Technical New engine troubles - missing when rev'd up

[marcus550]

This little car is really trying to kill me. I have a new, complete, 650cc Sport engine from Mr Fiat. Installed it two weeks ago and after a little tinkering and getting the 650 to fit my old 500, it started and ran fine. Took it for a 15 mile trip to dinner, then 15 miles home, and it ran fine. Next day, took it 5 miles to a friends house, turned it off, started it 15 minutes later and it hasn't run right since. Down on power (WAY down) and won't rev up without missing, bucking and mild backfiring. We drove below 1k RPM bucking and puttering the whole way home.

A timing light on #1 showed it was missing, so I chased that for a while. Points set, timing checked, even tried a new coil and a 2nd distributor and wires from the old 500 engine. Now the ignition is perfect, but the problem remains.

Checked the head bolt torque and did a valve adjustment just in case. No change.

So, fuel. I've got a clear fuel filter that looks empty all of the time. I don't see how the car runs without this being full of fuel, but it does.

When I installed the new engine, I ran the 28IMB's return line to the main fuel line entering the engine bay, before the filter and the pump. Thought this might cause issues, so pinched that off. No change.

To eliminate the fuel line in the car, the fuel tank and the pickup, I ran a hose from a gas can right in to the fuel pump. No difference. Rev it up and it doesn't like it.

Sprayed carb cleaner at the base of the carb and idle changed. Tightened the carb mounting screws and it no longer reacts to the carb cleaner sprayed here. No change in my problem, though.

The Haynes manual says my symptoms point to a clogged main jet. Chris posted lots of info on the 28IMB, so I looked up how to get the main jet out. Chris shows access to the main jet from outside the carb, but apparently this doesn't apply to my new Polish Weber! Before I pull it apart, I thought I would try the old 26IMB. It starts and idles fine, but as soon as I crack the throttle open it dies. Easily restarts, but won't rev above idle at all.

I'm suspicious of the fuel pump but need your feedback. Should the fuel filter have a mist, a little or a lot of fuel in it? I pulled the filter out just in case, but no change. The engine has a new fuel pump. New everything! In an attempt to raise fuel pressure, I removed one gasket from the pump's mount. No difference. There is a screw on top of the fuel pump - what is this for? I haven't tried it yet.

The new engine originally ran FINE for three short trips. What changed? I can see dirt clogging the main jet of the 28IMB, so I'll chase that next. What else can suddenly do this? What have I missed?

 

[Franko500]

The backfiring usually points to ignition. Have you checked the plugs & dizzy leads/cap. If a plug is breaking down that can prevent engine from revving

 

[fiat500]

You aren't having a lot of luck.
1. I would ask the seller of the engine.
2. If you're still doubtful about the pump, with care you could try running it by siphonage direct to the carb.
3. Having only two cylinders, there's no leeway if the slightest thing goes awry. So as Sean says, even a faulty plug, which needn't be visually obvious, would be enough to send the engine crazy. You do seem to have covered all bases though.

 

[Vitesse]

I wondering whether where you have the in-line fuel filter maybe causing a problem. The majority of them are usually positioned between the fuel pump and carb, including mine.

Having it before the pump maybe restricting the flow of fuel to the pump. The fuel pump pushes fuel to the carb, so therefore it is forced through the fuel filter at greater pressure. The fact that you say it is virtually empty all the time is odd, if fitted after the pump then mine is normally half empty when cold, then I see it bubbling away until it is normally full up once the car is fully warmed up and the engine rev'd a few times. Probably isn't this as you said it drove for 15 miles ok before but just a thought.

But what you also describe when driving sounds like it is missing on one cylinder, the fact that you discovered it was doing that on number 1 and then you checked all the associated timing bits but say the problem still remains. It's a little unclear are you saying it is still missing on number 1 or that has now gone?

I've had exactly the same issue before where it misses on one cylinder and the revs are really low, then you attempt to rev the engine up and just sort of burbles, coughing and spluttering the occasional back fire and if you are really lucky the missing cylinder will burst into life.

