Hi The 390 in my 63.5 Galaxie has a 600 cfm summit racing carb Choke has been adjusted so it behave "normal", ie closed when cold, open when hot When the engine is cold, the carb pops a couple of times, and the gas pedal also have to be kept a bit down to avoid that the engine stops, i.e. the idle speed is too slow when starting But when the engine is hot, the popping is gone and the idle speed is ok What could be the reason? Too lean idle mixture? If itsactual to adjust the idle mixture screws, how to measure the vacuum to be highest possible? Two actual links are listed in the bottom Carb Link: https://bit.ly/2XulivA Instructions link: https://bit.ly/2FpKFVc
Too lean an idle possibly... But first check the choke is not coming off too soon ie before the motor is warned up.
Will check the RPMs soon. The choke can come off too soon and the fast idle screw may have to be adjusted to get high enough idle rpm But I have started different engines without any chokefunction, and they dont pop, Doesnt it have to be another reason to make popping, than the choke?
Hook up a vacuum meter to a port with manifold vacuum (below throttle) and adjust until you get as high vacuum as possible.
Popping and stalling when cold may be sticky intake valves. They may not be fully closing until the engine warms up and loosens the gummy stems and guides.
Adjust the choke to stay on longer (rich) , increase fast idle speed . All engines are different , adjust each as to thier individual preference .
You might try using manifold vacuum , if you are using ported vacuum. This should cause the idle speed to increase. After you lower the idle speed back to where you want it, the throttle blade will be less open, leaving you more transfer slot for the transition circuit. All 360's /390's I have ever owned have been cold-natured and often did what you describe. Good Luck
Its probably your technique. When you first start the car the gas pedal gets pushed to the floor and that engages the choke, the choke flap closes and you start the engine. The choke now sees heat from an element the wire is connected to, a spring reacts to the heats and slowly opens the choke flap allowing more air to the engine. When you engaged the choke before starting the engine a cam mechanism rolled around and the fast idle screw is keeping the throttle blades partially open and that is why your idle speed increases. If you tap the gas pedal during any part of this process the high idle cam drops out closing the throttle blades - usually resulting in your popping and then you have to 'feather' the gas pedal to keep it running. You have to have a properly tuned engine that runs well to have the choke working right, it is intended for the driver to get in the car hit the gas once and start the car, let him go back in the house for coffee while the car warms up, when he hears it up on high idle then he hops in and drives. The choke won't work as intended 'as you drive' its more a stationary thing. In brief, if you touch the gas pedal the high speed idle cam drops out allowing the throttle blades to close while the choke flap is still partially open so the engine gets no air, thats why you have to feather the throttle and that action sends raw fuel into the engine and it 'pops'.
I agree with the above. A little fine tuning can make a big difference. If you are unable to make it the way you want it to be, as is, it might be worth putting one size larger jets in the primary side and re-adjust again. I have mine working pretty well (it has 3 Holley 2 bbl carbs) but if the temp is close to freezing I do let it warm up a bit before driving.
Some adjustment is necessary, maybe the simplest type of adjustment, like adjust the electric choke and the fast idle screw helps. If just tapping the gas pedal once, the engine stops and several starting attempts is necessary
In 2018, a complete MSD "pack" was installed including 6AL ignition box and Pro-Billet distributor 8594, plugs and plugwires. Will a change like using manifold vacuum instead of ported vacuum, also make a complete new timing/curving necessary?
