25 replies to this topic

[babsbrown]

Evening guys I'm at my witts end with this and it's my own silly fault haha.


The 998 was running ok before I messed with it but
I fitted a 12g940 1275 head and swapped my inlet manifold for a minispares one.

I pocketed the block a little as the valves where glancing the block.

When trying to start the car
I have no fuel getting to the cylinders.
I have fuel getting to dash pot
I have adjusted my valve clearances.
Torqued the head.

My compression is not great averaging 90psi over the 4 bores.

I did this for an experiment as I'm rebuilding a 1098 engine and was just doing it out of interest, as I heard different opinions on it.

If compression is that low, will that prevent the fuel getting to the cylinders? I'm thinking a lack of venturi effect.

The valves all move freely when it turns over.

Is there anything else I've missed?

I'm going to strip the carb and reassemble but the dash pot moves freely. The jet is clear.

It's timed to 6deg tdc and has a powerspark electric ignition module.

Plugs are gapped and getting a spark.
Putting fuel directly down the Chambers doesn't help.

I don't even get a cough or splutter. I'm tempted to give up on the whole idea and put my old head back on.

Cheers, Babs.

[absx2]

I would expect to see 180 PSI plus on a 998 with a 940 head so there`s something going on there for a start.

How did you pocket the block, did you keep well above the top piston ring and did you seal the bores and pistons from swarf ?

Is the head sitting on the water pump ?

Did you use a 1275 head gasket because you need to ?

Are you sure the valves are correctly adjusted ?

Is the inlet manifold tight on the head, is there a miss match between the inlet and exhaust manifold meaning the inlet might leak ?

Is the carb sitting on the new manifold okay with the spacer in the correct place ?

Are the plug leads in the correct order ?

 

That should get you started

Edited by absx2, 10 November 2024 - 08:11 PM.

[babsbrown]

Thanks for the reply @absx2 I've checked all of the above with no avail.
I think I'm just going to put the old head back on and be done with it.
I had my reservations and I should have known better. But least I know the head I have ran and I can put it back to how it was

[Lplus]

Evening guys I'm at my witts end with this and it's my own silly fault haha.


The 998 was running ok before I messed with it but
I fitted a 12g940 1275 head and swapped my inlet manifold for a minispares one.

I pocketed the block a little as the valves where glancing the block.

When trying to start the car
I have no fuel getting to the cylinders.
I have fuel getting to dash pot
I have adjusted my valve clearances.
Torqued the head.

My compression is not great averaging 90psi over the 4 bores.

I did this for an experiment as I'm rebuilding a 1098 engine and was just doing it out of interest, as I heard different opinions on it.

If compression is that low, will that prevent the fuel getting to the cylinders? I'm thinking a lack of venturi effect.

The valves all move freely when it turns over.

Is there anything else I've missed?

I'm going to strip the carb and reassemble but the dash pot moves freely. The jet is clear.

It's timed to 6deg tdc and has a powerspark electric ignition module.

Plugs are gapped and getting a spark.
Putting fuel directly down the Chambers doesn't help.

I don't even get a cough or splutter. I'm tempted to give up on the whole idea and put my old head back on.

Cheers, Babs.

I'd expect it to at least start even with 90psi

 

I assume by fuel to the dashpot you mean fuel to the float bowl.

 

When you say putting fuel down the chambers, I assume you mean actually pouring fuel down the plug holes.  If that is the case I wouldn't expect it to fire as the fuel needs to be atomised by passing through the carb to produce a fireable mixture.  Any fuel poured into the cylinders will also wash oil off the bores and reduce the compression even further.

 

Easy start or even carb cleaner through the carb is the best way to check if it's a fuelling problem.

[bpirie1000]

Are you getting sparkmat all.?

Are the plugs wet?

Is the inlet all tight? No air leaks.

Did you have distributor out (is it 180 out)

Is it fresh fuel? (lasts about a week these days.)

[babsbrown]

Hi again guys,

Ah yes I meant the dash pot. I'm getting fuel there. But as I've had my carb and inlet manifold off I have felt down the throat of the manifold and its completely dry.

Ahh thanks for clearing that up regarding the fuel down the bores. I used to do that with old 2 stroke bikes. I guess I didn't even think about the problem doing it on a mini.


I'm getting a spark and it's yellow and sometimes blue

The plugs are bone dry.

I've double checked all the inlets for leaks. I didn't remove the dizzy at all.

