9 replies to this topic

[Arthy]

Hi guys, first post on this forum so be gentle 

 

I currently have a standard 1275 (94' Sprite) which have recently been fitted with twin HS4's and a Maniflow exhaust system. (Yes I know twin HS4's are not needed, but they look + sound cool   )

 

Anyway to the problem. The engine seems to misfire/judder when under load. The best way to explain this is this; 

 

I'm cruising at 40mph in 4th gear. I want to get to 50mph so I accelerate. It is fine, but now I want to get to 55mph. I accelerate more and now the engine judders and shakes the whole car. If I ease off and feather the throttle, I can slowly get to that speed. 

 

Another situation it is from accelerating hard from a complete standstill. It is fine going from 1st to 2nd, but as 3rd is about to run out of steam the engine judder is there. I go to 4th and it disappears. 

 

The car is running slightly rich, the plugs tell me that along with the sooty exhaust but the car runs like a dream as long as I don't accelerate hard. The idle is smooth, and has a crisp throttle response. It's recently had a new fuel pump this week and new rotor arm, cap, HT leads and coil around 6 months ago.

 

I know minis fairly well, and my dad even more so as he grew up around them. The judder was never there before with the single carb which leads me to thinking its the twins. Maybe it's running too rich and choking the engine or leaning out at top end? Or the plugs? 

 

ANY help or ideas would be appreciated. Thanks for your time reading this 

Edited by Arthy, 23 March 2016 - 01:21 PM.

[gazza82]

Rich isn't good for the engine .. too rich as you start to create bore-wash and will wreck it.

 

The carbs need to be perfectly in balance as well .. twin HS4s (1 1/2 SUs) may be a bit too much for a standard 1275 ..

[Arthy]

Rich isn't good for the engine .. too rich as you start to create bore-wash and will wreck it.

 

The carbs need to be perfectly in balance as well .. twin HS4s (1 1/2 SUs) may be a bit too much for a standard 1275 ..

 

Thanks for the reply! 

 

This is what I thought, it is running too rich. I plan to replace the cam + head later on so twin HS4's will be needed then. 

 

I'm going to start again with the mixture and see if that solves the problem. 

Edited by Arthy, 23 March 2016 - 01:00 PM.

[cal844]

Sounds electrical to me! Try a known working coil to test

[Arthy]

Sounds electrical to me! Try a known working coil to test

 

Will do, I have lying around. However it never did it before I fitted the twins? Or maybe a more powerful one? Worth a shot anyway, thanks! 

 

Edit: The ignition does stay on sometimes until the voltage picks up when I accelerate. I recently replaced the alternator around 3 months ago too. Surely that can't be linked? 

Edited by Arthy, 23 March 2016 - 01:37 PM.

[slidehammer]

Sounds like mixture to me, as the problem isn't there with the factory carb. I would put the standard carb back on and save the twins until more tuning is done and then get it set up properly on a dyno. Wrong mixture is not good for the engine either too weak or too rich.

[Arthy]

Sounds like mixture to me, as the problem isn't there with the factory carb. I would put the standard carb back on and save the twins until more tuning is done and then get it set up properly on a dyno. Wrong mixture is not good for the engine either too weak or too rich.

Thanks for your help, the mixture is slightly rich but as the plugs indicate but one of the plugs is quite sootier than the others? I'm guessing this indicates an ignition problem?

It's a bloody right job taking those twins off, I've already done it twice and don't really want to do it again :)

[adam_93rio]

The twins are more than likely over fuelling it. I had almost the same problem with a worn carb. Causing it to richen up at higher load and bog down.
The over fuelling with the twins isn't good for your engine at all, as has been said above

Personally I'd be either swapping back to the single carb, or be taking it to a rolling road and having new needles fitted

[Arthy]

The twins are more than likely over fuelling it. I had almost the same problem with a worn carb. Causing it to richen up at higher load and bog down.
The over fuelling with the twins isn't good for your engine at all, as has been said above

Personally I'd be either swapping back to the single carb, or be taking it to a rolling road and having new needles fitted

 

I don't suppose anyone knows what type of needle I would need to run? I'm currently running AAA which I suspected is too rich. 

 

Thanks for your help! 

 

Edit: Would needle AUD1004 be about right? 

Edited by Arthy, 23 March 2016 - 11:38 PM.

[giannoutsos]

You could try AAU.
Good luck