Engine sputters and dies when at high rpms and under load

[bavArian]

Hello everyone,


2 weeks ago the A ran fine, but in the last few days I've been having a problem with the engine.

Current conditions: ~50 fahrenheit ambient temperature, dry, fuel has max. 5% ethanol in it and is fresh





When driving under high load and at mid to high rpms it tends to die while driving with missfire from the exhaust. If I lower the rpms, the engine stays on most of the time but I can't accelerate anymore. It's especially bad uphill. If it stays running I get missfire and a bad throttle response, making it impossible to keep moving. If I stop the engine and let it sit for a while or remove the drain plug of the carb and screw it back in (fuel running from the carb obviously) it usually starts right up and runs fine for a short time.



I've done/checked the following things:


- filled the tank to 3/4
- checked for a steady stream of fuel from the sediment bowl, looks good
- disassembled the car, everythings clean (jets as well) and gaskets are in place
- checked the float, it's the right height above the upper housing and moves freely
- cleaned fuel strainer
- float valve seems to work fine if carb is disassembled
- gas cap tight or missing doesn't make a difference







It idles perfectly but will make problems when it's running above what I guess are 1200 (??) rpm. Downhill/level roads with low revs are no problem.



Am I correct that it's not getting enough fuel? I've read that the float valve can work fine if the carb is disassembled but cause problems when running? I don't know how old the float valve is but I assume ->very<- is the correct answer.



If that's unlikely, what else could it be? Ignition seems unlikely because why would it run well for half a minute or so before making problems?


Hope that someone can guide me in the right direction.


Greetings from Germany,


Daniel

[History]

You may have a gasket that is compromised between carburetor and intake or between intake and head? I’ve not experienced this but it sounds reasonable to me?

[bavArian]

Carb to intake gasket is new and the surfaces look good. I'll check for vacuum leaks tomorrow, but 2 or 3 weeks ago I didn't find any except for the throttle shaft.

And if there is a vacuum leak, shouldn't it always run bad? It runs fine at high rpms for a short time, maybe half a minute uphill and 1 minute on flat roads?

[History]

It sounds like fuel starve. I would look for fuel flow, make sure the float is set correctly and isn’t sticking and vacuum leaks. Wish I could tell you more precisely, I think it has to be one of those.

[Jacksonlll]

Check the end of the fuel line. If it extends too far past the ferrule, it can hit the screen and restrict flow.

[livinginthepast]

Sponsored Links (Register now to hide all advertisements)

You seem to have checked out the fuel system pretty well...assuming you haven't got a grose jet fitted which are sometimes troublesome...I would suggest you try replacing the condenser in the ignition system

[wwirz]

Replacing the condenser is an easy fix/try. I had a similar issue a few months back. I was driving the car and I commented the car had no power. When hill climbing it was worse....I could take no hill at all except in 1st gear and then no throttle. The car was sputtering like crazy. Barely limped home. What it turned out to be was that the distributor cam had rotated. The car would start and idle (?) but also smoked. I retimed the engine and all was OK. I think if I were Henry F., I would have had a left hand thread on the cam screw because in the direction of cam rotation the cam screw can loosen. I ran this phenomenon of cam rotating with other club members and they had it happen to them too. Bottom line: Check your timing. If it is not the condenser.
Let us know how this turns out.

[lrf]

I had a problem with my "A" running fine for about a mile then she would run terrible, the condenser was replaced and now she runs fine. The cold condenser showed it was fine on the ohm meter, but when heated with a match, showed a short. Also check your point gap, I've seen the stationary point screw back-out and change the gap. Also, know that the distr. cam screw must have a thin lock washer under it to keep the screw tight. With 50 years of "A"ing, I've seen a lot as have most in the old car community. LRF

[nkaminar]

This may help: http://model-a.org/default.html

[bavArian]

Sorry, forgot to mention that the fuel line to the carb hasn't changed and it is ending well before the fuel strainer. So that can't be the problem either.


The float valve looks similar to this one:
https://www.macsautoparts.com/model-...8-24872-1.html
With the exception that it hasn't got the two cutouts on the float-side of the valve. Also not cross-drilled.





I'll switch the condenser for a spare new one I have here, maybe that solves it although I doubt it. The problem seems too random for that to be the cause. After all why should it return to perfect working order after sitting just half a minute, it won't cool off much inside the distributor. But maybe it really overheats because of low resistance...



Timing has been checked last week, but I'll double check when switching out the condenser.


Thanks for now, I'll report back.

