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Carburetor Question

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stevebn2bj7

Jedi Warrior
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On the way back from Canclave had an interesting problem crop up. After about ten hours of close to non-stop driving my throttle started to get very stiff. It felt like it had notches in the motion of it and would require a hard tap to get it to throttle down at stops. Then while on the freeway it stuck altogether and had to pull over to try and correct it. Once we were stopped it was easy to get the throttle to stick in the wide open position while the engine was off. Then when I started it with the throttle closed it would start and the revs would climb very quickly to 3000 or so and I shut it off. This was without touching the acclerator pedal. One of the guys in the group took off the dash pots and cleaned them up and put it back together and it started and ran fine. This after sitting for about an hour. The throttle was still a little stiff but it returned OK. After about another hour and within sight of home it happened again. At this point I cleaned the dampers and limped it home. The next day the throttle is very loose and no problem to drive.

Any thoughts?
Throttle shafts and busings improperly installed and reamed?

Thanks in advance.

Steve
 
Hummmmm, odd. But, if simply cleaning the dash pots worked, I would be suspicious of the needle sliders, I forget what they're called, but if there are ridges or dingleberries in them, they might get stuck. But not rev up without input.
Odd. I don't know, but would be carefully cleaning them carbs out, checking all the return springs and linkages, too! Must be british. does it leak? /ubbthreads/images/graemlins/wink.gif
 
Hello Steven, You didn't say what model SU's you have but I'm guessing that you have HD6's. Since there is no direct connection between the throttle shaft and the dashpots, I'm thinking that the tightening up is occurring in the jets themselves, or the jet actuating arm. When your friend cleaned up the dashpots he probably helped to loosen things alittle so that it stayed that way for awhile. When you had the throttle wide open with the engine off, you may have lifted the dashpots up also and the engine ran to 3000 rpms before the kneedles dropped back into place. I wonder if the jets in the diaphram have varnished up and the jets now are acting as if they are off center. The heat from long term running at constant state with the kneedles in the same position for a long time may be a factor. You may have picked up some setiment that have caused the jets to hang up.
Just some ideas,
Dave.
 
yeah, check the oil in the pots, too! couldn't hurt!
 
If the pistons are stuck up in the dashpots, but the butterflies are closing, you'll get a weak mixture but that's all. It will be a devil to start and run.
It sounds like your butterflies aren't closing, so check throttle linkages. Sounds like something's seizing when hot.
 
Thanks for all the suggestions guys. I will start to work through all of them and hopefully find the root cause. I am thinking that the throttle shafts may need to be rebushed and properly reamed or that they were improperly done two owners ago. There are other issues as well with the carbon and other dirt build ups in them.

Steve
 
[ QUOTE ]
If the pistons are stuck up in the dashpots, but the butterflies are closing, you'll get a weak mixture but that's all. hot.

[/ QUOTE ]

If the pistons are stuck up in the dashpots, that means the needles are pulled all the way out of the jets. And with the "butterflies" closed that means minimal air getting in. That's a RICH condition to me, not leaness.
/ubbthreads/images/graemlins/cheers.gif Dave C.
 
[ QUOTE ]
[ QUOTE ]
If the pistons are stuck up in the dashpots, but the butterflies are closing, you'll get a weak mixture but that's all. hot.

[/ QUOTE ]

If the pistons are stuck up in the dashpots, that means the needles are pulled all the way out of the jets. And with the "butterflies" closed that means minimal air getting in. That's a RICH condition to me, not leaness.
/ubbthreads/images/graemlins/cheers.gif Dave C.

[/ QUOTE ]
If the butterflies are closed but the pistons are up, the air velocity over the jet bridge would likely be so low that no fuel would be pulled out of the jets, no matter where the needle is positioned. The space between the piston & the jet bridge IS the venturi which causes fuel to be drawn out of the jet. (A huge venturi without the necessary air flow to pull fuel out.)
D
 
same thing happened to a friends healy while i was sitting in it,his problem was that after rebuilding the carbs the linkage was misadjusted and actually somehow locked over center giving full throttle,i would start there,check all alignment of rods,pushers,bushings,springs,etc.up to the carbs,then the carbs themselves as others have recomended,funny thing,when it happened to us,my friend horrified in less then a blink of an eye opened the bonet jumped out of the car with engine running at full throttle,in an attempt to fix something in there i simply leaned over and turned of the key.that helped!!that was 1968 i was all of 16 years old,remember it like it was yesterday.good luck /ubbthreads/images/graemlins/hammer.gif
 
Finally got around to taking the carbs apart and rebuilding them. The throttle shafts are definitely the problem. They are very worn and very tight in the bushings. I have a new master rebuild kit from SU and am in the process of installing that.

My question is how do yu remove the old throttle shaft bushings? Everthing else seems pretty straightforward.

Thanks,
Steve
 
Well lets see Steve, having worn throttle shafts and tight bushings is a bit odd me thinks. However, the throttle disks must be removed and the shafts extracted from the carbs to release the bushings .If you have a manual study the disassembly and reassembly steps presented in there or perhaps the kit includes instructions.--Fwiw--Keoke
 
steven, my unathoritive responce to your question, I'm sure others have some experience replacing the bushings, I have never needed to, but the bushings probably need to be pressed out, and then reamed and honed to fit the shafts. I have replaced distributor bushings this way, but not the carb bushings. This is tricky especially because the bushings absolutely must be reamed "in-line". I bought a special long reaming rod when I did the distributors.
 
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