18A Zenith downdraught eratic running

[Sunnysoutheast]

I’ve just replaced a zenith 36vn with a “new” one and I’ve got the tick-over perfect and runs smoothly. Great.

But on the road I’m getting an erratic misfire all the time in all gears. Not a bang but a miss. There are few hills around here so I’m not able to check for pinking just yet. Otherwise it drives ok.

So could it be a fuel pressure issue? I do have a small plastic inline filter just before the glass topped mechanical pump on the flexi pipe. Is that causing a drop in pressure that may be causing the incessant miss?
Note: new fuel tank and new fuel (E10 and wynns additive)

Or is it more likely to be ignition?
Note: new leads, plugs, dizzy cap, rotor arm, breaker and condenser. Timing spot on via strobe gun. 🤞
The Coil and dynamo however are original and of unknown integrity.

Any help or ideas would be good.

I’d like to keep the Zenith carb so don’t state the obvious; get a real carb! I’m hoping this can run for another 58 years. 🙌

Thanks in advance 😀

[Rustinmotion]

The in-line filters are so cheap I’d replace just to eliminate
Is the misfire revs related? Or load related? If it happens at higher revs regardless of load that would point to ignition, if it’s only when the engine is pulling hard that’s more likely fuel starvation check pump and fuel system

If you can get hold of a spare coil it does not need to be Volvo, just wire + to the battery and the dizzy wire to negative the car will run like this but be aware it won’t turn off with the key, it’s enough to rule out the coil
When you installed the new carb did you set the float level? this can make the engine run bad if it’s not correct

[Othen]

Originally Posted by Sunnysoutheast

I’ve just replaced a zenith 36vn with a “new” one and I’ve got the tick-over perfect and runs smoothly. Great.

But on the road I’m getting an erratic misfire all the time in all gears. Not a bang but a miss. There are few hills around here so I’m not able to check for pinking just yet. Otherwise it drives ok.

So could it be a fuel pressure issue? I do have a small plastic inline filter just before the glass topped mechanical pump on the flexi pipe. Is that causing a drop in pressure that may be causing the incessant miss?
Note: new fuel tank and new fuel (E10 and wynns additive)

Or is it more likely to be ignition?
Note: new leads, plugs, dizzy cap, rotor arm, breaker and condenser. Timing spot on via strobe gun. 🤞
The Coil and dynamo however are original and of unknown integrity.

Any help or ideas would be good.

I’d like to keep the Zenith carb so don’t state the obvious; get a real carb! I’m hoping this can run for another 58 years. 🙌

Thanks in advance 😀

Good morn,

My motor car is similar but has the B18d motor with twin SU carburettors; the rest of the fuel system will be the same.

I would not have thought it a fuel pressure problem, the output from the mechanical pump is only a few PSI, but I suspect you are on the right lines with the fuel filter and have a little air leak. When I first bought my Amazon (GAM) it has an inline filter fitted upstream of the fuel pump (perhaps like the one you describe), it was fitted like this, and led to one failure:



... as you may see, it had inadvertently been fitted back to front. After the failure I noticed it was on backwards, turned it round and tightened everything up:



This seemed to fix the problem, but I never really trusted the filter after that so soon afterwards I changed the pipe back to the original design:



The pipe is available (fairly inexpensively if I recall) from Brookhouse Volvo. Since then the problem has vanished, I can still keep an eye on the fuel supply via the glass topped mechanical pump (you may have the same). I have concluded that Mr Volvo's original design was best, if there had been a need for an inline filter then one would have been fitted 60 years ago.

I'd suggest first checking the fuel filter you have is the right way round, and that whatever clamping system you have is done up tight. You could also try changing the filter in case it is leaking air (they will be available for a few pounds from your local motorcycle dealer). In the fullness of time (this week) I'd suggest ordering an original fuel pipe and fitting it.

Good fortune, I hope this helps fix the issue; do report back to let us know how you fix it.

Alan

[Sunnysoutheast]

Originally Posted by Rustinmotion

The in-line filters are so cheap I’d replace just to eliminate
Is the misfire revs related? Or load related? If it happens at higher revs regardless of load that would point to ignition, if it’s only when the engine is pulling hard that’s more likely fuel starvation check pump and fuel system

If you can get hold of a spare coil it does not need to be Volvo, just wire + to the battery and the dizzy wire to negative the car will run like this but be aware it won’t turn off with the key, it’s enough to rule out the coil
When you installed the new carb did you set the float level? this can make the engine run bad if it’s not correct

The misses seem only to happen under load. I will exchange for the new flexi fuel hose and test that first.