[old ugly]

sounds like fuel starvation.

but try the condenser first that is easy. if that doesn't help

pull all the lines off. all the way back to the gas tank, blow them out, things like a small spider web in there or gunk may slow the fuel down just enough that it can only maintain a low rpm.
put a pail under the gas tank tap and open it, see if it runs freely. it should pour out fast.
then put the line back on the tank with a hose into a bigger pail and let it run into the pail for as long as you can. if the flow slows down the cap is not venting. BUT you said you have run it without the gas cap, if so likely not the problem.
then put the line back on to the sediment bowl, put the engine fuel line on and into the pail again, if the flow slows down then the sediment bowl screen or orifice is plugged.
then put the line to the carb. with the gas tank tap still shut off, remove the bottom half of the carb. put your pail under the carb and turn the tap on, watch and see how fast the fuel runs out. if it runs out as a good stream then the flow into the carb is fine.
this is a good time to put a small jar under the float and valve, held on with and elastic or something and check the float height make sure it is set to 5/8"floating in the gas.
once that is all done drive it. if then it dies the carb may still have dirt in it.

it may sound time consuming but sometimes there are no shortcuts.

[bavArian]

Originally Posted by old ugly

then put the line back on the tank with a hose into a bigger pail and let it run into the pail for as long as you can. if the flow slows down the cap is not venting. BUT you said you have run it without the gas cap, if so likely not the problem.

My car has a split fuel line from sediment bowl to carb, flow from the bowl is good. Carb-flow was checked with water when I completely dismantled it to check why it wouldn't run properly. Couldn't find a problem there, but will check through the drain plug of the main jet tomorrow. That seems to be the easiest way to check the entire carb for flow.


As the car only has a seasonal registration which ends with October, the 31st is the last day to test it on the road this year. If I don't find the problem today (tomorrow for you guys in the US) it's back to the drawing board for the winter.

[old ugly]

if you run it on fast idle with the hand throttle does it die?

does adjusting the GAV change anything?

[eagle]

Also check point gap. The point block wears, then the points close up.

[Rob Doe]

Originally Posted by eagle

Also check point gap. The point block wears, then the points close up.

There are some good points in this thread. Pardon the pun.

I've been in the thick of it all summer as we drove our coupe almost everyday for 30-150 miles. When the points close up, the points arm can short against the cam, rub block worn down. The stationary side of the points can back up on you if not tightened securely. I've had both these conditions in the last couple months. However, the symptoms were an occasional stumble and / or a plain old "stop dead in the water"

[old ugly]

you had mentioned that when you drive and try to achieve higher speed it pops in the exhaust and will not rev higher. backfire in the exhaust could be spark is to retarded, spark too retarded will also limit the rpms.
BUT i am thinking that you would have the spark set rite, i am just thinking out loud.

inside the bottom of that carb there are orifices and a secondary well that leads to the main jet and the cap jet. when you take off the bottom plug you say the fuel runs out but that does not mean those items are not free from obstruction. when you have the bottom of the carb off remove the cap jet, main jet, secondary well, and compensator jet. make sure the bottom of that well is clean AND that all the jets are clean. i have had carbs apart and back together so many time thinking that i had them clean just to find i missed something buried in one of those holes.

if the car is now off of the road just take your time, and don't try to short cut the fix.

[old ugly]

Originally Posted by eagle

Also check point gap. The point block wears, then the points close up.

this is a good thing to check.
over years of use that point block cinch bolt will stretch the little aluminium block at the pinch point gap. you will tighten the cinch screw down tight and think it is ok BUT it may not be holding the adjustable points contact tight.
if you ever have this issue it is easy to fix. just run a thin hacksaw blade in the cinch gap to make it a bit wider and it will now hold the adjustable point contact tight.

that little block is also prone to crack. if it does then it needs to be changed.

[bavArian]

-- Update --


Tested several things today, the gas cap was open for the fuel tests.
I'll just list everything up:


- Fuel flow at carb drain plug
The fuel flow is very high when opening it up with a closed fuel shut-off, the entire carb is dry within around 5 seconds. If I open the shut-off and let it run, it's not a steady stream but more like a steady dribble. Seems a bit low compared to testing the carb while having the bottom half removed. I'll try again with a new float valve, if the fuel flow increases that could be a part of the problem.



- Vacuum leak
Sprayed brake cleaner around the intake manifold and carb while having the warmed up engine idle. It did not rev up, instead the rpms dropped.



- Condenser
After provoking the problem out of town I stopped and removed the condenser. The engine seems to have more torque in low revs but the problem remained, couldn't feel a difference at mid to high speeds / rpms.



