General Mystery Misfire

[ianhunterv8]

My car has decided it doesn't want to idle any more, no idea why. It seems to go well if your foot is on the throttle, but on the overrun you can feel it jerking and hear popping and at idle it hops from one cylinder to two. If you blip the thottle it repsonds on two perfect cylinders. If you don't, it stops.

I have changed the spark plugs and the old ones were a bit white, which suggests a weak mixture, which means air getting in somewhere. This matches the misfire when not under load I think - but where has the air started to come in from?

Anyone got any ideas?

Ian

 

[oopsadaisy]

Initially I would try a new gasket between the carburettor and inlet manifold.

However, it is possible that the face of the carburettor flange may have become distorted and may need skimming.

 

[jjacob]

Could be any of the following:

Worn ignition points (gap is incorrect)
Worn distributor cap and rotor
Gasket (vacuum leak) leak at carburetor
Plugged idle jet in carburetor
Carburetor flooding at idle (float valve bad)

To check for the gasket leak just spray a little carburetor cleaner around the gasket while the engine is running. If the engine speeds up or changes at all then the gasket is leaking.

John

 

[Toshi 975]

I think that every single used Weber 26IMB cab that I have rebuilt had a distorted mounting flange. Pretty easy fix , I strip the carb , use my big vice to press the flange back as much as possible then use a sheet of wet-or-dry on a proper flat surface and run the flange face over it pressing down on the stud holes till it is flat. Even the thicker flange 28IMB (126) carbs get distorted. You have the gaskets above & below the bakelite spacer plus sometimes the spacers themselves crack. I have also had the carb work a little loose which also gives the effect you describe.

 

[ianhunterv8]

Made a start this evening. Did the tappets first as the engine was cold - slightly tight, but not enough of a problem. Managed to get a quarter turn on the carb retaining nut while the rocker cover was off. Started the car, made no difference.

Took the dizzy cap off and saw the carbon brush was a lot shorter than the new one, so fitted a new cap and rotor arm. Noticed the old cap had a perfect drilled 1.5 mm dia hole in the side - don't know why?

Made some marks on the pulley to check the timing (no marks, dots, cuts or anything from Fiat) and found 20 degrees advance or so instead of 10. Couldn't turn the dizzy enough to adjust it properly, so have removed it to see why.

Will solve that, renew points and condenser tomorrow, get the timing right and see what happens!

Part two tomorrow evening!

Ian

 

[tjmra]

Ian,

I hope someone with more technical experience chips in, but I would have though if your advance is set at 20 degrees (at idle?) you would have a real hard time even starting the car.

If your issue started with the distributor in the position it is in now and it is a timing issue, you should at least have enough adjustment to get the timing correct. I would take a couple of minutes to make sure your static marks are correct.

Regards

Joe R

 

[ianhunterv8]

Fitted the points and condenser today and put the dizy back in. I was pretty happy that my timimg marks are accurate. Fiddled about to get the timing spot on and started her up, problem is the same.

Sprayed around carb with cleaner, no change.

Removed carb, stripped and cleaned it, saw no obvious problems and put it back again. Ran a block of wood with fine sandpaper across all faces, seemed OK.

Started her up, problem is the same.

To summarise. Starts well hot or cold, idles well as long as some choke is applied, otherwise pops and misses. Idle mixture screw appears to have no effect whatsoever. I noticed on replacing carb that the carb rocked on its plastic flange rather than sitting dead flat. Maybe I'll order a plastic flange and new gaskets and try again.

Any brighter ideas?

Ian

 

[disgusted]

Mine did/still does this.

My 500L would often die when I came to a stop. It "hunted" a bit and popped on the overrun.

Partially fixed by increasing the slow-running speed, which is probably the same as applying the first bit of the choke.

I am planning to look at the carb/manifold interface/gasket. Horror of horrors, I may gloop it up with some liquid gasket stuff...