Then borrow the spare coil from the mini if the coil needs to be checked.

Thanks 🙏

[Sunnysoutheast]

Originally Posted by Othen

Good morn,

My motor car is similar but has the B18d motor with twin SU carburettors; the rest of the fuel system will be the same.

I would not have thought it a fuel pressure problem, the output from the mechanical pump is only a few PSI, but I suspect you are on the right lines with the fuel filter and have a little air leak. When I first bought my Amazon (GAM) it has an inline filter fitted upstream of the fuel pump (perhaps like the one you describe), it was fitted like this, and led to one failure:



... as you may see, it had inadvertently been fitted back to front. After the failure I noticed it was on backwards, turned it round and tightened everything up:



This seemed to fix the problem, but I never really trusted the filter after that so soon afterwards I changed the pipe back to the original design:



The pipe is available (fairly inexpensively if I recall) from Brookhouse Volvo. Since then the problem has vanished, I can still keep an eye on the fuel supply via the glass topped mechanical pump (you may have the same). I have concluded that Mr Volvo's original design was best, if there had been a need for an inline filter then one would have been fitted 60 years ago.

I'd suggest first checking the fuel filter you have is the right way round, and that whatever clamping system you have is done up tight. You could also try changing the filter in case it is leaking air (they will be available for a few pounds from your local motorcycle dealer). In the fullness of time (this week) I'd suggest ordering an original fuel pipe and fitting it.

Good fortune, I hope this helps fix the issue; do report back to let us know how you fix it.

Alan

I say, you have the exact same installation as the one I thought up exactly (as on your photos)
I did have a very dirty fuel tank,(16+ years standing idle, lots of white oxide? And occasional rust) hence the filter, but I’ve now fitted a new tank so the filter is really redundant. I bought a new hose and will try that as a matter of priority.
Thank you 🙏

[Othen]

Originally Posted by Sunnysoutheast

I say, you have the exact same installation as the one I thought up exactly (as on your photos)
I did have a very dirty fuel tank,(16+ years standing idle, lots of white oxide? And occasional rust) hence the filter, but I’ve now fitted a new tank so the filter is really redundant. I bought a new hose and will try that as a matter of priority.
Thank you 🙏

Super duper.

[Sunnysoutheast]

Originally Posted by Sunnysoutheast

I’ve just replaced a zenith 36vn with a “new” one and I’ve got the tick-over perfect and runs smoothly. Great.

But on the road I’m getting an erratic misfire all the time in all gears. Not a bang but a miss. There are few hills around here so I’m not able to check for pinking just yet. Otherwise it drives ok.

So could it be a fuel pressure issue? I do have a small plastic inline filter just before the glass topped mechanical pump on the flexi pipe. Is that causing a drop in pressure that may be causing the incessant miss?
Note: new fuel tank and new fuel (E10 and wynns additive)

Or is it more likely to be ignition?
Note: new leads, plugs, dizzy cap, rotor arm, breaker and condenser. Timing spot on via strobe gun. 🤞
The Coil and dynamo however are original and of unknown integrity.

Any help or ideas would be good.

I’d like to keep the Zenith carb so don’t state the obvious; get a real carb! I’m hoping this can run for another 58 years. 🙌

Thanks in advance 😀

I have now replaced the fuel hose and eliminated the inline filter. The test around the block has proven that no change has happened sadly.

I’ve reset the mixture screw but without much change.

Here’s an image of the before and after.
And the coil for good measure.

Is there any other worries with the carb or should I investigate the ignition next?

666B5373-B986-47D3-A5AA-28339B5FFC18.jpg

8F29F994-D1E9-4BA4-8B94-FF109E3C22DB.jpg

2C50F578-2B91-4FA9-B31D-76BBE9CCB763.jpg

PS

On turning off the engine in now runs on a bit after ignition is off. Has done since the new fuel. Should I stick to E5 if possible?

[Othen]

Originally Posted by Sunnysoutheast

I have now replaced the fuel hose and eliminated the inline filter. The test around the block has proven that no change has happened sadly.

I’ve reset the mixture screw but without much change.

Here’s an image of the before and after.
And the coil for good measure.

Is there any other worries with the carb or should I investigate the ignition next?

PS

On turning off the engine in now runs on a bit after ignition is off. Has done since the new fuel. Should I stick to E5 if possible?