- Running rich / lean
While backing up into the garage I noticed that there's some black smoke coming out of the exhaust if I give it more gas. When running the carb dry inside the garage there's a lot of fumes after just a few seconds but I don't know if that's normal or not.

Also I took a look at the spark plugs when the engine was warm, they were dry but there was a lot of carbon build up on them. Changed them for new ones and maybe half a mile later already had a bit of carbon on the new ones too.



- GAV
Turning the GAV seems to not change much. To be honest there's barely any change when driving. Should there be a noticeable change when turning the GAV?



- Point gap
After the first or second high speed run the original points failed. The resin paper block probably got brittle and sheared off. Replaced it with a modern style distributor plate and points set from Werner (thanks again Werner )
If the new style distributer set is a possible error source I'll buy an original points set over the winter and switch it in spring, but I don't see why that should be an issue to be honest. Should I measure the point gap? (it's fixed and not adjustable)



- Timing
I've timed the engine at least 3 or 4 times since the problem appeared. I'm quite sure that's note the problem because advancing the timing doesn't help when it starts to miss, neither does retarding it actually.



- Carb
I'm sure the carb itself is clean, checked every pathway with a small wire as shown in the tutorial. I'll check the small hole in the secondary well but that looked clean as well.

Will do so when I disassemble the carb again to flow test the jets.





What I think at the moment is this:
Removing the condenser seemed to give the engine more torque in lower rpms, is that normal or does that point to a weak coil? Neither the condenser nor the new plugs did change the problem of it dying at higher speed.


With the carbon build-up on the spark plugs and the smoke from the exhaust the engine seems to run fine in idle but too rich when increasing the speed. I'll flow-check the jets in the next few weeks. (earliest test for the car is on March 1st anyway as it's a seasonal registration 03-10)
It could be possible that the inside of the jets were still slightly dirty before driving the car and the consistent stream of fuel cleaned them up, incresing their flow rate.
If it runs too rich it could well suck the carb dry when under high load and/or high speed.




Another thought I had yesterday would be stuck valves. Is it possible for them to start to get stuck open when the motor is turning fast and get unstuck when the rpms drop again? Seems highly unlikely though...

[old ugly]

if adjusting the GAV does not change the running condition that leads me to think that you still have an issue inside that carb.
that GAV should change the way the engine runs.

you can check the compression if you think it is valves don't expect high compression on a model A. but im guessing not valves.

[Big hammer]

Originally Posted by bavArian

-- Update --


Tested several things today, the gas cap was open for the fuel tests.
I'll just list everything up:


- Fuel flow at carb drain plug
The fuel flow is very high when opening it up with a closed fuel shut-off, the entire carb is dry within around 5 seconds. If I open the shut-off and let it run, it's not a steady stream but more like a steady dribble. Seems a bit low compared to testing the carb while having the bottom half removed. I'll try again with a new float valve, if the fuel flow increases that could be a part of the problem.



- Vacuum leak
Sprayed brake cleaner around the intake manifold and carb while having the warmed up engine idle. It did not rev up, instead the rpms dropped.



- Condenser
After provoking the problem out of town I stopped and removed the condenser. The engine seems to have more torque in low revs but the problem remained, couldn't feel a difference at mid to high speeds / rpms.



- Running rich / lean
While backing up into the garage I noticed that there's some black smoke coming out of the exhaust if I give it more gas. When running the carb dry inside the garage there's a lot of fumes after just a few seconds but I don't know if that's normal or not.

Also I took a look at the spark plugs when the engine was warm, they were dry but there was a lot of carbon build up on them. Changed them for new ones and maybe half a mile later already had a bit of carbon on the new ones too.



- GAV
Turning the GAV seems to not change much. To be honest there's barely any change when driving. Should there be a noticeable change when turning the GAV?



- Point gap
After the first or second high speed run the original points failed. The resin paper block probably got brittle and sheared off. Replaced it with a modern style distributor plate and points set from Werner (thanks again Werner )
If the new style distributer set is a possible error source I'll buy an original points set over the winter and switch it in spring, but I don't see why that should be an issue to be honest. Should I measure the point gap? (it's fixed and not adjustable)



- Timing
I've timed the engine at least 3 or 4 times since the problem appeared. I'm quite sure that's note the problem because advancing the timing doesn't help when it starts to miss, neither does retarding it actually.



- Carb
I'm sure the carb itself is clean, checked every pathway with a small wire as shown in the tutorial. I'll check the small hole in the secondary well but that looked clean as well.

Will do so when I disassemble the carb again to flow test the jets.





What I think at the moment is this:
Removing the condenser seemed to give the engine more torque in lower rpms, is that normal or does that point to a weak coil? Neither the condenser nor the new plugs did change the problem of it dying at higher speed.