The fuel Is around a month old. But started fine a couple of days ago prior to changing the head.

I'm wondering whether lack of compression is preventing the carb for pushing fuel into the Chambers.

I've turned it over without the rocker cover on and I can see the valves are moving up and down freely too.

Would it be worth lightly oiling the cylinders to prevent me fully bore washing them?

[Lplus]

Hi again guys,

Ah yes I meant the dash pot. I'm getting fuel there. But as I've had my carb and inlet manifold off I have felt down the throat of the manifold and its completely dry.

Ahh thanks for clearing that up regarding the fuel down the bores. I used to do that with old 2 stroke bikes. I guess I didn't even think about the problem doing it on a mini.


I'm getting a spark and it's yellow and sometimes blue

The plugs are bone dry.

I've double checked all the inlets for leaks. I didn't remove the dizzy at all.

The fuel Is around a month old. But started fine a couple of days ago prior to changing the head.

I'm wondering whether lack of compression is preventing the carb for pushing fuel into the Chambers.

I've turned it over without the rocker cover on and I can see the valves are moving up and down freely too.

Would it be worth lightly oiling the cylinders to prevent me fully bore washing them?

oiling the cylinders would be a good idea as it would help seal the rings. I doubt it will have any effect on suction but it will improve compression as long as enough gets down there to go right round the cylinder walls  to oil the whole length of the rings.

 

It sounds more like a blockage between the float chamber and the carb jet bridge.  Is fuel visible inside the jet? and if so does it go down and rise again if you blow down the jet with a small flexible pipe?

[Earwax]

Double check you haven't swapped fuel lines over ( tank return line ?) and if the minispares inlet manifold has a vacuum plug I would reseal it .

[68+86auto]

90psi should be sufficient to at least run. Pouring a bit of fuel down the bore is ok, you just don't want to keep doing it. If it didn't start after two attempts like that, stop.

 

Is it a reconditioned head? what condition are the valves and seats in? were they lapped?

 

Now before going further, you need to check that the ignition and starting system is actually working correctly. What voltage do you get at the coil positive terminal when cranking? What rpm is it cranking at?

[babsbrown]

Hi again guys,

Ah yes I meant the dash pot. I'm getting fuel there. But as I've had my carb and inlet manifold off I have felt down the throat of the manifold and its completely dry.

Ahh thanks for clearing that up regarding the fuel down the bores. I used to do that with old 2 stroke bikes. I guess I didn't even think about the problem doing it on a mini.


I'm getting a spark and it's yellow and sometimes blue

The plugs are bone dry.

I've double checked all the inlets for leaks. I didn't remove the dizzy at all.

The fuel Is around a month old. But started fine a couple of days ago prior to changing the head.

I'm wondering whether lack of compression is preventing the carb for pushing fuel into the Chambers.

I've turned it over without the rocker cover on and I can see the valves are moving up and down freely too.

Would it be worth lightly oiling the cylinders to prevent me fully bore washing them?

oiling the cylinders would be a good idea as it would help seal the rings. I doubt it will have any effect on suction but it will improve compression as long as enough gets down there to go right round the cylinder walls to oil the whole length of the rings.

It sounds more like a blockage between the float chamber and the carb jet bridge. Is fuel visible inside the jet? and if so does it go down and rise again if you blow down the jet with a small flexible pipe?

Ah I'll try that tonight, should I disconnect the pipe from the jet to the dash pot and blow down that with the flexible pipe attached?

[babsbrown]

Double check you haven't swapped fuel lines over ( tank return line ?) and if the minispares inlet manifold has a vacuum plug I would reseal it .

Thanks the pipes are definitely on the right way. I'll attempt to reseal the plug tonight with some new crush washers. Cheers

[babsbrown]

90psi should be sufficient to at least run. Pouring a bit of fuel down the bore is ok, you just don't want to keep doing it. If it didn't start after two attempts like that, stop.

Is it a reconditioned head? what condition are the valves and seats in? were they lapped?

Now before going further, you need to check that the ignition and starting system is actually working correctly. What voltage do you get at the coil positive terminal when cranking? What rpm is it cranking at?

Thanks I'll have a look into that. I haven't checked that at all, I just went by the fact I had a spark. I'll connect the multimeter tonight and see what I'm getting.

When you say check the rpm, is that a case of connecting a dwell gauge with rpm reading? Where would I attach it?