 

[jjacob]

idles well as long as some choke is applied, otherwise pops and misses. Idle mixture screw appears to have no effect whatsoever.

This is not normal. You should be able to idle when hot with no choke. The idle mixture screw should definately make a big difference when adjusted.
This is an indication that the idle circuit in the carburetor is plugged, or you have a big vacuum leak.

Also, I see you are still using points. Point dwell can change quite a bit with small changes in gap, this can lead to a bad idle condition. Setting the point gap with a feeler gauge is good to start with, but a dwell meter is much better. You should consider putting in a Pertronix or similar hall effect unit to replace the points. Never have to set them again.

I would forget about the base idle timing. You should set the dynamic timing at 3000+ rpm to 18 degrees BTDC, and let the idle timing fall where it does.

The fact that your idle screw makes no change is possibly a major part of your problem. It is definitely not right. And may indicate other carburetor issues. I know you said you cleaned the carburetor out, but sometimes the small air passages are very hard to clean. I had to soak my carb body in lacquer thinner to dissolve the blocked passages, regular parts cleaner did not do it. Then blow out every passage with compressed air.

Another area to look at is if the timing chain timing is correctly set. But first I would first look at the carburetor and convert to pointless ignition.
John

 

[ianhunterv8]

John

Everything you say is spot on! I'll take the carb off when the new gaskets arrive and have another clean.

Any thoughts on where I ought to concentrate? Everything looked OK, but clearly isn't. I wish I could see a problem, then I would know I had actually fixed something!

There must be some specific things to check in that carb that would make that idle mixture screw redundant.

This big a problem would suggest a visible blockage, would'nt it?

I don't want to have to put it all back together and just hope!

Thanks for your input so far. Who sells the Pertronix or similar?

By the way, the timing gear is new and we have done over 1,000 miles since then!

Best regards

Ian

 

[jjacob]

In the UK I believe that Middle Barton Garage sells the Pertronix kit. They also sell the Pertronix coil if you are inclined to replace that also.
John

 

[Indyspecracer]

Fuel filter? Rusty fuel tank?

 

[ecdy]

If the idle speed adjustment screw does not adjust it is possible that the seat for the needle valve (the needle is at the end of the screw) is excessively worn or has been over-enlarged in the past. You might consider replacing the carburetor, a very easy job, or of the old carb is built for it, replacing the idle-screw valve seat.

 

[ianhunterv8]

Whilst that may well be the cause, how come the car has idled perfectly for the last 1,000 miles and now doesn't?

It hasn't suddenly worn, has it?

It looks like that screw has a tiny rubber ring on the end of it, which I guess would absorb most of the wear anyway?

The mystery remains!

The fuel filter in the carb was perfectly clean and I have no evidence to suspect a rusty tank. The inline fuel filter looks clear too.

Keep the suggestions coming!

Ian

 

[dunnah01]

Compression test?

 

[ianhunterv8]

The engine has recently (1,000 miles ago) had new piston rings, so I hope I don't need to check compressions! It runs OK, it just doesn't idle. Am I OK on ordinary unleaded fuel? I have had conflicting advice on whether or not to add an additive?

However, carb is off again and I notice the throttle spindle is really stiff and the throttle will stick open (had that happen on the road once). Maybe it isn't shutting off that perfectly either. Anyway, I have stripped it, cleaned it and rubbed the shaft with fine grit paper and now the return springs return the throttle from wide open or anywhere else to a firm stop against the idle speed screw.

Could that have been my culprit?

Thanks all.

Ian

 

[ianhunterv8]

Bingo!!

Cleaned everything once again and reassembled with new gaskets and a new orange plastic drip tray - started her up, got her warm and she seems to idle nicely!!

Will take her for a test drive this afternoon, but sounds like we are all sorted. Idle mixture screw still doesn't seem to do a whole lot, but who cares! Perhaps the rubber seal is past its best? Can't seem to find where to get a new one that doesn't come with a whole new carb attached!

Thanks for all your help. Regards

Ian