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| Frumpy idle | Rate Topic |
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| Posted: 04-22-2007 12:04 am |
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1st Post |
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smcmanus Member 
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I have the car running but it idles very lumpy at 1000 RPM. It seems to run rich at idle. I see the manual specifies "blue" springs. Is this a color or something else? I have passed the CL1 medical for years but I sure don'e see blue when I look at my springs. They seem to be bare metal. Oddly.........it idles very smoothly with a touch of choke! A slight lift of damper bogs it down with fuel. So it seems odd that it would be too rich at idle with the choke off. Any help is appreciated Steve
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| Posted: 04-22-2007 04:50 am |
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2nd Post |
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Mark Rosenbaum Member 
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Once warmed up, a JH engine with un-worn and properly set-up Strombergs should idle quite smoothly at 1000 rpm, and with increasing roughness down to 400 rpm or so, provided there are no air leaks at the throttle shafts or at the big o-rings between carbs and adapter plates. "Blue" refers to the paint or dye used to color one end of the spring. Usually this disappears after a few years of use. However, the correct spring for the JH should have wire that's 0.036" thick, should have a free length of 5.5 to 5.9 inches (which is not at all critical), and should compress to a length of 2.812" under a force of 117 grams. Please note that only the "blue" springs should be used in a JH. I suggest verifying that you have 2.0 to 3.5 psi fuel pressure at the tee that feeds the two carbs, that your ignition timing is correct, and that your cam timing is correct. Assuming the carb is properly assembled, unscrewing and lifting up on its damper rod shouldn't have any long-term effect on engine speed. OTOH, if you were talking about lifting the vacuum valve, meaning the cylindrical piston-like object visible from the carb's opening to the atmosphere, then a slight (1/16" or thereabouts) lift should cause engine speed to increase if the mixture is too rich, or decrease if it's too lean. So if you were lifting a vacuum valve and the engine bogged down, that says the fuel-air mix was too lean, and adding some "choke" would make the mix a bit richer. I'll note here that if at any time you can see liquid fuel flowing into the engine, there's something wrong with the carb. Given your description, and assuming that your carbs are in good condition, I'd expect them to be out of tune for air flow and/or mixture. This can be dealt with fairly quickly using the procedure in the shop manual, but bear in mind you may have to go through the adjustments several times before everything falls into place. If tuning the carbs doesn't help, it would be necessary to remove, disassemble and inspect them, rebuild them as indicated, then reinstall and re-tune them.
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| Posted: 04-22-2007 01:03 pm |
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3rd Post |
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smcmanus Member
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Mark Thanks for the info. You have straightened me out on the mixture thing. I thought that a lift of the vacuum valve would cause the mixture to richen. This explains why a little tiny bit of choke makes it run smoothly. I will adjust accordingly. BTW, the carbs are freshly rebuilt and everything has been gone through now so I'm almost on the road. Thanks Steve
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| Posted: 04-22-2007 03:16 pm |
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4th Post |
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Mark Rosenbaum Member
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smcmanus wrote: [...] I thought that a lift of the vacuum valve would cause the mixture to richen. That issue has tripped a lot of folks over the year, myself included. Fortunately, a sharp-eyed reader caught my error and called me on it, some time back. In a Stromberg, the vacuum valve and the jet bridge form a variable venturi whose size changes depending on how much the vacuum valve is lifted. Fuel delivery to the airstream flowing into the engine depends first on the speed of the air flow over the bridge, and second on the amount of fuel exposed to that air flow (i.e. the area of the annulus between main jet and mixture needle). When you lift the vacuum valve without altering the throttle setting, the air speed across the bridge decreases more rapidly than the annulus area increases, and the mixture becomes more lean.
