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Discussion Starter #1
My 76 Shadow has been running better than ever over the last two months. I got the carbs and idle tuned to perfection; running at perfect 600 RPMs when sitting at an intersection light.

Over the weekend, however, the automatic choke, which characteristically only requires a small tap on the gas pedal to turn off after a few moments of warming up, did not kick down to the normal 600 RPM speed. It’s stuck on fast idle at about 900-1,000 RPMs while in gear.

This issue popped up overnight. Nothing had been touched. No warnings or uncharacteristic behaviors.

I try adjusting the “idle stop screw” and nothing improved. I then remove this idle screw altogether, thinking the car would stall, and still no effect. I lubricated all the throttle controls to eliminate any sticking, but this did nothing.

I do not see much posted about this problem, so I am not sure how common this issue is.

I’d appreciate any first-place-to-start recommendations.

Thanks!
 

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A light oil is recommended for the choke cam mechanism, not engine oil or anything heavy as this can attract crud which can lead to a sticky choke cam. There should be no oil on the cam face as this can lead to it slipping. I would suggest that you clean the choke cam mechanism with carb cleaner and the give it a light oiling. If that doesn't sort it the problem might be at the choke stove pipe between the exhaust manifold and the choke spring. As heat builds in the manifold the stove pipe carries that heat to the choke spring and the spring opens to release the choke cam. If the insulation is missing from the choke stove pipe or if the stove pipe has disintegrated from corrosion or is blocked with crud the choke will not release as quickly as normal. Also, the choke spring may need to be cleaned by removing the cover and squirting plenty of WD40 or similar and blowing with an air hose. There's a heat element inside the exhaust manifold that delivers heat to the choke stove pipe, this element might have corroded or disintegrated but that's the last thing to check because the retaining nuts are probably seized and can be difficult to remove. If the ambient temperature has recently been unusually cold in your area lately then the
insulation on your choke stove pipe would be the next thing to check if cleaning and oiling the choke cam mechanism doesn't sort the problem. Good luck.
 

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Discussion Starter #3
I very much appreciate this helpful response and the excellent detail provided. This will be my weekend project. I'll report back. Many thanks!
 

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Discussion Starter #4
It's not a sticky choke cam. I will check the stovepipe next.

I did notice that a screw in front of the carb is missing. This might be the issue. Can any help me identify what screw goes here?

29436



Also here is a pic of the fast-idle mechanism. Not sure the adjusting screw should not be sitting on the cam.

29437
 

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Your choke cam appears to be working as designed. That missing screw might still be sitting in the "V" between the 2 rocker covers if you're lucky. Looks like you've identified the cause of the high idle problem.
 

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Love to see a bigger picture of your engine plumbing as some of those pipes and insulated pipes look different than the cars I have been looking at. Also to the right next to the missing screw in the picture that you show with the missing screw circled is what is called the throttle damper IIRC. It is a mechanical device similar to a kitchen cabinet and draw soft close mechanisms. Last summer I worked with a guy who rebuilds these carbs where this little simple part was rusted and was stuck so would not let throttle fully return. Can't wait till end of this lockdown and I can get back helping and also looking for my own Shadow. Just ordered an EV Tesla though for a daily runaround.....hey they put my bitcoin account up so much it is working out a freebee

Nick
 

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Discussion Starter #9
I had a mechanic friend look at the carbs today. He confirmed the screw I thought was missing, was not really missing. The threaded hole is not used on my car. We hooked up an engine analyzer (from the 1970s) to measure the RPMs, and used carb gauges from British Tool Works to rebalance the carbs and make adjustments. We got it running as smoothly as we could. However the idle fluctuated significantly after the adjustment; we could see it on the gauges jump by as much as 200 RPM. Gas was also leaking from the float chamber on the A bank side. It's been 8 years since the carbs received service. We going to re-build them next weekend.

I'm about to order the rebuild kits from Flying Spares, and I notice they have something called a OVERHEAT REDUCTION KIT (RH13183KIT). It offers a thicker gasket that insulates the carburetter from the inlet manifold and also gives a greater distance between the float chambers and the engine, thereby reducing the heat transferred from the engine and inlet manifold to the carburetor and float chambers. My question is, will incremental height from this thick gasket required resetting the linkage and lengthening the rods connected to the fast-idle mechanism? It does not look like there is enough thread length on the rods to accommodate adding too much more height. Please share what you might know about this option. It's pricey at $100 USD compared to the $4 standard gasket, and I wondering if it's worth it. If you have experience with this kit, please share your thoughts.

29456
 

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I would have thought your car already has the thicker gasket fitted. If you have not had a problem hot starting then you can be fairly sure it`s fitted. Wait until the carbs come off to be sure.

Jake
 

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Discussion Starter #11
Apparently these were introduced in 1977, a year after my model. I'm certain that I do not have this thinker gasket. I watched when the carbs were rebuilt and mounted years ago. The mechanic used a paper-thin gasket. I never had any hot starting problems living here in Southern Caifcorninam, so maybe I just don't really need it. I'll send an inquiry to the guys at Flying Spares to hear what they have to say and report back.
 

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Hi Mike

The only time I've had this vaporizing problem is when the ambient temperature has risen over 90F. I believe this is rare in SoCal, so since you have not had the problem before I would say you don't really need it. I'm in Tulsa at the moment where summer highs hit 100F so I will be fitting one this spring. When I lived in Tampa and Las Vegas it was a problem on my car, although the engine always eventually started and after a few minutes of erratic running would settle down as the cooler fuel was pumped through.

