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New to a Sprite, tune ups....

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robdmau

robdmau

Senior Member
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First off thank you all so much for putting up with my posts, I hope I am able to ask one more question.

Tune ups, I'm pretty sure it is running the stock carb but I am having a problem with consistent idle. Some times it will idle at 2K and others it’s at 2500. If it ever dips below 2K it dies. I was told by the original owner that the chock needs to stay out about an 1/8 of an inch. I've practiced this and can't seem to get it right. It starts to idle to high so I give it a tap in, then it wants to die so I pull it out a hair. Just never seems to be right. He did say it’s sat for about 3 years.

So I am just thinking it needs to be adjusted and or cleaned out. I am probably going to take it to a shop to have the carb adjusted but I figure I would check here and see if it was easier enough.

I know I need a sync tool to adjust but is it easy enough to do at home or should I not bother?
 
Couple of things to check.
Remove each carb dome (2 screws ea) and make sure the piston, dome, and tube are clean.
1 grain of grit will mess up your carbs.
I twist up a clean paper towel and fish it in and out the tube where the black cap is, and wipe the inside of the dome and rim of the piston with a clean paper towel.
Check your fast idle setting, there are 2 idle adjustments, 1 (on each carb) is in the back and points straight up, this is the regular idle screw.
The fast idle which come on when you pull the choke, is pointing at about 45 degrees down towards the engine, loosen those screws first and then set the regular idle to something like 850 rpms or so. no more than 1000.
Now pull out the choke and see what the idle is. You can adjust that with the 2 other screws, the ones pointing 45* down.
These carbs are so simple once you play around with them a little you will understand them.
To adjust the mixture, turn the nut on the bottom of the carb up for lean, down for rich.
A good starting point is 2 full turns down from fully up, or 12 flats of the nut.
But it is important that the pistons move freely, any dirt in there will cause them to stick and you can not adjust out a sticky piston.
 
There is a saying that 90 of carb problems are actually ignition related. You can go crazy here, but starting with a rebuilt distributor and Petronix from Jeff at Advanced Distributors.

Carb tuning can take some practice, but making sure you don't have any vacuum leaks and that the throttle leaks is a good idea before diving into anything else.
 
There is a lot that will contribute to your symptoms.

What I normally find is that twin SU's are never properly balanced and that previous attempts to make them work right involve making adjustments to one only or different, dissimilar adjustments to each. In order for them to work best, they should both be set up exactly the same. This needs start from "step one" getting the mechanical linkage and butterflies set the same for both. I can't tell you how many times I've seen only one butterfly that will completely close. That will definitely prevent you from ever getting a proper low idle. If you can back out the idle mixture screws all the way and the engine doesn't die, that's one sign the butterflies aren't closing all the way.

In any event, given the age and probable use, you should check for vacuum leaks at all the manifold gaskets and especially at the throttle shafts. It's not unusual for them to be worn out and if you have enough air leakage, you'll never attain a consistent idle either. Spray something like DW40 around the shafts on both sides of each carb and around the gaskets and see if the idle changes. If the idle momentarily drops, that's an indication you have a vacuum leak.

One other thing, if you have the original PCV valve installed, check the diaphragm for cracks/leaks. It's not usual for the diaphragm to be cracked, especially if it's the one that came with the car originally! I'm referring to this part:

It will be attached to the intake manifold.

pcvValve.jpg
 
WOW, great info here guys, thank you so much for the info. I'm thinking on maybe trying some of this on my own before I take it to a shop just to see if I can do it. Thanks again guys.
 
robdmau said:
I'm thinking on maybe trying some of this on my own before I take it to a shop just to see if I can do it.
Click to expand...
Go ahead and give it a try. I had never done more than some minor motorcycle maintenance before I dove in to all the overhaul work on the Tunebug. It's all learnable, and you'll get good responses from folks here if you have questions.
 
drooartz said:
robdmau said:
I'm thinking on maybe trying some of this on my own before I take it to a shop just to see if I can do it.
Click to expand...
Go ahead and give it a try. I had never done more than some minor motorcycle maintenance before I dove in to all the overhaul work on the Tunebug. It's all learnable, and you'll get good responses from folks here if you have questions.
Click to expand...