Ah, that is a pity, thank you for letting us know. Have you tested the fuel pump's output (just pull off the delivery hose to the carburettor, put it in a jam jar and have someone crank the starter a few times - there is nothing very scientific about it)?

The running on bit won't be anything to do with the coil. I think you have changed or checked everything to do with the ignition apart from the coil. I think you will have the armoured cable type, which is tricky access to, but as someone suggested above you could use any (old type) coil to eliminate it as an issue by bypassing the +ve wire from the ignition switch. If you have a spare coil around, or can borrow one then try that because it is free and would only take a few minutes.

You might check the rotor arm fits tightly on the spindle inside the distributor. I had an issue where the arm (fitted by the PO) wobbled around a little because the small metal tang inside didn't reach all the way down into the slot:



... it was exactly the same part number as another I bought from a local motor factor, where the tang was longer and fitted properly:

.

If the fuel pump, coil and rotor arm all check out then we are probably back to carburettor tuning. I don't know much about your particular carburettor, but they are all pretty simple beasts. If the dieseling is linked to the misfire then that would generally be due to the mixture being too lean, and so the spark plugs running too hot - hence igniting the mixture after the ignition switch is turned off.

There is a good section on the Volvo green book (Part 2A, B18A) covering the Zenith 36VN from pp 2-4 to 2-8. The set up procedure looks quite straightforward, I'd suggest just working through that methodically.

Good fortune,

Alan

PS. The correct fuel for your motor is 97 octane, which is E5. I nearly always run GAM on E10 with no issues, but perhaps change back to E5 and see if the problem goes away.

PS. If you don't already have a copy of the green book then you could download a PDF copy here:

https://volvo1800pictures.com/docume...ngine_B18A.pdf

... this is a different version to the one I have, I notice the part about the Zenith downdraft carburettor is from pp2-34 to 2-37, but the content is the same.

[Rustinmotion]

To run on you need a source of ignition and as the coil is off something is hot enough to ignite fuel and keep the engine running, usual suspect for this is a carbon build up that glows red hot
Can you do a plug cut, find somewhere safe to drive the car under load and switch the ignition off and dip the clutch then safely pull the car to a stop, take the plugs out and check the colour, this will tell you how it’s running under load rather than at idle
Can you blow out the pipe between tank and pump, that might be partially blocked
I’d still verify the float setting it’s not unusual for it to be wrong on a new carb

[Sunnysoutheast]

Originally Posted by Othen

Ah, that is a pity, thank you for letting us know. Have you tested the fuel pump's output (just pull off the delivery hose to the carburettor, put it in a jam jar and have someone crank the starter a few times - there is nothing very scientific about it)?

The running on bit won't be anything to do with the coil. I think you have changed or checked everything to do with the ignition apart from the coil. I think you will have the armoured cable type, which is tricky access to, but as someone suggested above you could use any (old type) coil to eliminate it as an issue by bypassing the +ve wire from the ignition switch. If you have a spare coil around, or can borrow one then try that because it is free and would only take a few minutes.

You might check the rotor arm fits tightly on the spindle inside the distributor. I had an issue where the arm (fitted by the PO) wobbled around a little because the small metal tang inside didn't reach all the way down into the slot:



... it was exactly the same part number as another I bought from a local motor factor, where the tang was longer and fitted properly:

.

If the fuel pump, coil and rotor arm all check out then we are probably back to carburettor tuning. I don't know much about your particular carburettor, but they are all pretty simple beasts. If the dieseling is linked to the misfire then that would generally be due to the mixture being too lean, and so the spark plugs running too hot - hence igniting the mixture after the ignition switch is turned off.

There is a good section on the Volvo green book (Part 2A, B18A) covering the Zenith 36VN from pp 2-4 to 2-8. The set up procedure looks quite straightforward, I'd suggest just working through that methodically.

Good fortune,

Alan

PS. The correct fuel for your motor is 97 octane, which is E5. I nearly always run GAM on E10 with no issues, but perhaps change back to E5 and see if the problem goes away.

PS. If you don't already have a copy of the green book then you could download a PDF copy here:

https://volvo1800pictures.com/docume...ngine_B18A.pdf

... this is a different version to the one I have, I notice the part about the Zenith downdraft carburettor is from pp2-34 to 2-37, but the content is the same.

Thanks sound advice, rotor then plugs then coil.
cheers
Dave