Engine not going above 5kRPM

[readymeal]

Hi fellow GSers,

I ve got a 79 GS 750, 78000 km

she has been working great last summer when i picked her up and fixed the few issues (timing, leaks, shims...) the best running bike i could get was running on 3 cylinder while cold...then on 4 when warmed up or above 4000 rpm when cold. running on the rich side though due to the previous owner installing 4 in 1, K&N filter and different needles and needle jets.

however in December i rigged up a side car so i could keep riding even in the snow but then on my way to Seattle the bike ran gradually rougher and rougher...barely able to keep any speed under load...even less when the road was going up... i did not finish my way home as the bike did not want to start again getting off the ferry...

after few investigations, the timing was a bit off but not that much, i cleaned up some connections and noticed there was a random intermittent spark not working...meaning sometimes there was no spark in 3 but 1,2 and 4 were fine... but the next time it would be another plug not working... which is strange... very weak sparks in fact overall...compressions were equal everywhere on the lower side though.

but then the bike went back to what it was before without any other fixing... for the last 2 month going well at high RPM...back to the best i ve known her.

until last week where while riding on a sunny day something similar happened... the bike this time suddenly ran rough... and not able to get into the rpm and losing nearly anything left when the road was going up. I check the timing on the side of the road... perfect...
The bike is however able to get well into high RPM in neutral or with the clutch in... but not under load...

any thoughts?

Thx

[Kbecker3]

it could be a few differant things. how is the charging system? is the battery staying charged? if the charging system is good i would venture to say it has got to be either air / fuel mixture, or some other electrical issue.

if i had to guess i would say electrical. maybe not a good ground? i have seen bad grounds do some funny things. i know its not much help.

did the plugs look fouled out? did they smell like gas? where they new plugs, or ones that were fouled out and cleaned?

how are the points and condensers?

[Big T]

I'd start with replacing spark plug caps. Cheap and know to short out after time.

The fact that it generally runs fine points to electrical, rather than mechanical problems.

So, a couple of questions:

1. Does the bike have points or an electronic ignition upgrade?
2. How old are the condensors?
3. Are you getting 12V at the coils?

[BassCliff]

Greetings and Salutations!!

Hi Mr. readymeal,

I'll take a couple of shots in the dark. Intermittent ignitors have been known to exhibit these symptoms. Do you have new plug caps? How close to 12v is your voltage at the coils? Now let me roll out the welcome mat for you...

Please click here for your mega-welcome, chock full of tips, suggestions, links to vendors, and other information. Then feel free to visit my little BikeCliff website where I've been collecting the wisdom of this generous community. Don't forget, we like pictures! Not you, your bike!

Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

[850 Combat]

Points, right?

When is the last time the condensers were changed?

I got a pretty decent '79 GS850G free because the PO didn't know the difference between a bad condenser and a blown engine.

I'm assuming that the points are opening, of course...

[Wingnut1]

Probably a long shot, but check the exhaust for mice nests or loose baffles or other restrictions.

[chef1366]

When your done eating there is some good advice above. ^^^^

[readymeal]

Thx very much for all your help,also BassCliff your links are greats...i got my shop manual... and some good step by step guides.

1 charging system works i get 12v and up while reeving... the battery is dying from old age though.

2 grounds...to be checked

3 new spark plugs, rich looking but not fouled

4 Points are good...not sure about the condensers as it s my first bike with points...(my old BMW has electronic ignition). I did not look at them as i did not know their existence...

5 spark plug caps are within spec about 5Kohms resistance

6 coils: i don t know how to check the voltage but the resistance of each of them was within specifications as well(can t remember the figure though)

7 Exhaust: it s a Jardin with 4 in 1 collector... more an empty can than a muffler... a lots of back fire and some flames coming out of it... very nice to watch at night from a following rider perspective...

8 Carbs, been cleaned twice and balanced, last time i drained them N 3 bowl had less gas in it compare to the others???? float a bit too low in this one? Also the bike is easier to start if i prime the carbs the morning...(they are not really leaking though...)

So the condensers are number 1 on the list followed by testing the voltage of the coils...(how do you check both of them....?)

Thx again

My winter rig... with home made chains...front and back

[82Shafty]

You're diehard riding in the snow on a bike.

P&C are cheap and worth every penny. Go with qaulity OEM.

As far as the cold starts, insure valves are adj to spec and throw new o-rings at the carbs and possible intake boots with o-rings. Adj fuel levels while you are at it.

Charging voltage should be 13.2-15 (anything lower or anything higher is unacceptable).

Also, have you vacuum synch'd your carbs yet? Perform this after you have installed new o-rings, adj valves, new P&C.

Have you checked for voltage at the coils yet. I find that on average most are 1 to 3 volts down on voltage at the coils. I was at the standard 1.5 volt loss. Check out the coil relay mod for your cold weather needs as well.

Also, what grade of oil are you running in this extremely cold temperature?

How long is your commute at this cold temperature? You will want to watch for a build up of condensation inside your engine.


Hope this helps.