Starts, dies on throttle

[mattfrme]

Hello all,

Ok, my 79 SF was running pretty well. It was running rich, but the idle was too low. I adjusted the idle from about 1100RPM to just over 1500. Next day bike ran fine but idle had creeped up to 2200 (most likely because i did not have the bike fully warmed up when I adjusted it previously). Backed the idle out to about 1500 again. shut off bike.

Then I checked the rear brake light relay (its dead) went to start the bike up again, it started then immediately died.

checked all the connections around the fuse pannel and brake relay, checked fuses, all seems ok. Started bike, it immediately dies on throttle.

Removed carbs, readjusted all of them, reinstalled. New plugs installed. On crank, alot of backfire, adjusted pilot jets out a bit, and it started with the throttle open a little, runs until you open throttle further, then immediately dies.

BIke has always been hard to start, in the am it requires a shot of started fluid, but then it runs fine.

I have no idea where to begin.... Help?

Thanks,

Matt

[Ken Talbot]

You'll need to remove the carbs for a thorough cleaning. Then you'll want to remove the carbs for a thorough cleaning. Finally, you can finish by removing the carbs for a thorough cleaning. Sound familiar?

Sorry, there is no short cut around dirty carbs.....

[mattfrme]

Ken,

I will again clean the carbs, however, I should mention that they have been cleaned about 5 times in the last 2 months...

At least I am getting good at taking them apart.

Matt

[CA.XSer]

Matt
Did you rebuild carbs with carb kits from K&L?

[mattfrme]

I am not sure who the rebuild kits were from, very much doubt they were K&N. That said, I did reuse the pilot Jets in carb 2 and 3 as the replacements were smaller then what was in there.

Somewhere I read that these were bigger so that the engine ran cooler.

In any event, the carbs are in soaking in carb cleaner, I hope this works.

Matt

[CA.XSer]

Matt

Recheck the sizes on the jets while you have the carbs apart. I had the same problem with my 79SF and the pilot jets from the kits were to big as were the main jets. The pilot jets should have 6 holes in them, on the sides. The jets in the rebuild kits I got had 8 holes.

I have a K & N air filter and Jardin 4 into 2 exhaust, with 140 mains, the jet needle up 1 notch and floats set at 26mm, stock pilot jets. Bike pulls fantastic above 4500 rpm, idle is smooth, but between 2000rpm and 4500rpm just pulls ok. I need to set the floats a little richer. HTH

[rover]

Matt, there are rubber o-rings in each end of the throttle shaft, carb cleaner will probably destroy these, causing a vacuum leak. This will make the idle erratic, and cause various other problems. After I get the cam chain on my 79F, I will take a look at the 79 SF carbs I soaked in cleaner, cause I had problems with them, so I used the carbs from my parts bike. I had no idea they were in there until I read a thread of someone having erratic idle after cleaning his carbs. I will post back if I find out how to cure this.

[randy]

Matt,

I also have a 79F. What is a rear brake light relay and where is it located?

Randy

[mattfrme]

Randy,

The relay is attached to the brake pedle by a spring. 2 wires go up behind the fuse panel.

Today I have recleaned the carbs, and reinstalled them. Bike was hard to start ( a squirt of starter spray helped) when it started, it idled at about 2000, but sounded like a high strung lawnmower. It took some throttle, and kept running.

I think it might be time for a professional...

Matt

[John]

Matt, the brake relay that you are referring to is actually the brake light switch. There is no brake light relay. There is also a switch in the front master cylinder/brake lever area.

[tab1324]

couple things to try

You might try tensioning the cam chain... A sligthly off timing could make it start hard. Also, you mentioned you cleaned the carbs, but when was the last time any of the parts were replaced? I did the triple clean and had similar problems until I removed the idle needle screw ass'ys, cleaned and reinstalled. Also the timing chain tensioning seemed to help. Mine still starts a bit difficult but I've got the routine now... Start on half choke, add a 1/8" turn of throttle for about 1 minute, then move choke to about 3/4. Then it will stay running.

Are all vacuum lines connected or plugged?

You mention the hight strung lawn mower... Does it sound like it's possibly missing in one cylinder. Mine did this also for a while. Or do the revs get real high? For mine when the revs got out of control, with the backfiring - was lean. How many turns out do you have the idle screws? Did the butterflys get adjusted when carbs removed? Might be worth a bench synch.

Just some ideas from my very limited experience. You'll get it running well soon, hang in... I dealt with the same thing for about a month.

Tom B.

[pgg]

Mattfrme, If you stick to basics you can't go wrong. Make sure all carbs are set up identical. Use mikuni jets. Use 137.5 mains and 42.5 pilots. Put all needles on the middle notch. Use 180 air pilot jets. Set all floats at 25 or 26mm. Smoothly operating carb slides. Diaphrams with no pinholes. No intake airleaks on hoses and boots. All hoses connected to the right places. Test the pickup coils, coils,spark leads and plug caps for uniformity. Fuel taps working properly. Spend cash on carb sync gauges - 4 cylinder bikes' need them. Get a timing light as well. Your motor will eventually purr like a cat.