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Mutley6

Help - Engine stutter below 3000rpm

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So, very frustrating but am experiencing a persistent misfire below 3000rpm and intermittent complete loss of power when moving off. Above 3000 engine revs freely. Dellortos have been fully refurbished and balanced, have changed plugs, plug leads, distributor cap, rotor arm and coil too. 

Any advice would be much appreciated. Ta

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If you have the lucas electronic rev limiter, try running it with that disconnected (on wheel arch nr battery).

Persisent sub 3K misfire points to idle circuit - Double check your idle jets are clear.   confirm that you have the correct idle jets (40) - consider a slight size increase to say 42/43.   Also confirm that your mixture screws are set to a reasonable datum point.  I'd suggest 3.5 full turns (from closed).

What does that leave?   Ignition amp and dizzy sensor.   I doubt these are the issue, but maybe worth testing via substitution. 

Check fuel pressure/regulator and fuel flow is normal.   Although the fact it runs OK a revs, suggests its fine

Rule out any mechanical issue by running a comp test.

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Thanks for the advice. I'll work my way through those steps and hopefully problems will be solved! 

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On 18/07/2020 at 22:05, Mutley6 said:

Dellortos have been fully refurbished and balanced

Has this issue started immediately after the carb refurbs?

Give us some info on when the car last ran ok and what happened immediately prior to the problem.

Andy.

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Hi Andy

The car is an 89 Stevens carb turbo and by the look of it the Dellortos had never been refurbed. I was experiencing the following issues prior to refurb:

- engine would start instantly when cold, but then immediately stutter and die. Warm starting generally difficult too

- persistent and quite severe shudder through back end of car when accelerating in 4th/5th at motorway speeds around 3000rpm. Engine would rev through to redline fine

- persistent misfire & hiccup when pulling away from idle at approx 1000 to 1500rpm

Now carbs have been refurbed cold starting problem better and hot starting improved a little. Above 3000rpm engine much more responsive. But I am still getting this pronounced shudder and low rev stutter. Lotus specialist I use can't trace fault but have said nothing obvious is untoward! Fuel lines are clear, C service carried out 12 months ago but petrol tanks are original.

All very frustrating as car is otherwise in excellent condition. 

Rejetting the carbs next step I guess.

Thanks

Mike

 

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10 hours ago, Mutley6 said:

I was experiencing the following issues prior to refurb

Hi Mike,

Did you buy the car with this issue? Has it been out of use for a while? Is the fuel fresh?

I've read quite a few similar scenarios and just because the carbs have been rebuilt, it doesn't rule them out as the issue.

I wouldn't rush into altering the jet sizes as they should be correct.

My approach would be to check the jet sizing is still standard, recheck that all jet orifices are clean/clear.

No point in looking at the main jets, concentrate on the much smaller idle circuits and emulsion tubes.

Check the carbs are assembled correctly and no parts missing.

Double check the float height and fuel pump delivery.

As Steve said, compression check would be good.

Andy.

 

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Thanks for your replies guys. I really appreciate your help.

The shudder I'm experiencing is a recent development which I am assuming is a misfire of some sorts. Was prevalent before carb refurb and is still a problem. Fuel is fresh. Usage fairly low; only 2500 mile last year. Compression check done recently & results ok.

Am of a mind to have it looked at by carb specialist familiar with working on Esprits. Anyone you could recommend? 

Thanks again

Mike

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On 18/07/2020 at 22:05, Mutley6 said:

am experiencing a persistent misfire below 3000rpm and intermittent complete loss of power when moving off. Above 3000 engine revs freely. Dellortos have been fully refurbished

Who refurbed the carbs?

What did the refurb entail?

Andy.

 

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Hi Andy

Weber Carbs Reconditioning Services Ltd did the refurb. Guy called Paul. He does have a good reputation.

Full refurb done using genuine Dellorto kit. 

Mike

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Were the the carbs then set up correctly after the rebuild?   i.e. linkage, balance and mixture can only be set up on a running car and are all crucial adjustments.  

As a crude & reversible test, you could turn all 4 mixture screws  exactly 1 full turn counter clockwise (richer) and see if the situation changes on the road.   (The mixture screws are only significantly operative in the sub 3k range)             

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Thanks, I'll try that tomorrow if I get the chance. The garage assured me carbs were set up properly and balanced but still the stutter persists.

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I purchased a carburettor synchrometer (ebay link below) an absolutely essential item as you must/can only balance the carbs once fitted in the car and linked.

https://www.ebay.co.uk/itm/STE-Carburettor-Synchrometer-Air-Flow-Meter-SK-40-42-45-48-SU-Weber-Dellorto-ADV/352992790086?hash=item5230028646:g:utkAAOSwjXNaW3~j

I then used this youtube as a guide (weber rather than delllorto but the same priciples apply) referencing my Lotus technical manual to get things right.

I got there eventually.

 

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Thanks for your advice. I am going to buy one of those and tackle my carbs issues within the next few days.

I'll keep you posted of progress, if any!

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10 hours ago, Mutley6 said:

Thanks for your advice. I am going to buy one of those and tackle my carbs issues within the next few days.

Also worth getting a Colortune to set the idle mixture correctly. :thumbup:


Cheers,

John W

http://jonwatkins.co.uk

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