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Engine Stutter At Specific Rpm


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#1 keule_k

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Posted 08 October 2018 - 03:05 PM

Hi Guys,

 

since I'm slowly running out of Ideas I would like to invite you to my guessing game:

 

1. The Car:

77 Clubman Estate fitted with a A+ 1293 engine - big valve Head but std. Metro cooper Cam. Fitted with a Bosch Alternator, 123 Ignition and A+ Starter. Also it has a 1275GT Speedo with Tachometer. I'm running a 123 Ignition with a Bosch Blue coil, NGK Sparkplugs and "High performance Leads".

The car is running on a Single HIF44.

 

2. The Problem:

The car runs fine most of the time. BUT when I hit around 4.000RPM and hold it on that RPM on the Motorway it starts to stutter. The Problem dissappears on full throttle.

Also it takes a while after starting the Car, before the Tachometer starts working. Under hard axceleration the Needle wobbles around quite a bit.

 

3. What have I changed so far:

- Ignition Coil, switched the 123 Dizzy with a fresh Standard Lucas 91 Carb-Cooper one (65MD-something)  same result.

- new HIF44 Carb with new Fuel Filter

- new Sparkplugs and Leads

- new Alternator

 

I'M GUESSING now something electrical?!?!

I'm happy for any suggestion

 

Cheers from Germany

Marc



#2 cal844

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Posted 08 October 2018 - 03:22 PM

It is indeed electrical, the tachometer bouncing around tells you that, (also if it bounces less than 100 rpm it is usually fuel issues)

Is the coil a new unit which is compatible with the electronic ignition?

I'd personally put a second hand coil from a known working car to try to fault find.



But first clean all the coil wires back to shiny clean metal(as much as possible) check ends for corrosion (green deposits)

Make sure the nuts on the terminals are cleaned and also tight (don't go crazy)

We have had it in the past where a new coil was faulty out the box, we fitted a second hand one from another mini in the fleet and the problem went away, it took us months of having a misfire in top gear which would act up but if we hit a slight downhill section (speed increases) it would be perfect again.

Edited by cal844, 08 October 2018 - 03:26 PM.


#3 MIGLIACARS

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Posted 08 October 2018 - 05:25 PM

try changing the plugs for new ones a cheap easy answer



#4 keule_k

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Posted 09 October 2018 - 09:58 AM

Guys,

thanks a lot.

The Coil was a fresh one and is recommended by the 123. Dizzy. Had that setup (123 and Bosch Blue Coil) on an Alfa Nord running perfectly.

Also the stutter accured before and after I installed the coil - but it doesn't hurt to try out another one.

 

Sparkplugs are new NGK BPR6ES. Tryed new BPR7ES but didn't make any difference. Spark gap is spot on.

 

I was wondering if a faulty Cable to the Coil and / or from the Coil to the Dash could be the problem, since the 1275GT Dials where retro fitted by one of the previous owners, as well as the dynamo was replaced by an alternator.

 

Cheers.



#5 nicklouse

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Posted 09 October 2018 - 01:45 PM

why did you try colder plugs? i would have put hotter ones in first to see if it was a fouling issue. 7s would just show the same problem.



#6 luismx123

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Posted 09 October 2018 - 02:41 PM

why did you try colder plugs? i would have put hotter ones in first to see if it was a fouling issue. 7s would just show the same problem.

whats the difference between and when would one use hotter/colder plugs? - sorry for the tangent



#7 nicklouse

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Posted 09 October 2018 - 03:00 PM

 

why did you try colder plugs? i would have put hotter ones in first to see if it was a fouling issue. 7s would just show the same problem.

whats the difference between and when would one use hotter/colder plugs? - sorry for the tangent

 

a very good question that i dont actually know the correct way of describing. plugs vary in design as to where the electrode is in relation to the "shroud" (ok only talking about NGK  plugs here) the higher the number (7,8,9) the colder the plugs run so can maintain their construction for longer before burning out. (melting) cars that need these plugs tend to be running high compression and use high revs for a long time. so circuit race cars. these pugs used in a road car would end up getting fouled (wet and oiled) as they could not ignite the fuel or burn deposits of the electrode as they did not get hot enough. now lets look at a hot plug 5 for example. if put in a race engine and raced on they would get too hot and melt in a road car they might just look a bit glazed over but could start causing Pinking (preignition) due to them glowing.

 

note that a race engine does not have to run cold plugs as the event may not last long. hillclimbs and drag racing for example. but for a road car the best way is to go for a spirited drive and then pull over and pull the plugs out and see how they look. please note this is immediately not after and pootle to get home you want to see them ASAP.



#8 ACDodd

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Posted 09 October 2018 - 04:41 PM

What you describe, sounds like part throttle lean misfire. It's very common.

Ac

#9 cal844

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Posted 09 October 2018 - 07:29 PM

Does sound like a fuel supply (lean) issue, I'd say that a work around would be to try turning the fuel up a flat, if it disappears it's been lean, if not you might need to change or alter the needle

#10 keule_k

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Posted 09 October 2018 - 07:31 PM

Hey there,

thanks a lot for the answers.

