20 replies to this topic

[Daveontoast]

Hi All,

Got a problem with my recently rebuilt engine. The engine spec is as follows:
998 block +0.40” overbore
Hepolite pistons
Fully balanced bottom end
MED centre main strap
MED duplex
MED damper pulley
Sw5 camshaft
Swiftune followers
MED lightened flywheel
12g940 rebuilt head opened to 23cc for 10:1 cr
Bk450 head gasket
MED road double valve springs
Alloy inlet manifold
Brand new HIF38 carb
K&N pancake filter
Maniflow exhaust manifold
123 distributor
Bosch coil (removed ballast ignition system)
Filter king fuel pressure regulator
Facet road fuel pump

The engine starts on the button and runs lovely up to 4K rpm, then it’s starts to splutter and cut out. I’ve put the colourtune on while revving and the flame all but disappears at 4K, leading me to think it’s being starved of fuel. I have made sure the filter king isn’t starving the carb of fuel. I changed the needle from an AAU to the richer AAA, while slightly better, the problem still persists. I richened the mixture to the point of smoke coming out of the exhaust at idle, and this seems to fix the issue at 4K (I’ve not gone higher as the engine had just been rebuilt). I’ve checked the timing, and tried adjusting, but this doesn’t seem to make any difference. The dash pot has oil (official su stuff). I’m now thinking I could have a vacuum leak. The servo banjo hole has been blanked of with a bolt and appropriate gasket, the carb has a spacer and abutment plate, with gaskets between each. The pcv hose is connected properly, as is the vacuum advance hose. I’ve triple checked all these points and everything seems ok. All of the hoses are brand new, so no splits or holes. Could there be enough of a leak from any of the gaskets to cause this issue? Is there a way to tell if a vacuum leak is causing the problem?
I don’t really know where else to look, I know I will need the needle customised, but can it be that far out? I will eventually get the car on a rolling road, but I’d at least like to get it running properly for the mot, and so I can drive it to said rolling road.
Any help would be greatly appreciated.

Dave

[cal844]

Try a heavier oil in the dashpot. Austin/BL Workshop Manual states 20w50 engine oil.

I suggest trying to slightly tighten the manifold nuts. You can try some flammable liquid on a COLD manifold to test for air leaks

Edited by cal844, 02 September 2020 - 06:08 PM.

[Daveontoast]

Ok, thanks. I’ve got some 20w50, I’ll pop some in and give it a try.
When you say flammable liquid on the manifold, you mean spray it around the manifold/gaskets and see if the engine revs change? What liquid would you recommend? Something like wd40?

[Spider]

Could there be enough of a leak from any of the gaskets to cause this issue? Is there a way to tell if a vacuum leak is causing the problem?

I don’t really know where else to look, I know I will need the needle customised, but can it be that far out? I will eventually get the car on a rolling road, but I’d at least like to get it running properly for the mot, and so I can drive it to said rolling road.

Dave

 

 

A Manifold leak you can pick up just as easy at lower speeds as you can at higher RPM, ideally, it needs to be spinning fast enough to be on the Cam. Spray some Carb Cleaner around the Joints and Gaskets with it's running, just be careful of the hot exhaust manifold, or use a plastic tube up to your ear (like a stethoscope) to listen all around the joints, though, given the way it runs at lower RPM, I really don't think / feel this is the issue.
 

The Needle definitely can be 'that far out' and I think you've proven that you yourself with the adjustment you made.

 

Just run it below the point where it starts to stave until you get it to the Rolling Road.

[cal844]

Ok, thanks. I’ve got some 20w50, I’ll pop some in and give it a try.
When you say flammable liquid on the manifold, you mean spray it around the manifold/gaskets and see if the engine revs change? What liquid would you recommend? Something like wd40?

Yes that's the method I use. I normally try wd40 or paint thinner(with extreme care not to get thinner on the exhaust) you can also use brake or parts cleaner but again take care not to have too much heat (IE don't run the engine too long.) I use an atomiser or spray bottle so you only use a small amount.

