Worn Throttle Shaft - carb balance problem

[undysworld]

I just got through rebuilding my carbs. The front seems fine, but we're having trouble balancing them. The rear carb won't return to it's lowest idle setting, without strong pressure being applied to the linkage arm. From reading lots of previous posts, I'm concerned that the throttle shaft is loose and causing problems. (I didn't see any problems with the intake manifolds.)

I've got about .07m up/down play on a throttle shaft. The shaft measured 7.94, pretty consistently. I can't detect any worn areas, so I'm guessing the base is worn. The i.d. of the throttle base is at about 8.04mm. There is some slight deviation, but those are close. There are seals at the outside of the housing, but both are broken.

Some questions please, for those of you who have done this already:

Am I correct, that this is too much movement of the throttle shaft?

Is this possibly caused by the ruined seals?

Can anybody tell me what the stock shaft dia. is?

Likewise, what is the stock bore of the base?

I noticed SAV sells an 8.3m OS shaft which I assume is the oversize shaft if you have to bore it out. Is this my only/best bet at this point?

If I do need to have it bored out:
Do I need to have the seal seat redone too?
Will stock seals and the remaining hardware still work?
If I cannot get the base bored out locally, who does this operation?

Thanks,
Paul

[Winter Beater]

Paul,

I could have answered a few of the diameter questions ... yesterday, sorry. Finished rebuilding mine earlier today. I had spoken with Jim L. prior to starting the job and complained I could never get the carbs to sync properly or to hold very long and it was frustrating. He indicated that the mating surfaces need to be flat so that the 1mm gasket can seal properly. The other advice was to replace the seals at the end of the throttle plate shaft; they are not included in the carb rebuild kit from SAV. 1 pair that I extracted could barely be recognized as a seal and could easily draw in air.

I started with 80 grit paper and worked up to 400 grit using a piece of glass plate to ensure the surface was flat. After 3 or 4 swipes on the 150 grit I determined if I need to go down to 80 or not. Just incase there was a good surface in the lot. I can tell you all the surfaces were deplorable, the choke was the worst but not the longest to correct.

I warmed up the Pinz and the carb's sync'd in a few minutes ( after reducing the idle mixture from 1.5 turns to 1 turn (Denver 5400 ft elev)) Won't know if this is a long term solution for another week. If not ... EFI here come!

Hope Jim's advice can help you too.

Terry.

[undysworld]

Terry,

Thanks for your input. My timing has never been ideal.

We'd checked the mating surfaces with a straightedge pretty carefully, and thought they looked ok. We also applied Hylomar on both sides of all the gaskets, so I doubt it's leaking there.

As I mentioned, the two seals on that carb were junk. I'm sure they must have been leaking air, but wouldn't that make the balancer read lower than normal? I'm getting higher balancer readings on the carb in question, unless we forcibly hold the throttle shaft closed firmly. Otherwise, the throttle plates in that carb seem to lift slightly off idle when we stop pushing it down. When it lifts, the balancer goes up.

We sprayed the outside of that carb to check for leakage, and found no effect from the spray.

I can't tell whether the seals are leaking and causing 1/2 the engine to go lean and rev higher, or if the wear at the throttle shaft is physically lifting the throttle plates thus increasing RPMs.

Paul

[Winter Beater]

I had a similar problem with the rear carb -couldn't get below 14 on the sync tool. Did 3 things at the same time (not scientific, but I didn't want to replace carb 3 times)

Removed tin shroud and cut notch for linkage which was binding at full throttle.
cut tin at rear where accelerator pump shaft is located, also turned washer & spring around so washer didn't rub on carb body.
disconnected choke cable to play with lever manually in case the brass ring had not seated correctly.

After all that it worked very well. The sync procedure was textbook after leaning out the idle mixture slightly

P.S. - I was also told not to use any gasket sealant on a Carb because of all the little passages and the ease of which you can plug them with goo.

Maybe someone else has better advice - this was my first proper rebuild as opposed to dropping in new parts and hoping.

[pcolette]

Removed tin shroud and cut notch for linkage which was binding at full throttle.

I also had this problem on the last pair I installed. The rear tin was rubbing against the linkage on the rear carb and was keeping it from returning all the way to idle. A long punch and a hammer were used to gently "massage" the tin to clear the throttle pivot. Not as nice as removing the tin and cutting but I was feeling lazy that day.

[undysworld]

Thanks guys for the input.

Well, I already pulled the carb back off to look for issues with the throttle shaft. But I can replace the base and see if things are interfering. I didn't look back by the accel pump linkage for obstruction. The chokes didn't seem to be interfering, as we had disconnected them as well. They did have normal travel.

As to the goo, I'm no pro either. I was assisted by a friend who's worked on a variety of old and new German-built vehicles, from top to bottom. A very careful application of this Hylomar goo is what he used. But I certainly understand your comment. I'm interested in any other feedback on this too.