If at some time in the future you decide you need it, it is not a big job to fit the thicker gasket as the entire central plenum and carbs can be lifted off as a unit once the fuel pipes and linkages have been disconnected - an afternoon's work.
 

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Discussion Starter #13
Good to know Geoff. Thank you for your insights. Based on your comments, I agree it's not "required" ... though it might be good insurance; I visit Las Vegas and Arizona several times a year. Also, the temps around here can exceed 100F+ when sitting in life-crushing standstill LA and Hollywood FWY traffic during the summer. I let you know what the Flying Spares folks tell me about the install. If they confirm no modifications to the linkage is required I will likely go for it. More to come.
 

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I'm presuming that you have HD8 carbs. Gas leaking from the float chamber is usually caused by a stuck inlet valve needle caused by crud or corrosion. Sometimes it can be unstuck by a gentle tap of a screwdriver handle on the outside of the float chamber to shock it into action but don't hit it too hard as the metal used to make the bowl is brittle. If that doesn't do the trick then usually a liberal squirting of carb cleaner on the needle valve will sort it. When the needle valve sticks in the open position the float chamber overfills causing the carb jets to allow too much gas to escape into the intake manifold which would enrichen the gas/air mixture causing the engine revs to fluctuate. Another cause for the fuel inlet needle valve to stick open is a punctured float in the carb float bowl and when gas enters a punctured float it weighs it down keeping the inlet needle valve open even though the bowl is already full. You may have to open your float bowl and inspect the float and needle valve. These plastic floats appear to perish from exposure to ethanol in modern fuels, Burlen The SU Carb Co. are now manufacturing an updated "stay-up float" to combat this problem. Opening the float bowl isn't a big job and if you do remove it you might as well clean out the filter in the end of the fuel pipe while you're at it.
 

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Discussion Starter #16
Thank you Shadow II. Yes, I have the HD8 carbs. I appreciate learning about the "stay-up floats" and practical ways to address this problem. After thinking about it, I'm going to take a little extra time and do a basic rebuild of the carbs and will use the new floats you suggested.

I'm also going to add the OVERHEAT REDUCTION KIT (RH13183KIT). Here's the response I received from Flying Spares regarding my question whether or not modifications were needed to fit this kit:

"This can be fitted without any modification, only as long as the original linkage/rods are still as when the car was built. This kit is a copy of the one supplied by the factory years ago and differs in no way from the original factory kit (which is no longer available) which also fitted without any modifications."

If the parts arrive by this weekend, I plan to dedicate my entire Sunday to this effort. I'm fortunate to have the help of a friend who is a real mechanic; and the service manual, thanks to the link provided by this site.

I'm sure many of you have rebuilt your carbs and learned things along the way that might not be in the manual. Please share other helpful tips/suggestions you have. Thanks!
 

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Hindsight....label, label, and label, especially the vac hoses. Oh, don't forget to take pics. They are invaluable!
Make sure each carb and its parts remain with the carb you are working on.
 

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Re: Please share other helpful tips/suggestions you have.

On some cars there is a strange plastic insert in the bottom of the float chamber with a plastic tube that extends along the passageway to the bottom of the carburetor jet (pics 1 and 2). This was discontinued on later cars. Cars with this insert use a shorter compression spring to hold the jet/diaphragm assembly in place but the kits come with the longer spring which is used on carbs without the insert. The upshot of this is if your car has the plastic insert, the spring will be too long. Flying Spares do sell the shorter spring, but at 17 gbp per spring, this adds 34 gbp to the cost of your rebuild. In my case, after some research and since I knew the plastic insert was discontinued on later cars, I did not refit the insert and used the supplied longer spring in the kit.

My guess is the Plastic insert (Part number CD1864, Assy Plug and Tube) was an attempt to prevent heat vaporization by speeding up the flow of fuel through the bottom of the float chamber to the jet. I suspect RR found it did not work so discontinued it's use and moved to the thicker gasket solution. As I said, this is a guess so if anyone has any better info it would be interesting to hear.

Mike - I suspect your 1976 model will not have this insert so there will be no problem (my car is a 1974 model). If it is there, then you will need to decide whether to buy the shorter spring or just not refit the plastic insert. Thanks for publishing the response from FS regards fitting of their kit.
 

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Thank you Shadow II. Yes, I have the HD8 carbs. I appreciate learning about the "stay-up floats" and practical ways to address this problem. After thinking about it, I'm going to take a little extra time and do a basic rebuild of the carbs and will use the new floats you suggested.

I'm also going to add the OVERHEAT REDUCTION KIT (RH13183KIT). Here's the response I received from Flying Spares regarding my question whether or not modifications were needed to fit this kit:

"This can be fitted without any modification, only as long as the original linkage/rods are still as when the car was built. This kit is a copy of the one supplied by the factory years ago and differs in no way from the original factory kit (which is no longer available) which also fitted without any modifications."

If the parts arrive by this weekend, I plan to dedicate my entire Sunday to this effort. I'm fortunate to have the help of a friend who is a real mechanic; and the service manual, thanks to the link provided by this site.

I'm sure many of you have rebuilt your carbs and learned things along the way that might not be in the manual. Please share other helpful tips/suggestions you have. Thanks!
I reckon that your problem may relate to the carb float or float jet needle or both. I would check these first and remedy if necessary and there may be no need to delve further if the problem is solved. No point in taking on additional work if it can be avoided.
 
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