If the issue isn't resolved check to see if the plugs are rich or lean. To do this you need to replicate the issue, then let it run like that for 10 seconds. Keep the fault active whilst you turn the key to the off position, now check each spark plug for colour. If you have a white or biscuit brown colour it isn't rich enough, black is too rich.

Hope this helps and as always be as safe as possible.

Cal

Edited by cal844, 02 September 2020 - 08:05 PM.

[Daveontoast]

I’ve got some carb cleaner and a day off Friday, so I’ll give it go. Definitely can’t hear any hissing coming from the assembly. I can’t get it to run for 10 secs, as it runs fine then dies pretty much straight away at 4K, but I’ll try stopping it immediately and checking the plugs. I’ve checked them previously, but they were black as I’m running it a little rich atm to help curb the issue, and I let it idle before doing so.
If not, like you say moke spider, I’ll just get it booked in at slarks rolling road and drive it as is, and hopefully it’ll just need the needle reprofiling.
Mot is booked for next weekend, so I’ll lean it out for the emissions test as I don’t believe they go over 2.5k rpm anyway.
Thanks again.

[Earwax]

Have you checked the float bowl fuel height?

[cal844]

Sounds like a fuel delivery issue to me, check the needle and float bowl for debris and then reset the float height (its in Haynes manual). With the HIF 38 you'll need to remove the carb to do this

Set your filter king pressure regulator to 3psi and try that.

Edited by cal844, 02 September 2020 - 10:07 PM.

[dyshipfakta]

Even AAA would be too lean for that spec I would imagine. It’s a stage 1 needle. You need to get it on the rollers.

[johnR]

This is almost exactly the problem I'm having with my newly built 1293 - almost identical in all but cc (SW5, 123 dizzy, lightened flywheel etc) but I'm using twin HS2 carbs.  I thought it was fuel delivery to start with, but after replacing the mech fuel pump with a facet and increasing fuel pipe dia. to 8mm the problem persisted with the revs collapsing at around 55mph. After months of trying different needles, multiple stripdowns of carbs etc I was taking it to a specialist with rollers when it started sounding like it wasn't getting oil so I aborted and ended up taking the  engine out. It's sitting in the garage waiting for me to summon the enthusiasm to investigate by stripping it down.

Edited by johnR, 03 September 2020 - 08:49 AM.

[timmy850]

When you say the PCV is connected properly- is it connected to the intake manifold or the carb? Can you describe how you have all the breathers set up?

An AFR gauge can help you out a lot. I have a mobile gauge - I helped out a guy near me who thought his van was running really rich - turned out the gauge said it was running super lean and bordering on doing damage. It can be hard to tell how the engine is running without reliable data

[Daveontoast]

The carb is brand new, I flushed the fuel tank before filling and the whole fuel line is new, so there shouldn’t be any debris. Might be worth checking just to rule out though. Maybe I’ll splash out on a richer needle, see if that makes any difference. I was under the impression that AAA was one of the richest needles? That’s why I went for an AAU to start with.
Sorry to hear that JohnR, hope you get it sorted. If/when I get mine sorted, I’ll let you know what the cause was.
The tappet case breather is connected directly to the carb vacuum connection (although I will be fitting a catch can eventually), that’s the only breather as it’s a 998 block. The carb came with the small breather at the top of the manifold side of the carb pre-connected to the lower breather connection next to the fuel connector.
I’ve been using a colourtune, so I can see (maybe not that accurately) how rich I’m running it at idle/lower rpm.

[timmy850]

Is the carb set up like this? Except connected to the tappet cover breather instead of rocker cover

 

 mbimage.php.jpeg   50.81K   5 downloads

https://www.mmoc.org...pic.php?t=51869

[Daveontoast]

Yes, exactly.

[Lplus]

so from your previous comment, the two vents A and B are connected together? 

 

If so, since these are apparently intended to weaken the mixture and you seem to have a weak mixture problem, the first thing I'd try is removing the interconnecting pipe and blocking off both A and B.

 

The AAA needle is about the richest needle there is for the .090 " swing needle jet, though the AAM and AAB are richer at large throttle openings they are both slightly weaker at small throttle openings; and I'm assuming this problem occurs at small throttle openings since you had problems whilst using a colourtune.