Rough running and needing loads of intermittent choke. 1972 MKIV Spitfire.

[Steve W]

My carbs are HS2. with tags, AUD517R. With the filters removed and opening the throttle I can see fuel vapour being drawn in at the front port but not  in the rear one. Sometimes it runs fine then acts up !! Any ideas please anyone ? I've checked the timing,dwell angle and adequate fuel to the float chamber ect.

Thanks in advance.Steve

 

 

[sulzerman]

Hello

I would check the colour of the spark plugs first, they should all be the same. A chocolate brown colour is about right. 

Then check the balance of the carbs, this makes a massive difference.

Good luck!!

[Pete Lewis]

has the hall marks of rubber slivers blocking the back of the float valve 

makes float filling slow  would remove the top cover and lift out the needle valve  to check  and do look at the jet feed tube is not kinked (very common)  or blocked ,  

and check the coil polarity is correct   ,if reversed you get some strange breaks in running 

Pete

[Mjit]

Sounds very much like my last missadventure in my big saloon - would idle fine and drive fine for a bit before losing all power.  Yank out the choke at that point and power would come back, then drop again as now over-rich.  Push back in and OK for a bit before losing power again.

Now I never found a smoking gun but had just replaced one of the fuel pipes due to it developing a pin hole leak/some of the rubber pipes with Gates E10 hose.  Pulling the carbs and giving them a good clean inside and out, then replacing all rubber pipes with Gates E10/new jets and it's been 'fine' since (still going in circles trying to get the mixture right bt that's classic ownership for you).

[Pete Lewis]

1 hour ago, Mjit said:

(still going in circles

is that the spitty or the 2.5Pi ?????

Pete

[Mjit]

22 hours ago, Pete Lewis said:

is that the spitty or the 2.5Pi ?????

Pete

This is my 2500S - though my Spitfire also has its issues

Changed the jets to what should be the correct ones for a 2500S on HS6s but just swapping had it running lean (confirmed by lifting pins).  Richened up and front carb seemed to do what it should WRT the lifting pins but engine note behaves the same regardless of how rich (so now think I have it running quite rich on the rear).  Also now even with the idle screws backed right off it idles at 1,000RPM.

Just needs more time but also been doing other jobs on the car/jobs on the Spitfire/the 2500 is outside and it keeps raining/I've occasionally thrown my toys out the param and put it off for another day   And I found a couple of rust spots I need to jump on...but it's forecast to rain this weekend...

[Pete Lewis]

so this is SU ,  have you checked the jet tube is not kinked and restricted other than rubber slivers getting in the float needles  etc

Pete

Edited Wednesday at 10:23 by Pete Lewis

[Bfg]

Have you checked the oil in the dash pots ?  I've done a lot of miles over the past three months and unknowingly had very little oil on one of the dashpots. The balance point may have been when I lifted the dashpots off to check the needle positions were correct, and when I laid it on its side - just a tiny dribble more oil came out.  As that was so little, I didn't think too much about it ..but the engine was intermittently running rough around town speeds (under 1800 rpm).  I happened to be talking this issue over with a retired mechanic friend of mine and as I stood there I unscrewed the cap ..and was sorta amazed to find almost no oil the one dashpot.  The oil acts as a damper to stop the piston and needle bouncing around, and so for smooth roads and steady speeds it was generally okay, but any change in conditions was noted by rough running.  I subsequently refilled both with SAE 5w30 engine oil and it's much better now.  I must get around to rechecking (use a flow meter) the carb balance at 1500rpm (ie., for town speeds) but otherwise the oil in the dashpot sorted the issue.  

Pete

[Mjit]

Another one to check are the breather hoses between the carbs and rocker cover.  With the car stationary/idling try "winging them around a bit" and if the revs. drop give the hoses a really careful check over.

Had one of them start to split in a sneeky way years ago.  Just looking at the hoses they looked perfect - and like that, with the split closed the car would run perfect too but sometimes vibrations would move things a fraction and let the split open up slightly.  Suddenly you have a load more air getting sucked in to the engine side of the carb. and so a weak mixture and choke was needed to drive (richening the mixture to counteract that extra air)