induction spit/cough No 4 Cylinder

[miked]

I am trying to bottom out a problem on my S4. If I start it from cold and let it warm with out revving it is fine and drives off and run well and goes like hell. 700 miles from fresh rebuild. It is fine after a hot start. However if I start it from cold and try and rev a little it causes a spiting and coughing back through one carb. Having Weber 151's I had the Carbtune (4 way device) fitted and noticed it was no 4 Cylinder as the slug bounced when the cough occurred. So I know it is No 4 only!

I thought I would go through the setting up procedure. Float levels are right per book and when lids are removed the levels are physically the same in each carb. I had previously set mixtures with colour tune so know they are reasonable. Misab plates on manifold are new appear to seal good.
When I lifted the carb lids I noticed the back carb had its lid gasket ripped in the fuel enrichment area and both the little locking plates that hold the enrichment pistons and springs in where pooped up and into the gasket. Had also damaged the gasket I have since re seated these. Symptom or cause?
I have the enrichment devices front mechanisms removed on both carb? per Des Hammill?s book. They are fitted with small gasketed plates.


My thoughts with only limited knowledge!

Enrichment device being lifted off their seats a causing fuel to be introduced?
Air leak on induction side in No 4 area!?
Slight exhaust leak at manifold?

I have a carb kit coming and will reseal everything.

Ideas appreciated as it ran sweet when warm and I could avoid it when warming. I can?t help thinking that something expanding with heat has an affect like a mild exhaust leak but thought that caused popping on the overrun which it is not guilty of. Maybe odd one.


Mike

[alexblack13]

Sounds like No 4 is running quite weak Mike.


You will find it,

Alex...

[twincamman]

Mike ?pick up a cheap remote thermometer and put the dot on the header bend ?.use that to set up mixture strength?around 450 480 deg
rees

[miked]

Thanks guys, I have rebuilt and moved up an idle jet size. Seemed to have cured most of it. I will have a go Ed with the temp'. Mike

[billwill]

How about the rear vacuum takeoff on the head inlet pipe (unused on an S4) is that leaking?

[rcourtney]

I had a similar issue. Caused by unbalanced idle and lean mix (someone had used the idle to compensate for mix?)

I was experiencing heavy popping on lift off and loud induction spit under load. I also had issues with starting, it required a lot of accelerator pumping before it would consider starting.

Over the weekend I checked the floats (set to 8.5mm), replaced the top cover gaskets, new filters, and new gaskets on the dustbin lids to create proper seals. I changed the idle (from 50F8 to 45F9) / main jets (from 120 to 115) and the air correctors (from 185 to 120). I think the carbs were setup for a 130S, I have an early car ('68).

First, I found the idle was not balanced. I reset the idle speed to 0 on both carbs, then turned the idle in 1/2 a turn. I set the mix to 1/2 a turn on each, then started it up. I found that i needed to turn the mix on each out to about 3/4 of a turn to get it rich enough to run, stop spitting / banging and start easily.

Increasing the mix seems to have sorted to induction spit that i was experiencing. The car is smooth, pulling without hesitation and holding good revs at idle when warm.

It still has a light burble on lift off. Before it was cracking and banging and scaring small children. I think a little bit of fine tuning and I will be there.

I'm not an expert, i'm more a trail and error guy, the above worked for me .. that's not to say its right !

[alan.barker]

Thanks guys, I have rebuilt and moved up an idle jet size. Seemed to have cured most of it. I will have a go Ed with the temp'. Mike

sorrybut i do not see why you have moved up on idle jet size. The problem was only n?4.
Maybe there is a blockage in cross drillings in carb for n?4, making it run weak.
When you adjust the mixture screw for n?4 is there any change on tickover?
Alan

[rcourtney]

This post has inspired me to get everything balanced properly .. I just pickup a Sychrometer inorder to check air intake and hopefully perform some further fine tuning.

I am going to try and balance 40mm DCOE 18's on my early +2 - Any idea what kg/h i should see / balance at idle and above?

I have looked online and cant find any tables etc for this.

[oldchieft]

After I set fuel level see: -

lotus-carbs-f40/help-with-webers-t31195.html#p218433

The engine was running like described in this post with a lot of spiting, in the past I could do the job with a bit of washer hose in my ear and listen to the hiss at the chokes.

I guess my hearing and my patience is not as good as it was, in the end I got fed up with guess work.

So I bought a Sychrometer, that got it done in ten minutes. I also bought a Colourtune that did the other part of the job in about 30 minutes, took longer to swop the plugs in and out then to adjust the mixture,

Not tools you often need but what a time saver, engine is running sweet now.

Jon the Chief

[ericbushby]

This post has inspired me to get everything balanced properly .. I just pickup a Sychrometer inorder to check air intake and hopefully perform some further fine tuning.

I am going to try and balance 40mm DCOE 18's on my early +2 - Any idea what kg/h i should see / balance at idle and above?

I have looked online and cant find any tables etc for this.

.
It should be about 3.5 kg/hour at 1000 rpm, mine is, with 40 DCOE 151's.
Eric in Burnley
S3SE DHC

[richardcox_lotus]

Yes I agree with Eric. The original documentation I had with mine suggested 2.5 but I struggled to tune it to that and it wouldn't run happily at that level.

Also 40DCOE 151s

Richard

[miked]

Thanks guys, I have rebuilt and moved up an idle jet size. Seemed to have cured most of it. I will have a go Ed with the temp'. Mike

sorrybut i do not see why you have moved up on idle jet size. The problem was only n?4.
Maybe there is a blockage in cross drillings in carb for n?4, making it run weak.
When you adjust the mixture screw for n?4 is there any change on tickover?
Alan

Not been on for a bit guys. Alan, just saw this and see what you are saying. Yes it did alter the RPM and when I had the colour tune plug in it altered the mixture strenght. I did so much at the time and it has now gone.
Bill, vac take off plug at back of No 4 is/was sealed well.

Mike