The head wasn't reconditioned, I mearly gambled and checked it sealed on the valves when I checked the chamber volume with liquid.

Cheers

[Lplus]

 

 

Hi again guys,

Ah yes I meant the dash pot. I'm getting fuel there. But as I've had my carb and inlet manifold off I have felt down the throat of the manifold and its completely dry.

Ahh thanks for clearing that up regarding the fuel down the bores. I used to do that with old 2 stroke bikes. I guess I didn't even think about the problem doing it on a mini.


I'm getting a spark and it's yellow and sometimes blue

The plugs are bone dry.

I've double checked all the inlets for leaks. I didn't remove the dizzy at all.

The fuel Is around a month old. But started fine a couple of days ago prior to changing the head.

I'm wondering whether lack of compression is preventing the carb for pushing fuel into the Chambers.

I've turned it over without the rocker cover on and I can see the valves are moving up and down freely too.

Would it be worth lightly oiling the cylinders to prevent me fully bore washing them?

oiling the cylinders would be a good idea as it would help seal the rings. I doubt it will have any effect on suction but it will improve compression as long as enough gets down there to go right round the cylinder walls to oil the whole length of the rings.

It sounds more like a blockage between the float chamber and the carb jet bridge. Is fuel visible inside the jet? and if so does it go down and rise again if you blow down the jet with a small flexible pipe?

Ah I'll try that tonight, should I disconnect the pipe from the jet to the dash pot and blow down that with the flexible pipe attached?

 

No, don't disconnect the jet from the float bowl, just blow down the jet and see if the fuel moves at all.  Better still remove the float bowl lid and see if you get bubble in the float bowl from blowing down the jet nad how much sediment there is in the floaat bowl base.

[68+86auto]

90psi should be sufficient to at least run. Pouring a bit of fuel down the bore is ok, you just don't want to keep doing it. If it didn't start after two attempts like that, stop.

Is it a reconditioned head? what condition are the valves and seats in? were they lapped?

Now before going further, you need to check that the ignition and starting system is actually working correctly. What voltage do you get at the coil positive terminal when cranking? What rpm is it cranking at?

Thanks I'll have a look into that. I haven't checked that at all, I just went by the fact I had a spark. I'll connect the multimeter tonight and see what I'm getting.

When you say check the rpm, is that a case of connecting a dwell gauge with rpm reading? Where would I attach it?

The head wasn't reconditioned, I mearly gambled and checked it sealed on the valves when I checked the chamber volume with liquid.

Cheers

Yes, connect a tacho like normal. Usually a function of a dwell meter or timing light. I suspect the rpm will be ok but it's best to know for sure.

[babsbrown]

Hi again guys,

Ah yes I meant the dash pot. I'm getting fuel there. But as I've had my carb and inlet manifold off I have felt down the throat of the manifold and its completely dry.

Ahh thanks for clearing that up regarding the fuel down the bores. I used to do that with old 2 stroke bikes. I guess I didn't even think about the problem doing it on a mini.


I'm getting a spark and it's yellow and sometimes blue

The plugs are bone dry.

I've double checked all the inlets for leaks. I didn't remove the dizzy at all.

The fuel Is around a month old. But started fine a couple of days ago prior to changing the head.

I'm wondering whether lack of compression is preventing the carb for pushing fuel into the Chambers.

I've turned it over without the rocker cover on and I can see the valves are moving up and down freely too.

Would it be worth lightly oiling the cylinders to prevent me fully bore washing them?

oiling the cylinders would be a good idea as it would help seal the rings. I doubt it will have any effect on suction but it will improve compression as long as enough gets down there to go right round the cylinder walls to oil the whole length of the rings.

It sounds more like a blockage between the float chamber and the carb jet bridge. Is fuel visible inside the jet? and if so does it go down and rise again if you blow down the jet with a small flexible pipe?

Ah I'll try that tonight, should I disconnect the pipe from the jet to the dash pot and blow down that with the flexible pipe attached?

Cheers for the help guys, I put some 5ml of fuel down the throat of the carb and a drop of oil down the bores.

It coughed and spluttered so I'm a step further. I also charged my battery a little which maybe gave it a little more cranking power. I've now had to come inside as I don't want to annoy the neighbours. The crush washer has been sealed better on the servo to inlet manifold pipe.

I'm hoping the splutter is the sign of something.

Going to check timing etc... and cranking amps and RPM.
Cheers