problem with P505, 32/35 tmima carburettor.

[2pac]

hello
revving the engine to about 4000 rpm, car is stationary, after releasing the pedal there are loud explosions from the exhaust.
the engine was rebuilt, carburettor rebuilt, timing is ok, never drove it yet.
any ideas?

 

[Beano]

So there are no explosions coming out of the top of the carburretor, if you leave off the intake ?

 

[2pac]

Now no, but when i first started the car after a rebuilt cylinder head it wouldn't want to fire up and there were some strong explosions and smoke from the carb. I tuned the carb and ignition and it went ok.
After retightening the head bolts it fired up yesterday immediately.
It seems that the head gasket doesn't leak.

 

[Beano]

It is very difficult to diagnose over the internet.

You said that "when i first started the car after a rebuilt cylinder head it wouldn't want to fire up and there were some strong explosions and smoke from the carb".
And this indicates a timing problem. Possibly valve timing rather than ignition timing.

Explosions on the exhaust side may suggest unburnt fuel being there, and then being ignited.
Perhaps the exhaust valves are still a little open when they are supposed to be closed ?

Double-check the way you did the tappet clearances. Are you 100% certain that the method yoiu used was correct ?

After that you could re-check the ignition timing. But it sounds like it could be the valve timing.
Did you align the timing chain marks correctly ? The chain is not one tooth out, is it ?

 

[2pac]

i'm sure i did the timing chain ok. it's a bit nasty to check it now (release all ancilaries, undoing the pulley nut, remove the radiators etc.). is there a way to tell by looking on the valves from above? i'm pretty sure they all close well.

could be valve seats leaking? although the machine shop checked them. they put new intake valves, one of them has a smooth plate while the others have a recess on the plates.

could be too thick a head gasket? it's 1.6mm and was bought when the head height was minimal, but since then the head was filled to bring it back to the normal height.

 

[Beano]

It won't be because of the head gasket, and the valves won't be leaking as the machine shop checked them.

I'll think about it for a while, and someone else may give a better answer.

 

[Peter C]

Despite your feeling that the timing chain and distributor timing are both correct I would check them both very carefully. I know of a case where the owner was positive the timing chain was set up correctly but, after lots of time spent looking elsewhere, found that the chain was out a notch after all.

 

[FIVEDOOR]

I know of a case where the owner was positive the timing chain was set up correctly but, after lots of time spent looking elsewhere, found that the chain was out a notch after all.

That's an easy mistake to make, which I did the first time I replaced a timing chain. I counted the links, but my mistake was I effectively started my link count 0,1,2,3.... instead of 1,2,3,4.... from the mark on the sprocket.

 

[2pac]

Could be spark plugs?
maybe ignition curve? I have the 123ignition dizzy and set it according to xn1 and M78 dizzy.
but the head is stamped xn1t, which has a different setting.
using an ignition light, the mark on the pulley isn't 100% stationary but vibrates a little bit.

 

[Peter C]

To me, it really sounds like a timing issue and, if you've replaced the head, it could well be the timing chain. However, before going to the trouble of taking off the timing cover etc, check the simple things first. If your worried about the valves sealing, do a compression test. Check the spark plugs, spark plug leads and distributor etc. I'm certainly no expert but I wouldn't have thought that the distributor curve you mention would be far out enough to cause your symptoms.

 

[Beano]

A basic rule is always to check the simple things first.....then the more complex.

Wrong ignition timing might produce those symptoms.

However if explosions are coming only from the exhaust, and not from the carby, then what is happening is that the fuel is sometimes igniting in the exhaust pipe. And the only way I can think of that happening is that the valves are still slightly open (instead of being completely closed) when the spark plug sparks. And that means the valve timing may be slightly out.

Could be spark plugs?

No. You're grasping at straws.

using an ignition light, the mark on the pulley isn't 100% stationary but vibrates a little bit.

That's normal. They all do that. A tiny amount of wear in the central shaft of the distributor will cause that.

maybe ignition curve? I have the 123ignition dizzy and set it according to xn1 and M78 dizzy.
but the head is stamped xn1t, which has a different setting.

There will be a very slight difference in ignition curve. Only one or two degrees, I think. But that won't cause explosions in the exhaust pipe. It will only affect performance......acceleration.

Does your vacuum advance unit work OK ? When you are using a timing light, if you rev the engine, can you see the timing mark (on the pulley) move to the left ? The timing should be advancing when you rev the engine.
If the vacuum advance is not working (which is common), and you have set the ignition too retarded, then you may get explosions coming from the exhaust.