Many examples of what are thought to be a carburetor defect are instead, an ignition related problem. The instructions for your carburetor suggest setting the idle mixture screws using a mechanic's vacuum gauge. This is a good idea, but let's back the truck up for a second. The vacuum gauge is also excellent for engine diagnostics or tuning derangements. Connect the gauge to a source of constant or full time manifold vacuum (i.e. below throttle plates) and measure the engine vacuum at idle. A stock, healthy engine in good condition, in good tune, should pull 18" to 20" Hg. at Sea level elevation. A correction factor is applied for things like performance camshaft, and of course higher elevations reduce the vacuum. The important thing to observe here though is that the manifold vacuum is steady (no vacuum leaks, valve problems, exhaust restriction etc) and high. Very little air velocity through a carburetor at idle so, the carburetor gets around this by employing a special, separate idle circuit that instead, uses engine manifold vacuum to achieve a high velocity for proper fuel atomization, it needs this vacuum for a crisp off idle response. I know this sounds long winded but if you want to tune the engine properly everything must be nominal and start from the beginning, to isolate or discover the fault through a process of elimination. A high steady engine manifold vacuum reading - do you have this measurement? Short answer: check your ignition timing carefully. Old Fords typically have defective harmonic balancers, where the ignition timing marks live. One of the characteristics of slow or late ignition timing is low engine manifold vacuum.
No, the manifold vacuum isnt measured yet. Never done that earlier, my level of knowledge and experience when it comes to adjusting engines is like adjusting the timing, using timing light and adjusting carburetors like fast idle/idle speed and choke You gave a relatively easy description of the vacuum function and it may not be so advanced as expected. In addition a vacuum gauge is cheap. It will be interesting to take a step further in knowledge and experience In some weeks when I get some free time, an attempt to adjust the engine, much based on what that has been described in this thread will be done The original harmonic damper wasnt good, the rubber had partially began to be squeezed. This damper was installet some years ago: https://www.summitracing.com/int/parts/pfs-80009/overview/
Well, this post is over a year old, but the same problem still exists Earlier this summer the manifold vacuum was adjusted to highest that was approximatly 18" It didnt help, but startet an additional fault, when driving downhills using the engine "rumbles" Before the adjustment, a professional stop had adjusted the idle mixture screws so that th CO2 - values was correct. Adjust these screws after that may not be the correct way to start diagnosing The screws will soon be adjusted back to default position which is 1- 1/2 turns out Next step will be to try using the manifold vacuum instead of ported vacuum and see if that help If non of these things help, buying and installing an EFI system like fitech will be actual Its ok the FE-engines are cold blooded, but i dont think the reason for this popping is my technicue Its almost like that the throttle anyway has to be "feathered" in spite that the choke have been adjusted quite high (staing on longer) and that the idle speed also is high My 63 Galaxie also use a lot of gas, so an EFI system wont hurt
Do a compression test cold and then hot,I think valves are to tight when cold. Does it have adjustable rocker arms?
"How" was the vacuum adjusted? Did you ever perform a compression test? The only direct adjustment to idle vacuum ordinarily would be the ignition timing. It is important for diagnostic purposes to measure the manifold vacuum at factory idle RPM and factory base ignition timing specification. For example I like to run my Y-Block engine at an initial setting of 12 degrees (or more.) But - the factory timing specification for Y-Blocks was 3 degrees or 6 degrees BTDC. The shop manual specification for vacuum is 19" to 21" (at sea level) Therefore a valid engine manifold vacuum test must be measured at say, 6 degrees. If the engine does not pass the vacuum test, find out why, because it will not run right. Keep in mind in this situation it won't ever run right with a Fitech either. For testing purposes at least set the ignition timing to the factory ignition timing, set the idle RPM to say 550, then see what the vacuum gauge indicates. It sounds like you've already replaced the defective crankshaft balancer, correct? So the ignition timing marks are verified to coincide with true piston TDC? That sounds like a feature, not a bug! 1.5 turns out is a preliminary bench setting only, and will always be far too rich in terms of idle AFR in a correctly functioning carburetor, a too-rich idle fuel mixture will tend to "load up" the spark plugs quickly, and soon cause plug fouling.