With the carbon build-up on the spark plugs and the smoke from the exhaust the engine seems to run fine in idle but too rich when increasing the speed. I'll flow-check the jets in the next few weeks. (earliest test for the car is on March 1st anyway as it's a seasonal registration 03-10)
It could be possible that the inside of the jets were still slightly dirty before driving the car and the consistent stream of fuel cleaned them up, incresing their flow rate.
If it runs too rich it could well suck the carb dry when under high load and/or high speed.




Another thought I had yesterday would be stuck valves. Is it possible for them to start to get stuck open when the motor is turning fast and get unstuck when the rpms drop again? Seems highly unlikely though...

Bingo !! Your first test, dribbles with the gas line valve open, you should have a good flow like when you drained the carb bowl ! Remove a gas line fitting after the gas valve and blow compressed air back into the tank, you'll hear junk pop back into the gas tank!

[old ugly]

Originally Posted by Big hammer

Bingo !! Your first test, dribbles with the gas line valve open, you should have a good flow like when you drained the carb bowl ! Remove a gas line fitting after the gas valve and blow compressed air back into the tank, you'll hear junk pop back into the gas tank!

i think he is saying that "with the bottom carb drain off its only a dribble"

so if you have good flow at the line into the carb but not at the carb drain then it most likely that the problem is inside the carb.

[Gene F]

Points too close.

[nkaminar]

If you get good flow with the bottom of the carburetor off but dribbles when the drain removed then it is likely that the float is hanging up. Take the carburetor off and shake it up and down and turn it over to check to see if the float moves freely. If not then take it apart to see if you can figure out where it is hanging up.

[nkaminar]

Deleted

[noboD]

You said you used brake clean to check for vacuum leak and it slowed down? I don't think brake clean burns. Better try something else, maybe propane.

[1crosscut]

Sponsored Links (Register now to hide all advertisements)

I'm not a big fan of the modern points. For me they are more difficult to set the point gap.
Your say your points gap isn't adjustable. You should be able to adjust it. What is the measurement of the gap you have now?

Pull the fuel line off at the carburetor and test the flow at that point. While you have it pulled free from the carburetor look to see how much of the fuel line sticks out past the ferrule. If it sticks out too far it can push against the fuel filter screen in the carburetor causing a lack of flow.

When you sprayed around the manifolds the engine speed didn't go up but went down. Just the fact that it changed speed when you did so it points to a leak in that area.

Leave the condenser in place when running the car. Swap it out for a new / different one instead.

You got to go for a test drive or two before March to make sure it's fixed!!

[Brad in Germany]

I had a similar problem where my engine started and idled just fine, but when I revved the engine to higher RPM's, it sounded like it was choking (starving for fuel?) and if I kept the accelerator pressed down, the engine eventually started to back fire (quite loudly).

I tried all of the things mentioned above with no success.

It turned out that my spark plugs were fouled (and possibly shorting at high RPM's). Once I thoroughly cleaned them and especially cleaned (and lightly sanded) the electrodes and then re-gapped them.....everything now runs perfectly.

Brad in Maryland

[old ugly]

the problem with the modern points is. good quality points are hard to find. most of them now have a plastic follower instead of the bakelite type. the plastic follower is rivet to the point spring and will cause a point of high resistance which will melt the plastic follower which will close up the point gap. i have changed so many of these junk points that now i will never put in a modern points plate. the original style point work just fine.

look at the points follower and see if it is plastic and if it is still intact, if they have melted and tightened up the point gap up throw them away and see if you can locate a good set that have the hard follower or change back to the original style points.

[super flat]

If your rpm changed with brake cleaner you have a vacuum leak!

[noboD]

Originally Posted by super flat

If your rpm changed with brake cleaner you have a vacuum leak!

Zactly. He said the idle went down when he sprayed brake clean. BC does not burn so it would go down not up.

[bavArian]

Originally Posted by noboD

Zactly. He said the idle went down when he sprayed brake clean. BC does not burn so it would go down not up.

I know that there's a small leak at the throttle shaft, but I'm still not sure if the manifold has a leak to the block because some of the bc-spray gets sucked in by the carb as well.


Guess I'll have to check again with a short hose connected to the carb to make sure. Why it should only occur at higher rpms/load is something that's beyond me though, I doubt the manifolds heat up so fast that a crack would open up.


Anyway, first on the list is flow testing the jets and changing the float valve.

[1crosscut]

What is the measurement of the points gap?

[dpson]

Beg, borrow or steal (ok, don't steal one) a known good running carburetor and put on your car and see what happens.

