1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

This is a new issue for me. The bike runs for about 15 min and then dies when it’s hot (I am assuming it's a heat related issue). I try to restart it, and it cranks just fine, but won’t run. Someone had suggested it was the condenser failing due to heat, so I swapped it out. Went on a test run and, same problem. Dies and won’t restart after 15 or so min of running. After about a 45 min cool off, it’ll fire right back up. But it dies again once it heats up.

I'd read elsewhere that if the condenser is bad, you won't get a spark when it's hot. So I tested for spark after it died. Got spark. Also, fuel is flowing. Compression is good.

What else could it be?

Thanks!

Sachs and Vanagon2.jpeg

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

Sometimes crank seals hold until the engine warms up. Just a guess.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

Richard Eberline /

one at a time: spark plug, heat range of the spark plug, external coil, ignition wire and cap. gap the plug correctly and check your timing. This happened to me once before, a new condenser was my fix. you need a big fat spark.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@don45640, if it were the seals would there be any other symptoms I could look for? Would compression be bad when hot, I'm guessing? <-- If so, this kinda makes some sense to me. Maybe I need a better compression test than the ol' thumb on the spark plug hole when cranking.

@rockettman, same question about other symptoms. Especially in regards to the coil, cap and ignition wire.

As for the plug, I suppose I could swap it, but when I check for spark, it looks good. Which doesn't make sense to me as why it won't run. If fuel is getting to the chamber and compressions feels good (using thumb test), then why no combustion when the plug sparks? And why does this only happen when hot? When it's cool, it starts up on first crank.

And when I fiddle the fuel tickler, the carb floods, so there is fuel getting to the carb. Would it make any sense that the fuel stops getting from the carb to the chamber when it's hot but flows properly when cool?

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@rockettman

Can you please explain what you mean when you say "heat range of the spark plug"?

Thanks!

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

OG Crazy Wayne /

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

if thers nice " spark " when it dies, then its not spark.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

OG Crazy Wayne /

A new condenser means absolutely nothing in today's world of junk parts.

If you don't have a nice blue Spark your condenser is garbage.

A blue spark represents a true plasma burn if it's orange or white replace the condenser.

And if your spark plug wire is not a directly to the coil in the flywheel most likely that HT coil is dying on its way out shortly.

There's some articles here on how to rewind those HT coils but I suggest replacing it with an original Bosch or installing a dual winding system with a separate winding in the stator that powers an external HD coil for best reliability and GAP your plug at 0.019" to 0.022" thousands of an inch.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

Too hard to determine useful compression using just thumb. When it is cold is it slow to start? you have to really pedal to get it to begin to run?

Compression on a worn motor can be like a fish in a frying pan. There is a thin line between not cooked and over cooked.

A worn bike can start with 90 psi but not start with 85.

After warming up the compression drops due to heat expansion. Though the bike will run after the compression drops to 85 or less, once it dies it will need to shrink back cold to get that 90psi back

Get your self a proper gauge. You may need a new piston and kit.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

OG Crazy Wayne /

Oops edit to my above post

And if your spark plug wire goes directly to the coil in the flywheel most likely the internal HT coil is dying on its way out shortly.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@cheetahchrome

Yeah, I need to get a compression gauge set to test compression on my old Vanagon. So I might at well get one. I'll let you know what I learn when I get to it.(edited)

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@ogcrazy

The spark is a very pretty and healthy blue spark. And it's had this issue with two different condensers. But the problem only started on the last two trips I took on it. Prior to that, it'd run fine.

When I pop the flywheel off, everything looks really nice in there, I'm out of town for the week, but am going to give it another look next weekend. Thanks for you input.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@cheetahchrome

As for starting the bike, it usually starts on the firt crank or two regardless of outside temp.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

Bob,compression will still read good with bad crank seals...………..the piston is pulling in leaked air instead of fuel/air mix.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

OG Crazy Wayne /

If it starts and runs awesome when cold with no choke chances are your exhaust is plugged up with carbon, time to BBQ the exhaust pipe and scrape the carbon out of the exhaust port......

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

...or the fuel tank cap vent may be clogged. Can you ride it with the cap off or real loose to test?

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

Good call Den but the bike runs for 15 min he says. A blocked vent would go on vacuum In about 1.5 miles. Yes?

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

OG Crazy Wayne /

Maybe your ankle bracelet is jamming the ignition system

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

> cheetahchrome . Wrote:

> -------------------------------------------------------

> Good call Den but the bike runs for 15 min he says. A blocked vent would

> go on vacuum In about 1.5 miles. Yes?