 

I think I have to give out some more Informations. When I bought the clubby it already had that problem.

I bought the car with an absolut standard 1275 early A+ engine. It had the same symptoms when I bought it. As the Dropgears where starting to sound a bit rough anyways I took out that engine and replaced it with my build Engine that has powered my SPi mini until the rust seperated us.

 

So the Problems stayed exactly the same while changing the Std. Engine to a modified one. The needle was changed accordingly. I also tryed out an even fater needle with the same result. A new mechanical fuel pump has been fitted also.

 

While doing so I replaced the Sparkplugs with the BPR6ES (Since I had a new set in the Workshop anyways).

Now, the engine has higher compression than the standard Setup and while experimenting around back when it was in my SPi I found that BPR6 as well as BPR7 work quite well on this engine. Since there is no NGK 6.5 I'm assuming that this engine is happy with both. I'm running the 7s now and they have a nice light brown colour without to much Carbon buildup. (The 6s looked nearly the same if I'm honest.) The 7s where the result of some talking about engine stress, now that it is powering a 10" Mini with 3.44 final drive, so that on longer Motorway journeys my engine builder and I thought that a colder Plug would be the better choice.

 

The engine leaning of on part throttle would sound like a too heavy Carb-Spring right?

Oh the car came fitted with an Oval K&N filter with a substack.


Edited by keule_k, 09 October 2018 - 07:33 PM.


#11 keule_k

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Posted 09 October 2018 - 07:37 PM

Does sound like a fuel supply (lean) issue, I'd say that a work around would be to try turning the fuel up a flat, if it disappears it's been lean, if not you might need to change or alter the needle

 I thought so also - I pulled the choke when the problem accured but hasn't realy helped - also turning it richer wasn't helping.



#12 luismx123

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Posted 10 October 2018 - 10:17 AM

 

 

why did you try colder plugs? i would have put hotter ones in first to see if it was a fouling issue. 7s would just show the same problem.

whats the difference between and when would one use hotter/colder plugs? - sorry for the tangent

 

a very good question that i dont actually know the correct way of describing. plugs vary in design as to where the electrode is in relation to the "shroud" (ok only talking about NGK  plugs here) the higher the number (7,8,9) the colder the plugs run so can maintain their construction for longer before burning out. (melting) cars that need these plugs tend to be running high compression and use high revs for a long time. so circuit race cars. these pugs used in a road car would end up getting fouled (wet and oiled) as they could not ignite the fuel or burn deposits of the electrode as they did not get hot enough. now lets look at a hot plug 5 for example. if put in a race engine and raced on they would get too hot and melt in a road car they might just look a bit glazed over but could start causing Pinking (preignition) due to them glowing.

 

note that a race engine does not have to run cold plugs as the event may not last long. hillclimbs and drag racing for example. but for a road car the best way is to go for a spirited drive and then pull over and pull the plugs out and see how they look. please note this is immediately not after and pootle to get home you want to see them ASAP.

 

 

Ok makes sense. So I'm guessing for someone who drives more aggressively, a colder or higher numbered plug would be better. For a 998 I doubt it'll make that much of a difference but its definitely something to consider next time!
Thanks for the explanation :)



#13 keule_k

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Posted 10 October 2018 - 11:42 AM

 

 

 

why did you try colder plugs? i would have put hotter ones in first to see if it was a fouling issue. 7s would just show the same problem.

whats the difference between and when would one use hotter/colder plugs? - sorry for the tangent

 

a very good question that i dont actually know the correct way of describing. plugs vary in design as to where the electrode is in relation to the "shroud" (ok only talking about NGK  plugs here) the higher the number (7,8,9) the colder the plugs run so can maintain their construction for longer before burning out. (melting) cars that need these plugs tend to be running high compression and use high revs for a long time. so circuit race cars. these pugs used in a road car would end up getting fouled (wet and oiled) as they could not ignite the fuel or burn deposits of the electrode as they did not get hot enough. now lets look at a hot plug 5 for example. if put in a race engine and raced on they would get too hot and melt in a road car they might just look a bit glazed over but could start causing Pinking (preignition) due to them glowing.

 

note that a race engine does not have to run cold plugs as the event may not last long. hillclimbs and drag racing for example. but for a road car the best way is to go for a spirited drive and then pull over and pull the plugs out and see how they look. please note this is immediately not after and pootle to get home you want to see them ASAP.

 

 

Ok makes sense. So I'm guessing for someone who drives more aggressively, a colder or higher numbered plug would be better. For a 998 I doubt it'll make that much of a difference but its definitely something to consider next time!
Thanks for the explanation :)

 

It's not a matter of Engine size but of Combustion pressure and therefor Temperature.
So a high compression 998 could also need a colder plug. My Engine builder ran 8 or 9's on his smal bore race engines if I remember correctly.

 

Don't be confused with the number since Bosch and many other Manufacturers rate their Plug the other way round compared to NGK.






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