[MichaelSchwarzer]

Hi, where are you located? I have some flow tested working carburetors here. We could do some road tests next spring.
Cheers Michael

[bavArian]

Originally Posted by MichaelSchwarzer

Hi, where are you located? I have some flow tested working carburetors here. We could do some road tests next spring.
Cheers Michael

Hi Michael,


thanks for the offer, I've sent you a pm.


To everyone else, I'll report back after checking the jets.



Greetings,


Daniel

[bavArian]

Update:


Last week I ordered a new float valve for the carb and installed it. The first test drive today was promising: It can now run up to around 45 mph without stalling and doesn't stall even when climbing a hill with ~10% with mid rpms in 2nd gear. (~15-20 mph)


I did notice however that the ignition advance lever didn't make a lot of difference. It is working, as I can feel the engine start to knock when I advance while idling, but there's not much difference on the road whether it's on 10 o'clock or fully pulled down.
So I'll check the timing tomorrow, maybe it's a bit retarded.




The torque at low rpms is good as it pulls away from a standstill nicely. As soon as I reach said 40 to 45 mph it just stops accelerating and starts to sputter, I think through the muffler, from time to time.

Also when running the carb dry to park (at ~1000 rpm I'd guess) the engine runs normal but just before it dies the revs climb noticably for maybe 1 second.

Am I right that it's still running too rich on at least the main jet and possibly also the others? GAV was completely closed by the way.
Haven't flow tested the jets yet, that's next on the to-do-list after an oil change.

[bavArian]

Another update:


In the meantime I disassembled the carburettor and flow tested the jets. Main and cap jet were relatively fine, with the main being a bit too lean. (fixed that, now they have the proper flow)
The comp jet flow was way too high, around tripple the normal flow rate. Soldered it up and tinkered with it until the flow was right.



Now the car won't go past 35 mph, it seems to run too lean. (getting lots of vibrations at ~30 - 35 mph without any backfire, no smoke as far as I could see)

MichaelSchwarzer sent me one of his spare carbs for testing and the engines runs fine with that one. Top speed was ~55 mph (measured via gps) and it pulls nicely as well.


That means it's definitely a carb-problem.

I've noticed there are 2 differences between Michaels carb and mine:


- the shaft of my throttle plate has a bit of play on the lever-side
- turning the GAV on his car seems to make a difference compared to the one on my carb


So I'll have to re-check the GAV on my carb, it seems to me that either the secondary well doesn't get enough fuel from the float bowl or the GAV won't allow enough fuel to pass to the cap jet.

Is that the right direction or am I totally wrong?


Thanks and have a nice day,


Daniel

[BillCNC]

This just happened to me, ... in the end, it was the Armored Ingnition Cable shorting out when ever the vibrations got to much. Drove fine under 35 mph, or under 1300 rpm.

Regards
Bill

[Gator6]

Originally Posted by bavArian

I know that there's a small leak at the throttle shaft, but I'm still not sure if the manifold has a leak to the block because some of the bc-spray gets sucked in by the carb as well.


Guess I'll have to check again with a short hose connected to the carb to make sure. Why it should only occur at higher rpms/load is something that's beyond me though, I doubt the manifolds heat up so fast that a crack would open up.


Anyway, first on the list is flow testing the jets and changing the float valve.

Daniel, I am having the same exact problem. I just bought my car about 6 months ago and haven't driven it much because I broke everything down, dropped the pan, drained the radiator, replaced the water pump, new battery, all new tires, etc etc. So While I was at it, I decided to rebuild the carb. Everything came apart fine and went back fine, but now when I drive at high RPMs, no power and it starts to backfire. So I'm going to replace the condenser, and then start on the carb. How exactly do you flow test the jets? Is there a link anyone can share?

Thanks. Newbie Rich

[CT Jack]

Please explain what you mean when you say you have a split fuel. I have one line exiting my fuel tank that feeds directly into the carburetor.
Do you have access to a Zenith carburetor that is known to be working properly? I would remove your existing carburetor and try running with the Zenith. It appears the carburetor you have been using has some internal flow problems at the higher flow operating range. As someone has mentioned you are starving for fuel at the higher engine speed range.

[nkaminar]

I don't know what bavArian meant by a split line but my first Model A had a rolled fuel line that a split the length of it. It may have been original. It eventually started leaking gasoline when the split opened up and I had to replace it with a regular tube.

If it runs well with the borrowed carburetor, as you say, and you cannot get the original carburetor to perform, then consider sending the carburetor in for repair to someone who specializes in Zenith rebuilds. They should test the carburetor on a car before sending back to you.