79 standard carb issues

[Dbshea]

Well, the carbs are currently soaking in pinesol. I picked up a magnifying glass to really check everything over. Other than the exhaust, is there anything you guys can think of to check on the rest of the bike?

[BA80]

One other thing I've run into ONLY ONCE. I had a set of slides that the diaphragms were loose on and could spin. they had spun on the slides and the slide wasn't properly oriented to work correctly.

There is a small tab on the outer edge of the diaphragm that fits in a little slot in the carb body.

The slide has a small hole in the bottom of it that has to be front and center on the engine side of the carbs. This small hole is what creates the negative pressure (vacuum) on top of the diaphragm that overcomes the spring and pulls the slide up.

I can get you some pics if you don't understand what I'm talking about

[Dbshea]

One other thing I've run into ONLY ONCE. I had a set of slides that the diaphragms were loose on and could spin. they had spun on the slides and the slide wasn't properly oriented to work correctly.

There is a small tab on the outer edge of the diaphragm that fits in a little slot in the carb body.

The slide has a small hole in the bottom of it that has to be front and center on the engine side of the carbs. This small hole is what creates the negative pressure (vacuum) on top of the diaphragm that overcomes the spring and pulls the slide up.

I can get you some pics if you don't understand what I'm talking about

I'm tracking. I'll check it out tonight. Thanks Greg.

[TopCatGr58]

Hey there,

Okay, you say the electrical was checked out, specifically the TCI, and coil wires, plugs. But have you verified that you ARE getting spark on all cylinders, spritzed the head pipes with water after idling and running a few minutes to verify that they all sizzle!? When there's no load on the engine, it can rev and respond to throttle seemingly just fine when in fact it's running on 2 cylinders, not 4. So...when put on the road/load, it craps out because it doesn't have enough power to really move the bike with just 2 cylinders firing.

Secondly, you fixed the pickup coil wires, but did you verify that the timing plate and centrifugal advance parts move easily and that the plate rotates easily! IT gets rusted, dirty, and can get sticky, and then you don't get any timing advance, so the engine can't rev very high because it's not able to advance the timing that allows it to run at higher rpms!

These are some of the things that can contribute to lack of rpm increase under load!

T.C.

[Dbshea]

Update for the night crew.

After another thorough cleaning I wanted to check these carbs against another set we got with a parts bike. Everything checked out until I pulled the slides out. Now, my disclaimer, these carbs are unproven. They were a loose rack that came with the bike. Well, comparing the slides from MY rack to the spare rack I noticed that the needles were .5 mm different in exposed length. So that's one thing, second, it would appear that the spring is not installed in the appropriate order on the needle. On MY rack, by pressing the tip towards the diaphragm it compresses, on the spare rack you pull the needle AWAY from the diaphragm. That correlates with this assembly picture:
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Now, I didn't separate the needles as my ring pliers don't have that reach. Assuming the carbs were legit as their condition was generally good, I let sleeping dogs lie. This is the ONLY discrepancy I can find, but it certainly makes sense. As always input is greatly appreciated.

Also, Greg, I verified that all the diaphragms do line up properly in the carb body and my mufflers are straight thru, I can see my header.

Updates to be posted. Fingers crossed.

[Dbshea]

Hey there,

Okay, you say the electrical was checked out, specifically the TCI, and coil wires, plugs. But have you verified that you ARE getting spark on all cylinders, spritzed the head pipes with water after idling and running a few minutes to verify that they all sizzle!? When there's no load on the engine, it can rev and respond to throttle seemingly just fine when in fact it's running on 2 cylinders, not 4. So...when put on the road/load, it craps out because it doesn't have enough power to really move the bike with just 2 cylinders firing.

Secondly, you fixed the pickup coil wires, but did you verify that the timing plate and centrifugal advance parts move easily and that the plate rotates easily! IT gets rusted, dirty, and can get sticky, and then you don't get any timing advance, so the engine can't rev very high because it's not able to advance the timing that allows it to run at higher rpms!

These are some of the things that can contribute to lack of rpm increase under load!

T.C.

Hey T.C.

I have a thermal gun and at idle all 4 pipes are at 440* +\-
I did verify the advance using a vacuum with engine running, and have used an inductive pick up to verify spark on the stand.

[TopCatGr58]

The vacuum advance is just a very small part of the timing advance. IT only provides about 16 degrees of advance in addition to the cent. adv.

The cent. adv. provides ~35 degrees on top of the static 10 degrees it's supposed to have.

You need to use a timing light, cap off the vac. adv., and then check the timing indicator at idle, and then again at around 2500 or so rpm to see and verify that the timing indicator moves to the more advanced position which will then show and prove that the mechanical centrifugal parts are moving and working properly!

T.C.

[Dbshea]

Because dead threads are no use to anyone I just wanted to confirm the fix for my issue. The slides/needles were set up incorrectly. The spring placement was wrong not allowing the bike to get into the mains. Got everything reassembled last night and just raised some hell in the neighborhood making sure everything is as it should be. 32* out with snow on the ground, I'm cold but happy. A couple new issues have shown up, nothing severe. Speedo and clutch, I'll investigate the and open a new thread if I dead end.

Thanks again to all that had input. Glad to be here!

[motoman]

Because dead threads are no use to anyone I just wanted to confirm the fix for my issue. The slides/needles were set up incorrectly. The spring placement was wrong not allowing the bike to get into the mains. Got everything reassembled last night and just raised some hell in the neighborhood making sure everything is as it should be. 32* out with snow on the ground, I'm cold but happy. A couple new issues have shown up, nothing severe. Speedo and clutch, I'll investigate the and open a new thread if I dead end.

Thanks again to all that had input. Glad to be here!

Thanks for reporting what needed to be done for success! Makes for good help for others.

[BA80]

Because dead threads are no use to anyone I just wanted to confirm the fix for my issue. The slides/needles were set up incorrectly. The spring placement was wrong not allowing the bike to get into the mains. Got everything reassembled last night and just raised some hell in the neighborhood making sure everything is as it should be. 32* out with snow on the ground, I'm cold but happy. A couple new issues have shown up, nothing severe. Speedo and clutch, I'll investigate the and open a new thread if I dead end.

Thanks again to all that had input. Glad to be here!

The diaphragm spring? I didn't know you could get that in wrong.

[Rasputin]

The diaphragm spring? I didn't know you could get that in wrong.

I have done that myself, where I have placed the spring for the needle (at the bottom of the slide) in the wrong order. It messes with your head for a while for sure.

[BA80]

OHHHHHH! That spring.

I've never made that error. I wouldn't think it would actually make all that much difference though.

Strange that the early carbs that spring goes on bottom of the clip and the late carbs it goes on the top. Never actually figured out why it was even there.