I can attest to the fact that the goo adhered the base gasket to the base so securely that I destroyed the gasket removing the carb. At a minimum, that's a PIA.

I'll go check for clearance this afternoon. I hope you're correct.

Paul

[audiocontr]

I have the same exact issue on my 710. Please keep us informed on how yours works out.

[Jim LaGuardia]

Throttle shafts are 8mm, if you have worn out Brass shafts, all you need in most cases is new Steel shafts(under $12ea at SAV). Over sized shafts (8.3mm)are also available($30ea), although I have not found anyone local that can re size the bores for the larger shafts or re bush them.
Shafts should rotate freely without play. A small amount of play is allowed, although the more play, the harder it will be to sync them. Most good carbs sync up around 6-11 Kg/h. Find a happy medium for worn units and remember Idle speed is 850-950 rpm warm.
One last not:No sealant is used on any part of the carb or it's mounting
Wet the gaskets and spacer with Brake cleaner or carb spray for best results.

[Jimm391730]

Enco (http://www.use-enco.com) has a 0.327" reamer (P/N 334-5092) which is 8.305mm in diameter for $20 if you get the 8.3mm oversized shafts. Of course you need someone who can use it properly!

[undysworld]

Thanks everybody for your input.

My shafts are not brass, they are a shiny silver, so I assume steel. I can't detect any wear in them to speak of. The measurements stay consistent along the shaft, although I can't really read it accurately in the center due to the slot for the throttle disks. It isn't a full 8mm. But even if it was 8, the hole is .04 larger still.

I'm sorry to read about not using any sealant. Live and learn I guess. So do I leave the upper rebuilt parts of this carb alone - hoping for no problems, and just deal with the shaft play, or do I go ahead and rip 'em both down immediately for this sealant issue?

I set the carb base back on the truck, to check for any interference with the throttle shaft linkages. The carb itself easily clears the tin. The link between the rear carb and the pivot comes close to the tin, but does not appear to interfere at all.

But I noticed a funny thing when I had the base in my hand and applied downward pressure to the shaft. If I pushed down in the front port, nothing much happened. But if I push downward on the throttle shaft in the rear port, it seems to pivot at the idle set screw and slightly open the throttle. My guess is, at idle, the vacuum increases enough to draw that plate and shaft downward, which forces it up off idle.

The local machine shop figured it would be more expensive to mill/install bushings than to bore it larger and use the oversized shaft from SAV. Willie tells me the stock seals will still work with it. I think if I want it to work right, I'll have to go this route.

It looks like that Enco reamer is about right. But I'm curious about the length, and am awaiting a reply from Enco. Did you already use one of these?

Thanks all.
Paul

[Jimm391730]

It looks like that Enco reamer is about right. But I'm curious about the length, and am awaiting a reply from Enco. Did you already use one of these?

No, I was just looking for what reamers I could find that sounded close...
Best of luck,

[undysworld]

Word from Enco, that one is only 1.5" long. Hmmm.

[Jimm391730]

Word from Enco, that one is only 1.5" long. Hmmm.

Although I bet that is the flute (cutting edges) length... but not certain.

[undysworld]

Well, proving once again, it's not what you know but who...

A good machinist friend stopped by yesterday to give me some advice. He was able to see that the holes were out-of-round even before he confirmed it with the calipers. His concern is that a reamer will follow the existing hole, and not be centered properly.

He also confirmed my opinion that the shaft itself does not show any appreciable wear.

He's taken the base, and plans to drill it oversize and install bronze bushings. Because it's out-of-round, he'll use the seal seat to center the o.s. hole. Then he'll drill out the bushings to fit the existing shaft. His plan is to leave the center hole as stock.

I'll let you know how it works out.

ps Jimmy, I suspect you're right. I naively asked how long the cutting surface was, not realizing that was not the overall effective length.

[milesdzyn]

Well, proving once again, it's not what you know but who...

A good machinist friend stopped by yesterday to give me some advice. He was able to see that the holes were out-of-round even before he confirmed it with the calipers. His concern is that a reamer will follow the existing hole, and not be centered properly.

He also confirmed my opinion that the shaft itself does not show any appreciable wear.

He's taken the base, and plans to drill it oversize and install bronze bushings. Because it's out-of-round, he'll use the seal seat to center the o.s. hole. Then he'll drill out the bushings to fit the existing shaft. His plan is to leave the center hole as stock.

I'll let you know how it works out.

Please do let us know how it turns out.

If your friend could take some pictures of how he is doing this it may be informative to the Pinz community. I personally am very interested as I have my own ideas on how to manage this repair, but my idea is based on the equipment I have on hand.

I am trying to find a drill manufacturer that will make a custom drill for me with a pilot guide shaft with a cutting bore set for a bushing. This would make it a one step drilling process. I may have to make it myself as I am having little luck so far.

Miles