car, then walk away, the choke will be opening, the fast idle linkage will be moving, but the fast idle cam will be held in place by the fast idle screw. If you blip the throttle the fast idle screw will drop as far as the fast idle linkage will let it, also lowering the idle.

The symptoms you describe about the idle getting faster if you let it sit sound normal to me, but also suggest a misadjusted fast idle linkage and choke.

The idle gets faster as the car warms up because the engine oil heats up, engine decreasing internal parasitic power loss, and the intake manifold heats up which assists with better fuel atomization. The throttle opening remains constant but the choke opens, allowing more air into the carb. This means the mixture is getting leaner while the engine is able to rotate easier. This leads to an increase in idle speed even though the throttle isn't opening.

A properly and well set up carburetor is capable of being start and let idle but automatically dropping the idle speed as the choke opens. I have had several slant six cars where this happened. The problem is that now most slant six carbs are getting pretty worn out with too much slop in the linkage for this to happen. The cure is to let the engine run at the highest idle for about 20-30 seconds, then blip the throttle, then drive.

I think you can cure most of your problems by (1) adjusting the choke and fast idle linkages and (2) making sure the heated air intake system is functioning correctly and (3) making sure the fast idle speed is set to about 1200-1300 RPM and (4) checking for vacuum leaks.

_________________
Casually looking for a Clifford hyperpak intake for cheap.

Hey Dan,

Yes, my car originally came with the Holley 1920 but unfortunately the previous owner ran it with a bad fuel filter and let it get all gummed up with rust before I bought the car. After several cleanings I still couldn't get it running right and was probably bitching on the forums when you offered to sell me a New Old Stock Holley 1945 which you said was from 1989 and the last revision of the 1945. Should have been more clear in my reply because I know you deal with a lot of people on this forum and probably don't remember all your transactions! But anyways, yeah, love the carb and its been great for me. I used the Chrysler Master Technicians Service Conference, Session 74-9 film that was linked somewhere on this site to tune all the settings initially, but haven't touched the carb much since. I think its time to give it a once over and check choke pull-off in particular. I'll let you know if my choke thermometer turns out to be no good.

I might have exaggerated a bit much when I said "higher and higher seemingly forever"... there's probably a point when it maxes out. I just know I've been shocked to return to my car and hear it revved up so high... sounds like its racing.

-Matt

Hey Reed,

Thanks for the info and clearing some stuff up for me. And YES, the only time I return to the car to find it racing is if I don't kick down on the pedal after letting it run that 10/15 seconds. I guess I was assuming incorrectly that the carb would "kick itself down" when it warmed up. Long ago I learned to start it and kick it down to normal idle before walking away from it (I'm talking dead of winter, temps in the negative). It's good to learn that my assumption that something was wrong was not necessarily correct. I plan to retune and check everything on the carburetor and I'll let you guys know how it turns out. Thanks for the help!

-Matt

That 1945 swap raises questions about the choke thermostat (the item bolted to the exhaust manifold with the rod that runs up to the carburetor). The 1973 and prior-year choke thermostats are not compatible with the Holley 1945 carb, so...what choke thermostat did you use?

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Oh wow. Now I feel stupid. Definitely used the original '73 thermostat. GEEZ. Are the '74 up thermostats easy to come by? Do you think there's any possibility that the '73 could me adjusted to work correctly? Thanks for pointing this out!

-Matt

Dan,

Was '73 the only year that had an electric assist choke or did '74-'76 also have an electric assist it just worked differently?

I just compared my old Holley 1920 with the 1945 currently on the car and I must say that the connector on the carb does seem to be in an almost identical location on both of them. I can also attest to the fact that the '73 thermostat WILL/DOES close the choke correctly when cold.

I guess I'm just trying to understand the physical difference between the '73/'74 thermostats.

Thanks!

-Matt

Dan,

Okay, I sincerely apologize for not getting my story straight immediately... I may in fact have a '74 thermostat. I just found the old e-mail exchanges we had when I bought the carb from you in 2010. You specifically told me at the time I'd need to replace the thermostat. I've never been one to ignore any advice I got from you which leads me to believe I may have, in fact, snagged my current thermostat from the junk yard when I got a '74+ air cleaner to properly fit on the 1945. Its frustrating that 2010 is already such a blur to me!

Sorry for so many words, I guess my questions remains the same: What is the difference between '73 and '74 thermostat.

THANK YOU!

-Matt

No, '73 was the first year for electric assist.
Worked the same, but the 1945 takes a different choke pushrod which hooks to the choke lever front-to-back instead of back-to-front like the pre-'74 setups.

If the junkyard '74+ thermostat is working for you, by all means keep it, but from your initial description of the symptoms it doesn't sound like it's working for you, at least not correctly.

Given you're running a 1945, I'd recommend a № 1234 Electric choke kit, which (unlike your present choke thermostat) is adjustable.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Okay, thanks Dan. Then I am definitely using the correct '74 Thermostat as the rod end is front-to-back.

I just took the choke thermostat out and looked it over. I actually used a hair dryer on it and it seemed to coil up correctly. This is interesting though, am I correct in saying that thermostat doesn't actually open the choke? I always thought it did, but now that I look at it it seems that it closes the choke when cold and simply ALLOWS it to open when hot. Is that correct? The choke pull-off is what actually controls the choke opening as it heats up?

-Matt

Your understanding of the choke thermostat is correct. The choke pull-off pulls the choke partway open (against thermostat spring tension) when the engine first starts. Then air flowing past the choke plate and the weight of the choke pull-off and fast-idle cam do the rest of the choke opening job as decreasing spring tension allows it. Notice how the choke plate is eccentrically mounted on the choke shaft so air flowing past it has more leverage on the side of the plate that tends to push it open.

Also note the choke thermostat's thermal calibration will shift after a few decades' worth of hot/cold cycles, so even if it passes the hair dryer test...

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Okay, thanks for clarifying that, Dan. I also won't rule out upgrading to a full-on electric, adjustable choke thermostat.

Checked pull-off with some suction and it's working... doesn't seem to leak. Also started car with air cleaner off and let it warm up. Choke DID open up although the process might have been a little slower than it's supposed to. Choke didn't open fully until everything was really, really hot. Maybe the pull-off rod has lost the adjustment I made.

I am going to do a total tune-up on the carb and redo all the procedures from the Chrysler training video. Only problem is I need to wait for gasket kit to arrive. I'll post back on the site as soon as carb is fine tuned to spec with a report.

Thanks again Dan and Reed for the valuable info!

-Matt