Ya, I tuned my 2001 Yamaha R1's carbs a few times, so hopefully I can convert it to my Sprite. Nothing but good responses here so far and I am very grateful!
 
SO this is kinda going ok, found a few issues but got it idling now. The distributor was barely on there so after getting that on now I have no idea where the timing should be on this thing... Got it idling at about 1000, to 1100 RPM but still sounds rough, lobbing pretty bad. Think I will just take it to the shop next weekend to get it setup perfect and then I can work from there.
 
So we went on a little drive, car idles so much batter, still runs SUPER rough but it doesn't dye. On our drive, the exhaust brackets that hold the muffler broke off, so I was dragging pipe so a block, had to strap it up to make it home lol lol man!

SO I feel this is running rich, any ideas on a good starting setting to lean it out and work from there?
 
Did you have a chance to watch the video from University Motors yet?

They will help on carb tuning. What happens now if you lift the carb piston behind the air filter a 1/8 inch while she is running?
 
check your sparkplugs first - sooty black is rich. SU's are easy to tune, though, I might check timing first. Timing marks are underneath. there are various threads about making a timing mark that you can see from above. I'm not near my manuals but you can probably get a good start timing by ear.
 
tdskip said:
Did you have a chance to watch the video from University Motors yet?

They will help on carb tuning. What happens now if you lift the carb piston behind the air filter a 1/8 inch while she is running?
Click to expand...

Ya I did, maybe i should watch it again... I learned where all the adjustments are... I'll watch it again to see if there is a good starting point for the lean/rich mixture...
 
JPSmit said:
check your sparkplugs first - sooty black is rich. SU's are easy to tune, though, I might check timing first. Timing marks are underneath. there are various threads about making a timing mark that you can see from above. I'm not near my manuals but you can probably get a good start timing by ear.
Click to expand...

Ya I know that the timing is off, the distributor was not on there right so ya, where are the marks for the timing?
 
robdmau said:
JPSmit said:
check your sparkplugs first - sooty black is rich. SU's are easy to tune, though, I might check timing first. Timing marks are underneath. there are various threads about making a timing mark that you can see from above. I'm not near my manuals but you can probably get a good start timing by ear.
Click to expand...

Ya I know that the timing is off, the distributor was not on there right so ya, where are the marks for the timing?
Click to expand...

https://www.spritespot.com/phpBB3/viewtopic.php?f=9&t=4361

someone can post a pic I'm sure. Looks like you need to be 5 deg BTDC. Once again, I know that you can get under the thing, but, I suspect you can loosen off the distributor and get it closer by ear.
 
robdmau said:
tdskip said:
Did you have a chance to watch the video from University Motors yet?

They will help on carb tuning. What happens now if you lift the carb piston behind the air filter a 1/8 inch while she is running?
Click to expand...

Ya I did, maybe i should watch it again... I learned where all the adjustments are... I'll watch it again to see if there is a good starting point for the lean/rich mixture...
Click to expand...

The good starting point is where you have it right now because it is running. check the plugs, they will tell you if you are running rich or lean.

https://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

assuming you are running SU's - the rich/lean adjustment in the nut on the bottom. anti-clockwise for lean, clockwise for rich, one flat at a time.
 
JPSmit said:
The good starting point is where you have it right now because it is running. check the plugs, they will tell you if you are running rich or lean.

https://www.verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

assuming you are running SU's - the rich/lean adjustment in the nut on the bottom. anti-clockwise for lean, clockwise for rich, one flat at a time.
Click to expand...

I am running the SUs, so this info will help thank you.

About the SUs, how do I know if there is enough oil in the top? I do not see a fill line or anything.
 
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