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Carb Issues
- Thread starter SaxMan
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Rut
Obi Wan
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I think Kurt offered the most logical solution and you can easily test the shaft fit by removing the butterfly and sliding the shaft to an unused portion.
Rut
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I have... and you should probably PM me if you want the dirt.
One of the tech tips is that poor compression or poor valve adjustments can cause a vacuum leak. I did take a warm compression test:
Cyl 1: 145
Cyl 2: 140
Cyl 3: 150
Cyl 4: 155
However, it is the rear carb that feeds 3&4 that is down on the vacuum compared to the front carb, yet those two cylinders have higher compression than the front cylinders, which is sucking down a ton more vacuum.
The other odd thing was that I accidentally started the car without the oil breather hoses hooked up to the carb, and the car's idle was more in the realm of reality. When I hooked the breather hoses up, the idle sped up.
I'll try the throttle shafts. After that, I'll likely take the car to a specialist. I got a couple of recommendations from the local British Car Club. I figure it would be better to have the carbs attached to the rest of the car rather than just send the carbs away, especially if I'm dealing with an engine issue, too.
Cyl 1: 145
Cyl 2: 140
Cyl 3: 150
Cyl 4: 155
However, it is the rear carb that feeds 3&4 that is down on the vacuum compared to the front carb, yet those two cylinders have higher compression than the front cylinders, which is sucking down a ton more vacuum.
The other odd thing was that I accidentally started the car without the oil breather hoses hooked up to the carb, and the car's idle was more in the realm of reality. When I hooked the breather hoses up, the idle sped up.
I'll try the throttle shafts. After that, I'll likely take the car to a specialist. I got a couple of recommendations from the local British Car Club. I figure it would be better to have the carbs attached to the rest of the car rather than just send the carbs away, especially if I'm dealing with an engine issue, too.
Which way does the throttle shaft come out? From the inboard (linkage) side or the outboard (float chamber) side? The manuals are mute on how to do this. The one on the front carb isn't going more than a 1/4" in either direction once the butterfly screws were removed.
EDIT: Disregard. I'm a moron. I didn't realize the butterfly went through the throttle shaft. Once I realized that I had to pull them through, they came out easily. Of course, I managed to trash one of the butterflies and the throttle shaft in the process.
EDIT: Disregard. I'm a moron. I didn't realize the butterfly went through the throttle shaft. Once I realized that I had to pull them through, they came out easily. Of course, I managed to trash one of the butterflies and the throttle shaft in the process.
Last edited:
Replaced the metering rods and the piston return springs tonight, also checked the float chambers. I'm trying to eliminate the possible variances between the front and rear carbs and see if that fixes the problem.
I've been mulling picking up a used second set of carbs just to do a true A/B test. There's a set off a '73 Midget on eBay right now that I'm going back and forth with on the "Best Offer" with the seller. I figure having spare parts can't ever hurt....as my garage slowly, but surely, fills up with them.
I've been mulling picking up a used second set of carbs just to do a true A/B test. There's a set off a '73 Midget on eBay right now that I'm going back and forth with on the "Best Offer" with the seller. I figure having spare parts can't ever hurt....as my garage slowly, but surely, fills up with them.
Roger
Luke Skywalker
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Yes, and I have found them helpful, prompt and courteous.
Parts came in today and I reassembled the carbs. While the carbs were out, I pulled the heat shield and checked the manifold bolts -- a couple of them worked their way loose, which could very easily be my vacuum leak. Carbs went back together and all the parts are moving smoothly. I put them back in the car and took considerable care to make sure the linkage was perfectly even. The carbs just look right. Now, we get to see if they are actually right. Hoping for some dry weather tomorrow morning to fire it up. Otherwise, it's going to be a couple of days.
I also pulled the trigger on the second set up SUs on eBay. They are not in bad shape, but I think they are certainly rougher than the original carbs. If nothing else, if my carbs are still off, they give me a way to truly A/B the carb setup. And, spare parts are certainly a good thing to have on hand.
I also pulled the trigger on the second set up SUs on eBay. They are not in bad shape, but I think they are certainly rougher than the original carbs. If nothing else, if my carbs are still off, they give me a way to truly A/B the carb setup. And, spare parts are certainly a good thing to have on hand.
Finally! Victory at last!
I could tell right off the bat as soon as the car barked to life this morning that things were different as the engine sounded a lot happier. Once the car was warm, it took about 10 minutes of tweaking to get her purring just under 1000 rpm.
It was certainly quite the odyssey -- 6 weeks since my initial post. It was frustrating at times, but I sure learned a heck of a lot about SU carbs along the way, and I guess that's what LBCs are about. That and the fun driving them. Thank you all for your feedback and support. As usual, I couldn't have done it without you guys!
I could tell right off the bat as soon as the car barked to life this morning that things were different as the engine sounded a lot happier. Once the car was warm, it took about 10 minutes of tweaking to get her purring just under 1000 rpm.
It was certainly quite the odyssey -- 6 weeks since my initial post. It was frustrating at times, but I sure learned a heck of a lot about SU carbs along the way, and I guess that's what LBCs are about. That and the fun driving them. Thank you all for your feedback and support. As usual, I couldn't have done it without you guys!
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woo hoo - now drive it like you stole it!
So, the high idle never quite resolved itself. After some driving, the Sprite idles around 1300 rpm, but sometimes as high as 1500 - 1600. At the end of the drive today in high 80 degree temps, it was idling around 11 - 1200 rpm. Otherwise, the motor and carbs are doing great. Acceleration is smooth and the motor has no problems running up to 4500 rpm. I haven't the nerve to push the old engine past that point, as previous runs past 4500 rpm seem to have only generated more noise than speed. In the past, I typically set the idle around 1000, even though that's a bit higher than spec. My thought process is 1) I'm not going to be spending a lot of time idling the car 2) Since everything else is working beautifully, am I just opening a Pandora's box trying to knock 200 - 300 rpm off the idle?
I'm still planning on a engine rebuild after this driving season, so everything is going to be coming out of the car anyway, so why not just drive it as I can right now?
I'm still planning on a engine rebuild after this driving season, so everything is going to be coming out of the car anyway, so why not just drive it as I can right now?
Jim_Gruber
Yoda
Offline
With those Compression Numbers what else is wrong unless you have a major oil leak leave it alone and drive.
Defintiely hot. Idle screws are completely backed out.
