Hi all, got a problem with a big valve motor. All standard on Dellortos. Misfire on idle. Tracked it to number 4 cylinder. Little or no suction at idle on the carb trumpet. Number 3 is what you would expect and is around the same as 1 and 2. ‘O’ rings replaced, carb stripped and cleaned. Still the same. Checked the compression on all four cylinders. All within a lb or so of each other. If anything number 4 is one of the higher ones. Getting good fat spark right up the rev range from idle to 4000rpm. Have an old colourtune kit. Put that on all cylinders, can fine tune the bleed screws on the other three cylinders, but not number 4. Can take the screw right out and it makes no difference you would notice. When revving the engine with the colourtune in number four, nothing shows on idle, rev it and we get the expected ‘spike’ from the pump jet, then normal mixture when the jets take over. Usually spraying a bit of WD40 or similar over the ‘o’ rings would cause a temporary reaction if the there was a leak there. Nothing changes when this is done, so they have not been nipped on fitting. Tried a brake pipe clamp tool on the servo pipe, no difference. Does anyone have any experience of the head cracking in the manifold area? Am going to remove the servo outlet pipe from the head tomorrow and see if anything is wrong there. Looks like it’s only a pipe thread size, so we will probably have a plug we can put in there to take that out of the loop. Guy is desperate for us to fit a X flow Kent 1700 motor to make for more reliable running, spoil the car I said, but he’s hissed off with the twincam at this point. Any ideas gratefully received. cheers. Rob 
Hi
You did not mention checking the butterfly settings. You should check for a twisted spindle as it sounds like No 4 is fully shut at idle.
I had the same issue on my S3 20 years ago, but my “miss” would dance around the other cylinders, very weird. Turned out to be the non-return valve for the headlamps, the one at the front of the inlet manifold next to number 1 cylinder. Don’t ask me why it danced around. But a new Norgren one way valve fixed the problem that I’d had for weeks & weeks.
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Interesting about the total close on the butterfly. Will have a look with the camera in the morning and compare it to number three. Carbs have done 103 thousand miles. He was thinking of replacing with new Webers. Thanks for the heads up.
Do you have an airflow gauge* for balancing the carbs? rather than the vacuum type. That would confirm airflow through the carb rather than indirect measurement of vaccum.
You could also intentionally unbalance the two carbs them to make sure the throttle is open for number 4 at idle and see if it fires reliably.
Could also use a bright light down the carb throat to compare the position of the butterfly v the progression holes.
Had a not dissimilar issue on my pre-crossflow with Webers - ran fine on 4 cylinders on the main jets but missing on 1 at idle. Compression was ok, but weak vacuum at idle on the missing cylinder. Leakdown testing revealed a weak valve spring on that cylinder.
If you can swap the carbs and have the same issue on the same cylinder that might eliminate a carb issue.
Blocked idle jet seems most likely. remove the idle jet/air bleed, and separate them, then inspect both for a bit of crud or rust flake. Flashlight and magnifying glass will help. Blow it out with an airline, use a very fine copper wire as a last resort.
Are Spark Plug colours all the same.
Alan
Had an hour on the Elan this afternoon. Removed the vac servo pipe and blocked it. No difference. Seems to be plenty of suction on the pipe, enough to hold your finger on the end and move the pipe around. Had the camera down the chokes. Seem to be at the same opening on idle and open pretty much the same. Also tried adjusting the idle screw to 1500rpm and the gaps look the same on 3 and 4. Still have the misfire at 1500. Don’t think the carbs will be quite on the main jet at that speed? We do have an airflow meter for our Escort carbs. Won’t fit in the Lotus sadly, no room between the master cylinder box and trumpet on 3 and 4. Had all the jets out and all clear of obstruction. Can’t really check the plugs when it’s sat at idle, but they all look pretty much the same. Looks like maybe as suggested, swap the carbs and see what happens. Not a job I relish! Or maybe just fit the new Webers he’s bought for the Kent motor? Either way it would rule the carbs out as the problem. Do these motors suffer with cam lobe wear? Just a thought. Would have thought if was that, the engine would complain enough to hear it, but maybe not? No top end noise at all. Cheers for your ideas.
There’s ports in the cylinder head on both the number 1 and number 4 inlet tracts. When you talk about the servo intake pipe on my engine at least it’s connected only to the number one tract via this port (Escort TC application, maybe Elan uses number 4 tract)
Did you check the number 4 inlet tract to ensure the brass plug (if fitted) for this port is installed and leak free?
Dellortos have a throttle bypass adjuster screw to enable balancing of the two butterflys in the one carb at idle. Have these become misadjusted ?
regards
Rohan
Thanks for the replies. Our Escort has only one vac port on number one for the servo. The Elan has two. One for the lights lifter on number one and another on number four for the servo.
Rohan, never messed with the balance adjuster on the carb. Where is it and how do you adjust the thing? Cheers. Rob
Back in the day, my sprint used to suffer a misfire due to no 4 plug sooting. This was caused by crank case vapour being drawn down no 4 inlet because the breather feeds into the air box adjacent to this inlet. I diverted the crank case venting to a condenser bottle and blocked off the vapour feed to the air box. Viola, no more misfire on no 4.
Hi all, just removed the Dellortos from the car. Got bored with messing on with them while fitted. Think we have found the problem. It was mentioned by Trevor early on in the post, regarding spindle being twisted. Seems the spindle on the back carb has been tweaked too far open at some point and has left the number three cylinder open and the fourth closed. The photo attached shows, if you can make it out, some WD40 sprayed in to the pair of them while stood on the trumpets. Clearly see that one is not draining through and the other has. There is no wear on the bearings at all, so all is well there. So…which way do we do it? Do we try and tweak it back on the open one, or open the closed one? Would seem to me to be easier to open the closed one? If it fails we just buy a new shaft, so not concerned about screwing it up at all. Many thanks to you all for your input. If we get it sorted will let you know. Rob
Maybe use the balance screw and see if you can get them running ok before bending the shaft. Can’t advise exact details on the balance screw in dhla but likely a screw set with locking nut some where upfront on top.
You could also just replace the shaft for a new straight one.
I’d take the spindle linkage nuts off - the spindle should have spanner flats on it. Try “tuning” the spindle with a pair of spanners working against each other. I’d do that with the throttle plates fully open so as not to damage the butterflies or the carb throats.
Hi
Why do a bodge for the price of a spindle, you also need to check the condition of the butterflies. If they are as twisted you say somebody without any appreciation of carburettors has been at work.
Hi all, managed to tweak the throttle spindle after a bit of fiddling. Have also changed the carbs front to back, just in case the fault was still there. Set the balance and mixtures and the car runs on four cylinders for the first time since he bought it. Now sounds fit and well. Starts better from cold as well, as we now have four cylinders working with the throttle shut. Thanks to all who posted regarding this and we hope it may help others if a similar fault occurs. All we need now is better weather. Cheers, Rob
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