So, as some of you may know, I recently acquired a nice condition all original 79 GS with relatively low mileage. Ever since I started driving it, it has had a bad high RPM stumble. I did a quick search and couldn't find anything that sounded like the problem I am having.

So a little history to start. The car is of course an all original 79, no mods to the engine, ignition, or fuel system at all. I've done all the regular maintenance stuff, replaced fuel filter and air filter along with checking the ignition system, at least as much as I could check. I've gone through the carb and vacuum hose spider for any vacuum leaks. I have not rebuilt the carb yet though. Also, about the only thing I haven't checked on the ignition system is the dwell on the points, I don't have a proper meter to do so.

So, here is the problem. Whenever I try to take the engine past 5500rpm it starts to stumble badly and looses a bunch of power, to the point it has a hard time making it to 7000rpm. I've actually not even had the engine up to 7000rpm. Now, it stumbles like this regardless of throttle input and the engine runs perfect anywhere under 5500rpm. There is no hesitation, stumbling and seems to have full power below this point. Also, it changes slightly every time I run the engine up above 5500rpm, sometimes worse than other times.

I know its not running lean, the engine still stumbles even with some choke input (easy way to check if its leaning out). I've blown some air into the air bleeds in the carb and I get a steady stream of fuel coming out the jets. I know that doesn't say much, but at least I know the main fuel circuits aren't clogged. Also, this is not a clogged fuel filter as I replaced it and this same thing happens even when free revving the engine with no load.

At this point I am leaning toward an ignition problem. I have to be honest, I haven't dealt with points before. I'm unsure if the points are all that good on this car, but they do look alright, no buildup or anything of that sort on them. I'm at a loss on this. Anyone have any ideas?

 

I would re-check timing to make sure you arent way too advanced and then manually run the secondary on the carb to make sure it isnt stuck. Those are my only ideas so far.

 

Done and done IanS. Timing was initially advanced when I got the car, about 15 degrees actually. Fixed that and the only thing that improved was the idle and low RPM throttle response. I can pop open the secondaries while the car is running and clearly see a good stream of fuel coming out and the engine quickly revs up. They are free to move as well.

Ok, one thought maybe someone can confirm as a possibility. I'm sure many experienced people here have heard of valve float on a piston engine. This is usually caused by the intake valve or exhaust valve springs being too weak to handle the high RPM they are asked to operate at. I would imagine this problem could exist on the breaker points on a points style ignition system? Another thing that I think could cause this problem, but is hard to diagnose, is contact breaking down. Maybe the surface on the points isn't in good enough shape to make a proper contact for the short amount of time they are contacting at that high RPM. Just throwing things out there to get others that know more about this than I do kinda thinking here.

 

Would be easy and cheap enough to convert to S2-S3 style dizzy. Then you could try that and if still happens then it most likely wasnt your original dizzy. I know Im telling you to just throw parts at it but honestly most of us have so many spare parts that it doesnt actually cost anything. Im sure someone local would let you borrow a dizzy for a couple days if need be. No idea how points systems work so cant help you on that part.

 

I've actually got a good S2 dizzy so I am set there. I don't have the coils though. As it turns out, the 79 coils are 9V coils. The electronic dizzy needs to use 12V coils, which I don't have any extra of (used to have 3, can't figure out where they went). I also have extra ignitors should the need for one arise lol. I would like to keep the stock points system as long as I can though, adds to the classic feel of the car in general when working on it, something I enjoy.

 

I have heard from a guy I know... Uh, yeah that FB coils fit into your pocket at the wrecker and are not noticeable if you have a baggy shirt on... Yeah...

 

lol that's a good one to remember. Unfortunately the 2 FB's in the scrap yard closest to me have been picked clean, very few good parts left, but then again the coils may be there. Will have to check it out tomorrow. Will bring wire cutters and an adjustable wrench to remove if they are present.

 

Hell just grab the entire upper left rail. They wont notice.

Edit: Man, I gotta stay out of the lounge.

 

lol, the lounge will do that.

 

point float as we call it in the racing world could be an issue. BUT are the secondarts really working on thier own while driving the car under load? could the vac diaphram be bad causing the secondarys not to open. this just an idea, had a simular problem some time ago with a 12a not going past 4500 rpm. do the nikki secondary mod( in the archives) and see if it cures the problem

 

I'm 90% sure the secondaries are opening at least most of the way. Like I said, up till I hit 5500-6000rpm it runs fine with seemingly full power as it gets up there pretty quick. After i got the timing adjusted in, it actually surprised me a bit in how much power it had between 4000 and 5500rpm. I will have to check the secondary diaphram when i rebuild the carb though. I should have a carb kit coming in the mail real soon. Also, as I posted, the problem is present regardless of throttle input. The 12A will rev out to 7000rpm without the secondaries, I know from experience. This problem is present even without opening the throttle enough to open the secondaries (there is a stop to prevent them from opening at all at low throttle input).