Did you set the tappets OK ? No problems while you were doing it ?
Because if the tappet clearance on an exhaust valve is too small, then the valve will remain open very slightly. (This is a bad thing.....if it is an exhaust valve, it can burn a crack in it if you drive around for a while......but I can't say how long this might take. You will be OK to start the car....to test the engine as you are now.... quite a few times though).
Can you do a compression test, as Peter suggested ? Do you have a compression tester ?

Since you have just rebuilt the engine, you should be setting the valve tappets with more of a gap than you ordinarily would. Then after 500 kilometres, you re-tighten the head gasket, and re-set the valves.

If you used a "mono-torque" head gasket (which you only tighten once), you don't have to re-adjust the tappets. But it's best to always make them a tiny bit loose rather than a tiny bit tight.

After you have thought about all this, then you can worry about checking the timing chain. Nobody wants to undo stuff like that, which they have only just done. So don't do it yet......

 

[2pac]

the dizzy is almost new, it does advance when revving, but i didn't check if the vacuum works.
after i installed the head i set the valve clearance to 0.15 and 0.30, and the explosion were already present. after running the engine i re-tightened the head bolts (they are now about 20 degrees tighter than before) and set the valve clearance to 0.1 and 0.15.

before i sent the head to refilling i don't think there were those explosions.

the ignition is set at 8 degrees as in 2.0 engines, should it be more advanced?

 

[Beano]

If it's a new dizzy then the advance will be working. Just check that the air tube is plugged in to the inlet manifold in the correct place.

Ok...the valves have been set alright.

The ignition seems to be ok. Eight degrees is right. Some people set it at ten degrees, but if your cylinder head has been shaved, it will be thinner and ten degrees will cause pinging. So just leave it at eight degrees.

I would hate to send you off on a course of action which would not be correct. So I don't want to tell you just yet to check the timing chain and whether it lines up with the timing marks on the sprockets. Perhaps you should first check that everything else is done 100% correctly ?

But could you please describe some details about how you put on the timing chain ?

Did you use a workshop manual ? And follow every step ? Was your mind calm while you were doing this ? Did anyone interrupt you and take your mind away from the job ?

How long have you run the engine ?

In the past, I confess that I put on a timing chain incorrectly. On two occasions.

I'm sure there is a way to check the valve timing by just looking at the tappets and valves as you turn over the engine. But I am not an expert on these things. It would be great if it was possible to do this, as you would only have to remove the tappet cover.
But you know, it's not the end of trhe world if you have to drain and remove the radiator, then the crank nut and timing cover.
It's irritating....yes....since you have just done it. But on older Peugeots it's a simple job.

Your spark plug leads are going to the correct plugs, aren't they ? You haven't got 2 of them
swapped around ?

 

[Beano]

Hang on a minute....you put in a new distributor. And rebuilt the engine.
Perhaps the drive at the bottom of the dizzie is one tooth out ?
Does it sit down on the block as far as it should ?

Undo the clamp at the bottom of the dizzie and pull it up a little bit. (Yes, I know....you will have to re-do the timing).
From the top, try to rotate the dizzie shaft a little. Then push the dizzie back down. Is it seating properly ?

I know it should only sit in there one way, but it is actually possible for it to sometimes sit wrongly.


Did you use the Haynes workshop manual ? Because it has a mistake in it.......regarding the shaft which goes down from the bottom of the dizzie to the oil pump.

Did you remove this shaft (it's a long, thin one) when you rebuilt the engine ?

Some of us call the Haynes manual "The Haynes Book Of Lies"

 

[robs]

Further to Beano's latest comment, if the rotor button isn't pointing at the correct terminal in the distributor cap it might send the spark to the wrong cylinder which can certainly lead to much banging.

Your distributor *probably* has a line imprinted on the top of its body aligned with the cap position for plug lead #1. If you position the crankshaft with the timing marks aligned, the rotor button should be pointing straight at the line on the distributor body. If it's exactly in the wrong direction you'll need to turn the crankshaft through another 360 degrees.

If the mark isn't very close to alignment you may be one or more teeth out in the distributor/oil pump drive shaft. Draw the shaft out by turning it with a large screwdriver and keep trying teeth until things line up.

If your distributor doesn't have a mark you can just make your own with a dot of "Liquid Paper" (say), lined up as well as you can with the #1 plug lead.

Hope that makes sense.

Have fun,

Rob.

 

[Beano]

^ Well said. But he can't be too far out as the car has been running. Certainly not 360 degrees.

What may have happened if 2pac has taken out the distributor/oil pump drive shaft during his engine rebuild.
The "Haynes Book Of Lies" tells people to align the slot in the top of that shaft (into which the dizzie fits) a certain way.
However if you do it that way, you do not have room to turn the dizzie in order to advance it while setting the ignition timing.

