Jerky / Stuttering Engine

[chrischew]

OK Guy, please go slow on me as I am very inexperienced but I would like to try. Here is my problem.

I have a 64 Corvair Coupe .. pretty much stock. During my 2 years ownership, I have experienced times when my Corvair engine will become jerky at low gear and speed. This happens during acceleration or even when I keep my pedal constant. It also happens when engine is cold or warm. I guess that it is still stuttering at higher speed except that the effect is not felt due to the momentum of the car.

The only thing I have done is balance or synchronize the air flow between the carbs using a gauge and adjusting the idle screws. Sometimes this seems to work .. and the car runs relatively smooth (relative because I can still fill very slight jerkiness) but truly I am not sure I have tackled the real issue.

As a novice it feels like one or several cylinders is starved of fuel or not firing. Appreciate your collective guidance as to how I can attempt to diagnose my problem.

One thing I noticed - with the air filter assembly removed, I gently covered the left carb with my the palm of my hand and slowly reduced the air flow. As I did so I hear the rpm increasing When I did the same with the right carb, I hear the rpm dropping and the engine stalling. What can I read out of this.

For full disclosure, I bought this 2 years ago and have pretty much kept it as is, driving it on weekends and did 1 oil change after 1 year of ownership (due for another soon) since the miles driven is less than 1000.

Thanks so much

[joelsplace]

The carburetor that increases engine speed as you cover it up is probably stopped up. It is certainly running lean which could also be an air leak.

[bbodie52]

Your description of the problems experienced could indicate a fuel starvation problem or possibly a carburetor in need of an overhaul/rebuild. The shop manual procedure shown below involves checking both the pump output pressure and fuel volume delivered. This procedure can test the fuel pump for proper operation. If the pump is found to be functioning properly, you can also check for vacuum leaks. If you methodically eliminate external faults with fuel delivery or vacuum leaks, rebuilding the two carburetors by chemically cleaning aol of the passages and metal components in carburetor parts dip and then carefully reassembling the two carburetors using rebuild kits may resolve your problems.

The Corvair mechanical fuel pump is usually pretty good at pulling gasoline the length of the vehicle to prime the fuel pump so that gasoline can be pressurized to fill the carburetor float bowls. But a small air leak or crack in one of the short rubber hoses at either end of the fuel line between the tank and the pump can create what amounts to a vacuum leak in the fuel feed line that supplies the pump. The pump can cycle many times as the engine cranks and the battery drains while sucking mostly air from a fuel line air leak instead of getting a good "drink" of gasoline from the tank.

An improperly inserted mechanical fuel pump may also limit the motion of the pump diaphragm, if the pump insertion is too shallow and not allowing the push rod to take a full stroke. Refer to the notes and illustrations below to see how the pump set screw is supposed to insert into the tapered hole in the side of the pump shaft, and not just push against the side of the pump shaft. Proper installation ensures that the pump is properly positioned in relation to the push rod, so that the pump gets a full stroke with each rotation of the crankshaft. Good fuel volume from the pump is considered to be 1 pint of fuel in 40 seconds or less at cranking speed.

To check for the possibility of fuel starvation, you should measure fuel pump pressure AND volume! It is easy to have a pump that delivers correct pressure, but cannot produce adequate fuel volume. A feed line leak or cracked/damaged/loose rubber hose between the tank and the pump can create an air leak that will keep the pump from being able to create a solid vacuum to pull a good supply of fuel from the tank. Without adequate fuel supply, the pump can generate adequate fuel pressure to the carburetors, but only until it is starved for gasoline from the tank. The pump must also be installed to the proper depth and anchored properly using the tapered bolt that is screwed into the tapered hole on the side of the pump.


https://ssl.corvair.com/user-cgi/catalo ... nd_page=65
https://ssl.corvair.com/user-cgi/catalo ... ow_page=57

The pump push rod is driven by a cam lobe on the crankshaft. The repeated "upstroke" causes the pump to form a vacuum in the feed line from the fuel source (tank or gas can). As the fuel is drawn into the pump chamber, the one-way valve in the pump inlet closes at the top of the stroke, and the spring in the pump forces the diaphragm back down to push the fuel out of the pump through the other one-way valve, toward the carburetors. The spring tension determines the fuel pump outlet pressure.

Check to make sure you are fully inserting the pump so that the pushrod driving the pump gets a full stroke to drive the fuel pump. Proper installation of the new parts is critical,,,

To remove and reinstall the fuel pump, be sure that you have installed it properly. It is important to ensure that the fuel pump is correctly seated and installed. There is a hole in the side of the pump shaft that the tapered bolt tip must seat into. If the pump is sitting too high and the bolt is simply pressing against the side of the pump housing, rather than seating inside the tapered hole, the pump push rod will not be doing its job. Fig. 57 in the shop manual page shows the tapered hole that the tip of the bolt fits into. This ensures proper installation and seating of the pump.