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| Posted: 04-22-2007 05:03 pm |
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5th Post |
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smcmanus Member
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Makes perfect sense now. What idle speed and ignition timing setting are you using? I have the stock distributor with the vacuum advance disconnected. Right now it is set at 12 degrees BTDC at idle speed of 1000 rpm. Thanks Steve
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| Posted: 04-22-2007 06:25 pm |
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6th Post |
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Mark Rosenbaum Member
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My car has stock cams and cam timing (110 IN / 110 EX). The vacuum retard is disabled, and the ignition timing is 12° BTDC. That's where I get what I consider the best combination of smooth idle, high manifold vacuum, and part-throttle engine pickup. In comparison, with the timing set to 16° BTDC, the vacuum, idle quality, and low-speed driveability are all noticeably worse. With the locally available regular fuel, performance does not improve, and I get hints of pinging ('pinking'). With mid-grade fuel, high-end performance is slightly improved but IMHO in a street car such as mine the tradeoffs are not worth the extra fuel cost.
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| Posted: 04-23-2007 09:55 pm |
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7th Post |
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smcmanus Member
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I had some time to fiddle with it today. I was able to turn the metering valve adjustment on the front carb, but the rear is stuck hard. I took it apart and still cannot turn it. Any hints? Thanks Steve
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| Posted: 04-23-2007 11:08 pm |
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8th Post |
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Mark Rosenbaum Member
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There's a set screw [7] in the side of the vacuum valve [6], which needs to be slightly loose if the mixture is to be adjusted. The directions in the carburetor overhaul kit are somewhat unclear, and people often overtighten this screw in consequence. If the set screw, mixture needle [8], or mixture adjustment screw [not shown] is frozen, a bit of penetrating oil may help. Attachment: set screw.jpg (Downloaded 284 times) Last edited on 04-23-2007 11:09 pm by Mark Rosenbaum |
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| Posted: 04-24-2007 04:27 pm |
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9th Post |
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smcmanus Member
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Well I can't get it to turn or get it apart. Anyone have as spare vacuum valve and needle? Thanks Steve
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| Posted: 04-25-2007 11:31 am |
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10th Post |
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smcmanus Member
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Following some expert advice I attempted to drive the needle out. Here is the result. Not what I desired. Yes, yes, yes, I was banging on the needle assy. It must be bonded into place. Next, I will apply heat to see if I can get it out. Of course it is already ruined but why give up now? Who has some spare parts to sell me?? Thanks Steve Attachment: zsdebacle.jpg (Downloaded 42 times)
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| Posted: 04-25-2007 11:33 pm |
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11th Post |
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John Finch Member
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Steve, I have a pair of strombergs that I've never run fuel through. They sat idle on the car in Phoenix for 15 years before I bought 18309 and converted to Dellortos. I don't know their condition but assume they were ok before the car was parked for engine work. I could send them to you and you can use what you need. I might even have a couple of rebuild kits lying around that I never used. I'll have to check. Shipping and some reasonable cost for whatever parts you use or buy the carbs and kits outright. Let me know John
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| Posted: 04-26-2007 12:00 am |
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12th Post |
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smcmanus Member
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OK, e mail me smcmanus at mis dot net. I'm definately interested in buying them. I may be in MSP soon so maybe I can get them there. My dad lives in Mendota. Thanks Steve
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| Posted: 05-03-2007 01:10 pm |
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13th Post |
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smcmanus Member
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Thanks to John for coming through with a set of carbs! Much appreciated Steve
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| Posted: 05-16-2007 04:30 pm |
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14th Post |
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smcmanus Member
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Rainy day today so I had a chance to fiddle with the carbs. I was able to balance them out then I tried to set the mixture. I'm able to get the front carb set pretty good, but the rear carb is too lean. I've cranked the needle CW quite a bit and the little washer is recessed up into the vacuum valve but still too lean. Am I missing something obvious? Any suggestions? Thanks Steve
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| Posted: 05-17-2007 04:53 am |
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15th Post |
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Mark Rosenbaum Member
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(1) Ensure that the fuel float in each carb is set to the identical level (0.688" nominal), and that the same thickness washer (0.087") is used under each carb's fuel inlet valve ('needle valve'). (2) Ensure that both carbs use mixture needles bearing the identical identification code (B1CM for early cars, B1DK for later ones). The needle identification code is stamped into the shaft just under the head of the needle, and may be viewed by pulling gently by hand on the needle shaft until the text becomes visible. (3) Gauge the main metering jet (the brass tube in the carb body, into which the mixture needle fits) for size and wear. A #40 (0.098" diameter) drill bit should always fit, a #39 (0.099") drill bit should be a tight to interference fit, and a #38 (0.101") drill bit should not fit. Circularity of the hole may be readily estimated by inserting the #40 drill bit into the jet and holding the carburetor body up against a bright light.
