14 replies to this topic

[jime17]

Hi folks.

Ive been trying to solve a missfire issue ive had since i did my auto to manual convertion.

998 hs4 d59 with powerspark module

It misses every 5 seconds or so at idle. Ive changed the plugs. Leads. distributor cap, rotor arm and checked valve clearance to no avail.

I was tinkering earlier and disconnected the vacuum advance hose. When i put my finger over the carb vacuum take off the missing stopped.

When i replaced the carb end of the hose it started missing again.

When i plugged the distributor end of the hose the missing stopped again.

This leads me to think that
A) the seal between hose and vacuum unit is leaking or

B) the diaphragm in the vacuum unit doesnt hold the vacuum properly.

I have two questions though.

1. Is there any other potential cause?

2. What are the downfalls of leaving it plugged and not having any vac advance?

Cheers

Edited by jime17, 13 June 2020 - 03:06 PM.

[GraemeC]

If you disconnect it from the carb and suck on the pipe, does it feel blocked off or can you draw air?

If you drive with it disconnected and plugged then you will see an increase in fuel consumption, part throttle driving may not be as responsive and the car may run a little hotter. But you are unlikely to do any damage.

Edited by GraemeC, 13 June 2020 - 05:23 PM.

[jime17]

If you disconnect it from the carb and suck on the pipe, does it feel blocked off or can you draw air?

I'll try that and report back.

Funny if it is that as its a replacement vacuum unit i fitted about 4 years ago. I guess new pattern parts dont have the longevity of the originals.

[jime17]

Right...

Vacuum unit holds a vacuum when i suck the pipe. Also advances the timing as the rpm increases when i suck.

If i connect the other end and disconnect the distributor end the pipe sucks onto my thumb and stays sucked.

Any ideas?

[ACDodd]

Easy fix, change the carb to ported vacuum, your engine has too much ignition advance at idle. Reducing the vacuum at idl retards the ignition timing to where it should be. Very common.

Ac

[jime17]

Thanks ac. Is that the reason for the random misfire?

Also what is ported vacuum?

[jime17]

Im a bit confused now. From what ive just read on a thread about someone fitting a vacuum gauge, the hs4 distributor vacuum take off is ported vacuum. This is what im using.

Is it manifold vacuum i need to use?

If so how do i achieve this?

Thanks

[Spider]

Also what is ported vacuum?

 

There's some info and explanations here;-

http://www.theminifo...dvance-fitting/

[jime17]

Thanks moke spider. I shall have a read.

[Mini Manannán]

I did the same thing as that thread {moved the vac advance to line up with the throttle) Instantly cured a persistent niggly misfire.

[jime17]

Thread resurrection.

Life got in the way of looking at this further, but I was looking at a you tube video about ported and manifold vacuum difference (thunderhead289 is the chap if anyone is interested) and it got me thinking about my misfire at idle again.

As I'm ignorant of much of this subject I'm hoping someone might be able to educate me.

My hs4 has the vacuum takeoff just in front of the dashpot. It was originally factory spec fitted to my 1991 998 auto.

I converted this to a 998 manual from what I believe is a 1984 metro. Well the block is anyway. I don't know the provenance of the head.

This setup worked fine on the auto but now misfires on idle randomly as previously discussed. Disconnecting and plugging the vac stops the misfire which kind of proves the reason.

I'm interested to know why it worked with the auto and but has an issue now?

1. Is it likely due to different cam and cr in the new engine?

2. Would the metro my new engine came from have had a different vac set up?

3. Is it possible my static timing is simply too far advanced for starters and the manifold vacuum is compounding this causing the misfire?

Thanks.

[ACDodd]

Just convert the carb to ported vacuum, and the problem will go away. It’s very common.

Ac

[jime17]

Just convert the carb to ported vacuum, and the problem will go away. It’s very common.

Ac

I'm looking to understand why it's a problem in some cars and not others. ,

[Spider]

The HS4 you have will already have Ported Vacuum.

 

I'm just wondering if it's from a worn spindle and / or bushes in the Carb ?

[jime17]

How could i tell that moke?