Rear cylinders not firing/misfiring

[Timok]

Hello,

As is probably typical of new users around here, my first post is a plea for help! I've done some searching across past threads, and a few older posts have been helpful, but now I'm resorting to this.

I've been going through the process of trying to get a 1970 Elan +2 back in good running condition (most things work remarkably well, to be honest). My bottleneck right now is trying to get the rear two cylinders going. Right now, at both idle and open throttle, they appear to be dead-weight.

Here are some observations:

• Weber carbs
• Front plugs (1 and 2) are dark but clean. Both spark, but I guess the mixture is a little rich?
• Rear plugs (3 and 4) are dark and wet. Both spark.
• Compression was about 120-160psi (my gauge is garbage) when I tested it. A previous shop's mechanical inspection also showed "good" compression.
• When I started, the mixture screws were set to 3/4 turns for 1, 2, and 3, but only 1/4 for 4. I've set them all to 3/4 based on the process outlined in the shop manual.
• Idle was high (1400), so I set that down to 1000.
• At idle, pulling the leads from plugs 3 or 4 causes no drop in RPM.
• Similarly, with throttle open providing 2500 RPM, pulling the leads from 3 or 4 causes no change.
• Blue/grey smoke is pouring out my tailpipe (unspent fuel?)
• Spring washers at the carb mounting flanges were all very tight on top. Manual recommends 1mm clearance. I played with these to test the seal, but no changes observed (not saying there isn't a leak somewhere).
• I suspect the carb balance is off, but I don't know how off and playing with the adjustment screw just changes idle speed and the amount of blue smoke coming out the exhaust.
• What little messing around I did do with the balance appeared not to engage the rear two cylinders. I have no means of checking air flow into the stack.
• I've not opened the carbs up, but at the very least everything suggests that all four cylinders are getting fuel.

I guess because this is an issue with both rear cylinders, on a common carb, I'm leaning towards the idea that it is an issue with the rear carb.

This thread seemed to describe a user's similar issues (on Cyl 1 and 2), but I am not certain as to how they were resolved.

I'm no mechanic, and I have little to no experience with carburetted engines. Any help is greatly appreciated!

-Timo

[PeterK]

If both rear plugs spark, but engine doesn't run, plus you've apparently got fuel passing through, then I would swap the plugs leads between 3&4 as a first step.

[mbell]

The fact the rear cylinders aren't firing suggests it's not just a tuning issue but something a little deeper. If the car has been sat for some time with fuel in the carbs you really need to strip and clean them. You'll not getting it running well if the carbs are dirty and clogged up.

It's one of those scary sounding jobs but it not actually that hard to do, I have very little experience of carbs but found it straight forward to strip and rebuild. (I haven't got them back on the car and tested jet thou).

I'd also be replacing the leads, dizzy cap and rotor arm. The access is a lot easier with carbs off and they aren't that expensive.

[john.p.clegg]

"If both rear plugs spark, but engine doesn't run, plus you've apparently got fuel passing through, then I would swap the plugs leads between 3&4 as a first step."

I'd second that...

John

[Timok]

Hey guys, thanks for all the swift replies.

I will give the lead swap a try tonight. It would certainly be nice (also somewhat embarrassing) if that ended up solving the issue! At first glance, it appeared to me as though the lengths of the lead wires would restrict which plug they could reach, but maybe they've settled down below and I've got some slack.

Hopefully I don't have to pull the carbs just yet!

-Timo

[mbell]

On my 23D the number 3 wire comes from the lower distributor cap connection nearest the block. Number 4 is lower connection furthest from the block.

[twincamman]

Ok ....first off buy a unysin about 40 bucks ...second DO NOT SET THE CARBS COLD .....drive it spirited ly with new plugs installed and then set them up Balance is where the idle screw is so turn and see what the unysin says and adjust the idle. Repeat as needed ....l...then remember carburetor is French for leaving it alone ...loss of performance is usually the points closing up ..If you had the leads crossed the back firing would have a swat team surrounding yourr garage .Ed. I just saw your from Ontario ,I m near the falls how close are you ? Maybe I can visit. And see what's a
What ....ed

[billwill]

If your rear cylinders are not firing properly a highly likely suspect is that the centre adjusting screw of the carbs, which controls the balance between the two carburettors is not set right.

Get yourself a proper balance meter. For over 40 years I thought I could do it by ear & by eye, then I finally bought a proper instrument and now balancing the carbs is a mere 15 minute job for perfect results.

Read all about it for me at this message and subsequent messages.

lotus-elan-f19/getting-ogu-roadworthy-again-t26101-105.html#p191111

[Timok]

Update: The rear plug leads were correct.

I've already started looking into a tool for synchronizing the carbs so thanks for the ideas and info w.r.t. that.

