RED HOT BUT NOT ROCKING

blacklav · May 29, 2012

I have a 74 MK2A with twin concentrics set as standard. The ignition is Boyer mk 4 micro, set up spot on at 28 deg before tdc at 5000rpm.
The motor runs fine ,pulls well and idles when warm. The problem is the right side cylinder gets very hot. So hot it has started to melt the plug cap. This is a plain soft rubber one as the ht lead is the resister.
The left side is ok. I have set the carbs up to the amal instructions as well as i can and balanced them with vacuum gauges.
If the right side was running rich would it cause this overheating if not what would.
It does smoke quite a lot when hot on both cylinders , probably inlet valve seals melted as the compression is ok.

Any ideas much appreciated.
chris r

Dances with Shrapnel · May 29, 2012

The right side being rich would not necessarily cause it to overheat. Overheating can be due to a variety of things including too lean, ignition is too far retarded, burnt exhaust valve(s), not enough exhaust valve lash and one cylinder pulling much harder than the other (carbs unbalanced).

Have a look and see if both slides are lifting reasonably close togther. A quick and dirty check is to listen to the slides as they close; you should hear one sound, not one slide cosing and then the next slide closing.

shrugger · May 29, 2012

Pull that spark plug and share with us what it looks like.

Hobot · May 29, 2012

Richness famously tends to damp down the fire. Lean-ness is first suspect, so that implies an air leak if not a carb problem. Spark timing too retarded on adv'd can do this but both sides if electro brain sparker, so rule that out. Don't know how but may be a plug is bad so we'd like to see it as we are just as stumped as you are, again.

concours · May 29, 2012

What does the chrome on the pipes look like? Even? Do a compression test. IR temp gun is only $29 now, shoot both sides of the head/cyl, etc., report back.

Josh Cox · May 30, 2012

Too rich will not run hot, too lean will definately run hot.

Matt Spencer · May 30, 2012

Being cautious so as not to strip the tread , so the plugs NOT TIGHT , could be the ' issue ' .

Of Course , We all wash & clean the plug threads & put a drop of grease ( or better still , Anti - Sieze ) on them before installing .

A not tight plug can cause ' bothers ' . Id Use ' Copper Kote ' as its copper graphite - pressure sealing AND Anti Seize combined . Gauled Freds arnt tolerated .

Josh Cox · May 30, 2012

I hope I'm not asking a stupid question, how long are you running the engine for ?, do you have a fan blowing onto the engine whilst running it in your shed ?.

As these are air cooled bikes, they do really really need airflow when running for more than a few minutes.

With your mixture concern, lean the mixture until that cylinder starts to splutter then richen it a a turn or two, the bushmans carby tuning tricks better describes the process.

blacklav · May 30, 2012

Thanks for the responses, just a quick update.
The plugs are both sooty looking i.e. running rich when at idle . From 2000rpm upwards, when i have pulled the clutch in and stopped the motor, removed the plugs, they are the correct darkish brown .
When listening to the engine at all revs, the right sounds louder and more powerfull. Maybe raising the throttle slide slghtly to increase fueling to the left might balance the load a bit more .
I did have it on a fast tickover whilst setting the carbs up for about 5 mins, but it still should`nt melt a plug cap.
The pipes are evenly coloured going light blue, but the right has a few dark blue spots about 1/4 by 1/8 inch approx 5 inches away from the exhaust port.
With it burning oil, i would have thought this would aid in cooling the head, but maybe not.
Apart from that, any other suggestions please.

chris r

sole survivor · May 30, 2012

Are you getting oil to the heads? Maybe overfilled the oil tank? What color is the smoke coming out of the pipes?

Josh Cox · May 30, 2012

Which plugs are you running ?, the Iridiums are well worth the money.

Where are you located, there maybe somebody from here near by..............

concours · May 30, 2012

What plug caps are melting? Pics please...

blacklav · Jun 5, 2012

Just a quick update.
There is plenty of oil getting to the head.
It is blue smoke.
The plug caps are of soft rubber, they only protect the connection to the plug from the weather. It was melted at the bottom 3/8 inch on to the sparkplug.
The plugs are the standard recommended champions.

Since i adjusted the leftside throttle slide approx 1/16th inch, the motor runs smoother and has a more even temperature between plugs.
So much for my cheap vacuum gauges.

chris r

1up3down · Jun 5, 2012

This may have nothing to do with the right side issue as presumable the left would react the same way but:

Is your timing too retarded at the 28 degrees at 5000rpm you say it is set at?

I remember my older analog Boyer specified 31 degrees at 5000rrpm, did this change with the digital boyers?

Check your boyer instructions to verify 28 degrees is the correct timing.

Secondly, I would put a compression gauge in the plug holes and full open throttle kick four times no ignition and see what your reading are to determine if there is a noticable difference between the cylinders.

rpatton · Jun 5, 2012

blacklav said:
Just a quick update
Since i adjusted the leftside throttle slide approx 1/16th inch, the motor runs smoother and has a more even temperature between plugs.
So much for my cheap vacuum gauges.

chris r

There was a really cool balance method on the BMW Airhead list. Get around ten feet of clear tubing that fits the balance tube barbs. Find the exact midway point and mark it. Mount the tubing so that it forms a U on a piece of wood about three feet long with the mark at the bottom. Put enough oil in the tubing so it's filled about one foot up on each side of the U. When you have the motor running the oil levels will go up and down on alternate sides. You can tell which side has the edge as far as vacuum goes. I tried it and it works great.

RED HOT BUT NOT ROCKING

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