When some mechanical pumps aren't pumpin', the pump itself may not be bad. Check the two short fuel hoses in the line between the tank and the engine compartment. They sometimes deteriorate and develop a leak or crack that allows air to enter and makes it impossible for the pump to develop the vacuum needed to draw fuel from the tank to the pump. Some new short hoses and clamps may be all that is needed!

[Nashfan]

A easy thing to try before ripping into the carbs is to run a couple of bottles of Techron Fuel injector cleaner through it... one bottle per tankful. I've found that this stuff does a great job on carbs also, and did it many times when my engine was still carbed (EFI now).

[chrischew]

Thank you all ... looks like I have my homework for the next few weekends. I will be sure to post updates and or more questions.

I am going for the vacuum leak angle, starting with low hanging fruits:

1. Check the 2 flex hose from the tank
2. Tightening the diaphragm screws on the pump (i remember seeing some fuel leak stains)

[joelsplace]

Be careful with those fuel pump screws. I've heard of people pulling the threads out of the base.

[chrischew]

I had a chance to work on my Corvair last weekend. The things I discovered were:

1. Tapered locking screw/bolt to hold fuel pump was loose, so I tightening it. If I recall correctly, there is a locking nut that helps hold the bolt once in position. I assumed (I realized that this assumption may be erroneous as I type this message) that the positioning of the bolt is correct so I simply tightened it until I felt some resistance.

2. I also notice fuel vapor escaping from the side of the fuel pump. I tightened diaphragm screws very carefully but could not eliminate the vapor leak.

While not eliminating the vacuum side issue (I plan to replace the 2 short rubber hoses), what is the best way to address the pressure side of the fuel pump?

1. Should I purchase a replacement pump from Clark Corvair? or Buy a Repair Kit? What are your recommendations.
2. What are the steps to remove and reinstall the pump and hoses (I assume that I will have to drain the fuel tank or is there a way to avoid it?)

Thanks again.

[joelsplace]

Last I checked no one makes a repair kit for the fuel pump. One company was making them a few years back but they did the hole spacing wrong and they didn't work.

[Deadwolf]

If your still using the standard mechanical fuel pump, you should not have to drain the tank to remove it or disconnect it. Only places you have to worry about it is if you disconnect the rubber line between the tank and the fuel line under the passenger side foot well.

[chrischew]

I came across a 2008 article about Corvair Fuel Pump written by Richard Finch for the Tuscon Corvair Association. There he recommended Antique Auto Parks Cellar in Massachusetts (781-335-1579) as place to buy good repair kits with the diaphragm made with non-silicone (link below if anyone is interested). I called them today and they still make and sell them. The person I spoke to asked me to check my fuel pump model and he said that I would have to take is out of the engine to see the numbers. Is this the only way to get the fuel pump model number? Thanks

http://www.corvairs.org/Corvairsation/08Dec.pdf

[chrischew]

Got around to removing my fuel .. with intention of either rebuilding or replacing. In the process I found one of the screws has stripped.

Does this mean the pump is now useless?

What brand of replacement pump is good from experience? Airtex, Precision, Carter ... other?

When re-installing the fuel lines back to the pump, is there a need for any sealant? Should I replace the 2 brass connectors (T for the 2 outlets and one of the inlet)

Can anyone suggest what to buy to replace the 2 flexible hoses from the fuel line coming from the tank.

Should I add an inline fuel filter? Is this a good addition?

Thanks
Chris

[bbodie52]

» Tue Apr 06, 2021 2:32 pm

Got around to removing my fuel .. with intention of either rebuilding or replacing. In the process I found one of the screws has stripped.

Does this mean the pump is now useless?

I saw this on Facebook, and thought it was worthy of a reprint here on the Forum...

Not sure if anyone else has already done this?

I was tired of the fuel pump bolts stripping the softer aluminum base, so I replaced them with 50 mm bolts threaded all the way though with lock nut on the other end to "clamp" the pump between them.

45 mm bolts would work, but 50 was all I could find, so I'll just cut off the extra 5 mm.

I did kind of the same thing on the oil pan bolts, since they thread into aluminum also.

This happened even with a new fuel pump -
This one was particularly bad but was replaced quickly, but prompted a different approach?
https://www.youtube.com/watch?v=ZIC7XmPFriw

» Tue Apr 06, 2021 2:32 pm

...What brand of replacement pump is good from experience? Airtex, Precision, Carter ... other?

Personally, I have had very good luck with the original GM mechanical fuel pump reliability, and never had one fail in more than 10 Corvairs since 1961, when my parents purchased their first... a 1961 Monza coupe. While switching to an electric pump is possible and often recommended, it can mean trying to find a safe, protected mounting location close to the fuel tank outlet (recommended, since electric pumps do a better job of pushing the fuel long distances instead of pulling the fuel all the way from the tank to the engine compartment). But this means pressurizing the long fuel path for the length of the Corvair. An electric pump mounted near the fuel tank can also cause pump noise to telegraph into the passenger compartment, bringing unwanted electric pump noise that may bother the passengers.

