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MG MGB Technical - Rough Idle-Colortune-weak spark?

I just purchased a colortune kit to try to find what is going on with my rough idle.

My idle fools me a lot - but I'm learning some of it's tricks! When it's cold the idle is not bad (not perfect either, but not bad). After warm up for say 20 minutes it gets very rough! Mostly in traffic, stop lights etc. If I disconnect the vacuum hose to the distributor - the engine runs much better - which leads me to believe I'm running rich for some reason?

I installed the Colortune plug and watched the sparks! But it doesn't look right - then again this my first time. I get what appears to be a series of inconsistant small blue sparks. Mixed in are much larger white flashes. So it's like this: small blue sparks pecking away - then big white flashes - small blue sparks peck for a while - then white flashes. It's not very consistant - it varys quite a bit. But this happens on ALL FOUR PLUG LOCATIONS.

Questions: Are the small blue sparks normal? Or is the colortune plug suppose to "glow" like the literature says. The white flashes look like a ring of color around the plug. The small blue sparks look like little pecks of sparks. I thought the ring of color was correct? And, I suspect these small blue sparks mean I have a weak spark?

Could this be a weak ignition coil giving too small of spark and gets worse after warmup? Would this explain why my engine runs better by disconnecting the vacuum hose - not a big enough spark to burn the gas so it runs rich?

BTW: Wires, cap, points and condensor were replaced this spring.
Rolin

What carbs, what year?
Dan Robinson

It's a 71' all stock and original. HS4 SU's....
Rolin

Replaced the coil - same problem. Rough idle and misfires. Colortune flashes bright white on every misfire.

What am I missing??

Rolin

Rolin. I am not a great fan of the Colortune, never having been able to get it to work properly with what passes for gasoline around here.

Yes, it sounds like you are running excessively rich. This could be caused by either a malfunction of the choke mechanism (jets did not come fully up when the choke was released) or basic jet adjustment. A set of dial calipers will allow you to measure how far below the "bridge" the jets are sitting. Peter Burgess recommended an initial tuning setting of .060" below the level of the bridge. I find that my final setting is about .053"-.055". This can be measured with an inexpensive set of dial calipers. (Which cost lots less than a Colortune, works much better and can be used for a large number of automotive related measurements). So, find out if the choke is sticking downwards (they often do) or whether you have a basic adjustment problem. Do your basic mixture adjustment. (Paul Hunt's excellent website, "The Pages of Bee and Vee" has the procedure on it." Then see what is happening.

In my opinion, you are better off spending a little money to have the car hooked up to an engine analyzer and exhaust gas meter than to use the Colortune. You will get more accurate and more representation readings. The Colortune was a good hobbyist grade tuning tool in its day--but, that day was 40 years ago. Les
Les Bengtson

Les, Thanks for the comments.

Right now it's impossible to even check the mixture using the colortune. I'm just not getting enough consistant spark in the plug to see whether it's rich or not. I wish I knew what the colortune plug is suppose to look like - it's such a small spark ticking away then the flashes of color - somethings not right. I should clarify that the "flash" isn't necessarily white - it's a ring of color and changes from bluish to varied colors.

The biggest question I have is about the small sparks I get - they just seem too small. Shouldn't it be a consistant pulsating "ring" of color rather than these little tiny sparks?! If the small tiny sparks are correct for the colortune, then what are the larger rings of color that spashes in from time to time?? I guess at this point I'm using Colortune as a diagnostic tool for the misfire rather than the mixture. It's certainly showing that there is a misfire of some sort.

So anyway, I did remove the colortune and went back to the basics. I adjusted the mixture the old fashioned way by slightly pushing up the pistons - but again it's very difficult to adjust since the engine is bouncing around so much. Something is WRONG...

Replaced the condensor again with a spare I had. No effect. So far I have replaced the cap, rotor, wires, points and condensor this a couple of months ago. Today, I replaced the coil, condensor (again). Yesterday I replaced the wires (again) and the cap (again). Checked the compression again today and all is around 150-155 on all 4. Dwell is 60, and timing is around 20 BTDC.

What the heck is left??
Rolin

Is the dizzy worn ? Bearing wear can give variable timing .
S Best

I'm to the point where I need a professional.

Again, I must stress that I don't even think the colortune plug is "flaming" correctly at all. It's just very small sparks. A flame in the shape of a ring happens very inconsistantly. Mostly it's just little blue sparks.

I found this webpage with images of how it's suppose to look. Mine looks nothing like this. Tiny sparks instead of the pictured "flamed" ring of color.

http://www.surenet.net/~verhey/colourtune/Intro.htm

At this point I have a problem with my spark being inconsistant and too weak on ALL cylinders. Or I have a defective colortune.

I'm at a loss, so I will contact a shop next week.
Rolin

Rolin. An experienced professional mechanic can put the car on an engine analyzer and four or five gas analyzer and determine in what area the problem lies. I used a ColorTune many years ago and gave it away because it did not work well with our "blended" fuels. It might be interesting to see what the fuel pumps in your area show. Ours show that all fuels are, depending on the time of year, either a blend of petroleum and ethnol or petroleum and MTBE (ether). With either blend, the ColorTune does not read the correct "bunsen blue" when the engines are properly tuned. I suspect that you may have a similar problem along with some basic tuning problems which need to be diagnosed and corrected. With a properly tuned engine, you can then take your ColorTune and see what it looks like. Les
Les Bengtson

Interesting thread. My Colortune has been a godsend allowing me to tune all sorts of carbs in many configurations with a much shorter "curve" to get in the proper range.

The problem is not the colortune as the symptoms appear when he is driving.

Rolin, check to see that your engine(tranny) to chassis ground is good first.

I think you should hook up a strobe (timing light) to your ignition and see if you have much flutter in the timing. If the mark appears to be all over the place (much more than a couple degrees) then you have an issue in the distributor that must be addressed like worn shaft bushes, weak points spring or (ta daaaaa) perforated vacuum advance/retard diaphram.

I have ALWAYS advocated and been rewarded by performing a complete tune-up whenever something is amiss (unless it is obvious). I understand that you have replaced much of it already but doing the tune-up in the "proper" order/sequence may just point up a problem you have been skirting.

Adjust the carb last.

A ground fault in the hot lead to the coil from the ignition or a failing ignition switch can also cause an irregular spark.

Mike!
mike!

Oh, Lordy. Try gas line dryer first. Water in fuel will not ignite and will give similar symptoms.

Mike!
mike!

This thread was discussed between 03/07/2004 and 06/07/2004

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