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| Posted: 05-20-2007 05:41 pm |
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16th Post |
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smcmanus Member
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Mark (et al) More time to play around today so I checked the items on the list. Float height is good. Needles......oops B1CM front which came with the car; B1DK rear which came with the spare carbs. OK, that's a problem. Can I use the B1DK needles in the carbs which originally came on the car equipped with the B1CM needles? Are the metering "jets" the same even though the needles are different? I was thinking of using the spare carbs which came with the B1DK needles. They have been rebuilt but almost everything is suspect; the float height is way off, O-rings in the wrong place etc. One thing is the metering jets are very slightly elevated in the venturi where on the old carb the metering jets sit down about a 16th of an inch into the body which is about correct by the book. Can the metering jet height be adjusted? Additionally the needles are very easy to turn in the spare carbs. How easy should they turn? I seems like they could easily vibrate off of the correct settings. Thanks for the help Steve
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| Posted: 05-20-2007 07:22 pm |
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17th Post |
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Mark Rosenbaum Member
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smcmanus wrote: Needles......oops B1CM front which came with the car; B1DK rear which came with the spare carbs. [...] Can I use the B1DK needles in the carbs which originally came on the car equipped with the B1CM needles? You can use either type as long as both carbs use the same type. The B1DK gives you a somewhat richer idle which might conceivably cause emissions concerns for some cars. Ideally both needles should show about the same amount of wear. Also, if you care about the exact differences, I wrote an article on the topic some time ago, which IIRC can be found somewhere in the old JHPS archives. Are the metering "jets" the same even though the needles are different? [...] One thing is the metering jets are very slightly elevated in the venturi where on the old carb the metering jets sit down about a 16th of an inch into the body which is about correct by the book. Can the metering jet height be adjusted? AFAIK, Stromberg main or metering jets come in only two bore sizes, 0.090" and 0.100". Only the latter is used in the JH's carbs. Ideally, both jets should sit at the same height/depth with respect to the top of the bridge into which they are pressed, though small differences can be tolerated provided they're within the range of adjustment for the mixture needles. The jets are pressed into the body of the carb. I've never seen that done so am not certain from which direction the jet is pressed. It might be possible to adjust the jet position in the carb body at home, assuming one had a press or a really big vise, but there'd be some risk of ruining both jet and carb body. Just FYI, I've attached a photo of a jet that I found on eBay awhile back. Additionally the needles are very easy to turn in the spare carbs. How easy should they turn? I seems like they could easily vibrate off of the correct settings. Normally, the mixture needle adjusters should turn quite easily -- they're just brass-on-brass screw threads. The anti-leak o-ring on the adjuster body seems to damp vibrations well enough that the mixture setting doesn't change much, if at all, between normal service intervals. I think you'll be very pleased with your car's performance once you get the carbs fully dialed in. Attachment: Main jet.jpg (Downloaded 151 times)
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| Posted: 05-20-2007 09:29 pm |
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18th Post |
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smcmanus Member
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OK, I put the matching needles into my old carbs and fiddled and tweaked and balanced and it now idles quite nicely. My ears will never be the same. Thanks Steve
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| Posted: 06-28-2007 06:08 pm |
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19th Post |
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sjensen24 Member
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thanks for the clear explanation, Mark. I have been working on a similar problem. A mechanic suggested to me that my car might be stuttering because it is too lean. I did the test suggested and the engine slowed down. This is really helpful.
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