The factory mechanical pump has been tested in millions of Corvairs over decades of operation, and does a very good job of sucking the fuel from the tank to the engine and then delivering it reliably to the carburetors. A leak in either short flexible hose at the tank end or next to the starter motor will only be a VACUUM LEAK, and will not tend to leak much gasoline or create a fire hazard. Such a vacuum leak in one of these fuel hoses will appear to be a fuel starvation problem, since the fuel will not make it to the pump inlet (like a drinking straw with a split or hole in its side). Owners may think the mechanical pump has failed, when the problem is actually just a cracked or split rubber fuel hose at either end of the long steel fuel line.



A proper electric fuel pump installation should also include one or more electric safety switches to automatically cut off the electric pump if the engine stops. A primary safety cutoff switch senses the loss of engine oil pressure, and then cuts power to the fuel pump. A secondary safety switch can also be installed that senses a vehicle impact in an accident and cuts power to the fuel pump if a collision occurs. All of this adds up to extra cost and fuel pump complexity, which can increase the possibility of a failure that can be difficult to troubleshoot (i.e. pump failure, electrical component failure, safety switch failure, etc.) Although there was a period of time where a batch of faulty, poorly made replacement mechanical pumps made it into the supply chain, those poorly made pumps have been all but eliminated.

Clark's Corvair Parts and other suppliers have made a strong effort to provide good replacements for Corvair mechanical pumps, if one is needed (mechanical pump rebuild kits are generally unavailable). But you may find that a stripped screw can be replaced with a longer screw and nut, and the internal pump components (diaphragms, valves, etc.) prove out to be serviceable. Be sure to check the two feed hoses periodically to prevent vacuum leaks on the fuel delivery side between the tank and the pump. You should also be aware that a rusty, dirty aging tank (inside the tank) can cause a blockage in the fuel inlet filter inside the tank, and that a 50+ year old tank may be ready for a replacement.

https://ssl.corvair.com/user-cgi/catalo ... ow_page=65

The filter inside the fuel tank can also deteriorate and may be clogged...



Strainer Sock, Fuel Sending Unit, 1954-88 GM, 5/16"

https://ssl.corvair.com/user-cgi/catalo ... ow_page=69

Fuel Pumps

We have only been able to locate 2 manufacturers of original style Corvair fuel pumps. Since 1973, we have consistently seen 1-2% of new fuel pumps fail. From about 1998-2001, the failure rate went to nearly 10%! We finally convinced the pump manufacturer that they were using the wrong diaphragm material! Currently, about 1%-2% of the fuel pumps continue to have problems. The problems have usually been seepage of fuel or complete failure resulting in no fuel or a rupture of the main diaphragm. The main supplier (C3403) is now using "antiwicking" diaphragms & has returned to all 3 diaphragms having fabric reinforced material. We've also added a pump from a 2nd supplier (C3403A). Our experience with both is nearly identical. A spare pump is always a good idea.

FINALLY...

The pump push rod is driven by a cam lobe on the crankshaft. The repeated "upstroke" causes the pump to form a vacuum in the feed line from the fuel source (tank or gas can). As the fuel is drawn into the pump chamber, the one-way valve in the pump inlet closes at the top of the stroke, and the spring in the pump forces the diaphragm back down to push the fuel out of the pump through the other one-way valve, toward the carburetors. The spring tension determines the fuel pump outlet pressure.

Check to make sure you are fully inserting the pump so that the pushrod driving the pump gets a full stroke to drive the fuel pump. Proper installation of the new parts is critical,,,

To remove and reinstall the fuel pump, be sure that you have installed it properly. It is important to ensure that the fuel pump is correctly seated and installed. There is a hole in the side of the pump shaft that the tapered bolt tip must seat into. If the pump is sitting too high and the bolt is simply pressing against the side of the pump housing, rather than seating inside the tapered hole, the pump push rod will not be doing its job. Fig. 57 in the shop manual page shows the tapered hole that the tip of the bolt fits into. This ensures proper installation and seating of the pump.

[chrischew]

Fixed!!!
Thank you to all who posted here and elsewhere. It was a great help collectively fir this wannabe mechanic.

Tried replacing screws n nuts to see if a tighter fit can save my pump but alas no. Got a replacement Carter pump, remember to tighten the screws because someone here advised.

At first pump worked but then died. I then remembered the advise here about not seating the pump properly. Undid the set screw, managed to push the pump down another 1/4” n this did the trick.

Will try to rebuild the old pump for spare where the kit is available from the source I posted before. That gentleman told me it will be available in 2 months.

Best