As Sean and Peter suggested it definitely sounds like a timing issue still.

Tony

 

[F123C]

I agree with all the other members who suggest a spark plug problem.

Iirc the screw on top of the fuel pump holds the dome cover in place, underneath is a gauze/mesh screen filter.

The new engine looks amazing !!

Al.

 

[F123C]

I agree that the fuel filter is normally fitted between the pump and the carb.

In the absence? of a fuel pressure gauge we used to check fuel pump volume (output) by running the fuel pipe/hose from the carb into a measuring container ( a 1 pint milk bottle!) and feeding the carb using a gravity-feed ( lawnmower fuel tank!), engine idling for 1 minute should give circa 1 pint.
To check pressure the hose when detached from the carb should throw the fuel iirc at least 2 feet horizontally (2-4 psi). You do this last test at your own risk.

Al.

 

[Vitesse]

I agree that the fuel filter is normally fitted between the pump and the carb.

In the absence? of a fuel pressure gauge we used to check fuel pump volume (output) by running the fuel pipe/hose from the carb into a measuring container ( a 1 pint milk bottle!) and feeding the carb using a gravity-feed ( lawnmower fuel tank!), engine idling for 1 minute should give circa 1 pint.
To check pressure the hose when detached from the carb should throw the fuel iirc at least 2 feet horizontally (2-4 psi). You do this last test at your own risk.

Al.

From reading your other posts Al you previously used to work or still do on servicing Fiats, including the little 500?

My understanding is that the fuel pump is basically a bowl that the fuel flows into and the the diaphragm driven by the engine pushes fuel up to the carburettor but as it is just a bowl when the engine stops most fuel will drain back away from the carb by gravity and a small amount will be retained in the fuel pump bowl for the next start up. Hence why you mostly see an empty or half full fuel filter before starting up.

Do you reckon there is any advantage to be gained by fitting an in-line fuel non return valve either before or after the fuel pump to give you a better startup from cold or are you just going to flood the carb?
Tony

 

[fiat500]

Tony, the fuel pump has two, simple , spring-loaded valves internally. One on them is pushed shut after every drawing in of fuel the other is spring shut as soon as fuel stops being pushed through it. the problem is that these are very lightly loaded springs and the valves are susceptible to being kept open by very slight particles of debris. But in theory there is nowhere for the fuel to go in any case because siphonage will keep a pressure at the pump inlet and the only place to go is through the diaphragm to the engine or out of pipe unions or the pipe itself. Both of these are disastrous and soon identified so I doubt any additional help is needed.
i suspect that any need to crank petrol into the engine at startup is most likely related to evaporation from the carb bowl but stand to be corrected.

 

[marcus550]

I forgot to mention I tried new plugs, the plugs from my old 500 engine which were practically new. Checked gap on both sets. No difference.

I originally thought I had an ignition problem. A timing light on #1 showed it missing (light would go out occasionally.) My first thought was the old coil, the only part NOT new here, was cooked. I changed it and the problem remained. I then tried the timing light on #2 and it was also missing. Tried it on the coil-to-distributor cap wire and it showed the same thing. Sounds conclusive. I changed all of the plug wires, added a ground strap to the distributor, new cap and rotor...no change. Tried the 500's distributor. No change. I ordered a Pertronix module just in case. In the mean time, I found the gap was WAY off on both distributors and set that per the manual. Tried both of them again, no change in behavior of the engine but I no longer see the timing light go out at random. The timing light shows I'm timed perfectly and never misses a beat. The engine still runs poorly when rev'd up, misses and coughs through the carb occasionally. Idles perfectly.

That is when I started chasing the fuel. I can't blame the filter as I had removed it completely at one point, but if it belongs on the other side of the pump I can easily change that. It will allow seeing the fuel being pushed up to the carb which is more meaningful right now anyway.

I'll tear in to the carb as well. Hopefully I can get to the main jet without damaging the gaskets. I don't have a rebuild kit for this guy.