If this has occurred, you just have to figure out how far to rotate that shaft so that the dizzie sits in the position it USED to sit in. That is what I did.
Then the problem is that the slot in the bottom will not go into the oil pump......though you may be able to get it in by pressing down while rotating the engine by hand, using the crankshaft pulley nut.

Draw the shaft out by turning it with a large screwdriver and keep trying teeth until things line up.

When this happened to me, I used a round piece of wood and hit it into the hole in the centre of that shaft. But not too hard. It stuck there and then I was able to pull up the shaft.


2pac : first....before you do anything.....just do as Robs said and, when your crankshaft timing marks are at 8 degrees before "top dead centre" (TDC), check that the dizzie rotor button is pointing directly at the correct lead inside the dizzie cap. Not before it, and not after it.

 

[2pac]

i put the timing chain as per the manuals, cam sprocket's mark up, crank sprocket's mark down, both aligned with the axes, 2 white marks on chain up, one white mark on chain down.
in the parts catalogue they show a different cam sprocket for xn1 engines. i have 2 sprockets from xm7, 1 from xn1, although i managed to mix them there is one where the timing mark is very slightly rightwards than the other two so i used it, but i really can't see the difference, it doesn't change the position of the teeth.

the dizzy shaft was of course removed, and i inserted it with the slot parallel to the block, bigger half away from the block, engine in tdc at firing stroke, etc., but it matters only for the purpose of the dizzy's capsule not hitting the inlet manifold. the postion of the shaft has nothing to do with ignition timing.

i replaced some of the push rods with used ones from another engine, because they looked bent, but didn't replace the tappets respectively. any relation to the problem?

2pac : first....before you do anything.....just do as Robs said and, when your crankshaft timing marks are at 8 degrees before "top dead centre" (TDC), check that the dizzie rotor button is pointing directly at the correct lead inside the dizzie cap. Not before it, and not after it.

that's only the initial adjustment, isn't it? after you start the engine it may have to be altered.

 

[robs]

that's only the initial adjustment, isn't it? after you start the engine it may have to be altered.

Yes -- IF everything else is correct, in particular the shaft tooth. If the distributor shaft were on the wrong tooth, the *correct* spark position will have the rotor button pointing somewhere between the terminals and might lead to cross-firing. Checking that the rotor is pointing right at the terminal at spark time is an easy sanity check.

But, I'm pretty sure rotor button phasing is NOT your problem. The banging comes when you close the throttle from fairly high revs, no load. That'll immediately cut the vacuum advance. Since it's idling ok (full retard) and revving well (quite a bit of advance) your problem on overrun (somewhere in between) can't very well be distributor phasing.

The classic causes of backfiring in the exhaust on overrun are retarded ignition and vacuum leaks, but given that the engine's newly rebuilt, I'm afraid cam timing has to be considered.

Vacuum leaks don't seem likely since you have a reasonable idle. On ignition timing, did you make sure the crank pulley gauge agreed with the dowel hole in the bell housing? Also, you ARE timing to the timing mark not the TDC mark aren't you?

I'm afraid I can't think of any quick and easy way to be sure the cam isn't out by a tooth. There's a nice YouTube of a guy diagnosing this problem with about $1000 worth of electronics (oscilloscope, current probe, pressure transducer). For the rest of us, there's always the hard way.

Have fun,

Rob.

 

[2pac]

Vacuum leaks don't seem likely since you have a reasonable idle. On ignition timing, did you make sure the crank pulley gauge agreed with the dowel hole in the bell housing? Also, you ARE timing to the timing mark not the TDC mark aren't you?

i don't have a hole in the bell housing, it came from a very late model, and anyway i didn't mind the dowel position when i fitted the flywheel. i'm going by the marks on the timing cover, and of course i;m timing the timing mark.

could be something in the carb.? maybe the enrichment valve? when rebuilding the carb i used an old diaphragm there because i didn't have a new one.

 

[Beano]

You seem to have done everything correctly.

Didn't you have an odd problem at one stage with fuel coming out somewhere ? Somewhere in the cooling system ?

All the same, even if you have too much fuel, and too rich a mixture, you will not get explosions coming from the exhaust pipe.
That is either valve timing, ignition timing, or something related to ignition timing, like ....as Robs said....cross-firing in the distributor cap. (Perhaps in conjunction with too much fuel).

Check simple things first. Make 100% sure of each one, then move along to the other problems.

The whole thing is very frustrating, I know. It is happening because you do not work on cars every day. This happens to me also. These days, I go for a couple of years between engine rebuilds, and then when I do one, usually I forget something and stuff it up. Then (after kicking myself ), I have to go backwards, do it correctly and then move on.

Can you get somebody with more experience than you to have a look at it ?