Empirical evidence trumps theory.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

I really appreciate all y'all's input here. When I get back to town I'm gonna give many of these things a look and see if I can narrow it down.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@don45640

"the piston is pulling in leaked air instead of fuel/air mix."

Is this something that would only happen when the engine is hot?

I've only had the bike up and running since last fall. And it's always had kind of a metallic rattly sound at high RPMs. Maybe whatever was causing that noise has developed into this new problem. Does that sound match up with a wonky piston?

Thanks!

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@ogcrazy

You know, I recently lost the muffler end piece and replaced it. I wonder if there is some exhaust issue... is this something that would only happen when the engine is hot?

Thanks again!

Re: 1979 Sachs Prima G3 dies

plug your decomp my man.

clean your points well and gap em check that the points lobe is in good shape inside the flywheel. If you can't get it better with those things honestly it'seasier to probably bail and replace the whole ignition. but plug the decomp, clean the points and swap a new spark plug first

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

I did some stuff:

1. Swapped out the condenser

2. I cleaned the exhaust pipe. Wasn’t toooo dirty.

3. I checked the gas tank vent tube and it is clear.

I’ve not been able to commit much time to the bike, obviously, and I’ve done (nearly) the least I can do. But I have eliminated three of the possible options.

After doing the above, I took it on a test run:

When I cold crank it, it fires up no problem (as it always did). With a crank or two it goes quickly from put-put to the crisp popping sound of combustion in the chamber and it’s running great.

When I ride it for even only a few minutes and then kill the engine, it won’t start back up. I can hear the put-put-put as I pedal crank it trying to get it to run again, but there’s no combustion happening. It never fires back up... Endless crank putting.

When it cools off, it fires up again.

So, I’m still inclined to think it’s a heat related problem. There are many things I intend to try (based on y’all’s suggestions), but if anyone could help me prioritize my tasks based on the symptoms and what most likely would fix the thing, I’d much appreciate it. I guess I’m trying to do the “fixes” that would cure the problems (dying when too hot, not starting when warm/hot) that are only cause by the engine being hot first. Then go from there.

1. New spark plug, properly gapped at 0.019" to 0.022" thousands of an inch. <--(the old spark plug is only one year old, but I can change it)

2. Get a compression gauge and check compression and hopefully I don’t need a new piston and kit <--(are there other symptoms/signs that this might be the problem? Rattly noise at high RPM, maybe?)

3. If bad compression (from above test) could be the seals... “Sometimes crank seals hold until the engine warms up.” Check for bad crank seals to see if the piston is pulling in leaked air instead of fuel/air mix. <--(not sure yet how to go about this yet, but will figure it out, I suppose...)

4. Clean the points well and gap them, check that the points lobe is in good shape inside the flywheel. <--(visually speaking, everything looks nice and clean in here, but I haven’t checked the gaps... Is this something that would only be a problem when hot?)

5. Plug the decomp

6. Check to see if the internal HT coil is dying <--(any tips on how to check this? And, again, would heat make this a prob?)

7. External coil?

8. Ignition wire and cap?

9. Check the timing?

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

Wayne mentioned spark color above.

My question is have you pulled the plug and watched spark quality when its cold and compared it to when its hot and dies????(edited)

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

Resistor in the antiparasite cap can also fail when cooking hot and work once cooled down again

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

You might replace the condenser .

While checking point gap , insure the cam wiper is lubed . If it's dry the point gap could change due to heat build up from the points heel riding on a dry cam .

And , yes , coils can and do change resistances when hot . Think expansion ...

If they change too much , they don't like working so good until they cool down , which puts the resistance back in the proper zone . Same thing happens to condensers . Maybe not all , but , it's something to keep in mind .(edited)

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@themotorhead

I just watched the plug spark when cold vs when hot.

cold: bright yellow/white with occasional blue tint

hot: mostly purple-ish blue, sometimes white-ish.

Both hot and cold there's a good spark.

What does it all mean???

Thanks!

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

@pdcomm

I've put two different condensers in there and the problem is the same. As I dig further, I'll see what I can do about the other stuff you mentioned.

Re: 1979 Sachs Prima G3 dies when hot, won’t restart until cooled off. Not condenser

> Bob Ray Wrote:

> -------------------------------------------------------

> @pdcomm

>

> I've put two different condensers in there and the problem is the same.

> As I dig further, I'll see what I can do about the other stuff you

> mentioned.

What brand of condensors?

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