I'm not ready to meter the fuel pump. Hate working around hot engines and flowing fuel to start with. Good to know the results, in case I get desperate and go for it.

 

[fiat500]

Marcus, you're now focusing, in as I did a couple of weeks ago, with the same problem...idle good....revs rubbish.
I had already dismantled the whole carb so doubted it was wrong. Sounds mad, but I boiled the carb body minus all jets and seals in a pan of water over the kitchen hob with a dishwashing thrown in. It worked great for me.
Good luck.

 

[FiorinoDan]

I'm assuming from what you have said that you are using the mechanical fuel pump? It may pay to try (even temporarily) an electric fuel pump and see if this helps. At least it will rule out the pump. Clean out the carb, and if all is not well, I would do a compression test to check for a sticky valve, particularly if the engine has not long been rebuilt.

 

[F123C]

From reading your other posts Al you previously used to work or still do on servicing Fiats, including the little 500?

My understanding is that the fuel pump is basically a bowl that the fuel flows into and the the diaphragm driven by the engine pushes fuel up to the carburettor but as it is just a bowl when the engine stops most fuel will drain back away from the carb by gravity and a small amount will be retained in the fuel pump bowl for the next start up. Hence why you mostly see an empty or half full fuel filter before starting up.

Do you reckon there is any advantage to be gained by fitting an in-line fuel non return valve either before or after the fuel pump to give you a better startup from cold or are you just going to flood the carb?
Tony

Hi Tony,
fiat500's explanation on the mechanical fuel pump is spot-on!! so I won't waffle on (as I usually tend to do!)
In the old days it used to be possible to rebuild these pumps,later pumps were sealed or internal parts not available.
There shouldn't be any need to fit a non-return valve as there is effectively one internally,so not sure if it would help or cause a problem.
I served an apprenticeship as a car mechanic with a Fiat/Lancia main dealer in the mid-late '70s and worked on other makes subsequently.
Retired some years ago,health reasons,now a fulltime carer.
My knowledge of Fiat 500 etc is therefore from a long time ago and my memory is maybe a bit hazy/incorrect on some aspects but I volunteer any vague recollections/ideas/tips in case they may help someone!!
Regards,
Al.

 

[F123C]

From reading your other posts Al you previously used to work or still do on servicing Fiats, including the little 500?

My understanding is that the fuel pump is basically a bowl that the fuel flows into and the the diaphragm driven by the engine pushes fuel up to the carburettor but as it is just a bowl when the engine stops most fuel will drain back away from the carb by gravity and a small amount will be retained in the fuel pump bowl for the next start up. Hence why you mostly see an empty or half full fuel filter before starting up.

Do you reckon there is any advantage to be gained by fitting an in-line fuel non return valve either before or after the fuel pump to give you a better startup from cold or are you just going to flood the carb?
Tony

Hi Tony, some further info on the working of a fuel pump in case you're interested.

The pressure generated by the pump is determined by the strength of the spring underneath the diaphragm (not a lot of people know this!)

The engine,usually an eccentric on the camshaft,therefore half engine speed, operates a lever or pushrod to pull down the diaphragm thus drawing fuel in to the pump chamber. The internal spring forces the fuel out.

So briefly the engine causes the fuel to be drawn into the fuel pump, the internal spring causes the fuel to be forced out of the pump under pressure.
The pressure is controlled by the internal spring, the volume(output) in determined by the chamber volume and the diaphragm stroke(slight adjustment via gaskets under pump mounting flange).

As others have said fiat500? the little automatic disc valves can stick or be held slightly open by a little dirt etc, especially on an old pump left unused for some time.

Al.

 

[F123C]

Re:- Position of in-line fuel filter.
Had a think about this, any I've seen were positioned between fuel pump and carb. i.e. the fuel is forced through the filter.

