Lotus Elan

Slow runniing problems Stromberg carbs.

PostPost by: saildrive2001 » Thu Oct 27, 2011 6:19 pm

Just started my twink after a complete rebuild (been dormant for over 20 years.)
Engine started using choke & some throttle, would not run on choke alone.
When it was warm I could get it to run but it would die under 2000rpm if I let the throttle close. Engine runs fine @ 2500 - 3500. I am now going through things to check if they are to spec.
The carbs are 175CD-2 UK spec. I noticed when I took off the plate that covers the idle by-pass location that the gasket had a passage connecting the two holes in the carb body. Being a UK spec engine mine does not have an idle by-pass valve. ? is should the gasket cover the holes & therby be blanking them off?
When it was running the ignition timing was good. I am now checking the valve timing just to make sure that it is correct but I don't believe that is a problem.
I did install a new jet & needle assembly but used a tool to make sure the height of the jet was the same as the original one. Set up the floats to the spec (did have to adjust them from what they were before the rebuild) The needle in my carbs is of the non adjustable type.
Any thoughts would be appreciated.
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PostPost by: miked » Thu Oct 27, 2011 7:11 pm

Keith,

I am no great brain on these but did live with two for about 6 years.
As I recall the blank and gasket do cover the bypass holes.
Re: Jet. My old research gave the jet height 100 thou' below the body at that point. The shoulder of the needle holder sits flush across the body of the slide (with a steel rule). Still need a mixture check (colour tune etc). Though the needles are not adjustable from outside, I recall them being able to be set from the position of the side grub scew in the slide (as above). Have you got the tops on the right way so that both slide lift and fall freely. There is a nib mark on the side of the top that lines up to the inlet nib mark (I think). They are machined in this position. Matched pairs!
If not in this position they can catch the case and the diaphragm can't lift the slide.

Mike
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PostPost by: saildrive2001 » Thu Oct 27, 2011 11:38 pm

Mike,
Jet & the needle are positioned as you suggest.
The piston that is attached to the diaphragm lifts freely when I remove the damper piston. I will take off the top cover & check the position of the makn piston.
I have a colourtune so should try that next.
I was hoping that someone who has easy access to their carbs could take off the cover plate & confirm whether the gasket blanks off the two holes or not.
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PostPost by: rgh0 » Fri Oct 28, 2011 3:29 am

IIRC correctly from similar non temperature compensator bypass Stromberg carbs i have had on other cars the gasket is the same as used on the bypass carbs but a flat steel plat replaces the bypass mechanism so the two holes are sealed by the plate from not connecting ?

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PostPost by: saildrive2001 » Fri Oct 28, 2011 3:54 am

Hi Rohan,
The gasket I got with the gasket set has a slot in the middle where the two holes in the carb body are located. This allows them to be connected all the time. I don't have the original gaskets that I took off the carbs so I can't check to seee if they were the same. The plate only holds the gasket in place.
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PostPost by: saildrive2001 » Fri Oct 28, 2011 3:56 am

Rohan,
Forgot to add that my carbs do have the temperature compensators on them but not the by-pass valve.
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PostPost by: rgh0 » Fri Oct 28, 2011 4:42 am

I am doing this from memory at work - I have a few Strombergs in my "old carbs" parts bin of various types - I will go and have a closer look tonight to refresh my memory
cheers
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PostPost by: miked » Fri Oct 28, 2011 8:07 am

Keith, It was not the piston position I was talking about but the top cover. They locate to the input side with a nib on the casting. I believe that when they are bored, this is the position they are in. However if they are free, then you are fine.
I do think, like Rohan, that mine had a flat plates on the bypass and a gasket that split the holes apart (no bypass). Must be some one in the UK with a spare set to look at since this was not fitted here. All my bits are gone. As I recall the gasket set had all the types of gaskets (bypass valve included), so it can be confusing.

Re: Temp compensators. I tuned mine in hot water (from a Triumph forum) to make correct and also synch' but was always careful of the tiny seals that stop them from pulling air. Are the seal in? Two as I recall. These used to soften and distort with fuel.

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PostPost by: oldelanman » Fri Oct 28, 2011 8:38 am

miked wrote:The shoulder of the needle holder sits flush across the body of the slide (with a steel rule).


According to the workshop manual it's the shoulder of the needle which should be flush with the slide face not the needle holder.

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PostPost by: miked » Fri Oct 28, 2011 9:47 am

Yes Roger, you are right. It is the shoulder on the needle itself, not the holder. My mistake. Not done it for a while.

Mike :oops:
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PostPost by: oldelanman » Fri Oct 28, 2011 9:59 am

miked wrote:Yes Roger, you are right. It is the shoulder on the needle itself, not the holder. My mistake. Not done it for a while.

Mike :oops:


I only remember it because it's virtually impossible to do accurately as the needle is biased in the holder. It's not too critical with an adjustable needle as it's only the starting point for the fine tuning.
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PostPost by: rgh0 » Sat Oct 29, 2011 8:17 am

Here is a photo of a Stromberg off an Elan with the balnking plate and gasket shown - no connection of the 2 holes in the gasket

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Stromberg.jpg and
Strmber balnking plate and gasket
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PostPost by: saildrive2001 » Sat Oct 29, 2011 2:30 pm

Thanks Rohan, I thought that they had not included that gasket in my set. I will blank off the by-pass holes & try the engine again after I have double checked the valve timing.
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PostPost by: saildrive2001 » Sat Oct 29, 2011 11:07 pm

Made two new gaskets as per the one on Rohan photo. Also checked the neddle height & noticed that I had set the height of the housing level with the bridge & not the shoulder of the needle. Fixed both of the carbs.
Started the engine again & it ran better under choke than before. When warm also would run slwer with some throttle but still spit back through the carb & wouldn't really run at low rpm. Also fuel showing in the carb intake. I slightly retarded the ignition but the same problem.
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PostPost by: saildrive2001 » Wed Nov 23, 2011 7:35 pm

I did have the 3 & 4 plug leads mixed up originally.
Triple checked the carbs, jets within .001" of each other, needles set with the shoulder flush with the piston & the floats at the correct height. Checked & made slight adjustment to the valve timing (only a couple of degrees) Ignition timing is ok.
Engine starts on choke & runs cleanly at 1500 rpm. When it is warm it will hunt between around 300 up to 800rpm & sometimes stall at the low rpm. If I cover the rear carb intake almost completely the revs will rise to around 1100 & it runs smoothly. I have a Colourtune in both the #2 & #4 plugs. The #2 shows a nice blue flame at all rpms the #4 varies but is predominatley yellow with some blue streaks in it. This would indicate to me that the rear carb is running rich but why do I have to blank off the intake to get it to run smoothly? Also I never can get a good blue flame as per the #2 cyl.
Puzzled
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