Having played with the screw used to set the balance, and seen no impact (on the rears), I'm wondering how "off" the balance would be to cause no action whatsoever from the rear cylinders. I certainly feel as though it is off by a lot...

Is this something that is either right or wrong, or should I feel the rears coming to life as I slowly turn the screw? Is there a way to mechanically estimate a reasonable starting location for the screw (ie: full left or full right, or measure the plate gaps on the levers)?

Ed, how critical is it to set the carbs warm? I am hesitant to drive "spiritedly" on two cylinders, particularly in an unlicensed car that leaves a smoketrail. I can let it idle for a while, though. Also, thanks for the offer to drop by - I'm just 30 mins west of Ottawa, though, so it might be a bit of a drive! Drop me an email if you're out this way.

-Timo

[rgh0]

Hi Timo
You can get the carbs balance by listening to the "hiss" at idle from each barrel and adjusting the balance screw until they are similar. Balance really only affects the carbs at idle as once the throttles start to open the difference in insignificant. But if balance is wrong such that the rear two cylinders do not fire at idle then you can end up with wet plugs like you have that will not then ignite the mixture when you open the throttles also and will continue to misfire.

If the two rear cylinders were not firing at all it would be almost impossible to drive the car on just 2 cylinders. Pull off the 2 rear plugs leads and try it !!!!

cheers
Rohan

[twincamman]

Ok....once a plug is fouled it usually stays so . Clean the plugs with quick start and a rag and an old tooth brush ...set the gap , put the rear plugs in the front and see how it idles then . A hot engine is easier to get the set up correctly as the carbs are designed to richen at high revs to avoid burning pistons . sat idle at 900 rpm .....so before you get into float levels and jetting and needle and seat and timing investigation put a spare plug in the 3 and 4 plug lead and place it on the valve cover start the car . And see it there is spark ...if there is then Back the balance screw off and Set the idle screws out 1 1/4 turns to start and turn the balance screw out until it is just free of touching the front actuating lever and start the car Get an idle at 2000 rpm then turn the balance screw in and back the idle off so that should be in the region for some kind of an idle that's where the balance is not the mixture screws on. Top. Be prepared to shut the ignition off quickly if the idle takes off You can wear out the mixture screws winding them in and out if the balance is off ..If it won't idle then there may be a. Ring or valve problem so do aleak down or compression test ,,,or an intake leak at the o ring area ...are you a member of the Ottawa club?...ed

[twincamman]

Also check there is. Good ground at the chassis and there is a full charge in the battery

....I found that not enough hydro can cause this problem ...Ed

[Timok]

So, I've had a chance to try a few things based on the input here.

• All plugs and plug leads are firing. I cleaned the two rear plugs and swapped them into the front two cylinders, just to be sure.
• When I was removing the #4 plug, a reasonable stream of smoke (or vapour?) was released from the cylinder. I also saw a puff of smoke out of the carb "trumpet" earlier. No idea what causes this.
• I tried running through the steps outlined by Ed: Got an idle ~2000RPM with the balance screw backed most of the way out; gradually turned the balance screw in (increases idle RPM) and the idle screw out (decreases idle RPM); I can do this to achieve some form of idle, but I just seem to be playing with the idle on the front carb directly. I never notice any RPM gain when pushing the balance towards the rear carb, nor do I see a drop when I pull the lead wires from rear plugs.
• Battery is new. I've check a couple circuits for a high impedance, or any signs of a bad ground, but nothing seemed off.

Ed, I haven't come across an Ottawa Lotus or Elan club. Sounds like something I should be looking in to, though!

[twincamman]

It's the. ottawa motor sport club .....active in CASC events but there is or are some lotus owners there. ........ok now let's check the timing ... Set number one at TDC remove the cam cover and check the cam marks re correct and the distributor rotor is pointing at 10 o'clock and the crank pulley is at 5 degrees. And put the balance screw back to its happy place (backed out ) and leave it there until you are sure the the timing is right or you will have two problems to deal with . Do this now before checking the timing....read the webber set up tool post .if you can get through the foolishness It works ,,,,,,.these engines leap into start situation when set correctly

[steve.thomas]

I would suggest removing the top of the rear carb and checking that there is in fact fuel in the float chamber. I had a similar issue a while back and found that the float arm had bent causing it to shut off the fuel supply completely (the car had been running fine). I reset the float height and all was well again. I never found out or can even imagine how it occured.

As a side note, i found it interesting that i was able to achieve a smooth tickover and good carb balance using a manometer, even though the engine was only firing on cylinders 1 and 2. It was only when the idle mixture screws on the rear carb didn't make any difference that i got suspicious and pulled the plug leads.