If the filter was to be fitted between tank and pump, then it would be under suction (depression?). The filter being bigger in cross-sectional area would mean the suction is reduced across the filter (pressure = force/area). Maybe this is why the filter never seems full if fitted before the pump?? IDK, just guessing!!

Al.

 

[F123C]

Re:- Clearing carb blockages without stripping down the carb!!!!

A friend of mine in the trade used to clear some carb.blockages by revving up the engine then placing his hand over the choke opening i.e. the open top of the carb , until the engine had nearly stopped then repeating as necessary until the blockage? had been cured. IDK if this would work on a Fiat 500 carb but it's worth a go?? Worked sometimes for him......

Al.

 

[Vitesse]

Thanks Al. As confirmed but yourself and Peter there is one already built in internally and the requirement to crank a few times is probably due to evaporation post the fuel pump or some leakage back through the disc. Let's face it, it is never going to start like a modern car as everything is mechanical.

Tony

 

[marcus550]

Update - I moved the fuel filter to the pump-side and it still isn't full of fuel when the car is idling. It gets about 1/5 full and never changes.

I also installed a Pertronix Ignitor last night. The wandering idle has been cured, it idles rock steady now.

With the smaller 26IMB carb still installed, it starts and idles fine. As soon as I crack the throttle it dies instantly. I was able to get it to rev up to about 2000 RPM using the choke if I moved it slowly.

So, if you have a clear filter after the pump, is your filter FULL when the car is idling?

 

[Vitesse]

I have just been out and checked on my car Marcus.

Its laid up for winter in the garage now so hasn't started for a couple of weeks.

Initially all the fuel had drained out of the filter before starting, as soon as it was running ok for me take the choke off and get around the back of the car, about a minute, it was probably a third full and then started bubbling away and filling up for the next few minutes.

After 5 mins and the engine was ticking over nicely it was almost completely full as you can see in the picture below.

I now stink of Fiat 500 exhaust fumes!!!
Tony

 

[F123C]

I have just been out and checked on my car Marcus.

Its laid up for winter in the garage now so hasn't started for a couple of weeks.

Initially all the fuel had drained out of the filter before starting, as soon as it was running ok for me take the choke off and get around the back of the car, about a minute, it was probably a third full and then started bubbling away and filling up for the next few minutes.

After 5 mins and the engine was ticking over nicely it was almost completely full as you can see in the picture below.

I now stink off Fiat 500 exhaust fumes!!!
Tony

If the filter was mounted vertically, the last air bubble would probably disappear!
Did some further checking, some US cars had a non return valve built-in to the inline filter and some US cars had a check-valve(Non-return valve?) fitted at the carb inlet. So maybe there is some benefit in fitting a non-return valve after all providing its compatible with modern fuel and low (relatively) pressure i.e circa 4psi iirc.
Nice clear pic, by the way!
Al.

 

[Vitesse]

Yeah it probably would disappear Al but as it is a metal and glass filter it is a bit heavier than normal so tends to flop to one side, seems to run ok though. Btw picture was taken with a phone.

I have been googling around Marcus and come up with a possible cause. That would really fit in with the pattern of what has happened to the new engine. Hopefully!!!!!

Are you running the coil from the original 500 engine? One suggestion I have found on another forum is that when changing from a 500 to 650, so by default you are going to go from dynamo to alternator. The output of the alternator charges the Battery to a higher voltage approx 14-15v. The coil from a 500 setup isn't expecting much more than 12v, anything greater then the coil starts to overheat and breakdown. See if your coil is really hot whilst the engine is running. Also measure the LT side of the coil if it is anything higher than about 12.4v it will damage a standard 500 coil. The resolution is to fit a ballast resistor on the positive LT wire to the coil that will reduce the voltage down.

I don't know if this is the problem but it would certainly fit in with what you have experienced. First drive was ok as the coil was ok initially, then started to breakdown, then you have seen misfiring on both cylinders at some point and now it won't rev, which could be why you are not seeing any fuel going through the filter, as it can't get up enough head of steam to draw a lot of fuel.

Tony