A compression was done a couple of years ago, each cylinder wasnt identical but quite correct for an over 50 year old engine. Doesnt have the data now So another (correct?) way to adjust manifold vacuum is to change the timing? Yes, as mentioned earlier in this thread, a new harmonic balance was installed and the marks are correct. A test as you advice me to do may be done before the winter comes, a lot of other things to do also, renovation of my newly bought residence would mostly be prioritized What I ment with "rumble" was that while coasting down a hill, not pushing the throttle pedal, with the stick in drive-position, "pops" are heard from the rear parts of the exhaust pipes. Probably wrong english word to describe what I ment. Its unburn fuel in the pipes. Because 1.5 turns is too rich, thats probably the reason
The engine manifold vacuum isn't so much a thing to be adjusted, it is something to be measured for diagnostic purposes. It is a direct indication of the internal engine health and overall state of tuning. The base ignition timing setting does have a direct influence on the average vacuum. Generally more ignition advance = higher manifold vacuum. Many people like to advance the base ignition timing to a higher setting than the factory data specification, mainly to improve performance and optimal fuel economy. But for testing purposes this will skew the measurement, for any diagnostic purpose. It's "cheating". A factory stock engine should be able to achieve 19" to 21" of steady vacuum when set at the base factory ignition timing. Many things will affect engine vacuum and have to be taken into account, including elevation or altitude above sea level, a performance camshaft with lots of valve overlap will alter engine vacuum. Short Answer: Set the base ignition timing to 6 degrees and measure the engine vacuum. Report back.
When you start the motor with the choke on, as soon as the motor starts, there is a small vacuum diaphragm that will pull the choke open so the motor will run. There is a specific amount the "choke pull off" is suppose to open when the motor is cold. If it is opening too far, the motor runs too lean, regardless of how tight the choke is set, which will cause a lean condition backfire. If the gap is set too tight, the motor will run rich the first minute or so of operation, until the choke starts to warm up. Every carb rebuild kit I've ever seen explains this adjustment and gives the proper gap dimension, but few people actually do it. With a couple pairs of pliers, you can bend the choke pull off linkage and change the amount the choke is pulled open. You can close up the gap a bit to help with a cold engine backfire or stumble, or you can open the gap a bit to help with a rich condition. A small amount of change of the open gap makes a big difference in how the car runs when its cold. The proper way to set the choke pull off is to manually close the choke, disconnect the hose to the vacuum diaphragm and connect a vacuum pump and compress the plunger in the diaphragm. While holding the choke plate closed, the correct amount of the gap opening is measured with the round end of a drill bit between the carb body and the choke plate. The drill bit is usually a 1/4" bit, but I've seen other sizes used. The correct fit is a slight drag on the drill bit as it is removed, without the choke plate moving after the bit has been removed. If its set too loose, (which I suspect yours is) bend the linkage to achieve the correct drag on the drill bit. .005 to .010 is a huge change in this adjustment. You need to adjust the choke pull off to make your motor happy, every one is different, and the only time you can tell if its right is the first 2 minutes of start up in cold weather. It may take a few cold mornings to get it adjusted for your motor. Once you have properly set the choke pull off gap, I suggest to readjust the choke back to the original setting, choke at the normal range rather then towards the rich like you have it now. If everything in the choke system is in proper adjustment and functions as designed (that includes the open heat passage in the intake) from a cold start up to fully warm should be a flawless operation. Yes, that can be done. I spent my teens adjusting choke systems for customers at the gas station I worked at. Gene
A very useful description how to adjust a choke. But the choke may not be the reason for those pops. Had a 1969 mustang with 351 windsor and drove it without choke. What happended if I started the driving too early, the engine just died. No popping
After doing the test and reporting, if no solution appears then, my last try to "avoid" buying a fitech injection system, it to swap a quite new edelbrock carb from my 1976 Torino Wagon to my Galaxie. The 351M-engine in the wagon works great, cold starts is no problem. It has a manual choke.
Remember, the old saying goes "90 per cent of carburetor problems are ignition related" for a reason. Carburetors too often get unfairly blamed. I suppose the carburetor manufacturers don't mind. One of the first things everyone usually did when they were still green when faced with a poor running engine, was replace the carburetor. Swapping out a known good carburetor is pretty fast. Not as fast as a vacuum test, which needs doing anyway. If there are internal engine defects that result in a low engine vacuum, it still won't run correctly after installing an EFI system. And ... (shhh!!) I think probably EFI off topic and verboten to discuss here.
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